Motion control for robot arms and cobots

In short:A robot or cobot joint is a tightly packed module: a frameless motor, a strain-wave gear, an encoder on each side of the gear, a fail-safe brake and a compact drive, all around a hollow centre for cables. Size it from the worst-case gravity and acceleration torque at the joint, then check heat, stiffness and what the safety functions need from the feedback.

Typical requirements

Why robot joints are a packaging problem

An articulated arm is a chain of rotary axes, and every joint carries the mass of the joints beyond it. A heavy wrist makes the elbow bigger, a bigger elbow makes the shoulder bigger, and so on down to the base. That is why robot and cobot designers care so much about torque density, axial length and mass, and why the drivetrain is built into the arm structure rather than bolted on.

For a sense of scale, Universal Robots publishes its UR10e at 12.5 kg payload, 1,300 mm reach, ±0.05 mm pose repeatability to ISO 9283 and ±360° on all six joints, with base and shoulder joints at 120°/s and the wrist at 180°/s. The whole arm weighs 33.3 kg. Hitting numbers like that is mostly a question of what goes inside each joint.

What goes into a cobot joint module

Most joints in collaborative and light industrial arms follow the same pattern, described in both the Akribis and maxon joint design articles:

Frameless motors

A frameless torque motor is a rotor and stator with no housing, bearings or shaft. The joint housing and bearings do that job, so nothing is duplicated. Torque rises steeply with air-gap diameter, so a flat, large-diameter motor gives more torque in less axial length and leaves a big hollow centre. Low-voltage kits such as the Akribis ADR-F (45–90 mm OD, 48 V DC bus) are aimed at this size of joint. Our frameless motor sizing guide for robot joints covers the maths.

Strain-wave gears

Strain-wave gear sets give zero backlash and ratios of roughly 30:1 to 160:1 in one stage, which suits a joint that must reverse constantly and hold position accurately. Component sets such as the Harmonic Drive CSG, the flat CSD and the Leaderdrive LCS/LCSG are built into the joint around the motor (Harmonic Drive is a trademark of Harmonic Drive LLC; strain-wave gearing is the generic term). Their weak points are torsional stiffness at low load, efficiency that changes with speed and temperature, and a ratcheting torque limit you must never exceed in a collision. Compare options in harmonic vs planetary vs cycloidal and browse gearboxes.

Dual absolute encoders

The standard arrangement puts an encoder on each side of the gear. Akribis describes its ARJ cobot joints as using two 19-bit encoders, one on the motor side and one on the load side, and using the difference across the strain-wave gear to estimate payload torque for compliance control. maxon’s example joint uses a 2,560-count incremental encoder on the motor and a 21-bit absolute encoder on the output.

The output encoder must be thin and hollow. Ring and capacitive designs such as the Netzer VLX and the off-axis magnetic RLS AksIM-4 are built for this layout. Read absolute vs incremental encoders before you choose an interface.

Brakes and torque sensing

Every joint that carries gravity load needs a power-off brake. Akribis contrasts pin brakes, which can allow a small amount of free movement, with spring-engaged friction-disc brakes that lock fully. For sensitive force control, KUKA fits joint torque sensors in all seven axes of the LBR iiwa. Lower-cost arms rely on motor current, dual-encoder estimation or a wrist force/torque sensor; the UR10e’s tool-flange sensor, for example, is specified at ±100 N and ±10 Nm range with ±5.5 N and ±0.5 Nm accuracy.

Compact drives

Putting the drive in the joint shortens the motor leads and turns the arm harness into power plus a fieldbus. Board-level drives such as the Novanta Everest (8–72 V DC, 45 A continuous, EtherCAT or CANopen) are designed for cobot joints and humanoids. Browse servo drives.

How to size a robot joint: a worked example

Take the shoulder of a 10 kg payload arm with 1.2 m reach, and assume the links and outer joints add the equivalent of 8 kg acting at 0.5 m.

