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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 term Servo is short for Servomechanism, a device that uses internal feedback to ensure that its mechanical output follows an input control setting. That’s what I’m talking about here.

A servo motor is the source of motion in a servomechanism, in this case, a small PMDC motor.

A Servomotor is usually a PMDC motor optimised for use in a servomechanism, perhaps featuring rapid acceleration and built-in feedback sensors. They tend to cost a lot of money are definitely not found in hobby servos!

These definitions are important, particularly when searching for suitable devices on the Internet. Use the search term ‘rc servo’.

Position-Control Servo

This type of servo has a rotary output, but the shaft can only rotate a maximum of half a turn or 180°. With a lever called a ‘horn’ attached to the output shaft it can provide a linear push-pull action, hence its original purpose as an actuator for the flight controls of a model aircraft. Outwardly, all ‘standard’ size hobby servos look the same: a rectangular black plastic or alloy box with two sets of fixing lugs, a 3-wire flat cable connection with 0.1in pitch socket header, and a splined output shaft protruding from one side. The original standard size, for example 

(781-3058)

 designed for fairly large radio-control model aircraft with IC engines, now sits alongside more compact versions termed ‘Mini’, ‘Micro’ and ‘Sub-Micro’. For really heavy applications there are sizes even larger than the standard.

All analogue servos are controlled in the same way: angular position is encoded in a constant repetition rate (50Hz) pulse train where the width of a pulse carries the information. A pulse width of 1ms corresponds to maximum anticlockwise while 2ms turns the servo to the maximum clockwise position. It’s often assumed that a servo will turn through 180° with these numbers, but that is usually not the case (see later).

The Motor

This is usually incredibly weedy and cheap-looking because it has to fit into the case with a whole lot of other components. Fortunately, the gearbox which is necessary to reduce its rotational speed from about 6000rpm to an output in the region of 30rpm, also multiplies the torque available by the same factor.

The Geartrain

A lot of gears have to be crammed in to achieve a reduction factor of about 180:1. Of course, the output shaft cannot rotate more than half a turn, so datasheets normally describe servo speed in terms of how long it takes to turn by 60°. For example, a typical figure might be 0.15 secs/60°.