chapter  8
16 Pages

Low Temperature Motors and Actuators: Design Considerations and a Case Study

ByJoe Flynn, Yoseph Bar-Cohen

The general distinction between a motor and an actuator is that a motor is the element that makes the motion (mostly rotation of an electromagnetic driver) while an actuator is a complete activation device with a gear that enables the movement of a mechanism. Mobility, manipulation, and articulation of mechanisms require the use of actuators and, in some applications, they enable extremely high precision displacement (e.g., optical devices). Each degree of freedom (DoF) of a mechanism typically requires a dedicated actuator and, with the rise in the number of DoF, the complexity signi†cantly increases. In such systems as robots, the mass and volume of the actuators take up a signi†cant percentage of the total system. Actuators function as the equivalent of muscles and they are operated by a source of energy (typically electric current, hydraulic, heat, or pneumatic pressure) that is converted into motion. The control system that drives an actuator can be a simple †xed mechanical or electronic system, computer driven or operated by a human user. The general types of actuators that are used include electric (such as ac, dc, brushed, and brushless motors), pneumatic, hydraulic, piezoelectric, and shape-memory alloys. The operation of actuators is signi†cantly different from the operation of natural muscles, which are both compliant and linear in behavior [Full and Meijer, 2004]. There are many low temperature applications where a motor is needed to perform

mechanical functions. For applications on Earth, 180 K (–135.8°F) is the lowest temperature

CONTENTS

8.1 Introduction ........................................................................................................................ 213 8.2 CTE and Material Selection Considerations .................................................................. 215 8.3 Fabricating Low temperature Motors and Actuators ................................................... 216 8.4 Testing the Motors and Actuators and Analysis of the Test Results .......................... 220

8.4.1 Cryogenic Test Results .......................................................................................... 220 8.4.2 Summary of the Test Conclusions .......................................................................222