Patent Publication Number: US-2023139395-A1

Title: Method for Minimizing the Dither Hum on a Valve

Description:
This application claims priority under 35 U.S.C. § 119 to patent application no. DE 10 2021 212 309.0, filed on Nov. 2, 2021 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
     The disclosure relates to a method for operating a, preferably hydraulic, valve as disclosed herein. 
     BACKGROUND 
     A corresponding valve is known, for example, from EP 2 280 179 B1 or DE 10 2019 204 246 A1. In this case, a linearly movable control slide is preloaded into a zero position by means of at least one spring. By means of two solenoids, the control slide can be adjusted in two opposite directions starting from the zero position, it being possible for the position of the control slide to be measured by means of a position sensor. The solenoids are coupled to the control slide via a pilot valve, it being possible for the pilot valve to be designed for example according to EP 2 740 980 B1 or as a 4/3 proportional directional control valve. The present disclosure can also be used for a valve which is actuated directly by means of the at least one solenoid. In the disclosure, a single solenoid can be provided, the zero position being an end position of the control slide. 
     It is known from U.S. Pat. No. 4,960,365 to actuate the solenoid of a valve using what is known as a dither. In this case, a periodic dither current control variable is superimposed on an effective current control variable. In this case, the effective current control variable represents the position of the control slide actually desired by the user of the valve. The dither current control variable serves to keep the control slide continuously in motion, such that a lubricating film always remains between the control slide and the surrounding housing, no static friction occurring, which would influence the control behavior of the valve in a very disadvantageous manner. It should be noted that U.S. Pat. No. 4,960,365 discloses a controlled adjustment of the control slide, the present disclosure preferably being used with a position-controlled adjustment of the valve in order to achieve a high adjustment accuracy. 
     The disadvantage of the dither is that what is known as a dither hum can be excited as a result. As a result, parts of the machine in which the valve is installed can be excited to oscillate, which can cause a considerable noise nuisance. It is therefore typically attempted to select the frequency of the dither such that no resonances are encountered, the amplitude of the dither being selected to be so small that the above-explained effect of minimizing the friction still occurs. 
     In the case of the use of pilot valves, it has been shown that the required amplitude of the dither is subject to considerable serial scattering. Furthermore, the required amplitude of the dither is dependent on the instantaneous operating conditions of the valve, such as operating pressure and set volume flow. Thus, if an amplitude of the dither excitation is to be firmly specified, this is too large in the vast majority of the operating time, the dither hum being unnecessarily loud. 
     The object of the disclosure is that of minimizing dither humming, the valve nonetheless operating reliably under the influence of serial scattering and changing operating conditions, in particular the dither movement of the control slide being reliably maintained. Furthermore, the wear occurring during operation, in particular on the pilot valves, can be compensated for. 
     SUMMARY 
     It is proposed for an actual dither amplitude to be calculated from the actual instantaneous position using a band-pass filter, a dither controller being provided, which, by adjustment of an amplitude of the dither current control variable and/or a waveform of the dither current control variable, causes the actual dither amplitude to not fall below a predetermined minimum dither amplitude. The proposed control makes it possible for the micro-oscillations of the control slide to be reliably maintained. The relevant actual variable of the control can be determined in a simple and cost-effective manner by means of the band-pass filter; in addition to the position sensor, which is present in any case, no further measuring means are required for determining the actual variable of the dither control. 
     In the context of the present application, an effective value is to be understood to mean a low-pass filtered and/or a time-averaged instantaneous value. 
     The magnetic force of the solenoid can act on a ferromagnetic armature which is motion-coupled to the control slide (direct actuation). The solenoid can be a component of a, preferably hydraulic, pilot valve, an output pressure of the pilot valve acting on the control slide in the adjustment direction. The pilot valve is preferably designed as a pressure reducing valve. If two solenoids are provided, their adjustment directions are preferably directed opposingly. A corresponding pilot valve can be designed according to EP 2 740 980 B 1. 
     The frequency of the dither current control variable can be between 70 Hz and 500 Hz, preferably between 100 Hz and 250 Hz. The term “quasi-periodic” is intended to describe the deviation from an ideally periodic dither current control variable caused by dither control according to the disclosure. A center frequency of the mentioned band-pass filter is preferably equal to the frequency of the dither current control variable. 
     The dither amplitude and/or dither waveform found in the context of the dither control is preferably stored when the system is switched off and used as a starting value for the dither control when the system is switched on again. 
     Advantageous developments and improvements of the disclosure are specified in the dependent claims. 
