DE102009047035A1 - Hydraulic control system for controlling one or more consumer loads, has directional valve, where each consumer load is assigned to directional valve during insert of two-way valves - Google Patents

Abstract

The hydraulic control system has a directional valve, where each consumer load is assigned to the directional valve during the insert of two-way valves. The supply flow in piston- or piston rod chamber determines the consumer load.

Description

There are many functional system solutions for the control of several consumers connected in parallel in the working and comfort equipment of working machines, all of which have reached a high level of development. A common problem of the systems used so far is the comparatively low energy efficiency. Some systems are susceptible to vibration and require a high development effort to ensure a stable system. The machines considered here include in particular hydraulically or pneumatically operated machines. Characteristic of this is that a medium, preferably hydraulic oil, compressed air or other liquid or gaseous substances are used for their operation.

A particular problem for controls, in particular mobile controls, are the pulling loads which frequently occur in mobile machines, in which the load acts in the direction of movement of the load. Even with low pulling loads, cavitation phenomena can occur in a displacement space of the actuator (cylinder, hydraulic motor). A throttling of the flow on the discharge side should avoid this. The speed control of the consumer is then realized via the process control edge (tank edge). Since the inlet control edge (measuring orifice) and the tank edge are arranged on the same valve slide, the edge contour can only be designed for a specific load case. If one controls a pressing load via a control slide with control edges designed for pulling loads, high throttling losses occur. Consequently, the control valves used today have the problem that they can not be designed simultaneously for low throttle losses, prevention of cavitation and good controllability. Furthermore, the adaptation of the control edge geometries due to the difficult to predict load cases is often made experimentally by costly experiments.

To overcome the problem of difficult to design and inefficient inlet and outlet control edges, control systems with resolved control edges have been developed. This inlet and outlet control edge can be operated independently. Thus, a targeted movement of pulling loads is possible without causing cavitation phenomena. At the same time, only the lowest possible throttling losses are permitted when controlling pushing loads.

The main advantage of a control system with resolved control edges over a system with a valve spool for all control edges is the increased efficiency, the reduction of the pressure level, the possibility of decentralization as well as the possibility of simple energy regeneration and energy reclamation from sinking movements.

Basically, there are two ways to run valves with resolved control edges. On the one hand, these are systems with 4 2/2 directional control valves, ( 1b ), on the other hand systems with 2 pieces 3/3 way valves, ( 1a ). For this purpose, numerous patent applications are known.

Here and in the following, a designation "X / Y-way valve" indicates that the valve in question has X connections and can be switched to Y valve positions (switching positions). If only one number is given, this information refers to the number of valve positions (switching positions).

For example, in the EP 1 710 446 A2 a hydraulic control arrangement and a control block presented, are arranged in the inlet to the consumer and in the course of the consumer valve spool. The two valve spools are preceded on the pump side by an individual pressure compensator. The pressure compensator has the task to achieve a load independence of the system with several parallel connected consumers. This is done by an individual control of the volume flow through the control valves. For this purpose, the load pressure and the pressure drop across the control valve (control pressure difference) set by a spring are compared with the pressure in front of the control valve. Via the spring preload one chooses the desired pressure drop over the control valve. A disadvantage of systems with upstream pressure compensator is the behavior of undersupply. The pump can then not provide the total required volume flow for all consumers. The highest-end consumer is first supplied less and in the worst case even stops.

The DE 102 96 738 T5 This invention provides a control edge solution in which a proportional valve is addressed via an electro-hydraulic control unit and controls the direction of movement of the hydraulic piston by releasing the fluid flow in one or an opposite direction. Only in the flow direction - as seen from the pump - behind this valve, the bidirectional proportional control valves are mounted, through which the flow in the rod or piston chamber of the cylinder can be controlled. A disadvantage of this arrangement, however, is that no load compensation is done with consumers connected in parallel. In addition, a high component cost, such as a variety of throttle valves and hydraulic Pilot controls and a directional control valve on the inlet side necessary.

Many of the known solutions with resolved control edges have so far found no comprehensive input into the machines on the market. One reason is the additional equipment expense. Instead of a valve and a pressure balance, two valves and two or more pressure compensators are required. In addition, if you want to consistently use the technical advantages of such systems (energy efficiency, energy recuperation potential, improve controllability), additional sensors and complex controls (multi-variable control) and accordingly more know-how of the commissioning technician required.

