LowPass1

LowPass1 smooths a noisy signal before a controller consumes it. One cut-off frequency governs every channel, and the channel count is fixed by the machine configuration. It also publishes the rate of change of its own smoothed output, so you never have to differentiate output yourself.

flowchart LR
    i1(["input — signal to smooth"]) --> B["LowPass1"]
    i2(["disable"]) --> B
    B --> o1(["output — smoothed signal"])
    B --> o2(["outputDot — rate of the smoothed signal"])
    B --> o3(["isEnabled"])

$f_c = \omega/2\pi$ — omega 6.283 rad/s is a 1 Hz cut-off. Time constant $\tau = 1/\omega$ [s]: 63% of a step in $\tau$, 90% in $2.3,\tau$, 99% in $4.6,\tau$. At the cut-off: gain 0.71, lag 45°, which is a delay of $0.125/f_c$ [s]. Below the cut-off the delay is $\tau$. That delay is the whole trade of this block.

Signals

Inputs

Outputs

Parameters

omega and enable are persistent — both survive a controller restart. The inputs and outputs do not; input and disable return to their linked sources and output starts from zero. No parameters exist below this block.

Setup

1. Link the source signal: …/actuator/actualVelocity → …/velocityFilter/input. input follows the source on a trace.

2. Set enable false. output equals input sample for sample and isEnabled reads false.

3. Set omega to 300. Read it back; a lower value means your task rate capped it, and tells you the highest cut-off this task rate allows.

4. Set enable true. isEnabled reads true and output still tracks input closely, with the noise still visible.

5. Halve omega in steps, watching the noise and the loop consuming it. Stop one step above where that loop feels soft or hunts.

   Step 5 changes what the controller sees. Lower the cut-off with the axis at rest before trying it in motion — the added delay can destabilise a tightly tuned loop.

6. Trace every element of input and output. An element that reads zero while the machine moves is an unwired channel, not a filtered one.

Tuning

1. Read the frequency of the ripple you want gone, in Hz, off a trace of input. Set omega to $2\pi$ times that frequency as a starting point, then lower it until the ripple is gone.
2. Measure the delay you have bought. Step the source and read the time output takes to cross 63% of the step; that time is $\tau$, and the delay below the cut-off equals it.
3. Keep that delay below a tenth of the response time of the loop consuming output. Above that, the loop pays for the smoothing in stability.
4. Check the loop in motion, not at rest. Delay costs phase margin only while the axis is moving fast enough to need it.
5. Re-check omega after any task-rate change. The upper bound scales with the task period, so a slower task can clamp a cut-off that used to be accepted.
6. If you need the noise gone and the delay, fix the noise at its source — shielding, grounding, or a higher-resolution sensor. No cut-off gives you both.

Read the time constant off any curve as the moment it crosses 0.63.

A conservative starting point for a velocity signal on a 1 ms task, feeding a position loop with a 100 ms response time:

omega  = 60.0
enable = true

This is a starting point, not a final tuning. Work step 1 with a trace of your own signal.

Limits and errors

The block raises no errors or warnings and logs nothing. Every failure above shows as a value on a trace, not as a message.

Verified against motorcortex-control3 3.30.0 (bc348fd).