Hi all,
I'm new to this forum, so if I'm posting in the wrong place my apologies, please point me in the right direction.
I'm in the process of building a repstrap, and since getting everything put together I've become baffled by the way the machine is behaving. I think there's something funny going on with my stepper motor drivers, but I don't know what (and I could be wrong about them even being the culprit).
When I got started on this project, I ordered one Pololu DRV8825 and used it to try out the motley collection of scavenged stepper motors I'm using for the machine. I saw that I didn't need much current to drive them and ordered DRV8824's for the other axes, keeping the 8825 for the y axis (which has the largest stepper motor).
When I got the machine together and started configuring the axes, I started with the y axis (the one driven by the DRV8825). I calculated number of steps/mm the axis should have (based on belt pitch, pulley teeth, motor's degrees/step, etc.), tried a test move, and everything looked right. When I commanded a 50 mm move, the axis really moved 50 mm (within the uncertainty of eyeballing it with a ruler). This, I believe, is the expected (and correct) behavior.
Then I did the same with the x axis, and the motion was way off -- the axis only moved about 55% of the commanded distance. I checked and adjusted everything I could think of (current limit, speed, set screws, that I had entered the correct steps/mm, etc.), but with no change. The axis moved pretty consistently about 55% as far as it should. (Note, the distance isn't exactly 50% off -- it's not a factor of two error as would come from e.g. getting the microstepping wrong.)
At this point I've removed the belts from the pulleys and adjusted the steps/mm setting so that commanding a 10 mm move should give me exactly one full rotation of the motor. What I see consistently is that, everything else being equal, when the stepper driver is a DRV8824 the motor turns only about 55-60% as far as when the DRV8825 is used. I get the same behavior with all of my motors (x, y, and z), using any of the slots in the RAMPS board (x, y, z, and extruder 2 which I have slaved to z), at high and low currents, at high and low speeds, and all four of my Pololu DRV8824 boards behave exactly the same. (So it's not one bad board. Could it be a bad batch?)
So ... what am I doing wrong?! I thought the 8824 and 8825 were supposed to behave exactly the same except for the current limit. I would just chalk it up as a mystery and manually fiddle with the steps/mm in the firmware until each axis was right, EXCEPT -- when I initially was trying to troubleshoot the x axis, a couple of times I caught it moving much further and faster than normal, almost as though it were suddenly going the correct speed. If that continues to happen it will likely make printing anything cleanly impossible. I suppose I could just buy more DRV8825's, but I don't like throwing money at the problem when I don't know what the problem really is. I've probably overlooked something basic but I can't guess what it would be. Anyone have any ideas?
(Some background: The repstrap's configuration is modeled on the Prusa i3: y axis moves the bed with a belt drive; x axis moves the printhead with a belt drive; z axis moves the x axis vertically with two motors turning threaded rods. I'm using a RAMPS 1.4 from Sainsmart, Pololu stepper motor drivers, and so far I've been using Repetier firmware and host software.)
I'm new to this forum, so if I'm posting in the wrong place my apologies, please point me in the right direction.
I'm in the process of building a repstrap, and since getting everything put together I've become baffled by the way the machine is behaving. I think there's something funny going on with my stepper motor drivers, but I don't know what (and I could be wrong about them even being the culprit).
When I got started on this project, I ordered one Pololu DRV8825 and used it to try out the motley collection of scavenged stepper motors I'm using for the machine. I saw that I didn't need much current to drive them and ordered DRV8824's for the other axes, keeping the 8825 for the y axis (which has the largest stepper motor).
When I got the machine together and started configuring the axes, I started with the y axis (the one driven by the DRV8825). I calculated number of steps/mm the axis should have (based on belt pitch, pulley teeth, motor's degrees/step, etc.), tried a test move, and everything looked right. When I commanded a 50 mm move, the axis really moved 50 mm (within the uncertainty of eyeballing it with a ruler). This, I believe, is the expected (and correct) behavior.
Then I did the same with the x axis, and the motion was way off -- the axis only moved about 55% of the commanded distance. I checked and adjusted everything I could think of (current limit, speed, set screws, that I had entered the correct steps/mm, etc.), but with no change. The axis moved pretty consistently about 55% as far as it should. (Note, the distance isn't exactly 50% off -- it's not a factor of two error as would come from e.g. getting the microstepping wrong.)
At this point I've removed the belts from the pulleys and adjusted the steps/mm setting so that commanding a 10 mm move should give me exactly one full rotation of the motor. What I see consistently is that, everything else being equal, when the stepper driver is a DRV8824 the motor turns only about 55-60% as far as when the DRV8825 is used. I get the same behavior with all of my motors (x, y, and z), using any of the slots in the RAMPS board (x, y, z, and extruder 2 which I have slaved to z), at high and low currents, at high and low speeds, and all four of my Pololu DRV8824 boards behave exactly the same. (So it's not one bad board. Could it be a bad batch?)
So ... what am I doing wrong?! I thought the 8824 and 8825 were supposed to behave exactly the same except for the current limit. I would just chalk it up as a mystery and manually fiddle with the steps/mm in the firmware until each axis was right, EXCEPT -- when I initially was trying to troubleshoot the x axis, a couple of times I caught it moving much further and faster than normal, almost as though it were suddenly going the correct speed. If that continues to happen it will likely make printing anything cleanly impossible. I suppose I could just buy more DRV8825's, but I don't like throwing money at the problem when I don't know what the problem really is. I've probably overlooked something basic but I can't guess what it would be. Anyone have any ideas?
(Some background: The repstrap's configuration is modeled on the Prusa i3: y axis moves the bed with a belt drive; x axis moves the printhead with a belt drive; z axis moves the x axis vertically with two motors turning threaded rods. I'm using a RAMPS 1.4 from Sainsmart, Pololu stepper motor drivers, and so far I've been using Repetier firmware and host software.)