     It can be provided for the determination of the actual dither amplitude to comprise a determination of extreme values of the filtered actual instantaneous position. In this way, the actual dither amplitude can be calculated easily, in particular with low computing power. 
     It can be provided that the determination of the actual dither amplitude comprises a difference formation of directly successive extreme values. In this way, the actual dither amplitude can be calculated easily, in particular with low computing power. The corresponding peak-to-peak amplitude is nevertheless a measure for the strength of the dither movement of the valve slide that can be advantageously used within the context of the dither control. 
     It can be provided for the determination of the actual dither amplitude to comprise averaging over several of the mentioned differences. This avoids fast or high-frequency changes of the actual dither amplitude. A comparable effect can be achieved by means of low -pass filtering. However, this results in a significantly higher latency. After the system has been switched on, it would take significantly longer until the system is adjusted. In the case of sudden changes in the operating conditions, the dither movement could briefly come to a halt. These disadvantages are avoided by means of the proposed averaging. 
     It can be provided that the dither controller regulates the actual dither amplitude to a target dither amplitude, which is greater than or equal to the minimum dither amplitude. The dither controller is preferably a continuous, linear controller, in particular a PID, a PI or a P controller. By virtue of the fact that the target dither amplitude is at least temporarily, in particular immediately after the system is switched on, above the minimum dither amplitude, a dither movement of the control valve can be brought about very quickly, the desired noise minimization nonetheless occurring some time later. 
     It can be provided for the target dither amplitude to be reduced as long as the actual dither amplitude is above the minimum dither amplitude. The reduction of the desired dither amplitude preferably takes place continuously at a predetermined speed. If the dither controller is designed as a PID controller or as a PI controller, such that it has no remaining control deviation even in the event of interferences, the target dither amplitude can be reduced to the minimum dither amplitude. 
     It can be provided for the dither controller to be activated after the control slide is deflected from the zero position for a predetermined period of time. Thus, in the zero position, the dither movement and the associated noise development come to a halt. In this case, it must be taken into account that the valve is not used for the vast majority of the time, such that the dither oscillation of the valve slide is not required. If the valve slide returns to the zero position, the dither controller is preferably deactivated again only after a predetermined further period of time. This further period of time is preferably selected so as to be of such a length that it is comparatively certain that the valve is subsequently not used for a longer time. 
     It can be provided for the amplitude of the dither current control variable to be selected to be so large during the specified period of time that the actual dither amplitude is reliably above the minimum dither amplitude. The control of the dither amplitude accordingly begins with this amplitude of the dither current control variable. At the start of the dither control, it is ensured that the dither movement of the valve slide is present. In this way, the desired state of a minimum dither movement is brought about very quickly, in particular more quickly than if the dither control were to be implemented when the control slide is absolutely stationary. 
     It can be provided for the waveform of the dither current control variable to be able to be changed in steps. In this way, a very fine adjustment can be achieved in a simple manner, compared with an adjustment of the dither amplitude. Preferably, a plurality of value tables are specified, each describing a period of the dither current control variable. The waveform of the dither current control variable can accordingly be changed in discrete steps. The user of the valve can select the particular waveform which results in the smallest possible noise formation in the system in which the valve is used. Especially when the dither control is already in the vicinity of the desired minimum dither amplitude, an automated adjustment of the waveform, as a replacement for an automated adjustment of the dither amplitude, can be used to adjust the dither excitation as finely as possible. 
     It can be provided for the current control device to comprise a switching device by means of which a supply voltage can be switched on and off at the relevant solenoid, a pulse width modulator being provided, which is actuated by the instantaneous current control variable, the pulse width modulator actuating the switching device. The instantaneous current control variable thus corresponds to the duty cycle of the pulse width modulator. The frequency of the PWM can be fixedly predefined, it being for example 10 kHz. The supply voltage is preferably a DC voltage, the voltage level of which is highly preferably substantially constant. The PWM frequency and the dither frequency are preferably synchronized according to DE 10 2008 013 602 B4. 
     It can be provided for each solenoid to be assigned a current sensor and a current controller in each case, it being possible for an instantaneous current actual value flowing through the solenoid in question to be measured using the current sensor, the current controller regulating an effective current actual value, determined, in particular calculated, from the instantaneous current actual value, to a target effective current, by adjusting an instantaneous current control variable. The current sensor can for example comprise an ohmic measuring resistor, which is connected in series to the solenoid, the voltage drop at the measuring resistor being measured, the current flowing through the solenoid at that time being determined, in particular calculated, from the voltage drop. The effective current actual value is determined from the instantaneous current actual value, for example by low-pass filtering. This is intended to in particular eliminate the fluctuations of the current which are caused by the PWM. The current controller is preferably a continuous, linear controller, in particular a PID, a PI or a P controller. 
