Real-time estimation method of knee joint flexion and extension angle based on angular velocity

The real-time estimation of knee flexion and extension angle using wearable equipment plays an important role in rehabilitation evaluation and real-time control of the exoskeleton rehabilitation robot. Two angular velocity sensors on the thigh and shank are used to realize real-time estimation of the knee joint angle. Firstly, the joint axis of knee joint model is optimized by genetic algorithm offline. Then, the data of these two angular velocity sensors are projected on the joint axis online to calculate the angular velocity relative difference of knee joint flexion and extension angle. The knee joint flexion and extension angle can be achieved by integrating the angular velocity relative difference. Meanwhile, the angular velocity sensor on the shank is used to detect heel-strike in real time, and the angle is reset at this time to eliminate the integral drift. Five healthy people and two patients with knee osteoarthritis are recruited for experiments. Results show that the angular velocity sensor on the lower leg can accurately determine the event of heel-strike. Compared with the time of heel-strike determined by the pressure sensor, the average root mean square error (RMSE) of the healthy group is 27.88±19.64 ms. And RMSE of the patient group is 54.60±7.21 ms, which accounts for about 3% of the gait cycle. It has high accuracy and can meet the application requirements. The average RMSE of the healthy group is 2.86±0.53° and RMSE of the patient group is 2.00±0.78°. Experimental results show that the wearable position does not affect the angle estimated by the model. This method is easy to operate. The integral drift can be effectively eliminated. The real-time accurate estimation of knee flexion and extension angle can be realized. © 2020, Science Press. All right reserved.