Proportional electro-hydraulic valves: from analogue to digital control

Proportional electro-hydraulic valves are ubiquitous as flow actuators in hydraulic systems. Flow regulation is the result of the accurate positioning of a spool driven by a solenoid and a position sensor, usually a Linear Variable Differential Transformer. The overall control consists of two hierarchical loops: the inner loop is the solenoid current regulator with a closed-loop bandwidth close to 1 kHz. A model-based digital regulator of this kind has been presented elsewhere: requirements and performance are here reminded. The outer loop is a position tracking control, in charge of an accurate positioning of the spool with respect to the valve openings. The paper addresses the outer loop and concentrates on the conversion of an existing industrial analogue controller into a digital one. The analogue controller is a nonlinear proportional, integrative and derivative controller including a second-order derivative, and is capable of recovering a dead-band hysteresis. The digital conversion provides the necessary position derivatives through a state predictor, in order to withstand the 5-kHz Nyquist limit of the power supplier. As such it departs from traditional conversions dating back to more than ten years ago. The digital control law is fed by the state predictions and repeats the analogue control law with some improvements. Preliminary experiments prove that the conversion repeats and improves analogue performance. Some flaws of the resulting digital controller are outlined and discussed in view of a model-based conversion.

Proportional electro-hydraulic valves: from analogue to digital control

Proportional electro-hydraulic valves are ubiquitous as flow actuators in hydraulic systems. Flow regulation is the result of the accurate positioning of a spool driven by a solenoid and a position sensor, usually a Linear Variable Differential Transformer. The overall control consists of two hierarchical loops: the inner loop is the solenoid current regulator with a closed-loop bandwidth close to 1 kHz. A model-based digital regulator of this kind has been presented elsewhere: requirements and performance are here reminded. The outer loop is a position tracking control, in charge of an accurate positioning of the spool with respect to the valve openings. The paper addresses the outer loop and concentrates on the conversion of an existing industrial analogue controller into a digital one. The analogue controller is a nonlinear proportional, integrative and derivative controller including a second-order derivative, and is capable of recovering a dead-band hysteresis. The digital conversion provides the necessary position derivatives through a state predictor, in order to withstand the 5-kHz Nyquist limit of the power supplier. As such it departs from traditional conversions dating back to more than ten years ago. The digital control law is fed by the state predictions and repeats the analogue control law with some improvements. Preliminary experiments prove that the conversion repeats and improves analogue performance. Some flaws of the resulting digital controller are outlined and discussed in view of a model-based conversion.

Digital current regulator for proportional electro-hydraulic valves featuring unknown disturbance rejection

Solenoid current regulation is well-known and standard in any proportional electro-hydraulic valve. The goal is to provide a wide-band transfer function from the reference to the measured current, thus making the solenoid a fast and ideal force actuator within the limits of the power supplier. The supplier is usually a Pulse Width Modulation (PWM) amplifier fixing the voltage bound and the Nyquist frequency of the regulator. Typical analogue regulators include three main terms: a feedforward channel, a proportional feedback channel and the electromotive force compensation. The latter compensation may be also accomplished by integrative feedback. Here the problem is faced through a model-based design (Embedded Model Control), on the basis of a wide-band embedded model of the solenoid which includes the effect of eddy currents. To this end model parameters must be identified. The embedded model includes a stochastic disturbance dynamics capable of estimating and correcting the electromotive contribution together with the model parametric uncertainty, variability and state dependence. The embedded model which is fed by the measured current and the supplied voltage becomes a state predictor of the controllable and disturbance dynamics. The control law combines a reference generator, state feedback and disturbance rejection to dispatch the PWM with the appropriate duty cycle. Modeling, identification and control design are outlined together with experimental result. Comparison with an existing analogue regulator is also provided

Digital current regulator for proportional electro-hydraulic valves featuring unknown disturbance rejection

Solenoid current regulation is well-known and standard in any proportional electro-hydraulic valve. The goal is to provide a wide-band transfer function from the reference to the measured current, thus making the solenoid a fast and ideal force actuator within the limits of the power supplier. The supplier is usually a Pulse Width Modulation (PWM) amplifier fixing the voltage bound and the Nyquist frequency of the regulator. Typical analogue regulators include three main terms: a feedforward channel, a proportional feedback channel and the electromotive force compensation. The latter compensation may be also accomplished by integrative feedback. Here the problem is faced through a model-based design (Embedded Model Control), on the basis of a wide-band embedded model of the solenoid which includes the effect of eddy currents. To this end model parameters must be identified. The embedded model includes a stochastic disturbance dynamics capable of estimating and correcting the electromotive contribution together with the model parametric uncertainty, variability and state dependence. The embedded model which is fed by the measured current and the supplied voltage becomes a state predictor of the controllable and disturbance dynamics. The control law combines a reference generator, state feedback and disturbance rejection to dispatch the PWM with the appropriate duty cycle. Modeling, identification and control design are outlined together with experimental result. Comparison with an existing analogue regulator is also provide

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