Building a DIY public domain low cost stepper motor analyzer



  • I am toying with the idea of having a public domain demo design of an inexpensive stepper motor analyzer so others can offer inexpensive kits or finished products, for example for 3D printers builders and tweakers.

    The idea is to sense the current of the stepper motor coils, performa simple analysis in realtime and display useful information and graphs on a small screen (e.g. 1.8" color TFT).

    This is what I have so far, a board with two +/-5A, AC/DC isolated current sensors (ACS70331) and a microcontroller (Teensy 3.2) that make sense of the signals and send the results to a computer via a serial port (the TFT screen is on order, should arrive in a few weeks).

    Currently it decodes accurately full steps but there is sufficient data to interpolate fractions of steps using the current ratios.

    Here is a picture of the prototype board (the two knobs are for testing only, not part of the final design)

    IMG-1671.JPG

    And here is an example graph of extruder steps while printing (0.9deg, 3:1 gear). The large retraction at the end is the end of print retraction.

    Annotation 2019-09-17 210546.png

    Should be easy to compute for example the actual max retraction, that is length of filament that was in the extuder and is now out, due to pressure advance, retraction etc. Other analysis can be for speed, acceleration, RMS current at high speeds and so on).



  • The analyzer, as primitive as it is now, already helped me diagnose a problem. I often had a general issue of under extrusion so the extruder path was clean and fully functional. Looking at the E steps counter on the analyzer while printing one of the problematic prints, I notices this:

    As you can see, the retraction was significantly longer than the unretraction which results general trend of the filament going OUT of the extruder.
    Annotation 2019-09-18 201652.png

    That also caused errors in the tracking of the stepper steps so it was easy to sync to that point of time with a scope and see this:

    SCR03.PNG

    The unretraction steps are too fast compare to what a that stepper with 24V supply can handle which results in reduced and distorted current patter through the coils.

    Looking at my config.g I found this line which sets the max extruder speed.

    M203 X15000 Y15000 Z3000 E15000

    And reducing it to E1500 solved the problem (the upper limit of reasonable stepper signal on my printer is about E2000). If my calculations are correct this is about 25mm/sec on my extruder with 0.9deg and 3:1 gear reduction.

    Hopefully we will have one day analyzers that will do all this process automatically or even Duet itself since the coil currents are available to it (not sure how though how much information the existing TMC drivers convey to the CPU).



  • @zapta Thanks a lot for your interesting analysis. Please go on like this, I'm curious to see what else comes!

    I try to minimize the vibrations of the hotend in my printer, so a vibration sensor coupled with your analysis could be interesting to find the best solution for configuration.



  • After further examination of my printer I realized that the insufficient current swing for fast extruder moves is probably caused by the very high inductance of the stepper I am using (~12mH) which prevents fast current changes through the coils.

    https://www.omc-stepperonline.com/nema-17-bipolar-09deg-36ncm-51ozin-09a-54v-42x42x40mm-4-wires-17hm15-0904s.html

    I placed an order for a E3D pancake extruder which has ~2mH inductance

    https://wiki.e3d-online.com/images/3/36/Slimline_Datasheet.pdf

    In retrospect I should have spent more time reading this https://duet3d.dozuki.com/Wiki/Choosing_and_connecting_stepper_motors instead of installing arbitrary steppers. I was lucky though with X/Y and Z steppers, they are good 😉



  • Had more progress with the analyzer's prototype. It now has a small display and shows information in real time. It shows current in both coils, number of quadrature state errors (e.g. if moving too fast for the stepper), the number of times the stepper was idle and the current step counter. Next I will try to show a graph on the display, of the step counter and/or of the coil currents. May also display aggregated values such as max current, RMS currents, speed, etc.

    The prototype board with the display:
    IMG-1696.JPG

    A close up of the display
    IMG-1698.JPG

    The data the analyzer sends in real time via the USB port is also plotted on the computer. This graph shows the extruder steps while printing Calicat. The big deeps are retractions. The ripples is the the pressure advance (set to 1.5).
    IMG-1699.JPG



  • Implemented the first analysis screen, still crude but shouldn't be difficult to clean. It shows the histogram of the stepper speed. vertical axis is speed from 100 steps/sec (bottom) to 2000 steps/sec (top). The length of the bars is the relative time the stepper was at that speed.

