Suggestion for improvement to SOFT LIMIT feature

math_indy

On a stepper CNC system like DDCSV2.1 it is always good to avoid "emergency stops" as they do not decelerate smoothly and risk losing HOME position calibration.  


As such I admire the SOFT LIMIT feature very much but I think it could be improved:

I have "Soft-limit stop mode" parameter #2030 set to 0 (deceleration) and am running V2018-10-12-102.


Currently when a G01 code orders an axis to travel way-beyond the soft limit it appears that there is no deceleration phase or, perhaps, the deceleration is agressive or perhaps something different from what I have programmed into my axis-specific deceleration parameters #263-#266.

When operating in CONT mode I notice that the deceleration works but DDCSV2.1 only begins decelerating when the soft limit is exceeded.  Because of this I have to put my soft limits more than 25mm away from the hard limit switch point.  With this large 25mm buffer I can approach the limit at full speed using CONT mode and it will decelerate almost exactly to, but slightly before, the hard limit.  This operation is wasteful of table space as if forces me to give up an inch or more on each side of my table.  


Since, for every axis, the DDSCV2.1 "knows" both the "current speed" and "allowed deceleration" I suggest that the soft limit feature always "look ahead" to, regardless of axis speed and operating mode, begin its deceleration in advance so that any axis that is trying to move beyond its soft limits will reliably decelerate, trip the alarm, and arrive exactly at the soft limit position.  


The exact distance required to stop is, at all times, known by DDCSV2.1:
Stop Distance[mm] = 0.5 * (Speed[mm/min]/60)^2 / (deceleration[mm/s^2])


In G-Code operation this would be easy to implement as any command that orders an axis to go beyond its soft-limit could instead just-order that axis smoothly to the soft limit and then trigger soft limit alarm.

In MPG and CONT modes the DDCSV2.1 would have to always look at the "current axis speed" and "allowed deceleration" and intelligently begin deceleration so that both MPG and CONT commands, no matter what axis speed, also arrive exactly, and gracefully, at the soft limit (then trigger the alarm).


If implemented properly this would allow DDCSV2.1 users to put their soft limits very close to the hard limits--like within 1mm or 2mm of the hard limit.

TOM

math_indy

I tested the soft limit changes in version V2018-10-12-102-1 and I like what has been done--a big improvement for STEP, MPG, and CONT modes and I was able to safely move my soft limits much closer to the hard limits.

It is OK with me if soft limit behavior in automatic (G-CODE) operation is updated later.  I am unlikely to write a program that would command to a location outside of the soft limits anyway.  However, in the G-CODE case, it would be OK with me if the offending G-CODE triggered an immediate soft limit error without ever moving the axis at all.  This or maybe go to the soft-limit, stop there, and trigger the alarm--either behavior would be good.