MODULE 03 / FOUNDATIONS
Actuators & QDD
Connect motors, gearing, encoders and current control. Learn why low-ratio integrated actuators are useful in legged robots, and what their specification tables do and do not promise.
By the end, you can…
- Explain motor, gearbox, encoder and driver roles in an integrated actuator.
- Calculate ideal torque and speed changes through a reduction ratio.
- Compare actuator specifications without treating peak torque as a continuous rating.
Forces, units & motion
Nine turns in.
One turn out.
Explore how a compact motor and a modest gear reduction work together.
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.
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.
The motor is only part of the actuator
An electric motor converts electrical input into rotating mechanical motion. In a brushless permanent-magnet motor, a controller switches currents in stationary windings to create a moving magnetic field. The rotor follows the resulting electromagnetic torque. BLDC describes a motor/control family; it does not tell you the complete mechanical reduction, sensing or command interface.
A useful robot joint combines the motor with bearings, a housing, an output connection, sensing and control electronics. It may also contain gears. An integrated actuator packages many of these responsibilities together. “Servo” describes controlled behavior using feedback; it is not a universal shape or a promise that every servo accepts the same signal. A small hobby servo and a networked robot actuator have very different interfaces.
What quasi-direct drive means
Direct drive connects the motor to the load without a reduction gearbox. Quasi-direct drive, or QDD, generally combines a motor with a relatively modest reduction ratio. It keeps some benefits of gearing while aiming for useful responsiveness and backdrivability. There is no single magic ratio that guarantees QDD performance: motor design, friction, bearings, control bandwidth and the mechanism all matter.
In an ideal 9:1 reduction, the motor makes nine turns for one output turn. Output speed is divided by nine and output torque is multiplied by nine. Real losses reduce available output power. Reflected motor inertia at the output grows with the square of the ratio, so more reduction is not a free improvement. A joint that can be pushed backward can still have friction and resistance.
Ideal speed ωout = ωmotor / N; ideal torque τout = N × τmotor; real torque is reduced by efficiency and limitsPlanetary gears share a compact axis
A simple planetary arrangement has a central sun gear, planet gears around it, an outer ring gear and a carrier holding the planet axes. Which element is fixed and which are input/output determines the ratio and direction. A sun-driven, fixed-ring system commonly uses the carrier as output. The teaching figure illustrates this mechanism; its exposed components are not a certified teardown of a specific RobStride revision.
RobStride’s manufacturer information describes integrated QDD products and a 9:1 ratio for RS03. The BDX-R audit’s current fourteen-motor map uses eight 03, three 02 and three 05 units. Those names require revision-specific datasheets before purchasing: current product labels and ratings should not silently replace the hardware assumed by an older project.
Read the whole operating envelope
Compare continuous torque, peak torque and permitted peak duration; output speed; voltage range; current limits; mass; encoder arrangement; communication protocol; thermal limits; and mounting dimensions. Ask whether a speed is no-load speed or speed at a rated load. A peak torque figure and a no-load speed generally cannot be demanded simultaneously.
The controller may accept position, velocity and torque-related commands. In impedance-style control, position error and velocity error contribute to a torque request, often alongside feedforward torque. This is why gains and units are part of actuator compatibility. Changing the sign of a joint or supplying degrees where radians are expected can transform a gentle target into a large error.
A simplified gear calculation
A hypothetical motor producing 1 N·m at 90 rpm through a 9:1 reducer gives 9 N·m at 10 rpm ideally. At 85% mechanical efficiency, that operating point becomes approximately 7.65 N·m at 10 rpm. These are exercise inputs, not a RobStride rating.
WORDS YOU NOW KNOW
- QDD
- Quasi-direct drive: an actuator approach usually combining a motor with modest gear reduction.
- Backdrivability
- How readily external output motion can drive the transmission and motor backward.
- Encoder
- A sensor that reports rotational position or changes in position.
- FOC
- Field-oriented control: a method that regulates motor currents in a rotating reference frame.
YOUR CALL
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Primary references for this module. Use the version and hardware revision named in the lesson.
RobStride product information and drawingsManufacturer RS03 pageAudited BDX-R hardware implementation