EP0122807A1 - Hydraulic system with proportional control - Google Patents
Hydraulic system with proportional control Download PDFInfo
- Publication number
- EP0122807A1 EP0122807A1 EP84302591A EP84302591A EP0122807A1 EP 0122807 A1 EP0122807 A1 EP 0122807A1 EP 84302591 A EP84302591 A EP 84302591A EP 84302591 A EP84302591 A EP 84302591A EP 0122807 A1 EP0122807 A1 EP 0122807A1
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- European Patent Office
- Prior art keywords
- motor
- valve
- pump
- load
- speed range
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- 238000006073 displacement reaction Methods 0.000 claims abstract description 12
- 230000007704 transition Effects 0.000 claims abstract description 12
- 239000012530 fluid Substances 0.000 claims description 31
- 230000001965 increasing effect Effects 0.000 claims description 13
- 230000003247 decreasing effect Effects 0.000 claims description 3
- 230000003213 activating effect Effects 0.000 claims description 2
- 230000008878 coupling Effects 0.000 claims description 2
- 238000010168 coupling process Methods 0.000 claims description 2
- 238000005859 coupling reaction Methods 0.000 claims description 2
- 230000001105 regulatory effect Effects 0.000 claims 4
- 230000001276 controlling effect Effects 0.000 claims 3
- 230000007935 neutral effect Effects 0.000 description 18
- 238000010586 diagram Methods 0.000 description 5
- 230000004044 response Effects 0.000 description 5
- 230000008859 change Effects 0.000 description 3
- 230000000994 depressogenic effect Effects 0.000 description 2
- 230000001976 improved effect Effects 0.000 description 2
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 230000001939 inductive effect Effects 0.000 description 1
- 230000000977 initiatory effect Effects 0.000 description 1
- 230000009467 reduction Effects 0.000 description 1
- 229910052710 silicon Inorganic materials 0.000 description 1
- 239000010703 silicon Substances 0.000 description 1
- 238000011144 upstream manufacturing Methods 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
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Classifications
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F15—FLUID-PRESSURE ACTUATORS; HYDRAULICS OR PNEUMATICS IN GENERAL
- F15B—SYSTEMS ACTING BY MEANS OF FLUIDS IN GENERAL; FLUID-PRESSURE ACTUATORS, e.g. SERVOMOTORS; DETAILS OF FLUID-PRESSURE SYSTEMS, NOT OTHERWISE PROVIDED FOR
- F15B21/00—Common features of fluid actuator systems; Fluid-pressure actuator systems or details thereof, not covered by any other group of this subclass
- F15B21/08—Servomotor systems incorporating electrically operated control means
- F15B21/087—Control strategy, e.g. with block diagram
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B66—HOISTING; LIFTING; HAULING
- B66F—HOISTING, LIFTING, HAULING OR PUSHING, NOT OTHERWISE PROVIDED FOR, e.g. DEVICES WHICH APPLY A LIFTING OR PUSHING FORCE DIRECTLY TO THE SURFACE OF A LOAD
- B66F9/00—Devices for lifting or lowering bulky or heavy goods for loading or unloading purposes
- B66F9/06—Devices for lifting or lowering bulky or heavy goods for loading or unloading purposes movable, with their loads, on wheels or the like, e.g. fork-lift trucks
- B66F9/075—Constructional features or details
- B66F9/20—Means for actuating or controlling masts, platforms, or forks
- B66F9/22—Hydraulic devices or systems
Definitions
- This invention relates to hydraulic systems for work vehicles such as lift trucks; more particularly, it relates to a control system which allows gradual energization of an hydraulic load circuit with reduced power loss.
- the lift motor In conventional lift trucks it is common practice to use an electric motor to drive a positive displacement lift pump which supplies pressurized fluid to the hydraulic load device including the lift motor.
- the hydraulic lift motor requires greater hydraulic flow than the other hydraulic load devices, such as the power steering system which typically requires the least hydraulic flow.
- Certain auxiliary load devices such as a load handling clamp, require an intermediate value of hydraulic flow.
- the lift motor is energized with pressurized fluid through a manually controlled lift valve.
- the lift valve is an open centre, proportioning valve which provides full flow from the inlet port to the tank outlet port with no flow to the load outlet port when the valve is closed. It provides full flow from the inlet port to the load outlet port with no flow to the tank outlet port when the valve is open.
- a proportioning valve In the range between the open and closed conditions there is a functional relationship between the flow to the load port and the valve movement and, even though the functional relationship may be nonlinear, the valve is called a proportioning valve.
- a proportioning valve permits feathering operation of a load device, i.e. gradual energization by gradual opening of the valve in the transition region between the open and closed conditions.
- the lift motor is started by moving the lift control lever from the neutral position in the lift direction. During first increment of movement the control lever actuates a switch which energizes the pump motor with full battery voltage to produce maximum pump flow. At first, the entire hydraulic fluid circulates from the pump outlet through the open centre lift valve and returns to the tank. As the control lever is moved further, the lift valve progressively closes the tank return port and progressively opens the lift motor port so that there is a decreasing flow to the tank and an increasing flow to the motor in correspondence with the movement of the lift control lever. When the flow resistance at the tank return port is sufficiently great, the back pressure causes the lift motor to lift the load. During the transition from full flow to the tank to full flow to the lift motor, energy is lost in the lift valve in the form of heat due to the pressurized fluid being reduced to the low pressure of the tank.
- a general object of this invention is to provide an improved hydraulic control system which overcomes certain disadvantages of the prior art and provides for gradual energization of a load device with minimized power loss.
- the present invention provides an hydraulic system of the type having first and second hydraulic load circuits, the second circuit having a greater hydraulic flow requirement than the first circuit, a positive displacement hydraulic pump connected with the load circuits, a changeable speed electric motor connected with the pump, said pump being adapted to supply the flow required by the first load circuit when the motor is operated in a low speed range and to supply the flow required by the second load circuit when the motor is operated in a high speed range, a proportioning valve for controlling flow from the pump to the second load circuit, said valve having an inlet port in fluid communication with the pump outlet, a return port in fluid communication with the pump inlet and a load port in fluid communication with the load device, said valve including means for progressively opening the load port while progressively closing the return port, and control means for energizing the motor for operation in the low speed range during opening of the load port and for energizing the motor for operation in the high speed range when the load port is fully open and the return port is fully closed whereby the pump is operated in the low speed range during transition of the return
- the present invention also provides a work vehicle of the type having first and second hydraulic load circuits, said second load circuit having a greater hydraulic flow requirement than the first circuit, a positive displacement hydraulic pump for supplying pressurized fluid to the load circuits, a changeable speed electric motor connected with the pump, said pump being adapted to supply the flow required by the first load circuit when the motor is operated in a low speed range and to supply the flow required by the second load circuit when the motor is operated in a high speed range, a first manually actuable selector means for operatively coupling the first load circuit with said pump and for causing said motor to be operated in the low speed range, an open centre proportioning valve for controlling flow from the pump to the second load circuit, said valve having a valve body including and inlet port in fluid communication with the pump outlet, a return port in communication with the pump inlet and a load port in communication with a load device, said valve including a spool movable between a closed position and an open position, said closed position having the load port closed to the inlet port and the return
- an hydraulic load circuit requiring a relatively high flow is energized from a changeable speed pump by opening a proportioning valve through its proportional range with the pump operating in a low speed range and then, when the valve is fully open, increasing the speed of the motor to its high speed range.
- feathering operation of the valve occurs a reduced flow level with a consequent reduction in power loss; initiation of the high speed after the valve is fully open produces the higher flow level required for the load circuit.
- a motor control circuit is provided to increase the motor torque in response to increased load so that a heavy load device can be actuated during operation of the motor in the low speed range. This is accomplished by a motor control system in which the motor voltage is increased in response to increased motor current to regulate the motor speed at a substantially constant value or within predetermined limits.
- an hydraulic control system provides for energizing a load device by a proportioning valve with a minimum of power loss during the transition from zero to full energization.
- the system comprises a proportioning valve for controlling the flow from a positive displacement pump to the load circuit, the valve having an inlet port communicating with the pump outlet, a return port communicating with the pump inlet and a load port communicating with the load device.
- the valve includes means for progressively 3 opening the load port while progressively closing the return port.
- Control means are provided for energizing the motor for operation in a low speed range during opening of the load port and for energizing the motor for operation in a high speed range when the load port is fully open and the return outlet is fully closed.
- an improved proportional lift control is provided for an electric lift truck in which the lift cylinder is energized through its feathering range with a minimum of power loss.
- An electric motor drives a positive displacement hydraulic pump which supplies the flow requirements of the power steering circuit when the motor is operated in a low speed range. It supplies the flow requirements of the lift circuit when the motor is operated in a high speed range.
- a lift valve for controlling the flow from the pump to the lift circuit is an open centre, proportioning type valve and provides full flow to the tank when the valve is closed and full flow to the lift cylinder when the valve is open.
- a manually actuable lift lever actuates the lift valve through the feathering range with pump motor in the low speed range and actuates a speed control switch to operate the motor in the high speed range when the lift valve is fully open.
