SetpointGenerator4thOrder

SetpointGenerator4thOrder generates a motion profile bounded in four derivatives: velocity, acceleration, jerk and snap. Bounding the fourth means jerk itself ramps instead of stepping, which is what a machine with a lightly damped structure needs — a stepped jerk rings it, a ramped one does not.

It moves to a position, or holds a velocity as a jog, and switches between the two while running.

flowchart LR
    i1(["command/start"]) --> B["SetpointGenerator4thOrder"]
    i2(["command/abort"]) --> B
    i3(["command/type"]) --> B
    i4(["command/jogVelocity"]) --> B
    p1(["targetProfile/position, velocity, acceleration, jerk, snap"]) --> B
    B --> o1(["outputProfile/position, velocity, acceleration, jerk, snap"])
    B --> o2(["status/isDone, isAborted"])
    B --> o3(["status/isMovingToPosition, isMovingToVelocity, isAtConstantVelocity"])
    B --> o4(["status/currentSample, currentIndex"])
    B --> o5(["calc/sampleArray, snapArray, numberOfSamples, totalTime"])

The four numbers under targetProfile other than position are bounds, not targets. The generator uses as much of each as it needs and no more, so a short move may never reach any of them.

Two things about this block will catch you out. Read them before using it.

command/start must be held true for the whole move. It is not a pulse. Clearing it mid-move aborts the move as soon as the axis reaches its cruise speed.

command/abort only works in jog mode. To abort a position move, clear command/start. And an abort of either kind only takes effect during a constant-speed phase — requested during acceleration or braking, it waits. This is not an emergency stop.

Signals

Inputs

Outputs

Parameters

All five are persistent and survive a controller restart. Negative values are used as their magnitude. No parameters exist below this block.

Setup

1. Set the four bounds from what the machine can take. All four have non-zero defaults, so the block will move without being configured — check them before you start it.

2. Set command/type to 0 for a first move, so the distance is relative to wherever the axis already is.

3. Set targetProfile/position to a small distance.

4. Set command/start true and leave it true.

   Step 4 moves the machine, immediately and at the bounds you configured. Set them well below the machine’s capability first, and use a distance you can afford to overshoot.

5. Watch status/isMovingToPosition go true, then status/isDone go true when it arrives. The block clears command/start itself.

6. Read calc/totalTime and check it against the distance and the speed bound. A much larger number means a bound is far smaller than you meant.

7. Confirm outputProfile/position ends exactly at the target. It should be exact, not close.

Tuning

1. Set targetProfile/velocity and targetProfile/acceleration first — they are what the machine’s mechanics allow.
2. Set targetProfile/jerk to what the drivetrain tolerates. Lower means gentler load reversals.
3. Set targetProfile/snap last, and use it to trade smoothness against move time. It is the only bound the third-order generators do not have, and the reason to use this block at all.
4. Run a move and watch outputProfile/jerk. With snap bounded it ramps between levels; the flatter the ramps, the gentler the move.
5. If the machine rings at a known frequency, lower targetProfile/snap until the ringing stops. Every reduction lengthens the move.
6. Compare calc/totalTime before and after each change, so you can see what the smoothness cost.
7. Short moves take longer than the bounds imply. Every profile segment is rounded up to a whole cycle, which matters most when the move is only a few cycles long.

The gentler the curve leaves zero, the less the machine is excited — and the longer the move takes.

A starting point for an axis that may move at 0.5 units per second:

targetProfile/velocity     = 0.5
targetProfile/acceleration = 5.0
targetProfile/jerk         = 100.0
targetProfile/snap         = 1000.0
command/type               = 1

Limits and errors

The block logs an error, repeatedly, when a bound is 0 or the move command is not valid. Every other condition above shows as a value on a trace, or not at all.

Verified against motorcortex-control3 3.30.0 (bc348fd).