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DC Servo Motors

TRM is a specialist in the design and manufacture of Motion Control, Tension Control and Temperature Control products and associated equipment for industrial machine and robot control.

The servomotors are different from the motors. The servomotors are designed to accelerate and decelerate faster. To achieve this the servomotors have bigger brushes and the armature is made of better materials. Generally the servomotors are smaller than the motors with same characteristic and therefore they weigh less and they have less inertia. For these characteristics servomotors are used in applications of motion control, numeric control and robotics. To close the control loop an encoder is used. Commonly, to achieve the precision wanted in the system, and the required torque, gears, pulleys, etc., are used. It is necessary to have clear that a servomotor is different from a motor. Motors can be used in feedback systems, but the performance of a motion control system will be never the same to the performance of the same system using servomotors. Since servomotors are special motors for motion control, they are also more expensive. Servomotors are divided in 3 classes: Brushed DC servomotors, Brushless DC servomotors and AC servomotors. We will talk about Brushed DC servomotors in this issue.

Fig 1. Servo Control

The brushed DC servomotor is the best for applications that require very high precision and low speed this is because this motor type is the most stable and has the lowest cogging factor. The Brushed DC servomotors offers the lowest cost in the whole system compared with the Brushless DC and AC servomotors. The cost of the brushed DC servos is similar at the cost of the brushless DC/AC servos, but the amplifier is relatively simple and it is around half of the cost of the amplifiers for the other two servos. The motion control system consists of a motion controller, an amplifier and a motor with coupled encoder, please see Fig 1. The motion controller is the one in charge of sending the motion signal to the motor, but before, the signal has to go through the amplifier. The controller signal is generally among + / - 10 V, where - 10 volts mean all the power in one direction, +10 volts all the power in the other direction and 0 volts at rest. This signal arrives to the amplifier (driver) which takes charge in transforming it into a PWM signal (Pulse Width Modulator)

Servomotors are special motors that adapt better to the requirements of motion control, numeric control and robotics for their acceleration and deceleration characteristics and low inertia.

Fig 2. PWM signal controls speed & direction of servomotor

This signal is the one that takes charge of running the motor varying the duty cycle. The duty cycle is the time in that the signal remains in "1", being a duty cycle of 50% when the signal time in "1" is similar to the signal time in "0" in a complete period of time. At rest the motor receives full power of the amplifier, the PWM signal corresponding to this state is like is shown in the figure 2.A, with a duty cycle of 50%. In 50% of the time the power is applied in one direction and for the other 50% in the other direction. When the controller command the motor to move, the duty cycle of the PWM signal changes, applying a higher percentage of the power to one pole of the motor. The bigger or smaller the duty cycle the bigger will be the power applied in one or another direction and for this reason the motor will rotate faster, please see figures 2.B and 2.C, where the duty cycle is of the 25 and 75% respectively. In the figure 2.B the servo rotate at high speed in one direction and in the figure 2.C The servo rotates to the same speed but in the opposite direction. The proportion of the duty cycle is based on calculations that include the wanted speed, acceleration, true speed measured by the encoder, and an algorithm PID to stabilise the system. It is possible to over stress the motor for a limited time, in some cases it is possible to obtain as much as 200% of the rated torque from the motor. Careful design is required to ensure that the motor is not over stressed which can cause the motor to overheat and damage the motor.

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