Brake vs. shorting stepper coils
Posted by Schild0r
|
Brake vs. shorting stepper coils October 21, 2020 03:33AM |
So I am building a new printer with triple z motors and SFU1204 ballscrews for independent movement and true bed leveling.
I recently noticed that my build plate may be a little bit heavy and fall when the steppers are powered off and I am now considering to implement some sort of braking mechanism, that brakes z movement once the 24V main power is cut (I still have 5V auxiliary power wired to the board for standby).
As I see it there are two possibilities for that.
1: Buy stepper motors like this one
With an integrated brake and connect the brake wires to the 24V PSU so the brake engages when the PSU is turned off.
2: Someone told me that Lulzbot had an approach where they had designed a breakout board which connects to power and the steppers and when it senses a power drop it will cut the connections from the steppers to the drivers and will instead connect/short each stepper coil which results in greater rotational resistance preventing the bed from falling.
Well since my motors are independently controlled (whereas Lulzbot had them connected in series on a single driver) I think I could get away with one of these relays
With 4x change over contacts per motor and connect it the following way:
EDIT: I did an oopsie when sketching things up. The relay needs to be rotated 180° so that the motor coils are at the common contact of course. I don't mean to short the driver /EDIT
Each of the four wires from the motor run to one of the change over contacts.
In engaged state (24V on the relay) the stepper coils will be connected to the driver as usual.
In disengaged state (0V on the relay/ PSU off/ see picture for reference) each change over contact disconnects the stepper wire from the driver and instead the two coils of the stepper are shorted by a bridge (orange in the picture) which leads to higher rotational resistance -> braking moment -> the bed won't fall.
First of all: is there any misconception or did I understand anything wrong so that these approaches would not work in the way I described it?
If both approaches do work, which one would you go for and why?
----------
Lemme start:
The "shorting coils" approach is cheaper at 10 bucks per motor whereas a new 0.9 NEMA17 with integrated brake is rare and 50 bucks a piece
Edited 2 time(s). Last edit at 10/21/2020 03:37PM by Schild0r.
I recently noticed that my build plate may be a little bit heavy and fall when the steppers are powered off and I am now considering to implement some sort of braking mechanism, that brakes z movement once the 24V main power is cut (I still have 5V auxiliary power wired to the board for standby).
As I see it there are two possibilities for that.
1: Buy stepper motors like this one
With an integrated brake and connect the brake wires to the 24V PSU so the brake engages when the PSU is turned off.
2: Someone told me that Lulzbot had an approach where they had designed a breakout board which connects to power and the steppers and when it senses a power drop it will cut the connections from the steppers to the drivers and will instead connect/short each stepper coil which results in greater rotational resistance preventing the bed from falling.
Well since my motors are independently controlled (whereas Lulzbot had them connected in series on a single driver) I think I could get away with one of these relays
With 4x change over contacts per motor and connect it the following way:
EDIT: I did an oopsie when sketching things up. The relay needs to be rotated 180° so that the motor coils are at the common contact of course. I don't mean to short the driver /EDIT
Each of the four wires from the motor run to one of the change over contacts.
In engaged state (24V on the relay) the stepper coils will be connected to the driver as usual.
In disengaged state (0V on the relay/ PSU off/ see picture for reference) each change over contact disconnects the stepper wire from the driver and instead the two coils of the stepper are shorted by a bridge (orange in the picture) which leads to higher rotational resistance -> braking moment -> the bed won't fall.
First of all: is there any misconception or did I understand anything wrong so that these approaches would not work in the way I described it?
If both approaches do work, which one would you go for and why?
----------
Lemme start:
The "shorting coils" approach is cheaper at 10 bucks per motor whereas a new 0.9 NEMA17 with integrated brake is rare and 50 bucks a piece
Edited 2 time(s). Last edit at 10/21/2020 03:37PM by Schild0r.
|
Re: Brake vs. shorting stepper coils October 21, 2020 10:29AM |
While you're solving chicken and egg dilemmas, here's another one to think about:
Why does shorting the coils brake the motor? When the bed drops the motor spins and generates EMF through the shorted coils that create a magnetic field that bucks the rotation of the motor and the dropping of the bed. But, if the motor isn't spinning, there's no current being generated and no magnetic field, so no braking. In order for the braking to work at all, the motor must spin, so the bed must drop. At best, shorting the coils will slow the drop- i.e. it adds friction. There are other ways to add friction if you want. Otherwise, it would be much easier and more reliable to simply fit the bottom of the bed or Z axis with springs so it won't do any damage when the bed hits the bottom of the Z axis, assuming it will, which isn't necessarily a good assumption.