Due to the increased component costs (valves, magnets, sensors), the costs for solutions according to the prior art are higher than in today's controls. The harsh working conditions of mobile work machines also require special solutions in the electronic controls.

• – Vermeiden der nicht prinzipbedingten Druckverluste an der Tankkante des Systems und damit Reduzierung des Energiebedarfs,
• – Senken des Druckniveaus an der Pumpe in einigen Arbeitszuständen,
• – Verbesserung der Steuerbarkeit von ziehenden Lasten und Reduzierung der Kavitationsneigung in den Aktuatoren,
• – Reduzierung des Abstimmungs-/Inbetriebnahmeaufwandes des Systems (Kantenauslegung zueinander wird vereinfacht) und
• – Möglichkeit zur Energierekuperation/Energieregeneration.

The main requirements for control systems with resolved control edge can be named as follows:

• Avoidance of non-principle pressure losses at the tank edge of the system and thus reduction of the energy requirement,
• Lowering the pressure level at the pump in some working conditions,
• Improving the controllability of pulling loads and reducing the cavitation tendency in the actuators,
• - Reduction of the coordination / commissioning effort of the system (edge design to each other is simplified) and
• - Possibility for energy recuperation / energy regeneration.

It results in the task of meeting these requirements by systems with resolved control edges, which manage with less component complexity (magnetics, sensors, valves, pressure compensators), than previous known systems with resolved control edges. This facilitates the practical implementation even with mobile machines.

The object is achieved by the solution according to claim 1. Advantageous embodiments are shown in the dependent claims.

In the following, the volume flow fed into the overall system by the pump is referred to as the delivery volume flow of the pump. This designation is retained until the first valve associated with a single consumer. Starting with this point is then spoken of the feed stream for each consumer. It is immediately obvious that the flow rate of the pump in circuits without recuperation is equal to the sum of the supply currents of all consumers. In circuit variants with recuperation, the flow rate of the pump results directly at its connection as the sum of the feed streams of all consumers reduced to the recuperation streams, which are branched off from the drainage streams and fed back into the flow of the pump.

The invention enables system solutions both for the principle of 3-way valves and for the principle of 2-way valves, which manage with only one downstream pressure balance per consumer and unlike solutions according to the prior art, a lower cost of inserted magnetics and / or Have sensor. The advantages of good controllability of an electro-hydraulic control system (electro-hydraulic control unit) are maintained. By deliberately raising all consumer pressures to a uniform pressure level and the possible use of the returning volume flow, there is the possibility of energy recuperation. It is known to the person skilled in the art that the proposed circuits with 2-way valves or 3-way valves, in addition to the solution variants presented here, also function if further connections are added which only control auxiliary currents.

It is proposed a circuit implementation of a control system with resolved control edge, which allows for low component complexity, the properties of high energy efficiency and load independence, the control of several parallel connected consumers.

It is within the scope of expert action to change the flow directions and valve positions while maintaining the principles presented below.

All new system solutions presented here are shown in simplified form, ie. H. without pilot control of the control valves, without complex load-sensing valves, without load-holding valves and without secondary pressure relief valves. These auxiliary devices are known in the art and their arrangement is part of the prior art and the usual design work for hydraulic and pneumatic control systems.

The terms piston chamber and rod chamber are known in the literature as piston chamber or rod space.

The system according to the invention with 2/2 way valve technology requires only 2 proportional valves and not, as before, 4 pieces. However, a cost-effective switching valve is additionally used. The number of proportional solenoids is reduced from 4 pieces to 2 pieces, plus one switching solenoid. Only one pressure balance is needed and not, as with other systems, at least 2 pieces. Only one pressure sensor per consumer is needed. In the simplest case, it is even possible to completely dispense with the use of sensor technology, which likewise makes no known solution possible.

The basic principle of the control is a classic load-independent flow divider principle. In the case of an upstream pressure compensator, however, the system can also be changed to a classic flow control principle. The valves are advantageously designed in panel construction and can be integrated into existing valve blocks for controlling selected consumers.

