Power management and hot steppers

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  • July 26, 2013 at 4:49 am #4350
    Roland
    Member

    Hello,

    I had some problems with missing steps that I noticed while trying to mill some pcbs. The more turns (parts) I had on the board, the more offset I had at the end. After a lot of thinking (didn’t help) and even more time surfing the web I found the hint to leave power on for the steppers. So I tried it out and am now getting all the precision I could wish for.
    Now my question: The steppers are getting pretty hot. I don’t have an exact temperature (really bad at guessing), but I can barely touch them. Still they are running fine and don’t stutter. In total they were running for about an hour.
    So one of my questions is: Do all steppers have thermal shutdown and what temperature is OK for the steppers?

    The other question: Is there a way to switch the steppers off e.g. while changing the work piece? Right now the only method I know is turning power management on for all steppers and moving them a short distance.

    As for my configuration: I have a TinyG v7 running the latest firmware from github. When configuring the axis according to the Wiki, I ended up setting the current to maximum.
    All my steppers are KH56QM2-851 from Servo Japan with 4.8V, 1.6A according to the datasheet.

    Thanks in advance,
    Roland

    July 26, 2013 at 9:16 am #4351
    alden
    Member

    Yes, the steppers do get hot, that’s normal. But it does have to be manageed, and it sounds like yours might be too hot.

    The maximum torque for a stepper is at standstill; and hence the maximum power draw. And heat. In addition to the motors heating up, the stepper chips also heat up. They will go into thermal shutdown if they are too hot, but they don’t know anything about motor heating, so there is no shutoff if the motors overheat.

    There are 2 things here: One, you might have your motor current set too high from the potentiometers. Try lowering it. You wat to find the balance point where the motion is powerful enough to maintain motion without skipping, but not over-powered to overheat the motors at standstill and low speeds.

    The second thing is not quite out yet, but watch for it. In the edge branch we have made some changes to the power management so that motor power can be shut off after a user-defined time if idle. This doesn’t change the heating when you are doing an hour-long run, but it makes sure the machine doesn;t stay powered up when idle.

    • This reply was modified 11 years, 3 months ago by alden.
    July 26, 2013 at 9:39 am #4353
    Roland
    Member

    Thank you for your suggestion, I will do some tests with lowered current.

    As a suggestion/feature request: How about a command to (de)activate all power management until the next command? This way I could just issue this command right after having finished milling one piece. As soon as I would issue the commands to continue with the next one, the usual settings would kick back in and reactivate it.
    My problem with the timeout solution is, that I have to change tools (graver, drills, …) while having to maintain the exact same position – 0.1mm difference could be too much already. I think this shouldn’t really be a problem with switched off steppers, but I also wouldn’t have thought I would have the problem while milling 😉

    July 26, 2013 at 7:05 pm #4354
    alden
    Member

    We are experimenting with power management that will do just that. The build currently in the edge branch has $me and $md commands which enable and disable the motors, respectively. Please note: edge is experimental, there may be bugs (In fact I’d be really surprised if there aren’t). But a lot of people have been banging on it, so it’s pretty stable.

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