Current Sense Recommendations

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I'm looking for a good solution for high-side current monitoring of DC
brush motors, clutches, and brakes driven with a 20 KHz PWM at 24 ~ 30
volts.

The motors are driven bi-directionally, that is to hold position the
H-bridge is driven in one direction for 25 uS (minus a switching dead
time delay), then in the other direction for the same time.

Brakes, clutches, and probably solenoids are driven unidirectionally,
that is in one direction only for some percentage of the PWM cycle and
not at all for the other.

I'd prefer to use a high-side switch, since the supply is chasis
grounded and a low side switch will never see common cable problems,
that is a power lead shorted to chasis somewhere.

We've tried the Maxim MAX4080 and MAX4081 with very poor results.
Despite a single line on the first page of the data sheet that they
are suitable for bidirectional motor drive sensing, they are not
usable at all.  They ring and oscillate like crazy 20 KHz, in addition
to the response and settling times being too slow.

Any recommendations appreciated.

--
Jack Klein
Home: http://JK-Technology.Com
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Re: Current Sense Recommendations

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If cost is not critical, you could use a LEM www.lemusa.com




Re: Current Sense Recommendations

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What does this accomplish beyond what you would get if you just turned
off the current and left it off?

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If you are using an H-bridge, then aren't you using both high-side and
low-side switching at once?

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Where did you put the Maxim chips?  In series with the positive rail
above the H-bridge?  Or in series with the motor winding?  The
difference would be whether you want direction sense in the current
sense, or just the absolute value of the motor current.  Settling time
is probably a factor of your external components and not a function of
the Maxim chip.  The implementation is very important.


-Robert Scott
 Ypsilanti, Michigan
(Reply through this forum, not by direct e-mail to me, as automatic reply
address is fake.)

Re: Current Sense Recommendations
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It provides a break action. Just like shorting the motor windings.

Meindert



Re: Current Sense Recommendations
On Wed, 15 Sep 2004 19:24:20 +0200, the renowned "Meindert Sprang"

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How is it different from just turning on both low or high switches and
leaving them on?

Best regards,
Spehro Pefhany
--
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It's not. But depending on the hardware implementation, you might no be able
to do that. If you drive both halves of the bridge from two CPU pins you can
do it, but if you have only one pin and drive the other half of the bridge
through an inverter, applying 50% duty-cycle would do the trick.

Meindert



Re: Current Sense Recommendations
snipped-for-privacy@interlogDOTyou.knowwhat says...
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It's a little smoother going through zero (you don't have to flip the
bridge around)and the ripple current is a little higher.

Also at zero you are actively driving it so you don't just rely on the
generating capability of the motor to provide the braking.

It's a useful configuration if you spend a lot of time either near zero
or if you reverse directions frequently.

Robert

Re: Current Sense Recommendations

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I usually monitor the current flowing in the common bridge return leg. The
sensing element is usually a low ohm resistor or, if the current is too
high for such considerations, a hall effect device stuck on the copper bar.
If monitored through an Isolation amplifier (like the HCPL7800) you have a
galvanically isolated means of monitoring motor current (what you see is
the combined mmcurrent through all the bridge legs and should be equivalent
to the motors current demand).

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Re: Current Sense Recommendations
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A couple of questions.  

What sort of current? 0.5A, 5A, 500A?  Given that you are using a high
side amplifier that suggests a sense resistor and thus a lower current
but...

What kind of bandwidth?  Are you trying to respond quickly to shorted
loads and protect the power devices or limit the current and protect the
motor?

And finally motor current or supply current?  Most of the hi-side current
sensors I've seen require a minimum voltage which would make them
unsuitable for use inside an H-bridge and so would only be useful for
measuring supply current.  A quick look at the Maxim parts suggests they
are cut from a similar mold.

For small quantity and high bandwidth a LEM module (as suggested by
another poster) is a good bet.  They are naturally isolated and
realtively easy to work with.  For larger quantity and high current (cost
sensitive) I'de suggest a linear hall sensor and a core to confine the
magnetic field.  For moderate currents Allegro has a 75-100A hall current
sensor in a solderable package that looks quite nice.

For coils take a look at Micrel. I've used the MIC5020 (a driver with
current sense for overcurrent protection)for low side switching
successfully.  It might be adaptable for high side use.

Robert

Re: Current Sense Recommendations
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I'm probably not understanding some important point here, but what
about the usual method of a small resistance in series with the load
and either a DAC on each side of that resistor or some analog
differencer driving a single DAC?

Re: Current Sense Recommendations
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the MAX408x contain just that: a differential amplifier with some extra
features for good common mode operation.

Meindert



Re: Current Sense Recommendations
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I have used the MAX4081T succesfully with a 0.015 ohm sense resistor
in a 24V 11A H-bridge DC brush motor amplifier.  This is in a high
side sense configuration.  I'll leave it as an exercise to figure out
how to make the bidirectional part work.

