SmoothStop

SmoothStop brings a moving axis to a controlled standstill on command. When you raise gotoStandstill it takes over from the live motion reference, follows a jerk-limited braking profile down to zero velocity, and holds there. Release the command and it fades back to the live reference.

The stop is bounded three ways at once — by deceleration, by jerk, and by whichever of a time budget or a distance budget runs out first. If the natural profile would exceed a budget, the block computes a shorter one rather than letting the stop overrun.

flowchart LR
    i1(["u — live position reference"]) --> B["SmoothStop"]
    i2(["uDot — live velocity reference"]) --> B
    i3(["uDDot — live acceleration reference"]) --> B
    i4(["gotoStandstill"]) --> B
    i5(["resetStandstill"]) --> B
    B --> o1(["y — position output"])
    B --> o2(["ydot — velocity output"])
    B --> o3(["yddot — acceleration output"])
    B --> o4(["yss — braking profile position"])
    B --> o5(["yssDot — braking profile velocity"])
    B --> o6(["yssDDot — braking profile acceleration"])
    B --> o7(["smoothStopActive — per channel"])
    B --> o8(["isAtStandstill"])
    B --> o9(["standstillSwitchIsOn"])
    B --> o10(["standstillSwitchIsOff"])

Stopping distance from velocity $v$ with deceleration $a$ is roughly $v^2/2a$, and jerk rounds the corners at each end. The stop takes about $v/a + a/j$ seconds. maxStopJerkFactor is the fraction of the stop spent ramping the deceleration, from 0 (a pure trapezoid, sharpest) to 0.5 (a pure triangle, gentlest).

Both limits default to zero, which means “stop instantly”. With smoothStopAcc and smoothStopJerk at 0 the block jumps the output to where the axis was and holds it — a hard stop from a block called SmoothStop. Set both before you use it.

gotoStandstill is a level, not a pulse. Hold it true and the axis stays stopped; release it and the block fades back to the live reference.

Signals

Inputs

Outputs

Parameters

All six are persistent and survive a restart, and the first five are per channel.

One parameter lives below this block: the changeover’s fade time, in the standstillSwitch sub-tree. See soft-switch.md.

Setup

1. Set smoothStopAcc and smoothStopJerk first. Both default to zero, which makes the block stop the axis instantly. Start with the deceleration the machine uses for a normal decelerating move, and a jerk about ten times that.

2. Set maxStopTime and maxStopDistance from the machine’s real safety budget — how long and how far the axis may keep moving after a stop is commanded.

3. Set maxStopJerkFactor to 0.25 as a starting point. It only matters when a budget forces a shorter profile.

4. Link u, uDot and uDDot from the motion source, and y, ydot, yddot to whatever follows. Confirm the outputs track the inputs exactly with gotoStandstill false.

5. With the axis moving slowly, raise gotoStandstill. Watch yssDot come down to zero and isAtStandstill go true.

   Step 5 stops a moving machine. Do it at low speed first, with the axis clear, and confirm the distance travelled is inside your budget before trying it at full speed.

6. Measure the stop: time it, and measure how far the axis travelled. Compare both against maxStopTime and maxStopDistance.

7. Release gotoStandstill and confirm the outputs fade back to the live reference without a bump.

Tuning

1. Set the deceleration from the machine — what the mechanics, the drive and the load can take. This is a physical limit, not a preference.
2. Set the jerk from what the structure tolerates. Lower jerk gives rounder corners and a longer stop; higher jerk stops sooner and shakes more.
3. Check the resulting stopping distance at full speed: roughly $v^2/2a$ plus a little for the jerk ramps. If it exceeds the budget, either raise the deceleration or accept that the block will override your limits.
4. Watch yssDot on a trace during a stop. It should fall smoothly to zero with no overshoot and no residual crawl.
5. If a budget is forcing a recomputed profile, the actual deceleration and jerk used will be higher than what you configured — and the block does not publish them. Infer them from the trace.
6. Use maxStopJerkFactor to shape those forced stops: lower for the shortest stop that fits, higher for the gentlest.
7. Set the changeover fade time in the standstillSwitch sub-tree so releasing the stop is smooth. The engagement is instant by design — the profile starts exactly where the axis is, so there is nothing to fade.

Read the deceleration off the slope of the straight section and the jerk off how rounded the corners are.

A starting point for an axis that decelerates at 2 units/s² and must stop within half a second and a quarter of a unit:

smoothStopAcc     = 2.0
smoothStopJerk    = 20.0
maxStopTime       = 0.5
maxStopDistance   = 0.25
maxStopJerkFactor = 0.25
startAtStandstill = false

This is a starting point, not a final tuning. Work Setup step 6 with your own machine.

Limits and errors

The block raises no errors or warnings. It writes one debug log line per channel at the moment a stop is triggered, recording the position, velocity and acceleration it latched. Every failure above shows as a value on a trace, not as a message.

Verified against motorcortex-control3 3.30.0 (bc348fd).