Torque evaluation method of spherical motors using six-axis force/torque sensor

Abstract

Spherical motors are classified as multi-degrees-of-freedom actuators. Several types of spherical motors are under investigation in the world because they will be useful for robots that currently use combination of 2 or 3 one-degree-of-freedom motors. However, most of the previous works mainly demonstrated how the motors rotate or how they output torque without giving enough specifications. Torque, velocity, efficiency, controllability, and response are important issues to determine how they can be used for robots or practical mechatronic systems. The objective of this paper is to propose a method to measure three-degrees-of-freedom torque of spherical motors while they rotate with arbitrary angular velocity, which can evaluate torque in practical situations and can be used to derive efficiency of the motor. These measurements are important for comparison among the implementations and improvement of the systems. The idea consists of two parts. One is torque measurement using a 6-axis force/torque sensor. It enables off-axis simultaneous measurement of torque. The other is an external 3-degrees-of-freedom rotor driver using wheels for omnidirectional mobile robots, which is installed between the rotor and the sensor. Detailed principles and a set of experimental measurements of our spherical induction motor demonstrating usefulness of the proposed method are described in this paper.