What Is a Servomotor?
A servomotor is a motor coupled to a feedback sensor and a controller, and it forms the actuating part of a servomechanism. The controller compares the commanded position or speed with what the sensor reports and corrects the difference continuously. The output can be rotary or linear.
The "system, not a motor class" point matters when you read a datasheet or a quote. Several motor technologies can serve as a servomotor, including AC synchronous, brushless DC and brushed DC. What makes a motor a servomotor is that it is built and paired with feedback so it can run in a closed loop. Low inertia, fast response and a mounting-ready flange are typical traits.
Why is it called a servo motor?
"Servo" comes from servomechanism, a feedback-controlled system in which a small command signal drives a larger output and the system corrects its own error. The word traces back to the Latin servus, "servant": the motor does what the command says and checks that it did.
Servo Motor Working Principle
The three building blocks
Motor. Produces torque. In industrial units this is usually a permanent-magnet synchronous motor.
Feedback device. An encoder, resolver or, in simple hobby servos, a potentiometer. It reports shaft position, and often speed. Industrial encoders commonly resolve 17 to 23 bits per revolution, which is 131,072 to 8,388,608 counts per turn.
Controller or drive. Takes the command, reads the feedback and sets the motor current. Industrial servomotors rely on a separate servo drive for this role. Hobby servos build a small control circuit into the case.
The closed-loop cycle
The controller receives a command: a target position, speed or torque.
The feedback device reports the actual shaft position.
The controller calculates the error, which is the gap between command and reality.
It sends corrective current to the motor. As the error approaches zero, the correction fades and the shaft settles on target.
The labeled loop below shows where each block sits and which signal passes between them.
| Block | Role in the loop | Signal passed on |
|---|
| 1. Command | Sets the target position, speed or torque | Setpoint to block 2 |
| 2. Controller / drive | Subtracts feedback (block 5) from the setpoint and turns the error into current | Motor current to block 3 |
| 3. Servomotor | Converts current into torque | Shaft motion to block 4 |
| 4. Load / axis | Table, joint, screw or spindle being moved | Motion sensed by block 5 |
| 5. Feedback device (encoder) | Mounted on the motor shaft; measures actual position | Position back to block 2 |
Hobby servos typically use a potentiometer and a simple control law in which the motor works harder the further it is from target. Industrial servomotors use higher-resolution encoders and tuned control loops, which let them reach a target faster and with less overshoot.
Servomotor vs. stepper motor
| Servomotor | Stepper motor |
|---|
| Feedback | Always closed-loop | Usually open-loop; closed-loop versions exist |
| Torque at speed | Holds torque across the speed range | Torque typically falls as speed rises |
| Failure behavior | Controller sees and corrects position error | Open-loop steppers can lose steps without detecting it |
| Cost and complexity | Higher; needs tuning | Lower; simpler to set up |
Steppers suit light, predictable loads. Servomotors suit axes that must accelerate hard, hold position under varying load, or prove they arrived where they were told.
What Are the Three Main Types of Servo Motors?
By motor technology, the three types are AC, brushless DC and brushed DC servo motors. Hobby sources use a different split, by motion output, covered after the table.
| Type | Commutation | Maintenance | Typical use |
|---|
| AC servo motor | Electronic, via the drive | Low; no brushes | CNC axes, automation lines, industrial machinery |
| Brushless DC servo motor | Electronic | Low; no brushes | Robotics, compact and mid-power axes |
| Brushed DC servo motor | Mechanical brushes | Brushes wear and need replacement | Light-duty and legacy equipment |
AC servo motors. Permanent-magnet synchronous designs dominate industrial work. They are the usual choice for CNC machine tools and automated production. Haozhi's BL 30 high-response AC servo motor and large-frame AC servo motors are examples of this class.
Brushless DC servo motors. These use electronic commutation, so there are no brushes to wear. They are compact and common in robotics. The line between "brushless AC" and "brushless DC" is largely a naming convention, and many vendors use the terms loosely.
Brushed DC servo motors. Simple and inexpensive, but the brushes wear. They now appear mostly in light-duty or older designs.
The other framing. Hobby and RC sources classify servos by motion: positional rotation (limited angle), continuous rotation, and linear. That is why articles disagree about "the three types." Both are valid; they answer different questions.
What Is a Servo Motor Used For?
Each application leans on a specific property of closed-loop control:
CNC machine tools: feed axes need accurate positioning and fast acceleration.
Robot joints and arms: they need torque holding and smooth, repeatable motion.
Packaging and automated lines: repeated start-stop moves need quick response.
Semiconductor and PCB equipment: small, precise moves benefit from fine feedback resolution.
If you are building or retrofitting a machine, explore Haozhi's range of servo motors for CNC, robotics and automation. For robotic arms specifically, a servomotor can also come packaged with a reducer, brake and drive as an integrated robot joint module.
