DE102012009729A1 - Pressure-flow regulator, adjustment unit for an adjustable hydraulic displacement machine with a pressure-flow regulator and method for controlling such an adjustment - Google Patents

Abstract

Disclosed is an adjusting unit for an adjustable hydraulic displacement machine with a pressure-flow regulator. The adjustment has at least one actuating cylinder for adjusting a displacement of the positive displacement machine. The actuating cylinder is controlled via switching valves.

Description

The invention is based on a pressure-flow regulator for adjusting an adjustment of an adjustable hydraulic displacement machine. Furthermore, the invention is based on an adjusting unit for such a displacement machine with such a pressure-flow regulator and on a method for controlling the adjusting unit.

In the DE 10 2010 048 070 A1 is disclosed an adjustable positive displacement machine in the form of a hydraulic pump. Their swing angle can be changed via two actuating cylinders. For adjusting the adjusting cylinder and thus for adjusting the pivot angle of the displacement machine, a pressure-flow regulator is provided. This has two proportional valves, via which a cylinder space of the adjusting cylinder can be connected to a pressure line connected to the displacement machine or to a tank line. The proportional valves are hydraulically actuated.

A disadvantage of such a pressure-flow regulator is that the proportional valves are relatively expensive and have a low robustness. Furthermore, a leakage of the proportional valves is comparatively high, which leads to a loss of efficiency of the positive displacement machine.

In contrast, the invention has the object to provide a pressure-flow regulator for adjusting an adjustment, which eliminates the disadvantages mentioned above. Another object of the invention is to provide an adjusting unit for an adjustable hydraulic displacement machine with such a pressure-flow regulator, which also eliminates the disadvantages mentioned. In addition, it is an object of the invention to provide a method for controlling such an adjustment, with the extremely simple way a displacement machine can be controlled.

The object with regard to the pressure-flow regulator is achieved according to the features of patent claim 1, with regard to the adjusting unit according to the features of patent claim 6 and with regard to the method according to the features of patent claim 12.

Other advantageous developments of the invention are the subject of further subclaims.

According to the invention, a pressure-conveying flow regulator for adjusting an adjusting unit of an adjustable hydraulic displacement machine has a switching valve for actuating the adjusting unit.

This solution has the advantage that the pressure-flow regulator is relatively inexpensive due to the switching valve, since a switching valve has comparatively low production costs and is simple in terms of device technology. A leakage of the pressure-flow regulator is also relatively low due to the switching valve. Furthermore, switching valves have a very short positioning time, which is why the pressure-flow regulator also has a very short positioning time. Due to the robust switching valve, the pressure flow regulator has a high shock resistance. Due to a low susceptibility of the switching valves against soiled hydraulic fluid maintenance of the pressure-flow regulator is low. A change in the switching behavior of the switching valves due to temperature changes is also very low compared to proportional valves, whereby the pressure-flow regulator can be used safely at different temperatures. Switch valves are also very robust against electromagnetic interference. Further, it is possible to control the switching valves of the pressure-flow regulator directly via a digital control unit or a mobile controller, in contrast to the prior art, where a digital / analog converter is necessary to control the proportional valves.

The switching valves are, for example, 2/2-way valves, via which a pressure fluid connection is up and is zuusteuerbar. The switching valve may be configured as a slide valve or preferably as a virtually leak-free seat valve. Preferably, a valve body of the switching valves is acted upon in the direction of a closed position with a spring force of a valve spring and can be acted upon via an electromagnetic actuator in the direction of an opening position with an opening force.

The at least one switching valve can be used flexibly in pulse width modulated mode (PWM) or in ballistic mode (BaM). Thus, different opening cross-sections can be provided in the switching valve.

In a further embodiment of the invention, at least one switching valve for controlling a Connection between the adjustment and a pressure medium source and at least one switching valve for controlling a connection between the adjustment and a pressure medium sink provided. In this way, for example, a setting cylinder of the adjustment can be adjusted in a simple manner.

The switching valves for controlling a pressure medium connection between the pressure medium source and the adjusting unit or the pressure medium sink and the adjusting unit can be controlled in the meter-in / meter-out mode (MIO).

In order to control a connection between the adjustment unit and the pressure medium source or the pressure medium sink pulse code modulated (PCM), at least two switching valves are provided in a pressure medium flow path between the adjustment unit and the pressure medium source and / or between the adjustment unit and the pressure medium sink. The switching valves of a pressure medium flow path in this case preferably have different nominal sizes in order to use them precisely in pulse-code-modulated mode. Alternatively, the larger switching valve can be used, for example, for coarse adjustment of the adjusting cylinder of the adjustment and the smaller switching valve for fine adjustment of the actuating cylinder.

According to the invention, an adjusting unit for an adjustable hydraulic displacement machine has a pressure-flow regulator according to the invention. The adjusting unit in this case has at least one actuating cylinder for adjusting a displacement of the positive displacement machine, in particular for setting a pivot angle. The actuating cylinder has a piston space limited by a cylinder, which is displaceable for displacing the piston in the direction of increasing cylinder space via the switching valve of the pressure-flow regulator with a tapped by a pressure line of the positive displacement pressure medium and the discharge of pressure medium from the cylinder chamber another switching valve with the pressure medium sink is connectable. For example, in a displacement of the piston of the actuating cylinder in the direction of the increasing cylinder space, a displacement of the positive displacement machine is increased and reduced when moving in the reverse direction.

