Engineering · Defect
Backlash is the free travel in a mechanism — the distance the input moves before the output notices. Machine tools live with it by never reversing under load. A motion platform reverses under load hundreds of times a minute.
What it is
That distance is backlash. It exists wherever two parts transmit force by touching without being rigidly joined: gear teeth need clearance to turn at all, a belt needs slack to run, a ballscrew needs a fit between ball and race. Take the clearance away and the mechanism binds. Leave it in and you have a small region, at every change of direction, where the motor is moving and the load is not.
Why it matters here
This is the difference that decides whether backlash is a tolerable specification or a design-defining one. Backlash is a fixed distance, so its importance is set entirely by how often you cross it and how small the motions around it are.
Most of what a rig does under you is not the big heave of a kerb strike. It is the continuous, low-amplitude texture of a surface. Those movements can be smaller than the dead zone itself, which means they do not happen at all — they are absorbed.
Six legs, each changing direction independently, many times a second. Every crossing is another pass through the gap. What is a one-off error in a machine tool is a continuous noise floor in a simulator.
You do not perceive backlash as a position error. You perceive it as vagueness — a rig that feels approximately right rather than connected. That is the hardest kind of fault to diagnose from a spec sheet, because no line on the sheet is wrong.
What it costs you
Backlash and encoder resolution work against each other directly. A fine encoder tells the controller precisely where the motor shaft is. If there is a transmission after the shaft, none of that precision reaches the platform until the gap is crossed.
| Quantity | Value | Where it comes from |
|---|---|---|
| Encoder word length | 21-bit | Absolute magnetic encoder on every motor |
| Discrete positions per turn | 2,097,152 | 2 to the 21st power |
| Angular resolution | 0.0001716° | 360° divided by 2,097,152 |
| Drive loop | 8 kHz | The encoder is read every 125 µs cycle, not on a slower timer |
| Mechanism between shaft and leg | none | No gearbox, belt, ballscrew or reducer to add a gap |
We publish no backlash figure for the PDK: the architecture removes the parts that produce it, which can be checked by looking at the machine. A measured figure is published with its rig and method when the measurement is made.
Keep reading
The architectural answer to this page: what removing the transmission actually removes, and what it asks of the motor instead.
The sensor that makes fine control possible, and why absolute rather than incremental matters at power-on.
What the loop can and cannot see once the shaft has turned.