Hobby Servos vs. Industrial Servomotors
| Hobby servo | Industrial servomotor |
|---|
| Feedback | Usually a potentiometer | Encoder or resolver |
| Control | Built-in circuit, pulse-width command | Separate drive with tuned loops |
| Motion | Often limited to roughly 180° | Continuous rotation, multi-turn |
| Duty cycle | Intermittent, light loads | Continuous, high-dynamic duty |
The command interface differs too: a hobby servo takes a PWM pulse, typically 1–2 ms repeated every 20 ms, while an industrial servomotor is commanded through its drive, which accepts drive commands such as pulse/direction, analog or fieldbus signals.
How to Choose a Servomotor
Work through six checks before comparing models:
Required torque and speed. Calculate continuous and peak torque for the real move profile, not just maximum speed. Many industrial servomotors can deliver roughly 300% of rated torque for short bursts, but the RMS torque over the whole cycle must stay within the rated value.
Load inertia match. A load far heavier than the rotor's inertia is harder to control, and reflected inertia matters most with gearing or screws. A common guideline keeps load-to-rotor inertia at or below about 10:1, and lower for highly dynamic axes.
Feedback resolution. Encoders of 17 to 23 bits per revolution are typical for industrial servos. More counts support finer positioning and smoother low-speed motion, but resolution is not the same as accuracy.
Frame size. Mounting flange and envelope must fit the machine layout.
Drive compatibility. The drive must support the motor's feedback type and current rating.
Duty cycle. Continuous heavy duty and short intermittent moves call for different thermal margins.
Worked sizing example
The numbers below are illustrative and do not describe a specific Haozhi model. Take a horizontal axis: a 100 kg table on a 10 mm-lead ball screw, 0.5 m/s maximum speed, 5 m/s² acceleration, 0.05 friction coefficient and 90% screw efficiency.
Speed: 0.5 m/s ÷ 0.010 m = 50 rev/s, or 3,000 rpm.
Force: 100 kg × 5 m/s² = 500 N, plus friction of about 49 N, gives roughly 550 N.
Peak torque: 550 N × 0.010 m ÷ (2π × 0.9) ≈ 0.97 N·m. With a 30% margin, about 1.3 N·m.
Rated torque: if the motor's peak is about 3× rated, the rating must be at least about 0.43 N·m. Then check RMS torque across the full move profile.
Reflected inertia: 100 kg × (0.010 m ÷ 2π)² ≈ 2.5 × 10⁻⁴ kg·m² (2.5 kg·cm²), before adding screw and coupling inertia. At 10:1, the rotor needs roughly 0.25 kg·cm² or more.
Resolution: a 17-bit encoder gives 131,072 counts per revolution, or about 0.076 µm of screw travel per count. A 23-bit encoder is about 0.001 µm per count.
With those figures, you can shortlist a series. Haozhi series to check against your numbers:
To compare series side by side, see the full Haozhi servo motor lineup. If you have a torque, speed and frame target in hand, send it to our engineers and we can help match it to a catalog model.
FAQ
What is a servo motor used for?
Servomotors move a load to a precise position, speed or torque and hold it there. Typical jobs include CNC machine axes, robot joints, packaging and assembly lines, and semiconductor or PCB equipment. They are chosen where accuracy, fast acceleration or reliable holding under changing load matters more than low cost.
Why is it called a servo motor?
The name comes from "servomechanism," a system that uses feedback to correct its own error. "Servo" derives from the Latin servus, meaning servant. The motor follows a command and the feedback loop confirms it got there, which separates it from a motor simply switched on and off.
What are the three main types of servo motors?
A common industrial split is AC servo motors, brushless DC servo motors and brushed DC servo motors. AC synchronous designs dominate CNC and automation, brushless DC suits compact robotics axes, and brushed DC is mostly light-duty. Hobby sources instead list positional, continuous-rotation and linear servos.
How expensive is a servo motor?
It varies widely. Cost depends mainly on torque class, feedback type, frame size and brand, and the drive and cables add to the total. A small hobby servo and a large industrial unit with a high-resolution encoder are in different price worlds. Servomotor cost breaks down what drives the price.
Does a servo motor need a drive or controller?
Yes. The closed loop only works if something compares the command with the feedback and sets motor current. Hobby servos have that circuit inside the housing. Industrial servomotors use a separate servo drive matched to the motor's feedback type and current rating.
What is the difference between a servo motor and a stepper motor?
A servomotor runs in a closed loop and corrects errors using feedback, and it keeps its torque at higher speeds. A stepper moves in fixed increments and usually runs open-loop, which is simpler and cheaper but can lose steps under overload. Closed-loop steppers exist and narrow the gap.
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