What Motors Do Robot Arms Use And Why It Matters

When someone asks what motors a robot arm uses, the honest answer is: it depends on the arm, the application, and the tradeoffs the manufacturer made between cost, precision, speed, and longevity. The motor choice is one of the biggest differentiators between an arm that runs reliably for years in production and one that struggles under real-world loads.

Here's a breakdown of the main motor types used in robotic arms, what each is good at, and what it means for the cobots you're evaluating.

The Three Main Motor Types in Robot Arms

Servo motors are the dominant choice in industrial and collaborative robot arms. A servo system combines a motor (typically a brushless DC or permanent magnet synchronous motor) with a high-resolution encoder and a closed-loop control system. The encoder continuously reports the actual shaft position back to the controller, which makes real-time corrections to eliminate any error between where the joint should be and where it actually is.

The result: high precision, high repeatability, and reliable performance under varying loads. Industrial arms pair servo motors with harmonic drives (strain wave gears) that achieve high gear reduction ratios with minimal backlash, critical for accuracy at the end of a long kinematic chain. The UFactory xArm series uses geared servo motors with 17-bit encoders at each joint, contributing to its ±0.1 mm repeatability specification.

Stepper motors divide one full rotation into fixed discrete steps. They're open-loop by default, the controller counts pulses sent to the motor and assumes the motor reached the target position. No encoder needed, which simplifies design and reduces cost. Steppers deliver high torque at low speeds and hold position when stationary without power draw, making them common in 3D printers, CNC machines, and some lighter robot arms.

The tradeoff: under high loads or rapid acceleration, stepper motors can "skip" steps, meaning the arm moves to the wrong position and the controller doesn't know it. For production environments with consistent, well-defined payloads, steppers can work well. For variable or unpredictable loads, the closed-loop feedback of a servo is safer.

Brushless DC motors (BLDC) offer a middle path: high efficiency, long lifespan (no brushes to wear out), and good power density relative to their size and weight. Many collaborative robots use BLDC motors paired with encoders and servo drives, creating a system that functions as a servo but is built around a BLDC core. This is increasingly the standard in modern cobots, where weight constraints at the distal joints are significant.

Harmonic Drives: The Hidden Factor in Precision

The motor itself is only half the equation. In most high-precision robot arms, the motor's output shaft runs at thousands of RPMs but needs to be slowed down by a ratio of 50:1 to 150:1 to produce useful joint torque. How that gearing happens matters enormously.

Harmonic drives (strain wave gears) are the gold standard for robotic joint gearing. They achieve very high reduction ratios with near-zero backlash, meaning the joint doesn't have any "play" when direction reverses. This is critical for accurate, repeatable positioning. The tradeoff is cost: quality harmonic drives are expensive, which is part of why precision industrial cobots cost more than basic models.

Cheaper arms may use planetary gearboxes or timing belt drives instead, which introduce more backlash and limit long-term positional accuracy, especially as the gears wear.

What This Means When You're Buying a Cobot

When evaluating a cobot, motor type and gearing are worth asking about directly:

• Does it use servo motors with encoder feedback, or open-loop steppers?

• What type of gearing is used at each joint, harmonic drives, planetary, or belt?

• What's the stated repeatability, and under what payload conditions was that measured?

For production automation, pick and place, machine tending, welding, you want a servo-driven arm with harmonic or high-quality planetary gearing and a repeatability spec of ±0.1 mm or better. Both the UFactory xArm series and Fairino cobots meet this standard, starting at $3,500 and scaling up to larger payload models like the Fairino FR30 at 30 kg capacity.

Our Cobot Selector compares arms by payload, reach, and specification, or you can book a demo to see the hardware in action.

Motor Type by Application

As a quick reference:

• Precision assembly, pick and place, welding: servo motors with harmonic drives, the standard for any production cobot

• Light inspection or tabletop tasks: BLDC with encoder feedback is sufficient; harmonic drives are optional at this payload range

• Educational or prototyping applications: stepper motors are adequate and cost-effective

• High-speed packaging: high-torque servos with fast amplifier response, look for rated cycle times, not just repeatability

CONCLUSION

The motor inside a robot arm determines how accurately it positions, how reliably it performs under load, and how long it lasts in production. Servo motors with encoder feedback and quality gearing are the standard for any production-grade cobot, and both UFactory and Fairino arms are built to that standard. If you're comparing specs across manufacturers, repeatability and motor type are two of the first things to look at.

Have specific questions about what's inside a particular model? Contact us or run your application through our Automation Analysis Tool to get a realistic picture of what you need.

FAQ:

Q: Are servo motors better than stepper motors for robot arms? 

A: For production applications with variable payloads, yes, closed-loop servo control catches and corrects position errors that open-loop steppers can't detect. For light, well-defined tasks, steppers can be cost-effective.

Q: What is a harmonic drive? 

A: A type of strain wave gear used in robot joints to achieve high gear reduction ratios with minimal backlash, contributing directly to positional accuracy and repeatability.