# Which signal shows that a physical action produced the expected result?
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description: The command sent, the actuator moved and the object in place are three facts that often get merged into one.

## Why it matters

A controller can confirm that it executed the movement without the object having moved at all. The record says everything went well and the environment says otherwise.

Without an independent signal, the system accumulates correct executions and absent results, and the difference only shows up when somebody looks.

## What we know so far

The check has to come from a channel other than the one that executed the action: another camera, another sensor, a weight, a later reading. Designing that signal before the movement changes the design of the whole quite a lot.

When there is no independent signal, we prefer to declare it as a limit rather than take the action as checked.

## What remains open

It is still open what to do when the checking signal is as noisy as the original perception.

And how much checking is proportionate when each additional verification costs cycle time.

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