162,300 ₹

The RC Servo or Hobby Servo has been used to move the control surfaces of Radio-Control (RC) model aircraft for many years. It has since become very popular for driving the limb joints of small humanoid robots, and when converted for continuous rotation, the wheels of mobile robots. However, some features optimal for aircraft control are less than ideal for [url=http://www.borunte.net/industrial-robot/]industrial robot[/url]s. First, let’s get the terminology straight:

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).