Advantageously, the piston of the adjusting cylinder is acted upon by a spring with a spring force in the direction of the decreasing cylinder space. It may be provided a second actuating cylinder, which has a piston defining a cylinder space. The cylinder chamber can in this case be connected to the pressure line of the displacement machine. The piston then counteracts, for example, the piston of the first actuating cylinder. The displacement machine can be an axial piston machine of swash plate construction, wherein the pistons of the actuating cylinder then engage a swash plate for controlling the displacement of the displacement machine.

To control the displacement machine, for example, an electronic control unit is simply provided, which controls the switching valves as a function of a pressure, in particular in the pressure line of the positive displacement machine, and / or as a function of a displacement path of the piston of the actuating cylinder or a swivel angle of the swash plate. For measuring the pressure, a pressure sensor is preferably provided, which taps this in the pressure line of the displacement machine. For measuring the displacement path, a displacement transducer can be used which measures a displacement path of the piston of the actuating cylinder.

So that the adjusting unit can react in a simple manner to sudden pressure peaks in the pressure line, an overflow valve can be provided, which can be arranged fluidically parallel to at least one switching valve arranged between the pressure line and the cylinder space of the adjusting cylinder. The overflow valve is, for example, a non-return valve which opens in the direction of flow toward the cylinder chamber.

To control the adjustment by the control unit, the switching valves are easily controlled, for example, with a three-step control. In this case, the switching valves can be regulated in a pressure-dependent manner, in particular as a function of a pressure in the pressure line or of a load pressure. It is also conceivable to regulate the actuating cylinder path-dependent from the displacement of the piston.

In order to avoid overheating of a switching valve, a switching cycle of the switching valve may have a predetermined ratio of a turn-on to a turn-off duration. D. h., The switching valve is energized with a certain period of time and is energized with an adjoining period of time.

To easily control multiple switching valves, a switching frequency of these is the same.

A sampling of the pressure and / or the displacement with a certain sampling frequency can be approximately synchronous with a control of the switching valves with a certain switching frequency, resulting in a extremely simple regulation leads. Preferably, the sampling frequency is a multiple of the switching frequency or corresponds to the switching frequency. Due to the switching frequency, a control pulse is delivered to the switching valves only at predetermined time intervals.

It is also conceivable that the sampling frequency and the switching frequency do not run synchronously, which makes it possible that the switching frequency is triggered only when needed, while a sampling of the pressure and / or the displacement takes place with a certain sampling frequency.

Preferably, the sampling frequency is greater than the switching frequency, which means that during the energization of the switching valve, the pressure and / or the displacement is further scanned and, if necessary, a duty cycle of the energized valve can be interrupted. For this purpose, it is advantageous if the duty cycle of the switching valves is variable during a switching cycle. The switching valve can close prematurely, if, for example, the measured pressure requires this.

In a further embodiment of the invention, the switching cycles of the switching valves are independent of each other, whereby the switching valves are thus independently controllable and thus can switch independently.

• – Abtasten des Verschiebewegs des Kolbens des Stellzylinder und/oder des Drucks, insbesondere mit einer bestimmten Abtastfrequenz;
• – Steuern der Schaltventile in Abhängigkeit des Verschiebewegs und/oder des Drucks, wobei die Steuerung insbesondere entsprechend der Dreipunktregelung erfolgt.

An extremely simple method according to the invention for controlling an adjusting unit according to the invention has the following steps:

• - sensing the displacement of the piston of the actuating cylinder and / or the pressure, in particular with a certain sampling frequency;
• - Control of the switching valves in response to the displacement and / or pressure, the control is carried out in particular according to the three-step control.

In the following, preferred embodiments of the invention are explained in more detail with reference to schematic drawings. Show it:

1 a hydraulic circuit diagram of a positive displacement machine with an adjusting unit according to the invention according to a first embodiment;

2 in a schematic representation of a three-step control of the adjustment;

3 a block diagram of a controller structure of the adjusting unit according to the invention;

4 a regulated position profile of a piston of an actuating cylinder of the adjusting unit according to the invention;

5 a block diagram of a controller structure of the adjusting unit according to the invention;

6 to 9 a regulated pressure profile of the displacement machine;

10 schematically a functional principle of a digital pressure control;

11 schematically a current waveform of an output stage of a control unit;

12 schematically switching cycles of a switching valve;

13 a block diagram of a controller structure of the adjusting unit according to the invention and

14 a hydraulic circuit diagram of a positive displacement machine with an adjusting unit according to the invention according to a second embodiment.

According to 1 is a hydraulic circuit diagram with an adjustable positive displacement machine 1 shown. This can be an axial or radial piston machine, which can be used as a hydraulic pump or hydraulic motor.

