Patent Application
20140130484
The invention concerns a pressure medium system, in particular a hydraulic system of a clamping device for mechanical clamping of workpieces or workpiece holders, such as workpiece pallets.
Such clamping devices having a hydraulic system are, for example, known from DE 31 36 177 A1 and contain a hydraulic pump, a pressure sensor and a pressure-limiting valve as well as a control unit.
The hydraulic pump generates the hydraulic pressure required for operation of the clamping device, wherein the hydraulic pump can be driven, for example, by an electric motor.
The pressure-limiting valve is arranged between the hydraulic pump and the hydraulic consumer of the clamping device and leads the hydraulic oil at exceeding of a predetermined maximum value back into a hydraulic oil tank in order to limit the hydraulic pressure to the admitted maximum value.
This pressure limitation can, for example, be required when the hydraulic pump, due to a malfunction, delivers a larger volumetric flow than is necessary for maintaining a predefined target value.
Furthermore, this pressure limitation can, however, also be required when the hydraulic oil enclosed in the hydraulic system expands due to heating, which is associated with a corresponding pressure rise.
The control unit measures, by means of the pressure sensor, the hydraulic pressure generated by the hydraulic pump and switches the hydraulic pump on when the hydraulic pressure falls below a predefined minimum value (switch-on pressure). During the subsequent pressure build-up, the control unit continuously measures, by means of the pressure sensor, the actual hydraulic pressure and switches off the hydraulic pump when the hydraulic pressure measured by the pressure sensor exceeds the predefined target value (switch-off pressure). In this manner, the hydraulic pressure is maintained between the minimum value and the target value during the operation of the clamping system.
This known hydraulic system has different disadvantages, which are described shortly in the following.
On the one hand, part of the volumetric flow delivered by the hydraulic pump is discharged via the pressure-limiting valve when the hydraulic pressure exceeds the predefined target value. This pressure limitation is, however, associated with a corresponding dissipation power of the pressure-limiting valve.
On the other hand, the hydraulic pump is mostly operated at a high hydraulic pressure near the target value, which is associated with a correspondingly high load of the hydraulic pump and with a correspondingly high energy expenditure.
Furthermore, there is the problem that the hydraulic pump must be turned on again when the hydraulic pressure has fallen below a predefined minimum pressure. It is problematic in this case that the so-called subsequent switching of the hydraulic pump does not immediately lead to a pressure rise, which has various causes. On the one hand, the motor relay of the hydraulic pump has a certain dead time, whereby the start-up of the hydraulic pump is delayed. Beyond this, due to its mass inertia, the hydraulic pump needs a certain start-up time. On the other hand, however, the hydraulic pressure also has a time constant in the hydraulic system and rises linearly after the start-up of the hydraulic pump. This temporal delay can cause for subsequent switching of the hydraulic pump that the predefined minimum pressure is fallen short of.
DE 199 59 706 A1 and DE 10 2005 060 321 A1 reveals pressure medium systems for a motor vehicle brake system, wherein the phenomenon also occurs that a hydraulic pump does not immediately stands when switched off, but rather has an overrun. The possible pressure rise during this overrun when switching off is, however, compensated for in these documents by the fact that the control times for downstream valves is modified accordingly. In this case, the pressure rise is thus not prevented during the overrun, but rather compensated for through suitable control measures.
Reference is also made to DE 20 2008 011 507 U1, DE 697 15 709 T2 and DE 197 13 576 A1 concerning the prior art from other technical fields.
Therefore, the invention is based on the object of creating a correspondingly improved hydraulic system, which avoids these disadvantages as far as possible.
The invention is based upon the technical insight that the fluid pump (e.g. hydraulic pump) still has an inertia-induced overrun also after the switching off of its drive, so that the fluid pressure (e.g. hydraulic pressure) still rises a little bit also after the switching-off of the fluid pump during the overrun of the fluid pump.