- an electric lift truck is depicted to show the driver's station and the manual controls for operating the lift truck.
- the lift truck is provided with an extendible upright with a lift carriage which is raised or lowered by an hydraulic motor.
- the upright is mounted on the vehicle frame for pivotal motion about a horizontal axis for tilting forwardly or rearwardly by a pair of hydraulic motors.
- the lift truck may be provided with an auxiliary load handling device such as a side shifter, load camp or rotator.
- the lift truck is provided with a driver's station including a seat 12 and a steering wheel 14 which is coupled to the dirigible wheels (not shown) through an hydraulic power steering system which will be described subsequently.
- the manual controls for load handling functions of the lift truck include a lift control lever 16, a tilt control lever 18 and an auxiliary control lever 22.
- a drive selector lever 24 is mounted on the steering column and is coupled with a controller 26 which provides starting and speed control of the traction motor.
- the controller 26, per se, forms no part of the present invention; it does, however, control the actuation of certain switches which affect the operation of the hydraulic system.
- the drive selector lever 24 is coupled by a suitable linkage 28 with the controller 26 for the selection of neutral, forward, reverse or park modes of operation of the traction motor.
- a seat occupancy detector 32 is provided as a safety device in the control of the traction motor and the hydraulic system.
- the detector 32 includes a suitable linkage 34 coupled between the seat 12 and the controller 26.
- An accelerator pedal 36 is connected through a suitable linkage to the controller 26 for starting and speed control of the traction motor.
- the driver's station additionally includes a foot brake pedal 38 for operation of the service brake of the vehicle.
- FIG. 2 is a block diagram of the hydraulic system of the lift truck. It comprises a single hydraulic pump 42 of the positive displacement type. The pump is driven by an electric motor 44 which is a DC series motor.
- the system further comprises a steering unit 46 and a main valve 48. Hydraulic fluid is supplied from the pump 42 to the steering unit 46 and through the steering unit to the main valve 48.
- the steering unit 46 includes a steering control valve (not shown in Figure 2) which supplies fluid to the hydraulic motor or steering cylinder 52 for power steering.
- the steering unit also includes a return to the hydraulic reservoir or tank 54.
- the main valve 48 supplies fluid to the hydraulic motor or lift cylinder 56 for the lift carriage of the lift truck.
- the main valve 48 supplies fluid to the hydraulic motors or tilt cylinders 58 and 58' for tilting the upright of the lift truck. Additionally, the main valve 48 supplies fluid to an auxiliary hydraulic motor or cylinder 59. The fluid from the main valve is returned to the tank through a return line.
- Figure 3 shows the electrical circuits, in block diagram, for energizing the pump motor 44 and for energizing the traction motor 62.
- the voltage from the vehicle battery 64 is supplied through a key switch 65 to the controller 26 and thence through a pump switch 82 to the pump motor 44.
- the energizing circuit for the pump motor 44 is completed through a voltage control circuit 66.
- the pump 42 is connected with the hydraulic system 100.
- the battery voltage is applied from the controller 26 through the motor control circuits 68 to the traction motor 62.
- the controller 26 includes switching circuits which are controlled by the drive selector 24, the seat occupancy detector 32 and the accelerator pedal 36 in a manner which will be described presently.
- the controller 26 includes a start switch 72 between the battery 64 and the motor control circuit 68.
- the start switch is actuated by movement of the accelerator pedal 36.
- a speed control member 74 of the ) inductive type is coupled with the accelerator pedal and supplies a speed control signal to the motor control circuits 68.
- Direction control for the traction motor is provided by a forward control switch 76 and a reverse control switch 78. These switches are connected in parallel with each other and in series with the start switch 72 between the battery 64 and the motor control circuits 68.
- the traction motor 62 will be energized through the motor control circuit 68 when the key switch 65 and the start switch 72 are closed with either the forward control switch 76 or the reverse control switch 78 closed.
- the controller 26 also includes a pump control switch 82 which is serially connected between the key switch 65 and battery 64 and the pump motor 44. Accordingly, when the pump control switch 82 is opened, the pump motor 44 is turned off and the fluid to the hydraulic system 100 is cut-off.
- the pump control switch 82, the forward control switch 76 and the reverse control switch 78 are selectively actuated by the drive selector lever 24. With the drive selector lever in neutral, both the forward switch 76 and the reverse switch 78 are open and the pump control switch 82 is closed. With the selector lever in the forward or reverse positions the respective forward and reverse switches 76 and 78 are closed and the pump switch 82 is closed. With the selector lever in park, the forward and reverse switches 76 and 78 are open and the pump control switch 82 is open.
- the selective actuation of the forward and reverse switches 76 and 78 and the pump control switch 82 is also controlled by the seat occupancy detector 32.
- the occupancy detector is operative, when the selector lever 24 is in neutral, to cause the pump control switch 82 to open and thereby turn off the pump motor 44 when the driver dismounts from the seat.
- the detector 32 is operative, when the selector lever 24 is in either forward or reverse, in response to the driver dismounting from the seat, to cause the forward and reverse switches 76 and 78 to open and deenergize the traction motor and to cause the pump control switch 82 to open and deenergize the pump motor.
- the electrical circuit of Figure 3 is operative to deenergize the traction motor 62 unless the key switch 65 and the start switch 72 and one of the forward or reverse switches 76 or 78, respectively, are all closed. Also, it is operative to deenergize the pump motor 44 unless the key switch 65 and the pump switch 82 are both closed; the pump switch 82 is closed only when the driver's seat is occupied and when the drive selector lever 24 is in forward, reverse or neutral. The pump switch 82 is open when the drive selector lever 24 is in park regardless of driver's seat occupancy. As a result of this control, hydraulic fluid to the hydraulic system 100 is cut-off unless the driver occupies the seat and the drive selector lever 24 is in forward, reverse or neutral.
- the hydraulic system 100 includes a power steering circuit for the vehicle, a lift circuit for the lift carriage, a tilt circuit for the upright and an actuation circuit for the auxiliary load handling device.
- Manual control of the power steering unit 46 is exercised by the steering wheel.
- Manual control of the lift, tilt and auxiliary load handling functions is exercised by respective hand levers 16, 18 and 22 (see Figure 2).
- the steering wheel 14 is coupled with the steering valve 84 for actuation of the valve in response to steering wheel motion.
- the lift control lever 16 is coupled with a lift valve 86 and the tilt control lever 18 is coupled with a tilt valve 88.
- the auxiliary control lever 22 is coupled with an auxiliary valve 92.
- the hydraulic system is shown schematically in Figure 9.
- the system comprises the positive displacement pump 42, the steering unit 46 and the main valve 48.
- the system also includes the hydraulic reservoir or tank 54.
- the hydraulic system is arranged so that the power steering circuit has priority over all other hydraulic functions in the lift truck.
- a priority demand valve 102 is provided and for design purposes it is located in the steering unit 46.
- the steering unit 46 also includes a steering valve 84 which controls energization of the steering cylinder 52.
- the main valve 48 includes the lift control valve 86 which controls the lift cylinder 56 and the tilt control valve 88 which controls the tilt cylinders 58 and 58'.
- the main valve also includes the auxiliary control valve 92 which controls the auxiliary cylinder 59.
- the priority demand valve 102 is located upstream of the steering control valve 84 and the main valve 48.
- a pressure relief valve 104 is coupled between the priority demand valve 102 and the steering control valve 84.
- a two-stage pressure relief valve or hydrastat is located in the main valve 48.
- the two-stage pressure relief valve includes a main relief valve 106 which is controlled by a pilot relief valve 108 and a pilot relief valve 112.
- the pump 42 has its inlet connected to the tank 54.
- the outlet of the pump 42 is connected to the inlet port 114 of the priority demand valve 102.
- the priority demand valve has a primary outlet port 116 connected with the inlet port 118 of the steering control valve 84. It has a secondary outlet port 122 connected through a supply line 124 to a control inlet port 150 of the lift control valve 86.
- the priority demand valve 102 is adapted to give priority to the flow requirements of the power steering system through the primary outlet port 116. If the input flow to the priority demand valve is greater than that to be allocated to the power steering system, the excess flow is diverted by the priority demand valve to the seconday outlet port 122 and line 124 for use by other hydraulic functions.
- the power steering circuit may be rated for a maximum of five gallons (22.75 litres) per minute and a maximum of 1,000 PSI (70.4 kg/cm 2 ). It will be appreciated that the actual flow and pressure will be determined by the actuation of the steering control valve 84 and the load imposed by the steering system. The flow produced to the inlet port 114 of the priority demand valve 102 depends, of course, upon the speed of the pump 42. As will be described further below, in the example of this illustrative embodiment, operation in a low speed range of about 800 RPM is used for the power steering and tilt functions.
- This provides about five gallons (22.75 litres) per minute and the pressure may vary over a range up to 1,000 PSI(70.4 kg/cm 2 ), depending upon load.
- Operation of the pump in an intermediate speed range of about 1,200 RPM is used for the auxiliary load device such as a load handling clamp; the flow requirement may be about nine gallons (40.95 litres) per minute and the pressure may range up to 2,000 PSI (140.8 kg/cm 2 ).
