Design, Comparison, and Evaluation of Controllers for Direct Current Servomotors

Structure Of A Dc Servomotor

This section describes how a DC servomotor operates, regardless of whether it is of the analog or digital type. A servomotor is composed mainly of three elements: a DC motor; a position sensor to measure the rotational displacement; and an electronic amplifier or servo amplifier consisting of a set of microprocessors and power electronics that is in charge of coupling and conditioning to the motor the impedance and low power voltage signal coming from a digital system (Reyes, 2011).

Figure 1 shows a schematic representation of a servomotor. Physically, it is composed of a DC motor; a gear train to reduce the speed and increase the torque on the axle; a potentiometer connected with the output axle to know the position; and a control circuit that converts a PWM signal into voltage, comparing it with the feedback position and then amplifying it to activate an H bridge to produce a turn at a given speed (Urrea & Kern, 2014).

Figure 1.

Schematic representation of a servomotor

Source: Urrea & Kern, 2014

Modeling An Unloaded Servomotor

The DC motor is a power actuator device that provides energy to a load. The DC motor converts electric energy (DC) into mechanical rotational energy. A large part of the torque generated in the motor's rotor (included) is available for moving external loads. Due to characteristics such as high torque, speed controllability over a wide range, portability, well behaved couple-speed characteristics, and adaptability to diverse methods, DC motors are used in general in numerous control applications, including robotic manipulators, belt transport mechanisms, disc units, and machine tools (Dorf & Bishop, 2005). Figure 2 shows a diagram of a DC motor.

Figure 2.

Schematic representation of a DC motor

Source: Urrea and Kern, 2014