Input saturation is a well-known nonlinearity in mechanical control systems; it constrains the maximum acceleration, which results in a limitation of the system response time. Input saturation has been considered in controller design in various ways, e.g., anti-windup control. In addition to the input, the state variables of mechanical systems are often subject to saturation. For example, the maximum angular velocity of electric motor systems is limited by the maximum voltage provided for the motor. In the case of electronically commutated motors (i.e. brushless DC motors) the maximum speed is additionally constrained by limitations of the servo amplifier output. If gears are utilized, further constraints are introduced due to resonances in ball bearings and/or velocity dependent friction. Although such factors are significant in practice, they have not been fully considered in controller design. This paper investigates the input and output saturations and presents how they may be considered in the controller design; a Kalman filter, a PID controller, and a disturbance observer are designed in view of input/output saturations. A case study is provided to verify the proposed methods.

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