RC Servo
A conventional RC position servo drives an output shaft toward a position requested by a control pulse. Its internal controller uses feedback to correct position error. Motor, gearing, sensor, permitted travel and control behaviour depend on the model.
For the common RC interface, pulse width carries the position command; duty cycle alone does not. A pulse near 1.5 ms commonly represents neutral and a period near 20 ms is common, but neither the allowed endpoints nor the angle and direction per pulse width are universal. Use the servo's documentation and calibrate within safe travel limits.
In plain terms
Like someone turning a knob while watching its position, the controller keeps comparing the target with the result and correcting the difference.
Why designers use it
- Positioning a suitable RC steering or control linkage.
- Moving a lightweight mechanism with integrated feedback control.
- Prototyping a rotary position actuator whose ratings fit the load.
Best for
- RC vehicles
- Robot joints
- Animatronics
Key specifications
- Control signal: Check pulse width, repetition rate and logic levels
- Motion: Allowed travel, direction, speed and positioning behaviour
- Load: Rated torque, duty and mechanical limits
- Power: Rated supply and peak/stall current requirements
When not to use it
- When required torque, speed, travel, duty or environment exceeds the selected servo's limits.
- When the application needs a safety-rated actuator or feedback system not provided by the hobby servo.
Common mistakes
- Assuming 1 ms, 1.5 ms and 2 ms always mean 0°, 90° and 180°.
- Continuing to command motion into a mechanical stop.
- Powering the servo motor through a microcontroller signal pin.
- Ignoring peak current, supply dips, signal-ground reference and compatible input levels.
- Treating a continuous-rotation version as a position servo.
Where you will find it
- An RC steering linkage uses a position servo, with endpoints calibrated to avoid pushing against the mechanism's stops.
- A pan-tilt mount can use two appropriately rated servos. Load, supply current and safe travel must be checked for both axes.
A short history
The familiar RC control interface communicates through pulse duration. Pololu's measured examples show why neutral, mechanical centre, travel direction and total range must not be treated as identical across every servo. The interface's common conventions are useful starting points, not universal angle specifications.
Good to know
- Neutral is not necessarily the exact centre of the available mechanical travel.
- Keeping duty cycle constant while changing pulse duration can change the position command.