EP0091018A1 - Position control for a double acting hydraulic motor - Google Patents

Abstract

A control system for controlling a double-acting cylinder includes four pilot-operated, proportional-type poppet valves for controlling fluid flow between the cylinder, a pump and a reservoir. Four solenoid-controlled pilot valves operate the poppet valves in response to error signals generated by a control circuit. The control circuit receives a cylinder position feedback signal and an operator-generated command signal. The control circuit provides for float, shutdown, variable deadband and pressure adjustment operation.

Description

The invention relates to an arrangement for controlling a double-acting hydraulic cylinder with an extension and a retraction chamber, which are connected via a valve device to a hydraulic pressure source and with a sump, and with a control circuit controlling the valve device.

Such arrangements are known in numerous embodiments and with different valves in the valve device.

It is generally known to control the flow to such a cylinder and also the outflow of the hydraulic oil with a slide valve, the slide valve being operable directly by means of electromagnetic actuation devices or with the aid of a pilot pressure, which in turn is controlled by an electrically actuated pilot valve.

Valve devices of this type are known in connection with position control systems for such hydraulic cylinders with a closed flow circuit. Gate valves are known for their sensitivity to contamination in hydraulic oil. It is also difficult to design such control systems with slide valves in such a way that they ensure a bumpless, stable operation, especially when the system is intended to withstand heavy loads that tend to overrun the drive factor. to control, for example when a heavy load is to be lowered with the help of the hydraulic cylinder. If a system is designed in such a way that it meets the latter requirement, however, a control system results which is too slow in the opposite sense, for example if the hydraulic motor is to lift a heavy load. Another disadvantage of such valve devices is that fairly complicated slide designs or additional valves are necessary to achieve a mode of operation in which the hydraulic motor can also assume a floating state.

As an alternative to slide valve devices, it is known to actuate double-acting cylinders via four pressure-reducing or non-return valves which can be switched on and off and which are controlled by pilot valves which can be actuated by means of an electromagnet. Such a valve device consisting of four valves can control a movement of the cylinder in both directions and, in addition, enable a floating state for the cylinder as well as a locking state for the cylinder. However, such valves that can be switched on and off can generate undesirably high pressures when used in systems which require high flow rates for the hydraulic fluid. In systems with high inertia, such valves can also lead to instabilities and thus the target positions being overrun. It is therefore desirable to obtain a stable system which is suitable for a closed fluid circuit and whose functions are extremely flexible without the need for slide valves.

It is therefore an object of the invention to provide an arrangement of the type specified in the introduction which is suitable for ensuring working characteristics both in the case of overflow and underflow conditions, and in which the valve device shows a high degree of flexibility in the functional sense, without the use of the system being restricted by high flow velocities or high moments of inertia.

This object is achieved in that the valve device has four independently operable valves for controlling a flow connection between each of the two chambers of the cylinder and pressure source and sump, that an arbitrarily actuatable control device for generating a position command signal and a sensor device for generating a Actual position signals are provided, and that the control circuit controls the four valves as a function of an error signal generated from the desired and actual signals.

The four independently operable valves are expediently each electrically controlled pressure reducing valves, a first of which is connected on the inlet side to the pressure source and on the outlet side to the draw-in chamber; a second one is connected on the inlet side to the feed chamber and on the outlet side to the sump; a third is connected on the inlet side to the extension chamber and on the outlet side to the sump, and a fourth is connected on the inlet side to the pressure source and on the outlet side to the extension chamber.

Each of the electrically controlled pressure reduction valves is preferably formed in the form of a pilot-operated, proportionally operating ring valve and each has an electromagnetically actuable pilot valve.

In order to prevent any malfunction due to undesirable short-circuiting of oil flows, it is expedient to have a first check valve to prevent pressure medium flow from the feed chamber to the first pressure reduction valve and a second check valve to prevent pressure medium flow from the extension chamber to the fourth pressure reducing valve.

The individual four valves control the fluid flow between the double-acting cylinder, the pump and the sump. The position of the hydraulic cylinder is continuously reported back to the control circuit by the actual position signal. This control circuit also receives the desired position signal generated by the operator.

