Ten teaching models to revisit as you work through the course. Predict the effect of a control before moving it.
MOTION BENCH / 01Interactive teaching model
Nine turns in. One turn out.
Explore how a compact motor and a modest gear reduction work together.
Planetary reduction
The outer ring stays fixed. The 12-tooth sun drives three 42-tooth planets inside a 96-tooth ring. Their carrier makes one turn for every nine turns of the sun.
Reduction9:11 + 96 / 12
Output speed10.0 rpm90 ÷ 9
Output torque3.60 N·m0.40 × 9
Mechanical power3.77 W out3.77 W in · 0.00 W lost
Why “quasi-direct drive”? A relatively low reduction combines the motor’s torque with gearing while limiting some of the friction and reflected inertia of a very large reduction. Backdrivability still depends on the entire actuator. RobStride identifies RS03 as a QDD actuator with a 9:1 reduction; the tooth counts and construction drawn here are an explanatory example. Gear teeth are stylized, not manufacturing profiles. Separated parts retain linked motion only to illustrate the ratio. Manufacturer reference ↗
Try it yourself
At 90 rpm and 0.40 N·m, predict the output before reading the numbers. Then switch on losses. Which number changes: speed, torque, or both? Explain where the missing power goes.
Check your reasoning
The ideal output is 10 rpm and 3.60 N·m. With the example 90% efficiency, the output remains 10 rpm under the assumed steady rotation, while available torque becomes 3.24 N·m. About 0.38 W is lost, mainly as heat. This simplified transmission model omits the motor’s torque–speed curve, current limits, thermal limits and acceleration.
These are educational illustrations and calculations. They send no motor commands and do not replace measured physics or a verified hardware configuration.