Follow the explanation
Back to the model ↑How the parts work together
Trace the complete assembly
Separate the components. Electronics sit behind the motor, the gearbox sits in front, and the bearing-supported output hub connects to the load. Select a part to read its role.
Turn electricity into shaft motion
The copper windings stay still while permanent magnets and the shaft rotate inside them. The controller times current using position feedback. This animation prescribes rotation; it does not solve electromagnetic forces.
Hold the ring, drive the sun
In gear-train view, watch a planet spin while its centre travels around the sun. The fixed ring constrains the motion. Three planets share the architecture; their number does not set the ratio.
Count four turns for one
The 4:1 set has an 18-tooth sun and 54-tooth ring. Four motor turns correspond to one carrier turn. The output rotates in the same direction as the sun, while each planet also spins on its own pin.
Trade speed for torque
Switch from 4:1 to 6:1 at the same motor speed and torque. Output speed drops, available output torque rises, and mechanical power is reduced by the selected efficiency. These are assumed operating points, not a motor performance envelope.
The main parts
- Rear cover
- Closes the back of the actuator and carries the cable connector. It stays fixed to the robot structure.
- Motor controller
- Power transistors switch current through the three motor phases. The controller uses rotor-position feedback to time that switching; the board here illustrates component placement, not an electrical circuit.
- Position encoder
- A patterned disk follows the motor shaft while a stationary sensor reads its position. This motor-side encoder sees the fast shaft; gearbox backlash could still introduce output-position error.
- Motor housing & supports
- Supports the stator and motor-shaft bearings. The ribbed shell protects the internals; cooling performance is not modeled.
- Stator & copper windings
- Stationary coils surround the rotor. Controlled three-phase current produces the rotating magnetic field that exerts torque on the permanent-magnet rotor.
- Rotor, magnets & shaft
- The permanent magnets and central shaft turn together inside the stator, separated by an air gap. The shaft directly drives the sun gear.
- Sun gear · 18 teeth
- Receives the motor rotation. Its teeth drive all three planets; the sun makes four, five or six turns for one carrier turn in the selectable configurations.
- Fixed internal ring gear
- Internal teeth mesh with the outside of every planet. A locating key on the housing seats in the ring's outer notch, holding it stationary and carrying reaction torque into the fixed structure. Separate the components to see the keyway.
- Planet gear 1
- Spins on its carrier pin while its centre orbits the sun. Both motions are needed to mesh simultaneously with the moving sun and the fixed ring.
- Planet gear 2
- The second identical planet is spaced 120 degrees around the carrier. Multiple planets provide parallel load paths; they do not multiply the reduction ratio.
- Planet gear 3
- The third planet completes the symmetric gear set. Actual load sharing depends on manufacturing accuracy and compliance, neither of which is solved here.
- Planet carrier & pins
- Three fixed pins on the carrier support the planet bores. Their centres orbit with this carrier, which takes the combined gear action to the output shaft.
- Output bearing
- An inner race supports the output shaft while the outer race is held by the front flange. Balls separate the rotating and stationary races; bearing contact forces and life are not calculated.
- Front mounting flange
- Locates the output bearing and bolts to the housing. Its centre opening lets the output shaft rotate independently of the fixed casing.
- Output shaft & hub
- Rotates with the planet carrier and carries motion to the joint or load. The reduction lowers speed and increases available torque, with the selected efficiency accounting for an assumed aggregate loss.
Questions worth exploring
What is the difference between a motor and an actuator?
A motor supplies rotation. This rotary actuator adds a gearbox, bearings, an output connection and position feedback around that motor so it can serve a robot joint. This explorer covers a brushless motor with a planetary stage.
Oriental Motor · Brushless motor structure, switching and feedbackMIT · Slocum, FUNdaMENTALS Topic 6, planetary drives (PDF pp. 32–35) and backlash (pp. 52–53)
Why does a 4:1 planetary gearbox reduce speed by four?
With the ring fixed and sun driven, the carrier reduction is one plus ring teeth divided by sun teeth. The 18-tooth sun and 54-tooth ring give 1 + 54/18 = 4. A 600 rpm motor therefore produces 150 rpm at the carrier. Adding more planets does not change that ratio.
MIT · Slocum, FUNdaMENTALS Topic 6, planetary drives (PDF pp. 32–35) and backlash (pp. 52–53)
Does multiplying torque create more power?
No. Mechanical power equals torque times angular speed. An ideal 4:1 reduction offers four times the torque at one-quarter the speed. With an assumed 90% gearbox efficiency, 1 N·m at the input becomes 3.6 N·m at the output; ten percent of the input mechanical power is lost.
MIT · Slocum, FUNdaMENTALS Topic 6, planetary drives (PDF pp. 32–35) and backlash (pp. 52–53)OpenStax · Rotational power, §10.8, equation 10.31
What does the position encoder measure?
The illustrated encoder reads the motor shaft. Its signal helps the controller determine rotor position. It does not directly measure joint output torque, and a motor-side encoder cannot by itself remove mechanical backlash between the motor and the load.
Oriental Motor · Brushless motor structure, switching and feedbackMIT · Slocum, FUNdaMENTALS Topic 6, planetary drives (PDF pp. 32–35) and backlash (pp. 52–53)