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.
Questions
Backlash is free travel: the distance an input moves before the output starts to follow. It appears wherever two parts pass force by touching rather than by being rigidly joined — gear teeth need clearance to turn at all, a belt needs slack to run, a ballscrew needs a fit between the ball and the race. Remove the clearance and the mechanism binds. Leave it in and there is a small region, at every reversal, where the motor is moving and the load is not yet.
There is no gearbox, belt, ballscrew or reducer between the motor shaft and the leg, so there is no transmission backlash by construction — the parts that produce it are not in the machine, and that is something you can check by looking at it. We publish no backlash figure, because we have not measured one; when it is measured it will be published with the rig and the method that produced it. The legs are not infinitely stiff either — platform mass and leg compliance store strain energy through each stroke — so this is a statement about transmission backlash, not a claim that nothing anywhere gives.
Hold the output still, move the input backwards, and measure how far it travels before the output follows. That distance is the backlash. On a gear pair it is done by locking the driven gear and reading the drive gear's free travel, or with a feeler gauge between the teeth, but the principle does not change with the mechanism — which is why it is a test you can run by hand rather than a figure you have to be given.
Because at every change of direction the drive has to cross its own clearance before it takes hold again. For that moment the input is moving and the output is not, so what reaches you is the crossing rather than the motion you asked for. The smaller and the more frequent the reversals, the larger a share of the movement that crossing becomes — which is why a mechanism can feel fine on a long sweep and coarse on short, quick ones.
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.
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