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An electro-hydraulic proportional actuator in which the actuator piston position is hydraulically controlled by fluid flow through solenoid valves operable by an electronic control system. An operator control provides an input voltage representing a desired piston position and a transducer generates

An electro-hydraulic proportional actuator in which the actuator piston position is hydraulically controlled by fluid flow through solenoid valves operable by an electronic control system. An operator control provides an input voltage representing a desired piston position and a transducer generates a voltage representing the actual piston position. The difference between the input and output voltages generates a difference or error signal that is fed into a feedback system including a sample-and-hold circuit. A clock logic system is provided for transmitting the signal from the feedback system to the solenoid valves at predetermined intervals to activate either valve to move the actuator piston in either direction or to activate both valves to hold the piston at a desired position.

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1. An electro-hydraulic proportional actuator, comprising: an actuator assembly having a double-acting, linearly-movable piston contained in a cylinder having a chamber at each end; a pair of three-way solenoid valves each of which has an actuator port, an inlet port and an outlet port; each en

1. An electro-hydraulic proportional actuator, comprising: an actuator assembly having a double-acting, linearly-movable piston contained in a cylinder having a chamber at each end; a pair of three-way solenoid valves each of which has an actuator port, an inlet port and an outlet port; each end chamber of the actuator assembly cylinder being coupled to the actuator port of one of the valves, and each valve inlet port being coupled through a check valve to a fluid pressure source and the return ports being coupled to each other with the fluid flow therefrom being routed to a flow control valve; and an electronic control system for operating the solenoid valves to direct fluid flow into and out of the cylinder chambers, comprising: an operator control means providing an input voltage representing a desired actuator assembly piston position, a transducer means coupled to the actuator assembly to generate an output voltage representing the actual position of the piston, the difference between the input and output voltages generating a difference signal representing the difference between the actual and desired positions of the piston, a feedback system including a sample-and-hold circuit in a feedback path having an input to receive the difference signal, and a clock logic means for transmitting the difference signal from the feedback system to the solenoid valves at predetermined intervals to activate either valve to move the actuator piston in either direction or to activate both valves to hold the piston at a desired position. 2. An electro-hydraulic proportional actuator according to claim 1, in which: the electronic control system further comprises an "OR" gate having an output and two inputs; the clock logic means includes a clock means and a digital control means; the clock means provides a sampling frequency to one input of the "OR" gate; and the digital control means has an input coupled to the output of the sample-and-hold circuit and outputs coupled in parallel to the solenoid valves and to the second input of the "OR" gate; the output of the "OR" gate being coupled to an input of the sample-and-hold circuit. 3. An electro-hydraulic proportional actuator according to claim 1, in which: the electronic control system further comprises an "OR" gate having an output and two inputs and an "EXCLUSIVE-OR" gate having an output and two inputs; the clock logic means includes a clock means and a digital control means; the clock means provides a sampling frequency to one input of the "OR" gate; and the digital control means has an input coupled to the output of the sample-and-hold circuit and outputs coupled in parallel to the solenoid valves and to the inputs of the "EXCLUSIVE-OR" gate; the output of the "EXCLUSIVE-OR" gate being coupled to the second input of the "OR" gate. 4. An electro-hydraulic proportional actuator according to claim 2 or 3, in which: the flow control valve coupled to the outlet ports of the three-way solenoid valves limits the velocity of movement of the actuator piston. 5. An electro-hydraulic proportional actuator comprising: an actuator assembly having a double-acting linearly movable piston contained in a cylinder having a chamber at each end; a pair of three-way solenoid valves each of which has an actuator port, an inlet port, and an outlet port; each end chamber of the actuator assembly cylinder being coupled to the actuator port of one of the valves; and each valve inlet port being coupled through a check valve to a fluid pressure source and the return ports being coupled to each other with the fluid flow therefrom being routed to a flow control valve; and an electronic control system for operating the solenoid valves to direct fluid flow into and out of the cylinder chambers, comprising; an operator controlled means for providing an input voltage V 1 representing a desired actuator assembly piston position, a transducer means coupled to the actuator assembly to generate an output voltage Vv representing the actual position of the piston, the difference between the input V 1 and output Vv voltages forming a difference signal Ve 2 representing the difference or error between the desired and actual positions of the actuator piston, a sample-and-hold circuit having an input to which the Ve 2 signal is coupled, a clock logic means including a clock means providing a sampling frequency coupled through first input of an "OR" gate having an output and two inputs to an input of the sample-and-hold circuit, the output of the "OR" gate comprising a sample-and-hold circuit, the output of the "OR" gate comprising a sample-and-hold circuit input control signal V sh, the sample-and-hold circuit passing the Ve 2 signal when the V sh signal is in a logic "1" state and retaining the last value of Ve 2 when the V sh returns to a logic "0" state, during a sampling instant, when the clock means sampling frequency pulse is high or in a logic "1" state, the control system operating to turn either solenoid valve on or off so as to move the piston in either direction or to hold the piston at a desired position until the next sampling instant. 6. An electro-hydraulic proportional actuator according to claim 5, in which: the clock logic means includes a digital control means, one input of which is coupled to the output of the sample-and-hold circuit and two outputs of which are coupled in parallel to the solenoid valves and the inputs of an "EXCLUSIVE-OR" gate; the output of the "EXCLUSIVE-OR" gate being coupled to the second input of the "OR" gate; the "EXCLUSIVE-OR" gate ensuring the frequency of solenoid operation to be the same as clock frequency and no faster, whereby the life of the solenoids is extended especially at low frequency input signals generated by an operator. 7. An electro-hydraulic proportional actuator according to claim 6, in which: the digital control means has a circuit corresponding to the following logic table: [TABLE] ______________________________________ DIRECTION SOLENOIDS OF PISTON V 1 V e2 15 16 MOVEMENT ______________________________________ |V 1 | < V 3 ALL 0 0 HOLD IN NEUTRAL POSITION |V 1 | > V 3 |V e2 | < V 4 1 1 HOLD IN ANY POSITION OTHER THAN NEUTRAL |V 1 | > V 3 V e2 > V 4 1 0 RIGHT |V 1 | > V 3 V e2 < -V 4 0 1 LEFT ______________________________________ in which for the solenoids: 1 is "On", 0 is "Off", V 3 sets the neutral deadband and V 4 sets the deadband outside neutral; the circuit producing digital output signals for activating the solenoids in accordance with the predetermined values of the neutral deadband and the deadband outside of the neutral zone and in conjunction with the operator controlled input position values. 8. An electro-hydraulic proportional actuator according to claims 5 or 7, in which: the flow control valve coupled to the outlet ports of the solenoid valves limits the velocity of movement of the actuator piston.