A New Method to Improve the Accuracy of Torque Estimation for Robotic Joint with Harmonic Drive Transmission
Fei Yu, Shu Xiao, Minghong Zhu, College of Automation, Harbin Engineering University, China
Abstract—Joint torque feedback(JTF) is widely considered as an effective technique for improving the performance of robot control. To impplement JTF to robot control schemes, how to get the measurement or estimation of the output torque transmitted by the system is essential. And there exist two popular methods to obtain the joint torque needed for feedback. One way is to equipe joint torque sensors which will certainly change the robot joint mechanical structure. Another method for estimating joint torque that adopts harmonic drive transmision based on motor-side and link-side position measurements is proposed in this paper. In the previous researchs, the output torque is obtained by sensors directly or by harmonic drive compliance mechanical model transimission. First method will have drawbacks in the sensor accuracy and environment temperature disturbance, and the other will have time delay and low output torque accuracy. What’s more , the first method that can be called built-in torque sensing also have a great cost for the force/torque sensor. So the second method provides a way to reduce the cost and also keep the robot joint mechanical structure properly.
For the second, harmonic drive is adopted to make the base of the whole tansmission system. Due to the trosional compliance of the harmonic drive of which are wave generator, flexspline and circular spline can be utilized for torque sensing, in recent years comercial high-resolution absolute position encorders are being applied in the hamonic drive trasmission system. The encorders are mounted in moter-side and link-side respectively, and input and output position measurements are directly obtained to provide the elements needed for caculating the tortional torque. Then the torque can be used together with the harmonic drive compliance model to achieve output joint torque.This method has been proved effective to get similar torque precision with the built-in comercial sensors. Because of advantages, the harmonic drive based transmission is widely adopted and more researches are being taken in this filed.
In this joint torque estimation method, a new filter mehod is adopted to improve the joint torque accuracy based on precise model and position measurements. More specifically, a new filter based on Unscented Kalman Filter ( UKF) is used together with the harmonic drive compliance model to achieve higher output joint toque accuracy. In this paper, the dynamic model of the harmonic drive –based joint model is developed at the beginning. Based on the proposed dynamic model , torsional deformation model and kinematic error model are developed to make compensations for the whole model. Then UKF technique is used together with the position measurements to estimate the output joint torque. This a new method of making a torque estimation based on the whole system dynamic model, which will effectively improve the accuracy of the output joint torque. The proposed method has been experimentally studied and its performance is compared with measurements of a commercial torque sensor. The results have verified the effectiveness of the proposed torque estimation method.