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DRV8880: Method for finding proper settings

Part Number: DRV8880

I have an product that I'm trying to find the "right" settings for the DRV8880. I have the DRV8880EVM and was testing different conditions with my motor (see attachments). I'm using a custom application to generate the control signals to the DRV8880 and the currrent code does not have any acceleration controls, just PWM frequency. The prior prototype was based on the DRV8825 and the last PWM setting yielded ~4kHz pulse rate (see attached scope plot).

I was using a different motor and slide for the DRV8825 prototype but by sending this step and varying the mode pins (using AutoTune) the motor just whines and does not move. Using the EVM and GUI connected to the new motor/slide arrangement I can get the same results but by adjusting the starting speed, stopping speed, and acceleration I can make the motor move but at this point I'm just making random uneducated guesses.

The motor doesn't have to move fast but with a 6" travel I don't want the operator to have to sit and twiddle their thumbs while the motor moves. The basic operation is that the motor moves a stage that exits a chamber. The operator then places a sample on the stage, pushes GO, and the stage retreats back inside the chamber where it will make a series of small moves (~15mm) back and forth for several hours.

I don't see any faults when the motor whines and I'm currently using a bench top power supply set at 24V and a current limit of 1.1A. For the most part the power supply sits at supplying ~350mA.

I'm hoping that someone can help me learn the best way to adjust and set the appropriate parameters as I was hoping that by switching to the DRV8880 and using AutoTune I wouldn't have to become a motor expert. We can add code to apply some acceleration parameters to adjust the PWM but of course if that's not needed it's preferred not to add additional work.

Thank in advance for the help and education!

3122.PK245-01AA,_StepperMotor.pdf

XSlide_owners_manual.pdf

10 Replies

  • Hi George,

    What starting, stopping, steps, acceleration, and steps to move settings have you tried with the EVM and GUI?

    How many turns of the lead screw are required to move 6 inches?

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Rick,

    I've been playing more with the GUI today in between watching some football and trying to not work so much! I should have saved the settings but I started using full steps and found that when using full steps everything seemed to be more sensitive to changes. I'll have to go back and see what I had but right now I'm experimenting with 1/4 step and I have the following settings:

    When I was at full step I was having trouble setting target speed more than ~4500.

    The lead screw is "2mm" but it doesn't appear to be trapezoidal like I thought it would be so it might just be a standard 2mm pitch screw? With the settings in this screen shot 40,000 steps moves the stage ~100mm

    I don't have a current probe for my scope (just too damn expensive but it would be nice to have one now) but I could check some things if needed.

    The current settings seem to be OK but I'm in a learning phase now and trying to determine how best to approach this and what kind of reasonable speed I can get out of this system. Right now I've just been poking around and changing settings on a trial and error basis.

    I forgot to mention that this system will have a very light load, under 0.5lbs.

  • In reply to George Ioakimedes:

    As a reference point I changed back to Full Step and even with these setting I'm not able to move the stage.

    I did have some combination of setting that allowed Full Step to work before but I foolishly didn't write them down or take a screen shot.

  • In reply to George Ioakimedes:

    Hi George,

    At full speed and 4500 PPS what problem did you see? Did the motor stop moving?

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Yes, the motor did not move and only whined at me!

    Here is a combination of settings that almost work using full step. My method for testing is that I get the motor moving and then find a number of steps that give me a fair amount of travel. I then adjust settings while in reciprocal mode until I start to hear noises or the motor doesn't move. With some additional experimenting I found the following combination of settings to yield good speed as well as low current (~200mA/24V). My tests are with "no load" (meaning it is the factory slide from Velmex with nothing added).

    Here is an additional 1/4 Step plot and I'll test more with Full Step later this morning.

  • In reply to George Ioakimedes:

    Hi George,

    Your method is sound. It is important to determine the starting/stopping speed, target speed, and acceleration rate.
    All of the values set typically scale based on the step mode selected.

    As you make the changes to the settings, toggle the ENABLE to re-calculate the acceleration ramps.

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Rick,

    OK, glad to hear I'm approaching this correctly although it is somewhat just guessing at settings. I'll work some more with Full Step today but I was surprised how "sensitive" things seemed to be with Full Step. I know the torque will be the highest and speed the slowest but in general it did seem more difficult to find setting that allowed smooth movement.

    In our initial code on our processor we did not make any settings than the PWM rate applied to the STEP pin. It seems as though that not the best approach but I'll have to see how hard it would be to rewrite code to mimic what this GUI is doing. I can see where these settings are more critical when we move our stage out of and back into the chamber (the travel will be ~4-6 inches) but once the sample is moved under our sensor we'll only have to move the stage ~10mm back and forth over and over again over a period of up to 4 hours. In this case I don't think any of the other setting will come into play since there isn't enough time to ramp up/down, right?

    Thanks,
    George
  • In reply to George Ioakimedes:

    Just wanted to post an initial Full Step settings experiment. With these settings the stage moves well but if I change the start speed to 2000 the stage stops moving. There's more experimenting to do but I'm still a bit unsure as to why the starting speed seems to matter so much.

  • In reply to George Ioakimedes:

    Hi George,

    A quick analogy is accelerating your vehicle on ice. You cannot simply release the brake and fully depress the accelerator; the tires will spin and you have no movement. If you release the brake, allow the vehicle to begin rolling while the engine is idling and slowly depress the accelerator, the vehicle can move and accelerate to the desired speed.

    Starting a stepper is similar. By starting at a slow speed, the rotor can follow the magnetic fields and accelerate. If you start at speed that is too fast, the rotor cannot follow the fields and stalls.

    Some motor manufacturers provide the maximum starting speed in their datasheet. If yours does not, you can contact them for this information. Even when provided, you should note what voltage is used to determine this speed.

    Rick Duncan
    Motor Applications Team

  • In reply to Rick Duncan:

    Hi Rick,

    Thanks for the analogy, that helps. The motor is this one but I don't see anything about speed. Looking at the GUI differences I see that the 1/4 Step is roughly 4x what I found with Full step so I guess things are making more sense now. I see that this motor has 11.2mH which seems like a lot and I imagine that this is greatest impact on the acceleration and speed settings. Looking back at the specs on the initial motors I was testing I see they only had 2.0mH and 4.5mH so if I'm right that would explain why things "seem" more difficult to dial in now.

    George

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