Servos In Brief:
A servo is a device rotates a shaft to a position set by the user, and holds this position until further input is given. Servos usually consist of a small DC (direct current) electric motor, several gears, and a head where an arm or wheel can be attached. When the user tells the servo what angular position to move to, the servo rotates and holds position until further input is specified. Servos are designed to hold position because external forces are always interacting with the aircraft, and would set control surfaces to undesired positions unless stopped. Servos exert a torque on external forces, that prevents them from changing the position of any control surface.
How RC Helicopter Servos Work
A servo’s job is to convert the angular movement of a servo arm to the linear movement of a control surface. This is done by attaching linkages, called control rods to the servo arm and the associated control surface. When the servo head rotates, it pushes the control rod back and forth. The rod is linked to a control surface, and can move it up or down as the servo rotates.
Three wires control a RC helicopter servo: two to provide the DC electricity that the motor needs, and one that sends the signal, controlling the servo. The signal wire works by sending the servo a series of pulses, which are interpreted by it’s internal circuitry. By varying the timing of each pulse, the servo knows exactly which position to move to.
Selecting the Right RC Helicopter Servo
Servos have a number of qualities that make them suitable for different applications:
- Torque – This is a measure of the servos “strength”, or how much “push” it has. Torque is the product of force and the radius at which it acts, or the . This is shown graphically in the figure on the right. Bigger planes need high torque servos to move their large control surfaces. In general, servo size goes up with rated torque.
- Speed – Speed measures how fast the servo can move from one position to another. Different RC airplanes and helicopters will need servos with different speeds. For example: a trainer doesn’t need to change control surface positions rapidly, but a 3D helicopter or plane does. High speed servos are many times more expensive than standard ones.
- Dimensions – As stated previously, the dimensions of a servo increase with the torque that it provides.
- Weight – The weight of a servo depends on several variables. Most often recorded in grams, the weight of a servo is always reported on the package.
- Bearings – There are two ways to support the output shaft of a servo – bearings and brushes. Brushes are cheaper, but bearings last longer and operate more smoothly. Very small and very cheap servos tend to be brushed, while high end and very large servos generally have bearings. It’s possible to upgrade a brushed servo to bearings, with several upgrade kits being available on the internet.
- Gears – Most hobby grade servos use nylon gears, while higher end servos use metal gears. Metal gears add more weight, but their advantage is that they can’t “strip”, causing an RC helicopter or airplane to crash. Metal gears wear over time, which can cause “slop” in their rotation, but the gears can be replaced somewhat economically. In general, nylon servos are adequate for sport flying. If you’re particularly worried about losing a model in a crash, or are flying intense aerobatics, a metal geared servo could be the right choice.