# SERVOMOTOR

## What is a servomotor?

It is an electromechanical device consisting of an electric motor, a set of gears and a control card, all constituted within a plastic housing. It can control its angular positions, that ism it can be located in any position within a range of operation generally of 180° but can be easily modified to have a 360° free rotation. Due to their high accuracy of angular position they are usually used in robotics, automatics and modeling (vehicles by radio-control, RC).

Its operation is due to the pulse width modulation (PWM), they have three cables, two for power Vcc and Gnd (varying from 4.8 V to 6 V) and a third cable to apply a train of control pulses, with the purpose of the internal differential control circuit locating the servomotor in the indicated position.

The frequency used to send the pulse sequence to the servomotor is 50 Hz, which means that each cycle lasts 20 milliseconds. These durations of each pulse are interpreted as motor positioning commands, while the spaces between each pulse are disregarded.

In general, a pulse between 0.5-1.0 ms corresponds to 0°, a pulse of 1.5 ms corresponds to 90° and a pulse between 2.0-2.5 ms corresponds to 180°.

Due to the variety of brands of servomotors there are small differences between the duration of the cycles, colors to identify the cables among others, therefore Table 1 specifies some of the technical characteristics of various brands in the market.

There are many types of servomotors but its main feature is the ability to precisely control the position of its axis. It must be taken into consideration that a servomotor is a closed circuit system with position feedback to control its movements and know the position.

## PARTS OF A SERVOMOTOR

In general, servomotors are usually composed of 4 fundamental elements:

### CONTINUOS CURRENT MOTOR (DC)

It provides mobility to the servomotor depending of the polarity of its connection, this means that when a potential is applied to its two terminals the motor returns in one direction at its maximum speed, if the voltage applied to its two terminals reverses the direction of turn is also reversed.

### REDUCING GEARS

The gear train is responsible for reducing the speed of the engine its torque capacity (torque).

### DISPLACEMENT SENSOR

It can be a potentiometer placed on the servomotor output shaft, it is usually used to identify the angular position of the motor (encoder).

### CONTROL CIRCUIT

It is an electronic board that adjusts an angular position control strategy through feedback. To do this the circuit compares the reference input signal or desired position with the current position measured by the potentiometer. The difference between the current position and the desired position is amplified and used to move the motor in the direction necessary to reduce the position error.

## TYPES OF SERVOMOTORS

Analog and digital servomotors are equal in terms of the user level: they are made up of the same structure (DC motor, reduction gears, potentiometer and control board) and are controlled with the same type of PWM signal. The main difference is due to the addition of a microprocessor in the control circuit of the digital servomotors, which is responsible for processing the input PWM signal and control the motor by pulses with a frequency 10 times higher than the frequency of servomotors analog. There are two types of DC servomotors: the classic brushless ones, and the new BLDC brushless ones.

### CLASSIFICATION ACCORDING TO TOPOLOGY

• Three phase AC induced engine
• Motor type brush DC
• Brushless servomotor (AC and DC)
• Engine by steps
• Switchedreluctance Motors
• Linear engine

### CLASSIFICATION ACCORDING TO TYPE OF MOVEMENT

• Rotary engines
• Linear engines
• Angular engines

Erick Méndez