Modeling and Identification of Wear in a Robot Joint under Temperature Uncertainties

Abstract

This paper considers the problem of wear estimation in a standard industrial robot joint. Motivated by the observation that the wear processes in a robot actuator cause an increase of the friction levels in the joint, static friction observations are used to infer the wear levels. An existing static friction model is extended to include the eects of wear, which are analyzed and modeled. The resulting model can explain friction in a robot joint with respect to changes in speed, load, temperature and wear. Based on this model and a test-cycle that generates static friction observations, a modelbased wear estimator is proposed. The performance of the estimator is found both by means of simulations and experiments on an industrial robot. As it is shown, the method can distinguish the eects of wear even under large temperature variations, opening up for the use of robust joint diagnosis for industrial robots. Abstract: This paper considers the problem of wear estimation in a standard industrial robot joint. Motivated by the observation that the wear processes in a robot actuator cause an increase of the friction levels in the joint, static friction observations are used to infer the wear levels.An existing static friction model is extended to include the effects of wear, which are analyzed and modeled. The resulting model can explain friction in a robot joint with respect to changes in speed, load, temperature and wear. Based on this model and a test-cycle that generates static friction observations, a model-based wear estimator is proposed. The performance of the estimator is found both by means of simulations and experiments on an industrial robot. As it is shown, the method can distinguish the effects of wear even under large temperature variations, opening up for the use of robust joint diagnosis for industrial robots.

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“…Effectively, the optimal weighting function selects a speed region that is more relevant for robust wear monitoring, giving less relevance for data collected at speeds close to zero or higher than 100 rad/s. A similar behavior was found in [2] for the quality (variance) of a wear estimate for different speeds under temperature disturbances.…”

“…Coriolis and centrifugal), gravity-induced torque, nonlinear stiffness and friction at that joint. The friction term in (11b) considers its relation to joint speedφ, the manipulated load τ l , the temperature inside the joint T , and the wear levels w. These dependencies of friction are motivated from the experimental studies presented in [3,2] and are illustrated in Figure 2 * . The deterministic unknown input of interest, i.e.…”

“…where w c = 35 is a wear level considered critical to generate an alarm (see [2] for details of the wear model). Here, T is considered random, with uniform distribution given by T = 30…”

“…Parameter estimation is a natural approach because it can use the physical interpretation of the system, see e.g. [12,2,14]. Deriving reliable robot models from physics and identification experiments is however an involving task, see e.g.…”

“…An example of such fault is studied in [14], where an observerbased method is suggested for health monitoring of the actuators' lubricant. In [2], a method is proposed for robust wear identification in a robot joint under temperature disturbances; the method is based on a custom designed identification experiment, i.e. a test-cycle, and a known friction model which can describe the effects of speed, load, temperature and wear.…”