Servo (radio control)

A servo (Latin servus, " servant, slave " ) refers to a composite of electrical drive and drive unit. This may be for example an electric motor together with its control electronics. In common parlance, servos are often equated with servo motors.


In model special servos are used. Its compact and lightweight design is mainly for aircraft models of great advantage.

Installation and connections

To assemble classic servos have with which they can be bolted to a pad on two sides fasteners. Servos can be connected directly to the receiver. The cable consists of three lines: the supply (VCC ), earth (GND ) and a signal line (PWM). The receiver provides the power supply voltage and the control signal. The plug shapes, connectors and wire colors are not uniform among the different manufacturers, which can lead to Kompatiblitätsproblemen.


Servos via a pulse width modulation (PWM ) can be controlled. Across the width of the pulses, the angle on the servo arm is to be placed, controlled. There is usually a 50 - Hz signal (20 ms cycle length ), which is between 1 ms ( left), and 2 ms ( right stop ) to a high level and the rest of the period length on low- level. Many servos have not exploited their full range of motion in this range of values. The values ​​at which the servo is on the far left or right, can also be below or above 1 ms 2 ms. The period of 20 ms is not a critical value, and must not be applied strictly in the control.

Position control

For servos, the angle of the output shaft is controlled. For the determination of the angle is located in the interior of a servo potentiometer which is connected to the output shaft. About this potentiometer the servo electronics determine the actual angle of the output shaft. This is compared with the target angle, which is determined from the PWM signal. With a deviation between actual and target angle, the electronics on the engine and transmission controls by the angle of the output shaft. A disadvantage of servos is that it can not query whether the servo is overloaded. This means that it can not determine whether the servo is at all able to approach the desired position.

Analog and digital servos

The electronics of servos can be digital or analog. Thus servos are classified as digital or analog servos. Digital servos have a higher angular resolution and the servo position is controlled quickly and accurately. Digital servos can often be reprogrammed, which can be adapted to the application ( depending on the power model), the speed, the overload protection and the like. However, the need for auxiliary power and also the price of digital servos is higher than that of analog servos.

Servo modification

A common modification to servos is the conversion into a gear motor without position control and endless rotation angle. For this purpose, a mechanical stop must be removed at the Endachsenzahnrad and the potentiometer is replaced by two fixed resistors in the servo housing. In the hobby electronics chopped servos are often used as wheeled mobile robots. Advantages over commercially available gearmotors include lower purchase price and easy mounting on the chassis, easy assembly of the wheels to the power axis, the motor drive is integrated in the servo. Drawbacks are the weak plastic gears with cheap servos and often not entirely uniform rotary motion, which can be caused by inappropriate modification of the servo mechanism.


In the automotive industry, more and more use variants of servos to be tapped. Operations of the retractable radio antenna, power windows, seat adjustments, etc. These servos. To control and monitor these servos they are usually connected to control devices. For simple cases such as the power windows no potentiometer for position feedback is used, but only Endpositionskontakte.

Hard Drives

In English-speaking countries of mechanical drive for positioning the write heads of hard disks is also referred to as a servo. Here initially came mainly stepper motors with a threaded spindle used nowadays almost exclusively plunger coil with position control ( " voice coil ").