By Richard Lindberg

Servos are small electro-mechanical devices that are mounted to your RC airplane. When connected with push rods, they move control surfaces such as elevators, rudders and ailerons.

A typical servo consists of a motor, drive gears, output shaft and arm, a circuit board and a potentiometer. The circuit board contains a signal amplifier and comparator circuits. These items are configured as in the following diagram:

Position + Position

Reference Output

Primarily the motor, then the amplifier type. Inexpensive servos contain cored motors, which are incredibly common and cheap. The core, or armature, is comprised of metal plates (poles) sandwiched around a metal shaft that’s supported by bearings at both ends, and each pole is wrapped with wire. The armature spins inside a hollow center permanent magnet that lines the inside of a metal can (the enclosure). Power is introduced to the windings, generating an electro-magnetic field, which is opposed by the permanent magnet field, thus causing the armature to rotate. More expensive servos contain coreless motors. These motors have the armature on the outside (imagine a hole saw with an arbor attached) that’s very light, and rotates around the fixed permanent magnet. Because of the larger size (diameter) of the armature, a coreless motor has a higher torque rating, and the lack of poles allows the motor to center more accurately while maintaining or holding position with increased authority.

There are two basic kinds of amplifiers: analog and digital. Analog (conventional) amps interpret RX commands and pulse power to the motor armature at 50 cycles per second. The space between pulses is known as the dead-band. If a signal is received from the RX or the servo arm is deflected the amp pulses power to either move the armature or resist the opposing force. The duration of the pulse speeds up the motor (longer pulse) or slows it down (shorter pulse). Digital amps interpret RX commands and pulse power to the armature at 300 pulses per second (!). The increased pulse cycles command the servo motor to react and perform with more precision. This results in faster response to control command signals, lower dead-band numbers, increased holding power and much better resolution. Also, these digital amps are microprocessor controlled, and some can be externally programmed. Center and end-point positions, speed, dead-band, rotation, failsafe and more are programmable.

In a word, cost. While there’s no inherent reason why a digital amp couldn’t be incorporated in a cored motor servo, the built-in limitations of such a servo would make the benefits very small. So, multi-pole (>3 poles) motors or coreless motors are used, and the cost is commensurately higher. The microprocessor cost is somewhat higher, too, but that’s coming down as more servos come to the marketplace.