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DRV3946-Q1: Valve showing open/short error after some period of time...

Richard Dorfner
Prodigy 50 points
Part Number: DRV3946-Q1

Tool/software:

Ibinu,

It's been a while and the drv3946 is working very well for us.

However! (There's always a however, isn't there? :) We are seeing some strange behavior that we are at a loss to understand.

The behavior is a bit of a story, so please bear with me on this.

We are running valves that operate at 24 volts. What we noticed is that when the valves are placed close to each other, and one of the valves is turned on or off, the adjacent valves respond with an "Open/short" error. THis turned out to be due to the EMI field generated by the coil being controlled would generate a spike on the adjacent coils due to the growing/collapsing magnetic field of the operated coil causing the adjacent coils to generate a voltage that the drv3946 interpretted as an open or short condition and would then disable the valves.  The fix was to use EMI shielding around the coils. This worked to mitigate this particular error.

What we are seeing now, though, is a single valve that is NOT in proximity to any other valves, nor is the wire connecting it running near any of the other valves, yet, the valve will generate the open/short condition error on it's own when the valve is activated for about 12 minutes (This allows it to heat up) and then when it's deactivated, it generates the open/short condition error.

I can imagine a ton of scenarios where this might be true, marginal connections, something wrong with how the valve was made etc etc etc.. But it gets.. weird. I can drive an identical valve and cycle it every one second.. after 40,000+ iterations of this test, the error never shows up. The same is true with the failing valve! I can cycle it continuously for extended periods of time with no error.  However, if I start with it as a cold valve (Like, physically cold not been run for a while) and energize it for 12 minutes, then turn it off, it throws this error. Once the error has occued, I can clear the error and then cycle the valve continuously every few seconds and it does not throw this error.

What I am looking for from you is ANY list of ideas of things I can look at either on the drv3946, or in regards to electrical measurements I can make on the valve itself or in conjunction with being operated by the drv3946 to determine exactly what is going wrong with this valve.

I look forward to your thoughts on this matter.

Thanks

Rick

  • 0
    TI__Mastermind 18505 points

    Hi Rick,

    Let me review this and get back to you.

    Best,

    Keerthi

  • 0
    Prodigy 50 points

    I have a bit of an update. After some digging around, it turns out that there are differences in the IPROPI resistors being used between a board that shows this failure right away, and a board that does not show this failure at all. It's also with one specific valve which I have not yet been able to measure the inductance of.

    Please let me know your thoughts as to why a valve might show this particular error (open/short) yet continue to be operable.

    I'm REALLY curious about this one!

    Thanks

    Rick

  • 0 in reply to Richard Dorfner
    TI__Mastermind 18505 points

    Hi Richard,

    Let me review this tomorrow and get back to you.

    Best,

    Keerthi

  • 0 in reply to Keerthivasan Kumanan
    Prodigy 50 points

    And more information..

    I have found that the "bad" valve when tested statically shows an impedance of about 450Milli Henries but when the plunger is pushed in, the impedance climbs to about 950Milli henries... The other valve only shows a difference with the same test of about 100 millihenries.  I have also been digging through the documentation for the drv3946 and noticed a reference to "Large impedance changes" can potentially cause the device to throw errors. Is this correct?

    Still anticipating some feedback from. you.

    Thanks

    Rick

  • 0 in reply to Richard Dorfner
    Prodigy 50 points

    yet MORE information. I think we have a working solution in hand and it has to do with the way we have a recirculation diode being used. In a previous revision of our hardware we were strapping the recircuation diode to be directly across the valve. This never saw the error occur. The newer revision of the hardware followed TI's recommendation to have the recirculation diode bleed throug the drv3946.

    Now, due to the very large change in inductance of the valve, the inductance would go from 900mh when active to 500mh when not active which induced a very large voltage spike that the drv3946 interpretted as a short circuit.

    THis working theory is based on this snippet from the manual...

    3.17 On-state diagnostics 

    When internal regulation is enabled in the ACTIVE state, the device can detect abrupt changes in the load impedance or supply conditions or IPROPI resistor value that may result in loss of driver control. 

    We will be strapping the recirculation diode to be across the valve coil and not let it go through the drv3946 and expect this to resolve our issue. If not, I'll resubmit this ticket.