Resistor for current measurement
 

Sorry if there is already a thread like this (I searched, but not hard.) I was wondering if it possible for us to place a resistor in series with the motor to test current. One rule says (I don't know which one, too late at night to look at the manual) that a "low impedence" current monitoring device is fine, but would a low value resistor (<1 Ohm) count? Thank you very much for your help.

Re: Resistor for current measurement
 

Try lower yet, like 0.01 ohms or less. Depending on what you're controlling, you could easily have 40 amps going through your sense resistor, and the power loss can be significant. A brief 100 amp stall current on a powerful motor with a ten milli-ohm sense resistor will drop a volt across the resistor and dissipate a hundred watts.

Re: Resistor for current measurement
 

Alan has pointed out the serious nature of power in your attempt to get valid current data. The Jaguar uses a 0.0005 ohm/2watt resistor. AT CIM stall current of 129 amps, that is almost 8.5 watts. One foot of #10 wire on the other hand is 0.001 ohms, will drop 0.1 volt at 100 amps and can easily dissipate the resultant heat.

Re: Resistor for current measurement
 

For current sensing on previous generations of the PD I used half milliohm resistors rated for 3 or 5 Watts and current sense amplifiers. As Alan said, 100A slugs (or worse) can and do happen.

Re: Resistor for current measurement
 

Might I suggest an off the shelf solution? I would recommend this product as an example.

It's max spec is a bit below stall current, but, with the rover wheels on a regolith surface, I would not expect a stall to ever take place.

If this is not acceptable, do some Googling and I'm sure you can come up with a COTS engineered solution.

JMHO

Mike

Re: Resistor for current measurement
 

We know we would have to use a value way below 1 Ohm to not drop significant load. However, if we can get current sensors we would use them. The only problem is it is too late, by the time they get here the robot would either be shipped, or we wouldn't have enough time to program. If we can find one at a local electronics store (like RadioShack), we will buy a current sensor there. Thank you for your replies.

Re: Resistor for current measurement
 

You are not going to get precision, high power, low ohm resistors any faster...

My recommendation remains the same...

Good Luck,

Mike

Re: Resistor for current measurement
 

Maybe something like this?

http://www.coilws.com/index.php?main...dex&cPath=2_14

I know they're designed for AC, but I think they may work for DC. An engineer would need to clarify this first, as I really don't know.

Re: Resistor for current measurement
 

Those work by measuring voltage generated in a coil of wire wrapped around the wire you're trying to sense. To induce this voltage, you need an AC signal in the wire who's current is being measured.

The motor controllers output a pulsed DC signal though, so they may work (Our mentor was formerly a EE prof, and thinks pulsed DC may work through a coil of wire for sensing). We haven't tested this yet.

There are also hall effect devices that measure the magnetic field generated by the wire (and work for DC applications) and I think you can get a hall effect IC for ~$2-5

Re: Resistor for current measurement
 

You will get an output from the pulsed DC, but I think it will be more of a pain to use than it is worth. I'd use this approach for measuring commutation, not current - each time the brushes hit the next coil, a small current pulse is created that could be detected with this coil. The number / rate of pulses tells you distance / speed of the motor.

For nasty high currents I've used this guy: http://www.gmw.com/magnetic_sensors/...csa/CSA-1.html

The eval board can be zip tied to a wire and get reasonable results. My interns used this in the battery monitoring board they developed to help characterize loads the PD would experience.

Re: Resistor for current measurement
 

Don't forget that the pulsed DC has very rapid rise times and in the Victors, full throttle is still DC. The Jaguars have a very short pulse at full throttle (to assist with internal FET control) but this pulse is so short it likely would not be sensed by a current transformer. At less than full throttle, a current transfomer would still be confused by direction changes, and induced currents flowing into the speed controller while the motor is winding down to a lower speed. As always, transformers operate by coupling changing magnetic fields into induced currents in the wires in that changing magnetic field. No changing field means no current.