GB2221518A

GB2221518A - Engine transmission control system

Info

Publication number: GB2221518A
Application number: GB8912190A
Authority: GB
Inventors: B J Alcock
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1988-09-07
Filing date: 1989-05-26
Publication date: 1990-02-07
Also published as: GB8821027D0; GB8912190D0

Abstract

An engine/transmission control system for a skid-steered vehicle, comprises (a) an engine (2), (b) at least one continuously variable transmission (CVT) system (18, 28) connected to and for transmitting drive from the engine, (c) means (40) for sensing engine speed and for generating a first signal (VES) representative of engine speed (d) means (50) for sensing power demand placed on the engine and for generating a second signal (VPD) representative of power demand, (e) a CVT ratio controlling (38) for generating a third signal (VC), derived from the difference between the first and second signals, for controlling means for varying the transmission ratio of the CVT system (18, 28) between maximum and minimum values in such a manner that when CVT ratio is within a range between said values, the CVT ratio governs the torque reflected through the CVT system to the engine such that engine speed increases with increasing power demand but is controllably maintained within a narrow range of speeds at constant power demand, the CVT controlling means including a phase advance system and a phase retard system, which systems limit the rate of change of the CVT transmission rate to an extent dependent on the rate of change of engine speed, and (f) a steer system responsive to steer demand for generating differential speeds between left-hand side and right-hand side drive wheels (24, 34). The system is responsive to rapid change in power demand, and provides skid steering control under fluctuating power demand. The CVT system may be hydraulic, mechanical or electrical and the vehicle may have independent drive and steer CVT systems (Fig. 6). <IMAGE>

Description

Engine/Transmission Control System This invention relates to a engine/transmission control system for a skid-steered vehicle incorporating a continuously variable transmission (hereinafter referred to as "CVT") system, which is suitable for use in vehicles and paticularly in skidsteered vehicles.

In a drive system which drives a rotary output, through a transmission system, the parameter of primary interest to the drive system operator is power which is the product of the speed of the rotating output and torque delivered by the output. If the drive system is incorporated in a vehicle, then power is the product of vehicle speed and tractive effort at the vehicle/ground interface.

Vehicle speed is directly related to engine speed by the transmission ratio, whereas tractive effort is related to the transmission ratio, engine torque and driveline efficiency. The speed of a running engine is determined by two factors: the torque delivered by the engine which may be increased or decreased in accordance with the drive system operator s demand for power (for example, the position of an accelerator pedal), and the torque reflected to the engine through the transmission system by the load placed on the rotary output. When the drive and reflected torque are equal, the engine speed remains constant.

Transmission ratio control systems employing a variety of control algorithms are known for controlling the transmission ratios of CVT systems widely used in vehicles. However, it is a feature of known CVT ratio control systems that they respond poorly to sudden increases in load placed upon the system, which leads to loss of engine speed and power and can result in engine stall, and sudden decreases of load which tend to cause over-revving of the engine. One control algorithm which may be used to obviate this problem is an engine speed control algorithm, in which transmission ratio is varied to maintain constant engine speed, and so control vehicle speed as a function of transmission ratio alone.This algorithm is undesirable in transmission systems used in vehicles because changes in ratio are found to produce significant changes in engine speed, due to the referred inertia of the vehicle being much greater than that of the engine in some areas of the operating envelope of the drive system. However, there is some advantage in being able to operate the engine within a narrow range of speeds as the design of the engine can be optimised to reduce its volume and weight. This allows the engine to be run at speeds at or just above its maximum power output speed to reduce the problems of transient response of the engine.

One known application of CVT ratio control systems in which the above mentioned-problem of poor system response exists is in the transmission of power within skid-steered vehicles which may be wheeled but are more commonly provided with tracks encompassing drive wheels. A skid-steered vehicle relies upon a differential speed being set up betwen the sets of wheels on either side of the vehicle in order to create the required steering effect. It is a feature of skid-steered vehicles that very high forces are generated at the track-to-ground or wheel-to-ground interface when steering, so that any steer demand (for example, created by the operation of a steering wheel) requires a rapid increase in power from the engine if the vehicle is to maintain constant speed.The increase in resistance to motion due to skid steering can create a situation whereby the power required to maintain the same vehicle speed can exceed that available from the engine and can result in the engine stalling. Even if a correctly-timed increase in power demand is effected to compensate for the increased load on the engine, a complex power redistribution system may be necessary to prevent an undesirable imbalance in the distribution of power delivered to the wheels on either side of the vehicle which might otherwise alter the steer radius of the vehicle set by the steer demand. Conversely when the steering manoeuvre has been completed there will be a rapid decrease in load placed on the system.

