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Geeth Kavinda

How does a servo motor work?

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How does a servo motor work?

The simplicity of a servo is among the features that make them so reliable. The heart of a servo is a small direct current (DC) motor, similar to what you might find in an inexpensive toy. These motors run on electricity from a battery and spin at high RPM (rotations per minute) but put out very low torque (a twisting force used to do work— you apply torque when you open a jar). An arrangement of gears takes the high speed of the motor and slows it down while at the same time increasing the torque. (Basic law of physics: work = force x distance.) A tiny electric motor does not have much torque, but it can spin really fast (small force, big distance). The gear design inside the servo case converts the output to a much slower rotation speed but with more torque (big force, little distance). The amount of actual work is the same, just more useful. Gears in an inexpensive servo motor are generally made of plastic to keep it lighter and less costly (see Figure 3 below). On a servo designed to provide more torque for heavier work, the gears are made of metal (see Figure 4 below) and are harder to damage.

Robotics gears in a servo motor
 The gears in a typical standard-size servo are made of plastic and convert the fast, low-power motion of the motor (on the right) to the output shaft (on the left).
Robotics gears in a servo motor convert motor speed to torque
 In a high-power servo, the plastic gears are replaced by metal ones for strength. The motor is usually more powerful than in a low-cost servo and the overall output torque can be as much as 20 times higher than a cheaper plastic one. Better quality is more expensive, and high-output servos can cost two or three times as much as standard ones.

With a small DC motor, you apply power from a battery, and the motor spins. Unlike a simple DC motor, however, a servo's spinning motor shaft is slowed way down with gears. A positional sensor on the final gear is connected to a small circuit board (see Figure 5 below). The sensor tells this circuit board how far the servo output shaft has rotated. The electronic input signal from the computer or the radio in a remote-controlled vehicle also feeds into that circuit board. The electronics on the circuit board decode the signals to determine how far the user wants the servo to rotate. It then compares the desired position to the actual position and decides which direction to rotate the shaft so it gets to the desired position.

Robotics  internal parts of a servo motor including circuit board
 The circuit board and DC motor in a high-power servo. Did you notice how few parts are on the circuit board? Servos have evolved to a very efficient design over many years.

Imagine you are playing catch with a friend on a sports field. You stand at one end and want your friend to go out for a long throw. You could keep calling out "farther, farther, farther" until she got as far away as you wanted. But if she went out farther than you can throw, you would have to call out "closer" until she got back to the right spot. If she were a simple motor in a robot arm and you were the microprocessor, you would have to spend some of your time watching what she did and giving her commands to move her back to the right spot (this is called a feedback loop). If she were a servo motor, you could just say "go out exactly 4.5 meters" and know that she would find the right spot. That is what makes servo motors so useful: once you tell them what you want done, they do the job without your help. This automatic seeking behavior of servo motors makes them perfect for many robotic applications.

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