The control circuit is expediently designed in such a way that it has a differentiating circuit for converting the actual position signal into a signal representing the speed of movement of the cylinder and a differentiating circuit for generating the error signal from the desired and actual signals, the outputs of which have a further differentiating circuit for generating a speed-compensated error signal, which is fed to a reversing converter, that two first driver circuits are also provided for controlling two selected pressure reducing valves, to which the compensated error signal is present, and that two further driver circuits are provided for the other two pressure reducing valves to which the reverse compensated error signal is present.

The control circuit thus generates reverse and non-reverse speed-compensated error signals, which are each fed to two corresponding actuation solenoids via the pulse width modulating circuits.

The invention is explained in more detail below with the aid of schematic drawings using an exemplary embodiment.

• Figur 1 ein vereinfachtes schematisches Diagramm der Anordnung gemäß der Erfindung und
• Figur 2 ein schematisches Schaltblockdiagramm des zugehörigen Steuerkreises.

Show it:

• Figure 1 is a simplified schematic diagram of the arrangement according to the invention and
• Figure 2 is a schematic circuit block diagram of the associated control circuit.

In Figure 1, a double-acting cylinder 10 is provided, which is controlled via a valve device 12. The cylinder is connected to a pressure medium source or pump 14 and a pressure medium sump 16 via this valve device. The pump 14 is preferably a conventional hydraulic pump delivering pressure on demand, or a type of pressure medium source. The cylinder 10 includes a position feedback sensor or a potentiometer 80, which is shown and described in detail in US-PS 37 26 191.

The valve device 12 comprises four pressure-reducing or proportionally operating ring valves 20a to 20d, which are controlled by electromagnets and can be actuated by means of pilot valves. These four valves can be operated independently of one another and are each electrically controlled. The valve 20a controls the flow connection between the pressure medium source 14 and a drawing chamber 11 of the cylinder. The valve 20b controls the flow connection between the sump 16 and an extension chamber 13 of the cylinder. A first check valve 22 prevents flow between the chamber 11 towards the valve 20a. Valve 20c controls the flow between chamber 13 and sump 16, while valve 20d controls flow between sump 14 and chamber 13. A second check valve 24 prevents flow from chamber 13 to valve 20d. The pressure reducing valve 20d controls the flow from the pump 14 to the chamber 13. The check valve 24 prevents a flow reversal in the direction of the pressure medium source 14.

The pressure reduction valves 20a to 20d are controlled in each case by pilot valves which can be actuated by electromagnets 21a to 21d. For example, when power is on When coil 21a is placed, armature 100 moves proportionally against the bias of spring 102 to expose opening 104. As a result, a pressure difference occurs across the opening 106 of the valve body 108. The valve body can thus move proportionally away from the valve seat 112 against the pretension of the spring 110, so as to open the valve 20a in relation to the pilot control and thus the pressure difference. The valves 20b to 20d operate in the same way.

The control circuit 30 generates the necessary control signals as a function of a position signal X, which is obtained from the converter 18 on the cylinder 10, and a control signal C, which is supplied by a converter 28, which is controlled by the operator. The transducers can each be designed as a potentiometer. The control or target signal C represents a desired position of the cylinder 10 or the associated piston.

FIG. 2 shows the control circuit 30. This comprises an isolation amplifier 32 with a unit gain factor, which is intended to separate the position signal X from the position converter 18. Amplifiers (not shown) with a predetermined gain factor may be necessary to influence one or both of the signals (position signal X and control signal C) and to use them for a single voltage range of e.g. Convert 0 to 8 volts. The actual signal X is differentiated by a differentiator circuit 34 and amplified by an inverting amplifier 36 with an amplification factor of approximately minus 0.6.

An error signal E is generated in that the actual signal X is subtracted from the desired signal C at the difference-forming connection point 38. The error signal E is then amplified by amplifier 40 factor of approximately 2.0 amplified and reversed by a reversing amplifier 42 with a unit amplification factor. A differential connection 44 includes a (-) input that receives the output from the inverter 42 and a (-) input that receives the output of the inverter 36. An inverted combined or speed-compensated error signal -E 'thus appears at the output of the differential connection 44. The inverted signal -E 'is inverted by a unit gain amplifier 46 to obtain an uninverted combined or speed compensated error signal + E'.