Patent specifications GB 1600699 and EP 0062072 address the problem of controlling a CVT tranmission system with reference to the difference between engine speed and operator's power demand.

However they are not directed to skid-steered vehicles in which the rapid rise and fall of load placed on the system is of such importance as described above.

Patent specification US 3914938 refers to the control of two CVT transmission systems for the propulsion of a tracked vehicle. The control system includes an anti-stall device but no means of controlling the engine speed when the engine speed rises above that required resulting from the reduction of load on the system or an incorrect gear change. Rate limiter circuitry is also included which limits the rate of change of the signal from the CVT ratio command signal varied by the operator. The anti-stall device output is not however subject to such control and is thus liable to reduce the CVT ratio further than necessary resulting in overrevving of the engine.

It is one object of the present invention to obviate at least some of the aforementioned disadvantages associated with known CVT ratio algorithms used to control skid-steered vehicles by providing an engine/transmission control system which provides a CVT system with a greater degree of responsivity to changing demands and loads placed on an engine, particularly rapid changes.

It is one further object of the invention to provide a skid steered vehicle which by the incorporation of an engine/transmission control system overcomes the aforementioned problems of stalling, over-revving and loss of steer control.

According to a first aspect of the present invention, there is provided an engine/transmission control system for a skid steered vehicle comprising (a) an engine, (b) at least one CVT system connected to and for transmitting drive from the engine, (c) means for sensing engine speed and for generating a first signal representative of engine speed (d) means for sensing power demand placed on the engine and for generating a second signal representative of power demand, (e) a CVT ratio controlling means for generating a third signal, derived from the difference between the first and second signals, for controlling means for varying the transmission ratio of the at least one CVT system between maximum and minimum values in such a manner that when CVT ratio is within a range between said maximum and minimum values, CVT ratio governs torque reflected through the at least one CVT system to the engine such that engine speed increases with increasing power demand but is controllably maintained within a narrow range of speeds at constant power demand, the CVT controlling means including a phase advance system and a phase retard system which systems limit the rate of change of the CVT transmission rate to an extent dependent on the rate of change of engine speed, and (f) a steer system responsive to steer demand for generating differential speeds between left-hand side and righthand side drive wheels.

In this way, the engine is capable of responding to changing power demands placed on the engine/transmission system but at the same time the at least one CVT system is controlled to produce a significant reduction in CVT ratio, and hence torque reflected to the engine, when sudden loads are placed on the at least one CVT system which could otherwise reduce engine speed to unacceptable levels. Furthermore, because engine speed is controlled within a narrow range, which range preferably spans over less than 10X of maximum engine speed over a ratio range of between 20X and 80X of maximum CVT ratio, the CVT ratio controlling means effectively governs the power developed by the engine in accordance with power demand.

In this specification, the term "CVT ratio" or "transmission ratio" means the ratio of output speed to input speed of the transmission system concerned. The CVT system may be of any type known to those skilled in the art, including hydraulic, mechanical and electrical CVT systems.

According to a second aspect of the present invention, there is provided a skid-steered vehicle incorporating the engine/transmission system of the first aspect. More particularly the skid-steered vehicle has a steer system for generating steer demand which produces a differential speed between left-hand side and right-hand side drive wheels connected to the engine and situated on either side of the vehicle, wherein the magnitude of the differential speed is dependent on the output from the CVT ratio controlling means to provide a ratio of wheel speeds between the left-hand side and right-hand side drive wheels which varies in accordance with steer demand but is substantially independent of both the load reflected to and power demand placed on the engine.

