Servo Motors

When a little Negative Feedback is a Good Thing

The technical definition of a "Servo Motor" is a motor which employs negative feedback to control motor speed and/or position. Servo feedback senses the difference between the actual and desired speed or position so that the controller can adjust the output to correct the 'error'. Servo control systems are capable of very precise motion control. They are used in everything from high-speed CNC machines to positioning motors for telescope mirrors.


RC Servos:

The most common and economical type of servo motor in the hobbyist world is the RC Servo. These were originally designed for hobbyist Radio Control systems. But these days they are widely used for all sorts of small-scale projects where moderately precise positioning is required.

Most RC servos provide position control over an approximately 180 degree range. They do not provide speed control or continuous rotation. Inside, an RC servo typically consists of a brushed DC motor with a gear train and a built-in H-Bridge motor driver. A potentiometer attached to the shaft provides position feedback to the control circuit.

RC Servos are available in a wide variety of sizes, torque and speed ratings. Case dimensions and control interfaces are mostly standardized. RC servo case sizes are generally categorized as 1/4 scale, Standard, Mini and Micro.

These motors are simple and economical to drive, either with dedicated servo controllers or with a GPIO pin from a microcontroller.

Analog Feedback Servos

One problem with RC servos is that there is only feedback to the internal controller. There is no feedback to your program, so you can't be sure that it is performing as expected. Analog feedback servos solve this problem with an extra feedback wire that you can connect to an analog input pin.
For more details on these specialized servos and how they work, see Analog Feedback Servos Guide in the learning system!

The following summary applies to RC Servo Motors suitable for hobbyist use. Industrial servo motors are far more capable - but at a much higher cost!


Applications of RC Servo Motors:

  • Robotics
  • Animatronics
  • Radio Control Cars/Boats/Planes

Advantages:

  • Low cost - (RC Servos) Smaller sized servos can be purchased for just a few dollars.
  • Variety - There is a wide range of sizes and torque ratings
  • Simple to control - using logic level pulses from a microcontroller or a dedicated servo controller

Limitations:

  • Limited range of motion - Most RC servos are limited to 180 degrees of motion.
  • Moderate precision - Positioning accuracy and repeatability of +/- 1 degree is typical.
  • Jitter - The feedback mechanism in the servo will actively try to correct any drift from the target position. This constant adjustment can create annoying twitches while trying to hold a steady position. If this is a problem for your application, consider a stepper motor instead.
Last updated on May 04, 2015 Published on May 21, 2014