James Torres

High-Gain, High Transmissibility PZT Displacement Amplification Using a Rolling-Contact Buckling Mechanism and Preload Compensation Springs

A novel design concept of piezoelectric actuators producing large displacement while transmitting a significant amount of energy is presented. A rolling-contact buckling mechanism with a novel preload mechanism can amplify the PZT stacks displacement on the order of 100 times while transmitting several times larger work output than conventional flexure-type displacement amplification mechanisms. Existing displacement amplification mechanisms are analyzed in terms of transmissibility and are characterized with two lumped-parameter elements: serial and parallel compliances. The maximum transmissibility is attained when the parallel stiffness and the serial compliance are zero. An existing flexure mechanism using structural buckling, that produces a large displacement but a low transmissibility, is replaced by a rolling-contact mechanism that approaches the maximum criterion. Furthermore, a mechanism is presented to apply a constant preload to each PZT stack despite their movement. A prototype has been built to implement the design concept and verify the theoretical results. Experiments using the prototype demonstrate that it produces a 4.2 mm free displacement with over 60% transmissibility.

Design of a Piezoelectric Poly-Actuated Linear Motor

Design and analysis for an efficient and force dense piezoelectric poly-actuated linear motor is presented. A linear motor is constructed with multiple piezoelectric actuator units engaging a rod having gear teeth. The multiple piezo-units are placed along the geared rod with a particular phase difference such that a near constant force is generated regardless of the rod position by coordinating the multiple piezo-units. Rolling contact buckling mechanisms within the piezo-units provide large displacement amplification with high energy transmission and low loss properties from the piezo-units to the geared rod. This piezo-based motor has capacitive actuator characteristics which allow it to bear static loads efficiently. Furthermore, the poly-actuator architecture presented provides for scalability through modular design.First, the basic design principle describing the engagement of buckling amplification mechanisms to a phased array-shaped gear rod is presented, and the resulting force and displacement characteristics are analyzed. Design methods for creating a piezoelectric poly-actuated linear motor are then summarized. A prototype design is presented for which a maximum mean force of 213 N, a maximum velocity of 1.125 m/s, and a force density of 41 N/kg is calculated.Copyright

Harmonic PZT poly-actuators

Capacitive actuators, such as piezoelectric stack actuators, provide an efficient solution for robotic and mecha-tronic systems that typically require large forces and minimal velocities over long periods of time. To overcome the stack actuators limitations, particularly their limited stroke, a harmonic poly-actuator design is presented. This design utilizes a multitude of intermediate buckling amplification mechanisms in a parallel arrangement to create a large stroke, high force actuator. The redundant system architecture combined with a particular spatial and temporal coordination allow for a number of salient features, including robustness to failure, backdrivability, and continuous force control via elementary ON-OFF control. A prototype was built using six intermediate buckling amplification mechanisms and was able to produce over 100 Newtons of force over a stroke of 450 mm.

Closed loop position control of a linear PZT poly-actuator

Piezoelectric stack actuators have many advantageous characteristics including power density, backdrivability, and their ability to efficiently hold a force for long periods of time. The stack actuator was integrated into a high-gain amplification mechanism which in turn was integrated into a poly-actuator, an actuator made up of several units. A proportional controller was used to close the loop around position, but it was unable to combat several significant disturbances. A PI controller was implemented to overcome steady-state error and two dynamic regimes were found due to the non-linear friction at the output. This friction was characterized and possible uses of the two regimes were explored, particularly at resonance.

Maximizing output work of PZT stacks while gaining large displacement amplification

A novel design concept is presented for amplifying the displacement of a PZT stack over 50-fold while transmitting a significant amount of work per cycle. Piezoelectric devices, such as PZT stack actuators, have a competitive power density, while consuming virtually no energy for generating a force at a constant position. Despite the salient features, the actual work usable for activating a load is significantly reduced when displacement amplification mechanisms are used for attaining displacements large enough to drive a macroscopic robotic system. This paper addresses how the work produced by a PZT stack is transmitted to a load without attenuation. The theoretical limit of maximum work is obtained and experimentally verified. Conditions for transmitting the maximum work, i.e. maximum work transmissibility, are obtained based on a simple model. A novel displacement amplification mechanism using a flexure-free, buckling mechanism is then presented as a solution that satisfies the maximum energy transmissibility conditions. A prototype device is designed and tested. Initial experiments show promising results.