In this preferred skid-steered vehicle, the engine/transmission system will include two or more CVT systems the transmission ratio of at least one of which is controlled by said regulated steer demand.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which Figure 1 is a schematic plan view of a drive/transmission system for a skid steered vehicle, having a CVT ratio controller and two independent CVT systems which are shown driving two drive wheels in a forward direction, Figure 2 is a block flow diagram of the CVT ratio controller schematically represented in Figure 1, Figures 3 and 4 are circuit diagrams of parts of the drive/transmission system of Figure 1 which are associated with the CVT ratio controller, Figure 5 illustrates diagrammatically one example of a control algorithm for the drive/transmission system illusrated in Figures 1 to 4, in which R represents transmission ratio, N represents engine speed, PD represents power demand, and the shaded region of the diagram represents the operating envelope of the system, and Figure 6 is a schematic plan view of a second drive/transmission system for a skid-steered vehicle, having independent drive and steer CVT systems.

Referring first to Figures 1 to 4, a drive/transmission system for a skid-steered vehicle includes an engine 2, preferably an internal combustion engine, which drives a drive shaft 4 mechanically linked to a first 6 and second 8 hydraulic transmission through a 1:1 fixed ratio splitter gearbox 10, first secondary drive shafts 12, bevel gearboxes 14, and second secondary drive shafts 16.

The first hydraulic transmission 6 comprises a first variable flow swash plate hydraulic pump 18 hydraulically interconnected by hydraulic lines 20 to a first fixed flow hydraulic motor 22 which in turn drives, at identical speeds, first drive wheels 24 on the lefthand side of the vehicle via a first output shaft 26 and bevel gearboxes 27 connected by a shaft 29. Similarly, the second hydraulic transmission 8 comprises a second variable flow swash plate hydraulic pump 28 hydraulically interconnected by hydraulic lines 30 to a second fixed flow hydraulic motor 32 which in turn drives, at identical speeds, second drive wheels 34 on the righthand side of the vehicle via a second output shaft 36 and bevel gearboxes 37 connected by a shaft 39. The capacities of the motors 22, 32 and the responses of the transmission systems to variation in swash plate angle are both identical.Each of pumps 18, 28 includes a swash plate 18a, 28a set at an angle 0 to the position of the plates at which no fluid is pumped (transmission ratio = zero). The swash plates 18a, 28a are pivotable between a maximum positive 0 angle(+ ) corresponding to the maximum ratio (=1) of the transmissions 6 and 8 for forward drive and a maximum negative angle ( 0) corresponding to the maximum ratio (=1) of the transmission for reverse drive.

A transmission ratio controller 38 receives electrical signals derived from the power demand placed on the engine 2 and from the speed of the drive shaft 4. A tachogenerator 40 on the drive shaft 4 of the engine 2 generates a tacho voltage VES of a magnitude which is directly proportional to the speed of the engine 2. This voltage VES is transmitted to the transmission ratio controller 38. An accelerator pedal 42 sets the power demand placed on the engine 2 by a direct mechanical linkage 44 linked to a throttle consisting of a butterfly valve 46 on the inlet manifold 48 of the engine 2. An accelerator (power demand) potentiometer 50 is linked to the accelerator pedal 42 to generate a power demand voltage VpD of a magnitude which is dependent upon the degree of depression of the pedal. The power demand voltage VPD is also transmitted to the transmission ratio controller 38.

The transmission ratio controller 38 processes the speed and power demand voltage signals and generates an output voltage which essentially comprises the difference between the power demand voltage signal VpD and the engine speed voltage VES when the latter PD exceeds the former. The controller 38 consists of a differential amplifier circuit 38a whose phase inverted input is connected to the tachogenerator 40 and whose non inverted input is connected to the accelerator potentiometer 50. A phase advance circuit 38b and a tickover offset circuit 38c are connected between the tachogenerator 40 and the comparator 38a, and a phase retard circuit 38d and a throttle offset circuit 38e are connected between the accelerator potentiometer 50 and circuit 38a. The output of the circuit 38a is connected in series through a diode 38f.

