Re: MicroControllers at competitions
 

It's not practical. .025 ohms will have a significant effect on motor torque.

25mOhm is .025 ohms.


A clamp-on DC ammeter uses a Hall Effect device to sense the static magnetic field created by the DC current.

Re: MicroControllers at competitions
 

Sorry. I confused mOhms with uOhms!

I think it would be better to skip the resistor and bring along a foot of gauge 8 wire. That aught to do it without over-dropping the voltage.

Using a hall-effect sensor is a different way of thinking about it! I was thinking about measuring current by creating a transformer, with the second winding as a coil around the main wire!

Re: MicroControllers at competitions
 

If you are truly looking at shunt resistors <<25mohm you absolutely should use the voltage across the wire from the PDB to the speed controller. The resistance of the wire per foot is easily found online and will be just as accurate as a resistor you could afford to add - plus its FREE. Just put a volt meter across the wire (or add your micro-controller). If you are looking for a failure, the difference between 133 amps and 125 amps is not really relevant anyway.

Re: MicroControllers at competitions
 

Assume you use a 25µΩ resistor. At 100 amps, how much voltage drop would there be? How many useable bits of resolution would there be between 0 amps and 100 amps with your ADC?

Re: MicroControllers at competitions
 

To increase the 12 bit resolution, I want to decrease the vRef voltage to something like 1v or .1 v, etc.

Re: MicroControllers at competitions
 

As someone who designs ADCs for a living, I strongly recommend against using a Vref less than 1v. At 1volt each count of yor ADC is already 25uv and noise of a CMOS amplifier is about 10uV. The best answer is to use a resistor that gives you a voltage that matches the input range of your ADC. I still think the suppy wire is in the right range. The key is to use a high impedance sense circuit with a Kelvin connection in parallel with the supply routing to the speed controller so a not to influence the system performance.

Re: MicroControllers at competitions
 

Dev,
Your original statement about DC is true for transformers. You only get an output when the input (primary) is a varying voltage. In a transformer, when you switch on a DC current, the secondary will produce an output during the transition and then settle back to zero. That is why Hall effect devices are used for current monitoring in DC applications.

Re: MicroControllers at competitions
 

That makes sense. Does anyone have a good place for me to grasp info on how hall-effect sensors work? I am interested in these neat devices! :)

Re: MicroControllers at competitions
 

2 for one - tutorials and something specific for hall effect sensors.

http://www.electronics-tutorials.ws/...ll-effect.html

So has anyone used a length of wire, resistor or hall effect sensor on a fielded robot as a current sensor?

Re: MicroControllers at competitions
 

At IRI a few years ago, I know a few robots had self-contained logging hall-effect current sensors strapped against the battery leads. I don't remember finding out who was responsible for them.

Re: MicroControllers at competitions
 

FIRST was trying to determine current draw in order to set the requirements for the 2009 PDB.

Re: MicroControllers at competitions
 

WildStang manufactured a current monitor called StangSense a number of years ago. We used a one foot piece of #10 wire and a current monitor circuit from Maxim. We then ported the data to the robot data link and recorded the data at the driver's station. I believe the file is still on CD someplace.
The current monitor that was added to robots at IRI was from HQ. They wanted to gather data to confirm the high current spikes I had mentioned in conversation. The data recorders were supervised by Wayne Penn as I remember.

Re: MicroControllers at competitions
 

http://www.andymark.com/product-p/am-2709.htm

New product from AndyMark, using a Hall-Effect, but needs to be inserted in the circuit.