In the following, the invention will be explained with reference to a pivotable axial piston machine with a swashplate engine. For pivoting the swash plate of the displacement machine, not shown 1 is a positioning cylinder 2 intended. This one has a piston 4 limited cylinder space 6 , The piston is via a one-sided piston rod 8th with the swash plate of the displacement machine, not shown 1 connected. A displacement of the piston 4 in the direction of an increasing cylinder space 6 causes the swash plate in the direction of increasing the displacement of the displacement machine 1 is pivoted. In contrast, a displacement of the piston leads 4 in the direction of the decreasing cylinder space 6 to a pivoting of the swash plate in the direction of a decreasing displacement of the positive displacement machine 1 , About a spring force of a return spring 10 in one of the piston rod 8th interspersed annulus 12 of the adjusting cylinder 2 is arranged, is the piston 4 in the direction of the decreasing cylinder space 6 acted upon by a spring force.

To the positive displacement machine 1 is to a pressure port P a pressure line 14 and to a tank connection T a tank line 16 connected. The pressure line 14 is connected to a hydraulic consumer, not shown. The tank line 16 flows into a tank 18 , The cylinder space 6 is with the tank line 14 via a first and second switching valve 20 respectively. 22 connectable. The switching valves 20 and 22 are arranged fluidly parallel and each control a pressure medium connection between the cylinder chamber 6 and the pressure line 14 , That in the 1 upper switching valve 20 here has a larger size than the lower switching valve 22 on. Fluidically seen between the smaller switching valve 22 and the cylinder space 6 is a throttle 24 intended. Fluidic parallel to the switching valves 20 and 22 is an overflow valve 26 between the pressure line 14 and the cylinder space 6 arranged. This is one in a pressure medium flow direction towards the cylinder chamber 6 opening check valve. Between the cylinder room 6 and the tank 18 is another switching valve 28 provided, with which a pressure medium connection between the cylinder space 6 and the tank 18 is on and zuusteuerbar.

The switching valves 20 . 22 and 28 are designed as 2/2-way valves. A valve body of the switching valves 20 . 22 and 28 Here is a valve spring 30 acted upon in the direction of a closed position with a spring force and with a force against the spring force in the direction of an opening position via an electromagnetic actuator 32 acted upon. The switching valves 20 . 22 and 28 are thus closed in de-energized state. The switching valve has a flow direction and has an opening cross-section of, for example, 3.34 mm ² , for example, at a valve lift of 0.2 mm. In the case of a pressure difference of 90 bar applied to the switching valve, for example, a flow rate of 20 l / min results. The pull-in and fall times amount to, for example, about 0.3 ms. The relative duty cycle (ratio of energized state to de-energized phase) is set to a maximum of 20%, otherwise the overheating of a solenoid coil of the actuator threatens. The switching valve may be designed, for example, to about 10 ⁹ switching cycles.

For measuring a displacement of the piston 4 or the piston rod 8th of the adjusting cylinder 2 is a transducer 34 intended. This measures the displacement with a certain sampling frequency and reports this to an electronic control unit, not shown. Furthermore, there is a pressure sensor 36 provided with the pressure in the pressure line 14 is measured and also reported to the electronic control unit. The measurement of the pressure takes place here also with a certain sampling frequency.

So that the switching valves 20 . 22 . 28 can be switched quickly, the electronic control unit is preferably designed specifically. This is, for example, an output stage electronics in which each electronic card in the European format, two programmable electronic components are located, which can generate a special current waveform. In particular, four switching valves can be controlled via a respective electronic component, which is, for example, the type CJ840. For example, the CJ840 devices can be programmed in an application-oriented manner via an SPI bus. The output variables are in particular a maximum voltage of 65 volts and current peaks of up to 12 amps possible. The control is usually done via a microcontroller, which is why working with voltage levels based on TTL.

The basic control concept for a digital pressure control of the positive displacement machine 1 can be described as a control with three-point behavior, see 2 , The upper (switch-on) point (P2) in 2 marks the limit for activation of the large switching valve 20 , The lower point (P1) causes the activation of the output side switching valve 28 , Between the switching points is the so-called dead zone of the scheme. The dead zone can also be seen as a tolerance range for the control deviation, since the control tolerates deviations within this range. In this dead zone, the control pauses until the controlled variable exceeds a switching point due to interference, etc. again. With an additional small switching valve 22 for fine metering, this three-point control will be extended. The initial idea is that the small switching valve 22 for fine metering through the large switching valve 20 to mitigate caused overdoses with combined switching. The switching valve 20 , responsible for the coarse dosage (and thus for the fast filling of the cylinder space 6 ), should be deactivated earlier and the balance of the remaining control deviation takes over the small switching valve 22 , This remains activated until the actual switch-on limit (P2), while the switching point for the large switching valve 22 (P3) is higher. On the whole, it has the switching valves 20 . 22 and 28 controlled adjustment, however, a three-point behavior. The three-step control therefore does not act with a single actuator, which executes the input, off and hold phases. Rather, the adjustment system as a whole, consisting of three actuators and the adjustment of the displacement machine 1 , this behavior on.

For controlling the adjusting unit with the adjusting cylinder 2 and the switching valves 20 . 22 and 28 a wide variety of control methods are provided, which are explained below.

In a first regulatory procedure is according to 3 and 4 the displacement machine 1 out 1 depending on the displacement of the piston 4 controlled, with the displacement of the transducer 34 is detected.