• 1. Spalte: Druckwaage vor den Ventilen: Die Pumpe fördert gegen die Druckwaage IDW, die vor dem 2/2 Wegeventil VW_zu (Zulaufventil) für den Zulauf angeordnet ist. Die Druckwaage vergleicht den Druck nach dem Zulaufventil mit dem Druck vor dem Zulaufventil und regelt den Volumenstrom über der Zulaufblende, indem eine konstante Druckdifferenz über dem Zulaufventil des Systems aufrechterhalten wird. Nach dem Zulaufventil ist das Richtungsventil VD angeordnet, das den Medienstrom abhängig von der gewünschten Bewegungsrichtung des Kolbens in die Kolben- oder die Stangenkammer des Zylinders presst.
• 2. Spalte: Druckwaage hinter den Ventilen: Die Pumpe fördert gegen das 2/2 Wegeventil VW_zu (Zulaufventil), das der Druckwaage IDW vorgeschaltet ist. Die Druckwaage vergleicht den Druck nach dem Zulaufventil mit dem höchsten Lastdruck im System und regelt den Volumenstrom über der Zulaufblende, indem eine konstante Druckdifferenz über dem Zulaufventil des Systems aufrechterhalten wird. Nach der Druckwaage ist das Richtungsventil VD angeordnet, das für diesen Lastfall den Medienstrom in die Kolbenkammer des Zylinders drückt. Das Medium wird durch den sich bewegenden Kolben aus der Stangenkammer verdrängt und über das Richtungsventil VD zum 2/2 Wegeventil VW_ab (Ablaufventil) geleitet und von diesem weiter in den Tank gedrückt.

• 1. Zeile: drückender Lastzustand: Das Medium wird durch das Richtungsventil in die Kolbenkammer des Zylinders geleitet. Durch den sich bewegenden Kolben wird das Medium aus der Stangenkammer verdrängt und über das Richtungsventil VD zum 2/2 Wegeventil VW_ab (Ablaufventil) geleitet und von diesem weiter in den Tank gedrückt. Das Ablaufventil ist vollständig geöffnet, um Drosselverluste weitgehend zu vermeiden.
• 2. Zeile: ziehender Lastzustand: Das Medium wird durch das Richtungsventil in die Stangenkammer des Zylinders geleitet. Durch den sich bewegenden Kolben wird das Medium aus der Kolbenkammer verdrängt und über das Richtungsventil VD zum 2/2 Wegeventil VW_ab (Ablaufventil) geleitet und von diesem weiter in den Tank gedrückt. Der Druck in der Kolbenkammer des Zylinders wird gemessen und beim Unterschreiten eines zulässigen Wertes wird das Ablaufventil teilweise geschlossen, um die Ablaufgeschwindigkeit des Mediums aus der Stangenkammer soweit zu reduzieren, dass der zulässige Mindestdruck in der Kolbenkammer erhalten bleibt.

The pressure compensator can be connected upstream or downstream of the valves. Together with the two load cases (pulling and pushing), four circuit variants result ( 2 ).

• 1st column: Pressure compensator in front of the valves: The pump delivers against the pressure compensator IDW, which is located in front of the 2/2 directional valve VW _to (inlet valve) for the inlet. The pressure compensator compares the pressure downstream of the inlet valve with the pressure upstream of the inlet valve and regulates the volume flow through the inlet orifice by maintaining a constant pressure difference across the inlet valve of the system. After the inlet valve, the directional valve VD is arranged, which presses the medium flow depending on the desired direction of movement of the piston in the piston or the rod chamber of the cylinder.
• 2nd column: Pressure compensator behind the valves: The pump delivers against the 2/2 directional valve VW _to (inlet valve), which is connected upstream of the pressure compensator IDW. The pressure compensator compares the pressure after the inlet valve with the highest load pressure in the system and regulates the volume flow over the inlet orifice by maintaining a constant pressure difference across the system inlet valve. After the pressure compensator, the directional valve VD is arranged, which presses the media flow into the piston chamber of the cylinder for this load case. The medium is displaced by the moving piston rod from the chamber and passed via the directional valve VD to 2/2 way valve VW _from (drain valve) and further pushed from this into the tank.

