Actuator anatomy: what is inside a robot joint
A humanoid joint is a motor, a gearbox, a position sensor and the drive electronics packed into one housing. Pull it apart, spin it up, and tap any part to see what it does. Dimensions are for an 80 mm reference design; the drive board is the real ulna-60-25.
The parts, in order from the back of the joint to the output
Closes the electronics bay and carries the cable gland. Also a heat path: the drive board conducts into the housing through its standoffs and the cover keeps dust and fingers out.
The brain and the muscle driver in one disc. A microcontroller runs field oriented control; three GaN half bridges switch up to 25 A peak into the coils. The encoder chip sits at the exact center, facing the shaft magnet.
Holds the rear shaft bearing and the three standoffs for the board. Its only real job is to keep the shaft magnet centered on the encoder chip to within a fraction of a millimeter.
Aluminum shell the stator is pressed into. Copper losses become heat in the coils; the fins move that heat to the air. In a humanoid this surface is often the limb itself.
Twelve steel teeth, each wound with copper. The coils are electromagnets whose polarity the drive flips thousands of times per second. The stator never moves; it is bolted to the world.
Fourteen permanent magnets glued around a steel ring, alternating north and south. The rotating field in the stator drags them round. The shaft carries the sun gear at the front and the encoder magnet at the back.
Twenty teeth on the motor shaft. Small, so the motor runs fast and the gearbox does the torque multiplication.
One hundred internal teeth, held still. The planets roll around inside it. Making the ring part of the housing saves a part and keeps the stack short.
Three forty tooth gears sharing the load, which is why a planetary stage is compact for its torque. They spin backwards relative to the sun while orbiting forwards with the carrier.
The spider that holds the three planet pins. Its orbit is the output motion: one turn of the carrier for every six turns of the motor.
A thin section bearing that takes the limb loads so the gears do not have to. Large diameter, little height, the usual trade in a joint.
Locates the output bearing and closes the gearbox. Eight bolts tie it back through the ring housing into the stator housing.
What the next limb segment bolts to. Six holes on a 34 mm circle, driven by the carrier through the bearing bore.
Four numbers that explain the design
Twelve stator teeth and fourteen rotor magnets. The counts are deliberately unequal so the magnets never line up with all the teeth at once, which keeps cogging torque low and the joint smooth at low speed.
Sun 20 teeth, ring 100 teeth, ring fixed. Ratio = 1 + 100 / 20 = 6. Output torque is six times motor torque and output speed is one sixth. Low ratios like this keep the joint backdrivable, which matters when a robot falls or is pushed.
The space between magnets and teeth. Smaller is stronger, but tolerances, thermal growth and bearing play set the floor. Torque scales with current, magnet flux and radius, which is why joint motors are wide and flat rather than long and thin.
A small diametral magnet on the shaft end faces a sensor chip at the exact center of the drive board. The drive reads rotor angle thousands of times per second and uses it to decide which coils to energize. No angle, no torque.
How it turns
Press Run motor above. The drive pushes current through the stator coils in a rotating pattern. Each coil becomes an electromagnet (red for north, blue for south) that pulls the nearest rotor magnet toward it. The rotor and its shaft spin the sun gear; the three planets roll around inside the fixed ring and carry the output flange round at one sixth of the speed. The whole thing is a field oriented control loop: measure angle and current, compute the three phase voltages, switch the GaN half bridges, repeat.
Reference actuator geometry is illustrative. Board geometry from the ulna-60-25 rev 1.2 STEP export.Questions and answersMore in the Lab: GaN Analytics · Zina AI