Servo motors are widely used in ROBOTICS as well as in INDUSTRIES, servo is the heart of ROBOTICS projects nowadays such as robotic arm, RC cars, radar dish, conveyor belts, antenna positioning, camera autofocus, printing process etc.
In many hobby projects, servo motors are widely used. I hope this article will provide you the brief information of what actually a servo is, the basic working principle of a servo motor, different variety of servo’s, basic comparison between servo motor and a normal DC motor.
Servo Motor Defination:
“ It is a device which provides motion in response to a command when regulates the motion in response to a negative feedback. “
The main feature of servo is to precisely control the position of its shaft. It is basically a closed loop system (i.e, negative feedback).
There are four main parts in closed loop system of servo:
- DC motor.
- Control circuit.
- Position feedback sensor.
The main part of a servo motor is small DC motor. These dc motors are of high speed (i.e, high RPM) and very low torque (i.e, twisting force responsible for doing work) . Basically, small dc motors are of very high speed but not have much higher torque (i.e, small force and greater distance ). But the design of gears inside servo convert this high speed and low torque of a dc motor to a slower speed and higher torque (i.e, a small distance, greater force).
Many dc motors are faster around 5000 RPM, but the gears reduce this very high RPM to about 60 RPM and high torque. Servo’s which are inexpensive have it’s gears made up of plastic in order to keep it’s weight lighter and low cost, while the servo’s designed for heavier work (i.e, very high torque) have their gears made up of metal for eg, servo’s used in industries.
When the power is first applied to the servo through input line, the dc motor rotates to a very high RPM and very small torque but due to gears, the shaft of the servo is slowed down.
Since the position feedback sensor is connected in a negative feedback arrangement in order to control the position of a servo precisely to the desired position. In industrial servo’s the position feedback sensor is a high precision encoder but in a hobby, servo’s the positional sensor is a potentiometer. The positional sensor (i.e, potentiometer) on the final gear is connected to the circuit which contain error detector ( i.e, this error detector is basically a comparator which compares two signals one is from input line also called target position and another is from the output through potentiometer also called actual position ) and a controller IC embedded on the board.
Due to the rotation of gears, the knob of the potentiometer rotates, as a result, the voltages is produced which is connected to the error detector. Now the error produced due to the comparison of two signal by the comparator, which is connected to the controller IC (i.e, this controller is an H-Bridge integrated on a chip). After comparison, an error is produced which enable the controller which decides the rotation of the shaft in either direction in order to achieve the target position.
Types of of SERVO’S:
Servo are available in different varieties.
- POSITIONAL ROTATIONAL SERVO:
The output shaft will rotate only 180 degrees. Additional stops are situated to limit the further rotation of shaft outside its range (for eg: ROBOTIC arms, rudders etc )
- CONTINOUS ROTATIONAL SERVO:
It is similar to positional rotation servo, but it can rotate for 360 degrees continuously. The commands sent to the servo will cause it to rotate in either clockwise or anticlockwise direction at varying speed ( for eg: RADAR dish etc).
- LINEAR SERVO:
The motion of this servo is back and forth direction rather than in circular direction (for eg: CNC machine, 3D printers etc).
BASIC COMPARISON BETWEEN SERVO AND DC MOTOR:
Servo motors can rotate 180 degrees back and forth but dc motors can rotate 360 degrees as long as powered is applied. Servo nowadays is also available in the continuous rotation which can rotate for 360 degree but due to the gear design servo’s speed is lesser as compared to dc motor. DC motors are the best choice for free running robots whereas when we need precision (for eg: ROBOTIC ARM, wipers boat rudders etc) servo is the best choice.
The speed of the dc motor is controlled by PWM (pulse width modulation) technique (i.e, by varying duty cycle we can control dc motor’s speed). For eg: duty cycle greater than 50% means speed is faster, a duty cycle less than 50% means speed is slower.
Whereas the PWM technique in servo motor determines the position of its shaft rather than speed. Once the command is sent to the servo, it will precisely set the position of the shaft to the desired position. Thus, this automatic adjustable behavior of its shaft makes the servo to be the perfect choice for ROBOTICS application.