Field of the Invention

The invention relates to a method for digitally controlling a hydraulic ON/OFF valve. Prior Art The invention lies in the field of digital hydraulics. Digital hydraulics implies the usage of

ON/OFF valves in a closed or open loop control for controlling an actuator (e.g. a piston of a main stage). A method for controlling an actuator using ON/OFF valves is disclosed in WO 02/086327 A1. For ON/OFF valves digital control methods are common, because the ON/OFF valves have two steady states which can be compared with the "one" and the "zero" of the digital control methods. The most common digital control methods are the pulse width modulation (PWM) and the pulse frequency modulation (PFM). Like in all digital control methods just two levels exist represented by "one" and "zero". During a pulse t, the signal is on the high level and during a pause t _p the signal is on a low level. When a signal is pulse frequency modulated the duration of the pulse t, is fixed but the duration of the pause t _p varies. When a signal is pulse width modulated the cycle duration (sum of pulse and pause) is fixed.

Typical for prior art digital control methods is the discontinuous movement of the actuator which is the result of the discontinuous volume flow which in turn is the effect of switching ON/OFF valves. It is desirable to avoid any discontinuity in the actuator movement.

Disclosure of the Invention

CONFIRMATION COPY According to the invention a method for digitally controlling an ON/OFF valve according to the independent claim is provided. Advantageous embodiments are defined in the dependent claims. A computing unit according to the invention is, in particular programmatically, adapted to carry out an inventive method.

Advantages of the invention

The invention is based on the finding that the discontinuities in the movement of the actuator originate from the pauses in the control signal. This problem occurs especially with ON/OFF valves, since the switching times (time for state change) are relatively long and cannot be neglected. The typical PWM cycle durations must therefore be relatively long in order to account for switching times.

The invention achieves to optimize the pause duration. The basis of the digital control method is to generate a digital control signal having a number of successive cycles, wherein each cycle j consists of a pulse with a pulse duration t, _j and of a pause with a specific pause duration t _pj , wherein the pause duration t _pj is set as a function of the pulse duration t _is so that the valve is, reliably, in the OFF state at the end of the pause. The resulting digital control method differs from usual PWM in that the sum of pulse and pause duration is not fixed, and differs from usual PFM in that the pulse duration is not fixed. Advantageously, the dependency tp _j (tj _j of the pause duration t _pj on the pulse duration (i.e. function) has both a minimum pause duration and a maximum pause duration and at least one monotonically increasing portion. (The function of usual PWM is strictly monotonically decreasing; usual PFM does not have such a dependency). Advantageously, the dependency f _p; ( ) of the pause duration tp _j on the pulse duration ^ (i.e. function) is monotonically increasing in its entire domain.

For each cycle j the following equations are valid for cycle duration 7}, cycle frequency f} and c cle duty g/. It has to be noted that in the last equation two out of the three parameters 7}, f,y and t _pj can be set variably for each cycle.

Advantageously, the pulse duration ¾, tf) or the pause duration t _pj is further set so that an application criterion is fulfilled. An advantageous application criterion is minimizing the cycle duration T _s . It is impossible to simply decrease the cycle duration, because the switching times of the ON/OFF valves are too high for such small PWM cycle durations. The pause duration t _pj and in turn the cycle duration 7} can be minimized if a new pulse j+1 starts at the time when the valve just reaches the OFF state. By minimizing the pauses between pulses, the discontinuity of the actuator movement is reduced and speed and quality of control can be improved.

A further advantageous application criterion is selected from the group comprising steady volume flow rate, minimized pressure pulsation, maximized control accuracy (i.e. the degree of correspondence between the ultimately controlled variable and the ideal value in a feedback control system), desired averaged flow cross-section, minimized time for reaching a steady regulation control state. The operation of the valve, e.g. providing a specific volume flow rate, fluid pressure or fluid mass, can be achieved by different signals consisting of pairs of pulse duration and pause duration. E.g. the volume flow rate Q can be formu- lated as a function of the pulse duration and the pause duration, Q^, t _p j). Advantageously, based on the application criterion, the pulse duration is maximized and/or the pause duration is minimized so that the application criterion is fulfilled. E.g. steady volume flow or maximized control accuracy can be achieved by using a minimum pause period. Minimized pressure pulsation or minimized time for reaching a steady regulation control state can be achieved by using a maximum pulse period.

