A quasi-direct-drive (QDD) motor combines a torque motor with low-ratio reduction. Estimate its output quantization from encoder bits and gear ratio, then check startup ambiguity and choose where your robot joint needs feedback.
Published by QDD Motors · Published · Reviewed
A smaller calculated step is not an accuracy specification. Ask for loaded output error and repeatability.
Motor single-turn data can represent several joint poses after reduction. Verify the turn state before enabling motion.
Consider an output sensor when joint position must be observed independently of the gear.
Dual feedback requires calibration and tuning. It does not guarantee a fixed arcminute accuracy.
Let b be the motor encoder’s bits within one turn, and N the exact motor turns per output turn. For ideal rigid gearing:
Motor counts = 2^b
Equivalent output counts = 2^b × N
Output step (°) = 360 / (2^b × N)
Arcminutes = degrees × 60; arcseconds = degrees × 3,600
Equivalent counts express an angular increment, not unique absolute output positions. A non-integer ratio may yield fractional equivalent counts. The tool assumes ideal transmission and excludes encoder error, backlash, compliance, temperature, sampling and controller behavior. Displayed angles use six significant figures.
| Motor bits / ratio | Equivalent counts / output turn | Output step (degrees) | Output step (arcsec) |
|---|---|---|---|
| 14 bits / 6:1 | 98,304 | 0.00366211° | 13.1836 |
| 17 bits / 9:1 (default) | 1,179,648 | 0.000305176° | 1.09863 |
| 17 bits / 1:1 (direct drive) | 131,072 | 0.00274658° | 9.88770 |
| 24 bits / 100:1 (tool limit) | 1,677,721,600 | 2.14577e-7° | 0.000772476 |
| Decision | Motor encoder only | Motor + output encoder |
|---|---|---|
| Measured position | Motor shaft; joint angle inferred through N | Motor shaft and joint angle measured separately |
| Gear error visibility | Downstream backlash/deflection not directly measured | Joint sensor exposes downstream position error; mechanics remain |
| Positioning accuracy | Unknown without a gear and sensor error budget | Unknown without sensor calibration and loaded closed-loop tests |
| Startup reference | Needs valid turn state or an unambiguous travel/reference constraint | Joint reading may establish pose within its measurement range; check offsets and turn coverage |
| Integration cost | Fewer sensors; budget for reference strategy and mechanical error | Extra sensor, mounting, interface, calibration and dual-loop tuning |
| Selection trigger | Consider if verified joint error meets the application budget | Consider if direct joint observability is required |
Controller example: Evidence & limits. Compatibility with a particular drive or motor module must be confirmed. This comparison makes no universal accuracy or price claim.
Need to choose a feedback architecture? Request a joint feedback review with your travel, accuracy target and power-off motion conditions.
For an ideal 9:1 gear, one motor turn moves the joint 40°. With no turn history and a full output revolution allowed, the same motor single-turn reading corresponds to nine possible joint positions, spaced 40° apart. Higher bit depth does not identify which turn occurred.
This is derived from N = 9 and a single-turn sensor, not a measured fault rate. Restricted joint travel plus a validated reference can remove the ambiguity; the gear ratio alone cannot.
| Configuration | Question before motion | Minimum next step |
|---|---|---|
| Single-turn motor encoder | Can two allowed joint poses share this motor reading? | Prove unique mapping over travel or establish a reference |
| Multi-turn motor encoder | Are turns retained and unpowered movements captured? | Test the documented retention method and power-loss cases |
| Output absolute encoder | Does its range cover joint travel and is zero calibrated? | Verify mounting, offset, wraparound and startup validity |
Evidence & limits. An absolute reading alone is not a robot safety function or proof that motion can start.
Illustrative engineering scenarios, not customer results or product promises.
The tool gives 0.00366211° per count, about 27 steps across the target. Quantization fits that target, but total accuracy is still unproven. Request loaded reversal error and encoder accuracy before accepting the joint.