According to 3 the three-position controller controls actuators in the form of the switching valves 20 . 22 and 28 , please refer 1 , with a control size u. This in turn leads to a manipulated variable y for the actuating piston or piston 4 of the adjusting cylinder 2 , This then results in the displacement of the piston 4 as a "controlled variable way s", from which together with a "reference variable way s _soll " a control deviation e is determined for the three-point controller. Depending on the controlled variable path s is the pivoting cradle or swash plate, not shown, the positive displacement machine 1 out 1 triggered, resulting in a swivel angle α for the engine of the positive displacement machine 1 leads. This in turn results in a certain volume flow Q, which is led to the consumer, which can be reacted to a pressure p of the consumer.

Thus changes in proportion to the change in position of the piston 4 the swivel angle, resulting in a modified flow. Depending on the consumer then results in the pressure. The piston 4 should within its adjustment range (for example a maximum of 20 mm travel) via the filling of the cylinder chamber 6 by means of switching valves 20 . 22 . 28 be brought into a predetermined position.

The 4 shows how the piston position follows the setpoint as a controlled variable. In this case, a sawtooth curve of the controlled variable can be seen. This course results from the sudden filling via the switching valves 20 . 22 (Rising flanks) and the emptying through the switching valve 28 (falling flanks) or the leakage over the piston 4 (slightly sloping course). Because the switching valves 20 . 22 . 28 With fixed set duty cycle of 20% (maximum possible duty cycle) are switched, resulting from overdoses resulting overshoot in the course. To reduce these overshoots and to prevent leakage through the piston 4 compensate, should the small switching valve 22 , as a throttled inlet valve for the fine dosing, are used. The sampling of the control variable is made only at the beginning of the switching cycle (for example, with a switching frequency of 10 Hz). Therefore, in 4 correspondingly detect late reactions of the controlled variable to changes in the reference variable. All switching valves 20 . 22 . 28 are driven with the same timing and duty cycle. That means that no matter which switching valve 20 . 22 . 28 all have subsequently followed the pause phase of 80% relative to the switching period, even if now a switching valve 20 . 22 . 28 would have to react in a way that did not work before. This need not be a major drawback, as a common switching of switching valves 20 respectively. 22 and 28 usually not necessary. As already stated above, the switching frequency of the switching valves 20 . 22 and 28 10 hertz. A sampling frequency of the transducer 34 on 1 is also 10 hertz.

In the case of setpoint jumps, partial overshoots resulting from overdoses result, see reference numeral 1 in 4 , caused by the rigidly determined switch-on durations. Due to the comparatively low sampling and switching frequency, delayed reactions of the controlled variable occur, see reference signs 2 in 4 , The advantage of this low switching frequency is the resulting low switching frequency. Due to the discrete dosing steps, only as much volume is admitted into the control chamber as necessary. This results in the great advantage of being able to save the losses of the control oil to a large extent.

With the help of the position control of the piston 4 or the derived therefrom swivel weighing control can primarily the flow rate of the displacer unit 1 be regulated, since each piston position or each swivel angle a corresponding flow can be assigned. However, the pressure builds up differently depending on the load.

According to 5 is in a further control method instead of the displacement of the piston 4 the pressure in the pressure line 14 out 1 as a controlled variable from the pressure sensor 36 detected.

In displacement machines with a hydraulic pressure regulator this main task is to adjust the pressure to a specified set point. It is therefore not a behavior-oriented regulation, but a regulation oriented to a disturbance behavior. With a digital pressure regulator, a management-oriented control is possible.

In 6 the control method with controlled variables is considered without additional interference influences (eg by pulsation etc.). The setpoint is set to 100 bar, for example. The switching frequency of the switching valves 20 . 22 . 28 has, for example, 10 Hz. Depending on the state of the controlled variable is at the beginning of the switching cycle that switching valve 20 . 22 . 28 which is necessary to bring the controlled variable back into line with the reference variable. In addition, for all switching valves 20 . 22 . 28 the same timing and switching cycle generation. If the pressure increases faster than the control can react to it, the overflow valve offers 36 out 1 a protection against too high pressure peaks. The overflow valve 36 works like a safety valve. It causes an addition to the switching valve 20 . 22 taking place filling of the control chamber when the pressure has exceeded a certain limit. To avoid overdosing, the switch-on times of the switching valves are to be avoided 20 . 22 and 28 variably adjustable. That is, the switching valves 20 . 22 . 28 can switch off earlier if necessary, before the maximum switch-on time has been reached. This is done in that after the necessary switching of a switching valve 20 . 22 . 28 the control variable is interrogated in the sampling cycle and, if necessary (depending on the result of the interrogation), the switching valve is switched off again. The sampling frequency is preferably 1 kHz. It will continue to be the small switching valve 22 used for fine dosing. However, this is thanks to the variably adjustable duty cycle of the large switching valve 20 only necessary for the intervalwise subsequent dosing of the leak. The limits of the controller dead zone (switching points), see 2 , for example, are set to 5 bar above and 10 bar below the setpoint.

