A servo (servomechanism) is a small electric motor with integrated position feedback, gearing, and control electronics. It receives a commanded position (typically as a PWM signal from the flight controller) and rotates its output shaft to match. In fixed-wing UAVs, servos deflect control surfaces — ailerons, elevators, rudders, elevons — to control the aircraft’s attitude and trajectory.
Servo selection for UAV design involves:
- Torque — the force the servo can exert, typically 1–20 kg·cm for small UAVs. Must exceed the hinge moment of the control surface at maximum airspeed with a margin. Under-torqued servos cause control flutter or loss of authority at high speed.
- Speed — angular velocity, typically 0.05–0.20 seconds per 60° of rotation. Faster servos provide better control response but draw more current.
- Weight — from 4 g for micro servos to 80+ g for standard metal-gear servos. For a small expendable drone, the total servo weight can be a significant fraction of the airframe mass.
- Gear material — plastic gears are lighter and cheaper but strip under shock loads (catapult launch, hard maneuvers). Metal gears survive abuse but add weight.
- Digital vs. analog — digital servos update their position at higher rates (typically 300 Hz vs. 50 Hz), providing stiffer, more precise hold and faster response. Cost premium is modest.
For expendable airframes, servo selection is driven by minimum-acceptable quality. A single servo per control surface (no redundancy) with the cheapest metal-gear unit that meets the torque and speed requirements. For reusable tactical and MALE UAVs, redundant servos, waterproofing, and EMI shielding become relevant.
Multirotor UAVs generally do not use servos — they achieve all attitude control through differential motor speed, with the flight controller commanding each motor’s ESC directly.
Related terms
- Flight Controller — the device that sends position commands to servos
- ESC — the motor controller equivalent for brushless motors on multirotors