I have four MD01B controllers. Two of them work perfectly. Two of them begin to work at low power but as it increases they go into failure mode. Sometimes they will work to full power, but fail as I drop power down again. If they make it to full power they seem to run indefinitely.
The input voltage is 15.5 - 16.0 volts.
They are driving two brushed DC motors with a combined resistance of 5 ohms.
Lower voltages make no difference in making the two problematic MD01Bs work.
I watched the current sense line behavior and the ones that work are pretty much identical to the failing ones (until they fail when CS drops to 0)
I have tried driving a single motor with roughly the same resistance with the same results.
I have tried PWM frequencies varying from 7KHz to 20KHz which makes no difference in failure.
Again, two of them work perfectly, two don’t.
Have they simply failed? If so I’m a little concerned. Unfortunately I didn’t mark them so I don’t know whether ones I bought first failed or the more recently purchased ones.
Any help would be appreciated.
The only description of the problem I see is “go into failure mode”. How are the units failing? Are the drivers getting hot? What are the diagnostic lines doing? Do the units reset as expected when you toggle the inputs?
The drives don’t get hot at this point. I’m not sure if they got hot at some point before they started acting this way. They typically fail at approximately 80% power. When I reduce power to about 25% they begin working again, I don’t toggle the inputs, just reduce PWM duty cycle. When they go into failure, Diag A is low, Diag B is high, strangely, both motor outputs (a and b) measure high compared to ground (i.e. the motor input voltage) Motor controller voltage input is at 14.2 volts.
After checking quite a few things I found that there is a possibility that the motor controller input voltage could have been as much as 18.5 volts. In my original testing if the voltage went high they would protect themselves. If these two continued to operate at 18.5 volts, 2.5 volts over the target voltage, could that have caused the failure? The specifications for this driver indicate that they will protect themselves if too high a voltage is used. I know that the max PWM rate of 20 KHz was never exceeded. Typically I ran them at about 13-15 KHz.
The motor drivers have over-voltage protection that can kick in as low as 16V. That’s not a failure mode. If you’re already close to the threshold, the noise on the line might change with your use and cause the over-voltage shutoff to kick in at some duty cycles and not at others.
Shouldn’t the motor controller have shut down at those same duty cycles when it was new? Now they shut down with an input voltage of under 10 volts using a 15 KHz PWM when it reaches about 80% duty cycle.
I want to find out what caused the change (failure) so that I don’t do it again, these controllers did work perfectly through all the same power and frequencies they now stop at. I love this controller, it’s the only one I can find that has good current handling, current feedback, a small size, and a good price point!
I think that the over voltage cutoff didn’t kick in early enough when I had them connected at higher voltages and allowed the chip to self destruct but that is only a theory. Unfortunately I didn’t get any temperature info from the two that have problems. I was just wondering if anyone else had that problem.
From now on I’m going insure that the voltages can never get to the top range of the cutout (19 volts) but have a max of 16 volts. I will see how the ones I get from here on out hold up. (I need to remember to write a date on them this time )
Sorry, I didn’t realize the problem was happening at under 10V; that’s definitely not right. The chip shouldn’t be self-destructing at 19V. From what you’ve said so far, I don’t know what would have caused the problem. We’ve driven these quite hard, and I think the PWM frequency really shouldn’t be an issue. If you send them back, we can take a look at them and fix them or send you new ones (make sure you reference this topic and include all your contact info!)
I have been very impressed with the chips and your board design. I was surprised two had problems.
I wound up ordering a VNH2SP30 chip thinking I might take a try at replacing it on one of the boards. I should have known this would be a bad idea. Unfortunately I started cutting the leads on one side before I realized that the heat slugs underneath were soldered (of course) and there was no way I could replace them myself!
I’ll send them back, the one won’t give you much info though. I’m just concerned I did something to cause them to fail but can’t figure out what it is.
Well, it looks like it was my bone headed mistake on this one!!!
In my two test setups I forgot to put the noise suppressing caps on the motor! It seems the first MD01Bs are pretty immune to the noise, the later ones are a bit less and fell victim to my mistake. Hopefully this will help someone else not make the same mistake I did!
I’ve now decided to put the caps on the control board instead of relying on the motors getting them installed. They should be at the motors since that is the source but it is worse to not have them at all!
Thanks to Jan for all his help, I continue to be impressed every time I work with this board and the growing range of products Pololu offers.
I thought I would post a quick video showing what I have been building with the MD01B motor controller. After help from Pololu to diagnose my bone headed mistake I was able to film the following video.
Nice video. Your Cab Commander keeps getting cooler!