The MAX4081 does not have enough bandwidth if you connect it after the
switches; you need to connect it at the high or low side supply.  High
side of course is better because the most common short is to ground.
The bandwidth on the 4081 is 150kHz.  It can't deal with the slew rate
of the switches.

I have a single pole low pass filter with a cut-off of 30kHz after
this device.  I get better than 10 bits of resolution.

Ryan

Re: Current Sense Recommendations
wrote in comp.arch.embedded:

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A follow-up, and thanks for the surprising number of replies.

The cause of the ringing and oscillation was our fault.  My colleague
Greg, who shall remain nameless, doesn't usually make mistakes but
must have been asleep when he put together this particular circuit.

First he put a MAX4081 across a shunt resistor in series with one of
the motor leads, to get 0 volts at -15 amps to 3 volts at +15 amps,
feeding the ADC inputs of a TI 2812 DSP.  That's fine in one
direction, as the +RS line it one FET voltage drop below the motor
power supply.  In the other direction, however, that same input is one
FET drop above ground.  Even though the chip is powered separately
from a 5 volt supply, it is not spec'ed to work with +RS below +4.5
volts, and it most surely shuts down.

We found that one, when the output was screwy, and I went over the
data sheet.  We switched to the high side, between the positive motor
supply and the top of the H-bridge.  We also changed to a 4080, since
there wouldn't be any reverse current other than a little regen that
didn't interest us.

But it oscillated at an almost perfect 100 KHz every time it turned
on.

Second, he clamped the output with a 3.3 or 3.6 volt zener, to protect
the ADC input in the event of a major over current, and that turned
out to be the cause of the oscillation.  The data sheet mentions a
maximum output capacitance of 500pf for "no continuous oscillations",
and the zener had a nominal capacitance of 495pf!  The oscillations
might not have been continuous, but they lasted through the 50 uS PWM
period without noticeable decay.

We are now protecting the ADC with a very low capacitance (14pf max)
Schottky diode to the 3.3 volt supply rail, and the ringing went away.

For those who asked questions about the design and would like answers,
here are a few.

We are familiar with various hall-effect current sensors, and use them
on a companion board with the same DSP that drives a three phase
brushless motor at 250 ~ 300 volts.  They work quite well, but are
both relatively large and relatively expensive, though absolutely
necessary to high performance space vector modulation control of the
three phase motor.

I have neither the space nor the budget for 8 of these four motors,
four brake/clutch/solenoid drivers per board.  The system physical
layout and covers are too far advanced, and I can't expand the board
10 mm in any direction.

As for why the bidirectional drive, it's provided in hardware by the
DSP and needed for out application.  Each of the DSP's event managers
(there are two) has three pairs of PWM outputs driven by a common
period register, needed for driving three phase motors.  Each pair is
driven from a single compare register to set the duty cycle.  Each
pair can also nicely drive a single servo motor.  One output is set to
PWM active high, and is connected to AHI and BLO FETs, the other to
PWM active low and drives the ALO and BHI FETs.  A dead time value is
programmed into another DSP register and it all works quite nicely.

As to why we want bidirectional drive, there are several advantages.
At 50/50 duty cycle, each side is driven for 24.7 uS (25 uS minus 300
nS dead band time).  In situations where the control loop has to hold
position, one can respond to a disturbance quicker because the drive
circuit is already on, only the PWM compare register needs to be
changed to apply torque in one direction or the other.

The other important reason is much better control at very low power
levels.  This is a medical imaging device, and very accurate velocity
control is a necessity during image acquisition.  One of the
clinically most important drives has a velocity range of 200 to 1, and
the slowest velocity is used clinically in scanning.

With single side drive, very low duty cycles tend not to be precise,
as you are switching transistors on for an extremely short time, and a
few nanoseconds difference in rise and fall times is a significant
percentage of the on-time, and precise velocity control gets very
difficult.

With bidirectional drive, you apply low power in one direction by
setting the PWM compare register a little above or below the midpoint.
So instead of trying to deliver to turn a FET on and off to deliver a
200 nS pulse in one direction, you are applying a 24.8 uS pulse in one
direction and a 24.6 uS pulse in the other.  No worries about narrow
pulses being different from one board to the next, or even varying
with board temperature.

Again, thanks for all the replies.

--
Jack Klein
Home: http://JK-Technology.Com
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Re: Current Sense Recommendations

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Have a look at the Allegro ACS750LCA device. It's bandwidth might be
too low for your application.

Regards
  Anton Erasmus




Re: Current Sense Recommendations
International Rectifier has a part - IR2175.

I also sampled the ZMC-10, which is galvanically isolated. Current
goes thru, and an isolated wheatstone bridge will give you a linear
voltage which you'll have to put thru an op-amp.

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