The circuit diagram of the accelerator potentiometer 50 and the CVT ratio controller 38 is shown in Figure 3 and consists of terminals T1-T10, resistors R1-R18, connectors L1-L3, potentiometers P1-P6, diodes D1-D3, capacitors C1-C4, amplifiers A1-A3, and earth connections E1-E3. The controller 38 is fed through terminals T3-T6 with a z-volt DC supply (typically z=12 volts). T1 and T2 are connected across the tachogenerator 40. The potential difference across T1 and T2 is directly proportional to engine speed. P1 acts as a gain adjuster which divides down tacho voltage Vexs. P2 acts as a counterbalance to the output from P1, and adjustment of P2 sets the tickover speed of the engine 2 below which no voltage signal is transmitted to Al.Above this speed, a tacho voltage signal is transmitted through A2 to the phase inverter input of A3. P3 represents the power demand potentiometer 50 connected to the controller 38 by connectors L1, L2, and L3. P4 connected in series with P3 sets the required maximum voltage drop across P3 whereas P5 sets the required throttle offset ie the fraction of maximum power demand below which no power demand voltage is fed to differential amplifier ciruit 38a. The tacho voltage signal and power demand signal are fed through identical resistors R13 and R14 respectively into the phase inverter input and non inverter input respectively of A3. P6 sets the gain of the differential amplifier circuit 38a. The controller 38 is earthed at E3 between two identical resistors R17 and R18 connected in series across the output of circuit 38a and T2.D3 allows the passage of negative current only from the circuit 38a. Capacitors C1 and C2 provide the phase advance and phase retard functions respectively of the controller 38. These functions are dependent on the capacitance of the capacitors selected.

The output voltage V from the controller 38 across T9 and c T10 is connected to a gear mode select mechanism 52 having a gear select lever 54, which processes the output voltage according to the following regime: D1 = drive position 1: Voltage from controller is transmitted through the gear mode select mechanism without modification D2 = drive position 2 Voltage from controller is halved, through resistors R20 (from T9) and R21 (from T10) N = neutral No voltage received R2 = reverse The polarity of the voltage from controller is halved and reversed, through resistors R20 (from T9) and R21 (from T10) The mechanism 52 consists of a multi-position switch S1 (see Figure 4) actuable by the gear select lever 54.The output voltage VG for the gear mode select mechanism 52 is connected to a steer box 56 containing left 57 and right 58 geared potentiometers fitted to the base of a steering column 60 which is manually rotatable by a steering wheel 62. These potentiometers 57, 58 each have their own output and are so arranged that only one potentiometer (the left potentiometer 57) is driven when the steering wheel 62 is turned to the left and only one potentiometer (the right potentiometer 58) is driven when the steering wheel 62 is turned to the right. The outputs from the left and right geared potentiometers 57, 58 are connected in series to left 64 and right 66 earthed amplifiers respectively which in turn are connected to left and right electro-hydraulic swash plate actuators 68 and 70 respectively.The left and right actuators 68, 70 in turn are mechanically linked to the left hand 18a and right hand 28a hand swash plates respectively. The actuators 68, 70 are connected to the swash plates 18a, 28a so as to generate a swash plate angle 0 of 0 + Oc when the amplified voltage is at its maximum positive value 0 0 (eg +5 volts), of 0 when the voltage is zero, and of -oC when the voltage is at its maximum negative value (eg -5 volts).

The operation in a skid-steered vehicle of the drive/transmission system illustrated in Figures 1 and 2 will now be described with reference to the control algorithm illustrated in Figure 3.

For normal forward drive, the gear select lever 54 is placed in drive position Dl. For zero and small amounts of pedal 42 depression, until engine speed exceeds a preselected tickover speed Nmin, the tickover offset circuit 38c suppresses transmission of tacho voltage to the differential amplifier circuit 38a. No output is generated by the circuit 38a, so that the swash plates 18a, 28a of both pumps 18, 20 remain closed, (swash plate angle 4 > = 00) and no drive is applied to the wheels 24, 34.

As the accelerator pedal 42 is depressed, the speed of the engine 2 will rise due to the mechanical opening of the throttle 46, and an output voltage V PD will be produced from the accelerator demand potentiometer 50. As the engine speed rises, the voltage produced by the tachogenerator 40 will also rise, but this voltage is opposed within the differential amplifier circuit 38a by the voltage from the accelerator demand potentiometer 50 and no output from the circuit 38a will occur until the tacho voltage V ES exceeds the accelerator demand voltage V by a minimum voltage Vmin PD (typically 1 volt) set by the diode 38f.The throttle offset 38e supresses output from the circuit 38a until the pedal 42 is depressed to a minimum level PD , which is shown in Figure 5 as min 1/4 of full depression. This is to ensure the engine 2 is developing sufficient power before any torque is transmitted through the hydraulic transmissions 6 and 8. The operational output from the CVT ratio controller 38, which is amplified through the swash plate control amplifiers 64, 66 before being fed into the electro-hydraulic actuators 68, 70, causes the swash plates 18a, 0 28a, to open (swash plate angle 8-) 0 ). The hydraulic pumps 18, 28 will then drive the motors 22, 32 and torque will be applied to the wheels 24,34.