Ball screws are usually low friction, relatively long lead devices. That almost guarantees the bed will drop when Z motor power is cut. If you use short-lead lead screws (1 or 2 mm, for example), the bed may not move at all when Z motor power is cut and Z axis resolution will increase (as maximum speed decreases). Using ball screws almost guarantees the bed will drop, but will it drop equally at each screw? Unlikely. Differences in friction at each screw will probably cause the bed to tilt as it drops, maybe to the point that it will bind and stop the movement at some odd bed surface angle, long before it hits the bottom of the Z axis. When you next power up the Z axis, the motors will jump (which is the reason for using autoleveling). Will they jump in a way that releases the binding or enhances it? Will your autoleveling be working against the bed being wedged tightly, every time? Maybe you can apply EM braking to one of the motors to ensure higher friction at that screw and force the bed to tilt so it binds and never hits bottom...
Some people think they want to do autoleveling, so they use multiple motors to drive Z axis screws. But it's actually the opposite- they have to do autoleveling because they used multiple motors. The screws get out of sync when the machine is powered up because there are multiple motors attached to them, and each motor jumps a little on power up. If the goal is to make youtube videos, autoleveling is OK, but it was more impressive a few years ago when the first one did it. Now there are lots of those videos and it's sort of "meh". If the goal is to make a printer that "just works" connecting all the screws to a single motor will keep them from ever getting out of sync so no autoleveling will be needed. You'll level the bed manually, one time, and never have to do it again unless you take the Z axis apart. If you have the funds to buy the extra motors, bed sensor, and controller expansion board, you could, instead, buy a flat bed plate and do without all the other junk and end up with a more reliable printer, probably for lower cost, definitely much simpler set-up, operation, and maintenance.
My corexy printer lifts the bed assembly with two belts driven by a single motor. The belts never get out of sync and the flat, tooling plate bed never tilts. The machine "just works". The single, Z axis motor is driven directly by one of the drivers on the Duet board. It has a 30:1 worm gear reducer that prevents bed movement when the Z axis is powered down. I routinely transport the machine laying on its back (Z axis is 695mm) in my car or in a truck and when I get where I'm going, take the machine out, stand it back up and start printing without any adjustments. It uses an old-school Z=0 switch (well, OK< it's a little more than that) mounted on the frame instead of a sensor on the extruder carriage, so doesn't require a moving cable.
Put a flat plate on a kinematic mount and lift it with a single motor. It's not hard to do...
This is the Z axis in my printer. The red box near the motor is the 30:1 worm gear reducer. A single long shaft drives the pulleys that drive the belts. It no longer uses steel core belts, and I have added a heat shield to protect the Z axis motor from the heat blowing off the chamber heater toward the back of the photo. You can see the two Z axis linear guides right next to the belts.
This is one of the two Z axis lifters:
And this is what they lift (bed plate removed):
The bed plate sits on the three screws with springs holding it down. The two ball-head screws sit in a chamfered hole and slot in the bed plate that allow the plate to expand along the X axis when heated without making anything flex or lifting or dropping the bed plate. The third screw just touches the smooth underside of the bed plate and allows expansion in X and Y.:
You can see most of the bed in operation here: [vimeo.com]
Edited 4 time(s). Last edit at 10/21/2020 11:07AM by the_digital_dentist.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Why does shorting the coils brake the motor? When the bed drops the motor spins and generates EMF through the shorted coils that create a magnetic field that bucks the rotation of the motor and the dropping of the bed. But, if the motor isn't spinning, there's no current being generated and no magnetic field, so no braking. In order for the braking to work at all, the motor must spin, so the bed must drop. At best, shorting the coils will slow the drop- i.e. it adds friction. There are other ways to add friction if you want. Otherwise, it would be much easier and more reliable to simply fit the bottom of the bed or Z axis with springs so it won't do any damage when the bed hits the bottom of the Z axis, assuming it will, which isn't necessarily a good assumption.
