What is a Servo Motor?
A servo motor is an electrical
device which can push or rotate an object with great precision. If you
want to rotate and object at some specific angles or distance, then you
use servo motor. It is just made up of simple motor which run through servo mechanism.
If motor is used is DC powered then it is called DC servo motor, and if
it is AC powered motor then it is called AC servo motor. We can get a
very high torque servo motor in a small and light weight packages. Doe
to these features they are being used in many applications like toy car,
RC helicopters and planes, Robotics, Machine etc.
Servo motors are rated in kg/cm (kilogram per
centimeter) most hobby servo motors are rated at 3kg/cm or 6kg/cm or
12kg/cm. This kg/cm tells you how much weight your servo motor can lift
at a particular distance. For example: A 6kg/cm Servo motor should be
able to lift 6kg if the load is suspended 1cm away from the motors
shaft, the greater the distance the lesser the weight carrying
capacity.
The position of a servo motor is decided by electrical pulse and its circuitry is placed beside the motor.
Servo Mechanism
It consists of three parts:
- Controlled device
- Output sensor
- Feedback system
It is a closed loop system where it uses positive
feedback system to control motion and final position of the shaft. Here
the device is controlled by a feedback signal generated by comparing
output signal and reference input signal.
Here reference input signal is compared to
reference output signal and the third signal is produces by feedback
system. And this third signal acts as input signal to control device.
This signal is present as long as feedback signal is generated or there
is difference between reference input signal and reference output
signal. So the main task of servomechanism is to maintain output of a
system at desired value at presence of noises.
Working principle of Servo Motors
A servo consists of a Motor (DC or AC), a
potentiometer, gear assembly and a controlling circuit. First of all we
use gear assembly to reduce RPM and to increase torque of motor. 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. 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
other source, will be processed in feedback mechanism and output will be
provided in term of error signal. This error signal acts as the input
for motor and motor starts rotating. Now motor shaft is connected with
potentiometer and as motor rotates so the potentiometer and it will
generate a signal. So as the potentiometer’s angular position changes,
its output feedback signal changes. After sometime the position of
potentiometer reaches at a position that the output of potentiometer is
same as external signal provided. 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, and in this situation motor stops rotating.
Controlling Servo Motor:
All motors have three wires coming out of them. Out of which two will be used for Supply (positive and negative) and one will be used for the signal that is to be sent from the MCU.
Servo motor is controlled by PWM (Pulse with
Modulation) which is provided by the control wires. There is a minimum
pulse, a maximum pulse and a repetition rate. Servo motor can turn 90
degree from either direction form its neutral 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° position, such as if pulse is
shorter than 1.5ms shaft moves to 0° and if it is longer than 1.5ms than
it will turn the servo to 180°.
Servo motor works on PWM (Pulse width modulation) principle,
means its angle of rotation is controlled by the duration of applied
pulse to its Control PIN. Basically servo motor is made up of DC motor which is controlled by a variable resistor (potentiometer) and some gears.
High speed force of DC motor is converted into torque by Gears. We know
that WORK= FORCE X DISTANCE, in DC motor Force is less and distance
(speed) is high and in Servo, force is High and distance is less.
Potentiometer is connected to the output shaft of the Servo, to
calculate the angle and stop the DC motor on required angle.
Servo motor can be rotated from 0 to 180 degree,
but it can go up to 210 degree, depending on the manufacturing. This
degree of rotation can be controlled by applying the Electrical Pulse
of proper width, to its Control pin. Servo checks the pulse in every 20
milliseconds. Pulse of 1 ms (1 millisecond) width can rotate servo to 0
degree, 1.5ms can rotate to 90 degree (neutral position) and 2 ms pulse
can rotate it to 180 degree.
All servo motors work directly with your +5V
supply rails but we have to be careful on the amount of current the
motor would consume, if you are planning to use more than two servo
motors a proper servo shield should be designed.
To learn more about servo motor working
principle and practical uses, please check below applications where
controlling of servo motor is explained with the examples:
