Keywords: piezo bender, hysteresis, linearization, neural network.

Piezoelectric element can be implemented as a micro drive in many precise applications. One of them is the implementation of a piezo bender actuator in the control stages of electrohydraulic servo valves, substituting torque motors. A piezo bender actuator is a flat beam-shaped, unilaterally mounted electromechanical drive. Consequently, the delineation of its free beam-end is ambiguous, and depends on the driving voltage as well as the retroactive status. The character of the piezo actuator performance applied to the control stage (the frequency limit reaching 200 Hz) of electrohydraulic servo valves permitted focusing on quasi-static non-linearity compensation A distortion having the biggest impact on the linearity of quasi-static characteristic is hysteresis. The compensating system, proposed in this paper, applies inverse profiling of the investigated drive developed through outer hysteresis loop transformation.

The processing of the modelling data, collected during the experimental drive testing, resulted in the determination of the quasi-static profile family, constituting a defined set of drive responses to the periodic constraints of various amplitudes. Subsequently, a hysteretic loop of the piezo bender controlled by a signal of maximal allowed amplitude (no system saturation) was selected. The loop was then divided into two arms, corresponding to the increasing and decreasing driving signal. Each point of both arms was the subject of calculation of the inverse function and then was used as learning data for two neural networks (for the increasing and decreasing driving signal).

The scaled response of the neural networks was added to the value of the linear function in the compensating system. The neural network input signal and direction factors of the linear function was based on the input signal monotonicity. Each consecutive local point of the input signal extreme was denoted as the turning point. Only the last turning point was saved by the system during the compensation progress. According to the input signal direction variation, the direction factors of the linear function crossing the last turning point and the minimum (for decreasing signal) or the maximum (increasing signal) of the hysteretic loop, were determined.

Simulations of the piezo bender hysteresis model responses to the compensated quasi-static sinusoidal signal (frequency 0.5Hz) were conducted. The real piezo bender actuator behaviour subject to the periodic signal with the compensating system operating as a real time machine based on Matlab - Simulink Real Time Workshop was examined.

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