- the flow In operation of the pump at high speed, which may be about 1,800 RPM for the lift function, the flow may be about 20 gallons (91 litres) per minute with pressures ranging up to 3,000 P S I (211.2 kg/ cm 2 ).
- the power steering circuit comprises the steering control valve 84 and the steering cylinder 52.
- the steering control valve 84 is a metering valve with an open centre spool adapted to bi-directional control of the double-acting steering cylinder 52 for actuating the dirigible wheels of the lift truck.
- the steering control valve 84 is provided with a primary return port 126 which is connected through a check valve 128 to the supply line 124.
- the steering control valve 84 is provided with a secondary return port 132 which is connected to the tank 54.
- the relief valve 104 is provided to prevent excessive pressure in the steering system. For this purpose, it has its inlet port connected to the primary outlet port 116 of the priority demand valve 102.
- the outlet port of the relief valve 104 is connected to the secondary return port 132. Thus, if the pressure at the primary outlet port 116 of the priority demand valve becomes excessive, the pressure relief valve 104 opens and dumps fluid to the tank.
- the steering control valve 84 has outlet ports 134 and 136 connected with opposite ends of the
- the lift circuit includes the lift control valve 86 which is a metering or proportioning valve having an open centre spool.
- the spool is normally centered and is movable in opposite directions by the lift control lever 16.
- the lift control valve 86 communicates with the single-ended lift cylinder 56 through a lift-lower valve 138 and a lowering flow control valve 142.
- the secondary outlet port 122 of the priority demand valve 102 is connected through the supply line 124 to the open centre inlet port 144 of the lift control valve 86.
- the open centre outlet port 146 of the valve 86 is connected to the open centre inlet port 148 of the tilt control valve 88.
- the priority demand valve 102 also has its secondary outlet port 122 connected with the control inlet port 150 of the lift control valve 86.
- the control outlet port 152 is connected with the inlet port of the lift-lower valve 138.
- the lift control valve 86 has a return line port 154 connected to a return line 156 which goes to the tank 54.
- the lift-lower valve 138 has a port 158 connected to a port 162 on the lowering flow control valve 142.
- the lowering flow control valve 142 has a port 164 connected with the lift-lower single ended cylinder 56. Operation of the lift-lower circuit will be described in greater detail subsequently.
- the tilt control circuit including the control valve 88 and the auxiliary control circuit including the control valve 92 will be described briefly.
- the tilt control valve 88 is a metering or proportioning valve of the open centre type with a spool which is bi-directionally movable by the tilt control lever 18.
- the valve 88 communicates with the single ended tilt cylinders 58 and 58' through a counterbalance valve 166.
- the tilt control valve 88 has an open centre inlet port 148 connected with the open centre outlet port 146 of the lift control valve 86.
- the tilt control valve 88 has an open centre outlet port 168 which is connected to an open centre inlet port 172 on the auxiliary control valve 92.
- the tilt control valve 88 has a control inlet port 174 connected with the secondary outlet port 122 of the priority demand valve through the supply line 124 to the supply passage 176 and thence through a flow control valve 178.
- the tilt control valve 88 has a control outlet port 182 which is connected through the counterbalance valve 166 to the tilt cylinder 58 and 58'.
- the tilt control valve 88 also includes a return inlet port 184 which is connected with the tilt cylinders 58 and 58'.
- a return outlet port 186 on the control valve 88 is connected to the return line 156.
- the auxiliary control valve 92 is a metering or proportioning valve having an open centre spool which is bi-directionally movable by the auxiliary control lever 22.
- the control valve 92 is adapted to control the energisation of the double-acting auxiliary cylinder 59.
- the control valve 92 has an open centre inlet port 172 which is connected with the open centre outlet port 168 of the tilt control valve 88.
- the control valve 92 also has an open outlet centre port 188 which is connected with the return line 156.
- a control inlet port 192 is connected with the secondary outlet port 122 of the priority demand valve 102. This connection extends through the supply line 124 to the passage 176 and thence through a flow control valve 194 to the control inlet port 192.
- the control valve 92 has a pair of control ports 195 and 196 which are connected respectively with the opposite ends of the auxiliary cylinder 59. It also has a return outlet port 198 connected with the return line 156. It also has a return inlet port 202 connected with the outlet port of the pilot relief valve 112. Also, the control valve 92 has a return outlet port 204 connected with the return line 156.
- the two-stage relief valve comprises a main relief valve 106, a pilot relief valve 108 and a pilot relief valve 112.
- the two-stage valve is adapted to relieve the pressure at the outlet port 122 of the priority demand valve 102 when it exceeds a predetermined operating value for different operating conditions.
- a predetermined operating value for different operating conditions.
- the pressure is to be limited, for example, to 3,000 PSI(211.2 kg/cm2).
- the pump is operated at an intermediate speed and the pressure is limited, for example, to 2,000 PSI (140.8 kg/cm 2 ).
- a lift truck with an hydraulic system of the type described above is disclosed in co-pending application United States Serial No. 291,681 filed August 10, 1981 and assigned to the same assignee as this application.
- the hydraulic pump 42 is operated at different speeds according to the flow demand which varies with the operating mode of the hydraulic system.
- a low motor speed is adequate.
- the power steering circuit requires a maximum of about five gallons (22.75 litres per minute).
- a pump speed of 800 RPM would be adequate for power steering.
- a low pump speed is adequate for other operating modes, such as tilt of the upright.
- pump 42 is driven by the series DC motor 44 having a series field winding 252.
- the motor is energized from the battery 64 through the voltage control circuit 66.
- the voltage control circuit is adapted to regulate the motor speed within low, intermediate and high speed ranges according to the operating mode of the hydraulic circuit.
- the voltage control circuit 66 is a thyristor-type pulsing circuit having presettable means for determining a mark/ space ratio for the different speed ranges and being provided with a feed back means for changing the mark/space ratio as a function of motor current.
- Such a motor control circuit is disclosed in the Morton et al United States Patent No. 4,119,898 granted October 10, 1978.
- the voltage control circuit 66 comprises a silicon control rectifier (SCR) 254 connected in series with the motor 44.
- a current sensing resistor 256 is connected in series with the SCR and develops at a feedback voltage corresponding to the value of motor current.
- a pulse generator 258 supplies a pulse train to the gate of the SCR 254 which controls the effective motor supply voltage in accordance with the mark/space ratio of the pulse train.
- a mark/space demand signal circuit 262 produces a demand signal which is applied to the pulse generator 258 and determines the mark/space ratio of the pulse train. The value of the demand signal produced by the demand signal circuit 262 is modified by the demand modifying circuit 264 in accordance with the feedback voltage supplied from the sensing resistor 256.
- the output of the demand signal circuit 262 is connected with the pulse generator 258 through a series connection of the key switch 65 and the pump switch 82 which were described with reference to Figure 3. Unless both the key switch and the pump switch are closed there is no demand signal input to the pulse generator 258 and the SCR is turned off and the motor 44 cannot be energized.
- the demand signal circuit 262 is provided with a speed range selection circuit 266. This circuit includes a pair of series resistors 268 and 272. It also includes an intermediate speed selector switch 274 and a high speed selector switch 276. When both switches 274 and 276 are open, the low speed range is selected. When switch 274 is closed with switch 276 open, the intermediate speed range is selected and when the switch 276 is closed the high speed range is selected.
- the selector switch 274 is closed by actuation of the auxiliary control lever 22 and the switch 276 is closed by the lift control lever 16, as will be described below.
- the demand modifying circuit 262 In operation of the voltage control circuit 66, the demand modifying circuit 262 produces an output signal which increases in magnitude as a direct function of the feedback voltage from the resistor 256 and hence as a direct function of the motor current.
- the mark/space demand signal circuit 262 produces a demand signal which increases from a minimum value when the motor current is zero to a maximum value when the motor current reaches a predetermined value 1 0 .
- This causes the pulse generator 258 to produce a pulse train having a mark/space ratio which increases from a predetermined minimum value to a maximum value in correspondence with a demand signal.
- the action of the demand modifying circuit 264 in response to the feedback voltage tends to maintain the motor speed substantially constant when it is operated in either the low speed range or the intermediate speed range.
- the lift control lever 16 is shown in greater detail in Figure 5.
- the lift control lever is adapted to actuate the lift control valve 86 and to exercise manual control over the operating speed of the pump motor.
- the lift control valve 86 is a spool valve having a spool 206. The spool is axially movable in either direction from a neutral position to select either the lift mode or the lower mode of operation.
- the lift control valve 86 will be described in greater detail with reference to Figure 4.
- the structure of the lift control valve 86 is depicted in Figure 4.
- the lift control valve 86 is a conventional proportioning valve of the open-centre, spool type and comprises a valve spool 206 in a valve body 208.
- the spool 206 as shown in Figure 4 is in a neutral position and is biased toward that position by a centering spring 282.
- the valve body 208 is provided with the supply line 124 which supplies fluid pressure from the secondary outlet port 122 of the priority demand valve 102.