The default step is 1.09863 arcsec. That fine increment does not resolve the nine possible startup poses over a full joint turn. Specify validated persistent turn tracking, output feedback, or a reference procedure.
At 9:1 the motor-referred step is 1.09863 arcsec; the independent output sensor step is 360/16,384 = 0.0219727°, or 79.1016 arcsec. The output sensor adds observability, even though its step is coarser. Choose it against the joint error budget and controller requirements.
| Risk | Consequence | Mitigation / alternative |
|---|---|---|
| Buying by bit depth alone | A fine number masks larger mechanical error | Allocate a joint error budget; request accuracy and repeatability tests |
| Unverified restart state | Joint pose may be wrong after a power cycle | Test retained turns or output reference before enabling motion |
| Dual-loop complexity | More commissioning time; poor tuning can cause oscillation | Verify drive support and test reversals with the intended load |
| Increasing ratio just for resolution | Changes motion, friction and reflected inertia tradeoffs | Select torque/speed and backdrive requirements first; compare 1:1 if feasible |
| Sensor integration cost | Added mounting, wiring, calibration and service effort | Request a complete integration quote and replacement procedure |
Low-ratio gearing is a QDD design approach, not proof of back-drivability: Evidence & limits. Tuning limitations: Evidence & limits. These are review actions, not certified operating procedures.
Sources reviewed . Manufacturer references describe principles and their own systems; they do not establish specifications for an unnamed QDD Motors product.
Separates measurement step, agreement with true position, and repeatability. These are different specifications.
Limit: Definitions apply to the calculation; this is not a QDD module accuracy test.
Single-turn readings repeat each revolution. Turn counting and its behavior across power loss depend on the encoder design.
Limit: Confirm retention and unpowered-motion behavior for the exact encoder. The 9:1 ambiguity example below is our kinematic derivation.
Documents motor-side commutation/velocity feedback and output-side position feedback, with the gear ratio explicitly configured.
Limit: This architecture reference does not specify a universal achieved positioning error.
Requires mounted and calibrated sensors and a correct gear ratio; strong nonlinearities limit a linear tuning model.
Limit: Validate reversal, settling and loaded motion on your assembly; feedback does not remove physical clearance.
Studies a high-torque-density motor with a low-ratio transmission, including measured backdrive behavior.
Limit: Research on one actuator supports the design approach, not a fixed QDD ratio range or performance guarantee for other joints.
| Known from the tool | Not established | Evidence to request |
|---|---|---|
| Ideal output quantization for your inputs | Actual positioning error and repeatability | Loaded joint test report with conditions |
| Motor turns per output turn | Startup pose and power-off tracking | Encoder datasheet and restart test |
| Single vs dual feedback roles | Interface compatibility, price and availability | Model-specific drawing, drive support and quote |
Only when the measured state uniquely identifies the required joint position, with valid calibration and any necessary turn information. Check the startup table above.
No. Enter bits within one motor revolution. Turn-counter bits increase the position range, not the angular resolution within a turn.
It can let the control loop observe and correct joint position error. Physical clearance remains; response under reversal and load must be tested.
No. Gear construction, friction, seals, bearings and motor behavior also matter. Ask for measured backdrive torque under stated conditions.
For an encoder mounted directly on the joint, use its per-turn bits with ratio 1. Do not multiply those bits by the upstream gear ratio.
Specify the exact sensor protocol and the drive’s supported inputs, timing and fault handling. A motor module’s network interface alone does not prove encoder compatibility.
Use the exact reduction from the assembly datasheet. Ask engineering to identify it before treating the calculated value as an input to selection.
Compare a gearless design when transmission error or backdrive behavior dominates. Verify torque, size, thermal capacity and controller requirements for the same duty cycle.
Send required joint accuracy, travel, torque/speed, power-off motion, encoder location, controller interface and quantity. Ask for model-specific feedback options, an output error report and a startup procedure.
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