RC Servo Fundamentals#

An RC servo is a self-contained position actuator: a small DC motor, a gear train, a position-sensing potentiometer, and a control circuit — all in one package. The control interface is a single PWM signal where the pulse width commands a shaft angle. No encoder, no PID tuning, no motor driver — just a pulse and the servo moves. This simplicity makes RC servos the standard actuator for hobby robotics, pan/tilt camera mounts, valve control, and any application needing moderate-precision angular positioning without a custom servo loop.

PWM Control Signal#

The standard RC servo protocol uses a 50 Hz (20 ms period) PWM signal where only the pulse width carries information:

The actual range varies by manufacturer. Many servos accept pulses from 500–2500 µs for a full 180° range; some are restricted to 1000–2000 µs for 120° or less.

/* STM32 — 50 Hz servo PWM on TIM3 CH1 */
htim3.Init.Prescaler = 71;        /* 72 MHz / 72 = 1 MHz tick */
htim3.Init.Period = 19999;        /* 20 ms period */
HAL_TIM_PWM_Start(&htim3, TIM_CHANNEL_1);

/* Set position (angle 0–180°) */
uint16_t pulse_us = 1000 + (angle * 1000 / 180);  /* 1000–2000 µs */
__HAL_TIM_SET_COMPARE(&htim3, TIM_CHANNEL_1, pulse_us);

/* Arduino — using Servo library */
#include <Servo.h>
Servo myservo;
myservo.attach(9);       /* Pin 9 */
myservo.write(90);       /* Center position */
myservo.writeMicroseconds(1500);  /* Same, but explicit µs */

Analog vs Digital Servos#

Digital servos run their internal control loop at a much higher rate than the incoming PWM signal. This means they resist external disturbances better (stiffer) but draw more idle current doing so.

Key Specifications#

Torque units: Servo torque is conventionally specified in kg·cm (kilogram-force × centimeters). To convert: 1 kg·cm ≈ 0.098 N·m.

Power Supply Considerations#

Servos draw significant current during motion — stall current on a standard-size servo can reach 1–2 A, and a sudden direction change can spike much higher:

Never power servos from the MCU’s 5 V regulator. The current draw exceeds what USB or LDO regulators can provide, and the voltage sag on the rail can brown-out the MCU. Use a dedicated servo power supply (5–6 V BEC, or a bench supply) with the ground connected to the MCU ground.

Multi-Servo Power#

Multiple servos amplify the supply challenge. Six servos moving simultaneously can draw 3–5 A total. Size the supply wiring and capacitance accordingly:

• Use 18 AWG or heavier wire for the servo power bus
• Add 470–1000 µF electrolytic at the power distribution point
• Add 100 µF at each servo connector if the bus length exceeds 20 cm

Continuous-Rotation Servos#

A modified servo with the position feedback potentiometer disconnected (or replaced with fixed resistors) becomes a continuous-rotation motor:

Continuous-rotation servos are not position-controllable — the pulse width sets speed and direction, not angle. They are useful for small wheeled robots but have poor speed precision (dead band around center is typically ±20 µs).

Tips#

• Use writeMicroseconds() instead of write(angle) on Arduino for finer control. The angle-to-µs mapping in the Servo library may not match the servo’s actual range, and microsecond control avoids this abstraction.
• Test the servo’s actual range by slowly sweeping from 500–2500 µs and noting where it hits the mechanical end stop. Commanding a position beyond the end stop stalls the motor and draws continuous high current.
• Place a 100 µF capacitor directly across each servo’s power pins to buffer current transients. This prevents voltage dips that cause the servo to reset or jitter.
• For smooth motion, ramp the commanded position over time rather than jumping directly to the target. A 1–2 ms update interval with 1–5 µs per step produces visually smooth movement.

Caveats#

• Cheap servos (SG90, MG90S) have significant gear backlash — typically 2–4° of free play. This limits effective positioning accuracy regardless of the PWM resolution.
• Servo jitter (small oscillation at rest) is caused by noise on the PWM signal, a noisy power supply, or the servo’s internal deadband being narrower than the PWM noise floor. Clean power and a stable timer-generated PWM minimize jitter.
• Servos have no feedback to the MCU — there is no way to confirm that the shaft actually reached the commanded position. If the load exceeds the stall torque, the servo stalls silently.
• The 50 Hz update rate is a convention, not a strict requirement for most servos. Many servos respond correctly at 100–330 Hz (faster update rate), but some older analog servos expect exactly 50 Hz and misbehave at other rates.

In Practice#

• Servo jitters continuously at rest. The PWM signal has timing noise wider than the servo’s deadband (typically 4–8 µs for analog, 1–2 µs for digital). This commonly appears when PWM is generated by software (bit-bang, delay()-based) or when the power supply has ripple. Switching to hardware timer PWM and adding capacitors on the servo supply eliminates the jitter.

• Servo moves to the correct position but buzzes audibly. This is characteristic of digital servos — the high-frequency internal PID loop continuously corrects for tiny position errors. If the load creates a spring-back force, the servo fights it at hundreds of Hz. Reducing the load or accepting the buzz as normal for digital servos is typical. Some high-end servos have a configurable deadband to reduce this.

• Multiple servos cause the MCU to reset when they move simultaneously. The combined inrush current sags the power rail below the MCU’s brownout threshold. The servos and MCU share a power source without sufficient decoupling. A separate power supply for servos (with common ground to MCU) eliminates the resets.

• Servo position drifts over several degrees after an hour of operation. The internal potentiometer wears or heats up, changing its resistance and shifting the feedback reference. This is a known failure mode of cheap servos under continuous duty. Replacing with a higher-quality servo with a longer-life potentiometer — or switching to a proper servo motor with encoder — addresses long-term drift.