The reliable operation of the sensors of robot joint drive systems (RJDs) is a key factor in ensuring the safety of equipment and personnel. Over the years, additional safety-related systems have been designed to prevent safety incidents caused by robot failures, ignoring the functional safety issues of the robot sensors themselves. In view of this, based on IEC61508, a functional safety analysis and design method for sensors of RJDs is proposed in this paper. Firstly, the hazard analysis and risk assessment clarified the goals that the safety protection function of the RJD’s sensor should achieve. Then, by establishing the motor drive model and transmission model, a model-based sensor fault diagnosis and isolation strategy is proposed. Considering the fault-tolerant operation of system, a fail-operational hardware architecture of the safety-related system is designed. Markov analysis shows that the safety integrity level (SIL) of safety-related systems can reach SIL3. Finally, experiments are designed to validate the proposed fault diagnosis and fault tolerance strategy. The results show that the safety-related system can effectively locate sensor failures, realize fault-tolerant control when a single sensor fails and perform safe torque off (STO) protection when multiple sensors fail.

Precise cable drive is a flexible frictional transmission method that transmits power from drive capstans to driven pulleys by a properly preloaded transmission medium. Since most of precise cable drive mechanism with high pointing precision proposed in previous studies suffer from low transmission stiffness and small output torque, this paper presents a novel many-to-one configuration precise cable drive mechanism (MCCDM) with high precision and large torque. The design methods of many-to-one configuration are studied including the system optimal design and cable wrapping design. The transmission characteristics of the developed configuration are analyzed, considering transmission capacity, transmission backlash, and transmission stiffness. The corresponding parameter sensitivities are also investigated. Moreover, a prototype of the MCCDM has been built, with which a series of experiments are carried out. It is noted that the developed mechanism has a motion range of ±2 rad. The steady-state error to a step reference of 1 rad amplitude is only 1.39 µrad and the position resolution is better than 0.5 µrad. This mechanism could achieve maximum output torque of 35 Nċm with compact size and light weight. It has reached a considerable toque-to-weight ratio of 8.997 N·m/kg. The experimental results show that the self-excited oscillation and backlash are dominant nonlinear effects in the MCCDM system. It shows that the self-excited oscillation is a function of the rotational displacement in such a system, with the highest peak at 0.1669 cycle/mrad and the amplitude of 34.63 µrad. The backlash could be measured about 60 µrad under exciting of a small amplitude and low frequency sinusoidal signal. The proposed technologies could have strong technological implications in some special applications such as antenna servo system.

Due to transmission backlash of precise cable drive in electro-optical targeting system, it is difficult to predict force and pointing precisely during target detection. This article aims to develop a rigorous model for predicting the cable tension and transmission backlash of precise cable drive in electro-optical targeting system to better evaluate the performance in the preliminary design stages. The model of transmission backlash in different phase is established by satisfying the system’s geometric and equilibrium constraints, which is applicable to cable drive systems with any external load, pretension, and motion trajectories. Simulations are conducted to reveal the changes of cable deformation, cable tension, and deflection angles. The model is validated by experiments using a suitable experimental setup, predicting transmission backlash with the error level of 8.27%. The sensitivities of the preload force and external load are tested, and corresponding results are in good agreement with the theoretical ones. The backlash plunges dramatically when the preload force is small and flattens out when the preload force is larger than the threshold value. A proper preload force should be applied to keep the backlash in stable state. The threshold value could be predicted by the proposed model. The method presented in this article could provide a theoretic guide for system design and nonlinearity compensation.

Friction is a complex nonlinear behavior and a significant factor that limits the performance improvement of servo systems. Drawing inspiration from the particular prestiction friction phenomenon exhibited by direct-drive systems upon sudden emergency stops, this paper introduces a dynamic and continuous friction model that includes pre-sliding and gross-sliding regimes. By analyzing the friction dynamics when the system velocity briefly reaches zero, a concave function related to the previous state of the system is used to describe the transition of friction in the pre-sliding regime. The Stribeck model is employed to represent the friction behavior in the gross-sliding regime, ensuring stationarity during friction regime switching. Based on the established friction model, a friction compensation method is developed in velocity control mode. The superior performance of this proposed friction compensation method is confirmed through sine-tracking experiments. Compared with the proportional integral controller and the Stribeck friction compensation method, the peak-to-peak value of the proposed method is reduced by up to 61.1 %, and the root-mean-square (rms) value is reduced by up to 81 %, with the smallest rms value reaching 0.13 mrad, significantly improving the dynamic tracking performance of the system.