This is something i always wanted to play with while i was on a FIRST team but never had time.
A few years back some hall effect current sensors were included in the kit. As mentioned earlier they were manufactured by allegro micro. You might want to look into those. The other thing you could look into is called a current shunt. Its basically a resistor of negligible but precise resistance. You measure the voltage drop across it to figure out the current going through it. It would probably take a little bit of added circuitry to isolate the signal and take advantage of the full input range of the RC. The allegro transducers on the other hand can be directly hooked up to the RC. With ALOT of cleverness you could figure out current using a speed sensor.
I guess the first step is to figure out why you would want to clamp current. Torque from our motors is directly porportional to the current flowing through them. It we are trying to push someone really hard we want all the torque we can get. We thus want all the current we can get. The problem is, if we use too much current, we trip the breakers which leads to no torque at all until they reset. If you can limit current you can get the most torque out of your motors. What you want in a pushing match is a way to operate as close to tripping the breaker as possible.
Now the real question is how would you implement such an algorithm. This is not as straightforward as it sounds. Bear with me.
The amount of time it takes the breakers to trip depends on how much the current exceeds the breaker rating by. You can find the datasheed for our breakers
here
Taken from the datasheet:
150 % overload = 3.9 - 47 seconds
175 % overload = 2.2 - 9.2 seconds
* 200 % overload = 1.5 - 3.9 seconds
250 % overload = 0.8 - 1.8 seconds
300 % overload = 0.5 - 1.1 seconds
400 % overload = 0.3 - 0.6 seconds
500 % overload = 0.2 - 0.3 seconds
Lets use a CIM motor for this example.
under full stall, it draws 133 amps. It must be connected through a 40 amp breaker. We get the following trip times for the cim at different loads. If you do at little math, you arrive at a startling table of % motor load vs breaker time. % motor load is taken as %torque/current of max.
30.8% - indefinately
45.1% - 3.9 – 45 sec
52.6% - 2.2 – 9.2 sec
60.2% - 1.9 – 3.9 sec
75.1% - .8 – 1.8 sec
90.2% - .5 – 1.1 sec
You don't just want to limit yourself to 40A. This would just unnecessarily limit your power output. you are going to have to have a fairly complex algorithm that takes time into consideration. you will also have to figure out a way to reliably limit your current consumption
your problem is threefold:
1.) Figure out what your current is
1.) figure out what you what your current to be
2.) figure out how to get your current to be what your want it to be.