Jeremy E. Frank

Design and performance of a high-force piezoelectric inchworm motor

A linear inchworm motor was developed for applications in adaptive, conformable structures for flow control. The device is compact (82 X 57 X 13 mm), and capable of unlimited displacement and high force actuation (150 N). The static holding force is 350 N. Four piezoceramic stack elements (two for clamping and two for extension) are integrated into the actuator, which is cut from a single block of titanium alloy. Actuation is in the form of a steel shaft pushed through a precision tolerance hole in the device. Unlimited displacements are achieved by repetitively advancing and clamping the steel shaft. Although each step is only on the order of 10 microns, a step rate of 100 Hz results in a speed of 1 mm/s. Since the input voltage can readily control the step size, positioning on the sub-micron level is possible.

Modeling and Design Optimization of a Bimorph-Driven Rotary Motor

A bimorph-driven rotary motor has been developed, and demonstrated in a flow-control actuation application. A rotary (roller) clutch rectifies bimorph-powered oscillation into rotational motion to convert electrical to mechanical power. Through a progression of varied designs, the development process was based on experimentation and engineering intuition. The resulting motor satisfied the application requirements, and thirty-two actuators were supplied. To further exploit the possibilities of the bimorph-driven motor concept, a mathematical model was developed. Using empirical data collected from a prototype actuator and a roller clutch, the mathematical model was adjusted so that it predicted the power output of the prototype. The model was then used to tune the design parameters using nonlinear optimization techniques. The results predicted that the power density could be increased by more than 30 times. An improved prototype based on the optimization results demonstrated a significant performance improvement over the original design. Though the predicted improvement was not achieved, the power density was increased by a factor of 10.

A Low-Order Model for the Design of Energy Harvesting Piezoelectric Devices

The use of energy harvesting devices is an attractive method of utilizing available mechanical energy by converting it into usable electrical energy. Numerous potential applications exist for energy harvesting devices, including structural health monitoring, discrete actuation systems, and wireless sensor networks, which are considered here. A piezoelectric element is employed to convert mechanical energy from a vibration environment to electrical energy, which is then converted to a regulated power source through an attached circuit. While the devices considered vary in configuration, the essential component is the piezoelectric unimorph or bimorph annular plate with an associated proof mass designed to be mechanically driven near its resonance frequency. The development of a low-order model is a critical step in predicting the device behavior for both the current and future generations of devices. Such a model, based on the assumed modes method, is developed and presented. As employed here, the assumed modes method uses Lagrange’s equations and a computation of the (both electrical and mechanical) potential and kinetic energy and virtual work of the device. The model provides a rapid computation of key parameters such as open- and short-circuit natural frequencies, device coupling coecient, and mode shapes for a device of circular geometry, as well as the ability to produce frequency response functions and time-varying responses to arbitrary forcing functions. Model predictions are compared with experimental data and the model is also used to analyze various physical connections of such plates in a manner like component mode synthesis. A particular strength of the model is the ease with which device parameters, such as the piezoelectric element thickness or device radius, can be changed to evaluate their impact on the device performance. As such, the model is useful when designing the next generation of devices or optimizing a particular configuration, an example of which is presented.

Modeling and simulation of a resonant bimorph actuator drive

A rotary actuator driven by piezoelectric bimorphs has been developed for various smart structure applications. A rotary (roller) clutch rectifies bimorph oscillation into rotational motion to convert electrical to mechanical power. While prototype actuators perform well, they were designed with just engineering intuition. Here, a mathematical model of the actuator is developed. Using empirical data collected from a prototype actuator and a roller clutch, the mathematical model was tuned so that it predicted accurately the performance of the prototype. The model was then used to perform parameter studies and optimize the design of the actuator. The model predicts that performance can be significantly increased by making slight modifications to the prototype. Work to verify these predictions of the mathematical model is underway.

