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Original question:

DRV8353R: Current sense offset

DRV8353M: Current sensing grief

John Skeldon
Prodigy 30 points
Part Number: DRV8353M
Other Parts Discussed in Thread: DRV8302, DRV8353

Hi, 

I thought i would start a new thread ... my old thread was DRV8302: failure of drive chip when running.

i can now run a motor but my current sense is not very good... Should i have filtering from drive chip to my micro i can see on the dev. kit that there is 56ohm and 2200pf.  With what i have round me i've tried 62ohm and 1000pf but my waveform hasnt changed much.

picture shows loaded up motor pulling nearly 5A at 12v i'm using a 0.001 ohm current sense  but with 33ohm in line of the op amp lines and a 10nf between (ive also tried it with these out). I'm thinking my choice of current sense is too small (i'm set at 40v/v) gain as well 

blue trace - to dspic 

yellow trace - h-bridge out to motor (the live side )

Also were driving the motor at 16khz ...  this was set by my predecessor using the old chip and i've left it.. Is there anything stopping me going up in frequency ? or is it an EMC thing

Suggestions ?? i'd be grateful.

i'll be re-reading the application notes and data sheets over the weekend 

John

  • 0
    TI__Mastermind 21290 points

    Hello John,

    The team is out of the office due to Thanksgiving holiday here in the US. We will be back in office on Nov 29th, thank you for your patience and understanding.

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    TI__Mastermind 21290 points

    Hey John,

    Thank you once again for the patience during the holiday. I am a little confused on your question regarding the current sensing. What is wrong with your current sense values, are you seeing offsets in your values?

    About the PWM switching frequency, typically we recommend using 20-25kHz. This keeps your switching frequency above the audible range of human hearing in the case that the switching is audible. The downside comes when you have a really high PWM frequency which increase the amount of switching losses.

    You can watch this TI precision labs video with more information on PWM: https://training.ti.com/ti-precision-labs-motor-drivers-motor-interfaces-and-pwm-frequencies

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    Prodigy 30 points

    i've re-read the datasheet and the worked example.. the data sheet mentions RMS motor current - which can be obtained by a calculation from RMS Torque divided by the torque constant.. I don't seam to have the torque figure for my motor.  as you can see i have nominal torque but not RMS torque 

    VALUES AT NOMINAL VOLTAGE
    Nominal voltage 48 V
    No load speed 4900 rpm
    No load current 88.4 mA
    Nominal speed 4620 rpm
    Nominal torque (max. continuous torque) 420 mNm
    Nominal current (max. continuous current) 4.58 A
    Stall torque 7370 mNm
    Stall current 78.9 A
    Max. efficiency 93 %
    CHARACTERISTICS
    Terminal resistance 0.608 Ω
    Terminal inductance 0.423 mH
    Torque constant 93.4 mNm/A
    Speed constant 102 rpm/V
    Speed / torque gradient 0.666 rpm/mNm
    Mechanical time constant 3.81 ms
    Rotor inertia 547 gcm²

    the offsets are not that bad.. I would just like to have a better range of current sense voltage.  As you can see from the voltage trace which also doesnt seam right.  I've changed the sense resistor for a big high wattage 0.1 ohm resistor just as a test and my scope trace is something i'd expect.  But i'd like to do the calculations so see what range i can use.

  • 0 in reply to John Skeldon
    TI__Mastermind 21290 points

    Hey John,

    Okay I seem to understand the problem a bit more now. If you cannot calculate the RMS current value of the motor, then perhaps understanding the relationship between the CSA gain values and your current sense voltage. Increasing the gain provides a more precise reading but greatly decreases your voltage range while decreasing the gain will provide a better range but in turn decrease the resolution.

    Increasing your shunt value can also limit your range, think of the V= I*R relationship if the current is constant but we increase the resistance R then we will have a higher voltage. Perhaps altering the gain voltage and your shunt resistor will help you obtain better values.

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    Prodigy 30 points

    we've looked at the graphs for the motor plus i've carried out some tests stalling the motor ect... So i know my max current so if i use that value plus a margin i should get my self in the correct ball bark to re-order some sense resistors .. inital calculation looks like i'm out by nearly a factor of 100. Will keep the thread posted Slight smile

  • 0 in reply to John Skeldon
    TI__Mastermind 21290 points

    Thanks John! Looking forward to it and let me know if you have any additional questions I can help out with.

    Best,

    Isaac

  • 0 in reply to Isaac Lara
    Prodigy 30 points

    Hi, After trying a couple of different values of shunt / current sense resistor we have something to work with.  I was able to chose a resistor that gave me a good range of ' load' though the motor but not going 'full scale to early'.   i'll get some waveforms at some point and post them up.  My next grief is how much filtering (if any to use and the micro 'seeing' the I_Sense voltage ) 

  • 0 in reply to John Skeldon
    TI__Mastermind 21290 points

    Hey John,

    Thanks for the update, glad you have found a combination that gives you a better sensing range so far. I would recommend starting with a 56 ohm resistor and a 2200pF cap to the micro, this is typically the low pass filter that we use on some our EVMs. This could be a good starting point and tweaking beyond this it does require some fine tuning. Looking back at your schematic and I realized that you are trying to filter at the input of the device. This is not what we typically do for our evaluation modules we normally implement the filtering at the output. I know this post is regarding DRV8353 but all the info is regarding DRV8302 and the schematic as well so I am not sure if DRV8353 was a typo. The filter info still applies to both devices though.

           

    I hope this helps!

    Best,

    Isaac