- the supply line 124 is connected with the open centre inlet port 144 and also with the control inlet port 150.
- the valve body 208 also provides an open centre outlet port 146 which is connected with the open centre inlet port 148 of the tilt control valve 88, as previously described.
- a control outlet port 152 is connected with the lift cylinder 56 through the lift-lower valve 138 and the lower control valve 142, as previously described.
- a return line port 154 is connected with the return line 156, as previously described.
- the valve spool 206 comprises cylindrical sections 284, 286 and 288.
- the spool defines an annular chamber 292 between the sections 284 and 286 and an annular chamber 294 between the sections 286 and 288.
- the valve spool 206 is movable by actuation of the lift lever 16 to a "fully open” condition by moving the spool 206 upwardly (as viewed in Figure 4) until the open centre inlet port 144 is fully closed to the open centre outlet port 146 by the spool section 286. With the spool in this position, the control outlet port 152 is fully open to the control inlet port 150 through the chamber 152 and the return line port 154 remains closed by the spool section 288. In this position, the valve is referred to as being "fully open”.
- the fluid flow to the lift cylinder 56 is controlled in a proportional manner.
- the valve With the spool 206 in the neutral position, the valve is fully closed and the fluid flows freely through the open centre inlet port 144 to the open centre outlet port 146 and there is no flow to the control outlet port 152. Upward movement of the spool 206 results in restricted flow from the open centre inlet port 144 to the open centre outlet port 146 which causes a back-pressure in the fluid and there is restricted flow from the control inlet port 150 to the control port 152.
- This movement causes the spool section 284 to close the open centre outlet port 146 to the open centre inlet port 144 and at the same time to open the return line port 154 to the control outlet port 152 through the chamber 294 so that the fluid pressure in the lift cylinder 56 is released.
- the driver may also exercise feathering control during the operation in the lowering mode.
- the lift control lever 16 ( Figure 5) is pivotallty mounted on a shaft 212 for pivotal motion in the fore and nfL directions by the lift truck driver.
- the lever 16 is provided with a slot 214 which receives a pin 216 extending through the end of spool 206 of the control valve. Accordingly, when the control lever 16 is pivoted in the rearward direction (phanton lines) the stem 206 is raised from the neutral position and the valve is operated in the lift mode. When the hand lever 16 is rotated in the forward direction, the stem 206 is lowered and the valve is operated in the lowering mode.
- a speed control switch 218 is provided.
- the speed control switch 218 is electrically connected in the speed control circuit described with reference to Figure 6.
- the speed control switch comprises switch contacts 222 which are biased by a spring 224 toward a closed position.
- a switch plunger 226 is adapted, when depressed, to hold the switch contacts 222 in the open position.
- the speed control circuit causes the pump motor 42 to operate in the high speed range.
- the lift lever 16 is adapted to close the speed control switch 218 concurrently with full opening of the valve 86.
- the control lever 16 is provided with an arm 228 which actuates a push rod 232.
- the push rod in turn, actuates the switch plunger 226.
- the push rod is mounted in the control lever housing 234 for reciprocal motion and is spring loaded by a bias spring 236 in the upward direction. With the lever in the neutral position, the upper end 238 of the push rod 32 is spaced from the lower face 242 of the arm 228. This provides a lost motion of predetermined distance D, as indicated in Figure 5, between the control lever 16 and the push rod 232.
- control valve 86 When the control handle 16 is returned to the neutral position after operating in the lift mode, the control valve 86 is returned to its neutral position in which the valve is closed. This movement of the control lever 16 allows the speed control switch 218 to open and the motor control circuit is switched for operation in the lower speed range.
- the control lever 16 When the control lever 16 is operated to lower the carriage by pivotal motion in the forward direction from the neutral position, the spool is moved downwardly and the cylinder port is connected to the tank return port. This releases fluid from the lift cylinder to lower the load carriage.
- the control valve 86 may be operated in the proportional manner for feathering the lowering control of the load carriage.
- the lift truck driver may operate the lift lever 16 to raise the lift carriage while the pump motor is operated in the lower speed range.
- the low speed motor operation obtains when the lift truck is operated in the idle mode (key switch 66 and pump switch 82 closed), power steering mode, or the tilt mode, or any combination of these modes.
- the lift control valve 86 is moved from the fully closed position to the fully open position through its feathering range while the pump motor is operated in the low speed range.
- the lift control valve is opened, the pressure in the lift cylinder 56 is progressively increased causing the load on the motor to increase.
- the motor speed control system increases the motor torque with increased load to maintain the motor speed substantially constant within the low speed range.
- the speed control switch 276 When it is fully open at time t2, the speed control switch 276 is closed and the motor speed increases in a ramp fashion to its maximum speed at time t3 and remains substantially constant at that speed. It will now be appreciated that similar operation will be obtained if the lift truck driver actuates the lift control lever 16 when the pump motor is operated in the intermediate speed range. In such case, the feathering operation of the lift control valve takes place at the relatively lower intermediate pump speed and the high speed pump operation is not initiated until the control valve is fully open.
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Abstract
Description
- This invention relates to hydraulic systems for work vehicles such as lift trucks; more particularly, it relates to a control system which allows gradual energization of an hydraulic load circuit with reduced power loss.
- In conventional lift trucks it is common practice to use an electric motor to drive a positive displacement lift pump which supplies pressurized fluid to the hydraulic load device including the lift motor. The hydraulic lift motor requires greater hydraulic flow than the other hydraulic load devices, such as the power steering system which typically requires the least hydraulic flow. Certain auxiliary load devices, such as a load handling clamp, require an intermediate value of hydraulic flow. The lift motor is energized with pressurized fluid through a manually controlled lift valve. The lift valve is an open centre, proportioning valve which provides full flow from the inlet port to the tank outlet port with no flow to the load outlet port when the valve is closed. It provides full flow from the inlet port to the load outlet port with no flow to the tank outlet port when the valve is open. In the range between the open and closed conditions there is a functional relationship between the flow to the load port and the valve movement and, even though the functional relationship may be nonlinear, the valve is called a proportioning valve. Such a proportioning valve permits feathering operation of a load device, i.e. gradual energization by gradual opening of the valve in the transition region between the open and closed conditions.
- The lift motor is started by moving the lift control lever from the neutral position in the lift direction. During first increment of movement the control lever actuates a switch which energizes the pump motor with full battery voltage to produce maximum pump flow. At first, the entire hydraulic fluid circulates from the pump outlet through the open centre lift valve and returns to the tank. As the control lever is moved further, the lift valve progressively closes the tank return port and progressively opens the lift motor port so that there is a decreasing flow to the tank and an increasing flow to the motor in correspondence with the movement of the lift control lever. When the flow resistance at the tank return port is sufficiently great, the back pressure causes the lift motor to lift the load. During the transition from full flow to the tank to full flow to the lift motor, energy is lost in the lift valve in the form of heat due to the pressurized fluid being reduced to the low pressure of the tank.
- A general object of this invention is to provide an improved hydraulic control system which overcomes certain disadvantages of the prior art and provides for gradual energization of a load device with minimized power loss.
- The present invention provides an hydraulic system of the type having first and second hydraulic load circuits, the second circuit having a greater hydraulic flow requirement than the first circuit, a positive displacement hydraulic pump connected with the load circuits, a changeable speed electric motor connected with the pump, said pump being adapted to supply the flow required by the first load circuit when the motor is operated in a low speed range and to supply the flow required by the second load circuit when the motor is operated in a high speed range, a proportioning valve for controlling flow from the pump to the second load circuit, said valve having an inlet port in fluid communication with the pump outlet, a return port in fluid communication with the pump inlet and a load port in fluid communication with the load device, said valve including means for progressively opening the load port while progressively closing the return port, and control means for energizing the motor for operation in the low speed range during opening of the load port and for energizing the motor for operation in the high speed range when the load port is fully open and the return port is fully closed whereby the pump is operated in the low speed range during transition of the return port from open to close to reduce the energy loss during the transition.
- The present invention also provides a work vehicle of the type having first and second hydraulic load circuits, said second load circuit having a greater hydraulic flow requirement than the first circuit, a positive displacement hydraulic pump for supplying pressurized fluid to the load circuits, a changeable speed electric motor connected with the pump, said pump being adapted to supply the flow required by the first load circuit when the motor is operated in a low speed range and to supply the flow required by the second load circuit when the motor is operated in a high speed range, a first manually actuable selector means for operatively coupling the first load circuit with said pump and for causing said motor to be operated in the low speed range, an open centre proportioning valve for controlling flow from the pump to the second load circuit, said valve having a valve body including and inlet port in fluid communication with the pump outlet, a return port in communication with the pump inlet and a load port in communication with a load device, said valve including a spool movable between a closed position and an open position, said closed position having the load port closed to the inlet port and the return port open to the inlet port, said open position having the load port open to the inlet port and the return port closed to the inlet port, a second manually actuable selector means coupled with said spool for moving it between the closed and open positions for progressively decreasing the flow to the return port and increasing the flow to the load port, and motor speed control means including switching means for operating the motor in the high speed range when the switching means is activated, said second selector means being coupled with the switching means for activating the switching means when the spool reaches the open position, whereby the pump is operated in the low speed range during transition of the return outlet from open to closed and thereby reduces the energy loss during the transition.