Resonant bimorph-driven high-torque piezoelectric rotary motor

A compact high-torque rotary motor was developed for use in large-displacement structural shape control applications. The main principle underlying its operation is rectification and accumulation of small resonant displacement of piezoelectric bimorphs using roller clutches as mechanical diodes. On the driving half of each cycle, the forward motion of the bimorph is converted to rotation of the shaft when the hub drive torque exceeds that of the load. On the recovery half of each cycle, a second, fixed, roller clutch prevents the load from backdriving the shaft. This approach substantially increased the output mechanical power relative to that of previous inchworm-type motor designs. Experiments to date, conducted under conditions of continuous operation at a 90 Vrms drive level, have demonstrated a stall torque of about 0.4 N-m, a no-load speed of about 750 RPM, peak power output greater than 1 W, and power density of about 5 W/kg. While not yet competitive with conventional motor technologies, this motor may also be fabricated in unusual (i.e., non-cylindrical) form factors, enabling greater geometric conformability than that of typical motors. The use of commercial roller clutches, piezoelectric bimorphs, and single frequency drive signals also results in a simple, inexpensive design.

Design and performance of a resonant roller wedge actuator

A compact rotary motor driven by piezoelectric bimorph actuators was developed for applications in adaptive, conformable structures for flow control. Using a roller wedge (rotary roller clutch) as its central motion rectifying element, the actuator converts electrical power to mechanical power by way of a set of resonating bimorph/mass systems. With this type of resonant drive system, the output mechanical power of the actuator was dramatically improved over previous inchworm-type designs. Also, the actuator cost was kept low by using commercial roller clutches and bimorph actuators instead of PZT stacks. Within an application size constraint of 4 x 4 x 1.75 inches, the unloaded speed was 600 RPM, the stall torque was 0.5 N-m, and the peak output power was nearly 4 watts. The motor is driven by a single frequency sinusoidal input, resulting in significant improvements of the cost, size and complexity over typical piezoelectric actuator drivers. Since the backlash of the roller clutch is a critical parameter in assessing the motor performance, an experimental study was performed to better understand its dynamics.

Design and Performance of a Linear Piezoelectric Wedgeworm Actuator

A new concept in linear piezoelectric actuators is developed for applications in adaptive, conformable structures for flow control. Motivated by a desire for high actuation force (>lkN) and simplified drive signals, the design takes advantage of self-locking wedges to lock the clamping elements. The concept relies heavily on knowledge and manipulation of the friction coefficients between several surfaces, so the choice of coatings and lubricants are a major part of the investigation. Since the wedges are self-locking in one direction, the actuation force is limited only by the size of the piezoceramic and the strength of the actuator structure. The device contains a single piezoceramic stack (8x8x42 mm, PZT 5H), so the drive signals and amplifiers are drastically simplified from previous designs. A prototype of the concept is developed and experimentally tested. At a drive frequency of 200 Hz, the free velocity is 8 mm/s with a travel of 25 mm. An actuation force of 250 N is achieved with the prototype. The wedge concept also reduces the amount of precision necessary in machining and assembling the device.

Determination of the driving signal parameters to maximize the performance of a piezoceramic inchworm actuator

A series of linear piezoceramic inchworm actuators with various applications have been developed. There are three active piezoceramic elements within the inchworm: two ‘‘clamps‘‘ and one ‘‘pusher,’’ each of which is driven independently with a voltage of up to 120 V. Large displacements are achieved by repetitively advancing and clamping the pushing element, with each small step on the order of 10 μm. A program using labview software is used to generate the driving signals to the clamps and pusher. With this program, experiments are performed to study the dynamics of the system, and in particular to identify the conditions for maximum force and velocity as well as the onset of resonance within the clamp and pusher elements. The maximum frequency, force, and velocity for the inchworm are shown to depend on the end loading condition, the type of load (mass or spring), as well as the input signal parameters, which are the frequency, phasing amplitude, and shape of the waveforms. The result of the experimenta...