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DRV8811 interface questions

Adam Reed
Prodigy 250 points
Other Parts Discussed in Thread: DRV8811

Hi, I am working with the DRV8811 eval board, and I also have a chip installed on the Avayan Electronics AE-MDL-STPR11 eval board.

My application is a pretty "plain vanilla" one, running a 1.8 degree stepper in full-step mode.  I am running at 29VDC, 2Amps.  The application is a fruit labeling machine.   It places the little labels on your apples and pears that drive us all nuts.  I am upgrading a design that currently uses the old L297/L298 chip set.

My first question is regarding the decay function settings.  I have been looking at the current across the sense resistor on my scope as I vary the voltage to the DECAY pin.  I can see no change in the waveform regardless of where I set the voltage.  Have tried in sync and async modes both.  Can you offer some guidelines as to how this should be set up?

Second question: What is the best way to measure the operating current limit in real time?  I am using 0.1 ohm resistors for sense, and the formula for calculating current limit is pretty simple: I=Vrefx/(8*Rsense).  so it seems I would want a vref of 1.6 volts for 2 Amp current limit.  When I set the board up this way, I measure approximately 160 mV across the sense resistor on my scope (1.6A).  To get what looks like 2 amps p-p on the scope,  I set vref at around 2.0 to 2.1V.

 

Many thanks for your help!

Adam Reed

Dendera Partners LLC

 

  • 0
    TI__Expert 7455 points

    Hi Adam,

    DECAY pin will only work on Synchronous mode (SRn = LO). On Asynchronous mode, the external body diodes take over and pretty much you will always run in fast decay mode, for which you will see no effect while changing the input voltage at the DECAY pin.

    If you are on synchronous mode, however, then you should see a change on how the winding current looks on sine wave quadrants 2 and 4. That is, when the current is discharding. For this you need a current probe. Notice that on Quadrants 1 and 3 (while the current is charging up), we are always in slow decay. On quadrants 2 and 4, however, we will be on FAST, SLOW or MIXED as defined by the DECAY pin. What you will see is the level of sine wave deformation changing. Please let us know if you still can not see thie behavior with your current probe.

    Now, ITrip is the maximum current the H Bridge will see before shutting down the H Bridge. It should not be confused with Peak to Peak current, neither the average current. Per example, if you set ITRIP to 2A, you will not get 2A at the windings. The average current will be close, but it will be a little be less than the 2A. Why? Because the current will be regulated from some value up to 2A. The low current value will depend on your motor inductance, the input voltage, the decay mode and the switching frequency as stated by the RC network at the RCx pin.

    Once you reach ITRIP, the H Bridge is disabled and the current starts to decay. How much it decays depends on the decay mode (slow, fast or mixed will have different decay rates so current will end up at a different value), the motor inductance ( the L on the Ldi/dt with the larger the L, the longer it takes to decay) and the TIME OFF as specified by the RCx pin (the dt on the Ldi/dt, with the larger the TIME OFF, the more the current decays). Hence it could be that the low value current ends up at 1.9A or 1.95A. I bet  you can easily see this on the scope. Whatever it is, you can then do the math and get your average current.

    Now, another very important aspect to have in mind is that you may be using full step. In this case the ITrip equation must be multiplied by 0.707 to take the RMS value. What happens is that the ITrip equation as ITrip = VREF / (8 * RSENSE) is meaningful only for the sine wave generation modes (e.g. half, quad and eight). This is what we call 100% Full Scale. On full step, however, we must take the RMS value of the similar sine wave (or multiply by 0.707). This can be seen on the lookup table on the datasheet on page 12, as the full steps values used are those of 71% and -71%. This would make sense with your reasings.

    Please let me know if there is anything else I can help you with.

    Jose I Quinones
    Applications Engineer - Analog Motor Drivers

  • 0 in reply to Jose Quinones
    Prodigy 250 points

    Jose, thank you for an excellent explanation.

    One final subject that I have not been able to find an answer to in the documentation or on the forums:

    What is the effect of varying the decay setting, in terms of enhancing the stepper motor performance? Why would we want to set the decay to FAST or SLOW or MIXED?

    I should add that I am running in FULL STEP mode.

     

    thanks again

    Adam Reed

  • 0 in reply to Adam Reed
    TI__Expert 7455 points

    Hi Adam,

    On full step, varing the DECAY pin plays no role whatsoever since you always operate at slow decay.

    The idea behind adding fast and mixed decay has to do with the ability to control the current and this is only meaningful while microstepping. On an inductance, it is easier to charge the inductor than it is to discharge it. Hence, when you are "drawing" the sine wave and the current is increasing, slow decay is good enough to generate all the different current magnitudes.

    When the sine wave is on the region of current discharge, however, then the wave shape gets distorted under slow decay. Fast decay or mixed decay allow us to control the current to the point where the sine wave shape is preserved. What does this means in terms of motor performance? Pretty much it translates to decreased torque ripple. If the sine wave is noisy, so will the motion profile. I don't have the numbers to specify how this degradation affects the performance, but I am of the opinion the cleaner you can make that wave shape the better.

    Another aspect of having this control is the ability to eliminate audible noise. If  you use fast decay, chances are the chopping frequency will get distorted. However, by using the right ratio of fast to slow decay on mixed decay, you can obtain a chopping frequency which is above the 20 KHz.

    Hope the info helps and Happy New Year!

    Best regards,

    JIQ