Experimental and statistical determination of the static position repeatability and a recommended specification for a SCARA robot

Abstract

This paper describes an experiment and the statistical analysis performed to evaluate the repeatability of a SCARA robot and proposes an alternative form of specification for repeatability. Repeatability has been chosen because the precision obtained from teach-and-play-back programming for pick-and-place tasks, which is still widely used, depends primarily upon this performance value. The measuring method uses two theodolites, which though laborious in set-up is nevertheless simple, economical and sufficiently accurate for the purpose intended. The results are used for comparison with the given specification and for studying the influence of factors such as load, speed and spatial location in the working envelope using analysis of variance; the estimate of the repeatability value is calculated using the standard deviations of the measured coordinates of representative points modified by a factor from Student's t curves. From the analysis, it can be inferred that the repeatability value is dependent upon operating speed, load and plane location. The experimental study also points to a possible form of specification. For example, for the specification to be useful in the study of arrangement of feeders of parts to be assembled by the robot and the estimation of throughput given part weight, it could be given in a table form showing the range of the repeatability value for the designed combinations of speed, payload and position in the envelope.