A servo consists of a small motor, a gearset, a feedback potentiometer (variable resistor), and some control electronics. The motor spins at variable speeds (much faster than the output shaft) and is coupled to a (reduction) gearset that converts the motor's high speed into something that is more usable for our purpose. When you reduce the motor's speed through a reduction gearset, you gain torque. Torque is the 'twisting' power of the servo -the more torque, the heavier the object the servo can move.

How They Work

A servo is a classic example of a closed-loop feedback system. The potentiometer is coupled to the output gear. It's resistance is proportional to the position of the servo's output shaft (0° to 180 °). This resistance is used by the control electronics to generate an error signal when the desired position isn't the same as the current position. If you send a servo a command to place itself at 90° and the head is actually at 45°, an error signal will cause the motor to move the head (via the gears) until the error signal is 0 (when the head has reached 90°). If the head had been at 180°, an error signal of opposite polarity would have been generated and the motor would have turned in the oposite direction to bring the head 'back' to 90°. As you can see, the current position is 'fed-back' to the control system in a loop to maintain a zero error signal. The farther the actual position from the desired position, the faster the motor turns to bring the error back to zero.

How are They Controlled

Servos have 3 wires coming out of them: Power, Ground and Control. The Control lead is used to send the positioning signal we're going to talk about. Servos are controlled using a system called Pulse Code Modulation (PCM). In order to understand this, you need to understand the terms &quotmiliseconds"(ms) and &quotmicroseconds"(µs). 1 ms is 1/1000th of a second; or put another way, there are 1000 ms in every second. 1 µs is 1/1,000,000th (one -one millionth) of a second, or, there are 1 million µs in each second. Servo manufacturers usually specify pulse-widths in µs, so it's hands to be able to convert between µs and ms. The servo's electronics work in 20 ms blocks (50 of them every second). For each 20 ms block, the servo needs to see a positive-going pulse who's period (width in ms) tells it where to position the head (output shaft).

The period of the pulse that is sent determines where (0° to 180°) to place the head. Different servo manufacturers require different pulse-widths, so you have to experiment a little to find the pulse-widths that correspond to each position. For example, the Futaba servo sold by HVW Technologies has a 90° position (middle) pulse-width of about 1.5 ms. This means that if you send a 1.5 ms pulse to the servo at least once every 20 ms, the servo will move to, and hold at, it's 90° position. If you try to turn the head with your hand you will feel the servo forcing against you, trying to keep the 90° position.

This is easily done with a BASIC Stamp 2, with a single line of code:

 PULSOUT pin, 750

where pin is the Stamp pin connected to the servo's control line. The PULSOUT command works in multiples of 2 µs, so we need 750 x 2µs = 1500 µS which is 1.5 ms

In practice, you can send pulses more often that once every 20 ms; you can send them less often as well, but if you don't send any pulses for about 50 ms or so, the control electronics in the servo will "go to sleep"(enter a power-saving mode). Don't forget, servos were designed for R/C airplanes and cars, where battery life is important. When a servo powers-down, it no longer works against an applied force to maintain it's position. You will find that even then the head is fairly difficult to turn.

Continuous Rotation

For many applications, modified servos make excellent drive wheels for mobile robots -but how do you get them to keep going around instead of stopping at 180° ? There are a couple of problems:

• The feedback loop: The servos' head gear is connected to the feedback potentiometer (so it can track the head's position and &quotfeed-back"this information to the control electronics). The potentiometer has a finite resistance -it can't keep increasing with successive rotations of the head gear.
• Mechanical Stop: The head gear has a little plastic tab on it that hits other plastic tabs on the servo's case at the 0° and 180° positions. This tab stops the head from going all the way around

Solution ? -Make the servo think that it's head is always at the 90° (middle) point (there are 2 ways to do this, see below) and cut-off the plastic tab. When you give it a command to go to 0°, the control circuitry will see that the head is at 90° and will turn the motor on to try and bring the head to 0°. What happens though is that since the resistance isn't changing as the head moves, the control system will keep the motor on forever. Telling the servo to go to 180° will cause the motor to spin (forever) in the opposite direction. The tab no longer mechanically stops the head. Presto ! a very nifty little gearmotor.

There are 2 ways to make the servo think it's head is always at 90°:

• Method 1: Remove the potentiometer and replace it with 2 resistors who's values match the "center" resistance of the potentiometer.
• Method 2: Leave the potentiometer in, but stop it from turning with the head gear.

Which method should you use ? -That depends on your application, but generally, the second method is much faster and simpler. Some things to consider:

Method 1		Method 2
PRO	CON	PRO	CON
Immune to vibrations You have a nice little pot left over	Can't be easily un-done Requires you find suitable resistors to replace pot. Requires soldering	Fast Easily reversed No soldering	Prone to vibrations

Drawbacks

• Servos can have their drawbacks; they're not incredibly powerful although they are ceratinly strong enough for most people's needs.
• Unless you spend considerably more for servos with ball bearings, the servo's lifespan may be less than what you might hope for. Back to our earlier comment about what servos are designed for, they are NOT designed to support several pounds of side force while continuously rotating. Realize that a few minutes of continuous rotation on a robot weighing 2 pounds is the equivalent in wear to many, many, many hours of use in it's intended area of use (R/C Planes, cars etc.). That said, you can still expect most servos -even those without ball bearings, to last several years of intermittent experimental or competition (not Sumo competitions !) use. If you have an application that will see extended, continuous, or heavy-duty use buy ball-bearing servos -they cost more but will last much longer and provide smoother performance.
• Servos require constant attention from the control system. A pulse must be sent every 20 ms or more so the control electronics must continuously allocate resources to do this. Not a huge problem in most cases.