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DRV8711 - Slow Decay works in a strange way

Nick Alford
Prodigy 60 points
Other Parts Discussed in Thread: DRV8711

Apologies if this has been discussed before but...........I couldn't find the answers in the datasheet or forum.

I'm trying to reduce motor heating and noise on stand still by firstly reducing the current in the windings using the Torque register and then switching to slow decay. (this is great as there is almost no audible noise). However.........When I switch to slow decay I get some very odd happenings with the current level. Please see what I mean below. There is a step clock input of 1Hz (in blue), after 0.4 seconds of no clock inputs the controller turns the current down to 50% and sets the decay mode to slow decay. This is what I get......

At most points the current increases and at the points in the table where the current does decrease it doesn't to the level requested.

To investigate further I just ran the driver IC on slow decay with the same 1Hz step input and I got this wave form out!?! We are still set at 1/16 micro stepping!!! It looks like a half stepping output?!?

If I stay in auto mixed decay mode and just reduce the torque register my plan work perfectly! See below:

Its almost as if ........when I'm using slow decay the IC uses a different lookup table for the sin and cos current tracking which makes it look like half stepping!?!

Is there a way round this or does slow decay work in this unexpected way for a reason?!?

Many Thanks

Nick

  • 0
    TI__Guru** 114905 points
    Hi Nick,

    Slow decay can act this way. It depends on the system voltage, the motor inductance, the blanking time, the off time, and the step speed.

    In slow decay, the DRV8711 is injecting more current during tblank than the off time can remove. In the next drive cycle the current continues to increase until an equilibrium is reached.

    Automixed decay prevents this loss of current regulation from occurring by adding a portion of mixed decay in each off cycle. You may be able to do the same thing by increasing the off time, but that may cause audible noise if you have to increase toff too much.
  • 0 in reply to Rick Duncan
    Prodigy 60 points

    Thanks Rich,

    I'm still a little confused how with slow decay we seem to be able to track the current amazingly well in a half stepping fashion (even though we are 1/16th micro stepping) but not to the desired sin wave form?!? Surely if it can track a current on stand still so impressively at these settings, it should be able move between the desired 1/16th micro step current levels and hold that current until the next step is received.

    Why is the driver IC so good at tracking these "equilibrium" levels? Surely if the charge cycle is too long and the decay is to short the current level would continue to climb out of control. Does the IC do something intentionally to limit the current to certain levels if this scenario occurs?!?

    Many Thanks

    Nick

  • 0 in reply to Nick Alford
    TI__Guru** 114905 points

    Hi Nick,

    It is sometimes confusing. Please note that while the waveform looks like half step, it is not really. The DRV8711 sets current at 71% and 100% for half step. This appears to be 50% and 100%. Also, the max current appears to be twice the current of the other scope captures.

    A quick experiment to see the effect of slow decay is to lower the voltage to 12V, assuming you are running at a higher voltage.

    An example of loss of current regulation can be seen in the blog ( HERE )