William P. Robbins

Restructuring of first courses in power electronics and electric drives that integrates digital control

Since 1994, the University of Minnesota has been undertaking a long overdue restructuring of power electronics and electric machines/drives courses. This restructuring allows digital control to be integrated into first courses, thereby teaching students what they need to learn, making these courses appealing, and providing a seamless continuity to advanced courses. By a concise presentation in just two undergraduate courses, this restructuring motivates students to take related courses in programmable logic controllers, microcontrollers and digital signal processor applications. This ensures a first-rate education that is meaningful in the workplace as well as in graduate education leading to a research and development oriented career. This restructuring has several components to it. Outdated topics that waste time and mislead students are deleted. To integrate control in the first courses, unique approaches are developed to convey information more effectively. In the first course in power electronics, a building block is identified in commonly used power converter topologies in order to unify their analysis. In the field of electric drives, the use of space vectors is introduced on a physical basis to describe operation of ac machines in steady state in the first course, and to discuss their optimum control under dynamic conditions in the advanced course. Appropriate simulation software and software-reconfigurable hardware laboratories using a DSP-based rapid prototyping tool are used to support the analytical discussion.

A linear piezoelectric stepper motor with submicrometer step size and centimeter travel range

A linear stepper motor capable of submicrometer controlled movement has been constructed using the piezoelectric material lead zirconate titanate (PZT). This motor consists of a 25.4-mm*12.7-mm*1.6-mm piezoelectric driving element connected between a glider base and an attached load. The device is inset in a trench to constrain motion to one dimension. An electrode on the bottom of the glider is used with an electrode on the top of the trench to implement an electrostatic clamp. This clamp enables the stepper motor to climb slopes of up to 12 degrees , whereas without the clamp only slopes of 6 degrees or less are tolerated. A linear inertial sliding motion can be achieved by expanding and contracting the piezoelectric bar, but the addition of the electrostatic clamp enhances the movement capabilities of the glider by the periodic clamping and unclamping of the glider. Glider velocities of 5.7-476 mu m/s are measured by timing the movement of the glider over a 1.0-mm portion of the track through an optical microscope. Displacement steps of 0.07-1.1 mu m are calculated by dividing the measured glider velocity by the frequency of the applied voltage pulses. Displacement step size and glider velocity are controlled by the application of PZT extension voltages ranging from +or-(60-340) V.<<ETX>>

High-displacement piezoelectric actuator utilizing a meander-line geometry I. Experimental characterization

The compact linear-motion piezoelectric actuator developed has relatively large displacement capabilities. It is composed of a number of parallel bars of lead zirconium titanate (PZT) connected together in a meander-line configuration so that they are mechanically in series and electrically in parallel. The polarity of the adjacent bars is arranged so that if a given bar expands under the applied voltage, the adjacent bars contract. An electromechanical model of the actuator predicted and measurements verified that stiffeners added to the basic meander line geometry significantly increased the force output without affecting the displacement versus applied voltage relationship.<<ETX>>

High-displacement piezoelectric actuator utilizing a meander-line geometry II. Theory

For pt.I see ibid., vol.38, no.5, p.454-60 (1991). A simple electromechanical model of the meander-line actuator is developed. The model consists of a voltage-actuated force source, which is the result of the piezoelectric properties of the bars that make up the actuator, in parallel mechanically with a stiff spring that models the compression-expansion response of the piezoelectric bars. This parallel combination is in series mechanically with a second spring. This series connection is subjected to large bending moments that cause the stiffness of this second spring to be much less than the stiffness of the spring in parallel with the force source. The addition of stiffeners to the actuator can significantly reduce these bending moments and thus increase the stiffness of this second spring. As a result, the force generation capability of the actuator is significantly increased without affecting its displacement capability.<<ETX>>

Ferroelectric-based microactuators

Abstract This paper reviews the current state of ferroelectric-based microactuators. The advantages of ferroelectric-based microactuation are considered. Several actuator concepts are discussed including unimorph-based cantilever actuators, membrane-based micropumps, linear positioning actuators using folded geometries of cascaded elementary actuators for both in-plane and out-of-plane displacements, linear stepper motors using inch-worm geometries, and ultrasonic-based micromotors. Optimistic, order-of-magnitude estimates of actuator performance are given based on simple, idealized analysis. Processing issues in fabricating ferroelectric-based microactuators are also considered.