1. Gravity torque, arm horizontal: payload 10 × 9.81 × 1.2 = 118 Nm, plus arm 8 × 9.81 × 0.5 = 39 Nm. Total about 157 Nm.
2. Acceleration torque: estimate the inertia about the shoulder (payload and arm masses at their radii come to about 16.4 kg·m²). At 3 rad/s² that adds about 49 Nm. Peak demand is roughly 206 Nm.
3. Gear: at 100:1 the motor sees about 2.1 Nm peak before gear losses. Divide by the gear efficiency read from the manufacturer’s curve at your speed and temperature, not a nominal figure.
4. Speed: 120°/s at the joint is 20 rpm, so 2,000 rpm at the motor. Check the motor’s torque-speed curve at the bus voltage you actually have.
5. Thermal: an arm rarely sits fully extended, so work out RMS torque from a realistic duty cycle. Then check that the motor’s continuous rating, de-rated for a sealed joint with little airflow, covers it.
6. Brake and gear limits: the brake must hold 157 Nm plus margin. The gear’s momentary peak rating must cover a collision or emergency stop, which is often the real sizing case.

Treat these numbers as an illustration. A published joint family such as Akribis ARJ spans about 12.7–248 Nm continuous and 32–383 Nm peak across five sizes, which shows how quickly torque falls from base to wrist.

Slip rings, cables and end effectors

Most arm joints have limited rotation and route cables through the hollow centre. Continuous rotation turns up at the last wrist axis on some designs, on rotary tool changers, robot-mounted turntables and positioners, and on mobile manipulator bases. Those axes need a compact through-bore slip ring, for example the Servotecnica SVTS C range with Ethernet and pneumatic passage options, or a capsule unit for small wrists. Gripper air or vacuum can cross the axis through a rotary union. Our page on slip rings for robotics and the Ethernet through slip rings guide cover signal integrity.

End effectors run on small motors and actuators: electric grippers, screwdrivers, press-fit heads. Linear-rotary motors such as the LinMot PR02 combine stroke and rotation for screw-driving and press-and-turn tasks. For linear tooling see linear actuators.

Safety: ISO 10218-1 and -2:2025

The 2011 robot safety standards were replaced in 2025. According to A3, Part 1 covers the robot manufacturer and Part 2 the integrator’s application and cell. Collaborative requirements formerly in ISO/TS 15066 are now part of ISO 10218-2, and the standard talks about “collaborative applications” rather than “collaborative robots”. ibf solutions summarises other changes that matter to component choice:

• The blanket PL d, Category 3 requirement has largely gone. Default levels are now set per function, for example PL d / SIL 2 for protective stop and reduced speed, and PL c / SIL 1 for emergency stop, with deviation allowed through a fuller risk assessment.
• Reduced speed is capped at 250 mm/s.
• A low-hazard robot class gets reduced control requirements.
• Force and pressure limits for contact situations sit in Part 2.
• ibf reports that the EN ISO versions were cited in the EU Official Journal in September 2026.

What this means for the joint: safety functions such as monitored speed, position limits and monitored standstill depend on trustworthy feedback. Ask encoder suppliers for safety data (diagnostic coverage, MTTFd, or a safety-rated protocol such as BiSS Safety) and drive suppliers for STO or Safe Motion functions with stated PL or SIL.

Typical pitfalls

1. Sizing for gravity only and forgetting acceleration and collision torque at the gear.
2. Relying on motor-side feedback and finding that gear wind-up spoils accuracy under load.
3. Ignoring heat in a sealed joint. Lubricant and encoder temperature limits often bind before the motor’s winding limit.
4. Leaving no room for cables through the hollow centre, including the slip ring or rotary union you add later.
5. Choosing a brake that allows free play on an arm that must hold pose precisely.

Send us your joint torques, envelope and safety targets and our specialist engineering and supply partners can propose matched motor, gear, encoder, brake and drive sets.

Standards to have in view

• ISO 10218-1:2025 (industrial robots: safety requirements for the robot)
• ISO 10218-2:2025 (industrial robot applications and robot cells, now including collaborative application requirements)
• ISO/TS 15066:2016 (collaborative robot applications; content folded into ISO 10218-2:2025)
• ISO 13849-1 (safety-related parts of control systems, performance levels)
• IEC 62061 (functional safety of machinery control systems, SIL)
• ISO 9283 (manipulating industrial robots: performance criteria and test methods)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.