Abstract:
Parallel, pulse-width-modulation controlled three-way valves each both supply and exhaust a pressure chamber of a fluid actuator. The phase difference of the oscillator carrier waves of the pulse width modulation input circuit effectively increases the frequency of a net actuator carrier wave so as to quicken the response of the actuator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a control apparatus for an actuator, and, more particularly, to a control apparatus for an actuator which is operated by controlling fluid compression. 
     In accordance with one known type of actuator which is operated by compressing fluid, there consists, for example, a single acting type fluid pressure cylinder wherein fluid pressure acts on one side of a piston and wherein the piston is returned by its own weight or a load or spring force. An example of means to control the feed of the fluid pressure with respect to the single acting type fluid pressure cylinder employs a three-way valve as described in the publication `Fluid Power Control` published in the U.S. in 1960, pp. 527-540. Regarding this, in a case where an actuator that is provided with an actuator driver having a long rise time or dead time is driven by a pulse width modulated control system, for example, in a case where an actuator such as single acting type air cylinder is driven according to the pulse width modulation control by an actuator driver such as proportional valve or solenoid valve, the period of a carrier wave must be several times longer than the response of the actuator driver. This has led to the problem where the responsiveness of the actuator to an input signal worsens drastically. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a control apparatus for an actuator which can result in enhancing the speed of the controlled response. 
     The present invention for accomplishing the object consists of a control apparatus for an actuator wherein a driver for the actuator is controlled in accordance with an error signal between a pressure reference signal and a pressure feedback signal of the actuator and wherein an operation of the actuator is controlled by a compressed fluid from the actuator driver, in a control apparatus for an actuator comprising a plurality of drivers for the actuator which are connected to the actuator in parallel with each other, a pressure reference signal generator which outputs a reference voltage signal corresponding to a pressure reference signal, a pressure sensor which detects a pressure of the actuator to output a pressure signal, an operational amplifier which produces an error voltage signal based on the difference between the reference voltage signal from said pressure reference signal generator and the pressure signal from said pressure sensor, an oscillator which outputs carrier wave signals corresponding to the actuator drivers, with a phase shift therebetween, and pulse width modulation circuits which respectively supply said actuator drivers with pulse trains having duty ratios proportional to the error voltage, on the basis of the corresponding carrier wave signals from said oscillator and the error voltage signal from said operational amplifier, whereby a frequency of a carrier wave is equivalently raised for the actuator so as to control it with quick response. 
     Other objects, features and advantages of the present invention will become apparent from the following description of embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit arrangement diagram of an aspect of performance of an apparatus according to the present invention; 
     FIG. 2 is a diagram showing the signal waveforms of portions in an embodiment of the apparatus of the present invention shown in FIG. 1; 
     FIG. 3 is a circuit arrangement diagram of another embodiment of the apparatus of the present invention; and 
     FIG. 4 is a diagram showing the signal waveforms of portions in another embodiment of the apparatus of the present invention shown in FIG. 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the drawings. 
     FIG. 1 shows one embodiment of a control apparatus for an actuator according to the present invention. Referring to the figure, the actuator 1 is a single acting cylinder in which a fluid pressure acts on a fluid chamber 1B located on one side of a piston 1A. Branched pipe lines 5A and 5B are connected to the fluid chamber 1B of the actuator 1. These branched pipe lines 5A and 5B are respectively provided with control valves 13 and 23 for controlling fluid pressures P s  from a fluid pressure source. By way of example, the control valves 13 and 23 are ON-OFF type three-way valves. The actuator 1 is furnished with a pressure sensor 2 for detecting a pressure in the fluid chamber 1B. A pressure reference signal generator 6 outputs a reference voltage e r  corresponding to a pressure reference signal. An operational amplifier 4 produces an error voltage e e  which is proportional to the difference between the reference voltage e r  and an output e p  from the pressure sensor 2. An oscillator 3 outputs triangular wave signals 1031 and 1032 which are carrier wave signals having a phase shift of 180 degrees therebetween. A comparator 11, corresponding a pulse width modulation circuit, generates a pulse train e e1  having a duty ratio proportional to the error voltage e e  on the basis of the triangular wave signal 1031 from the oscillator 3 and the error voltage e e  from the operational amplifier 4, while a comparator 21 generates a pulse train e e2  having a duty ratio proportional to the error voltage e e  on the basis of the triangular wave signal 1032 from the oscillator 3 and the error voltage e e  from the operational amplifier 4. Power amplifiers 12 and 22 control the control valves 13 and 23 ON and OFF in accordance with the pulse trains e e1  and e e2  from the comparators 11 and 21, respectively. 
     Next, the operation of the above embodiment of the apparatus of the present invention will be described. 