- According to this invention, an hydraulic load circuit requiring a relatively high flow is energized from a changeable speed pump by opening a proportioning valve through its proportional range with the pump operating in a low speed range and then, when the valve is fully open, increasing the speed of the motor to its high speed range. Thus, feathering operation of the valve occurs a reduced flow level with a consequent reduction in power loss; initiation of the high speed after the valve is fully open produces the higher flow level required for the load circuit.
- Further, in accordance with this invention, a motor control circuit is provided to increase the motor torque in response to increased load so that a heavy load device can be actuated during operation of the motor in the low speed range. This is accomplished by a motor control system in which the motor voltage is increased in response to increased motor current to regulate the motor speed at a substantially constant value or within predetermined limits.
- In accordance with this invention, an hydraulic control system provides for energizing a load device by a proportioning valve with a minimum of power loss during the transition from zero to full energization. The system comprises a proportioning valve for controlling the flow from a positive displacement pump to the load circuit, the valve having an inlet port communicating with the pump outlet, a return port communicating with the pump inlet and a load port communicating with the load device. The valve includes means for progressively 3 opening the load port while progressively closing the return port. Control means are provided for energizing the motor for operation in a low speed range during opening of the load port and for energizing the motor for operation in a high speed range when the load port is fully open and the return outlet is fully closed.
- Further, in accordance with this invention, an improved proportional lift control is provided for an electric lift truck in which the lift cylinder is energized through its feathering range with a minimum of power loss. An electric motor drives a positive displacement hydraulic pump which supplies the flow requirements of the power steering circuit when the motor is operated in a low speed range. It supplies the flow requirements of the lift circuit when the motor is operated in a high speed range. A lift valve for controlling the flow from the pump to the lift circuit is an open centre, proportioning type valve and provides full flow to the tank when the valve is closed and full flow to the lift cylinder when the valve is open. A manually actuable lift lever actuates the lift valve through the feathering range with pump motor in the low speed range and actuates a speed control switch to operate the motor in the high speed range when the lift valve is fully open.
- A more complete understanding of this invention may be obtained from the description that follows, taken with the accompanying drawings, in which:
- Figure 1 is a perspective view of a lift truck showing certain manual controls for use by the driver;
- Figure 2 is a block diagram of the hydraulic system of the truck;
- Figure 3 is a diagram of the electrical system;
- Figure 4 is a cross-sectional view of a lift valve;
- Figure 5 shows the lift control lever and associated speed selector switch;
- Figure 6 shows the voltage control circuit for the drive motor of the pump;
- Figure 7 is a graphical representation of the operational characteristics of the drive motor;
- Figure 8 is a graph of speed change for use in explaining operation of the invention; and
- Figure 9 is a schematic diagram of the hydraulic system.
- Referring now to the drawings, there is shown an illustrative embodiment of the invention in the hydraulic system of a work vehicle, specifically a lift truck. It will be appreciated as the description proceeds that the invention may be employed in other types of work vehicles and is useful in other applications.
- Referring now to Figure 1, an electric lift truck is depicted to show the driver's station and the manual controls for operating the lift truck. In a known manner, but not shown in the drawings, the lift truck is provided with an extendible upright with a lift carriage which is raised or lowered by an hydraulic motor. The upright is mounted on the vehicle frame for pivotal motion about a horizontal axis for tilting forwardly or rearwardly by a pair of hydraulic motors. In addition, the lift truck may be provided with an auxiliary load handling device such as a side shifter, load camp or rotator.
- The lift truck is provided with a driver's station including a
seat 12 and a steering wheel 14 which is coupled to the dirigible wheels (not shown) through an hydraulic power steering system which will be described subsequently. The manual controls for load handling functions of the lift truck include alift control lever 16, atilt control lever 18 and anauxiliary control lever 22. Adrive selector lever 24 is mounted on the steering column and is coupled with acontroller 26 which provides starting and speed control of the traction motor. Thecontroller 26, per se, forms no part of the present invention; it does, however, control the actuation of certain switches which affect the operation of the hydraulic system. Thedrive selector lever 24 is coupled by asuitable linkage 28 with thecontroller 26 for the selection of neutral, forward, reverse or park modes of operation of the traction motor. Also, aseat occupancy detector 32 is provided as a safety device in the control of the traction motor and the hydraulic system. Thedetector 32 includes asuitable linkage 34 coupled between theseat 12 and thecontroller 26. Anaccelerator pedal 36 is connected through a suitable linkage to thecontroller 26 for starting and speed control of the traction motor. The driver's station additionally includes afoot brake pedal 38 for operation of the service brake of the vehicle. - Figure 2 is a block diagram of the hydraulic system of the lift truck. It comprises a single
hydraulic pump 42 of the positive displacement type. The pump is driven by anelectric motor 44 which is a DC series motor. The system further comprises asteering unit 46 and amain valve 48. Hydraulic fluid is supplied from thepump 42 to thesteering unit 46 and through the steering unit to themain valve 48. Thesteering unit 46 includes a steering control valve (not shown in Figure 2) which supplies fluid to the hydraulic motor orsteering cylinder 52 for power steering. The steering unit also includes a return to the hydraulic reservoir ortank 54. Themain valve 48 supplies fluid to the hydraulic motor orlift cylinder 56 for the lift carriage of the lift truck. Also, themain valve 48 supplies fluid to the hydraulic motors ortilt cylinders 58 and 58' for tilting the upright of the lift truck. Additionally, themain valve 48 supplies fluid to an auxiliary hydraulic motor orcylinder 59. The fluid from the main valve is returned to the tank through a return line. - Figure 3 shows the electrical circuits, in block diagram, for energizing the
pump motor 44 and for energizing thetraction motor 62. The voltage from thevehicle battery 64 is supplied through akey switch 65 to thecontroller 26 and thence through apump switch 82 to thepump motor 44. The energizing circuit for thepump motor 44 is completed through avoltage control circuit 66. Thepump 42 is connected with thehydraulic system 100. The battery voltage is applied from thecontroller 26 through the motor control circuits 68 to thetraction motor 62. Thecontroller 26 includes switching circuits which are controlled by thedrive selector 24, theseat occupancy detector 32 and theaccelerator pedal 36 in a manner which will be described presently. - The
controller 26 includes a start switch 72 between thebattery 64 and the motor control circuit 68. The start switch is actuated by movement of theaccelerator pedal 36. Aspeed control member 74 of the ) inductive type is coupled with the accelerator pedal and supplies a speed control signal to the motor control circuits 68. Direction control for the traction motor is provided by aforward control switch 76 and a reverse control switch 78. These switches are connected in parallel with each other and in series with the start switch 72 between thebattery 64 and the motor control circuits 68. Thus, thetraction motor 62 will be energized through the motor control circuit 68 when thekey switch 65 and the start switch 72 are closed with either the forward control switch 76 or the reverse control switch 78 closed. - The
controller 26 also includes apump control switch 82 which is serially connected between thekey switch 65 andbattery 64 and thepump motor 44. Accordingly, when thepump control switch 82 is opened, thepump motor 44 is turned off and the fluid to thehydraulic system 100 is cut-off. - The
pump control switch 82, theforward control switch 76 and the reverse control switch 78 are selectively actuated by thedrive selector lever 24. With the drive selector lever in neutral, both theforward switch 76 and the reverse switch 78 are open and thepump control switch 82 is closed. With the selector lever in the forward or reverse positions the respective forward and reverseswitches 76 and 78 are closed and thepump switch 82 is closed. With the selector lever in park, the forward and reverseswitches 76 and 78 are open and thepump control switch 82 is open. - The selective actuation of the forward and reverse
switches 76 and 78 and thepump control switch 82 is also controlled by theseat occupancy detector 32. The occupancy detector is operative, when theselector lever 24 is in neutral, to cause thepump control switch 82 to open and thereby turn off thepump motor 44 when the driver dismounts from the seat. Also, thedetector 32 is operative, when theselector lever 24 is in either forward or reverse, in response to the driver dismounting from the seat, to cause the forward and reverseswitches 76 and 78 to open and deenergize the traction motor and to cause thepump control switch 82 to open and deenergize the pump motor. - In summary, the electrical circuit of Figure 3, as just described, is operative to deenergize the
traction motor 62 unless thekey switch 65 and the start switch 72 and one of the forward or reverseswitches 76 or 78, respectively, are all closed. Also, it is operative to deenergize thepump motor 44 unless thekey switch 65 and thepump switch 82 are both closed; thepump switch 82 is closed only when the driver's seat is occupied and when thedrive selector lever 24 is in forward, reverse or neutral. Thepump switch 82 is open when thedrive selector lever 24 is in park regardless of driver's seat occupancy. As a result of this control, hydraulic fluid to thehydraulic system 100 is cut-off unless the driver occupies the seat and thedrive selector lever 24 is in forward, reverse or neutral. - The
hydraulic system 100 includes a power steering circuit for the vehicle, a lift circuit for the lift carriage, a tilt circuit for the upright and an actuation circuit for the auxiliary load handling device. Manual control of thepower steering unit 46 is exercised by the steering wheel. Manual control of the lift, tilt and auxiliary load handling functions is exercised by respective hand levers 16, 18 and 22 (see Figure 2). For this purpose, the steering wheel 14 is coupled with the steeringvalve 84 for actuation of the valve in response to steering wheel motion. Thelift control lever 16 is coupled with alift valve 86 and thetilt control lever 18 is coupled with atilt valve 88. Also, theauxiliary control lever 22 is coupled with anauxiliary valve 92. - Before proceeding with the description of the lift valve and the pump speed control, it will be helpful to consider the overall hydraulic system of the lift truck.
