Reaction Force Estimation of Electro-hydrostatic Actuator Using Reaction Force Observer

Abstract

Electro-hydrostatic actuators (EHAs) are hydraulic actuators that are flexible and exhibit high backdrivability. Flexible operation and accurate detection of reaction forces are required for robots to be able to perform in environments in which they will be cohabiting with humans. However, nonlinear elements that degrade detection accuracy, such as static friction, backlash, and oil leakage, are present in hydraulic systems. In addition, pressure sensors in hydraulic systems are not very accurate at estimating reaction forces, because they cannot estimate internal forces and viscous friction. In this study, we propose a combination of control algorithms for accurately estimating reaction forces. Static friction is compensated by using feedback modulators. In addition, we use a backlash and oil leakage compensator, which do not require any models, to suppress the relative velocity between the motor-side and load-side. Then, the use of a reaction force observer (RFOB) that exploits both pressure sensors and encoders is proposed. The RFOB can be implemented because disturbances are linearized by the compensators. Experimental results show that reaction forces can be estimated with very high accuracy using the proposed RFOB. In addition, we implemented force control using the RFOB and evaluate the force tracking performances by improving the estimation accuracy.

Friction Compensated Force Control of Electro-Hydraulic System Using Fuzzy Controller

“…Friction affects the system performance, no matter the system is under position, velocity or force control. The mostly used method to counteract friction problems was to implement a model-based friction compensation [10][11][12][13][14][15][16][17] because of its less complexity compared to friction model-free compensation method. Implementing of a steady state friction compensation is straight forward and was effectively implemented in various applications [10][11][12][13].…”

“…The mostly used method to counteract friction problems was to implement a model-based friction compensation [10][11][12][13][14][15][16][17] because of its less complexity compared to friction model-free compensation method. Implementing of a steady state friction compensation is straight forward and was effectively implemented in various applications [10][11][12][13]. In the neural network training process of the five-axis milling center process force estimation, friction on each axis of the machine was estimated by Stribeck friction model with the use of velocity data on that axis [10].…”

“…The experiments in [11] were done under a constant known load force, therefore an accurate load force measurement is needed to guarantee its successful implementation in real-world applications. A proposed reaction observer consisted of a disturbance observer working in combination with compensations of static friction, backlash and oil leakage, could effectively estimate reaction force acting on a hydraulic actuator [12]. Steady state friction model of a flexible robot joint was constructed by curve fitting of piecewise continuous polynomial functions, and robot joint velocity control performance was improved with the friction compensation in the PI controller compared to the controller without compensation [13].…”

Backdrivability Improvement Method Based on Angular Transmission Error in a High Reduction Gear

“…This method achieves backdrivability by maintaining a constant position between the backlash, which requires a high-speed response to the load-side external torque, and is not easily accomplished in normal actuation. Another method assists the backdrive of an electrohydrostatic actuator using the transmission error of the angular velocity (14) . This method feedbacks the differentials of the input-and output-side velocities.…”

Fusion Hybrid Linear Actuator: Concept and Disturbance Resistance Evaluation

“…Pneumatic and hydraulic actuators have a higher power-to-weight ratio than electric actuators, but fluid actuators have a low control frequency. Electro-static hydraulic actuators, which have fluid transmission mechanisms to adjust the force using an electric motor, have been explored as a viable solution [12], [13]. Nevertheless, single actuators for simultaneous high thrust density and high control frequency remain far-fetched.…”