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Servo Motor

Basically is a regular DC motor, with the output shaft feeding into a potentiometer, which is in turn connected to some control circuitry. The control circuitry compares the output of the pot with a control signal, and makes the motor move so that the output of the pot will match the control signal.
Servo motor consist of potentiometer, servo case, drive gears, control circuit, and output spline. Servos are controlled by pulse of variable length sent from a computer of radio and must be connected to a source of power. The amount of power required by the motor is proportional to the distance it needs to travel. The pulse travel through a control wire, the parameters of the pulse are maximum pulse, minimum pulse and a repetition rate. The servo motor expects to see a pulse every 20ms and the length of the pulse will determine how far the motor turns. For instance, a 1.5ms pulse will make the motor turn to 90 degree position. A pulse less than 1.5ms will move it in the counter clockwise direction towards the 0 degree position and more than 1.5ms will cause it to turn clockwise direction toward the 180 degree position.
servos are designed motors to be used in the application of robotics .They are used for precise position and control of high toque.

Types of Servo Motors

DC and AC servo motors.


A DC servo motor is when a DC motor is controlled by a servo motor by the process of servomechanism . Servomechanism is a closed loop system consisting of a closed device which is the controller,output sensor and a feedback system.


servos are mostly used to control position.. Mechanical position of the shaft can be sensed by using a potentiometer, which is coupled with the motor shaft through gears. The current position of the shaft is converted into electrical signal by the potentiometer, and the compared with the command input signal.
Command input is given according to the required position of the shaft. If the feedback signal differs from the given input, an error signal is generated. This error signal is then amplified and applied as the input to the motor, which causes the motor to rotate. And when the shaft reaches to the required position, error signal becomes zero, and hence the motor stays standstill holding the position.
The actual input applied to the motor is the difference between feedback signal (current position) and applied signal (required position), speed of the motor is proportional to the difference between the current position and the required position. The amount of power required by the motor is proportional to the distance it needs to travel.

How it is controlled

Usually the maximum movement of a servo motor can be 180 degrees .Three wires are taken out of a servo: positive, ground and control wire. A servo motor is controlled by sending a Pulse Width Modulated (PWM) signal through the control wire. A pulse is sent every 20 milliseconds. Width of the pulses determine the position of the shaft. For example, a pulse of
1ms will move the shaft anticlockwise at -90, a pulse of 1.5ms will move the shaft at the neutral position that 0 and a pulse of 2ms will move the shaft clockwise at +90.
.When servo motor is commanded to move by applying pulses of appropriate width, the shaft moves to and holds the required position. If an external force is trying to change the position of the shaft, the motor resists to change. Pulses need to be repeated for the motor to hold the position.

Here is our report on the servo motor control after six weeks of project.

-- Ruth Obo Adjaye - 2017-03-27

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Topic revision: r3 - 2017-05-01 - uli
 
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