Servo motors are used for robotic applications that require precision positioning. Before diving too deeply into the ways servos are used in robotics, it’s helpful to first learn about the basic function and form of these critical components of motion control.
What Is A Servo?
At its most elemental, a servo is a precise and powerful way of converting rotational motion into linear motion. A servo (or servo motor, as it’s sometimes known) consists of:
The Electronic Assembly, which consists of an AC or DC electric motor, a controller board, and a potentiometer.
The Case, which is the plastic housing for the motor and other components.
The Drive Gears, which reduce the motor’s high-speed output to a lower speed, higher torque servo output.
The Output Spline, which is attached to the output shaft and is the final interaction point between the servo and object of the motion it is creating. A good example is a servo in a remote-controlled airplane. The output spline of the servo might be attached via a control rod to a control surface such as an aileron or rudder. The motion of the servo would, therefore, cause an equivalent movement of the airplane itself.
How Does A Servo Work?
The servo receives a signal from a motion controller.
Depending on the pulse width modulation (PWM) of the input signal, the servo will rotate a certain amount. At rest, the output spline of a servo is usually at 0°. Based on an expected pulse frequency of 20 milliseconds (ms), a pulse width of 1.5ms will make the output spline rotate 90° in one direction. A pulse width of 2ms will make the output spline continue rotating 90° further to the 180° position. A pulse width of 1ms will make the output spline rotate 180° backward to the 0° starting position.
The potentiometer constantly monitors the position of the output spline. When the output spline reaches the desired position, the power to the motor is cut and the servo robot will hold that position until it receives a signal not to. While stopped in a given position, a servo motor will actively try to hold that position.
A key feature of servos is proportional operation. A servo motor will operate only as fast as it needs in order to rotate from its current position to its desired position. If a servo is stopped at the 180° position but needs to be at the 0° position, the motor will rotate very quickly to get there. If stopped at a position that is already closer to 0°, the motor will rotate much more slowly to get there.