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Tech Info - RC Servos

RC ServoMy experience of using RC (radio control) style Servos is fairly limited and recently acquired, and I have to admit to previously having some confusion in my mind about these devices and their relationship with servo motors and servo drives more generally.

Servo motors are a type of electric motor designed to be used as part of a larger servo control system, often using closed loop control methods. These mainstream “servo” systems are widely used in industry to drive loads of various kinds and sizes with good control over the driven motion – for example by effectively controlling the position, speed or acceleration of the moving load.

The RC style Servo is, however, not an electric motor, although it does contain one – it is effectively an entire position feedback closed loop control system packaged in a compact casing. They contain the electric drive motor (usually DC PM) with mechanical reduction gearing, position feedback sensor and the closed loop control and motor drive electronics. The drive output is usually via a limited rotation shaft (to which various torque arms can be fitted) and the input signal is a position demand signal where the required position is encoded in a series of length controlled low voltage pulses.

The range of available RC servos is mind boggling, especially to a beginner such as me. Physical size, output torque, speed of movement, range of movement, position resolution, backlash and build quality seem to be the main parameters that differentiate the available choices. I did find a fair amount of information on the web about the use of RC servos in different applications and that helped in making buying choices - do your own search to see.

RC Servo Internal GearingGenerally they are not large devices, the above image shows a Hitec HS-815BB “Mega Sail Servo” which, with an output torque of about 25 (~2.5 Nm). This is pretty much at the top of the available torque output range. There are a few industrial sized servos around but these are much more expensive. The torque arm on the Hitec has a limited working travel of about +/- 90° (mechanical travel is larger) and has a no-load speed of about 0.14 sec/60° when working at 6V.

The image right shows the servo with the top cover removed. You can see the built in geared transmission – in this case a 5-stage reduction using plastic straight spur gears to reduce the high motor shaft speed to the approx 1 rev/s needed at the output shaft. This servo also has a ball bearing mounted final drive shaft although less expensive units may use plain bearings. The second image below shows the base of the servo removed where the PCB of the drive electronics can be seen. The drive motor and position feedback potentiometer lie within the casing. The gear train is of good quality and there is very little backlash noticeable at the torque arm even with the relatively large number of reduction stages.

RC Servo Internal PCBSo they are capable of fairly high speed motion over a limited range of travel. The speed of motion will slow at higher loads and when the applied load is too high the drive motor will stall. My interest in using this servo was for a joystick force feedback project and for this type of work where a smooth, vibration-free movement under load is required the servo is not an ideal solution. Under load the arm vibrates very slightly at the refresh frequency of the internal position control loop and this can be felt. For many less sensitive positioning applications the device would perform satisfactorily. This particular RC servo can be driven fairly easily from the torque arm when it is not powered, this is probably down to the straight spur gear transmission - other servos with other types of geared transmissions may not be so easily back-driven.

There are many proprietary RC servo controllers available to drive RC servos. Depending on type these can be interfaced with standard radio control receivers, several types of micro controller or to USB or serial ports on a PC, so the range of control options is fairly large. The image below shows a Pololu 16 Servo Serial controller which I used to drive the servos from a PC.


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