6 shows how after an initial pressure rise, see reference numeral 1 , by load increase the spill valve 26 secures the system against a pressure increase of more than 25% above the setpoint (100 bar). See reference numeral 2 , In addition to actual and setpoint is in 6 nor the pressure curve drawn, which would result without control. Only at the second switching cycle start after 100 ms can the controller react for the first time. The subsequent increase in pressure, see reference numerals 3 is the result of leakage over the piston 4 , which is the slow emptying of the cylinder space 4 entails. At the next beginning of the switching cycle, the leakage is the controlled variable above the upper switching point of the large switching valve 20 , see reference number 4 , climbed. The switching of the switching valve 20 causes the pressure to drop. At 300 ms, the controlled variable is still within the dead band of the controller. Only the additional dosing of the leak at each start of the switching cycles via the small switching valve 22 causes an adaptation of the pressure curve to the reference variable, see reference numeral 5 , When reducing the load and thus decreasing pressure, see reference number 6 , the regulation can react only with the delay of a switching cycle, see reference numeral 7 , The second load increase (noticeable by the increasing uncontrolled pressure) causes further deviations of the controlled variable, up to the pressure limit of the overflow valve 26 , see reference number 8th ,

The variable duty cycle contributes significantly to increase the responsiveness and accuracy of the pressure curve. In this case, in terms of the function for reducing overdoses even on the small switching valve 22 be waived. For the fine dosage, however, it is still used. The aspect of loss reduction in tax oil remains positive.

Below we will use the 7 a control method of pressure control of the displacer unit 1 described with a "nested switching cycle". To enable even faster reactions of the controlled variable, the following does not use a rigid switching cycle. If a new switching cycle begins and at the beginning no switching is required, not another switching cycle is completely paused, but from this time in the sampling clock of, for example, 1 kHz for a necessary switching request asked. If then, after a few polling times, a switching of the switching valves 20 . 22 . 28 is necessary, then the normal switching cycle is traversed, with the required duty cycle and the necessary pause ratio.

In 7 can be seen that, for example, a few milliseconds before the time 500 ms, a reaction of the switching valve 20 takes place, see reference number 1 in 7 because no switch was necessary after the last pause time. This reaction would have occurred with the previous control concept only at the time 500 ms. Thus, there is another measure for containing the leakage caused by the drift in the pressure curve.

By eliminating the rigid switching cycle, the reaction of the controlled variable is further increased. If in this case the small switching valve 22 is additionally used, the control process can be slightly further improved.

Based 8th is a control method of pressure control with a separate valve control of the switching valves 20 . 22 and 28 explained.

Considering the pure filling and emptying of the cylinder space 6 , is a common switching of switching valves 20 respectively. 22 and 28 usually not necessary and thus a common control not detrimental. However, a combined switching of the switching valves brings 20 . 22 . 28 in terms of targeted dosage and in terms of better response to fast multiple load changes an advantage.

By separating the control with respect to sampling rate and switching cycle generation, the switching valves 20 . 22 . 28 now switch independently and even simultaneously. Opening the switching valve 28 does not have an equal volume flow from the cylinder chamber 6 As a result, the volume flow in the cylinder chamber 6 , which is at the same time opening the switching valve 20 results. At the switching valve 28 there is a lower pressure difference (cylinder chamber pressure to tank pressure) than at the switching valve 20 (High pressure to cylinder space pressure). The lower pressure difference results in the smaller volume flow. By combined switching, a finely dosing filling is now possible. The pressure gradient in 8th shows how too large pressure drops (due to overdose of the switching valve 20 ) with immediately following reactions of the switching valve 28 be partially balanced, see reference numerals 1 in 8th , Fast pressure increases due to load increase still have to be provided by the overflow valve 26 be caught, see reference signs 2 in 8th ,

The combined switching increases the accuracy of the control curve and thus also makes the need for a switching valve used for fine dosing 22 superfluous.

By increasing the switching frequency, an even better responsiveness can be made possible.

In 9 a control method of a pressure control with a high switching frequency is explained. Optimizing the reactivity in the switching behavior can still not prevent rapid pressure increases, since large switching cycles worsen a fast reaction of the control. As the switching frequency increases, there are more opportunities to switch the switching valves within a certain period of time 20 . 22 . 28 and thus a greater responsiveness of the control to disturbances.

9 shows that the load increases, previously from the spill valve 26 can now be largely compensated before the hedge limit is reached, see reference numerals 1 in 9 , Where previously showed even larger pressure fluctuations in the course of time, hardly larger deviations can be seen, see reference numerals 2 in 9 ,

By increasing the switching frequency, for example to 100 Hz, fast pressure increases can be compensated in time, so that the protection by the overflow valve 36 less or not at all. The significantly increased responsiveness also makes the control process noticeably more accurate. Disturbances are corrected faster and the pressure curve adapts significantly better to the reference variable. The only disadvantage here is the increase of the maximum switching frequency. The total number of switching cycles is limited and the increased switching can shorten the life of the switching valves 20 . 22 . 28 result. However, the switching frequency is also dependent on the dynamics of the controlled variable ie dependent on the load cycles that the control has to cope with. Load changes result in interference in the controlled variable. Another influence on the switching behavior is the leakage via the actuating piston. The greater the failure due to design, the more must be readjusted accordingly. With this control concept, a relatively good course of the controlled variable can now be generated.

