Are acceleration and jerk applied to homing moves?
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Those are some good ideas, and I'm glad you've got them working. I for a brief moment considered thinking about some arrangement like that too, but for now I will just live with a slower homing on Z and a light clunk at the point the axis is halted abruptly. I hope this doesn't cause any damage to the balls in the ball nut. I know this can cause pitting and wear on the running surfaces so I wish to minimize it.
Also, I keep bringing up Jerk rate here but it seems it does not ever apply to moves from or to a standstill anyway. It seems like we could use some documentation outlining the intended behaviour of all the motion control system, because for those of us who are not coders, we have no way of knowing except by asking people who can parse the source code.
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@bot I'm sorry but we'll just have to agree to disagree.
An end stop is by definition an end stop - not some arbitrary position beyond which carriages can move by some further amount. Many people use contact probes such as Piezo sensors. I'm sure they will all be delighted if David changes the firmware so that axis movement carries on beyond the point where the Piezo triggers and wrecks the Piezo and/or damages the print surface when the nozzle crashes into it.
And what will your proposed change achieve? Simply allow you to home at a really fast speed. But if you home before a print and start the homing at a "normal" or even slow speed, then the entire sequence will be completed well within the time it takes to heat the bed and or the hot end. So you still have to wait for some period of time after homing and you will achieve nothing.
I have a very heavy 400mm x 400mm 10 mm thick bed and 750 mm travel in Z. I do a "fast" home at 600mm/min (10mm/sec) so the very worst case is that it takes 75 seconds to travel full distance. Then I back off 5mm and re-home at 180mm/sec. So it all happens well within the time it takes the hot end to reach temperature, let alone the bed.
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@deckingman the proposed changes would allow people to command their (usually) stepper motor driven axes without abruptly stopping them ON PURPOSE.
I wonder what industrial CNC machines do when homing. I have a strong sense that they do not abruptly stop unless the endstop is being used as an emergency stop.
We shouldn't even really use the term "endstop." It's a homing switch. Unless you specifically set it up as a trigger to perform an emergency stop, it ain't ever gonna stop your stuff from crashing.
We use stepper motors. They are torque limiting devices, but abruptly stopping them with all their force can damage ball screws. Having an OPTION, like a flag on the homing move command to allow deceleration is all that I'm asking for.
Your view is, IMO, opposite of what should be best practices. Best practices should be to ALWAYS respect the limits set in the config.g. IMO, the jerk rate is a limit. MAXIMUM INSTANTANEOUS SPEED CHANGE. That is a lie if we allow homing above the set "jerk" rate.
*not actually a robot
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@bot said in Are acceleration and jerk applied to homing moves?:
Why not force users to perform homing at the jerk rate? This is what the current scenario effectively does.
The difference is that when printing, the jerk rate must be low enough to avoid the risk of the motors skipping steps. Whereas during course homing, skipped steps don't matter.
If you are worried about damage, you can use the M913 command to reduce motor currents while homing, which will allow the motors to skip steps with less force.
I think you will find that all open source softwares for 3D printing and CNC control stop homing moves abruptly. Let me know if you find one that doesn't.
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@bot There is no need to start shouting which is what your use of capitals indicates. We can have this discussion in a dignified and civilised manner.
But taking the points you raise above. Firstly, if you take a look at the Duet board and all the documentation, you'll see that everything refers to "end stops" and not "homing switches". So "end stop" is indeed the correct terminology.
Secondly, I don't think anybody is talking about stopping stepper motors with all their force. I certainly didn't mention it. You do mention best practice and it is best practice to reduce the motor currents during homing.
Thirdly you mention that quote .."Best practices should be to ALWAYS respect the limits set in the config.g.". IMO that includes axis limits, yet what you propose would allow movement beyond the axes minima that are set in config.g.
And to put all this into perspective, taking a speed of say 60 mm/sec that's 216,000 mm/hour which is 216 metres/hour which is 0.216 km/hr. That's about 20 times slower than a brisk walking pace so even if you used full power on the motors, coming to a sudden stop is hardly likely to do any damage.
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I have a CR-10S with dual Z, setup as independent axis (Z & U), as well as a BL-Touch.
My homing routine for Z runs the bed down (towards home for me) until both Z endstop switches activate, then up a bit and final Z home with BL-Touch. This compensates for a very slight X carriage tilt that occurs when the right Z motor is released, an inherent problem in a lot of CR-10S's, and ensures a level X & bed.