One or more functions t _pj {tj _j ) can be predefined and be stored in the computing unit where one of the one or more functions can be chosen based e.g. on the application criterion. Examples of such functions are shown in figure 5. A function can be implemented as a (e.g. analytic or numeric) f _py (f<,)-function or as a table of values t _PJ {tij). A function can be determined heuristically.

The invention is described in relation to the ON state and the OFF state of the valve, wherein the ON state is the actuated stated caused by a pulse and the OFF state is the normal state caused by a pause. In NC (normally closed) valves, the OFF state is the closed state; in NO (normally open) valves, the OFF state is the open state.

For long pulses the switching off time of an ON/OFF valve is nearly constant. So in this case the minimum pause duration is the duration of the switching off time. When the pulses are very short so that the piston cannot reach the end stop of the ON state (so called ballistic operation mode) the pause duration t _pj must be reduced. Otherwise the pause duration t _pj is not minimized. Thus, the pause duration is chosen so that the valve reaches the OFF state just at the end of the pause.

In order to account for a change of the fluid temperature, the fluid viscosity or the flow forces which could change the switching times of the ON/OFF valve, the used pause duration t _p ' could comprise the determined pause duration t _pj and a robust parameter k _r , for example as a sum: t _Pj '=t _Pj +k _r . The robust parameter is an additional reliability duration ensuring that the valve reliably is in the OFF state at the end of the used pause duration t _pj '. Thus, instabilities of the system could be avoided.

One preferred possibility to determine the pause duration for a specific pulse duration is based on a novel model to describe the dynamics of ON/OFF valves during fast switching , control signals. This model is described under reference to figure 1.

Another preferred possibility to determine the pause duration for a specific pulse duration is based on a measurement of the piston stroke. E.g. the time when the piston reaches the OFF position can be measured.

By the digital control method according to the invention, one or more ON/OFF valves can be controlled as pilot valves, which in turn operate (e.g. movement or position control) a hydraulic actuator, e.g. a piloted valve, a linear or rotational hydraulic motor, a hydraulic (single- acting or double-acting) cylinder, a variable displacement pump or motor etc.

According to a preferred embodiment, a first group and a second group of ON/OFF valves are controlled, wherein the digital control signal for the second group has no pulse while the digital control signal for the first group has a pulse, and wherein the digital control signal for the first group has no pulse while the digital control signal for the second group has a pulse. Especially, the first group controls a movement of the actuator and the second group controls an opposite movement of the actuator. This allows for an improvement of speed and quality of control. Each movement of the actuator can be operated by a number of ON/OFF valves. These ON/OFF valves can be controlled synchronously, i.e. having the pulses at the same time, or can be controlled with a time shift between the pulses. This allows for a reduction of discontinuities of the actuator movement and an improvement of speed and quality of control. Also, the implementation of the invention in the form of software is advantageous because it allows particularly low costs, especially when a performing computing unit is still used for other tasks and therefore is present anyway. Suitable media for providing the computer program are particularly floppy disks, hard disks, flash memory, EEPROMs, CD-ROMs, and DVDs etc. A download of a program on computer networks (Internet, Intranet, etc.) is possi- ble.

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.

It should be noted that the previously mentioned features and the features to be elucidated in the following are usable not only in the respectively indicated combination, but also in further combinations or taken alone, without departing from the scope of the present invention.

In the drawings:

Figure 1 shows a relation between a piston stroke of an ON/OFF valve and a control signal according to a model underlying the invention.

Figure 2 schematically shows a hydraulic system comprising four ON/OFF valves for operating an actuator in a closed loop control.

Figure 3 schematically shows a preferred embodiment for the control structure of Figure 2. Figure 4 schematically shows an alternative embodiment for the control structure in case the time when the ON/OFF valve reaches the OFF state is measured.

Figure 5 schematically shows three functions between the pause duration t _pj and the pulse duration according to the present invention in comparison with one function between the pause duration t _pj and the pulse duration according to usual PWM method. Figure 6 schematically shows a dependency of a nominal averaged volume flow rate on a pulse duration for one digital control signal according to the present invention in comparison with three PWM signals having different frequencies.

Description of the drawings

In Figure 1 a relation between a piston stroke s of an ON/OFF valve and a digital control signal v according to a model underlying the invention is shown. The control signal v and the resulting stroke s are shown over time t. The control signal v has two successive cycles j with cycle duration T _j , and j+1 with cycle duration T _j+1 . The first cycle j has a pulse duration t _Sj and a pause duration t _pj . The start of the first cycle j is referenced by ΔΤ , the start of the second cycle j+1 is referenced by ΔΤ .

