A DOB-based Parameter Identification method for Series Elastic Actuators without Load-side Encoder

This paper presents a novel parameter identification method for series elastic actuators (SEAs). Conventionally, such devices are equipped with (linear or rotary) encoders to measure the displacement of both the motor and the load sides. However, in some applications (e.g. wearable devices), it is difficult to mount a load-side encoder, due to cost or manufacturing issues. MEMS accelerometers have recently attracted considerable attention in this field, due to their low-cost and add-on feature. The proposed method replaces the load-side encoder with a MEMS accelerometer and relies on disturbance observers (DOBs). DOBs are famous for their ability to nominalize the plant by feedforward of the computed equivalent disturbance. Instead, the equivalent disturbance is used here for parameter identification, in an iterative fashion. The identification process is performed with two ad hoc closed-loop motor position tracking experiments, one for identifying the motor-side parameters and the spring stiffness, and one for the load-side parameters, exploiting the orthogonality of position, velocity, and acceleration signals. A theoretical analysis is provided for the applicability of the method, also when tracking errors and noise are present. Experimental results are provided to validate the proposed method.