TECHNICAL SUPPORT
Published 2026-04-20
If you have ever wondered how a remote-controlled car steers or how a robotic arm moves precisely, the answer lies in a simple yet powerful technique called Pulse Width Modulation (PWM). This article explains the fundamental principle of how PWM controls aservomotor – a question many hobbyists and engineers ask. In essence, PWM controls aservomotor by varying the width of an electrical pulse sent to the motor every 20 milliseconds. Theservoreads this pulse width and turns its output shaft to a specific angle (e.g., 0°, 90°, or 180°). No complex digital signals or brand-specific protocols are needed – just a repeating pulse whose “on” time changes.
Pulse Width Modulation (PWM) is a method of reducing the average power delivered by an electrical signal by chopping it into discrete on-off pulses. Instead of a steady DC voltage, you get a series of square waves. The key parameter is theduty cycle– the percentage of time the signal is HIGH (on) within a fixed period.
For servo control, theperiodis almost always 20 milliseconds (ms), which means 50 pulses per second (50 Hz). Within each 20 ms period, the signal is HIGH for a short “pulse width” and LOW for the remaining time. The servo’s internal electronics measure that exact HIGH time and compare it to a target position.
The fundamental principle is a linear mapping between the pulse width (the “on” time) and the servo’s output angle. Here are the universally accepted values for standard hobby servos (common in everyday projects like robot arms, RC planes, and pan-tilt cameras):
0.5 ms pulse (5% duty cycle)→ Shaft rotates to0 degrees(full counterclockwise, typical limit)
1.5 ms pulse (7.5% duty cycle)→ Shaft rotates to90 degrees(center position)
2.5 ms pulse (12.5% duty cycle)→ Shaft rotates to180 degrees(full clockwise, typical limit)
Any pulse width between 0.5 ms and 2.5 ms will produce a proportional angle. For example, a 1.0 ms pulse gives roughly 45°, and a 2.0 ms pulse gives roughly 135°.
Imagine you are building a robotic gripper to pick up a small ball. You attach a standard servo to the gripper’s jaws. When you send a 1.5 ms pulse every 20 ms, the servo moves to 90°, keeping the jaws half-open. To close the jaws, you send a 0.5 ms pulse – the servo rotates to 0°, fully closing the gripper. To release, you send a 2.5 ms pulse – the servo goes to 180°, fully opening the jaws. This precise, repeatable control is why PWM is the industry standard.
Inside a standard servo motor, there are four main components: a DC motor, a potentiometer (position sensor), a gear train, and a small control circuit. The PWM signal enters the control circuit, which performs three steps every 20 ms:
1. Measurethe incoming pulse width (e.g., 1.5 ms).
2. Readthe current position from the potentiometer (e.g., currently at 70°).
3. Compare– if the desired position (from pulse width) is greater than the current position, the circuit powers the DC motor to rotate forward. If smaller,it rotates backward. The gear train reduces speed and increases torque, moving the output shaft.
4. Stopwhen the potentiometer voltage matches the voltage equivalent of the pulse width.
This closed-loop feedback happens continuously, holding the position even against external forces (up to the servo’s torque rating). No external encoder or controller is needed – the servo’s internal electronics do all the work.
Mistake 1: Using the wrong period.Some beginners use a period other than 20 ms (e.g., 10 ms or 50 ms). While some digital servos accept a wider range, standard analog servos will jitter or overheat.Always start with 20 ms (50 Hz).
Mistake 2: Sending pulses outside 0.5–2.5 ms.Pulses shorter than 0.5 ms may not move the servo, and pulses longer than 2.5 ms can drive the servo against its mechanical stops, potentially stripping gears.Stay within the safe range.
Mistake 3: Updating the pulse too slowly or irregularly.The servo expects a new pulse every 20 ms. If you pause for several seconds, the servo may lose holding torque.Maintain a steady 50 Hz refresh rate.
Because the PWM-to-angle mapping is a de facto standard (established decades ago by hobby servo manufacturers), it works across all generic servos you will encounter – whether you buy a low-cost servo for a school project or a high-torque servo for a robot. You do not need any proprietary software, drivers, or brand-specific hardware. Any microcontroller (or even a simple 555 timer circuit) that can generate a precise 0.5–2.5 ms pulse every 20 ms can control any standard servo.
1. Test with a known good setup:Use an oscilloscope or a logic analyzer to verify your PWM signal’s pulse width and period before connecting a servo. This prevents accidental damage.
2. Start with the center position:Always initialize your servo at 1.5 ms (90°) to avoid sudden jumps that could break linkages.
3. Add a small delay after each position change:When moving from 0° to 180°, send incremental pulse widths (e.g., 0.5 ms → 0.7 ms → 0.9 ms …) with 20–50 ms delays to allow the servo to physically move. Sudden large jumps can stall the motor.
4. Use a dedicated power supply:Servos can draw 200–500 mA or more when moving. Do not power them directly from a microcontroller’s 5V pin; use a separate 5V–6V battery or regulated supply with a common ground.
5. Measure your servo’s actual limits:Not all servos achieve exactly 0° at 0.5 ms or 180° at 2.5 ms. Use a simple test program to sweep from 0.4 ms to 2.6 ms and note where the servo stops physically. Then set your software limits accordingly.
To control a servo motor with PWM, you only need to remember:a 20 ms period, a pulse width between 0.5 ms and 2.5 ms, and a direct linear mapping to 0–180 degrees.The servo does the rest – reading, comparing, and holding position. This simple principle powers countless real-world devices, from toy car steering to industrial automation arms. Now that you understand the “why” and “how,” you can confidently implement PWM servo control in your own project. Start with a basic test: generate a 1.5 ms pulse every 20 ms, watch the servo move to center, then change the pulse to 2.0 ms – you will see the shaft rotate. That is the fundamental principle in action.
Update Time：2026-04-20
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