Characteristics of acoustic emission signals generated by termite activity in wood

A series of experiments has been performed to determine if acoustic emission (AE) monitoring can be used to detect termite infestations in wood. Results obtained with a computer-controlled measuring system indicate that termite activities in the wood generate a significant amount of AE which has frequency components extending to above 100 kHz. These emissions can be easily detected by commercially available AE sensors (50 kHz resonant sensors were used in these measurements). Spectral analysis of noise signals (including some deliberately generated) indicate that nearly all noise signals have most of their energy below 20 kHz. Thus, it appears feasible to separate termite-generated signals from noise signals by filtering. The ultrasonic propagation characteristics of wood, especially the large and frequency dependent propagation losses, have a significant impact on the detection process. However by working below 100 kHz, termite-generated signals could be detected at least 2 m away from the site of the infestation, as long as the sensor was on the same piece of wood. A practical hand-held, battery-powered detection system has been designed, fabricated, and tested successfully in the laboratory.<<ETX>>

Passive Wireless MEMS Microphones for Biomedical Applications

This paper introduces passive wireless telemetry based operation for high frequency acoustic sensors. The focus is on the development, fabrication, and evaluation of wireless, battery-less SAW-IDT MEMS microphones for biomedical applications. Due to the absence of batteries, the developed sensors are small and as a result of the batch manufacturing strategy are inexpensive which enables their utilization as disposable sensors. A pulse modulated surface acoustic wave interdigital transducer (SAW-IDT) based sensing strategy has been formulated. The sensing strategy relies on detecting the ac component of the acoustic pressure signal only and does not require calibration. The proposed sensing strategy has been successfully implemented on an in-house fabricated SAW-IDT sensor and a variable capacitor which mimics the impedance change of a capacitive microphone. Wireless telemetry distances of up to 5 centimeters have been achieved. A silicon MEMS microphone which will be used with the SAW-IDT device is being microfabricated and tested. The complete passive wireless sensor package will include the MEMS microphone wire-bonded on the SAW substrate and interrogated through an on-board antenna. This work on acoustic sensors breaks new ground by introducing high frequency (i.e., audio frequencies) sensor measurement utilizing SAW-IDT sensors. The developed sensors can be used for wireless monitoring of body sounds in a number of different applications, including monitoring breathing sounds in apnea patients, monitoring chest sounds after cardiac surgery, and for feedback sensing in compression (HFCC) vests used for respiratory ventilation. Another promising application is monitoring chest sounds in neonatal care units where the miniature sensors will minimize discomfort for the newborns.

An instructional laboratory for the revival of electric machines and drives courses

This paper describes the developments at the University of Minnesota of new approach in teaching of electric drives, focusing on the associate state-of-the-art laboratory. The mission of these developments is to nationally revitalize courses in industrially and strategically vital fields of electric drives (and power electronics). This is accomplished by making these courses appealing to students (undergraduate enrollments have significantly increased subsequent to adopting these approaches) where they receive a first-rate education in just one undergraduate course in a way that ensures a seamless continuity to advanced courses. The laboratory is based on a dSPACE development board and several custom designed power converter boards and electric motors, working on a 42 V DC-bus voltage system.

A building-block-based power electronics instructional laboratory

This paper describes a new power electronics instructional laboratory based on the building block (switching pole) methodology. This methodology provides a common basis for describing all practical converter topologies and gives the student a unified top-down presentation of power electronic converters. The novel features of this laboratory include: a unifying building-block methodology (the Power Pole), tight coupling with lectures, use of low voltages (<50 V) for enhanced safety, and ease of use and low cost. A key component of the laboratory is the use of a reconfigurable circuit board (the Power Pole board) which contains the Power Pole circuit as well as on-board isolated drive circuits, PWM generation, fault protection, output filter, and switched load. The detailed circuit board layout is described and several examples of its use are presented.

Design of linear-motion microactuators using piezoelectric thin films

Three types of microactuator for linear displacement are presented which use piezoelectric thin films for the electrical-to-mechanical energy conversion process. One actuator uses a folded-path or meander-line geometry to produce horizontal tethered linear displacements. The second geometry uses a number of unimorph bars arranged in a planar format and mechanically connected in series to produce a tethered displacement perpendicular to the plane containing the unimorphs. The third actuator uses an inertial recoil mechanism in conjunction with an electrostatic clamp to produce incremental stepping motion. Sufficient repetition of the stepping sequence produces virtually unlimited travel range, being limited by practical considerations such as electrical connections. Electromechanical models for all three actuators are developed and are used to quantitatively estimate the performance of microactuators designed to a particular set of dimensions. Fabrication procedures for the microactuators have been developed and the status of the fabrication efforts is presented.