The main task of the scheme is to control the deviation with the specified switching points of the switching valves 20 . 22 . 28 to compare. If the control deviation exceeds the value of a switching point, the following request will be made to open the switching valve 20 . 22 . 28 , whose switching point has been exceeded, forwarded. In the "level control" subsystem, the query result of the comparison result is made in the sampling frequency. If, at a polling time, the control is a reaction of the switching valve 20 . 22 . 28 requires, then a switching pulse is started. The switching pulse is logically coupled to a switch-on signal. This is started at the beginning of each switching cycle and lasts a maximum of 20% of the switching period. This ensures that no duty cycle exceeds the prescribed duty cycle, even if then the switching valve 20 . 22 . 28 should remain open according to the system deviation. The switching pulse is in turn fed back coupled with the sampling pulses, so that during the switch-on a query of the comparison result in the sampling clock is possible. As a result, a previous shutdown is possible if the control deviation was compensated before the end of the 20% duty cycle.

The activation of the switching valve 28 allows the option of a circuit breaker after the level control. In certain cases, the possibility must be taken into account that the controlled variable can not reach the reference value over a longer period of time. This is the case, for example, if, despite the swung-out state of the displacement machine 1 (maximum swivel angle) the delivery rate is insufficient to build up the pressure setpoint. So now the switching valve 28 not constantly asked to switch, although the cylinder chamber 6 is already emptied, the switch fuse helps unnecessary switching of the switching valve 28 to avoid. Simply put, the fuse counts with a positive comparison result (switching valve 20 must switch) the switching cycles. If the control deviation does not change despite a sufficiently large number of switching operations, the fuse assumes that the cylinder chamber 6 already emptied and further switching operations for the time being would not produce any changes. Another switching of the switching valve 28 is suppressed until the control deviation is again within the dead band of the controller.

In 10 the basic principle of operation is graphically clarified again. When the controlled variable (actual state) goes beyond the limit of the dead zone of the control marked by the switching points, the switching valves become the same according to the prescribed switching characteristics 20 . 22 . 28 activated in order to adapt the controlled variable again to the reference variable (setpoint).

The following is based on 11 a configuration of the valve control explained. 11 shows the characteristics of the current waveform that the output stage generates to allow the valves to rapidly switch. The programming (setting of bits in registers of the CJ840 electronic module) is performed manually, for example, via a computer. Different bit combinations in five registers stand for different properties, which are changeable. After switching on the power supply, the registers are set to a default setting. The current flow in 11 shows the current values resulting from the initial setting.

The switch-on behavior of the switching valves 20 . 22 . 28 is essentially divided into five phases. The control signal of the control gives the command to open the switching valve 20 . 22 . 28 to the power amplifier on. To the switching valve 20 . 22 . 28 To open, a large current is first required, which is achieved via a voltage of 65 volts maximum. The current of this so-called boost phase is adjustable as well as the time frame available to the boost. The greater this power boost, the faster the magnetic field in a solenoid of the actuator 32 out 1 build up and set the valve armature in motion. Also programmable is the subsequent "pick-up" phase, which occupies a certain current value via pulse width modulated voltage until the switching valve 20 . 22 . 28 guaranteed to have opened. Thereafter, a rapid quenching of the current takes place via a high negative voltage and persists at a likewise regulated via PWM holding current. The phase of the holding current control is used to maintain the open state and is also terminated by fast deletion when requested to close. Again, timeframes and current values are adjustable.

In the use of the positive displacement machine 1 Volume flow pulsations can occur, from which in turn a pulsation of the pressure can take place. The controlled variable has a signal-noise due to the pulsation. For the most accurate and efficient control but the scanning of the most accurate control variable is necessary. For smoothing the sample size, there are various possibilities, which are explained below.

It is conceivable to smooth the sample size with a hardware filter, such as the Bessel low-pass filter.

It is also possible to smooth the sample size by means of a software filter. The application of digitally programmed filtering methods offers various possibilities. Software filters, which calculate a smoothed signal by the moving average, achieve shorter runtimes than hardware-based filters. However, only sufficient signal smoothing is obtained, the more measured values are processed in the averaging. Since the digital pressure control works with a sampling frequency of 1 kHz, this would have one Delay of several milliseconds result. Such a time delay would make the control unstable. The instability would result from overdosage of the switching valves 20 . 22 . 28 , These would not switch on time because of the time delays caused by the filtering.

The best smoothing result is achieved if the sampling of the values to be averaged is just the same as the Nyquist-Shannon sampling theorem (formula 1), which in turn results in an excessive time delay of the signal. f _scan > 2 · f _max Formula 1

Although a higher sampling frequency allows the desired lower time delay, but this is the smoothing less and less efficient, since then again the high-frequency oscillations result from the average of the larger number of sampling points.

When smoothing with a low-pass filter, the filtering of the signal can be implemented by means of a simple PT1 element. The corner frequency of the transmission f _E is changed over the time constant T _z .

The noticeable in the transmission behavior of the PT1 element phase shift (= signal delay) is reduced with increasing corner frequency for relatively small frequencies to the microsecond range. However, it should be noted that the corner frequency may not be much greater than the pulsation frequency, otherwise the filter would be ineffective.

Another option for low-pass filtering is the compressibility and inertia of the hydraulic medium. The approach thus provides a smoothing or damping of the pulsation signal, which takes place before the measuring element in the transmission medium. Depending on the volume between the pressure pick-up and the engine (generator of the pulsation) and in general on the total system volume, the pulsation arrives correspondingly damped at the sensor.