     It can be provided for the target effective current to be formed by superimposing an effective position control variable and the dither current control variable. The aforementioned superimposition is preferably calculated by adding the stated variables. Alternatively, it is conceivable for the dither current control variable to be superimposed with the instantaneous current control variable, so as to act on the system in a more controlling manner. As a result of the inclusion, proposed above, of the dither current control variable in the current regulation, the corresponding adjustment is much more effective and, above all in the case of different operating conditions, always has substantially the same effect. This is often not the case in the alternative mentioned. 
     An adjustment controller can be provided which, by adjusting the effective position control variable, regulates an actual effective position, determined, in particular calculated, from the actual instantaneous position, to a target effective position. The actual effective position is preferably determined by low-pass filtering from the actual instantaneous position. This is intended to eliminate the fluctuations caused by the dither. The position controller is preferably a continuous, linear controller, in particular a PID, a PI or a P controller. 
     Of course, the features mentioned above and those still to be explained below can be used not only in the respectively specified combinations, but also in other combinations or alone, without departing from the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is explained in more detail below with reference to the accompanying drawings. In the drawings: 
         FIG.  1    is a circuit diagram of the valve on which the disclosure is based; 
         FIG.  2    is a control diagram illustrating the method according to the disclosure; and 
         FIG.  3    is a graph of different waveforms of the dither current control variable. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a circuit diagram of the valve  10  on which the disclosure is based. In the present case, the valve  10  is designed as a hydraulic slide valve, it also being possible for it to be designed as a seat valve. It has a middle zero position  12  which is preloaded by means of two springs  13 . The zero position  12  can be an end position of the valve  10  which is preloaded by a single spring. 
     In the present case, the valve  10  is a 4/3-way valve, the zero position  12  forming a blocking position in which the associated cylinder (not shown) is firmly clamped, hydraulically. Starting from the zero position  12 , the valve  10  can be continuously adjusted in two directions, the mentioned cylinder entering one valve position, whereby it moves out in the other valve position. The path by which the control slide  11  of the valve  10  is moved away from the zero position  12  determines the movement speed of the cylinder. 
     The present valve  10  is hydraulically adjusted in both adjustment directions, in each case by means of a pilot valve  15 , the pilot valve  15  in turn being actuated by means of an associated solenoid  14 . The two solenoids  14  are connected in parallel to a supply voltage  32  and to an electrical ground  33 . The supply voltage  32  is preferably a DC voltage, although an AC voltage is also conceivable, provided that the corresponding alternating voltage frequency is higher than the PWM frequency. Each solenoid  14  is assigned a switching device  31  and a measuring resistor  42 . The measuring resistor  42  is preferably connected between the relevant solenoid  14  and ground  33 , such that the electrical potential between the measuring resistor  42  and the solenoid  14  represents an instantaneous current actual value  43 . The explained arrangement forms a current sensor  41 , because the mentioned instantaneous current actual value  43  can be calculated from the mentioned electrical potential. 
     The switching device  31  is designed such that it can quickly switch on and off the occurring currents, it being formed, for example, by a field effect transistor. In the present case, the switching device  31  is connected between the solenoid  14  and the supply voltage  32 , because it influences the determination of the instantaneous current actual value  43  the least here. The two current control devices  30  are preferably designed identically. 
     Furthermore, a position sensor  20  is provided, which can for example operate according to the LVDT principle (https://de.wikipedia.org/wiki/Differential transformer). This converts the mechanical position of the control slide  11  into an electrical potential which corresponds to the actual instantaneous position  21  of the control slide  11 . In particular the micromovements of the control slider  11  are reflected in the actual instantaneous position  21 , which micromovements are caused by the dither explained further below. 
     The control device  11  preferably comprises a programmable digital computer. The control device  11  is connected to the two switching devices  31  in such a way that they can open and close the electrical circuits associated in each case. The control device  11  preferably comprises a plurality of analog-to-digital converters, by means of which the two instantaneous current actual values  43  and the actual instantaneous position  21  can be converted into digital values. The method according to the disclosure is preferably implemented digitally. The corresponding calculations are preferably carried out in a time-discrete manner. In this case, a plurality of calculation rounds are preferably carried out continuously at a fixed time interval of, for example, 1 ms. Each individual calculation round is preferably free of feedback. The feedback which occurs, for example, in the control loops explained further below is preferably made possible in that at least one calculation result of a calculation round is used at the earliest in the next calculation round. 
