A pose measurement system consisting of a small coordinate measuring machine, an IBM personal computer, and several fixtures is described. The pose measurement system is used to collect two data sets of 45 poses each. These data are then used to identify the parameters in five manipulator models of increasing complexity. The models consist of a nominal model, a model with six parameters to determine the actual robot location, a 12-parameter model including robot location and joint offsets, a 30-parameter kinematic model, and a 32-parameter model including compliance in two joints. The pose measurement system is used to evaluate the accuracy of each of the five models. The results demonstrate that, in general, increases in model complexity lead to enhanced accuracy, although the degree of improvement at any level is dependent on the particular robot under study.>

A calibration procedure for a manipulator in a typical industrial environment is investigated. The calibration problem is posed, and the constraints are listed for the particular task to be addressed. The robot geometry is investigated to locate the primary sources of inaccuracy. A calibration procedure is proposed that combines design modifications in the robot with active calibration fixtures in the workspace. Results of several tests are tabulated to demonstrate the success of the calibration procedure.>

There is a considerable amount of literature dealing with robot calibration. A classification hierarchy of calibration procedures is discussed which groups robot calibration by objective. The state of the art for each classification is discussed with citations from recent literature. Several proposed models are described. Based upon the common assumption of all lower pair joints, three concepts are discussed which may be applied for comparing most of the calibration models proposed in the literature. These concepts are completeness, equivalence, and proportionality. Each of these are discussed in detail in the text. Attention is also paid to the possibility that joints may actually be higher pairs. Although this possibility is commonly accepted in coordinate measuring machine calibration, it is not widely recognized in the robot literature. The paper discusses possible causes and effects of higher order joint pairs and demonstrates how they might be modeled.

An overview is given of the existing work on robot calibration, and some of the basic issues are identified in calibration and improvement of robot precision. Modeling, measurement, identification, and correction issues in robot calibration are discussed, and some of the unresolved questions are identified.

One of the most important specifications associated with a robot manipulator is repeatability. Unfortunately, there are no standard procedures for determining or specifying the repeatability of a robot. This situation has resulted in a good deal of confusion when attempting to use a published repeatability specification to determine if a robot will meet the requirements of an application. The purpose of this paper, therefore, is to propose a systematic procedure for the determination and specification of robot repeatability. To accomplish this, three aspects of repeatability analysis are examined. First, a method of collecting data to study repeatability is discussed. A data acquisition system is described as well as an associated test procedure. Next, a procedure for utilizing the acquired data to develop a repeatability specification for a robot is proposed. This procedure results in a specification of the robot's ability to orient objects as well as the classical specification for positioning repeatability. A methodical approach to determining where in the workspace the repeatability test should be made is also addressed. Finally, an approach is presented whereby the data collected during the repeatability test may be used to determine which joints contribute most to the variations in robot repeatability.