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

Hi, I'd like to measure the current which a motor-controller is applying to a m otor. 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 fe ed 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 one way, 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 r ange of [-2V,+65V]. So sounds great when I'm driving one direction, but i'll destroy it when re versing?

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

Thanks for any pointers, advice, and success stories.

John

Reply to
john.ruckstuhl
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motor.

my instrumentation-amplifier to map that into an ADC -appropriate range, say [0.1V,2.4V].

way, and -12V when driving the other way.

of [-2V,+65V].

reversing?

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...

Avago has some chips specifically for this.

There are also Hall-effect current sensors.

There's this, too:

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A flying-capacitor thing would be fun, and cheap.

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

motor.

my instrumentation-amplifier to map that into an ADC -appropriate range, say [0.1V,2.4V].

way, and -12V when driving the other way.

range of [-2V,+65V].

reversing?

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...

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

John S

Reply to
John S

motor.

my instrumentation-amplifier to map that into an ADC -appropriate range, say [0.1V,2.4V].

way, and -12V when driving the other way.

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 respectively.

range of [-2V,+65V].

reversing?

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...

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:

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If it's really +/-12V this kind of device won't work.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

Just measure the voltage on both sides of current sensing resistor by ADC with appropriate analog scaling, then do math in the software. For better accuracy, you may have to calibrate it for common mode voltage at zero current.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

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Reply to
Vladimir Vassilevsky

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 controller.

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

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

motor.

[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.

inductively?

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.

Reply to
whit3rd

motor.

dandy.

inductively?

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 :)

Jamie

Reply to
Jamie

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

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Detail: 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 recommendatio n (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 r esponding.

Joerg wonders if I might be wrong about my motor controller outputting a di fferential voltage. Joerg, I don't think so -- my reading of the Maxon LSC 30/2 "Operating Intr uctions" (

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220404.pdf wow, so hard to find this online! Maxon, could you make it easi er?)

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 shun t. 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, an d with great tact notes that I may have written nonsense about using a coil over 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 abo ve.

Jaime describes the current transformer also, Jaime, as whit3rd reminded me, I'm measuring a DC motor, so I'll go with on e 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

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

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)

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 measur ement 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 wit h sensed current. The device requires 5VDC for VCC and a couple of filter c apacitors.

Features:

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
Reply to
john.ruckstuhl

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 terminal.

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.

[...]
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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

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Reply to
Nico Coesel

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"

-Lasse

Reply to
langwadt

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 hacking.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

[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...

Reply to
whit3rd

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.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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!

Reply to
john.ruckstuhl

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.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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