A Force and Displacement Self-Sensing Piezoelectric MRI-Compatible Tweezer End Effector with an On Site Calibration Procedure

Copyright © IEEEDOI: http://dx.doi.org/10.1109/TMECH.2013.2257827Copyright © IEEEThis paper describes a self-sensing technique for a piezoelectrically driven magnetic resonance imaging (MRI)-compatible tweezer style end effector, suitable for robot assisted MRI guided surgery. Nested strain amplification mechanisms are used to amplify the displacement of the piezo actuators to practical levels for robotics. By using a hysteretic piezoelectric model and a two port network model for the compliant nested strain amplifiers, it is shown that force and displacement at the tweezer tip can be estimated if the input voltage and charge are measured. One piezo unit is used simultaneously as a sensor and an actuator, preserving the full actuation capability of the device. An on-site calibration procedure is proposed that calibrates the combined electromechanical model without requiring specific loading conditions on the inner piezoelectric actuators. Experimental validation shows an average of 12% error between the self-sensed and true values

Bistatic sonar and a novel form of variable depth sonar

This thesis relates to the sonar system research undertaken for a Naval Requirement for proposals to improve the cost effectiveness of the defence of shipping against submarine attacks. Defence systems evolved as a function of the developing technology of the opposition which in this instance is the ability of a submarine to remain undetected below the sea surface while searching for, tracking and attacking its targets. An inherent problem for underwater detection with Escort Ships hull type sonars is its location on the air-sea interface with the need of a two-way propagation path to access the depth-range volume available to a submarine. As the power of an Escort's sonar is increased so is the size of ship, 5000 tons and more, to accommodate the optimum size of transducers required.Sonar system research at all times is a multi-discipline task and in this particular case was further broadened with a requirement to review the possibilities for sources of energy other than underwater acoustics. The research confirms the dominance of sonar for underwater detection and establishes the feasibility of a Bistatic Sonar concept which replaces the two-way propagation path of a hull type sonar with a one-way path, source-target-receiver with a variable depth directive towed line receiver on a small ship as a distant receiver. A second objective which became feasible with the development of an adequate towed source was a variable depth sonar which has now been produced by British Aerospace for a world market. A summary of the thesis is provided as an introduction to the subject matter of the different sections

Dynamic modeling and bioinspired control of a walking piezoelectric motor

Szufnarowski F. Dynamic modeling and bioinspired control of a walking piezoelectric motor. Bielefeld: Universität Bielefeld; 2013.Piezoelectric motors have increasingly extended their field of applications during recent years. Improved material properties and manufacturing techniques have led to a variety of designs which can achieve theoretically unlimited displacements for moderate voltage levels while retaining a relatively high stiffness. In practical terms, this leads to stronger and faster motors which become a viable alternative to electromagnetic drives, especially if compact size and small weight are important. The piezoelectric motor considered in this work consists of four piezoelectric bender elements which can forward a ceramic bar by means of a frictional interaction. The drive elements can be compared to "legs" walking on a movable plane. The walking motor offers outstanding force generation capabilities for a motor of its size. Despite this fact, this motor has not been used in a force control scenario before and no motor models exist in the literature which can reproduce the effect of load on its performance. In this work, two dynamic motor models are developed to address the latter issue. Both of them faithfully reproduce the non-linear motor velocity decrease under load. The first model is based on an analytic approach and describes the low-level frictional interactions between the legs and the ceramic bar by means of several physically meaningful assumptions. This analytic model explains several non-linear phenomena in the operation of the walking motor within the full bandwidth of its rated operation. Non-linear influences due to the impact dynamics of the legs, ferroelectric hysteresis and friction are identified in the motor and new insights for an improved motor design as well as an improved motor-drive strategy gained. Moreover, the analytic model finds its application in a theoretical investigation of an alternative motor-drive strategy which is based on findings in insect walking. Specifically, it is shown that the performance of the motor can be improved by a half in terms of its force generation and doubled in terms of its maximal velocity, as compared to classical drive approaches, if the bioinspired drive strategy as proposed in this work is used. The second model is based on an experimental approach and system identification. Although less general, the second model is well-suited for a practical application in a force-control scenario. In particular, the experimental model is used in this work for the development of a load compensation strategy based on force feedback which restores the linearity of motor operation for moderate levels of loading. Based on the linearized motor model, a force controller is developed whose performance is evaluated both theoretically and experimentally. The developed force controller is also used in a bioinspired control scenario. Specifically, two walking motors together with their force controllers are employed in a 1-DOF antagonistic joint as force generators. The motors are supposed to partially mimic the functionality of a muscle based on the non-linear force-length relation as derived by Hill. A simple positioning task shows the feasibility of this kind of non-standard application of a piezoelectric motor. Beside the development of motor models and bioinspired control approaches, this work addresses the issue of drive-signal generation for the walking motor. Specifically, the development of motor-drive electronics is presented which supersedes the commercially available products due to its compactness and the possibility of waveform generation at much higher drive frequencies, above 50 kHz, as compared to the nominal limit of 3 kHz and commercial products. In this context, the possibility of motor operation at ultrasonic frequencies is discussed which would benefit the motor in terms of its speed and the absence of audible noises

Design of Electrical Contacts for Fast Mechanical Disconnect Switches

The objective of this research is to develop an understanding for the design considerations for electrical contacts with the goal to improve the performance of fast mechanical disconnect switches (FMS). The design of electrical contacts involves tradeoffs between current rating, voltage rating and speed of FMS, which are demonstrated for an FMS prototype based on piezoelectric actuators. The research focus involves the selection of optimal geometries (profile) for the contacts and selection of the most suitable contact material to achieve certain performance goals Contacts with uniform field geometries such as Bruce and Rogowski were shown to minimize the enhancement of the electric field when open and contact resistance when closed. The most suitable contact materials are identified by deriving the material indices that affect performance of FMS. The material selection process identified minimizing power loss, fretting wear and overheating as the major objectives and copper based contact materials as the most suitable for this application. The impact of fretting wear was further studied and was found to result in a derating of voltage and current rating of FMS. The potential performance gains with dissimilar contact materials are explored. Dissimilar contact materials were found to improve the life expectancy of FMS by delaying the increase in contact resistance caused by fretting wear. The use dissimilar contact materials to redirect heat away from the temperature sensitive components via thermal rectification is demonstrated through simulation and experiments. The combination of these two effects allow for the design of electrical contacts that can significantly improve the performance of FMS.Ph.D

Two-Port Network Models for Compliant Rhomboidal Strain Amplifiers

Copyright © IEEEDOI: http://dx.doi.org/10.1109/JSEN.2012.2218282Piezoelectric stack actuators have the advantages of zero backlash and no acoustic noise, but their stroke is too small to actuate robotic links directly. Because the force available is often more than the required, the stroke of the piezoelectric stack can be amplified by a compliant mechanism at the expense of force. It is not always clear what the geometry of this compliant mechanism should be. Compliant mechanisms have parallels in biology in that they describe two-way interactions between the actuator and the environment. In this paper, we employ the concept of a two-port network model from circuit theory to describe this two-way interaction and present a method to obtain each element of the two-port model as an analytical function of physical geometric parameters for a wide class of geometries. This method makes use of Castigliano's theorem and Euler-Bernoulli linearly elastic beam theory. To our knowledge, this is the first two-port representation of a compliant mechanism that is based on analytical expressions of geometric parameters. This analytical model agrees well with finite-element method calculations. We also examine a representative case experimentally and achieve accuracies better than 18%