Robotics servo control uses pulse width modulation, or PWM, to send position commands to a motor controller inside a servo. This cheat sheet helps students connect signal timing, angle control, wiring, and starter code in one reference. It is useful when building robot arms, steering mechanisms, grippers, pan-tilt cameras, and other moving parts.
Students need it because small timing or wiring mistakes can cause jitter, weak motion, or damaged components.
The most important idea is that a servo reads the width of a repeating control pulse, not the average voltage like a simple DC motor speed controller. A typical hobby servo expects a pulse about every 20 ms, with about 1.0 ms for one end, 1.5 ms for center, and 2.0 ms for the other end. Duty cycle is calculated with duty cycle = pulse width / period x 100%.
Calibration adjusts the exact pulse limits so the servo moves safely without hitting mechanical stops.
Key Facts
- PWM stands for pulse width modulation, and it controls a signal by changing how long the signal stays high during each repeating period.
- Period and frequency are related by frequency = 1 / period, so a 20 ms servo period equals 50 Hz.
- Duty cycle is calculated with duty cycle = high time / period x 100%, where high time is the pulse width.
- A common hobby servo range is 1.0 ms to 2.0 ms, with 1.5 ms usually near the center position.
- For a 20 ms period, a 1.5 ms pulse has duty cycle = 1.5 / 20 x 100% = 7.5%.
- A simple angle map is pulse width = min pulse + angle / 180 x (max pulse - min pulse) for a 0 to 180 degree servo.
- Servo power should usually come from a separate 5 V to 6 V supply, but the servo ground and controller ground must be connected together.
- Calibration means testing and setting safe min, center, and max pulse values before using the servo in a robot mechanism.
Vocabulary
- PWM
- Pulse width modulation is a control method that sends repeated on-off pulses and changes the high-time of each pulse.
- Pulse width
- Pulse width is the amount of time a PWM signal stays high during one cycle, often measured in milliseconds or microseconds.
- Duty cycle
- Duty cycle is the percent of one PWM period during which the signal is high.
- Frequency
- Frequency is the number of repeating PWM cycles per second, measured in hertz.
- Servo calibration
- Servo calibration is the process of finding safe pulse values that match the servo's real center and motion limits.
- Common ground
- Common ground means connecting the ground of the controller and the ground of the servo power supply so they share the same signal reference.
Common Mistakes to Avoid
- Using duty cycle alone to describe servo position is wrong because most hobby servos respond to pulse width, usually in microseconds or milliseconds.
- Forgetting to connect common ground is wrong because the servo signal has no reliable voltage reference and the servo may jitter or ignore commands.
- Powering several servos directly from a small microcontroller pin is wrong because signal pins cannot supply the high current a moving servo needs.
- Commanding 0 degrees or 180 degrees without calibration is risky because the servo may hit a mechanical stop and draw too much current.
- Confusing frequency with pulse width is wrong because frequency sets how often pulses repeat, while pulse width sets the commanded servo position.
Practice Questions
- 1 A servo signal has a 20 ms period and a 1.0 ms high pulse. What is the duty cycle?
- 2 A controller sends a 50 Hz PWM signal. What is the period in milliseconds?
- 3 Using pulse width = 1000 microseconds + angle / 180 x 1000 microseconds, what pulse width commands 90 degrees?
- 4 Why should a robot use a separate servo power supply with a shared ground instead of powering a servo only from a microcontroller signal pin?