What are servo motors and how do they work?

What are servo motors and how do they work?

How do servo motors work?

To understand how that servo works, you need to take a closer look at the engine. A fairly simple setup is found: a small DC motor, a potentiometer and a control circuit. The motor is attached to the setting wheel via gears. As the motor turns, the resistance of the potentiometer changes, allowing the control circuit to regulate exactly how much movement there is in which direction.

When the shaft of the motor is in the desired position, power to the motor is stopped. If not, the motor is rotated in the appropriate direction. The desired position is sent via electrical impulses over the signal line. The motor speed is proportional to the difference between the actual position and the commanded position. So when the motor is close to the desired position, it spins slowly, otherwise it spins fast. This is called proportional control. This means that the engine only runs as much as necessary to get the job done, which is very energy efficient.

How is the servo controlled?

Servos are controlled by sending a variable width electrical pulse or pulse width modulation through the control line. There is a minimum impulse, a maximum impulse and a repetition rate. Servo motors like those of GHV can usually only rotate 90° either way to move a total of 180°. The neutral position of the motor is defined as the position where the servo has the same potential clockwise or counterclockwise rotation. The pulse width modulation sent to the motor determines the position of the shaft and is based on the duration of the pulse sent over the control line. The rotor turns to the desired position. The servo motor expects a pulse every 20 milliseconds. The length of the pulse determines how far the motor turns. So a 1.5 millisecond pulse causes the motor to turn to the 90° position. If the value is less than 1.5 milliseconds, it is shifted counter-clockwise towards 0°. If the value is longer than 1.5 milliseconds, the servo will rotate clockwise towards 180°.

When these servos are commanded to move, they move into position and hold that position. If an external force pushes against the servo while the servo is holding a position, the servo cannot move out of that position. The maximum force that the servo can exert is called the torque of the servo. The position pulse must be repeated to tell the servo to stay in the desired position.

What types of servo motors are there?

There are two types of servo motors – AC and DC. AC servos can handle higher current surges and are used in industrial machinery. DC servos are not rated for high current surges and are usually better suited for smaller applications. In general, DC motors are less expensive than their AC motors. These are usually servo motors specifically designed for continuous rotation. They are fitted with two ball bearings on the output shaft to reduce friction and facilitate access to the potentiometer for rest point adjustment.

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