Introduction to RC servos
A “servo” is a generic term used for an automatic control system. It comes from the Latin word “servus” – slave. In practical terms, that means a mechanism that you can set and forget, and which adjusts itself during continued operation through feedback. Disk drives, for example, contain a servo system insuring that they spin at a desired constant speed by measuring their current rotation, and speeding up or slowing down as necessary to keep that speed.
What is “feedback”? Think about driving a car, and wanting to keep your speed smack on 55 MPH. You are cruising along, and occasionally glance at the speedometer. If your speed is under 55, you press down harder on the gas pedal. If your speed is over 55, you lift your foot. The speedometer gives you feedback – information about how fast you really are going. The brain uses this to decide whether to press down or raise the foot. This is also known as “closed loop” control.
An automobile’s “cruise control” is a closed loop system that works similar to the way that you do while driving. You establish a set point, and if the speed is below that, the cruise control presses down on the gas pedal.
The most common consumer-visible servo is that used to operate radio controlled (RC) model planes, boats, and other gadgets. These are small boxes that contain:
a DC electric motor,
gears with an output shaft,
and control circuitry.
The controlling intelligence, in this case the operator of the model, indicates to the servo the position that the output shaft should have. The position-sensing mechanism tells the servo what position the shaft currently has. The control circuitry notes the difference between the desired position and the current position, and uses the motor to “make it so”. If the difference in position is large, the motor moves rapidly to the correct position; if the difference is small, the adjustment is more subtle. As for the operator, all he knows is that he moved a slider half-way up, and the rudder on his model plane moved to the center position, and will stay there until he moves the slider again.
RC servos have a huge amount of haunt potential for precision animation. Just imagine a skull head with eyeballs that can track left to right, following guests as they walk by….
Here are a couple of Royal “Titan” servos.
Types and availability of RC servos
There are numerous types of servos. They differ in their precision, speed, and strength – all of which are reflected in price. This is as it should be – you shouldn’t have to pay for a ball-bearing, metal-gear, high-precision servo, when your application can get by with a cheaper kind.
The radio controlled model market is evidently a lucrative one – there are numerous companies making RC servos: Airtronics, Cirrus, FMA Direct, Futaba, Hitec, JR, Ko Propo, Multiplex, Tower Hobbies.
Although the array of manufacturers may seem daunting, it is only a good thing. It gives you lots of choices, and lowers the prices.
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Where to buy RC Servos?