A Guide to Using RC Airplane Servos

Remote Control (RC) airplane servos, like the one to the left, reliably move to a desired angular position, given the appropriate input. The same servos are also intended to be used in RC cars, boats, and other models.
Increasingly, these servos are being applied in non-traditional ways. This is largely due to several attractive features: However, they do have their limitations. The best way to explore this is to get one and experiment with it.

Basic Function of Servos

This is a block diagram, illustrating the basic idea of the servo's function. The desired position is compared to the output position of the servo shaft. A compensator adjusts this signal, and sets the input to the motor. The motor is attached to a gearhead, so that the output is high torque, although at low speed. A potentiometer is attached to the output shaft to provide the feedback signal.

Controlling Servos

Below is the most common kind of servo connector. It fits onto standard SIPP pins, and has three leads:
Servos works well in the range of 4V-7V. The signal is supposed to be at TTL levels. Usually, servos are run near 5V, for both power and signal high.

This is what the control signal should look like. Every 20ms, a new high pulse is sent. The length of this pulse sets the position of the servo. The servo is typically centered when the pulse is about 1.5ms long.

There are many ways to generate this signal. It may be generated with a signal generator, with digital electronics, with a PIC, with a microcontroller, or with an off-the-shelf RC transmitter receeiver pair. If you are interfacing a servo with other electronics, using a microcontroller (such as a Motorola 68hc11, Intel 8051, Zilog Z-80, etc.) or a PIC (such as a Microchip Technologies PIC16F84, or a Basic Stamp) is usually the best solution. If you are building a more sophisticated system, it is also easy to interface to a single board PC.

Control Codes

Here is an excellent reference on controlling a servo with a Basic Stamp.

Servo Limitations

Servos do not have infinite speed or torque. Experiment to see if a given servo will perform adequately for your application.

Some Applications

Hobbyists, researchers, and designers are coming up with very creative applications for these servos.

Some are making modifications to traditional RC model applications. It is possible to embed a microprocessor into these model vehicles to log data, and design innovative control techniques.

Many servos are finding their way into robots. Servos can effectively actuate arms, walking devices, and graspers.

With a small modification, servos make effective actuators for wheels. This is accomplished by disconnecting the potentiometer, so that there is no feedback signal. The result is a shaft speed proportional to the input signal.

Other Links

Ferretronics makes some nice chips which control servos, based on a serial command input. They explain the connection, etc.

Aquila Vision makes a similar product.

A Guide to wiring.

Yet another interface to PC

TIC Kit Program

A guide for a class


Hopefully, this guide provides enough information to get started at applying RC Airplane servos. Be creative, and build prototypes to try new ideas out.

Last updated circa 2000 by Michael Prados.