Another option for smoothing is provided by a numerical pulsation filter. The relevant disturbance factor in the controlled variable represents the pulsation. It is mainly dependent on the speed and the number of engine pistons. Therefore, the pulsation can be approximately mathematically described by a sine wave. As a result, the approach results from the pressure signal determined by the pressure transducer to effectively eliminate the pulsation. In actual cases, however, the pulsation does not represent an ideal sine, but a superimposition of several sine signals with different frequencies and amplitudes. Therefore, when filtering with a simplified sine, only the largest disturbances can be filtered out. For more effective smoothing, all other sine signals in the filtering would have to be calculated with, as well as other influences. Among other things, influences on the characteristic of the pulsation result from the load behavior, which, however, is difficult to predict.

Another way to optimize the control is to change the switching frequency, see 12 , A greater reactivity of the control should be able to bring a higher switching frequency. However, the switching frequency can not be increased arbitrarily. The dynamics of the switching valves dictate the limits of the maximum possible switching frequency. This means that the maximum possible duty cycle within the switching period must not be less than the switching behavior of the switching valve 20 . 22 . 28 it allows. As in 12 can be seen, the duty cycle is at the switching frequency of 200 Hz at one millisecond. At best, since the valve alone requires 0.6 ms for opening and closing, only 0.7 ms would be available for the open state and for the maximum on time.

In the end, at this switching frequency, the reaction of the control is no longer dependent on a variable duty cycle, but on the sum of the switching operations with constant duty cycles of one millisecond. The only way to influence the duty cycle is to increase the sampling rate by two times. As a result, opening times are possible with half of the maximum duty cycle. Since the opening and closing times of the valve remain fixed, this results in a reduction of the dosing process by 0.2 ms. Therefore, the switching frequency of 200 Hz can be considered as the upper limit become. The realization of larger switching frequencies would not be worthwhile because the switching dynamics of the solenoid valves could no longer meet the switching requirements.

Another way to increase the reactivity could be the extension of the control by an internal feedback with adjustable time response. Like the control loop in 13 shows, this is realized via an internal feedback with a PT1 element.

Depending on the parameter setting of the PT1 transfer function, the control deviation is modified accordingly, so that the switching points of the three-point controller are reached earlier. Due to the returning correction, the control deviation becomes, as it were, an anticipatory character. As a result, the control causes the manipulated variable change earlier and can thus more quickly counteract a controlled variable deviating from the setpoint. The magnitude of the correction step results from the feedback quantities, the gain K _r and the time constant T _r and is proportional to the size of the input jump. The P component of the controller results from the manipulated variable change resulting from the correction step. Due to its integrative character in the control loop, the adjustment is part of the controller function (I component).

As an approximation, the following transfer function results for a three-position controller with a downstream integral positioning system:

The transfer function shows that this controller has a PI behavior. The factor in front of the bracket from formula 3 can be equated with the gain K of the transfer function of the PI controller, as well as the time constants T _r and T _N :

The greater the gain of the feedback Kr (or the smaller Tr), the greater becomes the advance (correction step) of the control deviation, which results from the internal feedback. The overall gain of the quasi-PI controller becomes smaller. Due to the anticipatory character of the control deviation modified by the feedback, the switching points of the control are reached earlier.

With regard to the different volume flow via inlet and outlet valve 20 . 28 (caused by different pressure drop) is a double, for each valve 20 . 28 separate recycling better suited. A common return would not do justice to the fact that via the exhaust valve 28 due to the lower pressure drop, a smaller volume flow is to be expected. Differently large volume flows mean different influences on the gain of the adjustment.

The inlet valve has, as it were, a greater reinforcing effect than the outlet valve due to the larger volume flow it can provide 28 , That's why should be for the exhaust valve 28 a lower advance can be made possible, since the risk of overdosage is lower due to the smaller volume flow amplification than the inlet valve 20 , One for each valve 20 . 28 own feedback allows individual configurations that can cope with the different volume flow amplifications.

One way to increase the dosing resolution can provide a higher sampling rate. However, as with the switching frequency, limits arise here, which are dictated by the dynamics of the solenoid valves. A sampling frequency of more than 2 kHz would not be worthwhile, since the valves 20 . 22 . 28 require at least 300 μs until they are fully open or closed. A sampling time window greater than the opening or closing time guarantees that the valve 20 . 22 28 can not be interrupted during the opening or closing process. So it can be better ensured that the valve 20 . 22 . 28 is not unnecessarily often energized in vain.

By Verzweifachung the sampling frequency Dosierauflösung can also be doubled. This means that finer doses can be made and thus overdoses can be better avoided. At the same time, a larger scan would eventually increase the responsiveness entail. The control of the relative duty cycle can be made in a larger resolution, ie it may optionally earlier with the shutdown of the valves 20 22 . 28 to be started, as with the usual 1 kHz.

