Michael Goldfarb

A lumped parameter electromechanical model for describing the nonlinear behavior of piezoelectric actuators

A lumped-parameter model of a piezoeletric stack actuator has been developed to describe actuator behavior for purposes of control system analysis and design, and in particular for control applications requiring accurate position tracking performance. In addition to describing the input-output dynamic behavior, the proposed model explains aspects of nonintuitive behavioral phenomena evinced by piezoelectric actuators, such as the input-output rate-independent hysteresis and the change in mechanical stiffness that results from altering electrical load. Bond graph terminology is incorporated to facilitate the energy-based formulation of the actuator model. The authors propose a new bond graph element, the generalized Maxwell resistive capacitor, as a lumped-parameter causal representation of rate-independent hysteresis. Model formulation is validated by comparing results of numerical simulations to experimental data.

On the Efficiency of Electric Power Generation With Piezoelectric Ceramic

This paper analyzes the efficiency of piezoelectric ceramic for purposes of electric power generation. An analytical model is presented which suggests that the primary problem of using PZT for electric power generation is that most energy is stored in the ceramic and returned to the mechanical port. The efficiency as a function of force input frequency and resistive load are derived based upon a linearized model of a commercially available PZT stack. The analysis yields counterintuitive results in that maximum efficiency occurs in a low frequency region, several orders of magnitude below the structural resonance of the stack. The analytical results are followed by presentation of experimental data that substantiate the model. The model is then utilized to show that. due to hysteresis in the ceramic, the efficiency of energy transfer is dependent on the amplitude of force input, and that greatest efficiencies can be achieved with maximum input forces.

Design and energetic characterization of a liquid-propellant-powered actuator for self-powered robots

This paper describes the design of a power supply and actuation system appropriate for position or force controlled human-scale robots. The proposed approach utilizes a liquid monopropellant to generate hot gas, which is utilized to power a pneumatic-type actuation system. A prototype of the actuation system is described, and closed-loop tracking data are shown, which demonstrate good motion control. Experiments to characterize the energetic performance of a six-degree-of-freedom actuation system indicate that the proposed system with a diluted propellant offers an energetic figure of merit five times greater than battery-powered DC motors. Projections based on these experiments indicate that the same system powered by undiluted propellant would offer an energetic figure of merit in an order of magnitude greater than a comparable battery-powered DC motor actuated system.

The effect of force saturation on the haptic perception of detail

This paper presents a quantitative study of the effects of maximum capable force magnitude of a haptic interface on the haptic perception of detail. Specifically, the haptic perception of detail is characterized by identification, detection, and discrimination of round and square cross-section ridges, in addition to corner detection tests. Test results indicate that performance, measured as a percent correct score in the perception experiments, improves in a nonlinear fashion as the maximum allowable level of force in the simulation increases. Further, all test subjects appeared to reach a limit in their perception capabilities at maximum-force output levels of 3-4 N, while the hardware was capable of 10 N of maximum continuous force output. These results indicate that haptic interface hardware may be able to convey sufficient perceptual information to the user with relatively low levels of force feedback. The data is compiled to aid those who wish to design a stylus-type haptic interface to meet certain requirements for the display of physical detail within a haptic simulation.

Design, control, and energetic characterization of a solenoid-injected monopropellant-powered actuator

This paper describes a direct-injection configuration of a monopropellant-powered actuator that is intended to provide high-energy-density actuation for a self-powered position- or force-controlled human-scale robot. The proposed actuator is pressurized by a pair of solenoid injection valves (each of which control the flow of a monopropellant through a catalyst pack and directly into the respective side of a pneumatic-type cylinder), and depressurized via a three-way hot-gas proportional exhaust valve. A controller is described that coordinates the control of the two solenoid propellant injection valves, together with the control of the proportional hot-gas exhaust valve, in order to provide actuator force tracking. Experimental results are presented that validate the effectiveness of the force-control approach. Finally, energetic performance of the proposed actuator is experimentally assessed and shown to provide an energetic figure of merit, an order of magnitude greater than that of a battery-powered servomotor approach

Control Design for Relative Stability in a PWM-Controlled Pneumatic System

This paper presents a control design methodology that provides a prescribed degree of stability robustness for plants characterized by discontinuous (i.e., switching) dynamics. The proposed control methodology transforms a discontinuous switching model into a linear continuous equivalent model, so that loop-shaping methods may be utilized to provide a prescribed degree of stability robustness. The approach is specifically targeted at pneumatically actuated servo systems that are controlled by solenoid valves and do not incorporate pressure sensors. Experimental demonstration of the approach validates model equivalence and demonstrates good tracking performance.

Design of a PZT-actuated proportional drum brake

Presents the design of a piezoelectric-ceramic (PZT)-stack-actuated brake that is similar to a magnetic particle brake in dynamic range, size, weight, and cost, while providing a significantly larger bandwidth and requiring significantly less electrical power for a given continuous torque output. The device is essentially a single-pad drum brake that is actuated with a PZT-stack actuator. A significant component of the design is the compliant-mechanism-based transmission utilized to transmit the PZT-stack actuation into brake pad motion. Following the device description, experimental data is presented to characterize the performance of the brake. The performance characteristics are subsequently compared to those of a commercially available magnetic particle brake of comparable size and weight.

A unified force controller for a proportional-injector direct-injection monopropellant-powered actuator

This paper describes the modeling and control of a proportional-injector direct-injection monopropellant powered actuator for use in power-autonomous human-scale mobile robots. The development and use of proportional (as opposed to solenoid) injection valves enables a continuous and unified input/output description of the device, and therefore enables the development and implementation of a sliding-mode-type controller for the force control of the proposed actuator that provides the stability guarantees characteristic of a sliding mode control approach. Specifically, a three-input, singleoutput model of the actuation system behavior is developed, which takes a nonlinear noncontrol-canonical form. In order to implement a nonlinear controller, a constraint structure is developed that effectively renders the system single-input, single-output and control canonical, and thus of appropriate form for the implementation of a sliding mode controller. A sliding mode controller is then developed and experimentally implemented on the proposed actuator. Experimental results demonstrate closed loop force tracking with a saturation-limited bandwidth of approximately 6 Hz.

Design and Energetic Characterization of a Solenoid Injected Liquid Monopropellant Powered Actuator for Self-Powered Robots

This paper describes a direct-injection, liquid monopropellant powered actuation system, which was developed for the purpose of providing mechanical power to self-powered human-scale robots. The actuation system utilizes the catalytic decomposition of a monopropellant as a hot gas generator for powering pneumatic-type actuators. Specifically, pressurization of a pneumatic actuator is provided via solenoid injection valves, which control the flow of the monopropellant through a catalyst pack into the respective sides of the cylinder. Depressurization is provided by a three-way proportional spool valve, which can exhaust one of the two cylinder chambers. A prototype of the actuation system is described, and experimental data is presented that demonstrates good force and motion tracking performance. Experimental results characterizing the energetic performance of the system demonstrate that the prototype provides an energetic figure of merit an order of magnitude greater than that of battery-powered servomotors.

Predictive Control for Time-Delayed Switching Control Systems

A methodology is proposed for the control of switching systems characterized by linear system dynamics with a time delay in the input channel. The method incorporates a state predictor that at each switching period determines the effect that the next control input will have on the future output of the system, and chooses the input that will take the system closest to the desired future state. The resulting control action is suboptimal, but is computationally tractable and shown to provide a bounded tracking error for stable plants. The proposed predictive control methodology is demonstrated on a hot gas pressurization system that tracks a desired pressure trajectory via an on/off solenoid valve.