     Reference is also made to the reference signs a 1 , a 2 , b 1 , b 2  and c, the corresponding signal paths being included again in  FIG.  2    as a x , b x  and c, it being possible for x to assume the value 1 or 2. It follows from this that the arrangement according to  FIG.  2    is preferably present twice, specifically once for each of the two actuation directions of the valve  10 . 
       FIG.  2    is a control diagram illustrating the method according to the disclosure. 
     The current actuator  30  comprises a pulse width modulator  34 . At the input thereof, the instantaneous current control variable  35  is present, which corresponds to the duty cycle of the pulse width modulation (https://de.wikipedia.org/wiki/Pulse Duration Modulation). This is steplessly adjustable within the context of the digital resolution. A divalent signal is present at the output of the pulse width modulator  34 , by means of which signal the switching device associated in each case (reference sign  31  in  FIG.  1   ) is switched on and off in quick succession. The corresponding switching frequency can be for example 20 kHz. This frequency is so high that the valve slide substantially does not react to this excitation, due to its inertia. In contrast, the dither frequency of for example 140 Hz is significantly lower, such that the valve slide can perform the desired micro-oscillations. 
     The instantaneous current control variable  35  is provided by a current controller  40 , which in the present case is designed as a PID controller, it also being possible for a PI or a P controller to be used. The effective current actual value  44  flows into the target-actual comparison  46  of this control loop as the actual variable, the corresponding target variable being formed from the additive superimposition  36  of the effective position control variable  27  and the dither current control variable  64 . In this case, the effective current actual value  44  is determined by low-pass filtering  45  from the instantaneous current actual value  43  already explained with reference to  FIG.  1   . The cutoff frequency of the corresponding low-pass filter  45  is preferably selected such that the current fluctuations caused by the dither are contained in the effective current actual value  44 , but the current fluctuations caused by the pulse width modulation substantially are not. The mentioned cutoff frequency is for example 1 kHz. 
     In the present case, the position controller  24 , which provides the effective position control variable  27 , is designed as a PID controller, although it is also possible for it to be designed as a PI or P controller. Its target variable is the target effective position  25 , which is preferably predetermined by the user of the valve, for example by means of an operating lever. The actual effective position  22  flows into the corresponding target-actual comparison  26  as the actual variable. This is determined by low-pass filtering  23  from the actual instantaneous position  21  explained with reference to  FIG.  1   . The cutoff frequency of the corresponding low-pass filter  23  is designed such that the micromovements of the control slide caused by the dither are substantially no longer contained in the actual effective position  21 . The mentioned cutoff frequency is for example 100 Hz. The position control  24  then operates virtually independently of the dither. Only in the subordinate current regulation  40  is the dither taken into account in the context of the target variable. 
     One special feature of the present disclosure is that the intensity of the dither is not fixedly predetermined. For the large majority of the operating time, it is selected so as to be so large that the desired micromovement of the control slide is present, but not greater. As a result, noise caused by the dither can be avoided. 
     The dither current control variable  64  is a quasi-periodic signal. For this purpose, the dither oscillator  60  provides a strictly periodic signal which can have one of the waveforms explained with reference to  FIG.  3   . This periodic signal is adjusted or modulated, with respect to amplitude, by multiplication  61 . The frequency of the dither is for example 140 Hz. The amplitude of the dither is adjusted relatively slowly, even when the regulation is not yet in its stationary state. 
     In the present case, the dither controller  63  is designed as a PID controller, although it is also possible for it to be designed as a PI or P controller. The dither controller  63  is intended to ensure that the valve slide also actually mechanically performs the desired micro-oscillations, such that the entire valve control in each operating state responds quickly to changes in the target effective position  25 . In this case, the relationship between the amplitude of the dither current control variable  64  and the actual dither amplitude  54  is strongly non-linear. In particular, the actual dither amplitude  54  suddenly becomes zero if the amplitude of the dither current control variable  64  falls below a certain limit value. This limit value is difficult to predefine, which is why the dither controller  63  preferably ensures that it is not fallen below at all. 
     For this purpose, the actual dither amplitude  54  is first determined as the actual variable of the corresponding control. In this case, the actual instantaneous position  21  undergoes band-pass filtering  50 . The center frequency of the band-pass filter  50  is preferably equal to the frequency of the dither oscillator  60 , i.e. for example 140 Hz. The actual dither amplitude  54  is the averaged peak-to-peak amplitude of this filtered signal. By means of the extreme value determination  51 , first the extreme values or the peaks of the filtered signal are determined, i.e. the values at which the filtered signal assumes its largest or its smallest value. Subsequently, the difference  52  between a maximum and an immediately following minimum is formed. A mean value  53  is then formed from a plurality of these differences, such that the actual dither amplitude  54  changes comparatively slowly. For example, averaging over three of the mentioned differences is performed. 