The invention therefore provides for that the fluid pump is already switched off during pressure build-up before the fluid pressure has reached the predefined target value. During the subsequent overrun of the fluid pump, the fluid pressure then still rises from the switch-off pressure with a certain overrun pressure rise towards the predefined target value. The invention thus exploits the kinetic energy of the fluid pump, of the drive of the fluid pump and/or of the liquid column delivered by the fluid pump.
On the one hand, this offers the advantage that the fluid pump is operated less often at high fluid pressures near the target value, whereby the fluid pump is protected and less drive energy is consumed.
On the other hand, the invention also offers the advantage that less fluid (e.g. hydraulic oil) must be discharged via the pressure-limiting valve, whereby the pressure-limiting valve is protected and less dissipation power comes up.
In a preferred specimen embodiment of the invention, the switch-off pressure is dimensioned such that the pressure difference between the predefined target value and the switch-off pressure is smaller than the overrun pressure rise. This means that the fluid pressure after the switching-off of the fluid pump still rises at least up to the predefined target value. The overrun pressure rise should therefore be preferably large enough in order to bridge the pressure difference between the switch-off pressure and the target value.
In this case, it must be taken into consideration that the pressure rise during the overrun of the fluid pump runs asymptotically up to a final value, so that the pressure rise is continuously slower in the upper pressure range up to the final value. It is, however, generally desirable that the predefined target value of the fluid pressure is reached as fast as possible during the overrun. Preferably, the switch-off pressure is therefore dimensioned such that the overrun pressure rise exceeds the pressure difference between the switch-off pressure and the predefined target value by at least 1%, 2%, 5%, 10%, 20%, 50%, 100% or 200%. This offers the advantage that for bypassing the pressure difference between the switch-off pressure and the predefined target value, the relatively steep-running initial pressure rise during the overrun is exploited, so that the predefined target value is adjusted relatively quickly after the switching-off of the fluid pump.
On the other hand, it is not required that the fluid pressure still rises substantially after the switching-off of the fluid pump during the overrun further than up to the desired target value. The switch-off pressure is therefore preferably dimensioned such that the overrun pressure rise exceeds the pressure difference between the switch-off pressure and the predefined target value by at most 200%, 100%, 50%, 20%, 10%, 5%, 2% or 1%. This offers the advantage that during the overrun of the fluid pump, only little excess fluid comes up, which must then be discharged via the pressure-limiting valve.
The above-mentioned percent values are possible if one uses certain factors in the calculation. However, the invention is not restricted to fixed values. Depending on the stability and characteristic of the hydraulic system, there are different values. Preferably, however, the smallest possible value is used within the context of invention. This depends on the quality of the calculation, the constancy of the parameters of the hydraulic system and, here, in particular on the stiffness of the system, the reaction speed of the control unit and of the drive. Values below 5% are desirable.
In the preferred exemplary embodiment of the invention, the switch-off and/or the switch-on of the fluid pump resp. of the drive of the fluid pump are pressure-controlled. This means that the control unit measures the fluid pressure by means of the pressure sensor. The control unit then switches off the fluid pump during the pressure build-up when the measured fluid pressure exceeds the predefined switch-off pressure. Furthermore, the control unit can switch on the fluid pump again when the measured fluid pressure falls below the predefined switch-on pressure.
For specification of the switch-off pressure, it should be taken into account that the overrun pressure rise does not only depend on the inertia of the fluid pump and its drive, but rather also on the currently delivered and outflowing discharge flow. If, for example, a large discharge flow flows out via the consumer, the overrun pressure rise is only very low. For specification of the switch-off pressure, one therefore preferably takes into account the currently outflowing discharge flow of the fluid pump.
An option for determining the current discharge flow of the fluid pump consists in measuring the pump speed of the fluid pump or deriving it from the motor control, wherein the discharge flow can then be derived at least through approximation from the pump speed.
Another option for determining the current discharge flow of the fluid pump consists in the measurement by means of a volumetric flow sensor.
A further option provides for, in contrast, that the discharge flow of the fluid pump is assumed to be known.
