Servo Motor construction working and control

Servo motor is revolving or linear motion actuator, that allows the precisely control of its linear velocity, acceleration or angular position. This motor is available in market with coupled sensor for feedback control. This motor is control with sophisticated controller or a special kind of module is designed for controlling this motion. Servo motor is not a special class of motor, actually the servo word is used for that motors, which is used for control loop system. Servo motor is usually interfaced with microcontroller for control applications.  A simple servo motor with feedback sensor is shown in figure 1

A Simple Servo Motor with Feedback Sensor

Servo Motor Mechanism:

The servo motor basically composed of three basic parts.

  1. Controlling Device
  2. Output Sensor
  3. Feedback system

The Servo motor works on the basis of automatic closed loop system, for this system a controller is required, which is composed of a comparator and a feedback path. The controller has two inputs and one output. In controller the comparator compares the output signal with the desired reference signal for producing the input for servo motor. The output signal is sense through the sensor. The input signal for the motor is called the feedback signal. The motor works on the basis of this feedback signal. The comparator signal is basically logic signal for motor. If the logical difference is high, then the motor would be on for desired time and the if the logical difference is low then the motor would be off for desired time. Actually the in controller the comparator would be decided the motor would be on or off. A good controller is mandatory of the proper functional of this motor.

Construction of Servo Motor:

Servo motor is Dc motor which consist of following parts

  1. Stator winding
  2. Rotor winding
  3. Bearing
  4. Shaft
  5. Encoder

The servo motor consists of two winding stator and rotor winding. The stator winding is wound on stationary part of the motor and this winding is also called field winding of the motor, this winding could the permanent magnets. The rotor winding is wound on the rotating part of the motor and this winding is also called the armature winding of the motor. The motor consists of two bearing on front and back side for the free movement of shaft. Shaft is basically the iron rod on which the armature winding is coupled. The encoder has the approximate sensor for telling the rotational speed and revolution per minute of the motor. The construction of servo motor is shown in figure 2

2 Servo Motor Construction

Working Principle of Servo Motor:

Servo motors is actually the dc motor and all dc motors work on the principle of Fleming s left hand rule. This rule is used for determining the direction of force which act on the DC motor armature conductor. This rule tells us if we extend our left hand,index finger, middle finger and thumb in such a way that shown in figure 3

Figure 3 Fleming s Left Hand Rule

Then the index finger shows the magnetic field which is perpendicular to direction of current, shows the middle finger in figure. When the current carrying conductor is placed in magnetic field then the conductor experience a force in the direction, which is perpendicular to the both direction of magnetic field and direction of current that is shows by the thumb in figure 3

Servo Motor Control:

With the experimental point of view, the servo motor is control with the controller or module here for understating the servo motor control we will the explain the control with an example. Consider a motor is rotate with an angle 45degree by a given signal then stop for some time and wait for the further instruction. The shaft of this motor is coupled to another shaft with a gear assembly for decreasing the high rpm in to the low rpm (revolution per minute) at the output side and that shaft is called output shaft shown in figure 4

Figure 4 Servo Motor Control

According to the figure a potentiometer is adjust at the voltage adjusting knob and this knob is coupled with another gear assembly at the output shaft. During the rotation of the output shaft the potentiometer knob is also rotate and the potentiometer creates the electrical potential according to rotating output shaft. This electrical potential is basically the reference output signal. When the shaft rotates 0 to 45 degree then the signal is produced according to this rotation. The reference output signal and reference input signal is given to the error detector amplifier, this error detector amplifier decided when the motor would be on and when would be off. When the error detector amplifier shows the positive difference between input reference signal and output reference signal then the motor moves in forward direction to that angle according to the output shaft and when the error detector amplifier shows the negative difference then the motor moves in reverse direction for that angle. Similarly, when the error detector amplifier shows the zero difference then the servo motor remains stationary till for that time until the error of error detector amplifier goes to positive or negative. These are the simple control logic’s of servo motor control.

Servo Motor Interfacing with Microcontroller:

Servo motor is one of the most special motor that can be used for precise angular movement. The main advantage of this motor is that if we are controlling this motor with microcontroller then there is no need any feedback controller or mechanism. This motor is mostly used in industrial and commercial application. Its working principle and operation is very simple. It consists of three wires, two wires are used for power on the motor and third is used of control signal. A servo motor is control through ATMEL 8051 Shown in figure 5

Figure 5 Servo Motor Control with ATMEL8051

According to the figure the one wire is connected to the VCC of microcontroller, second wire is connected to the ground and third wire is connected to the any output port of microcontroller, which have configured in code. PWM (pulse width modulation) technique is used of controlling the servo motor control signal and its angular position. 11.0592 MHz crystal oscillator is used for clock pulse and 22 Pf capacitor is used for the stable operation of crystal oscillator. Here theservo is used for changing the angular position from 0 to 180-degree and the duty cycle is controlled between 1ms to 2ms shown in figure 6

Figure 6 Servo Motor Angular Rotation Control

Applications of Servo Motor:

  1. It is used in robotic industry of position control.
  2. It is used in robotic arms.
  3. It is used in press and cutting industry for the cutting and pressing the piece precisely.
  4. It is used in conveyer belt for start and stop the conveyer belt at every position.
  5. It is used in digital cameras for auto focusing.
  6. It is used in solar tracking system for tracking the sun at every precise moment of time.
  7. It is used in labeling and packing industry for labels the monogram and packing the things

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