According to 14 a hydraulic circuit diagram is shown, which is a positive displacement machine 38 shows that can be used as a hydraulic pump and motor. Next to the actuating cylinder 2 is another actuating cylinder 40 shown, a piston 42 having. From the piston 42 out extends a piston rod 44 , on a swash plate, not shown, of the displacement machine 38 attacks. The piston 42 limited with his from the piston rod 44 pioneering side of a cylinder room 46 that with the pressure line 14 the displacer machine 38 connected is. In the cylinder room 46 is a spring 48 arranged the piston 42 with a spring force in the direction of the increasing cylinder space 46 applied. The piston 42 of the adjusting cylinder 40 acts against the piston 4 of the adjusting cylinder 2 , A displacement of the piston 42 in the direction of the increasing cylinder space 46 thus leads to a displacement of the piston 4 in the direction of a decreasing cylinder space 6 and vice versa. Unlike the hydraulic circuit diagram 1 are according to 14 no second switching valve between the cylinder chamber 6 and the pressure line 14 and no overflow valve provided.

The switching valves 20 . 22 and 28 out 1 or the switching valves 20 and 28 out 14 can be controlled in Pulse Width Modulated mode (PWM), Meter In / Meter Out (MIO) or Ballistic Mode (BAM). According to 1 , at the two switching valves 20 and 22 at the pressure medium flow path between the pressure line 14 and the cylinder space 6 are provided, the switching valves can be controlled in pulse code Modulated mode (PCM). The control of switching valves in ballistic mode, for example, in the document DE 10 2009 052 285 A1 disclosed.

In particular, according to the invention, an adjusting unit for an adjustable hydraulic displacement machine with a pressure-flow regulator is disclosed. The adjustment has at least one actuating cylinder for adjusting a displacement of the positive displacement machine. The actuating cylinder is controlled via switching valves.

LIST OF REFERENCE NUMBERS

1

displacement

2

actuating cylinder

3

adjustment

4

piston

6

cylinder space

8th

piston rod

10

Return spring

12

annulus

14

pressure line

16

tank line

18

tank

20

first switching valve

22

second switching valve

24

throttle

26

overflow

28

switching valve

30

valve spring

32

actuator

34

transducer

36

pressure sensor

37

control unit

38

displacement

40

actuating cylinder

42

piston

44

piston rod

46

cylinder space

48

feather

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

• DE 102010048070 A1 [0002]
• DE 102009052285 A1 [0094]

Claims (12)

Pressure flow regulator for adjusting an adjusting unit ( 3 ) an adjustable hydraulic displacement machine ( 1 ), wherein a switching valve ( 20 . 22 . 28 ) for driving the adjusting unit ( 3 ) is provided. Pressure-conveying flow regulator according to claim 1, wherein the at least one switching valve ( 20 . 22 . 28 ) is used in Pulse Width Modulated mode (PWM) or in Ballistic mode (BaM). Pressure-conveying flow regulator according to claim 1 or 2, wherein the at least one switching valve ( 20 . 22 . 28 ) for controlling a connection between the adjusting unit and a pressure medium source ( 14 ) and at least one switching valve ( 20 . 22 . 28 ) for controlling a connection between the adjusting unit and a pressure medium sink ( 18 ) is provided. Pressure-conveying flow regulator according to claim 3, wherein the switching valves ( 20 . 22 . 28 ) are controlled in meter-in / meter-out (MIO). Pressure-flow regulator according to one of the preceding claims, wherein at least two switching valves ( 20 . 22 . 28 ) for controlling a connection between the adjusting unit and the pressure medium source ( 14 ) and / or two switching valves ( 20 . 22 . 28 ) for controlling a connection between the adjusting unit and a pressure medium sink ( 18 ) are provided. Adjustment unit for an adjustable hydraulic displacement machine ( 1 ) with a pressure-flow regulator according to one of the preceding claims with at least one actuating cylinder ( 2 ) for adjusting a displacement of the positive displacement machine ( 1 ), wherein the actuating cylinder ( 2 ) one of a piston ( 4 ) limited cylinder space ( 6 ) used to move the piston ( 4 ) in the direction of the increasing cylinder space ( 6 ) via the switching valve ( 20 . 22 ) of the pressure-flow regulator with one of a pressure line ( 14 ) of the positive displacement machine ( 1 ) tapped pressure medium and the for discharging pressure medium from the cylinder space ( 6 ) via another switching valve ( 28 ) with the pressure medium sink ( 18 ) is connectable. Adjusting unit according to claim 6, wherein an electronic control unit ( 37 ) is provided, which the switching valves ( 20 . 22 . 28 ) in dependence of a pressure, in particular in the pressure line ( 14 ), and / or a displacement path of the piston ( 4 ) of the adjusting cylinder ( 2 ) controls. Adjusting unit according to claim 7, wherein the control unit ( 37 ) the switching valves ( 20 . 22 . 28 ) with a three-step control. Adjusting unit according to one of claims 6 to 8, wherein a switching cycle of a switching valve ( 20 . 22 . 28 ) has a predetermined ratio of a turn-on to a turn-off. Adjusting unit according to one of claims 7 to 9, wherein a sampling of the pressure and / or the displacement with a certain sampling frequency and a control of the switching valves ( 20 . 22 . 28 ) takes place approximately synchronously with a certain switching frequency. Adjusting unit according to claim 9 or 10, wherein the switching cycles of the switching valves ( 20 . 22 . 28 ) are independent of each other. Method for controlling an adjusting unit according to one of claims 6 to 11, comprising the steps of: - sensing the displacement path of the piston ( 4 ) of the adjusting cylinder ( 2 ) and / or pressure - controlling the switching valves ( 20 . 22 . 28 ) depending on the displacement and / or the pressure.

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