AT. = ^J T = y ^J t. +t

T„ The dynamics of the valve stroke from different kinds of ON/OFF valves basically show similar characteristics. One main effect of the valve dynamics is a time lag between the rising and falling edge of the control signal v and the beginning of the valve stroke s. In general this lag is not symmetrical but different for activation and deactivation. Short pulses or pauses of the control signal are completely suppressed by the valve, thus, the lag is no sim- pie time delay. On the one hand the time lags which occur whenever the valve was fully activated or deactivated are independent from the width of the pulses and pauses of the control signal and therewith independent from the time the valve was in the activated (ON) or deac- tivated (OFF) state. On the other hand, if the control signal has its falling edge as soon as the valve is fully activated the time lag will be much smaller than in the case described above. The effects are mainly influenced by design parameters like spool or poppet mass, spring constant and spring preload or the orifice which controls the damping of the valve stroke but also by parameters which result in different solenoid forces like electric resistance, inductivity or voltage.

As shown in Figure 1 the cycle start is referenced by ΔΤ,. After the start of the pulse at t ₀ , the valve piston does not move for a constant time lag. This time lag is called 'switching on de- lay" t min. After the duration f _tmn the piston motion starts sharply. The observations of switching valves show that the period of acceleration is in many cases very short, so that this period is neglected in the model. The velocity of the piston during the period of activating the valve is modelled to be constant. The time between the start of the pulse and time when the piston reaches the ON end stop (s=1) is called 'switching on time' t _on .

On the other hand, the time between the end ti of the pulse and the piston starts to move to the OFF end stop is called 'switching off delay" t _p,min and the duration between the end of the pulse and the valve reaches the OFF end stop (s=0) is called 'switching off time' t _off . In order to minimize the pause duration in line with the invention, the following pulse should start at ΔΤ, ₊₁ when the valve is in the OFF state.

To account for the fact that the piston stroke of small pulses is suppressed virtual ranges 21 , 22 are implemented. Two virtual ranges exist, one virtual range 21 between the OFF end stop and a 'virtual OFF end stop' (s=0*) and one virtual range 22 between the ON end stop and a 'virtual ON end stop' (s=1*). In these virtual ranges no physical piston movement occurs. At the initial state ('virtual OFF state') the piston is at the virtual OFF end stop (s=0*). During the duration t _iimin the piston virtually moves with a constant velocity to the real OFF end stop (s=0) and reaches the real range after the time t _iimin . The chosen velocity for the virtual range and the duration t _Kmm define the height of the virtual range 21. To complete the model an 'additional switching off lag' t _Vi0ff is introduced to specify how fast the virtual OFF state is reached.

The same procedure is done for the virtual range 22 between the ON end stop and the virtual ON end stop. At the initial state ^virtual ON state') the piston is at the virtual ON end stop (s=1*). During the duration t _Pimin the piston virtually moves with a constant velocity to the real ON end stop (s=1) and reaches the real range after the time t _pmm . The chosen velocity for the virtual range and the duration t _Pimin define the height of the virtual range 22. An 'additional switching on lag' t _Vi0n is introduced to specify how fast the virtual upper stop state is reached.

The model parameters P=(t _i:min , t _on , t _Vi0n , t _Pimin , t _off , t _Vi0ff ) which are needed for the model can be identified in different ways, for example by measurements. With the identification of the parameters by measurements a model validation is done in the same iteration. One possibility is to measure the piston stroke. With these measurements the parameters can be identified easily. The parameters can be identified not only by measurements but also by simulations of a more complex and validated existing CFD/FEM-model, for example.

An ON/OFF valve according to the model only opens when the pulse duration t _i} exceeds t,

If a valve lag exists, i.e. if a movement of the piston does not immediately results in a hydraulic opening of the valve, operating the valve within the lag should be avoided. Thus, a minimum pulse duration defined by t _Kmin + t, _ag is preferably provided which substitutes t _iimin . Thus, each control output of the closed loop control is converted into a pulse having at least a pulse duration t _min + t _iag .