The inertia of the system consisting of the fluid pump and its drive reflects during the operation in the temporal pressure change during the pressure build-up, i.e. in the first temporal derivative of the fluid pressure. Thus, a rapid pressure rise during the pressure build-up indicates a correspondingly high inertia and a high overrun pressure rise. One preferably measures therefore the temporal pressure change during the pressure build-up and takes it into account as a dimension for the inertia of the fluid pump.
It should also be mentioned that the switch-off pressure during the operation of the pressure medium system according to the invention is preferably adapted dynamically to the current operating state. This means that the switch-off pressure is continuously adapted to the actual operating state (e.g. rotational speed, fluid pressure, pressure rise, etc.).
For this dynamic adaptation of the switch-off pressure, the following marginal conditions resp. optimization goals are preferably taken into consideration:
In the preferred exemplary embodiment of the invention, the switch-off pressure is therefore calculated according to the following formula and continuously adapted during the operation:
P
OFF
=P
REQ−(K1/PREQ+K2)·dPACTUAL/dt·1/Q
with:
The invention is, however, with respect to the calculation of the switch-off pressure, not limited to the above-mentioned formula, but rather can fundamentally be realized also with other formulae for calculation of the switch-off pressure.
In a variant of the invention, the control unit is constructionally integrated into the pressure sensor and generates a switch-off signal for the motor control. It is, however, alternatively also possible that the control unit is constructionally separated from the pressure sensor and receives from the pressure sensor a pressure signal as an analog signal.
For a consumer, it can be required that the pressure is subsequently switched again, for example that a replenishment results along with the temporal delay or that a small leakage occurs or that the pressure can be reduced a little bit through strong cooling. Such subsequent switching pressure typically lies 5-10% below the predefined target value PREQ, but above the switch-off pressure POFF. In this case, only a very small discharge rate is supplied in the system and needs further triggering when excess oil quantity should not be discharged via the pressure-limiting valve. For this case, the switching time of the drive motor of the fluid pump (“pressure motor”) is reduced to such an extent that only the rotational speed is reached in order to achieve a smaller pressure build-up through overrun. This happens by reducing the constant K1 of the displacement volume Q and proportionally reducing the start-up time of the pump motor drive.
The term switching on and switching off of the fluid pump used within the context of the invention preferably gears to fully switching on and switching off the drive of the fluid pump. The invention also claims, however, protection for variants for which the drive of the fluid pump is merely run up or shut down.
In the preferred exemplary embodiment of the invention, the pressure medium system is a hydraulic system. The invention can, however, also be realized with other pressure medium systems, such as with pneumatic systems. It is merely decisive that the fluid pump still has an inertia-induced overrun after switching-off, while the fluid pressure still rises.
It should also be mentioned that the pressure medium system according to the invention preferably comprises a consumer, which is supplied with pressurized fluid. The consumer is preferably a clamping system for mechanical clamping of workpieces or workpiece holders such as workpiece pallets. Such clamping systems are per se known and described, for example in DE 31 36 177 A1, so that the content of this publication is to be included in full in the present description. The invention, however, also claims protection for pressure medium systems with other types of consumers.
Another aspect of the invention deals with the problem that the fluid pump has an inertia-induced pre-run during switching-on (subsequent switching), so that the fluid pressure does not yet rise substantially during the pre-run although the fluid pump is already switched on. The reasons for this pre-run are—as was already explained briefly at the beginning—on the one hand the dead time of the motor relay of the fluid pump and on the other hand the delayed pressure build-up in the pressure medium system.
The invention therefore also provides for that the control unit switches on the fluid pump again already at the drop of fluid pressure when the fluid pump is switched off before the fluid pressure has fallen to a predefined minimum pressure (e.g. 5% below the target pressure), which should not be fallen short of. The switch-on pressure (subsequent switching pressure) of the fluid pump is thus preferably greater than the predefined minimum pressure, which should not be fallen short of. This offers the advantage that the possibly occurring further pressure drop during the inertia-induced pre-run of the fluid pump does not cause that the predefined minimum pressure is fallen short of.
