Servo Motor – Principle and Working with Some Important Concept

Many of us know a little about Servo Motors. So I thought of sharing complete information about the principle, construction ,working of Servo Motors and few interesting points about Servo motors.

The word servo is derived from the Latin word servus means slave. Slave means who works based on the order of owner.

Hey now it’s very simple to understand that is servo means slave implying that servo motor are the  things which listen our words through programming instructions.

Servo motor is a closed loop servo mechanism that uses position feedback to control motion and final position.

Designing a servo controller is a difficult process when it is  compared to other motors.

Most of the hobbyist servos have a box shape with three wires as in the below figure.

Have you ever tried to peep into the servo box? Then I will take you inside the box in this article.

We know the DC motors can be controlled by sending pulse width modulation signals (PWM).

What is PWM?

When you have a switch you can complete ON or OFF the motor but if you want to the motor to rotate 50% of the full speed what will you do?

let’s introduce controllers that govern the motors behavior by driving the pulses that open and close the switch for pieces amount of time. This modulation is called pulse width modulation.


The controller delivers a series of pulses to the gate of the switch. These pulses open and close this switch and switch tells us pulses of the current to the motor. To controller generate pulses at equal intervals, so that the pulse, the more current reaches the motor and fast it runs.

Duty cycle is the term used to describe the PWM.

In simple terms duty cycle is the ratio of the time which motor is on to the total time.

Choosing the right PWM frequency is critical. In making this decision, we have two important factors to consider:

• If the frequency is too low, the rise/fall of the power reaching the motor will cause it to rotate in a rough, jerky fashion.

• If the frequency is too high, the pulses will be too narrow to open and close the switches properly. In addition, the electromagnets will generate heat, decreasing the motor’s efficiency.

There are two types of servo motors analog and digital and according to the controller perspective there is no change in the functionality the changes are only in the internal circuit.

Rotatory encoders

To convert any hobbyist server into good servo motor, mechanism must be attached to the server that identifies the shaft angle and position.

These are of two types

1. Optical encoders

2. Magnetic encoders

Let us discuss only optical encoders as magnetic encoders are used in the air case.

Optical Encoders

Of the many encoders used with servos, optical encoders are the simplest and most common. Their operation is made possible by a disk connected to the motor’s shaft. This disk is transparent in some areas and opaque in others.

On one side of the disk, a light source directs light at one portion of the disk. On the other side, an optical sensor measures how much light passes through. The sensor delivers its results to a processor, which may assign a 1 to the presence of light and a 0 to the absence.

The disk’s pattern of transparent and opaque regions usually takes one of two forms shown below.

Optical Encoder Disks: Incremental and Absolute

The disk on the left has alternating stripes of transparent and opaque regions. As the shaft turns, the optical sensor measures the time between successive flashes of light. The processor uses this to determine how quickly the motor’s shaft is turning. Because it provides speed but not position, this type of encoder is called an incremental encoder .

In contrast, the disk on the right is used by absolute encoders because it identifies the shaft’s angle as well as its speed. In this case, the light shines along an axial stripe that has alternating transparent/opaque regions. The optical sensor detects this light and passes multiple readings to the processor. The microprocessor converts the pattern of light and darkness into a number and uses it to determine the shaft’s approximate angle.

Open-Loop and Closed-Loop Systems

If a controller receives feedback identifying a servo’s shaft angle, it can measure the angle over time

to determine the motor’s speed and acceleration. If the motor deviates from the desired behaviour, the controller will send control signals to reduce the deviation.

This exchange of information—the servo provides its position, the controller provides control signals—forms a loop. For this reason, systems with feedback are referred to as closed-loop systems.

Systems without feedback are open-loop systems. The block diagrams below illustrate the difference between these two systems.

Summary

With hobbyist servos, feedback isn’t a concern. These servos are simply DC motors that can be controlled with PWM signals. They come in two types: digital and analog. A digital hobbyist servo contains a microprocessor that receives the controller’s pulses and delivers pulses to the motor. Three advantages of digital servos over analog servos are that they’re more responsive, provide greater torque, and can be configured through programming.

Any electric motor can be converted into a servo by attaching a rotary encoder. This provides feedback to a controller that identifies the shaft’s angle. The most common rotary encoders use light sensors that measure light passing through a disk. These optical encoders may be absolute, which means they provide angular position and speed, or incremental, which means they provide speed without identifying the specific angle.

Final Note

Servos are the commonly used motors so we need learn more about them. If you have any queries about the article write down in the comment box below.