• 1st line: pressing load state: The medium is directed through the directional valve into the piston chamber of the cylinder. By the moving piston, the medium is forced out of the rod chamber and passed through the directional valve VD to 2/2 way valve VW _from (drain valve) and further pushed from this into the tank. The drain valve is fully open to largely avoid throttle losses.
• 2nd line: pulling load state: The medium is directed through the directional valve into the rod chamber of the cylinder. By the moving piston, the medium is forced out of the piston chamber and passed through the directional valve VD to 2/2 way valve VW _from (drain valve) and further pushed from this into the tank. The pressure in the piston chamber of the cylinder is measured and when falling below a permissible value, the drain valve is partially closed in order to reduce the flow rate of the medium from the rod chamber so far that the permissible minimum pressure in the piston chamber is maintained.

Pressing and pulling load conditions can occur in both directions of movement. The 2 shows this only as an example for the one and the other direction of movement. It is also conceivable that the direction of force reverses.

The operator uses the joystick to set the target speed of the consumers. In a control algorithm, the position of the inlet valve VW _to , the position of the directional valve VD and the required volume flow amount is calculated. The pump is swiveled to pump exactly the amount needed by all consumers in the system. About the directional valve VD, the direction of movement of the consumer is specified. The information about the position is taken from the operator signal.

The flow control edge is controlled so that the pressure on the inlet side does not fall below a certain value. This predetermined lower pressure limit depends on the hydraulic or pneumatic media used, the speed of movement, the chamber shape and other boundary conditions. At the latest when the pressure in the inflow volume flow falls below a cavitation-critical value, the sequence must be throttled. However, since this already carries the risk of damage, the throttling of the drain volume flow should be done before. In this case, a pressure sensor is used to determine the pressure p _A. In the case of a pressing load F in the opposite direction of movement x, the sequence control edge is opened completely in order to avoid unnecessary throttling losses. With a pulling load F (acts in the direction of movement x), the pressure p _{A can} drop sharply. To prevent this is from a certain pressure p _A (for example, 5 bar), the flow control edge regulated so that the refluxing flow is throttled to exclude cavitation states. This demand-based throttling is adjusted according to the load and thus offers considerable energy advantages compared to today's control systems with common control edges.

The control system then operates independently of the load when the same pressure drop occurs across all supply control edges to the parallel actuated consumers. This is achieved by individual volume flow control over the inlet control edge using individual pressure compensators (IDW). Their functionality has proven itself in today's load-sensing systems. The individual pressure compensators can be connected upstream, as in the system IDW_vor, which gives the system the character of a classic power control circuit, but also downstream, as in the system IDW_nach, which behaves the system like a classic power divider circuit.

The use of the pressure sensor can be dispensed with if a direct return of the load pressure of the inlet side to the valve of the outlet side is realized ( 3 ). The discharge side then has the character of a classic lowering brake valve. In this case, the circuit is completely without the use of additional sensors.

The circuits after 2 and 3 can be extended for multiple consumers by running the inlet flow line and the drain line as common lines. In each valve disc there is a load-sensing valve, which reports the highest load pressure to each valve disc (valve disc = valves per consumer, these are often designed as a common module).

The invention provides solutions for both groups of resolved control edges (2-way valve design and 3-way valve design) ready. Both solutions are designed on the control side as a demand-supplied electro-hydraulic control system.

The 3-way valve system works basically similar to the 2-way valve system. However, the directional control does not take place through an additional valve, but directly in the proportional valves. Due to the double flow valve slide used, it is possible to reduce the number of pressure compensators to 1 piece.

The double-flow valve slide are known from the prior art and are manufactured for example by the Bosch-Rexroth AG.

4 shows a preferred circuit variant, which is based on the basic principle of 2 pieces 3-way valves. So that only one (downstream) pressure compensator has to be used for both directions of movement, the valves are designed as double-flow 5/3 directional control valves. A directional control valve is not necessary. The basic control of the pump and the inlet and outlet control edge is carried out as in the described 2/2 way valve technology. The pressure sensor is retained in this circuit variant.

In contrast to the known from the prior art arrangement with upstream pressure compensator ( EP 1 710 446 A2 ), however, the behavior of a system with downstream pressure compensator is significantly different. Due to the self-adjusting classical current division, a load independence is achieved, in which all valves remain controllable even in the case of undersupply conditions.

