5 The Servo:
What is a servo? In the context of R/C equipment, a servo is an electromechanical device that sets the rotational position of a mechanical arm as a function of the command signal pulse width.

5.1 Position Feedback and Servo Error:
The servo uses an internal device to measure the arm position so that the position can be set in accordance to the command signal, and so that external forces applied to the arm will not effect its position. In the context of the servo, this form of measurement is called "position feedback" and is very important to the servo's operation.

The difference between the measured position and the commanded position of the arm is called "servo error", and it is the servo's job to drive the servo error to a value of zero at all times. When the servo error is zero, then the arm is exactly where it is supposed to be according to the control signal.

Position measurement is obtained by mechanically connecting a "potentiometer" or variable resistor to the shaft that the arm rotates about. The potentiometer will change in electrical resistance as a function of the servo arm position. What the servo does with this dynamic resistance varies from manufacturer to manufacturer. I will explain only one of several means of using this feedback resistance to precisely position the servo arm as a function of the input command signal.

It is not necessary to understand what electrical resistance is, just realize that it changes with the servo arm position. I will explain how it is used to change the operation of the one-shot circuit that has already been described - so let's move on.

5.2 Comparing the Command Signal to Position Feedback:
How can we translate a command signal to mechanical position? It's is like finding a common denominator in a math problem. We need to convert one of these known states into something that can be compared to by its counterpart. We either need to convert the command pulse to resistance, or the resistance to a command pulse. It is easy to do the later by using the one-shot circuit that was described earlier.

5.3 Creating a Comparable Command Pulse:
The one-shot can be used as a precision timer with a time-out period that is set as a function of resistance. In this case, varying the position of the feedback potentiometer can vary the time-out period. Doing so, we can create a pulse that exactly matches the command pulse when the servo arm is in a particular position. I will refer to this new signal as the "feedback pulse".

If the servo arm were allowed to freely rotate, the feedback pulse would vary from 1 to 2 milliseconds as the arm is moved from one rotational extreme to another. Like the command pulse, the feedback pulse will be high for 1.500 milliseconds when the arm in its center, or neutral position.

5.4 Comparing the Feedback Pulse to the Command Pulse:
In the real world, there would be a slight delay of about 10 or 20 nanoseconds between the low to high transition of the command pulse and the low to high transition of the feedback pulse. For now, however, let's ignore circuit response times and assume that the command pulse triggers the servo's one-shot so that the command pulse and the feedback pulse go high at exactly the same time. Now imagine the command pulse and the feedback pulse are exactly the same, same amplitude of 0 to 5 volts, the same high and low period, and transitioning at exactly the same points in time. This is the case when the servo arm is in a position that sets the time-out period of the feedback one-shot to exactly the same time period as the command pulse. Remember that the one-shot circuit has complimentary outputs of Q and Q!. In this case, the Q! output is equal and opposite in every respect as compared to the command pulse.

At this point, we need to go outside the digital domain a little bit and start thinking about an analog signal that is the average voltage value of the command pulse and the Q! feedback pulse. What would we get if we mathematically averaged their voltage values as if they were numbers on a chart? When both pulses are equal and opposite, as they are in this case, their average will always be a straight voltage level exactly half way between the two high and low logic levels, or 2.5 volts in this example. No matter where you compare the two signals, when one is at a 5-volt level, the other is at 0 volts - their average is always 2.5 volts.

5.5 Servo Error:
When the command pulse and the feedback pulse are exactly the same, the servo is in a stable state where no corrective action is required. However, this is not the case when the transmitter stick is moved and the command pulse changes. Now when we average the command pulse with the Q! feedback pulse, we no longer get a straight value of 2.5 volts.

Let's say that the command pulse changed from 1.5 to 1.7-milliseconds, and the servo arm has not moved. Now there is a 0.2-millisecond discrepancy where the command signal extends beyond the feedback pulse. Remember that the command pulse triggers the feedback pulse so they always start together. Now when the two signals are averaged, the average will be 2.5 volts up to the end of the Q! feedback pulse. During the time that the command pulse is high and the Q! feedback pulse is also high, the average of the two signals will be 5 volts. In this state we have a signal that is 2.5 volts for 19.8 milliseconds and 5 volts for 0.2 milliseconds.

Now let's go the other way where we leave the servo arm in place and change the command signal to 1.3 milliseconds. When the two signals are averaged in this state, we end up with a signal that is 2.5 volts for 19.8 milliseconds and 0 volts for 0.2 milliseconds.

In each example above, there is a 0.2-millisecond point where the average signal is above or below 2.5 volts. Each of these conditions illustrate a servo error where the servo arm position and the resulting feedback pulse does not match the command pulse.

5.6 Servo Mechanics:
Before we examine how to deal with servo error, the mechanics of the servo needs to be explained. The servo arm is driven by a small electric motor with a gear reduction to improve torque. The sizing of the motor and the gear reducer is important. The servo motor is geared down so that it can produce sufficient torque to physically move the control surfaces of a model. The gear reducer cannot be too slow or it won't be fast enough to do its job in a reasonable period of time. There must be very little physical lag between the transmitter stick position and the control surface.

The final stage of the gear reducer drives a shaft that connects to the servo arm outside the enclosure of the servo body, and also moves the feedback potentiometer inside the servo. This driving shaft is sometimes outfitted with ball bearings to handle a high physical load.

All of the servo mechanics as well as all of the electronics required to drive the servo motor and generate the feedback pulse are housed inside a common enclosure.

5.7 Compensating for Servo Error:
The servo error signal is amplified and applied to the servo motor in such a way that the motor will turn the servo arm in a direction that nulls the servo error. The direction that the motor turns is determined by the servo error signal magnitude. It will turn in one direction if the averaged signal reaches 5 volts, and it will turn in the other direction if the averaged signal reaches 0 volts.

5.8 Summary:
Typical R/C systems are comprised of three major subsystems including the transmitter, receiver, and a set of servos. The transmitter generates an electrical pulse stream that defines the position of each and every servo in the system. A command pulse of 1.00 to 2.00 milliseconds is decoded by the receiver and distributed to each servo individually. Each servo interprets that command signal and positions the mechanical arm on top of the servo body to an angle that represents the pulse width of the command signal.

Now that you know how these systems work, you may be able to create your own circuits that create or modify the existing command signals to do other things. Many of the products that VeeTail offers utilize microcomputers to measure the command signals and modify them in a manner consistent with a defined function (such as cross-channel mixing) to create new command signals that make servos do other things. Let your imagination go, be creative, and make your servos work for you in custom ways that are otherwise not possible.