Bernhard Gottlieb

Efficiency-Improved High-Voltage Analog Power Amplifier for Driving Piezoelectric Actuators

A novel high-voltage analog power amplifier derived from a well-known complementary class B amplifier is presented in this paper. The amplifier offers improved efficiency when driving capacitive loads such as piezoelectric multilayer actuators, while maintaining a high level of signal quality, peak power and low electromagnetic radiation. The main drawback of a class B amplifier in comparison to other power amplifiers is low efficiency, resulting in extensive power-loss in the two complementary transistors of the amplifier stage. Power-loss can be reduced by minimizing the voltage drop across the collector-emitter path of the transistors when a high collector current occurs. The proposed circuit uses an additional capacitor to recover up to one half of the charge stored in the actuator whenever the actuator is being discharged. The capacitor offers an additional power supply rail next to the positive and negative supply voltage of the amplifier stage. Switching the power supply net of the power transistors between the different supply rails, in order to reduce the voltage drop across the transistors, minimizes power-loss. The development of the amplifier was triggered by the introduction of a new piezoelectric motor and the requirements concerning signal quality and electromagnetic radiation. Therefore, in this paper, the circuit is analyzed using application-specific sinusoidal control signals which are used to drive the motor. Following the analytical investigation of the circuit, the amplifier is tested in an experiment measuring power consumption with a pure reactive load, and the level of distortion caused by the amplifier. In the experiment, a power-loss reduction of 47% was achieved in comparison with a class B stage. The amplifier processed a peak power of 160 W. The proposed amplifier showed total harmonic distortion of THD = 0.60% maximum. Distortions of a pure class B amplifier in the same setup were measured at THD = 0.25% .

Accurate Load Detection Based on a New Piezoelectric Drive Principle Employing Phase-Shift Measurement

This paper introduces a technique for measuring the torque applied to a piezoelectric motor while it is operating. The technique utilizes phase measurement rather than amplitude measurement and takes advantage of the kinematical principle of a piezoelectric actuator drive (PAD) later described in the paper. Piezoelectric actuators are bidirectional converters of electrical energy into mechanical energy and vice versa. Due to the special kinematical principle of the PAD, an applied external torque and internal torque lead to a phase-shift between the driving signals (a voltage applied to actuators) and the corresponding mechanical feedback (a modulated force). The actuator acts as a sensor to the feedback force converting it into a charge signal. The charge signal is measured and converted into a voltage by a simplified Sawyer-Tower-Circuit. The dc bias of both signals the driving actuator voltage, and measured charge signal is removed by a high-pass filter. The signals are then amplified and limited to form digital signals out of the sinusoidal input signals. The phase-shift between both signals is analyzed by a phase detector based on a zero-crossing time difference measurement. By employing the theory of electromechanic conversion of the piezoelectric actuators under the marginal conditions of the drive setup, the torque value is calculated, based on the measured phase-shift. The described technique offers highly accurate real-time torque measurement and additional information that can be used for an on-line diagnosis of the piezoelectric ring motor. The theory was validated in an experiment showing typical errors of 5% and 312 to 1248 measurements per 360 deg turn of the motor shaft. The piezomotor used during the experiment offered a maximum torque of 5 Nm

A method for auto-adjustment of a new Piezoelectric drive

Piezoelectric Actuator Drives (PADs) introduce a new kind of rotary motors, combining high dynamics, torque, and positioning accuracy. The working principle is based on a micro-toothed pair of ring and shaft, which guarantee form-fit and continuous force transmission. Due to miniaturization constant speed and absolute positioning accuracy depend on precise adjustment and therefore demand control to overcome mounting tolerances and thermal influence. In this paper a method for adjusting the PAD is presented, which makes use of the smart sensing function of the piezoelectric elements. In a simplified model the expected feedback will be calculated and an appropriate closed-loop controller will be proposed.

Piezoelektrischer Stellantrieb (PAD) (Piezoelectric Actuator Drive (PAD))

Der steigende Bedarf an miniaturisierten mechatronischen Antriebssystemen stellt eine besondere Herausforderung für die klassische Antriebstechnik dar. Zukünftige Antriebssysteme müssen nicht nur kompakter werden, sondern weitere Eigenschaften wie Stellpräzision, Echtzeit-Force-Feedback und Energieeffizienz vereinen. Bereits seit langem gibt es deshalb Bestrebungen, unkonventionelle Aktoren als Antriebselemente zu verwenden, die diese Eigenschaften intrinsisch aufweisen. Einen Beitrag zur Lösung dieses Zielkonfliktes stellt die PAD-Kinematik dar. Sie ermöglicht eine direkte Wandlung der Elongationen von Festkörperaktoren in präzise steuerbare Rotation, bei gleichzeitiger Erfassung der Lastmomente und hoher Energieeffizienz. Aus dem Zusammenwirken der Systemkomponenten PAD-Motor, PAD-Leistungselektronik, PAD-Sensorelektronik und PAD-Motion-Control-Software ergeben sich attraktive Systemeigenschaften (ähnlich Servosystemen). Today´s challenge in electrical drive engineering is the increasing market for small mechatronic systems. In the future these systems not only have to become smaller but also must offer higher precision, force feedback and high energy efficiency. Hence much attention has been payed on unconventional actuators with intrinsic sensor capabilities in the past. The PAD kinematical principle is a new tool to solve this trade-off and to convert the elongation of solid state actuators into rotation — precisely, efficiently and enabling real-time force-feedback. The interaction of the system components as PAD-Drive, PAD-Power-Stage, PAD-Sensorics and PAD-Motion-Control-Software provides an attractive set of system features (similar to servosystems).

A New Piezo Actuator Drive for Magnetic Resonance Imaging

In this paper a new piezo actuated drive the PAD is introduced. The PAD exhibits a combination of working principle and characteristics that makes it ideally suitable for robotics and positioning tasks in an MRI. Experiments in a 1.5 Tesla MRI showed that there is a high distortion in the images with the PAD next to the slice, but the image quality is improving rapidly, with an increasing distance between the PAD and the image by some centimeters. Connecting the PAD to the power supply does not affect the distortion, but increases the noise in the image.