The error signals E and -E are combined via two corresponding arrhythmic units 50, 54 and 48, 52, respectively, and correspondingly supplied to two identical driver circuits 80b, 80d and 80a, 80c for actuating corresponding electromagnets. These circles, which are described in more detail below, work to produce a change of e.g. 300 mA in the drive current for the electromagnetic windings. This drive current is designated Ic. The change in the current in the windings 21a to 21d takes place depending on the occurrence of a voltage change of e.g. 2.5 V in the error signal output of differential connection 44. The (-) inputs of arrhythmic circuits 48 and 52 both receive the reverse error signal -E ', while the (-) inputs of arrhythmic circuits 50 and 54 receive the non-inverted error signal + E 'received.

The (-) inputs of arrhythmic circuits 48 through 54 continue to receive a low or high level shutdown signal from an operator controlled bistable device 56, e.g., a switch. A low level signal from switch 56 will turn off all windings 21a through 21d. As a result, all valves 20a to 20d close. A shutdown state is thus achieved.

Another bistable device (e.g. switch 58) which can be controlled by the operator supplies a signal of a high or low level which is sent to the (+) inputs of the arrhythmic circuits 48 and 54 and to the (-) inputs of the arrhythmic circuits 50 and 52 is created. The operator can thus turn on switch 58 to close valves 20a and 20d while opening valves 20b and 20c, causing engine 10 to float.

The error signal E from the amplifier 40 is fed to the (+) input of a comparator 60 via the resistor R1. The inverted error signal -E from the inverter 42 is fed via resistor R2 to the (+) input of the comparator 62. The (-) inputs of comparators 60 and 62 are both connected to an adjustable contact of a variable potentiometer 64, which generates a variable response reference signal Vdb. The output of comparator 62 is connected to the (+) input of comparator 60. The signal at the output of the comparator 60 has a high value except for the moment when the error signals E or -E are within the response value range, the width of which is determined by the level of the response value reference signal Vab from the potentiometer 64. The output of the comparator 60 is connected via a resistor R to a voltage source of +8 volts and to the input of an integrator 66, which has a reverse gain factor of -0.3. Integrator 66 raises or lowers its output between voltage limits in response to abrupt changes in the output of comparator 60. Integrator 66 also performs an inverse function to produce an inverse response reference signal Vdb 'which assumes a low value if so the error signals E and -E are not within the response value range mentioned above. The reverse response reference signal Vdb 'turns on the (+) inputs of differential junctions 50 and 52 applied to turn off coils 21b and 21c and close valves 20b and 20c when error signals E or -E are within the response range.

A conventional pressure sensor 68 can be arranged to measure the outlet pressure of the pressure medium source 14 and to generate a pressure setting signal Vpa which is proportional to the outlet pressure of the pump. This signal is added to the reverse pickup reference signal Vdb 'at the summation point 70. The sum of these two signals is applied to the (+) inputs of summing connections 48 and 54. Thus, when the output pressure of the pump 14 decreases, the pressure sensor 66 amplifies the signal Vpa so that there is a proportional reduction in the state of the current supply to the coils 21a and 21d and thus a corresponding greater movement of the valves 20a and 20d in the direction of the closed position. This proportional closing of the valves 20a and 20d increases the pressure drop across these valves and thus compensates for the original increase in the pump pressure. Conversely, a decrease in the pump pressure leads to compensation by proportionally opening the valves 20a and 20d.

The output signals of the summation points 48 to 54 are applied to identical circles 80a to 80d, one of which will be explained in detail. These circles are driver circles. Circuit 80a includes an amplifier 82a with a gain factor of approximately 0.8 that amplifies the output of summing connection 48. The amplified error signal is routed to one (-) input of a summing connection 84a. The other (-) input of connection 84a receives an expanded dither signal of 200 Hz and a triangular waveform generated by the dither oscillator 72 and an inverter 74.

The output V3 of connection 84a is coupled to amplifier 86a with an amplification factor of approximately 20. This generates a signal V4, which is then connected to the input of a pulse-wide modulator 88a. The modulator 88a also receives the non-inverted signal from a pulse-wide oscillator 76 which generates a 300 Hz, triangular waveform signal. The modulated output Vc of the modulator 88a is a square-wave voltage signal of 3000 Hz with a percentage modulation or a duty cycle equal to 100 x ((V4-1.26) / (3.93-1.26)), in which 3.93 and 1, 26 are the high and low peak values of the signal from the oscillator 72, respectively. The output signal Vc reaches one end of the winding 21a.