I haven't implemented it yet (always other projects ... LOL) but I intend to setup an "Emergency Stop" if the Z switches ever actuate during BL-Touch homing, in case of sensor failure.
The BL-Touch's plastic probe is always exposed. On a failed print you can get plastic strings that can catch it and bend it (ask me how I know ...), which can cause the nozzle to smash into the glass if homed with a bend pin.
@bot Interesting thought,
and it's always cool to discuss these things, it's how the software advances, but I must disagree.Endstops are ENDstops, and should be respected. Compensating for Accell & Jerk during homing is an entertaining theoretical discussion but never gets off the ground in the real world. As @dc42 points out, every software out there does the same, including multiple million dollar machines.
The amount of force needed to damage the ball screws when stopping would require a very high rate of speed. Running at such speed past the endstops would cause a heck of a lot more damage than simply stopping the motor would create to the ball screw!
If this was a real problem, we would be seeing wear being reported by the hundreds of print farms that run dozens of machines 24/7, demanding a fix. We don't.
Endstop = Edge of World = Beyond here lie Monsters.
Endstop.
{Giggle} Love to know you're thinking about things, keep it up!!
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Larger CNC machines respect acceleration during homing moves, but homing typically involves moving past the home switch (sometimes by a considerable margin) then backing up to the nearest encoder index.
In larger CNC machines switches can never be crushed by over-travel, they activate and allow the axis to pass. Axis travel is typically limited by rubber bumpers. Here's a small linear stage with a representative arrangement:
As the machine gets heavier you need to think of endstops as more of a notification that something is wrong than an actual travel stop. Any fault should de-energize the axis drives, after which inertia will take over and allow the machine to coast (or smash) to a stop as physics dictates.
Homing is expected to overshoot the homing switch (which is not necessarily the limit switch). There is a very detailed overview of how homing and limit switches can be placed and configured starting page 88 here: https://www.parkermotion.com/manuals/Digiplan/ViX-IH_UG_7-03.pdf
That being said, this kind of setup is far more complicated than you want in a hobby 3d printer and rigid z-probes pretty much necessitate an immediate stop.
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@puterpro said in Are acceleration and jerk applied to homing moves?:
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The amount of force needed to damage the ball screws when stopping would require a very high rate of speed. Running at such speed past the endstops would cause a heck of a lot more damage than simply stopping the motor would create to the ball screw!
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This isn't exactly the case. The problem with ball screws is that there are lots of tiny, precise, hard balls running along precise, hard grooves. When they experience repeated excitation at high energy levels, they can develop pits in the raceways or on the balls themselves. Essentially, any repeated motion in the same part of the screw that is slamming it stopped at full force can cause wear. It's not an individual hit that will damage the screw, it's the constant hitting of the screw in the same way at the same point of travel that will cause damage.
Interesting discussion. It seems I'm not making headway, so I'll just accept the norm we adopted from ancient firmwares.
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@691175002 - Thanks! I'm CERTAINLY not a CNC expert, that's great info, Thx!
@bot - LOL! Sometime stopping hitting your head on a wall feels SSooooo good. LOL
I see what you're saying, but don't the balls have a tiny bit of movement during operation, or are they totally captive in position? One (well I) would think they'd shift a bit during long moves.
I'm pondering a linear rail setup on my CR-10S (well it started as a 10S ....).
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There may be some small amount of movement, but this is exactly what causes the problems. The same thing happens with helicopter rotor bearings (or any other extreme application of bearings). The blades slightly change pitch during their motion, and those bearings that allow that slight movement are exposed to lots of vibration and sometimes shock loading. This causes lines to develop on the inner race surface, where the needles of the bearing rested at the common positions of the blade pitch, that need to be taken out by hand when rebuilding the rotor.
Bearings, exposed to vibration = premature bearing wear. If those are shock loads, the risk is even greater.
BTW, my z stepper was not stalling when commanding the abrupt halt -- this is the problem. The stepper is able to slam the axis stopped and hold the position -- causing the most potential damage to the screw.
Yeah, yeah, it's my responsibility to control the speeds.
I thought I did -- it's called "Max instantaneous speed change." 
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@bot said in Are acceleration and jerk applied to homing moves?:
I thought I did -- it's called "Max instantaneous speed change."
Smartass. LOL
Good info, thx!