Based on the model above, a minimized pause duration t _PJ {tj _j ) can be calculated as a function of the preceding pulse duration t according the following method:

Control function S t _ih t _p j) of the valve can have the two different values "1 " and "0", wherein t means the actual time, t, _j means the pulse duration of cycle j and t _pj means pause duration of cycle j, AT _j means the start time of cycle j:

As mentioned above, the sum T _j of t, _j and t _pj for different cycles j usually differs. by:

As mentioned above, the actually used pause duration could comprise the determined pause duration t _PJ {tij) and a robust parameter k _r . As one can see, a minimum pause duration At is defined to avoid that a cycle duration becomes zero. In Figure 2 a control loop 200 is shown schematically to illustrate a preferred embodiment of the invention. The control loop 200 includes four ON/OFF valves \ - V ₄ for operating an actuator 210. The actuator 210 may be for example, a piston or a main stage valve of a pilot- operated valve assembly. A set-point value x _so n is compared with a feedback actual value x _ist and a control error e is calculated therefrom. The control error e is transmitted to a control element 220, e.g. a proportional controller. The control element 220 calculates a control output u based on the control error e. The control output u is transmitted to a calculating block 300 which is shown in more detail in Figure 3. In the calculating block 300 a digital control signal v is generated according to a preferred embodiment of the invention. The digital control signal v is used to control the valves Vi - V ₄ . The valves are connected in pairs, so either the valves V-i and V ₂ are open, while the valves V ₃ and V ₄ are closed, or vice versa. The actual position of the actuator is detected at 240 and feed back as the actual value. Under reference to Figure 3, a pulse duration t _rj (u) is calculated based on the control output u in a block 310. This calculation is preferably made taking into account the minimum pulse duration described above. Especially, a relation in the form of t, _j (u) = c u+ t _iim j _n +tiag having a suitable slope c can be used.

In a block 320 the calculated pulse duration tj _j (u) is quantized to t, * and transferred to a model block 330. The model block 330 includes the above described relation Further, the mentioned parameters P are transferred to the model block 330. Such calculated pause duration t _pj is combined with the robust parameter k _r to t _prj at 340 and quantized at 350.

Quantized pulse duration t, * and quantized pause duration t _pr * are combined in 360 for generating a digital control signal v. In the digital control signal, a new pulse t ₊ i* starts immedi- ately after the preceding pause duration t _pr *.

The digital control signal v and the control output u are transmitted to block 370, in which the control signals for the ON/OFF valves Vi - V ₄ are calculated. Which of the two pairs of valves is opened can be decided based on the sign of u. The chosen pair is then controlled with the digital control signal v. E.g. in case u is larger than zero, valves \ and V ₂ are controlled, and in case u is less than zero, valves V ₃ and V ₄ are controlled.

Under reference to Figure 4, an alternative embodiment 400 to the calculation of the pause duration is based on a measurement of the piston stroke. In this embodiment, the time when the valve reaches the OFF position is measured and the following pulse is started when each valve V V ₄ is OFF. Signals S1 -S4 representative of the piston strokes are transferred to a determination block 430. When each valve is OFF, a trigger signal is transferred to digital control signal generating block 460. This trigger signal defines the start of the pulse defined by pulse duration ¾ *.

In Figure 5, three functions 501 , 502, 503 between the pause duration t _pj and the pulse duration tj _j according to the present invention are shown in comparison with one function 504 according to usual PWM method. According to usual PWM method, the sum of the pulse duration tj _j and the pause duration t _pj always equals the constant PWM cycle duration T _PWM . Thus, function 504 is a strictly monotonically decreasing function. In contrary thereto, the cycles of a digital control signal according to the present invention do not have fixed cycle duration. Rather, pause duration is calculated as a function of the directly preceding pulse duration. Each function 501 , 502, 503 is a monotonically increasing function and has both a minimum pause duration t _min and a maximum pause duration t _max .

In Figure 6, a dependency of a nominal averaged volume flow rate Q on a pulse duration ty for one digital control signal 601 according to the present invention in comparison with three PWM signals 602, 603, 604 having different frequencies is shown. PWM signal 602 has a frequency f _PWM of 40 Hz, PWM signal 603 has a frequency f _PW M of 100 Hz, and PWM signal 603 has a frequency f _PWM of 200 Hz. It can be seen from Figure 6 that for PWM operation the ON/OFF valve is always in the ON state if the pulse duration is at least 1/fpwM- It can be seen from Figure 6 that lines 602 and 603 have a straight portion wherein a desired nominal averaged volume flow rate Q can be achieved by controlling the pulse duration. However, the possible pulse duration is upper limited by f _PW M and T _PWM . It can be further seen from Figure 6 that a desired nominal averaged volume flow rate Q can be achieved by line 601 using a method according to the present invention without any upper limitation. Further, it can be seen from Figure 6 that for the same pulse duration the flow rate of 601 is, within the controllable range, higher than for 602 and 603.