Prototype Drive and Modulation Concepts for DSP-Controlled Ultrasonic Motors powered by Resonant Converters

The driving principle of travelling wave ultrasonic motors (USM) is basically different from those of electro- magnetic motors and requires thus tailored control schemes. This paper deals with newly developed analog and digital modulation concepts to drive a two-phase resonant converter, allowing a feeding of the USM with variable frequency, amplitudes and phase. A flexible experimental set-up for ultrasonic drives controlled by a DSP is presented including a suitable digital measuring system in order to realize efficiently control algorithms for the high level prototype drive. Recently, USMs have attracted considerable attention as a new type of actuator in servo-systems. USMs combine features such as high driv- ing torque at low rotational speed, high holding torque without an applied electric power, extremely low noise in operation, simple mechanical design and fast response. In a travelling wave type USM two orthogonal vibration modes are excited to resonance by piezoelectric actuators. Due to the travelling wave the surface points of the stator perform an elliptic motion driving the rotor by frictional force, see (1),(2). The electrical excitation of motor vibrations is applied by a two-phase inverter adding two series inductors and using the capacitance of the piezoceramics. Thus the motor becomes integral part of a resonant converter (3). In practice, the inherent mechanical characteristic of USMs is widely nonlinear and time variant and change largely according to temperature effects due to operation conditions. Therefore the two-phase resonant converter feeding the USM must be driven by an adequate quantity of control parameters such as adjustable frequency, voltage amplitudes and phase difference to attain the best motor performance, especially in servo-system applications. After an introduction about an experimental set-up for ultrasonic drives, the inherent typical characteristics of USM are outlined which yield to tailored driving circuits for the feeding two-phase resonant converter. The paper deals in detail with feasible analog and digital modulation concepts for a two-phase resonant converter in order to realize a flexible control scheme for a high level ultrasonic prototype drive controlled by a DSP. Finally, an appropriate digital measuring system for the phase and amplitude control of the drive is presented, extracting the fundamental fourier coefficients of the measured quanti- ties by analog signal processing.