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current sensing when common-mode-voltage may be in range [-12V,+12V]

Discussion in 'Electronic Design' started by [email protected], Sep 29, 2012.

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  1. Guest

    I'd like to measure the current which a motor-controller is applying to a motor.
    Current range is [-2A,+2A].
    I imagine either
    (a) "map" this current into 0-4V, which I can read with my ADC, or
    (b) map this into a smaller output range, say [-0.02V,+0.02V], and I can feed my instrumentation-amplifier to map that into an ADC -appropriate range,say [0.1V,2.4V].
    If I'm using a shunt resistor then I suppose that leads me to (b) over (a).

    The motor-controller outputs a differential motor driving voltage.
    So, whichever motor wire that I choose to splice a shunt resistor into,
    the common-mode voltage might be as much as +12V when driving the motor oneway, and -12V when driving the other way.
    (that is, the Vdiff applied to the motor can be in [-24,+24]).

    How would you sense the current?
    I searched "High-Side Current Sensing" and found an Analog Devices AD8210,
    but that device, and its family of devices, permit common-mode voltage in range of [-2V,+65V].
    So sounds great when I'm driving one direction, but i'll destroy it when reversing?

    Or should I wind a coil of wire around a motor lead and try to sense it inductively? Then maybe I could ground one end of that coil and come away with a [-0.02V,+0.02V] signal that and I would calibrate it against a DMM. Maybe I'll reseach inductive current probes and see what I find...

    Thanks for any pointers, advice, and success stories.

  2. John S

    John S Guest

    I am using the Touchstone TS-1101 in my design. It is a remarkable device.

    John S
  3. Joerg

    Joerg Guest

    Are you sure about this? Most motor drivers I have dealt with apply 0V
    and 24V or so. Then when reversing polarity they flip this to 24V and 0V

    Some high-side sensing devices can be set up to measure bidirectionally
    but generally not (much) below 0V common mode. For example this one,
    although at 24V I'd pick one with a little higher abs max limit:

    If it's really +/-12V this kind of device won't work.
  4. On Fri, 28 Sep 2012 18:57:44 -0700 (PDT), the renowned
    The LEM Hall-effect current sensors work beautifully.. 1%-ish accuracy
    and bandwidth from DC to well into the 10's of kHz. Should be under
    $20 one-off. They're great for test rigs*.

    Sine the input is galvanically isolated from the output you don't need
    to worry about CM range.

    You'd probably use something cheaper if you were designing the motor
    controller, but it sounds like you just want to test an existing

    * Most of the specs on high-current BLDC motor controllers seem to be
    rather "optimistic".

    Best regards,
    Spehro Pefhany
  5. whit3rd

    whit3rd Guest

    [from +12V/ -12V supplies]

    Does that mean the motor is connected from the controller to GND? That
    would mean a small resistor in the ground lead will sense the current just dandy.
    Surely, you mean wrap a magnetic core around a motor lead, and a second
    coil of wire around the magnetic core, to make a current transformer?

    Works for AC pretty well, of course. A reversible DC motor, not so much.
  6. Jamie

    Jamie Guest

    What works well is if you take a donut large enough to pass a motor
    lead through it, 2 wound coils 180 degrees from each other, you can form
    a type of magnetic control, where the motor lead in the center will
    effect the two coils. You energize one coil with a sine wave REF and use
    the other to detect it.

    Years ago we needed to monitor the current on a heavy DC path with
    out making any connections to it. That is how we did it.. I am sure
    today it
    would be a big joke :)

  7. Guest

    I'm ordering "ACS712 Breakout" on a small pcb for $10

    John Larkin recommends:
    + Avago chips
    + Hall-effect current sensors
    + a TI LMP8602
    + a flying-capacitor thing...

    John, I looked at the TI LMP8602 data sheet after seeing your recommendation (thanks for the convenient
    link). Yes I see CMVR -22V-+60V (at Vs=5V). The flying-capacitor is
    beyond my ability right now. Didn't check Avago yet.

    John S recommends Touchstone TS-1101 --
    John S, sorry, I didn't explore this yet. I will, though, and thanks for responding.

    Joerg wonders if I might be wrong about my motor controller outputting a differential voltage.
    Joerg, I don't think so -- my reading of the Maxon LSC 30/2 "Operating Intructions" ( wow, so hard to find this online! Maxon, could you make it easier?)

    Vladimir Vassilevsky suggests measuring voltage on both sides of the
    shunt, then calculating.
    Vladimir, let's say I am comfortable losing 1%, or 0.1V of 10V, at the shunt.
    Won't I be giving up at least 6 bits of the ADCs to the common-mode voltage?
    (now two ADCs, to measure both sides)
    Right now that sounds like a lot to give away.

