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Old 01-12-2011, 10:26 PM   #5 (permalink)
DJBecker
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Current sensors

Cougar uses a LEM Hass current sensor. This is a premium, calibrated isolated current sensor.

This type of sensor is based on the Hall Effect -- it measures the current by sensing the magnetic field.

There are few alternate ways of measuring high current flows, especially with DC. The traditional approach is using a resistive current shut. These are very low value resistors made of specific alloys that have minimal resistance change with temperature. The construction is typically multiple wide thin strips separated by air gaps. At first glance they look like a solid rectangular brass bar.

We have a few current shunts, but use them only for bench tests. There they are hooked up to panel meters, not connected to the controller. Among their draw-backs is that they are part of the traction voltage circuit, and they create a voltage drop. Both are problems when you are trying to get accurate measurements to a mostly-isolated controller.

When selecting an alternate current sensor, read the specifications carefully. The aperture (opening) size and shape are often a limiting factor if you need to fit the sensor around a bus bar. Many older sensors require bipolar (e.g. both +15V and -15V) supplies, and can sometimes consume surprising amounts of power.

Hall effect sensors are not damaged by measuring currents above their rating. The rating is only the range over which they are linear and accurate. They typically will produce usable results at 50% to 100% overcurrent before they saturate. Above the saturation point they don't change output with increased current. Of course when you are pumping that much current through, something might be melting.

For smaller currents -- under 100 amps -- there are inexpensive alternatives.

One recent device is an integrated hall effect sensors that run current through the chip package itself. These are great, but don't handle overcurrent well. The conductor doesn't have enough cross-section to get rid of the heat as the resistive drop becomes significant.

Another option is hall effect sensors from DeviceTech. These are inexpensive, nicely calibrated, low power and flexible. If you are ambitious you can even change the comparator bias point by replacing the SMT resistors to allow a higher positive current measurement. Their drawbacks are the small aperture size (1/4" diameter for the low end) and exposed circuit board.

For the most part, we use current sensors that we make ourselves. We start with big toroids from discarded computer power supplies. We remove the copper windings, cut a slot through one side, and epoxy a A1302 ratiometric hall effect sensor the center of the cut. These are uncalibrated, and thus would be completely unsuitable for production use. But we need to do scaling and calibration in software anyway, so it just means entering a scaling factor.

It's also possible to measure current by just placing a hall effect sensor adjacent to the bus bar, in the right orientation. This is easy, but the reading will vary with outside influences. You wouldn't want to speed up while driving north and slow going south...

Last edited by DJBecker; 01-29-2011 at 12:26 AM..
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