The 4 , the circuit is able without media flow. In order to guide the media flow into the piston chamber, the valves VW and VW _to be made _from such that the, is released in the left-hand path representation. Now the medium moves through the valve VW _to , through the pressure gauge IDW and then again through VW _to (double-flow valve) in the piston chamber of the consumer. The medium is displaced from the rod chamber and flows via the drain valve into the tank. The pressure compensator detects the pressure difference of the inflowing medium after the inlet valve and the highest-load load of the inlet valve and regulates the permissible media flow accordingly. The pressure sensor detects the pressure conditions in the incoming media flow and transmits them to the electrohydraulic control device. When falling below a permissible pressure causes this, that the outflowing medium flow is throttled by the drain valve VWab. To reverse the direction of movement, the valves interchange their function and are set so that the, in the representation, right way is released. Since the load is not pulling in this case, however, no throttling of the running media flow is necessary. It goes without saying that the loads on each consumer based on the direction of movement of the piston can act pushing or pulling, the valve positions are then preferably adjusted by the electro-hydraulic control unit accordingly, do that the control principle described is realized.

A preferred system with 3-way valves and 2 consumers is in 5 shown. It is in principle expandable to any number of consumers. The pressure compensator of each valve section ensures that the pressure behind all inlet control edges in the system is the same. Thus, the volume flow distribution in the system only depends on the opening cross sections of the inlet control edges. The system behaves like a classic power divider circuit. Here too, a pressure sensor and a common electro-hydraulic control device are used per valve axis.

The dissolved construction of the valve spool allows an energy recuperation or regeneration (without intermediate storage) when pulling loads occur and / or a load-low parallel connected consumer.

A preferred circuit variant for energy recuperation is in 6 shown. There are numerous solutions for energy storage that could be implemented with this valve technology.

In the energy recuperation with dissolved control edge is raised preferably targeted the pressure level before the flow control edge of the lastniederen consumers. As a result, all consumers work with the same system pressure on the inlet side. The volume flow, which is throttled over the flow control edge, can then be made available preferably via a recuperation _valve V _rek or a _hydraulic motor which sits on the drive shaft and has a variable displacement volume. There are many other possibilities here.

In the preferred circuit variant shown in Figure 6, two 3-way valves per consumer are used. The circuit corresponds largely to the in 4 shown. However, a recuperation valve is now used which, if the pressure of the _{outflowing medium flow} is higher than the pump pressure, _{feeds part of the outflowing medium flow} into the inlet via the recuperation _valve V _rek . The recuperation valve is preferably designed in the manner of a pressure compensator, which compares the pressure in the inlet line and the outlet line and opens the valve at a higher pressure in the outlet line. A double check valve in the inlet of the recuperation valve ensures that the recuperation valve is always fed from the respective active drain. Further, to be supplied by the pump consumers would then connected with their associated valve circuits in parallel to the circuit shown to the delivery line of the pump or the drain line leading into the tank. This circuit variant is controlled by an electro-hydraulic control device.

If one waives the use of pressure compensators in the system, another preferred option for energy recuperation can be realized. For loads in the direction of movement, the return flow is directed directly to the pump side. In this case, the inlet edge must be integrated into the control of the valve to avoid a "falling through" of the consumer. The energy source is a motor / pump unit used to recover energy. 7 shows such a circuit variant. The valve positions are also indicated depending on the load direction. The line B acts in load in the direction of movement and the valve position for the inlet in the piston chamber as Rekuperationsleitung and the line A as a pump line. In a reversal of the direction of movement of the piston, ie at the valve position for the inlet into the piston chamber (but again with load in the direction of movement) interchange the functions of the lines A and B. The throttling of the outflow volume flow takes place via the control of the trailing edge of the 3 Directional valve, which has the function of the drain valve in the current situation. Further, to be supplied by the pump consumers would then connected with their associated valve circuits parallel to the circuit shown to the delivery line of the pump or the drain line leading into the tank. The pressure in the valve is adequately detected by one sensor per valve axis, since both the valve position and the quantity through the valve and the area ratio at the cylinder are known. This circuit variant is controlled by an electro-hydraulic control device.

