SERVO MOTOR

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.

Simple Definition of a Servo Motor

This is normally a simple DC motor which is controlled for specific angular rotation with help of additional servomechanism (a typical closed loop feedback control system). It has a rotary actuator that allows for precise control of angular position, velocity and acceleration with great precision. They are small in size but pack a big punch and are very energy-efficient. A simple Servo Motor is shown in figure 1a and the internal structure is shown in figure 1b.

How is it different from a normal DC Motor?

The main reason behind using a Servo is that it provides angular precision, i.e. it will only rotate as much we want and then stop and wait for next signal to take further action. This is unlike a normal Electrical DC Motor which starts rotating as and when power is applied to it and the rotation continues until we switch off the power. We cannot control the rotational progress of electrical motor; but we can only control the speed of rotation and can turn it ON and OFF.

Now what is Servo mechanism?

A Servo mechanism sometimes shortened to “Servo”, works on mainly three basic components – a controlled device, an output sensor, a feedback system.

There is an automatic closed loop control system. Here instead of controlling a device by applying 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 feedback system. This third signal acts as 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 output signal of the system.

After the device achieves its desired output, there will be no longer logical difference between reference input signal and reference output signal of the system. Then, third signal produced by comparing theses above said signals will not remain enough to operate the device further and to produce 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 a Servo Motor

A Servo Motor is basically a DC motor (in some special cases it is AC motor) along with some other special purpose components that make a DC motor a servo. In a servo unit, we can find a small DC motor, a potentiometer, gear arrangement and an intelligent circuitry as shown in figure 2. The intelligent circuitry along with the potentiometer makes the servo to rotate according to our needs.

As we know, a small DC motor will rotate with high speed but the torque generated by its rotation will not be enough to move even a light load. This is where the gear system inside a servomechanism comes into picture. The gear mechanism will take high input speed of the motor (fast) and at the output, we will get an output speed which is slower than original input speed but more practical and widely applicable. We will see the working as follows now:-

1. Now at the 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.
2. This output port of the potentiometer is connected with one of the input terminals of the Error detection Amplifier as shown in figure 3.
3. Now an electrical signal is given to another input terminal of the error detector amplifier.
4. Now difference between these two signals, one comes from potentiometer and another comes from external source, will be amplified in the Error Detection Amplifier and feeds the DC motor.
5. This amplified error signal acts as the input power of the dc motor and the motor starts rotating in desired direction.
6. As the motor shaft progresses the potentiometer knob also rotates as it is coupled with motor shaft with help of gear arrangement.
7. As the position of the potentiometer knob changes there will be an electrical signal produced at the potentiometer port.
8. As the angular position of the potentiometer knob progresses the output or feedback signal increases.
9. After desired angular position of motor shaft the potentiometer knob is reaches at such position the electrical signal generated in the potentiometer becomes same as of external electrical signal given to error detection amplifier.
10. At this condition, there will be no output signal from the Error Detection Amplifier to the motor input as there is no difference between external applied signal and the signal generated at potentiometer.
11. As the input signal to the motor is nil at that position, the motor stops rotating.
12. This is how a simple Servo Motor works.

How is the Servo controlled?

Servos are controlled by sending an electrical pulse of variable width, or pulse width modulation (PWM), through the control wire. There is a minimum pulse, a maximum pulse, and a repetition rate. A Servo Motor can usually only turn 90 degrees in either direction for a total of 180 degree movement. The motor’s neutral position is defined as the position where the servo has the same amount of potential rotation in the both the clockwise or counter-clockwise direction as shown in figure 4 below.

The PWM sent to the motor determines position of the shaft, and based on the duration of the pulse sent via the control wire; the rotor will turn to the desired position. The Servo Motor expects to see a pulse every 20 milliseconds (ms) and the length of the pulse will determine how far the motor turns. For example, a 1.5ms pulse will make the motor turn to the 90-degree position. Shorter than 1.5ms moves it to 0 degrees, and any longer than 1.5ms will turn the Servo to 180 degrees. When these Servos are commanded to move, they will move to the position and hold that position. If an external force pushes against the servo while the servo is holding a position, the Servo will resist from moving out of that position. The maximum amount of force the Servo can exert is called the Torque Rating of the Servo. Servos will not hold their position forever though; the position pulse must be repeated to instruct the Servo to stay in position.

Simulation Example of a Servo Motor

This design demonstrates the operation of a PWM controlled servo in figure 5 along with signal waveform in the Oscilloscope. The position of the motor arm is determined by the width of the control pulse. Typically, the pulse ranges from 1ms to 2ms and repeats at a rate of 50Hz. The motor arm can rotate over a range of 180 degrees.

In figure 6, we can see that when the potentiometer is set to a neutral position, the Servo Motor rotates to 0 degree.  As seen in Oscilloscope and waveforms below, a 1.5ms pulse will make the motor turn to 0 degree position or a neutral position as in figure 7.

And similarly in figure 8, we can see that when the potentiometer is set to low, the Servo Motor rotates at -90 degrees ans shorter than 1ms moves it to -90 degrees as in figure 9

But when a high voltage is supplied, the Servo Motor is at +90 degrees as shown in figure 10  and any longer than 1.5ms will turn the servo to +90 degrees as in figure 11.

Servo Motor Applications

Servos are found in many places: from toys to home electronics to cars and airplanes. If you have a radio-controlled model car, airplane, or helicopter, you are using at least a few servos. Servos also appear behind the scenes in devices we use every day. Electronic devices such as DVD and Blu-ray Disc players use servos to extend or retract the disc trays. In automobiles, Servos manage the car’s speed: The gas pedal, similar to the volume control on a radio, sends an electrical signal that tells the car’s computer how far down it is pressed.

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