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Published 2026-04-06
This article explains the fundamental working principle of Pulse Width Modulation (PWM) for controlling standardservomotors. You will learn how a simple varying pulse signal determines theservo’s shaft position, using common real-world examples such as robotic arms and remote-controlled (RC) vehicles. No brand or company names are mentioned, and all information is based on widely adopted industry standards for hobbyist and industrialservosystems.
Pulse Width Modulation (PWM) is a method of encoding a position command into a repeating digital signal. Servo motors use PWM because it requires only one control wire, is highly reliable,and is easy to generate with microcontrollers.
A PWM signal has two key characteristics:
Period– the time for one complete on-off cycle.
Pulse Width– the duration the signal stays high (on) during each period.
For standard servos, the pulse width directly translates to a specific angular position of the output shaft.
Almost all conventional servos follow the same PWM standard:
Important:The pulse width determines the angle, while the period stays constant at 20 ms. The servo ignores the remaining time (off period) until the next pulse arrives.
Inside a standard servo, there is a small DC motor, a potentiometer (feedback sensor), and a control circuit. Here is the step‑by‑step process:
1. The control circuit receives the PWM signal.
2. It measures the pulse width of the incoming signal.
3. It compares that pulse width with the current shaft position (reported by the potentiometer).
4. If there is a difference, the circuit drives the DC motor to rotate the shaft until the position matches the commanded pulse width.
5. The servo holds that position as long as the same pulse width is repeated every 20 ms.
Core principle: Pulse width equals target angle.The wider the pulse, the farther the shaft rotates in one direction; the narrower the pulse, the farther it rotates in the opposite direction.
A hobbyist builds a 3‑joint robotic arm. The base joint uses a standard servo. To rotate the arm 30° clockwise, the microcontroller sends a 1.0 ms pulse every 20 ms. To rotate it 150° counterclockwise, it sends a 2.0 ms pulse. The arm moves smoothly to each position and holds it firmly, even when carrying a light object.
In an RC car, a servo controls the front wheels. When the transmitter’s steering wheel is centered, the receiver outputs a 1.5 ms pulse – the wheels point straight. Turning the wheel fully left reduces the pulse to 1.0 ms, steering the wheels to the left stop. Turning fully right increases the pulse to 2.0 ms, steering to the right stop. The driver experiences instant, proportional steering response.
Although different servo models have slightly different ranges, the typical relationship is:
Linear relationship:Between 1.0 ms and 2.0 ms, the angle changes linearly. For example, 1.25 ms gives about 45°, and 1.75 ms gives about 135°.
Minimum pulse width:Sending pulses shorter than 0.5 ms may cause erratic behavior or no movement.
Maximum pulse width:Pulses longer than 2.5 ms may drive the servo beyond its mechanical limits, potentially damaging the internal stopper.
Signal frequency:The servo expects a 50 Hz signal (20 ms period). Higher frequencies (e.g., 100 Hz or 300 Hz) are used by special “digital” servos, but standard analog servos will overheat or jitter.
Voltage:Most standard servos operate at 4.8 V to 6.0 V. Lower voltage reduces torque; higher voltage can destroy the control circuit.
> The shaft position of a standard servo motor is determined solely by the pulse width of the PWM signal, provided the signal repeats every 20 ms. Changing the pulse width changes the angle; keeping the pulse width constant holds the position.
This is the single most important concept to remember. The servo does not care about the duty cycle percentage – only the absolute pulse width in milliseconds.
Based on the principles above, follow these practical steps to ensure your servo system works correctly:
1. Generate a precise 50 Hz (20 ms period) PWM signal– Use a dedicated servo library or hardware timer on your microcontroller. Avoid software delays that cause timing jitter.
2. Start with the neutral pulse (1.5 ms)– Before attaching any load, send a 1.5 ms pulse. This centers the servo and prevents sudden jumps.
3. Limit the pulse range to 1.0 ms – 2.0 ms– This respects most servos’ safe mechanical travel. Test your specific servo’s exact endpoints by slowly increasing from 1.0 ms to 2.0 ms while observing the shaft.
4. Use a separate power supply for servos– Servos can draw 0.5 A to 2 A or more during movement. Never power a servo directly from a microcontroller’s 5 V pin.
5. Add a large capacitor (1000 µF or more)across the servo power lines near the servo to absorb voltage spikes and prevent resets.
6. Refresh the signal at least every 20 ms– If the PWM signal stops, most servos will hold their last position but may become loose. Always send continuous pulses.
7. Calibrate each servo individually– Due to manufacturing tolerances, two servos of the same model may have slightly different 0° and 180° pulse widths. Write a calibration routine to find the exact min/center/max values.
PWM control of servo motors is a robust, industry‑standard method based on a simple relationship: pulse width equals angle. With a fixed 20 ms period, varying the high‑time from 1.0 ms to 2.0 ms rotates the shaft from 0° to 180°. Real‑world applications like robotic arms and RC vehicles rely on this principle every day. By adhering to the recommended signal specifications and following the actionable steps above, you can achieve precise, repeatable, and reliable servo positioning in your own projects.
Update Time：2026-04-06
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