Ball screws are usually low friction, relatively long lead devices. That almost guarantees the bed will drop when Z motor power is cut. If you use short-lead lead screws (1 or 2 mm, for example), the bed may not move at all when Z motor power is cut and Z axis resolution will increase (as maximum speed decreases). Using ball screws almost guarantees the bed will drop, but will it drop equally at each screw? Unlikely. Differences in friction at each screw will probably cause the bed to tilt as it drops, maybe to the point that it will bind and stop the movement at some odd bed surface angle, long before it hits the bottom of the Z axis. When you next power up the Z axis, the motors will jump (which is the reason for using autoleveling). Will they jump in a way that releases the binding or enhances it? Will your autoleveling be working against the bed being wedged tightly, every time? Maybe you can apply EM braking to one of the motors to ensure higher friction at that screw and force the bed to tilt so it binds and never hits bottom...
Some people think they want to do autoleveling, so they use multiple motors to drive Z axis screws. But it's actually the opposite- they have to do autoleveling because they used multiple motors. The screws get out of sync when the machine is powered up because there are multiple motors attached to them, and each motor jumps a little on power up. If the goal is to make youtube videos, autoleveling is OK, but it was more impressive a few years ago when the first one did it. Now there are lots of those videos and it's sort of "meh". If the goal is to make a printer that "just works" connecting all the screws to a single motor will keep them from ever getting out of sync so no autoleveling will be needed. You'll level the bed manually, one time, and never have to do it again unless you take the Z axis apart. If you have the funds to buy the extra motors, bed sensor, and controller expansion board, you could, instead, buy a flat bed plate and do without all the other junk and end up with a more reliable printer, probably for lower cost, definitely much simpler set-up, operation, and maintenance.
My corexy printer lifts the bed assembly with two belts driven by a single motor. The belts never get out of sync and the flat, tooling plate bed never tilts. The machine "just works". The single, Z axis motor is driven directly by one of the drivers on the Duet board. It has a 30:1 worm gear reducer that prevents bed movement when the Z axis is powered down. I routinely transport the machine laying on its back (Z axis is 695mm) in my car or in a truck and when I get where I'm going, take the machine out, stand it back up and start printing without any adjustments. It uses an old-school Z=0 switch (well, OK< it's a little more than that) mounted on the frame instead of a sensor on the extruder carriage, so doesn't require a moving cable.
Put a flat plate on a kinematic mount and lift it with a single motor. It's not hard to do...
This is the Z axis in my printer. The red box near the motor is the 30:1 worm gear reducer. A single long shaft drives the pulleys that drive the belts. It no longer uses steel core belts, and I have added a heat shield to protect the Z axis motor from the heat blowing off the chamber heater toward the back of the photo. You can see the two Z axis linear guides right next to the belts.
This is one of the two Z axis lifters:
And this is what they lift (bed plate removed):
The bed plate sits on the three screws with springs holding it down. The two ball-head screws sit in a chamfered hole and slot in the bed plate that allow the plate to expand along the X axis when heated without making anything flex or lifting or dropping the bed plate. The third screw just touches the smooth underside of the bed plate and allows expansion in X and Y.:
You can see most of the bed in operation here: [vimeo.com]
Edited 4 time(s). Last edit at 10/21/2020 11:07AM by the_digital_dentist.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
|
Re: Brake vs. shorting stepper coils October 21, 2020 10:36AM |
Quote
the_digital_dentist
While you're solving chicken and egg dilemmas, here's another one to think about:
Great writeup, food for thought
http://www.marinusdebeer.nl/
|
Re: Brake vs. shorting stepper coils October 22, 2020 09:21AM |
Quote
Schild0r
So I could indeed use an allen wrench to reach through the trough hole in the build plate to the hex socket of the maggot screw to adjust the levelness of the build plate and retighten the counter nut. This would be tedious compared to a leveling knob but hopefully I would not have to do that often.
I threaded the leveling screws into pilot holes in PTFE blocks so there's no need for counter nuts to lock them in place. The trick is to not tap the holes that the screws are going into, and to use a material that doesn't mind the heat transferred from the bed plate via the leveling screws. PTFE is just about perfect for this. The metal screws roll their own threads into the plastic and are gripped tightly but still easily adjustable. I tried this once using nylon instead of PTFE and quickly discovered why they use nylon in nylock nuts. I could no longer turn the screws once they got about 5 mm into the plastic. There are about 13 mm of threads in the PTFE and it is still pretty easy to turn the screws.
Edited 2 time(s). Last edit at 10/22/2020 09:25AM by the_digital_dentist.
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Sorry, only registered users may post in this forum.