The other end of winding 21a is grounded via current sensing resistor R4a and connected to the (+) input of connection point 84a via amplifier 90a and integration device 92a. The amplifier 90a has a gain factor of approximately 2.84, for example. Integrator 92a also receives a reference voltage Vref equal to 3.43 volts. It generates a voltage V2, which is defined by the LaPlace transformation equation V2 = 2Vref - V1 (6250 / (S + 6250)), in which V1 is the voltage at the output of amplifier 90a. The overall effect of circuit 80a is to supply coil 21a with a drive current, designated Ic, which is proportional to the combined signal from arrhythmic unit 48. The feedback provided by amplifiers 90a and 92a reduces the variation effects in the voltage supply and resistance of winding 21a and provides improved frequency response sensitivity for the system.

It is also noted that the (-) inputs of connections 84a and 84c receive the reverse dither signal, while the (-) inputs of ver bonds 84b and 84d receive the non-reverse dither signal. As a result, the dither signals keep the operation of valves 20a and 20c out of phase with the operation of valves 20b and 20d. This prevents valves 20a and 20b and similarly valves 20d and 20c from opening simultaneously, so as to prevent a short circuit current to cylinder 10 by direct overflow of hydraulic oil from pump 14 into reservoir 16. This reduces the flow required to achieve equivalent pressure control, which could otherwise be achieved without dither signals.

It should also be noted that while modulators 88a and 88b receive an uninverted oscillator signal, modulators 88c and 88d each receive an inverted oscillator signal via inverter 78. This means that the two pairs of valves are alternatively actuated in a pulsed manner, instead of simultaneously actuating in a pulsed manner. This also reduces the peak demand with regard to the power supply (not shown).

The system operates to produce a differential pressure drop across valves 20a through 20d that is inversely proportional to the magnitude of the winding current Ic. By controlling the pressure drops via the valves 20a to 20d, the pressure medium flow between the chambers 11 and 13 is controlled so as to retract or extend the cylinder as desired. When the control converter 28 is actuated to extend the cylinder, a positive, non-inverted error signal E is generated. If E is positive, the reverse error signal -E is negative and no current is generated in the windings 21a and 21c, so that the valves 20a and 20c remain closed. This positive signal E generates in the circuits 80b and 80d winding currents for the windings 21b and 21d, so that the valves 20b and Open 20d to create a proportional pressure differential across the piston of cylinder 10 and cause cylinder 10 to extend to a new position that corresponds to desired position signal C generated by control transducer 28. Conversely, if the transducer 28 requires the cylinder to retract, the reverse error signal -E becomes positive while the non-reverse error signal becomes negative. This leads to the opening of the valves 20a and 20c and the closing of the valves 20b and 20d. The cylinder 10 thus retracts as desired. The speed information returned by the differentiator 34 enhances the overall stability of the control system.

Claims (13)