Design Method for Buckling Amplified Piezoelectric Actuator Using Flexure Joint and its Application to an Energy Efficient Brake System

This paper shows a practical design method for a displacement amplification mechanism for a piezoelectric actuator which employs a buckling-like phenomenon. This mechanical singularity realizes a substantial displacement magnification, at least 50 times, within a simple structure. An SMA preload mechanism essentially provides potential for full range push-pull actuation to the piezoelectric actuator. This integrated actuator performs a high energy transfer ratio and is suitable for brake mechanisms due to their requirement of high force, specific displacement and energy efficiency. A practical design method is shown and is evaluated by comparing the analytical model with finite element analysis and experimental hardware performance. The actuator properties obtained by these methods fit well each other with errors less than 13%.The experimental actuators are applied to a brake for a commercial motor and its properties are evaluated. The brake can produce more than 2.5Nm in the displacement range of 0.5mm. These experimental results suggest that this novel piezoelectric actuator has potential for use in a wide range of applications.Copyright

Harmonic analysis of a PZT poly-actuator

Similar to combustion engines comprising multiple cylinders engaged with a crankshaft, multiple piezoelectric stack actuators (PSA) engaged with a common output rod can produce smooth, long stroke motion with desired properties. In particular, when equally spaced multiple units are arranged to push sinusoidal gear teeth on the output rod, the system exhibits unique collective behaviors thanks to “harmonic” effects of the multiple units. For example, although the force-displacement characteristics of individual units are highly nonlinear, the undesirable nonlinearity, including singularity, may be eliminated. Here we present harmonic analysis and control of a class of actuators consisting of multiple driving units engaged with a sinusoidal transmission, termed a harmonic poly-actuator. Through theoretical analysis we obtain 1) conditions on the unit arrangement to eliminate their nonlinearity from the output force, 2) control algorithms for coordinating the multiple units to generate a commanded force with desired force-displacement characteristics, and 3) a method for compensating for output force ripples due to possible misalignment and heterogeneity of individual units. The control algorithms are implemented on a prototype harmonic poly-actuator with six units of PSAs. Experiments demonstrate the unique features of the poly-actuator exploiting the harmonic properties of the system.

Damping Pressure Pulsations in a Wave-Powered Desalination System

Wave-driven reverse osmosis desalination systems can be a cost-effective option for providing a safe and reliable source of drinking water for large coastal communities. Such systems usually require the stabilization of pulsating pressures for desalination purposes. The key challenge is to convert a fluctuating pressure flow into a constant pressure flow. To address this task, stub-filters, accumulators, and radially elastic-pipes are considered for smoothing the pressure fluctuations in the flow. An analytical model for fluidic capacitance of accumulators and elastic pipes are derived and verified. Commercially available accumulators in combination with essentially rigid (and low cost) piping are found to be a cost-effective solution for this application, and a model for selecting accumulators with the required fluidic-capacitance for the intended system is thus presented.

Dynamic Analysis of a Buckling-Type Amplification Mechanism to Maximize the Power Output by Varying the Load Impedance and Control Timing

Due to the limited displacement of piezoelectric stack actuators, common practice is to use some form of displacement amplification mechanism. This paper focuses on an externally leveraged mechanism that utilized a buckling motion to achieve large amplification ratios within a single stage. This mechanism interfaces with a sinusoidal gear track that acts as the load. The dynamics of the system are derived and are shown to be fifth order. Due to the significantly nonlinear amplification caused by the buckling phenomenon and the gear, the dynamics are run in simulation to gain insight into the performance of the actuator. There is shown to be an optimal speed at which to run the actuator to maximize the possible power output. Furthermore, due to the simple binary control significant benefits are achieved by varying the control timing based on the velocity to ensure the force and velocity of the output are in phase.Copyright

A PZT Array Actuator Using Buckling Strain Amplification and Preload Mechanisms

Displacement amplification mechanisms have been a topic of research for piezoelectric actuators for decades to overcome their significantly small strain, but still utilize their high power density, force, and efficiency. This paper further analyzes a nonlinear buckling mechanism to improve its efficiency, defined as the ratio of mechanical work output of the buckling actuator to the mechanical work output of the PZT actuator, as well as, employing two methods, preload and loading conditions, that improve its work output per cycle. This is accomplished by running a numerical analysis of the geometry of the flexure joints in the buckling mechanism which found a maximum mechanical efficiency of 48%. The preload is applied using shape memory alloy wire to exploit the low stiffness of the super elastic regime; which in turn allows a larger work output due to a loading condition supplied by a novel gear design. Finally, a prototype was fabricated to provide a baseline of comparison against these concepts.Copyright