In principle a digital servo is mechanically the same as a normal servo. The difference is in the electric circuit that governs the motion of the output arm.

Information about the desired servo position is output by the transmitter approximately 30-50 times per second. Inside a "normal" servo, a servo amplifier compares the desired position with the actual position of the servo arm. It does this at the same rate as it receives information from the radio receiver, i.e. 30 - 50 times per second. If there is a difference between the desired and actual positions of the servo arm, a current is applied to the servo motor in order to move the output arm.

A digital servo utilize a microprocessor to do this comparison. The advantage is that the microprocessor can do it at a rate different from the rate of the incoming information. A typical value is somewhere between 200-1000 times per second.
There are several advantages to this:

- Any difference between actual and desired position can be detected and adjusted more quickly and more accurately (less dead-band).

- It is possible to use control algorithms to control the speed of the servo motor. For example, servo overshoot caused by motor inertia can be controlled in this way.

- The microprocessor can be programmed to ignore pulses outside the standard 1-2 ms range. Many functions that were previously included in the transmitter and/or receiver can now be included in the servo itself, e.g. travel adjustment, servo reverse, sub-trim, speed and fail-safe in case of corrupt signals or loss of signal.

- Often, but not always, low impedance MOSFET motor drivers are used in conjunction with the microprocessor. This will increase the speed and holding power of the servo. Apart from the "novelty"-factor this is one reason that digital servos are more expensive than normal ones. It is also the reason that digital servos often put a bigger load on the battery

What does a digital servo NOT do:

-It doesn't improve the rate of information transfer from the transmitter stick to the input of the servo itself. Irrespectively of what the internal frequency of the digital servo amplifier is, this rate is still only 30-50 Hz.

/Red B.