Servo Motor – Working, Advantages & Disadvantages

Servo implies an error sensing feedback control which is utilized to correct the performance of a system. It also requires a generally sophisticated controller, often a dedicated module designed particularly for use with servomotors. Servo motors are DC motors that allow for precise control of the angular position. They are DC motors whose speed is slowly lowered by the gears. The servo motors usually have a revolution cut off from 90° to 180°. A few servo motors also have a revolution cutoff of 360° or more. But servo motors do not rotate constantly. Their rotation is limited between the fixed angles.

The servo motor is an assembly of four things: a normal DC motor, a gear reduction unit, a position-sensing device, and a control circuit. The DC motor is connected with a gear mechanism that provides feedback to a position sensor which is mostly a potentiometer. From the gearbox, the output of the motor is delivered via servo spline to the servo arm. For standard servo motors, the gear is normally made up of plastic whereas, for high power servos, the gear is made up of metal.

A servo motor consists of three wires- a black wire connected to the ground, a white/yellow wire connected to the control unit, and a red wire connected to the power supply.

The function of the servo motor is to receive a control signal that represents a desired output position of the servo shaft and apply power to its DC motor until its shaft turns to that position.

It uses the position-sensing device to figure out the rotational position of the shaft, so it knows which way the motor must turn to move the shaft to the instructed position. The shaft commonly does not rotate freely around similar to a DC motor, however rather can just turn 200 degrees.

From the position of the rotor, a rotating magnetic field is created to efficiently generate toque. Current flows in the winding to create a rotating magnetic field. The shaft transmits the motor output power. The load is driven through the transfer mechanism. High-function rare earth or other permanent magnet is positioned externally to the shaft. The optical encoder always watches the number of rotations and the position of the shaft.

Working of a Servo Motor

Servo Motor consists of a DC Motor, a Gear system, a position sensor, and a control circuit. The DC motors get powered from a battery and run at high speed and low torque. The Gear and shaft assembly connected to the DC motors lower this speed into sufficient speed and higher torque. The position sensor senses the position of the shaft from its definite position and feeds the information to the control circuit. The control circuit accordingly decodes the signals from the position sensor and compares the actual position of the motors with the desired position and accordingly controls the direction of rotation of the DC motor to get the required position. Servo Motor generally requires a DC supply of 4.8V to 6 V.

Controlling a Servo Motor

A servo motor is controlled by controlling its position using Pulse Width Modulation Technique. The width of the pulse applied to the motor is varied and send for a fixed amount of time.

The pulse width determines the angular position of the servo motor. For example, a pulse width of 1 ms causes an angular position of 0 degrees, whereas a pulse width of 2 ms causes an angular width of 180 degrees.

Advantages:

• If a heavy load is placed on the motor, the driver will increase the current to the motor coil as it attempts to rotate the motor. There is no out-of-step condition.
• High-speed operation is possible.

Disadvantages:

• Since the servomotor tries to rotate according to the command pulses but lags, it is not suitable for precision control of rotation.
• Higher cost.
• When stopped, the motor’s rotor continues to move back and forth one pulse, so that it is not suitable if you need to prevent vibration

7 Applications of Servo Motors

Servomotors are used in applications requiring rapid variations in speed without the motor getting overheated.

• In Industries they are used in machine tools, packaging, factory automation, material handling, printing converting, assembly lines, and many other demanding applications robotics, CNC machinery, or automated manufacturing.
• They are also used in radio-controlled airplanes to control the positioning and movement of elevators.
• They are used in robots because of their smooth switching on and off and accurate positioning.
• They are also used by the aerospace industry to maintain hydraulic fluid in their hydraulic systems.
• They are used in many radio controlled toys.
• They are used in electronic devices such as DVDs or Blue-ray Disc players to extend or replay the disc trays.
• They are also being used in automobiles to maintain the speed of vehicles.

Application Circuit of Servo Motor

From the below application circuit: Each motor has three inputs: VCC, ground, and a periodic square wave signal. The pulse width of the square wave determines the speed and direction of the servo motors. In our case, we just need to change the direction to allow the device to move forward, backward, and turn left and right. If the pulse width is under a certain time frame, the motor will drive in a clockwise direction. If the pulse width exceeds that time frame, the motor will drive in a counterclockwise direction. The middle time frame can be adjusted through a built-in potentiometer inside the motor.

3 Differences between Stepper Motor and Servo Motor:

• Stepper Motors have a large number of poles, magnetic pairs generated by a permanent magnet, or an electric current. Servo motors have very few poles; each pole offers a natural stopping point for the motor shaft.
• The torque of a stepper motor at low speeds is greater than a servo motor of the same size.
• Stepper motor operation is synchronized by command pulse signals output from the pulse generator. In contrast, the servomotor operation lags behind the command pulses.

Now you have an idea about the working of servo meter if you have any queries on this topic or the electrical and electronics projects leave the comments below.

Photo Credit

• Servo Motor by Wikimedia