Servo Motor

Servos are position controlled actuators. They usually consist of a motor along with some form a feedback. These devices can produce high torques and so be used to create lifting and gripping weapons. Disconnecting the feedback turns the servo into a motor + ESC package.

Control and Feedback
To control a servo motor you must supply a PWM signal. The positions are encoded using the pulse width of the signal, with the minimum pulse width representing the lowest position and the largest pulse width representing the maximum position.

The feedback device is typically a potentiometer in cheaper servos. Hence often travel is limited between zero and 180 degrees. However this can be increased using mechanical advantage.

An onboard controller ensures that the motor turns to the position in the input signal.

Using Servos for Drive


With some fiddling it is possible to make a non continuous servo suitable for drive. To do this the travel restriction must be removed. This is normally a mechanical stop which can be filed down or sawn off. To enable the controller to move the motor beyond its travel limits you must remove the feedback to the controller. This will mean the feedback to controller will always be zero and hence any non zero position demand will create an error between the position demand and the feedback. Hence the controller will now act as a speed controller for the servo.

There are servos which manufactures have done this for you and these are called continuous servos.

Powering drive servos requires a quite a lot of current on the 5V rail which also supplies the receiver, this will often be supplied via a voltage regulator built into the ESC for any brushless motor, this is usually from a 7805 (max current 500mA). It is therefore important to check that you have a sufficiently powerful supply as running the servos can cause brownouts. A solution to this problem is to use a BEC (Battery Eliminator Circuit) which is a 5V regulator typically with an output of around 3A.

Using Servos for weapons
Non continuous servos can be used to operate mechanisms including weapons on your robot by providing reversible rotary motion. Example weapons that can be made using servos include lifters and axes.