M. S. Thesis Abstract

Development and Analysis of an Absolute Three Degree of Freedom Vision Based Orientation Sensor

Martin Klement

September 1997


The design of multi-degree-of-freedom actuators, such as ball joint-like spherical motors, is often hampered by the lack of accurate, non-contact orientation sensing methods suitable for use in a control system. Typically, specially designed mechanisms must be used to isolate and transfer specific motions to traditional single degree-of-freedom (DOF) sensors. A three DOF spherical motor, for example, requires a gimbal mechanism to isolate each axis of rotation for the use of incremental shaft encoders. This approach inevitably results in undesirable increases in mechanical complexity, friction and weight. Machine vision systems may overcome these difficulties with the possibility of highly accurate non-contact position sensing. This thesis presents the theoretical and practical requirements of a three DOF vision based orientation sensor, as well as the development and experimental investigation of hardware, software, and algorithms required to demonstrate a prototype working model.

The sensor system consists of an image sensor and a specially structured spherical grid. The grid is designed such that any position on its surface, equivalent to its orientation with respect to a fixed position, is uniquely determined by the line patterns at that position. The need for homing or the potential for accumulated errors found in incremental encoders is overcome. The algorithms necessary for defining the grid are presented, as well as the design of a mechanism, which includes mechanical, electrical, and software control components, used to produce grids. The orientation sensing itself consists of a machine vision algorithm for deducing the orientation of the grid by analyzing an image of the grid surface.

As part of a continuing investigation into the multi-DOF vision based sensor concept, this thesis accomplishes the following:

  • An introduction and review of the motivation for pursuing the sensor concept

  • The necessary theoretical background to support prototype development

  • Design issues allowing successful prototype implementation

  • The production and testing of essential sensor system elements

  • An analysis of design limitations and errors on system performance

  • A presentation of experimental results