    This is the result of stepper A of a CoreXY printer while printing Calicat. The movements are relatively and the insufficient time to accelerate result in low overall speed:

    IMG-1707.JPG

    This video shows it updated in real time while using PanelDue to move a few times 10mm (slow) and then a few times 100mm (fast):

    https://www.dropbox.com/s/mw2pdjfm5h5n9b8/stepper_speed_histogram.MP4?dl=0

    The blue button resets the histogram data when you want to start from scratch.

    I believe that the Duet boards have sufficient information to provide such histograms. This could be another useful differentiator.

    Next I will try to implement a similar histogram that show the peak current in each speed. I believe that this will be able to diagnose the problem I had with a high inductance extruder stepper as the bars will be significantly shorter for the higher speeds.

    BTW, the microcontroller in the video is Teensy 4.0, a 600Mhz M7 beast that leaves a lot of room for future expansion, both in memory and computation.



  • Interesting.

    I am still battling on with 12V on my current 'in progress' machine. I noticed similar issues with the commonly touted 3600 instant speed and 3600 max speed. I could hear it on my machine. Things got very rattly on retracts and things sounded so much better (clean tones) after I dropped the instant speed to 1100 and the speed to 2100.

    If you paired up what you are building with something like the test on this you tube video you could have a very powerful tool.

    https://www.youtube.com/watch?v=GVs2d-TOims

    In a crude manned you could train a USB camera at the scales display and capture X number of frames around the point where the requested speed no longer matches the actual.

    If you are looking at the current or voltage plots I would imagine it wouldn't be a huge stretch to investigate if the electrical characteristics of the motor indicate when the motor is about to stall. That said the effect of the current choppers may make things too muddied to see.

    In relation to the retracts there could be an argument for two speeds on the retract. Unloading the filament could be really quick as the torque requirements are just about over coming the rotor and gear inertia (which in itself may indicate mass dampers would help avoid resonances) and then the true retract where you are looking to pull the filament back up away from the nozzle during which you have to overcome the viscosity of the material, but not pull a vacuum in the hotend which would encourage out gassing of the polymer.



  • @DocTrucker said in Building a DIY public domain low cost stepper motor analyzer:

    I noticed similar issues with the commonly touted 3600 instant speed and 3600 max speed. I could hear it on my machine. Things got very rattly on retracts and things sounded so much better (clean tones) after I dropped the instant speed to 1100 and the speed to 2100.

    This is how the coils current look in a too-fast scenario

    1568863755179-scr03.png

    There is not enough time for the current to build up in the coils, the sine wave forms are distorted, the peek current are lower, and the torque level is ver low if at all. Currently the analyzer detects it by noticing invalid stepper phase errors and count them as bad steps. The current histogram by step speed will allow to diagnose it better and predict the max useful stepper speed. At lease I hope so 😉



  • Just measured the extruder stepper (~26 full steps per mm) stepper speed histogram when printing Calicat with pressure advance of 1.0.

    Majority of the time the speed is in the 100-200 full steps per sec range, with very short time going up to ~1800 full steps per sec (during retraction).

    This suggests to me that the retraction speed has little affect on the overall print time.

    IMG-1709.JPG



  • Thank you for this thread! It really shows how a simple tool used with intelligence gives a lot of insight.



  • The firmware of the stepper analyzer proof of concept is done and the prototype has four functioning screens (see below) which demonstrate the viability of the hardware design. Next I will work on a PCB layout.

    Screen1: general information in text form (current in coils A.B, total steps, phase errors, etc)
    IMG-1726.JPG

    Screen2: histogram of time spent in each speed range (botton = slow, top = fast).
    IMG-1727.JPG

    Screen3: histogram of coil current vs stepper speed. Will indicate issues such insufficient power voltage or too high inductance, which cause lower currents in high speeds.
    IMG-1729.JPG

    Screen4: general signal waveforms of coils A (green) and B (red).
    IMG-1721.JPG


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