1. Arrangement for controlling a double-acting hydraulic cylinder with an extension and a retraction chamber, which are connected via a valve device to a hydraulic pressure source and with a sump, and with a control circuit controlling the valve device, characterized in that the valve device can be operated four independently Valves (20a-20d) for controlling a flow connection between each of the two chambers (11, 13) of the cylinder (10) and pressure source (14) and sump (16), that an arbitrarily actuable control device (28) for generating a position Target signal and a sensor device (18) for generating a position-actual signal is provided, and that the control circuit controls the four valves in dependence on an error signal generated from the target and actual signals. 2. Arrangement according to claim 1, characterized in that the four independently operable valves are electrically controlled pressure reducing valves, of which a first (20a) is connected on the inlet side to the pressure source (14) and on the outlet side to the draw-in chamber (11); a second one (20b) is connected on the inlet side to the draw-in chamber (11) and on the outlet side to the sump (16); a third (20c) on the inlet side to the extension chamber (13) and on the outlet side to the sump (16) and a fourth (20d) on the inlet side to the pressure source (14) and on the outlet side to the extension chamber (13). 3. Arrangement according to claim 2, characterized in that each electrically controlled pressure reducing valve consists of a pilot-operated, proportionally operating ring valve (20a, 2Qd) and an electromagnetically actuated pilot valve (21a, 21d). 4. Arrangement according to claim 2 or 3, characterized in that a first check valve (22) for preventing a pressure medium flow from the drawing chamber (11) to the first pressure reducing valve (20a) and a second check valve (24) for preventing pressure medium flow from the extension chamber ( 13) to the fourth pressure reducing valve (20d) are provided. 5. Arrangement according to one of claims 1 to 4, characterized in that a differentiating circuit (34) for converting the actual position signal into a signal representing the speed of movement of the cylinder (10) and for generating the error signal from target and actual signal, a difference circuit (38) are provided, whose outputs are connected to a _'further difference circuit (44) for generating a speed-compensated error signal which is a reverse converter (46) is supplied, that two first driving circuits (80a, 80c) selected for controlling two Pressure reducing valves (21a, 21c) are provided, on which the compensated error signal is present, and that two further driver circuits (80b, 80d) are provided for the two other pressure reducing valves (21b, 21d), on which the reverse compensated error signal is present. 6. Arrangement according to one of claims 1 to 4, characterized in that a difference-forming device (44) for generating a non-inverted error signal corresponding to the difference between desired and actual signals and an inverting circuit (46) are provided for reversing the error signal, and that two first driver circuits (80a, 80c) receive the non-inverted error signal and two further driver circuits (80b, 80d) receive the inverted error signal for controlling the first two and the second two pressure reducing valves, respectively. 7. Arrangement according to claim 5 or 6, characterized in that each driver circuit has a modulation circuit (88a to 88c) for converting the received error signal into a pulse width modulated driver signal, the duty cycle corresponds to the size of the received error signal. 8. Arrangement according to claim 7, characterized in that the modulation gyroscope (88a, 88d) are each designed such that one of the modulation circuits of the modulation circuits of the first two and the second two driver circuits (80a-80d) operates out of phase by 180 ° . 9. Arrangement according to one of claims 5 to 8, characterized in that a control-side control circuit (64) is provided for generating a variable response value reference signal, which together with the error signal and the reverse error signal a generator circuit (66) for a response value Setting signal is supplied, and that means (48,52) are provided to combine the compensated error signal with the response value on signal and to supply the combined signal to one of the first two driver circuits (80a, 80c), and that means (50, 54) are further provided to combine the reversed compensated error signal with the response value setting signal and to supply the second combined signal to one of the two second driver circuits (80b, 80d). 10. The arrangement according to one or more of claims 5 to 9, characterized in that a user-controllable device (58) is provided whose output signal can be combined by combination circuits (48-54) with the input signals for all four driver circuits (80a-80d) to open two selected pressure reducing valves (20b, 20c) between the chambers of the cylinder and the sump and to close the other two pressure reducing valves (20a, 20c) between the chambers of the cylinder and the pressure source. 11. The arrangement according to one or more of claims 5 to 10, characterized in that a device-controllable device (56) is provided for generating a switch-off signal, which can be combined via combination circuits (48-54) with the input signals of all driver circuits (80a-80d) to generate a signal closing all pressure reducing valves (20a-20d). 12. The arrangement according to one or more of claims 8 to 11, characterized in that an oscillator (72) is provided for generating a dither signal of a predetermined frequency, the output of which is connected to an inverting converter (74) for generating an inverted dither signal which 180 ° out of phase with the dither signal is that combination circuits (84b, 84d) are provided for combining the dither signal with the signal and combination circuits (84a, 84c) controlling the second and fourth pressure reducing valves (20b, 20d), to combine the signals controlling the first and third pressure reducing valves (20a, 20c) with the reverse dither signal. 13. Arrangement according to one or more of claims 9 to 12, characterized in that the response value generator circuit has two comparators (60,62) with bistable output, one of whose inputs each connected to the generator (64) for the response value reference signal , and one (60) of which has not yet received the reverse error signal and the output of the other comparator (62) and the latter receives the reverse error signal, and that means are provided for integrating the output of the first comparator (60).

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