- The hydraulic system is shown schematically in Figure 9. In general, the system comprises the
positive displacement pump 42, thesteering unit 46 and themain valve 48. The system also includes the hydraulic reservoir ortank 54. - The hydraulic system is arranged so that the power steering circuit has priority over all other hydraulic functions in the lift truck. For this purpose, a
priority demand valve 102 is provided and for design purposes it is located in thesteering unit 46. Thesteering unit 46 also includes a steeringvalve 84 which controls energization of thesteering cylinder 52. Themain valve 48 includes thelift control valve 86 which controls thelift cylinder 56 and thetilt control valve 88 which controls thetilt cylinders 58 and 58'. The main valve also includes theauxiliary control valve 92 which controls theauxiliary cylinder 59. - For the purpose of determining flow priority, the
priority demand valve 102 is located upstream of thesteering control valve 84 and themain valve 48. Apressure relief valve 104 is coupled between thepriority demand valve 102 and thesteering control valve 84. A two-stage pressure relief valve or hydrastat is located in themain valve 48. The two-stage pressure relief valve includes amain relief valve 106 which is controlled by a pilot relief valve 108 and apilot relief valve 112. - As shown in Figure 9, the
pump 42 has its inlet connected to thetank 54. The outlet of thepump 42 is connected to the inlet port 114 of thepriority demand valve 102. The priority demand valve has aprimary outlet port 116 connected with theinlet port 118 of thesteering control valve 84. It has asecondary outlet port 122 connected through asupply line 124 to acontrol inlet port 150 of thelift control valve 86. Thepriority demand valve 102 is adapted to give priority to the flow requirements of the power steering system through theprimary outlet port 116. If the input flow to the priority demand valve is greater than that to be allocated to the power steering system, the excess flow is diverted by the priority demand valve to theseconday outlet port 122 andline 124 for use by other hydraulic functions. For example, the power steering circuit may be rated for a maximum of five gallons (22.75 litres) per minute and a maximum of 1,000 PSI (70.4 kg/cm2). It will be appreciated that the actual flow and pressure will be determined by the actuation of thesteering control valve 84 and the load imposed by the steering system. The flow produced to the inlet port 114 of thepriority demand valve 102 depends, of course, upon the speed of thepump 42. As will be described further below, in the example of this illustrative embodiment, operation in a low speed range of about 800 RPM is used for the power steering and tilt functions. This provides about five gallons (22.75 litres) per minute and the pressure may vary over a range up to 1,000 PSI(70.4 kg/cm2), depending upon load. Operation of the pump in an intermediate speed range of about 1,200 RPM is used for the auxiliary load device such as a load handling clamp; the flow requirement may be about nine gallons (40.95 litres) per minute and the pressure may range up to 2,000 PSI (140.8 kg/cm2). In operation of the pump at high speed, which may be about 1,800 RPM for the lift function, the flow may be about 20 gallons (91 litres) per minute with pressures ranging up to 3,000 PSI (211.2 kg/cm 2). - The power steering circuit comprises the
steering control valve 84 and thesteering cylinder 52. Thesteering control valve 84 is a metering valve with an open centre spool adapted to bi-directional control of the double-actingsteering cylinder 52 for actuating the dirigible wheels of the lift truck. Thesteering control valve 84 is provided with aprimary return port 126 which is connected through acheck valve 128 to thesupply line 124. Thesteering control valve 84 is provided with asecondary return port 132 which is connected to thetank 54. Therelief valve 104 is provided to prevent excessive pressure in the steering system. For this purpose, it has its inlet port connected to theprimary outlet port 116 of thepriority demand valve 102. The outlet port of therelief valve 104 is connected to thesecondary return port 132. Thus, if the pressure at theprimary outlet port 116 of the priority demand valve becomes excessive, thepressure relief valve 104 opens and dumps fluid to the tank. Thesteering control valve 84 has outlet ports 134 and 136 connected with opposite ends of thesteering cylinder 52. - The lift circuit includes the
lift control valve 86 which is a metering or proportioning valve having an open centre spool. The spool is normally centered and is movable in opposite directions by thelift control lever 16. Thelift control valve 86 communicates with the single-endedlift cylinder 56 through a lift-lower valve 138 and a loweringflow control valve 142. Thesecondary outlet port 122 of thepriority demand valve 102 is connected through thesupply line 124 to the opencentre inlet port 144 of thelift control valve 86. The opencentre outlet port 146 of thevalve 86 is connected to the opencentre inlet port 148 of thetilt control valve 88. Thepriority demand valve 102 also has itssecondary outlet port 122 connected with thecontrol inlet port 150 of thelift control valve 86. Thecontrol outlet port 152 is connected with the inlet port of the lift-lower valve 138. Thelift control valve 86 has areturn line port 154 connected to areturn line 156 which goes to thetank 54. The lift-lower valve 138 has aport 158 connected to aport 162 on the loweringflow control valve 142. The loweringflow control valve 142 has a port 164 connected with the lift-lower single endedcylinder 56. Operation of the lift-lower circuit will be described in greater detail subsequently. - The tilt control circuit including the
control valve 88 and the auxiliary control circuit including thecontrol valve 92 will be described briefly. Thetilt control valve 88 is a metering or proportioning valve of the open centre type with a spool which is bi-directionally movable by thetilt control lever 18. Thevalve 88 communicates with the single endedtilt cylinders 58 and 58' through a counterbalance valve 166. Thetilt control valve 88 has an opencentre inlet port 148 connected with the opencentre outlet port 146 of thelift control valve 86. Thetilt control valve 88 has an opencentre outlet port 168 which is connected to an opencentre inlet port 172 on theauxiliary control valve 92. Thetilt control valve 88 has acontrol inlet port 174 connected with thesecondary outlet port 122 of the priority demand valve through thesupply line 124 to thesupply passage 176 and thence through aflow control valve 178. Thetilt control valve 88 has acontrol outlet port 182 which is connected through the counterbalance valve 166 to thetilt cylinder 58 and 58'. Thetilt control valve 88 also includes areturn inlet port 184 which is connected with thetilt cylinders 58 and 58'. Areturn outlet port 186 on thecontrol valve 88 is connected to thereturn line 156. - The
auxiliary control valve 92 is a metering or proportioning valve having an open centre spool which is bi-directionally movable by theauxiliary control lever 22. Thecontrol valve 92 is adapted to control the energisation of the double-actingauxiliary cylinder 59. Thecontrol valve 92 has an opencentre inlet port 172 which is connected with the opencentre outlet port 168 of thetilt control valve 88. Thecontrol valve 92 also has an openoutlet centre port 188 which is connected with thereturn line 156. Acontrol inlet port 192 is connected with thesecondary outlet port 122 of thepriority demand valve 102. This connection extends through thesupply line 124 to thepassage 176 and thence through aflow control valve 194 to thecontrol inlet port 192. Thecontrol valve 92 has a pair ofcontrol ports 195 and 196 which are connected respectively with the opposite ends of theauxiliary cylinder 59. It also has areturn outlet port 198 connected with thereturn line 156. It also has areturn inlet port 202 connected with the outlet port of thepilot relief valve 112. Also, thecontrol valve 92 has areturn outlet port 204 connected with thereturn line 156. - The two-stage relief valve, as previously alluded to, comprises a
main relief valve 106, a pilot relief valve 108 and apilot relief valve 112. The two-stage valve is adapted to relieve the pressure at theoutlet port 122 of thepriority demand valve 102 when it exceeds a predetermined operating value for different operating conditions. In particular, when operating in a lift mode which calls for high speed pump operation the pressure is to be limited, for example, to 3,000 PSI(211.2 kg/cm2). When operating in the auxiliary mode, the pump is operated at an intermediate speed and the pressure is limited, for example, to 2,000 PSI (140.8 kg/cm2). - A lift truck with an hydraulic system of the type described above is disclosed in co-pending application United States Serial No. 291,681 filed August 10, 1981 and assigned to the same assignee as this application.
