Controller for a pulsed servovalve

[57] ABSTRACT

An exhaust and pressure solenoid is operated by a circuit which generates a repetitive sawtooth signal waveform upon which is superimposed an input representing a desired control signal for regulating the flow of an operating fluid to a device such as a tension controller or brake. The orifices of the exhaust and pressure solenoids are duty cycle modulated at the repetitive frequency of the sawtooth signal and their respective orifice sizes are varied in accordance with the input signal.

Claims, 4 Drawing Figures

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Patented Moy 2, 1972

3,669,631

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OLI

3,659,631

CONTROLLER FOR A PULSED SERVOVALVE

This invention relates to control systems and, in particular, to a circuitry for generating signals to control the effective orifice size of a solenoid servovalve wherein the valve member is duty cycle modulated or oscillated.

The control circuitry of this invention actuates servovalves for controlling the ingress and egress of a hydraulic or a pneumatic fluid by digital techniques without the need for converting digital signals into analog control signals. In conventional servovalve control systems, a mechanical member is moved from one position to another position to effect a change ofthe orifice size.

A primary advantage of such a digitally controlled servovalve is the elimination of the need for digital-to-analog conversion apparatus so that such a valve may be directly controlled by digital signals emitted from a control device. The control system enables simple on/off valve elements to be used, thereby enhancing reliability because the failure of one element results only in degraded over-all servovalve performance.

A feature of the control system is that duty cycle modulation is used to obtain infinite resolution ofthe effective orifice size from zero to its maximum opening. That is, an orifice of one thirty-secondth sq. in. which is closed 75 percent of the ²⁵time and open 25 percent ofthe time has an effective average orifice size of one one hundred-twenty-eighth sq. in. Therefore, since the on/off element may be fully closed or fully opened, or operated at any ratio of closed time to open time, it can be operated at any effective average orifice size from zero to the maximum opening, for example, one thirty-secondth sq. in. The infinitely variable regulation of the duty cycle advantageously provides a very high resolution of the effective orifice size, and therefore the control in accordance with this invention may be used in a proportional closed loop or servo system.

Further, if the frequency modulation in the control system is higher than the frequency response of other elements in the system, then the duty cycle modulated servovalve may provide system performance equal to a conventional servovalve with respect to frequency response, deadband, friction, drift, and resolution.

In accordance with the invention, a generator provides a sawtooth waveform output at a fixed frequency. In an alternative embodiment the frequency of the sawtooth waveform may be modulated by a sweep frequency. The sawtooth waveform is combined with a control signal which may be analog, digital, a.c., d.c., or a hybrid signal. The control signal varies the reference level of the sawtooth waveform and the combined signal is split and fed to each of two similarly constructed channels. The signal in each channel is compared with a reference threshold signal which is of different amplitude in each channel. The reference level difference is preferably selected so that one reference level forms an upper limit and the other reference level forms a lower limit for the combined signal. The signal in each channel is compared with its respective reference threshold level and if the signal level exceeds the reference threshold level, the resultant output is used to excite the control coil of a servovalve in each channel. This operation produces a pulsing or dithering ofthe valve in each channel and appropriate control of each valve by duty cycle modulation.

It is an object of this invention to provide an improved control circuit for regulating the effective orifice opening of a servovalve.

It is yet a further object of this invention to provide an improved circuit for controlling a servovalve to improve its drift and resolution and to decrease its deadband and friction.

It is still another object to provide an improved circuit ofthe type specified for controlling a pneumatically or hydraulically operated system by modulating the duty cycle of a servovalve member.

And yet a still further object is to reduce or eliminate beating between the modulated control signals and other frequen2 cies existent in the type control systems to which the invention relates.

The foregoing objects and advantages will be better unwhere^ ^W*^{th reference t0 the accom}Panying drawings

FIG. 1 illustrates an embodiment of the digital control circuitry;

FIGS. 2a-2c illustrate the relationship between the com> bined sawtooth waveform and input control signal and the reference level in each of two channels of the circuit illustrated in FIG. 1;

FIG. 3 shows a sweep generator circuit which may be used to drive the circuit of FIG. 1 in a modified embodiment of the invention; and

FIG. 4 is a cross-sectional view of a servovalve which is controlled by the circuit illustrated in FIG. 1.