     The oscillator 3 generates the triangular wave signals 1031 and 1032 as carrier wave signals having a relative phase shift of 180 degrees with respect to each other, and it inputs them to the comparators 11 and 21 as reference signals, respectively. The operational amplifier 4 operates the error voltage e e  which is proportional to the difference between the voltage e r  of the pressure reference signal generator 6 corresponding to the pressure reference signal and the output e p  of the pressure sensor 2, and it inputs the operated error voltage to the comparators 11 and 21. These comparators 11 and 21 compare the carrier wave signals 1031 and 1032 with the error voltage respectively, to generate the pulse trains e e1  and e e2  which have the duty ratios proportional to the error voltage e e  and which turn ON and OFF the control valves 13 and 23 through the power amplifiers 12 and 22. 
     Examples of waveforms of the respective voltages in the above operation are shown in FIG. 2. In this figure, (a) illustrates the error voltage e e , and (b) and (c) illustrate the carrier wave signals 1031 and 1032 in each of which the error voltage e e  is depicted by a dot-and-dash line to clarify the corresponding relation. (d) and (e) illustrate the outputs e e1  and e e2  of the comparators 11 and 21. 
     Owing to the arrangement stated above, the control valves 13 and 23 are individually controlled by the carrier waves having the phase shift from the pulse width modulation circuits. The effective opening ratio of the control valve 13 or 23 is proportional to the duty ratio of the pulse train e e1  or e e2  corresponding to the output of the pulse width modulation circuit, and is updated every cycle of the corresponding carrier wave signal 1031 and 1032. Thus, an effective opening ratio for the actuator 1 becomes the sum between the effective opening ratios of both the control valves 13 and 23. More specifically, in case of driving the actuator with a single control valve, only a responsibility longer than the cycle of a carrier wave signal can be attained. In contrast, the control valves 13 and 23 which are actuated by the carrier waves having the phase shift can equivalently raise the frequency of a carrier wave for the actuator 1 and enhance the responsibility of the control thereof. 
     FIG. 3 shows another embodiment illustrative of the practicable arrangement of the apparatus of the present invention, in which the same portions as in FIG. 1 are indicated by identical symbols. Numeral 103 designates a pulse width modulation input circuit, numeral 104 a digital subtractor, numeral 105 and A/D converter, and numerals 111 and 121 decrement counters. The pulse width modulation input circuit 103 outputs rectangular wave signals 1131 and 1132 which are carrier wave signals having a phase shift of 180 degrees therebetween, and which are respectively applied to pulse generator circuits 113 and 123 to produce pulses E sp1  and E sp2  synchronous with the rising edges of the rectangular waves. The respective pulses E sp1  and E sp2  preset the counters 111 and 121, and simultaneously set flip-flops 114 and 124. The error value E e  between a pressure feedback value E p  and a reference value E r , namely, the output of the subtractor 104 preset in the counters 111 and 121 is immediately counted down according to a clock, to reset the flip-flops 114 and 124 simultaneously with the generation of borrow pulses. That is, the outputs e e1  and E e2  of the flip-flops 114 and 124 become pulse trains whose widths are proportional to the error E e  and which turn ON and OFF the control valves 13 and 23 through the power amplifiers 12 and 22. 
     FIG. 4 shows examples of waveforms of the signals at the various parts stated above. In the figure, (a) and (b) illustrate the rectangular waves 1131 and 1132 generated by the pulse width modulation input circuit 103, and (c) and (d) the pulses E sp1  and E sp2  which the pulse generator circuits 113 and 123 generate at the rising edges of the rectangular waves 1131 and 1132. The waveforms (e) and (f) illustrate the situations of countdown of the counters 111 and 121 in terms of analog quantities, in each of which a dot-and-dash line indicates the error E e  that is preset in accordance with the pulses E sp1  (c) and E sp2  (d). Waveforms (g) and (h) illustrate the borrow pulses E ep1  and E ep2  which the counters 111 and 121 generate. Waveforms (i) and (j) illustrate the outputs E e1  and E e2  of the flip-flops 114 and 124 which are set by the set pulses E sp1  (e) and E sp2  (d) and reset by the borrow pulses E ep1  (g) and E ep2  (h). 
     Even when the apparatus is arranged as in this embodiment, an effect similar to that of the foregoing embodiment can be attained. 
     Although each of the above embodiments has illustrated the example in which the two control valves are arrayed in parallel, a similar effect can be produced also in case of arraying three or more control valves in such a way that driver circuits are disposed for the respective control valves, that the number of reference signals to be generated by an oscillator is equalized to the number of the control valves and that the magnitudes of phase shifts among the signals are made smaller. For example, when three control valves are employed, the pase difference between the adjacent ones of three carrier wave signals is set at 120 degrees. 
     Besides, although the embodiments are illustrative of an example in which the frequencies of the reference signals being generated by the oscillator are equal, a similar effect can be produced even with unequal frequencies. 
     Further, the actuator is not restricted to the single acting cylinder, but the invention is also applicable to an actuator made of an elastic cylindrical body. In addition, the control valves are not restricted to the ON-OFF type, but three-way valves of the proportional type may well be employed. 
     According to the present invention, the responsiveness of the operation control of an actuator of long rise time or dead time can be enhanced.