As the pedal 42 is gradually depressed further, the accelerator demand voltage will increase, but the engine 2 will also accelerate due to the mechanical operation of the throttle 46 and the output voltage from the tacho generator 40 will rise, causing the swash plates 18a, 28a to open further and the vehicle to accelerate. However, the increase in load on the engine 2 will limit the rise in speed and as the tacho voltage approaches the accelerator demand voltage the swash plates 18a, 28a will start to close.The gain of the differential amplifier circuit 38a is set to provide an output to the swashplate control amplifiers 64, 66 which will adjust the angle of the swash plates (which angle is equivalent to CVT ratio) and therefore the load on the engine to maintain an engine speed within a narrow range (eg from 2% to 10% of the maximum rated speed of the engine) between the closed and fully-open position of the swash plates. It can be seen from Figure 5 that because the output signal is dependent upon the difference between tacho voltage and power demand voltage, CVT ratio rises sharply with small increases in engine speed within the range, and the mid-point of the range increases with increasing power demand voltage.The swash plates 18a, 28a will not close until the tacho voltage has fallen below the accelerator demand voltage indicating that the engine speed (and therefore engine torque) is insufficient to move the vehicle.

The sudden depression of the accelerator pedal 42 causes a rapid rise in engine speed because of the mechanical throttle linkage 44. There will also be a sudden increase in the accelerator demand voltage which may momentarily exceed the tacho voltage to reduce the positive differential signal from the circuit 38a to zero and cause the swash plates 18a, 28a to close. However, the phase advance circuit 38b senses the rapid rise in power demand voltage and produces an output voltage in excess of the tacho voltage to increase the differential signal from the circuit 38a. This causes the swash plates 18a, 28a to open thus applying additional load on the engine 2, which will control the rate of rise in engine speed and cause the vehicle to accelerate smoothly. The effect of phase advance is shown in Figure 5 as the curved portions of the constant power demand lines as CVT ratio rises from zero.

Sudden release of the accelerator pedal 42 results in a collapse of the accelerator demand voltage, and a reduction in engine speed which because of engine and transmission inertia will be slower than the collapse in accelerator demand voltage. The tacho voltage will therefore exceed the accelerator demand, the output from the differential amplifier circuit 38a will be increased, and the swash plates 18a, 28a will be driven open to give a momentary increase in vehicle speed. To prevent this from occuring the phase retard circuit 38d is employed to control the decay in the accelerator demand voltage to more closely match the rate of decay in engine speed hence tacho output voltage. The effect of phase retard is shown in Figure 5 as the curved portions of the constant power demand lines as CVT ratio falls from unity.

When the gear select lever 54 is placed in drive position D2, the switch S1 in the gear select mechanism 52 divides the output voltage V from the operational amplifier 38g by two. The engine 2 c is therefore required to run at twice the speed to produce the same control voltage signal to the swash plate control amplifiers 64, 66 as that produced in drive position D1. As a result, the swash plate angle 4F hence CVT ratio will be reduced by 50%, providing an effective "low gear" operation.

When the gear select lever 54 is placed in reverse position R2, the polarity of the control voltage signal Vc generated in drive position D2 is reversed. This causes the angle 0 of the swash plates 18a, 28a to open in the reverse direction and therefore reverses the direction of fluid flowing from the hydraulic pumps 18, 28.

When the gear select lever 54 is placed in the neutral position ("N"), the output voltage from the tachogenerator 40 and power demand potentiometer 50 are earthed. No output voltage is produced, so that the swash plates 18a,28a remain closed (swash plate angle -8 = 0) and no drive is applied to the wheels 24, 34.