With reference to FIG. 1, the conduction of transistors 10 12 produces a rising potential at 14 through resistor 16 to ground. The collector current of transistor 12 charges capacitor 18 to produce a ramp voltage input to the base 20 of driver transistor 22. When capacitor 18 charges to the firing potential of unijunction transistor 24, it produces a positive spike on resistor 26, thereby discharging capacitor 18. Transistors 10 and 12 remain conductive. Transistor 10 is a temperature compensating transistor for the current source provided by transistor 12. Capacitor 18 discharges through resistor 26 and the aforementioned cycle is repeated as the potential at 14 becomes sufficiently negative to render transistors 10,12 conductive. The ramp or staircase signal across capacitor 18 is amplified by transistor 22 and provided to voltage divider 28 from the emitter 30 of transistor 22 via emitter resistor 32 and divider resistor 34. The components ofthe sawtooth generator circuit are selected to provide the desired sawtooth waveform output signal to divider 28 in accordance with characteristics of the system. For example, the frequency of the sawtooth waveform may be in the range consistent with the charactenstics of the servovalve.

A signal input, for example, from a controller, is applied at terminals 36,38. Terminal 38 is grounded and the input signal at 36 is superimposed with the aforedescribed sawtooth signal at junction 40 of voltage divider 28 through resistor 42. The combined singai is split into two channels and fed respectively 45 to high gain operational amplifiers 44,46 through resistors 48 SO. The network of resistors 52, 54, 56, 58, 60, 62 establishes different threshold operating potentials 49,51 for each of high gain amplifiers 44,46, respectively. Thus, high gain amplifiers 44, 46 function as comparator circuits in an on/off mode. The positive input to comparator 44 is connected to threshold potential 49 as is the negative input to comparator 46 connected to threshold potential 51. The combined input signal is then respectively applied to the negative and positive inputs of these same comparators. Thus signals more positive than threshold potential 49 are compared by comparator 44 and signals more negative than threshold potential 51 are compared by comparator 46.

The output from comparator 44 is supplied to transistor 64 via resistor 66 and diode 68. Transistors 64 and 70 are connected in a well-known Darlington circuit to provide current amplification of the output from comparator 44 to drive coil 72 of solenoid 74. Resistor 76 and diode 78 are connected in paralie¹ across coil 72 to dampen fly-back voltages across coil 72. Similarly, the output from comparator 46 is fed to transistor 80 through resistor 82 and diode 84. Transistors 80 and 86 also are connected in a Darlington circuit to provide current amplification of the output of comparator 46 to drive coil 88 of solenoid 90. Resistor 92 and diode 94 perform the same function as described above for resistor 76 and diode 78. Solenoid 74 represents an exhaust solenoid and solenoid 90 may represent a pressure solenoid whereby pneumatic or hydraulic fluid may be exhausted from, or applied to, a desired system such as a brake for a controller in accordance with the signal input from the controller, at 36.

3,659,631

FIG. 2a represents the sawtooth waveform in the case where the input at 36 is zero and, as illustrated, the waveform lies between threshold potentials 49, 51. The reference threshold potentials may be raised or lowered to obtain different operating conditions of the controlled servovalve mechanisms. For example, lowering each of the threshold potentials increases the duty cycle operation of the servovalves, whereas increasing the threshold potentials causes the duty cycle to decrease. It is apparent that the sensitivity and deadband are also affected by the particular setting of threshold potentials 49,51.

FIGS. 2b, 2c respectively represent cases wherein the level of the combined signal input at 40 has been raised and lowered so that the peak portions of the sawtooth waveform exceed the threshold potentials. The waveform shown in FIG. 2b will result in a duty cycle modulation of solenoid 74 in FIG. 1, whereas the waveform shown in FIG. 2c will provide a duty cycle modulation of solenoid 90. In an actual system the input signal at point 36 of FIG. 1 will vary in accordance with a desired control condition of the system operated by actuation of solenoids 74, 90. The signal input at 36 may be analog, or an a.c. or d.c. signal, or a hybrid signal representing a combination of signal types.

