Abstract-We are developing haptic interfaces compatible with functional Magnetic Resonance Imaging (fMRI) for neuroscience studies. The presented prototype with one rotary degree of freedom is actuated by a traveling wave ultrasonic motor operating under admittance control. Torque is sensed from the deflection of an elastic polymer probe via light intensity measurement over optical fibers. This concept allows us to place all electronic components outside the shielded MR room. Hence, the device can be used in conjunction with fMRI, providing torque and motion feedback simultaneously with imaging. Its compactness and simplicity facilitate the construction of multiple degree of freedom systems.
Based on a mathematical model, simulation results show how the mechanical performances of the motor are sensitive to the prestressing force. A test bench has been developed in the laboratory for experimental measurements. The motor is driven by a two-phase resonant converter controlled by a DSP in order to realize a flexible control scheme [1]. An hysteresis brake dynamometer controlled by PC make it possible to generate any load profile. Results obtained by measurement confirm the motor sensitivity to the variation of the prestressing force, as shown in the simulation result. Thus, it makes it possible to validate the model and proves that the choice of the prestressing force is very important to optimize the motor design.
Abstract-Ultrasonic motors are a good alternative to electromagnetic motors in medical robotics, since they are electromagnetically compatible. Estimating speed instead of using encoders reduces cost and dimension of the robot on the one hand and increases reliability on the other hand. However, no sensorless speed controller is yet industrialized. Analytical models of the traveling wave ultrasonic motor being too complex to be exploited for sensorless control purpose, we suggest speed estimation based on artificial neural networks. The artificial neural network is designed based on a sensitivity analysis using design of experiments methods. Factorial designs have been chosen to find out the effects of each input factor, but also the effect of their interactions. First results show that speed estimation using artificial neural networks is a promising approach. The artificial neural network optimized with design of experiments methods is a valid model of the traveling wave ultrasonic motor to estimate speed.
We will present a method allowing to evaluate the mechanical performances (torque, speed, mechanical power, dissipated losses in the surface of contact between stator and rotor) of the travelling wave ultrasonic motors. The visualization of the motor characteristics will make it possible to the manufacturer to optimize the operation point of the motor compared to these proper used criteria. The method was developed based only on the two-dimensional equivalent mechanical model of the motor.The piezoelectric motors are resonant vibromotors. They represent a new actuator generation in the field of the servodrives. In particular, the travelling wave ultrasonic motor presents a high torque at low speed, a zero speed torque without feeding, weak electromagnetic disturbances and, furthermore, this is a more compact solution compared to the conventional electromagnetic motors. Figure 1 shown the basic components for this type of motor. A piezoelectric ceramics ring (actuator) is stucked on the stator in order to induce in this last oscillations at the frequency of resonance. Piezoelectric ceramics ring is divided into two excitation systems (two phases). Each system is fed ideally by a sinusoidal voltage in the ultrasonic field (40-45 KHz), thus generating two standing waves in the stator. A space dephasing equal to the wave length quarter is then introduced between the two excitation systems in order to generate a travelling wave by the superposition of the two standing waves. The rotor is then in a hurry against the stator with the elastic ring. Had with travelling wave, the points on the surface of the stator describe an elliptic trajectory combining vertical and horizontal movements. Thanks to the pressure generated in the contact zone between stator and rotor by the vertical movement, it exists a force of friction. Thus the horizontal displacement prints a force of traction to the rotor. This interaction between stator and rotor is characterized by a strong non-linearity. Figure 1. Diagram of construction of the travelling wave ultrasonic motor [1]I. MODELISATION To study the model, we referred to the work carried out in [2]. The travelling wave ultrasonic motor can be represented with the functional diagram shown in Figure 2. The static converter (first block) generates the two phase voltages necessary to the motor supply. Then by the reverse piezoelectric effect (second block) we obtain an electric energy transformation into mechanical energy, allowing the creation of the two standing waves in stator ring. A space dephasing equal to the wave length quarter is then introduced between the two excitation phases in order to generate a travelling wave by the superposition of the two standing waves. The interaction between the travelling wave and the rotor (contact zone), generate a motor torque. 1907 0-7803-7420-7/02/$17.00 © 2002 IEEE
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