The description of operation has hitherto assumed that no steer demand is being placed on the drive/transmission system and the steering wheel 62 is at its central position. At this position, the outputs from both steer potentiometers 56, 57 are the same, so that the control signals to both swash plate control amplifiers 64,66 are also both the same.

As the steering wheel 62 is turned, for example to the right, the voltage to the right hand swash plate control amplifier 66 will be reduced, and the swash plate angle 0 will decrease. The displacement of the right hand pump 28 will as a result be reduced, causing the right hand drive wheel 34 to rotate more slowly than the left hand drive wheel 24 and the vehicle to skid steer to the right.

The further the steering wheel 62 is turned to the right, the further the voltage to the swash plate control amplifier 66 is reduced, and the radius of the skid steer turn become progressively smaller, until a point is reached when the right hand amplifier voltage is reduced to zero. At this point, the right hand swash plate 28 is closed and the right hand drive wheel 34 becomes stationary. Turning the steering wheel 62 further to the right reverses voltage polarity to the right hand swash plate amplifier 66 thereby causing the right hand drive wheels 34 to rotate in the reverse direction. At maximum turn of the steering wheel 62, the wheel 34 will revolve in the reverse direction at a speed equal to the forward speed of the wheel 24. The vehicle will then execute a neutral turn.

Turning of the steering wheel 62 to the left will produce the same effect on the left hand pump 18, and turn the vehicle to the left.

For a skid steer manoeuvre to the left, the lowering of the ratio through the first hydraulic transmission 6 causes a slowing of the left side drive shaft 26 with a consequent transfer of power from shaft 26, through the first hydraulic transmission 6, the fixed ratio splitter box 10 and the second hydraulic transmission 8 whose ratio remains unchanged. The resultant skid steer forces result in an increase of power required by shaft 36 and a net increase in power reflected to the engine 2 through the splitter box 10. A similar sequence occurs for a skid steer manoeuvre to the right with a net increase of power reflected to the engine 2. This causes the speed of the engine to decrease. The tacho voltage and hence output voltage falls as a result, so that the displacement of both pumps 18, 28 falls by the same proportion.In this way, a constant radius of vehicle turn is maintained, but at a slower vehicle speed so preventing the engine 2 from overloading or stalling. Power demand may be increased to compensate for the loss of velocity, but again the effect on the displacement of the pumps 18, 28 is such that the constant radius of turn is maintained.

Referring next to Figure 6, an engine transmission system for a skid-steered vehicle includes many of the same component parts as those shown in Figure 1 except that in Figure 6 these same components are prefixed with the number "1". In Figure 6, the second output shaft 136 drives a pair of drive wheels 180a and 180b through a drive differential 182, and the first output shaft 126 is connected to a pair of steer wheels 184a and 184b through a steer differential 186, a pair of steer shafts 188a and 188b, and, between the right hand steer shaft 188b and the right steer wheel 184b, a direction-reversing box 190 and a reverser output shaft 192. The wheels 180a and 184a are linked to rotate at the same speed by a shaft 181 and bevel gearboxes 183. similarly, the wheels 180b and 184b are linked to rotate at the same speed by a shaft 185 and bevel gearboxes 187.The right hand swash plate amplifier 166 is connected directly to the output of the gear select mechanism 152 whereas connected in series between the left hand swash plate amplifier 164 and the output of the ratio controller 38 is a steering mechanism 194 consisting of a geared potentiometer 196 fitted to the base of a steering column 198 which is manually rotatable by a steering wheel 200. The potentiometer 196 is so arranged that it produces no voltage when the steering wheel 200 is in its central position, produces a voltage which increases up to the controller 138 output when the wheel 200 is turned one way, and produces a reverse voltage signal which increases up to the controller 138 output but in reverse when the wheel 200 in turned the other way.

The particular advantage of the configuration shown in Figure 6 is that the combined power requirement of the two pumps is in the order of two thirds of the power requirement for the more conventional configuration shown in Figure 1. The reason for this is explained below.