     In addition, the target dither amplitude  66  flows into the target-actual comparison  65  of the dither control. It would now be conceivable to set this constantly to the desired minimum dither amplitude. However, this has the consequence that, after the system has been switched on, it takes a comparatively long time until the control slide performs the desired micromovements. Therefore, it is preferred for the target dither amplitude  66  to be selected, after the system is switched on, so as to be so large that the dither movement begins safely and very quickly. Thereafter, the target dither amplitude  66  is slowly reduced to the desired minimum dither amplitude, until this is reached. Although this results in a higher noise level immediately after switching on, the position control of the valve is optimal immediately after switching on. 
     One is added  62  to the output variable of the dither controller  63 , in order to obtain the desired multiplication factor for the multiplier  61 . In addition to this simple amplitude modulation, it is possible to adjust the waveform of the dither oscillator  60 . In this case, the adjustment of the waveform results in a more sensitive actuating behavior than the adjustment of the amplitude. Preferably, therefore, first the waveform is adjusted, the amplitude being adjusted only if the effect of the waveform adjustment is not sufficient to correct the system. This situation is intended to be indicated by the dashed line  70  in  FIG.  2   . 
       FIG.  3    is a graph of different waveforms  71 ;  72 ;  73  of the dither current control variable  64 . The time t is plotted on the horizontal, an entire oscillation period T and a half oscillation period T/2 being marked. The amplitude x of the waveform, which is standardized to +1 or −1, is plotted on the vertical. All the waveforms  71 ;  72 ;  73  are preferably point-symmetrical with respect to the zero crossing at T/2. The mathematical relationship x(t)=−x(t+T/2) preferably furthermore applies. 
     The first waveform  71  is shown by a solid line. It comprises exclusively linear portions. It could be characterized as a square wave oscillation in which the signal change rate is limited upwards. Due to its sharp corners and the associated harmonics, the first waveform  71  causes the strongest oscillation excitation of the valve slide. 
     The second waveform  72  is shown as a dashed line where it deviates from the first waveform  71 . It causes a weaker excitation than the first waveform  71 , but a stronger excitation than the third waveform  73 . The reason for this is that the corners of the first waveform  71  are somewhat rounded and/or beveled, such that the harmonic content is reduced relative to the first waveform  71 . 
     The third waveform  73  is shown as a dot-dash line where it deviates from the first waveform  71 . The mentioned corners are even more rounded or beveled than in the case of the second waveform  72 , such that the third waveform  73  brings about the smallest excitation of the three waveforms  71 ;  72 ;  73  shown. 
     Of course, further waveforms can be provided according to the same pattern, which waveforms differ in their excitation effect at the same amplitude. For example, seven different waveforms are used to adjust the dither excitation as finely as possible. 
     The different waveforms  71 ;  72 ;  73  are preferably stored in the form of value tables in the control device (no.  16  in  FIG.  1   ). 
     REFERENCE SIGNS 
       10  Valve 
       11  Control slide 
       12  Zero position 
       13  Spring 
       14  Solenoid 
       15  Pilot valve 
       16  Control device 
       20  Position sensor 
       21  Actual instantaneous position (c) 
       22  Actual effective position 
       23  First low-pass filter 
       24  Position controller 
       25  Target effective position 
       26  Target-actual comparison 
       27  Effective position control variable 
       30  Current control device 
       31  Switching device 
       32  Supply voltage 
       33  Ground 
       34  Pulse width modulator 
       35  Instantaneous current control variable 
       36  Superimposition 
       40  Current controller 
       41  Current sensor 
       42  Measuring resistor 
       43  Instantaneous current actual value (b1; b2) 
       44  Effective current actual value 
       45  Second low-pass filter 
       46  Target-actual comparison 
       47  Target effective current 
       50  Band-pass filter 
       51  Extreme value determination 
       52  Difference formation 
       53  Averaging 
       54  Actual dither amplitude 
       60  Dither oscillator 
       61  Multiplier 
       62  Amplification factor determination 
       63  Dither controller 
       64  Dither current control variable 
       65  Target-actual comparison 
       66  Target dither amplitude 
       71  First waveform 
       72  Second waveform 
       73  Third waveform