FIG. 3 illustrates a sweep generator circuit which may be used to modify the sawtooth waveform at 40 in the following manner. There are applications in which it is desirable to reduce or eliminate the effects of beating between the frequency of the sawtooth waveform with a resonant or other characteristic condition of the circuit which is controlled by the circuit of FIG. 1. Beating may be reduced or eliminated by βθ sweeping the sawtooth output from, for example, 10 to 25 Hz at a 1 Hz rate. The sawtooth generator of FIG. 1 will produce such an output if a ramp input of 1 Hz repetition rate is applied to control the conduction of transistors 10 and 12. The 1 Hz repetitive sawtooth waveform may be produced by the cir- 35 cuit illustrated in FIG. 3. This circuit functions essentially in the same manner as the previously described sawtooth generator wherein transistors 100 and 102 charge capacitor 104 to provide a ramp input to the base 106 of driver transistor 108. Unijunction transistor 110 fires when the potential at capaci- 40 tor 104 reaches its firing potential and discharges capacitor 104, with the resultant positive spike on resistor 107. Transistors 100, 102 remain conductive; transistor 100 provides temperature compensation, and transistor 102 is a constant current source. The aforementioned cycle is repeated 45 and the components may obviously be selected to provide the necessary 1 Hz repetition rate. The output from the emitter of transistor 108 is applied as the sweep generator input to the sweep generator circuit of FIG. latpoint 114.

FIG. 4 shows, a typical solenoid or servovalve which is represented by solenoid 74, 90 in FIG. 1. A pneumatic or hydraulic input is provided at port 120 of solenoid 122 and an exhaust is obtained at port 124. Magnetically inductive slug 126 includes valve seat 128 which is spring biased by spring 130 to seat against opening 132. Slug 126 is withdrawn from seat 128 by the application of a current to coil 134. Application of a pulsating current to coil 134 will cause slug 126 and valve seat 128 to oscillate or be duty cycle modulated, thereby effectively controlling the orifice opening at 132.

Typical component values and types for the circuit of FIG. 1 are listed in Table I.

Resistor 42

Resistor 48

Resistor 50

Resistor 52

Resistor 54

Resistor 56

Resistor 58

Resistor 60

Resistor 62

Resistor 64

Resistor 67

Resistor 69

Resistor 76

Resistor 82

Resistor 83

Resistor 85

Resistor 92

Transistor 10, 12

Transistor 22

Transistor 24

Transistor 64, 80

Transistor 70, 86

Amplifier 44, 46

Capacitor 18

Diode 68, 84

Diode 78, 94

Solenoid 74,90

Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm· Ohm·

18K 100K 100K 100K 887 13K 10K 3.92K 100K 4.7K 2.2K 560 100 4.7K 560 2.2K 100 2N4917 2N5306 2N2646 2N656 2N3O55

2P15A output/input 50K .47 uf, 50V 1N4148 1N482O

Allied Control Co., Inc. No. 21383

1% 1% 1% 1%

TABLEI

Component Values For Circuit of Figure 1

Component Value or Type

In a practical application, illustrative of the use of the aforedescribed control circuit, solenoids 74, 90 respectively are connected to the exhaust and pressure ports of a brake mechanism which is controlling the tension of a paper feed ' roller for a printing press. The brake mechanism is controlled by the aforedescribed operation of solenoids 74, 90 to regulate the egress and ingress of air to the brake mechanism in accordance with input signals at 36 (FIG. 1) from a tension controller associated with the paper feed roll. Such apparatus is well known to those skilled in the art and requires no further description to enable the control circuit of this invention to be used. Other applications of the control circuit, as well as modifications of the circuit described herein, will be apparent to those having familiarity with control system techniques.