- The
hydraulic pump 42 is operated at different speeds according to the flow demand which varies with the operating mode of the hydraulic system. When a light load on the hydraulic system is selected , a low motor speed is adequate. For example, in a typical lift truck hydraulic system the power steering circuit requires a maximum of about five gallons (22.75 litres per minute). With a positive displacement pump which is sized to deliver 20 gallons (91 litres) per minute at full speed of 1,800 RPM, a pump speed of 800 RPM would be adequate for power steering. Similarly, for other operating modes, such as tilt of the upright, a low pump speed is adequate. For a heavy load on the hydraulic system, such as that imposed by operation in the lift mode either alone or with simultaneous operation in the steering or tilt load, operation of the pump will be required at its maximum rated speed, such as 1,800 RPM. For operation of the hydraulic system with an intermediate load, such as that imposed by operation in an auxiliary mode with a load handling clamp, intermediate flow is required and would be obtained at an intermediate speed, such as 1,200 RPM. Accordingly, the operating speed range of the pump motor is determined by the selection of the operating mode of the hydraulic system. The motor speed control circuit will be described with reference to Figures 6, 7 and 8. - As shown in Figure 6, pump 42 is driven by the
series DC motor 44 having a series field winding 252. The motor is energized from thebattery 64 through thevoltage control circuit 66. The voltage control circuit is adapted to regulate the motor speed within low, intermediate and high speed ranges according to the operating mode of the hydraulic circuit. For this purpose, thevoltage control circuit 66 is a thyristor-type pulsing circuit having presettable means for determining a mark/ space ratio for the different speed ranges and being provided with a feed back means for changing the mark/space ratio as a function of motor current. Such a motor control circuit is disclosed in the Morton et al United States Patent No. 4,119,898 granted October 10, 1978. - The
voltage control circuit 66 comprises a silicon control rectifier (SCR) 254 connected in series with themotor 44. Acurrent sensing resistor 256 is connected in series with the SCR and develops at a feedback voltage corresponding to the value of motor current. Apulse generator 258 supplies a pulse train to the gate of theSCR 254 which controls the effective motor supply voltage in accordance with the mark/space ratio of the pulse train. A mark/spacedemand signal circuit 262 produces a demand signal which is applied to thepulse generator 258 and determines the mark/space ratio of the pulse train. The value of the demand signal produced by thedemand signal circuit 262 is modified by thedemand modifying circuit 264 in accordance with the feedback voltage supplied from thesensing resistor 256. The output of thedemand signal circuit 262 is connected with thepulse generator 258 through a series connection of thekey switch 65 and thepump switch 82 which were described with reference to Figure 3. Unless both the key switch and the pump switch are closed there is no demand signal input to thepulse generator 258 and the SCR is turned off and themotor 44 cannot be energized. Thedemand signal circuit 262 is provided with a speedrange selection circuit 266. This circuit includes a pair ofseries resistors speed selector switch 274 and a highspeed selector switch 276. When both switches 274 and 276 are open, the low speed range is selected. Whenswitch 274 is closed withswitch 276 open, the intermediate speed range is selected and when theswitch 276 is closed the high speed range is selected. Theselector switch 274 is closed by actuation of theauxiliary control lever 22 and theswitch 276 is closed by thelift control lever 16, as will be described below. - In operation of the
voltage control circuit 66, thedemand modifying circuit 262 produces an output signal which increases in magnitude as a direct function of the feedback voltage from theresistor 256 and hence as a direct function of the motor current. The mark/spacedemand signal circuit 262 produces a demand signal which increases from a minimum value when the motor current is zero to a maximum value when the motor current reaches apredetermined value 10. This causes thepulse generator 258 to produce a pulse train having a mark/space ratio which increases from a predetermined minimum value to a maximum value in correspondence with a demand signal. The action of thedemand modifying circuit 264 in response to the feedback voltage tends to maintain the motor speed substantially constant when it is operated in either the low speed range or the intermediate speed range. This is illustrated for the low speed range in the graph of Figure 7. As indicated, the mark/space ratio increases with motor current up to a predetermined current of 10 at which point it reaches a maximum value and remains constant. This change of mark/space ratio causes the motor voltage to change accordingly, as indicated in Figure 7. An increasing load on the motor is reflected by increased current which in turn produces an increased voltage and motor torque which tends to maintain the motor speed substantially constant, as indicated in Figure 7 for the low speed range. The same relationship obtains when the motor is operated in the intermediate speed range. When thespeed selector switch 276 is closed by thelift control lever 16, thedemand signal circuit 262 produces a demand signal corresponding to the high speed range. In this latter condition, the full battery voltage is applied to the pump motor so that the pump is operated at the highest speed capability of the motor for the particular load. - The
lift control lever 16, alluded to with reference to Figures 1 and 2, is shown in greater detail in Figure 5. The lift control lever is adapted to actuate thelift control valve 86 and to exercise manual control over the operating speed of the pump motor. Thelift control valve 86, as previously noted, is a spool valve having aspool 206. The spool is axially movable in either direction from a neutral position to select either the lift mode or the lower mode of operation. Thelift control valve 86 will be described in greater detail with reference to Figure 4. - The structure of the
lift control valve 86 is depicted in Figure 4. Thelift control valve 86 is a conventional proportioning valve of the open-centre, spool type and comprises avalve spool 206 in avalve body 208. Thespool 206 as shown in Figure 4 is in a neutral position and is biased toward that position by a centeringspring 282. Thevalve body 208 is provided with thesupply line 124 which supplies fluid pressure from thesecondary outlet port 122 of thepriority demand valve 102. Thesupply line 124 is connected with the opencentre inlet port 144 and also with thecontrol inlet port 150. Thevalve body 208 also provides an opencentre outlet port 146 which is connected with the opencentre inlet port 148 of thetilt control valve 88, as previously described. Acontrol outlet port 152 is connected with thelift cylinder 56 through the lift-lower valve 138 and thelower control valve 142, as previously described. Areturn line port 154 is connected with thereturn line 156, as previously described. - The
valve spool 206 comprisescylindrical sections annular chamber 292 between thesections sections spool 206 in the neutral position shown, the opencentre outlet port 146 is fully open through thechamber 292 to the opencentre inlet port 144. Also, in the neutral position, thecontrol outlet port 152 is fully closed by thespool section 286 to thecontrol inlet port 150 and thereturn line port 154 is fully closed by thespool section 288 to the control outlet port 294. In this condition, herein referred to as the "fully closed" condition, there is no flow to thelift cylinder 56 and there is full flow to thetank 54. Thevalve spool 206 is movable by actuation of thelift lever 16 to a "fully open" condition by moving thespool 206 upwardly (as viewed in Figure 4) until the opencentre inlet port 144 is fully closed to the opencentre outlet port 146 by thespool section 286. With the spool in this position, thecontrol outlet port 152 is fully open to thecontrol inlet port 150 through thechamber 152 and thereturn line port 154 remains closed by thespool section 288. In this position, the valve is referred to as being "fully open". - When the control valve is actuated by the
lift lever 16 between the fully closed and fully open conditions, the fluid flow to thelift cylinder 56 is controlled in a proportional manner. With thespool 206 in the neutral position, the valve is fully closed and the fluid flows freely through the opencentre inlet port 144 to the opencentre outlet port 146 and there is no flow to thecontrol outlet port 152. Upward movement of thespool 206 results in restricted flow from the opencentre inlet port 144 to the opencentre outlet port 146 which causes a back-pressure in the fluid and there is restricted flow from thecontrol inlet port 150 to thecontrol port 152. As thespool 206 is moved further, the back-pressure increases and there is less restriction to the flow between thecontrol inlet port 150 andoutlet port 152 so that the flow to thelift cylinder 56 is a direct function of the displacement of the spool. This so-called proportional control permits the driver to gradually increase the fluid pressure in thelift cylinder 56 and thus to exercise "feathering" control in raising the lift carriage. When the lift carriage is to be lowered, thelift lever 16 is moved in the lowering direction to move thespool 206 past the neutral postion. This movement causes thespool section 284 to close the opencentre outlet port 146 to the opencentre inlet port 144 and at the same time to open thereturn line port 154 to thecontrol outlet port 152 through the chamber 294 so that the fluid pressure in thelift cylinder 56 is released. The driver may also exercise feathering control during the operation in the lowering mode. - The lift control lever 16 (Figure 5) is pivotallty mounted on a
