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Brushless Motor Precise Current Sensing Monitor for RC Airplane Electric Power System

motionsense
Prodigy 20 points
Other Parts Discussed in Thread: INA282, TINA-TI

Specs: 
Load Current Range: 0 - 100 A

Load Voltage Range 0 - 45 V 

Expected Max Load Power: 3KW

Power supply 5 or 6 volts regulated 

Temp Range : industrial, the actual temperature of the sensing circuitry is expected to reach 80 deg C while operational (educated guess)

Required Accuracy:   max 1% or better, that is when the motor is run at max power, the accuracy should still be not worse than 1% 

Is the above possible at all? Accuracy can be relaxed if necessary ... 

 

Here is what I need:

Some application note detailed enough  so that I can learn how to analyze the accuracy of a  current sensing circuit that would meet the above specs. I am not an analog designer, so perhaps some reference design and general application note for current sensing solution would help. I was not able to find anything specific for this application ... perhaps someone has a suggestion where can I find reference material.

Thank you in advance

Best regards,

Paolo

PS: the current sensing circuit is meant to be put between the battery and the motor speed controller (high side current sensing), so this is a DC problem.

PS2: this is meant to be used as subsystem for a digital close loop power control system, which would regulate the max load current by changing the pitch of the spinning propeller. The intention being to design a system that based on the voltage input (controlled by the user, which defines how fast the propeller spins) automatically maximizes the output thrust of the propeller while automatically preventing  the motor from reaching an overcurrent/overheating condition (which should it occur, would burn the motor in a few seconds). The high accuracy is required because the goal is to achieve the highest possible performance.  

 PS3: I just need help with the current sensing circuit ;-) 

  • 0
    TI__Mastermind 25805 points

    Hello motionsense,

     

    At this time we do not have a published application note that meets your needs.  However, I would be happy to help you through the process.

     

    The first step you have already accomplished for the most part, which is defining the design criteria.  We typically need the following information to initially paint the picture of the application:

     

    a)      Vcm range (0V-45V in your case)

    b)      Load range (0A-100A in your case)

    c)      Accuracy at minimum-load? (1% or better…will need to be relaxed)

    d)      High or Low side?  (In PS1 you noted this is High-Side)

    e)      Frequency?  (In PS1 you noted that this is a DC measurement)

    f)        Directionality (uni or bi)?  (Let’s assume uni-directional)

    g)      Output (current or voltage?)  (Let’s assume voltage output)

     

    At this point I can make a part recommendation.  My suggestion is the INA282, assuming your system will not have any significant steps in Vcm.  It meets all of the aforementioned criteria.

     

    When looking at accuracy, we first need to take the initial Vos(max) into account.  I assume that your system can’t run with 0A, so let’s say your minimum load current is more like 5A.  The Vos(max) of the INA282 is 70uV.  In order to obtain 1% accuracy (and only taking initial Vos(max) into account and assuming your minimum load current is 5A), you would require a 1.4mohm shunt resistor.  From here we need to ensure that at maximum load current (100A) a 1.4mohm shunt resistor does not saturate the input/output specs of the device.

     

    100A load with a 1.4mohm shunt resistor would yield a sense voltage of 140mV.  This is within spec of the INA282 input.  When multiplying the input by the gain (50), however, we see that the output would have to reach 7V.  This is not possible with your 5V-6V supply specification.

     

    From here you can adjust the accuracy specification and shunt resistor value to obtain acceptable results.

     

    From there you will need to take the following into consideration:  Vos shift (with respect to time), Vos drift (with respect to temperature), other Vos contributors (e.g. CM & reference), gain error, and non-linearity errors.  There is a good discussion of most of these in the INA282 datasheet.  What is not covered are the topics of shift and drift.  Drift is straightforward, for there is a specification in the datasheet which shows how much Vos will change due to each degree change.  Shift (with respect to time) can be account for by doubling the Vos(max) specification in your calculations.  This will ensure that over a period of 10 years the initial offset and maximum shift will be accounted for.  

     

    You may want to download TINA-TI, which is a free SPICE simulator.  Here is the link:

     

    http://focus.ti.com/docs/toolsw/folders/print/tina-ti.html

     

    Also, here is forum post that may be useful as well:

    View this post at http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/108956/385128.aspx#385128

     

    I hope this helps.

     

     

  • 0 in reply to Pete%20Semig
    Prodigy 20 points

    Hi Pete, 

    thank you for your reply, it really helped with getting started.
    The SPICE simulator also helps.We'll relax the accuracy a bit, but it looks like we can get a first prototype started. I also  like that the INA282 comes with an evaluation  board.

     

    Thanks again,

    Paolo

    PS: your assumptions about the specs were all correct .

    The minimum current in theory is about 1A (no load current), but in practice the 5A (or possibly more) is a good guess.