In a preferred exemplary embodiment of the invention, the control unit detects the temporal change of the fluid pressure by means of a pressure sensor when the fluid pump in the switched-off state. The switch-on pressure is then calculated by the control unit preferably depending on the temporal change of the fluid pressure, the fluid pump being switched off, the switch-off pressure and the predefined minimum pressure, wherein the calculation can be done according to the following formula:
P
ON
=P
MIN−(k1+k2·POFF)·dPACTUAL/dt
with:
The switch-on pressure (subsequent switching pressure) is thus preferably dimensioned such that the fluid pressure after switching on the fluid pump does not fall below the predefined minimum pressure during the pre-run of the fluid pump.
It should also be mentioned that the invention also comprises a corresponding operating method, as can already be seen from the above description.
Other advantageous developments of the invention are characterized in the subclaims or are explained in more detail below together with the description of the preferred exemplary embodiments of the invention on the basis of the figures. The figures show as follows:
The hydraulic pump 1 is connected on the input side with a hydraulic oil tank 4 from which the hydraulic pump 1 extracts hydraulic oil and pumps via a back-pressure valve RV into a high-pressure area 5 to which the clamping system 3 is connected.
Furthermore, the hydraulic system has a pressure-limiting valve 6, which connects the high-pressure area 5 with the hydraulic oil tank 4. The pressure-limiting valve 6 is closed in the normal state and opens when the actual hydraulic pressure PACTUAL in the high-pressure area 5 exceeds a predefined maximum value PMAX.
Furthermore, the hydraulic system has a pressure sensor 7, which measures the actual hydraulic pressure PACTUAL in the high-pressure area 5 and transmits it to a control unit 8, which triggers a motor control 9 depending on the measured hydraulic pressure PACTUAL, wherein the control unit 8 optionally switches on or switches off the electric motor 2.
For the triggering of the electric motor 2, the control unit 8 also takes into account the actual discharge flow Q of the hydraulic pump 1, since the actual discharge flow Q influences the overrun pressure rise. For this purpose, the control unit 8 is connected with a rotational speed sensor 10, which detects the rotational speed n of the electric motor 2 and thus also the pump speed. From the pump speed n, the control unit 8 calculates then the actual discharge flow Q of the hydraulic pump 1.
In addition, a pressure-reducing valve 11 is provided for, which branches off between the hydraulic pump 1 and the back-pressure valve RV and recycles hydraulic oil back, in the opened state, into the system oil tank 4, wherein the pressure-reducing valve 11 is controlled by the control unit 8. The control unit 8 opens the pressure-reducing valve 11 when the target value PREQ is decreased. This is meaningful so that the hydraulic pressure IS is reduced as fast as possible to the new, lower target value PREQ.
The control unit 8 then continuously calculates during the operation (cf. step S1 in
P
OFF
=P
REQ−(K1/PREQ+K2)·dPACTUAL/dt·1/Q
with:
The device-specific constants K1, K2 can be determined previously in a calibration process.
In the switched-off state of the hydraulic pump, the control unit 8 continuously measures by means of the pressure sensor 7 the hydraulic pressure PACTUAL in the high-pressure area 5 (cf. step S2 in
The control unit 8 then continuously checks whether the measured hydraulic pressure PACTUAL falls below a predefined switch-on pressure PON (cf. S3 in
If this is the case, the control unit 8 sends a switch-on signal to the motor control 9, which then switches on the electric motor 2 in order to increase the hydraulic pressure (cf. step S4 in
During the subsequent pressure build-up, the control unit 8 then continuously checks whether the actual hydraulic pressure PACTUAL exceeds the switch-off pressure POFF (cf. step S5).
If this is the case, the control unit 8 sends a switch-off signal to the motor control 9, which then switches off the electric motor 2 (cf. step S6).
