Current Sense Recommendations

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.

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Jack Klein
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Jack Klein
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If cost is not critical, you could use a LEM

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Reply to
Gary Pace

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

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

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.)

Reply to
Robert Scott

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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Paul E. Bennett ....................
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Reply to
Paul E. Bennett

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

Reply to
R Adsett

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?

Reply to
Lewin A.R.W. Edwards

It provides a break action. Just like shorting the motor windings.

Meindert

Reply to
Meindert Sprang

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

Meindert

Reply to
Meindert Sprang

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 the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

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

Reply to
Meindert Sprang

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

Reply to
Ryan

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

Reply to
R Adsett

On Tue, 14 Sep 2004 20:28:00 -0500, Jack Klein wrote in comp.arch.embedded:

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.

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Jack Klein
Home: http://JK-Technology.Com
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Reply to
Jack Klein

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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

Reply to
Anton Erasmus

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.

Reply to
Mike V.

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