The directional control valves are preferably designed as electromagnetically controlled valves. However, it is also possible to use a motor control, a manual override, a hydraulic actuation or any other suitable drive for moving the valves. In these cases, however, displacement sensors at the inlet edges or displacement sensors on the individual pressure compensators are necessary in order to obtain electronic information about the valve opening and thus the flow rate requirement for the pump.

characters

1 Valve controls with resolved control edges (prior art)

2 Circuit diagram for the new control system with resolved control edges

3 Circuit diagram for a new sensor-free control system

4 Circuit diagram for the new control concept using 5/3 directional control valves

5 Exemplary system with two consumers

6 New circuit concept with possibility of recuperation

7 Valve concept without pressure compensators for easy energy recuperation

LIST OF REFERENCE NUMBERS

A, B

Pump line or recuperation line (depending on the flow direction of the media)

F

force exerted by the load

x

Direction of movement of the piston

Q _too

Volume flow of the supply line

Q _from

Volume flow of the drain line

Q1

Flow rate of the pump

Q _A1

Volume flow of the supply line of the consumer 1

Q _B1

Volume flow of the discharge line of the consumer 1

Q _A2

Volume flow of the supply line of the consumer 2

Q _B2

Volume flow of the discharge line of the consumer 2

p ₁

Delivery pressure of the pump

p _A1

Pressure in the supply line of the consumer 1

p _B1

Pressure in the drain line of the consumer 1

p _A2

Pressure in the supply line of the consumer 2

p _B2

Pressure in the drain line of the consumer 2

M

engine

VD

directional valve

VW _too

Valve for controlling the incoming media flow

VW _off

Valve for controlling the outflowing media flow

ECU

Electrohydraulic control unit

VW _zu1

incoming media stream for consumers 2

VW _zu2

incoming media stream for consumers 2

VW _ab1

Expiring medium flow for consumers 1

VW _ab2

running media stream for consumers 2

IDW

Individual pressure compensator

IDW ₁

Individual pressure compensator assigned to consumer 1

IDW ₂

Individual pressure balance assigned to consumer 2

V _rek

Rekuperationsventil

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1710446 A2 [0007, 0033]
• DE 10296738 T5 [0008]

Claims (5)

Hydraulic control arrangement for controlling one or more consumers wherein each consumer has at least one pressure gauge and a continuously adjustable directional control valve arrangement and the directional control valve arrangement inlet flow and outflow by means of separate directional control valves can control each other separately - When using 2-way valves each consumer a directional valve is assigned, which sets the inlet flow in the piston or rod chamber of the consumer and an individual pressure compensator is assigned, which is located in front of or behind the 2-way valve of the inlet stream, but always between the pump and directional valve wherein an electro-hydraulic control unit controls the direction of movement and the speed of the consumer by processing user inputs, pressure readings from the feed stream, and prescriptions to a lower pressure limit in the feed stream and controlling the position of the directional valves and the opening widths of the 2-way valves or - When using 3-way valves each consumer an individual pressure compensator is assigned, which is arranged behind the directional control valve of the feed stream, with an electro-hydraulic control unit direction of movement and speed controls the consumer by processing user inputs, pressure readings from the feed stream and specifications to a lower pressure limit in the feed stream and controls the opening widths of the 3-way valves. Hydraulic control arrangement according to claim 1, wherein the electro-hydraulic control unit throttles the outflow stream by means of the opening width of the directional control valve in the outflow stream in such a way that the predetermined lower pressure limit in the inflow stream is not undershot. Hydraulic control arrangement according to claim 1 or 2 with 2-way valves, wherein no pressure readings from the feed stream are processed by the electro-hydraulic control unit and the load pressure of the feed stream to the directional control valve of the drain stream is directed so that it throttles the flow with decreasing pressure of the feed stream. Hydraulic control arrangement according to claim 1 or 2 with 3-way valves and a recuperation valve, which is designed as individual pressure compensator, which opens the recuperation when the pressure of the outflow stream is greater than that of the delivery flow rate of the pump, so that at least a portion of the outflow stream in the flow rate the pump is fed. Hydraulic control arrangement according to claim 1 or 2 with 3-way valves and without individual pressure compensator, wherein at least a portion of the outflow stream is fed into the delivery volume flow of the pump when the pressure of the outflow stream is greater than that of the delivery volume flow of the pump by the electro-hydraulic control unit 3 Directional control valves so that the 3-way valve in the drain stream this divided depending on the pressure conditions in a recuperation and a tank flow and the 3-way valve throttles the supply flow to the extent that the predetermined speed of movement is maintained.

Images