2025

How Do Servos 'Obey'? An Introduction to Their Control Principles

This article explores how servos "obey" via PWM signals (1-2ms pulses) and closed-loop feedback, using a control circuit, potentiometer, and motor to align current and desired angles, ensuring precision.

Author:

Shenzhen XQ-POWER Technology Co., LTD.

How Do Servos "Obey"? An Introduction to Their Control Principles

Servos are renowned for their ability to move to precise angles and hold position—whether steering a model car, adjusting a robot arm, or tilting a security camera. But how do these compact devices "understand" commands and execute them with such accuracy? The answer lies in a clever interplay of electrical signals, feedback mechanisms, and mechanical coordination. Let’s break down the basics of how servos "listen" and respond.

At the core of servo control is a specific type of electrical signal: PWM (Pulse Width Modulation). Unlike continuous power, PWM sends rapid, repeated pulses of electricity. For servos, these pulses follow a strict pattern: a 20-millisecond (ms) interval (the time between the start of one pulse and the next) with a variable "on" time (pulse width) ranging from 1ms to 2ms. This pulse width acts like a numerical command: 1.5ms typically corresponds to the neutral position (e.g., 90° for a 0-180° servo), 1ms to the minimum angle (0°), and 2ms to the maximum angle (180°). Think of it as a language where "1ms" means "turn left all the way," "2ms" means "turn right all the way," and "1.5ms" means "stay in the middle."

But a signal alone isn’t enough. Servos rely on a closed-loop feedback system to ensure they reach and maintain the target angle. Here’s how the key components work together:

First, the control circuit (the servo’s "brain") receives the PWM signal, often from a microcontroller, remote control, or sensor. It decodes the pulse width to determine the desired angle.

Next, the potentiometer (a variable resistor attached to the output shaft) acts as a "position sensor." As the shaft moves, the potentiometer’s resistance changes, generating a voltage that the control circuit interprets as the current angle.

The control circuit then compares the desired angle (from the PWM signal) with the current angle (from the potentiometer). If there’s a mismatch—say, the signal demands 120° but the shaft is at 90°—the circuit activates the DC motor. The motor spins in the direction needed to close the gap (clockwise or counterclockwise), driven by power from the servo’s power supply.

As the motor turns, it moves the gear train (which reduces speed and increases torque) and, in turn, the output shaft. This rotation adjusts the potentiometer, updating the "current angle" signal sent to the control circuit. The process repeats—signal decoding, comparison, motor activation—until the current angle matches the desired angle, at which point the motor stops.

This closed-loop system is what makes servos so reliable. Even if external forces (like a light push on a robot arm) nudge the shaft off position, the potentiometer detects the change, and the control circuit restarts the motor to correct it.

Not all servos follow this exact pattern, though. Digital servos, for example, use a microprocessor to interpret PWM signals more precisely, often with faster response times than analog servos. Continuous-rotation servos (used for tasks like driving wheels) modify the system: instead of stopping at a set angle, they spin continuously, with pulse width controlling speed and direction (1.5ms for stop, below 1.5ms for one direction, above for the other).

In essence, a servo’s ability to "obey" stems from its ability to "speak" the PWM language, "sense" its position, and "correct" its movements—a combination of simplicity and sophistication that makes it indispensable in countless automated systems.

Keywords:

torque servo speed servo gear servo titanium servo waterproof servo digital servo brushless servo digit servo standard servo coreless servo metal servo horn metal servo arm 25t metal servo airplane servo 150kg 150kg servo motor 150kg torque servo 12v 150kg servo digital servo 150kg 150kg industrial servo digital servo motor digital servo for rc robot servo motor aluminum servo horn 25t aluminum servo arm