The power delivered by a shaft is related to VT, where V is its rotational speed and T is the torque. For the configuration shown in Figure 1 , operating on reasonable slopes with average adhesion limit, maximum torque is required during skid-steer manoeuvres. All wheels must be able to deliver the full torque range over the full rotational speed range. This results in a total power requirement of say VT, with 1/2VT provided at each side of the vehicle. For the configuration shown in Figure 6 however the front wheels need only apply approximately T/3 over the full velocity range V for climbing a reasonable slope, and the rear wheels need to supply the full torque range T over a velocity range of V/3 for a reasonable turning rate.The consequence is that the combined power requirement of the two pumps is given by 2VT/3 In operation and when no steer is being demanded, the engine/transmission system illustrated in Figure 6 behaves in much the same way as that illustrated in Figure 1. The transmission ratio controller 138 sets the angletFof the swash plate 128a,which in turn determines (in combination with engine speed) the speed of rotation of the four wheels 180a, 180b, 184a, and 184b. The reverser 190 allows both wheels 184a and 184b to rotate in the same direction and at the same speed when the shaft 126 is stationary due to zero voltage being supplied to the actuator 168 (ie swash plate angle 0 = 0) and therefore shafts 188a and 188b rotating in opposite directions at the same speed. When steer is demanded, the swash plate 118a begins to open in either the forward or reverse direction depending on the direction of steer. As a result, the shaft 126 begins to rotate in a direction dependent on swash plate 118a position, creating via the differential 186 a differenntial speed between the wheels 184a and 184b, and therefore between the wheels 180a and 180b via the shafts 181 and 185. This results in skid steer. The amount of skid steer is regulated by the control signal from the controller 138. In this way, the steer ratio between the wheels 180a, 184a and 180b, 184b remains unaffected by either load placed on the system or power demand placed on the engine 102. As before, the controller 138 also protects the engine 102 from stalling during skid-steering.

Claims

1. An engine/transmission control system for a skid-steered vehicle, comprising (a) an engine, (b) at least one CVT system connected to and for transmitting drive from the engine, (c) means for sensing engine speed and for generating a first signal representative of engine speed (d) means for sensing power demand placed on the engine and for generating a second signal representative of power demand, (e) a CVT ratio controlling means for generating a third signal, derived from the difference between the first and second signals, for controlling means for varying the transmission ratio of the at least one CVT system between maximum and minimum values in such a manner that when CVT ratio is within a range between said maximum and minimum values, CVT ratio governs torque reflected through the at least one CVT system to the engine such that engine speed increases with increasing power demand but is controllably maintained within a narrow range of speeds at constant power demand, the CVT controlling means including a phase advance system and a phase retard system which systems limit the rate of change of the CVT transmission rate to an extent dependent on the rate of change of engine speed, and (f) a steer system responsive to steer demand for generating differential speeds between left-hand side and righthand side drive wheels.

2. A control system according to claim 1 wherein the CVT ratio controlling means is electrical and the first, second and third signals are voltage signals.

3. A control system according to any one of the preceding claims wherein the CVT ratio controlling means includes a tickover offset mechanism for supressing transmission of power through the at least one CVT system when engine speed is at or below a preselected tickover speed.

4. A control system according to any one of the preceding claims wherein the CVT ratio controlling means includes a throttle offset mechanism for supressing transmission of power through the at least one CVT system when power demand is at or below a predetermined minimum value.

5. A control system according to any one of the preceding claims wherein said narrow range of speeds is less than 10X of maximum engine speed over a CVT ratio range of between 20Z and 80Z of maximum CVT ratio.

6. A skid-steered vehicle according to any preceding claim wherein the engine/transmission control system comprises at least two CVT systems and wherein the magnitude of the differential speed is dependent on the magnitude of said third signal to provide a ratio of wheel speeds between the left-hand side and right-hand side drive wheels which varies in accordance with steer demand but is substantially independent of both the load reflected to and power demand placed on the engine.

7. A skid-steered vehicle as claimed in any one of claims 1 to 5 having left and right-hand side drive wheels, wherein the at least one CVT transmission system provides drive to both left and righthand side drive wheels, a further CVT transmission controlling differential velocity between the left and right-hand side drive wheels in order to effect skid-steering.

8. A skid-steered vehicle as claimed in claim 7 wherein the magnitude of differential velocity is dependent on the magnitude of the third signal.

9. An engine/transmission control system substantially as hereinbefore described with reference to Figures 1, 2, 3, and 4.

10 An engine/transmission control system substantially as hereinbefore described with reference to Figures 2, 3, and 6.