Belt driven Z =) (dropping problems ofc.)



  • Hi,

    I just converted my bed design to use 3 seperate steppers with 3:1 gear reduction and belt drive 😃
    It allows me to get 200 steps/mm on Z and I must say the results are great compared to my old design with 3 leadscrews.

    But as I knew from the beginning, the bed falls like a stone when the power is cut off, so far I've manage this by simply not turning of the steppers when the print is completed and just removed the bed (sits on 3 magnets).

    Now to my question, would it be possible to let the Duet controll 3 NO relays to open them when it's time to print?
    And then as soon as power goes of they will short again?



  • I'm testing a belt drive on my Ormerod and joined in a discussion on RepRap suggesting stepper brakes but another raised the idea of a worm drive from the stepper to the drive pulley. So long as you control the axial motion of the worm well they normally self brake.



  • Stepper brake controls the hazardous situation whereas a suitably selected worm drive eliminates the risk entirely (up to the point of structural failure).



  • @doctrucker I've thought about a worm drive but it would not be manageable when I have 3 seperate steppers for the whole Z axis. I use them as bed leveling as well.



  • Certainly more costly yes.

    I wonder if all the stepper enable pins are fired at the same time? If so I'd consider getting and optical isolator driving your relay from on a spare external drive?

    If that were possible it would be a no cide solution as it'll click in as soon as the steppers are disabled. I'll have a nose at the schematics while I have my brew!



  • All stepper drivers are enabled from one signal from the processor ENN, that does go to the Duex5, which I assume you're running.

    Duet 2 1.04a https://github.com/T3P3/Duet/blob/master/Duet2/Duet2v1.04/DuetWifiv1.04a_Schematic.pdf

    This is pulled up to 3.3V and the processor pulls it to ground. Just looking to see where this would be available on the Duex5.



  • The test points seem to have the chip select line (used for SPI comms I thought...) on them rather than the stepper enable.

    https://github.com/T3P3/Duet/blob/master/Duet_Expansion/Duex5v0.8/Duex5v0.8_Schematic.pdf

    I can't see anywhere where ENN is available to hook into on the board other than the expansion header that you're already using.

    Other than using general IO fired by gcode I'll step back!



  • @doctrucker Hi!

    I use the Duex2 for now but will get a Duex5 soon for my IDEX setup.
    If I could get a 3.3V signal when the Duet bord enables the steppers that would be great!
    Then I could simply short the phases of each stepper when no power is delivered.



  • @doctrucker Ok what if I do it like this then:

    Use 3 NC relays for each of the 3 Z stepper, short 1A and 2A.
    Define a new tool that uses either a fan output or a heater output that will activate the relays and un-short the steppers.
    In config.g, as soon as the Duet fires upp I will activate the relays and at the same time energize theese 3 steppers.

    Would this work?



  • @haggan90 said in Belt driven Z 😃 (dropping problems ofc.):

    @doctrucker Ok what if I do it like this then:

    Use 3 NC relays for each of the 3 Z stepper, short 1A and 2A.
    Define a new tool that uses either a fan output or a heater output that will activate the relays and un-short the steppers.
    In config.g, as soon as the Duet fires upp I will activate the relays and at the same time energize theese 3 steppers.

    Would this work?

    @dc42 Do you think this would work?
    Could I activate the relay and at the same time energize the steppers as soon as the Duet fires up? 😃



  • You'll have to wait for them to comment on that because the usual advise is not to plug or unplug the steppers when the board is on. Technically you're pretty much describing unplugging and plugging a motor in electro-mechanically and so they may advise against it on those grounds.



  • @doctrucker Hmm.. but the steppers are still allways connected to the Duet board?
    I will only "un-short" the phases of each motor upon booting.



  • I'm going to guess they're not going to like that because any delay in unshorting could leave the drives shorted. Wait for someone from Duet3D to confirm.



  • I use a belt lifted Z axis in my printer. It has a 30:1 worm gear drive that lifts the bed with 2 belts. The bed is a piece of 8mm cast tooling plate and that's uses a kinematic mount/leveling system. There's no auto leveling because it is not needed. The system is extremely stable. Once the bed is set level (using RRF's manual bed leveling assistant), it stays that way.

    The worm gear prevents any movement of the bed unless driven by the motor, which means that resuming prints in the event of power failure, etc., should be easy (I've never actually tried it).

    alt text

    The bed support made of 4040 t-slot and PTFE leveling screw blocks:
    alt text

    The bed plate with heater and TCO mounted with high temperature silicone:
    alt text



  • @mrehorstdmd I've read your project on Hackaday many times, and it's the main reason why I choose to switch 😃
    My printerdesign don't really allow me to use your exact setup, and I really like the auto bed leveling function.
    Also, I'm not so keen on buying 3 pcs of that reduction gear 😛

    I also have a tooling plate, but it's 525x525mm large.
    I lock it in place using 3 round nuts and imbeded magnets, works really well.



  • @haggan90 A multimotor set-up needs active leveling (or frequent, tedious, manual leveling) because power cycling throws the motors out of sync causing the bed to tilt. With a single motor, that doesn't happen. A single worm drive will probably provide all lifting/holding power needed unless your bed system is extremely heavy. For your large bed, lifting and guiding along a third rail may be better that two. It would be easy and inexpensive to add a third drive pulley and belt to the output shaft of the worm drive.



  • Reading what I think Lulzbot has done, the big TAZ uses steppers with planetary gears at a ratio high enough that the bed doesn't drop.

    The mini Taz I believe uses a relay board that either passes drive or shorts across the windings to brake. The relay is controlled by a power fail circuit that is only activated at power loss.

    All this should be available at their site in the development area. I browse it with FTP.



  • Shorting the windings doesn't act as a brake. The motor has to be spinning to generate the voltage that would cause braking, so all it can do is slow the drop. If the fall is long enough, once the rpm gets high enough, it acts like the short isn't there any more and the thing drops fast and hard. I know this from testing on my machine. I looked at a lot of ways to stop the drop before I settled on the worm gear drive.

    Multimotor autoleveling is interesting technically and makes for great youtube videos, but like a certain politician, it merely solves a problem of its own creation. If you didn't have multiple motors, you wouldn't have to keep leveling the bed over and over again...


  • administrators

    You can get at the ENN signal on the expansion bus by using a 50-way ribbon cable with e IDC connectors fitted to it instead of just 2. This is easy to make yourself if you have a vice (or a vise if you are American). Then you can use ENN being low to deactivate the brake.



  • @dc42 I have several vices...


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