MATHEMATICAL MODELING OF HYDRAULIC DRIVE FOR THE MITSUBISHI RV-2FRL INDUSTRIAL ROBOT: INFLUENCE OF MOMENT OF INERTIA ON SERVO VALVE RESPONSE TIME

Abstract

This paper describes the application of alternative hydraulic drive systems to the Mitsubishi RV-2FRL industrial robot, a 6-axis manipulator with a payload of 2 kg and repeatability of ±0.02 mm. Although the robot originally utilizes AC servo motors, certain operating conditions (explosive environments, high force-to-weight ratio requirements) favour hydraulic actuation. The main contribution of this paper is a second-order mathematical model that explicitly links the robot's moment of inertia (J = 0.18 kg·m² for the J4 axis) with the servo valve time constant (tv). The model combines Newton's rotational dynamics, the hydraulic motor torque equation, first-order valve dynamics, and the oil continuity equation. The resulting transfer function yields the key relation: settling time ts. Simulation in MATLAB for varying tv (5 to 50 ms) and J (0.18 to 0.50 kg·m²) shows that to maintain the factory positioning accuracy of the RV-2FRL robot, the servo valve time constant must be below 10 ms. Valves with tv > 20 ms cause unacceptable phase lag and overshoot. It is demonstrated in this paper that hydraulic drive is theoretically feasible, but only high-performance servo valves (tv < 10 ms) satisfy the dynamic requirements of the robot, which explains why electric drives remain optimal for small precision robots.