Development of hysteresis based models for torque measurement in elastomer coupling

Abstract

This thesis evaluates the properties of elastomers which could be used in the drive train of a rehabilitation exoskeleton unit. They provide damping for safety purposes and, through angular displacement, provide an ability to measure the torque applied. The elastomers experience large amounts hysteresis as well as nonlinear strain. Consequently, extracting accurate torque data is complex and it is this challenge which forms the basis if this project. To extract this data accurately the aim was to measure the elastomers behaviour and then fit models to it. A testing rig was produced combining a displacement sensor and load cell. On this rig two commercially available polyurethane and two custom made spiders were tested. The data extracted and processed from this rig was fitted to a range of models incorporating multiple orders of springs, a damper, and two different hysteresis models. Additionally, the ability to incorporate a dead zone was designed into the model. These models were successfully fitted to all of the spiders. For both commercial spiders a variant of the Dahl model was the most effective model. We did however find both of these to be stiffer than ideal – hence making the less stiff spider the better solution of the two. This was tested for repeatability and found to be not perfectly repeatable, with an increase in error of around 11% for the best fitting model. For the custom models we found much lower errors – however there was a lower range of inputs that were suitable for these elastomers. In this case for both of these we found the best models to be second and third order damper models respectively, with a minimal difference between fits. The elastomer with the dead zone had a significantly higher error than the standard model, however we do see a definite improvement in accuracy as a result of the incorporation of the dead zone component in the model. In conclusion we successfully designed and implemented a modelling process that has provided models for the existing elastomers and provided a basis for evaluating future options and further development.