Servo Motor

A servo motor is a linear or rotary actuator that provides fast precision position control for closed-loop position control applications.

      There are some special types of application of electrical motor where rotation of the motor is required for just a certain angle not continuously for long period of time. For these applications, some special types of motor are required with some special arrangement which makes the motor to rotate a certain angle for a given electrical input (signal). For this purpose servo motor comes into picture.

Servo Motor Working Principle:

Before understanding the working principle of servo motor we should understand first the basic of servomechanism.

Servomechanism

A servo system mainly consists of three basic components – a controlled device, a output sensor, a feedback system. This is an automatic closed loop control system. Here instead of controlling a device by applying the variable input signal, the device is controlled by a feedback signal generated by comparing output signal and reference input signal. When reference input signal or command signal is applied to the system, it is compared with output reference signal of the system produced by output sensor, and a third signal produced by a feedback system. This third signal acts as an input signal of controlled device.

        This input signal to the device presents as long as there is a logical difference between reference input signal and the output signal of the system. After the device achieves its desired output, there will be no longer the logical difference between reference input signal and reference output signal of the system. Then, the third signal produced by comparing theses above said signals will not remain enough to operate the device further and to produce a further output of the system until the next reference input signal or command signal is applied to the system. Hence, the primary task of a servomechanism is to maintain the output of a system at the desired value in the presence of disturbances.

Working principle of Servo motor:

A servo motor is basically a DC motor along with some other special purpose components that make a DC motor a servo. In a servo unit, you will find a small DC motor, a potentiometer, gear arrangement and an intelligent circuitry. The intelligent circuitry along with the potentiometer makes the servo to rotate according to our wishes.

Say at initial position of servo motor shaft, the position of the potentiometer knob is such that there is no electrical signal generated at the output port of the potentiometer. This output port of the potentiometer is connected with one of the input terminals of the error detector amplifier.

Now an electrical signal is given to another input terminal of the error detector amplifier. Now difference between these two signals, one comes from potentiometer and another comes from external source, will be amplified in the error detector amplifier and feeds the DC motor. This amplified error signal acts as the input power of the DC motor and the motor starts rotating in desired direction.

As the motor shaft progresses the potentiometer knob also rotates as it is coupled with motor shaft with help of gear arrangement. As the position of the potentiometer knob changes there will be an electrical signal produced at the potentiometer port. As the angular position of the potentiometer knob progresses the output or feedback signal increases. After desired angular position of motor shaft the potentiometer knob is reached at such position the electrical signal generated in the potentiometer becomes same as of external electrical signal given to amplifier. At this condition, there will be no output signal from the amplifier to the motor input as there is no difference between external applied signal and the signal generated at potentiometer. As the input signal to the motor is nil at that position, the motor stops rotating. This is how a simple conceptual servo motor works.

The control input to a servo is a pulse width modulated (PWM) signal, generally of frequency 50 Hz. This means the pulse should repeat every 20ms. A typical value of the pulse width is somewhere in the range of 1.0 to 2.0 milliseconds (ms). For a standard servo, a pulse width between 1.0 ms to 1.5 ms makes the servo to turn clockwise (CW), between 1.5 ms to 2.0 ms makes it to turn counterclockwise (CCW), and a 1.5 ms pulse width turns the servo motor to its center.

Most servos rotate through 180°. However there are some that could rotate through a full 360° or more. Servos are widely used as the moving joints in robotic arms for their precise angular positioning. They also find applications in radio controlled (RC) toys. For example, in RC cars they are used in the steering mechanisms, and in RC boats to control the rudder.

A servomotor has three wires: two are designated for power supply (Vcc and Ground) and the third wire is for the control signal. The Vcc wire is usually red and the ground one is either black or brown. The control signal wire comes in white, yellow, or orange color.

Types of servo motors

Servos come in many sizes and in three basic types: positional rotation, continuous rotation, and linear.

• Positional rotation servo: This is the most common type of servo motor. The output shaft rotates in about half of a circle, or 180 degrees. It has physical stops placed in the gear mechanism to prevent turning beyond these limits to protect the rotational sensor. These common servos are found in radio-controlled cars and water- and aircraft, toys, robots, and many other applications.
• Continuous rotation servo: This is quite similar to the common positional rotation servo motor, except it can turn in either direction indefinitely. The control signal, rather than setting the static position of the servo, is interpreted as the direction and speed of rotation. The range of possible commands causes the servo to rotate clockwise or counterclockwise as desired, at varying speed, depending on the command signal. You might use a servo of this type on a radar dish if you mounted one on a robot. Or you could use one as a drive motor on a mobile robot.
• Linear servo: This is also like the positional rotation servo motor described above, but with additional gears (usually a rack and pinion mechanism) to change the output from circular to back-and-forth. These servos are not easy to find, but you can sometimes find them at hobby stores where they are used as actuators in larger model airplanes.

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