During the subsequent inertia-induced overrun of the hydraulic pump 1, the hydraulic pressure PACTUAL still rises in spite of the switched-off electric motor 2 due to inertia, wherein the overrun pressure rise ΔPOVERRUN (cf.
During the overrun, the pressure-limiting valve 6 continuously checks whether the hydraulic pressure PACTUAL exceeds a predefined maximum value PMAX (cf. step S7 in
If this is the case, the pressure-limiting valve 6 opens automatically and conducts the excess hydraulic oil from the high-pressure area 5 into the hydraulic oil tank 4 back in order to prevent any further pressure rise beyond the maximum value PMAX (cf. step S8 in
Furthermore, the pressure-limiting valve 6 continuously checks whether the hydraulic pressure has fallen below the predefined target value PREQ (cf. step S9 in
If this were the case, the pressure-limiting valve 6 automatically closes in order to prevent any further flowing-out of hydraulic oil from the high-pressure area 5 in the hydraulic oil tank 4, since the hydraulic pressure PACTUAL would thereby still fall below the predefined target value PREQ (cf. step S10 in
It is further apparent from
The exemplary embodiment in accordance with
A particularity of this exemplary embodiment consists in the fact that the control unit 8 is arranged in a common housing 11 with the pressure sensor 7.
In a first step S1, device-specific constants K1, K2 that characterize the pressure rise after switching-on of the hydraulic pump 1 during the pre-run of the hydraulic pump 1 are determined.
In a further step S2, the minimum pressure PMIN, which should not be fallen short of is predefined.
Furthermore, in a step S3, the switch-off pressure POFF, which leads to switching-off the hydraulic pump 1 during the run-up of the fluid pressure PACTUAL is calculated. The calculation of the switch-off pressure POFF was already explained in detail, so that, to avoid repetitions, reference is made in this respect to the preceding statements.
In a loop, the fluid pressure PACTUAL is firstly measured in a step S4.
Furthermore, the temporal change dPACTUAL/dt of the fluid pressure PACTUAL is then calculated in the loop in a step S5.
In a further step S6, the switch-on pressure PON is then calculated according to the following formula:
P
ON
=P
MIN−(k1+k2·POFF)·dPACTUAL/dt.
In a step S7, it is then checked in the loop whether the measured fluid pressure PACTUAL falls below the calculated switch-on pressure PON. If this is the case, the hydraulic pump 1 is switched on in a step S8. Otherwise, the above-mentioned steps S4-S7 are repeated in a loop.
In this manner, it is ensured that the fluid pressure PACTUAL will not fall below the predefined minimum pressure PMIN, which should not be fallen short of, in spite of the inertia-induced pre-run of the hydraulic pump 1.
Subsequent switching in the suggested manner is advantageous because the kinetic energy of the pump-motor unit is exploited again and there is no pressure that is substantially higher than the target pressure. Thus, with such a device, a pressure value can be adjusted without too much oil volume having to be delivered by the pump, which would then have to be discharged again via a limiting valve.
In combination with the switching-off of the pump according to the invention already before reaching the target value PREQ, a pressure adjusting system results for which the pressure-limiting valve 6 only serves for security purposes. The pressure setting is carried out through change of the target value PREQ.
Through the use of the switch-off pressure POFF from the initial pressure rise, some more energy is supplied to the hydraulic system, since the pressure must only be built up in the system consisting of the hydraulic pump 1 and the pressure tube and the whole hydraulic system is connected only after opening of the back-pressure valve RV.
The invention is not limited to the preferred exemplary embodiments described above. Instead, a plurality of variants and modifications are possible, which also make use of the concept of the invention and thus fall within the scope of protection. Furthermore, the invention also claims protection for the subject-matter and the features of the subclaims independently of the claims to which they refer.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2011 105 584.7 | Jun 2011 | DE | national |
| 10 2011 112 701.5 | Sep 2011 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/002598 | 6/20/2012 | WO | 00 | 12/20/2013 |