    Spehro Pefhany recommends LEM Hall-effect current sensors.
    Spehro, I went to the LEM website to explore.
    I don't doubt that LEM has what I want, but I had a hard time navigating
    that website to find what I want.

    whit3rd asks if one side of the motor is grounded, giving a simple soln, and with great tact notes that I may have written nonsense about using a coilover a DC current.
    whit3rd, Thanks for giving me a face-saving gentle reply :)
    No, the motor is driven by a differential voltage, two leads from the
    motor controller. At least that's how I read the Maxon doc I reference above.

    Jaime describes the current transformer also,
    Jaime, as whit3rd reminded me, I'm measuring a DC motor, so I'll go with one of the other options.

    Thank you, all, for taking the time to reply.
    I really appreciate your help.
    So in conclusion, your suggestions led me to seach more on the Hall-effect sensors, and I found this part...
    "ACS712 Breakout" on a small pcb for $10

    (See also the "ACS712 Low Current Sensor Breakout" for $15 with op-amp on board, )

    For the $10 item:
    Description: This is a breakout board for the fully integrated Hall Effect based linear ACS712 current sensor. The sensor gives precise current measurement for both AC and DC signals. Thick copper conductor and signal traces allows for survival of the device up to 5 times overcurrent conditions.

    The ACS712 outputs an analog voltage output signal that varies linearly with sensed current. The device requires 5VDC for VCC and a couple of filter capacitors.


    x05B (5 Amp) version
    Low noise analog signal path
    Device bandwidth is set via the FILTER pin
    5us output rise time in response to input current
    80kHz bandwidth
    1.5% output error at 25 degrees C
    1.2mOhm internal conductor resistance
    2.1 kVRMS minimum isolation voltage from pins 1-4 to pins 5-8
    5.0 VDC, single supply operation
    66 to 185 mV/A output sensitivity
    Output voltage proportional to AC or DC currents
    Factory-trimmed for accuracy
    Extremely stable output offset voltage
    Nearly zero magnetic hysteresis
    Ratiometric output from supply voltage
  8. Joerg

    Joerg Guest

    Good medicine, as Phil Hobbs would have said. Although you can get the
    bare ICs for less than $4 and invest the saved $6 into a couple of
    brewkys at the local pub :)

    Looking at the block diagram on page 14 it seems that it's unipolar. I
    wouldn't know where any negative voltage for the motor terminals should
    come from. AFAICS, the only voltage it can apply to either motor
    terminal is between 0V and the positive voltage level of the externally
    connected power supply.

    When the motor reverses they seem to simply flip the connection around
    in the bridge inside the controller, then a positive voltage is applied
    to the minus motor terminal and ground is applied to the plus motor

    If that's true then a simple device like the INA21x plus shunt should
    work. But mind that these should not be operated above 24V common-mode.
    There are others that can go higher.

  9. Nico Coesel

    Nico Coesel Guest

    Sense resistor, voltage divider followed by a bipolar instrumentation
    opamp with an offset input so you can shift the signal.
  10. Guest

    yeh, it can't really be anything but a h-bridge output, TI list
    several INA1x
    that are rated for +36V common mode

    looking at the block digram there's already current sense inside the
    box, so
    one could sneak a wire onto dip3 and get it for "free"

  11. Joerg

    Joerg Guest

    Yes, but that one is most likely unidirectional, just measuring the
    current into the lower input leg of the H-bridge. IIUC John.R would like
    to have directional info as well. One could also pipe out the
    directional info to the bridge controls but this would involve a bit of
  12. whit3rd

    whit3rd Guest

    [current sense resistor in H-bridge driver]
    Offhand, that would only take a comparator driving a DPDT analog
    switch (or maybe a relay). Motor output voltage is likely always
    in phase with output current...
  13. Joerg

    Joerg Guest

    Usually those current shunt resistors will not show the actual motor
    current at all when braking, when there is no power fed into the bridge
    but the upper two FETs are conducting.
  14. Guest

    Looking at the block diagram on page 14 it seems that it's unipolar. I
    Yes, I took measurements and confirm you are correct above, the motor driving voltage is inside the supply rails, but ...
    Not what I see.
    The motor driving voltage has a common-mode of very close to halfway betw the supply rails.
    In other words, I connect power supply 18V and GND to the controller, and it then provides a Motor driving voltage that is symmetric about 9V, like (5V,13V)
    Thanks for your replies, Joerg!
  15. Joerg

    Joerg Guest

    That would be the deluxe edition, leather seats and all. Instead of a
    simple bang-bang bridge it probably contains two synchonous buck
    outputs. Those can either be real regulators with feedback or just
    steered without feedback.

    You are welcome. Looks like you could do it with such a current sense
    device then. One of them in one of the motor leads should suffice,
    provided you bias it so that it can sense current in both directions.
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