shaft 212 for pivotal motion in the fore and nfL directions by the lift truck driver. Thelever 16 is provided with aslot 214 which receives apin 216 extending through the end ofspool 206 of the control valve. Accordingly, when thecontrol lever 16 is pivoted in the rearward direction (phanton lines) thestem 206 is raised from the neutral position and the valve is operated in the lift mode. When thehand lever 16 is rotated in the forward direction, thestem 206 is lowered and the valve is operated in the lowering mode. - When the system is operated in the lift mode, the
pump 42 is operated in the high speed range. For this purpose, aspeed control switch 218 is provided. Thespeed control switch 218 is electrically connected in the speed control circuit described with reference to Figure 6. The speed control switch comprisesswitch contacts 222 which are biased by a spring 224 toward a closed position. Aswitch plunger 226 is adapted, when depressed, to hold theswitch contacts 222 in the open position. - When the
speed control switch 218 is closed, the speed control circuit causes thepump motor 42 to operate in the high speed range. Thelift lever 16 is adapted to close thespeed control switch 218 concurrently with full opening of thevalve 86. For this purpose, thecontrol lever 16 is provided with an arm 228 which actuates apush rod 232. The push rod, in turn, actuates theswitch plunger 226. The push rod is mounted in thecontrol lever housing 234 for reciprocal motion and is spring loaded by abias spring 236 in the upward direction. With the lever in the neutral position, the upper end 238 of thepush rod 32 is spaced from thelower face 242 of the arm 228. This provides a lost motion of predetermined distance D, as indicated in Figure 5, between thecontrol lever 16 and thepush rod 232. When the control lever is pivoted rearwardly from the neutral position the first increment of motion imparts movement to thestem 206 of thelift control valve 86. Continued movement of thecontrol lever 16 operates thevalve 86 in its feathering mode between the closed and open positions. When thevalve stem 206 is moved a distance d, as indicateed in Figure 5, the valve is fully open and the arm 228 of thecontrol lever 16 engages the push rod 232 (phantom lines). Upon engagement of thepush rod 232 by thecontrol lever 16, thepush rod 232 is depressed against the resistance of thebias spring 236 and the lower end of the push rod moves away from theswitch plunger 226 allowing theswitch contact 222 to close. This closure of thespeed control switch 218 causes the motor control circuit to operate the motor in the high speed range. Thus, thepump motor 42 is operated in the high speed range when the lift control valve is fully open to apply maximum lifting force to the lift carriage. - When the control handle 16 is returned to the neutral position after operating in the lift mode, the
control valve 86 is returned to its neutral position in which the valve is closed. This movement of thecontrol lever 16 allows thespeed control switch 218 to open and the motor control circuit is switched for operation in the lower speed range. When thecontrol lever 16 is operated to lower the carriage by pivotal motion in the forward direction from the neutral position, the spool is moved downwardly and the cylinder port is connected to the tank return port. This releases fluid from the lift cylinder to lower the load carriage. Thecontrol valve 86 may be operated in the proportional manner for feathering the lowering control of the load carriage. - In operation, the lift truck driver may operate the
lift lever 16 to raise the lift carriage while the pump motor is operated in the lower speed range. The low speed motor operation obtains when the lift truck is operated in the idle mode (key switch 66 and pumpswitch 82 closed), power steering mode, or the tilt mode, or any combination of these modes. When the driver actuates thelift lever 16, thelift control valve 86 is moved from the fully closed position to the fully open position through its feathering range while the pump motor is operated in the low speed range. As the lift control valve is opened, the pressure in thelift cylinder 56 is progressively increased causing the load on the motor to increase. The motor speed control system increases the motor torque with increased load to maintain the motor speed substantially constant within the low speed range. This enables the lifting of heavy loads with the pump operating in the low speed range. When thelift control valve 86 reaches the fully open position, thespeed control switch 276 is closed and the motorspeed control circuit 66 is switched to operate in the high speed range. Accordingly, the motor speed is increased from the low speed range to the high speed range only after the control valve is fully open. This operation is depicted graphically in Figure 8 with the motor operated in the low speed range, the driver commences to move thelift lever 16 in the lift direction at time tl. The lift lever is moved gradually for feathering control so that the lift control valve moves from fully closed to fully open in the interval between time tl and time t2. When it is fully open at time t2, thespeed control switch 276 is closed and the motor speed increases in a ramp fashion to its maximum speed at time t3 and remains substantially constant at that speed. It will now be appreciated that similar operation will be obtained if the lift truck driver actuates thelift control lever 16 when the pump motor is operated in the intermediate speed range. In such case, the feathering operation of the lift control valve takes place at the relatively lower intermediate pump speed and the high speed pump operation is not initiated until the control valve is fully open.
Claims (9)
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US48620183A | 1983-04-18 | 1983-04-18 | |
US486201 | 1983-04-18 |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0122807A1 true EP0122807A1 (en) | 1984-10-24 |
EP0122807B1 EP0122807B1 (en) | 1987-09-09 |
Family
ID=23930995
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP84302591A Expired EP0122807B1 (en) | 1983-04-18 | 1984-04-17 | Hydraulic system with proportional control |
Country Status (7)
Country | Link |
---|---|
EP (1) | EP0122807B1 (en) |
JP (1) | JPS59217598A (en) |
AU (1) | AU2673284A (en) |
CA (1) | CA1221152A (en) |
DE (1) | DE3465969D1 (en) |
ES (1) | ES531559A0 (en) |
ZA (1) | ZA841931B (en) |
Cited By (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0228707A1 (en) * | 1985-12-28 | 1987-07-15 | Hitachi Construction Machinery Co., Ltd. | Control system of hydraulic construction machinery |
US4930975A (en) * | 1987-03-20 | 1990-06-05 | Nissan Motor Company, Limited | Control for load carrier for industrial vehicle |
EP0531746A1 (en) * | 1991-09-07 | 1993-03-17 | KIWA MASCHINENFABRIK GmbH | Switching method and arrangement for controlling, in particular a crane installation |
US6853271B2 (en) | 2001-11-14 | 2005-02-08 | Radio Frequency Systems, Inc. | Triple-mode mono-block filter assembly |
ITUA20164446A1 (en) * | 2016-06-16 | 2017-12-16 | Merlo Project Srl | VEHICLE LIFT CONFIGURED FOR THE ATTACHMENT OF HIGH POWER HYDRAULIC EQUIPMENT |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPH0326801U (en) * | 1989-07-27 | 1991-03-19 |
Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DE2318872A1 (en) * | 1972-04-14 | 1973-10-25 | Lancer Boss Ltd | IMPROVEMENTS TO OR RELATED TO HYDRAULIC OPERATING CIRCUITS |
FR2396881A1 (en) * | 1977-07-05 | 1979-02-02 | Towmotor Corp | PRIORITY HYDRAULIC CIRCUIT, ESPECIALLY FOR LIFT TRUCKS |
GB2102511A (en) * | 1981-07-29 | 1983-02-02 | Nissan Motor | Improved hydraulic control system for industrial vehicle |
EP0072233A1 (en) * | 1981-08-10 | 1983-02-16 | Clark Equipment Company | Vehicle and hydraulic system with single pump therefor |
-
1984
- 1984-03-14 CA CA000449624A patent/CA1221152A/en not_active Expired
- 1984-03-15 ZA ZA841931A patent/ZA841931B/en unknown
- 1984-04-11 AU AU26732/84A patent/AU2673284A/en not_active Abandoned
- 1984-04-12 ES ES531559A patent/ES531559A0/en active Granted
- 1984-04-17 EP EP84302591A patent/EP0122807B1/en not_active Expired
- 1984-04-17 DE DE8484302591T patent/DE3465969D1/en not_active Expired
- 1984-04-18 JP JP59076763A patent/JPS59217598A/en active Pending
Patent Citations (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
DE2318872A1 (en) * | 1972-04-14 | 1973-10-25 | Lancer Boss Ltd | IMPROVEMENTS TO OR RELATED TO HYDRAULIC OPERATING CIRCUITS |
FR2396881A1 (en) * | 1977-07-05 | 1979-02-02 | Towmotor Corp | PRIORITY HYDRAULIC CIRCUIT, ESPECIALLY FOR LIFT TRUCKS |
GB2102511A (en) * | 1981-07-29 | 1983-02-02 | Nissan Motor | Improved hydraulic control system for industrial vehicle |
EP0072233A1 (en) * | 1981-08-10 | 1983-02-16 | Clark Equipment Company | Vehicle and hydraulic system with single pump therefor |
Cited By (7)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP0228707A1 (en) * | 1985-12-28 | 1987-07-15 | Hitachi Construction Machinery Co., Ltd. | Control system of hydraulic construction machinery |
US4726186A (en) * | 1985-12-28 | 1988-02-23 | Hitachi, Construction Machinery Co. | Control system of hydraulic construction machinery |
US4930975A (en) * | 1987-03-20 | 1990-06-05 | Nissan Motor Company, Limited | Control for load carrier for industrial vehicle |
EP0531746A1 (en) * | 1991-09-07 | 1993-03-17 | KIWA MASCHINENFABRIK GmbH | Switching method and arrangement for controlling, in particular a crane installation |
US6853271B2 (en) | 2001-11-14 | 2005-02-08 | Radio Frequency Systems, Inc. | Triple-mode mono-block filter assembly |
ITUA20164446A1 (en) * | 2016-06-16 | 2017-12-16 | Merlo Project Srl | VEHICLE LIFT CONFIGURED FOR THE ATTACHMENT OF HIGH POWER HYDRAULIC EQUIPMENT |
EP3260410A1 (en) * | 2016-06-16 | 2017-12-27 | Merlo Project S.r.l. | A lifting vehicle configured for attaching high-power hydraulic implements |
Also Published As
Publication number | Publication date |
---|---|
ES8507234A1 (en) | 1985-08-16 |
AU2673284A (en) | 1984-10-25 |
EP0122807B1 (en) | 1987-09-09 |
ZA841931B (en) | 1984-10-31 |
DE3465969D1 (en) | 1987-10-15 |
CA1221152A (en) | 1987-04-28 |
ES531559A0 (en) | 1985-08-16 |
JPS59217598A (en) | 1984-12-07 |
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