I looked through the specs on the 884 on the IFI site and don’t see the output pulse rep rate. have you got a link to where it might be? I’d like to demo this to the students with a scope (if I can get near the robot this last week.) Thanks.
Bruce
I looked on the IFI website for both the 883 and 884. Last fall there was a post in the IFI forum looking at the differences between the 2. IFI detail the differences in the switching feq. Seams they have cleaned the posts up for the 2006 season and it’s not there. Would have to be re asked or give IFI a call to confirm. There where some threads where this topic was discussed on Chief Delphi. Again check the motor mounting blocking the vent slots. There was another post concerning this.
This stuff is legally “non-flamable”. But that doesn’t mean you can’t make a 5 foot fire ball in your kitchen when your wife isn’t home. Nevermind… (Don’t worry - you really need to try to make it burn like that - your robot won’t make it go whoosh. It’s OK… really it is.)
Has anyone ever thought of making aluminum blocks to mount to the motor, and storing them in a cooler of dry ice between matches? Actually… you may loose most of the cooling by the time the match gets started… just a thought…
I’d just like to re-iterate this. It is probably too late for you to do anything about it, but the vent holes in the front of the FP motor are critical to its cooling.
Matt
Try reading the current after the breaker, going into the victor.
This is not going to help. Actually, even starting the match with ice cold motors doesn’t help much either.
The problem is that the Cp of metal is just not that high – meaning you can’t store that much heat energy in them unless a phase change is involved (i.e. melting the metal).
In my opinion, even heat sinks on the outside of the motor can are of only marginal usefulness. Yes, heat sinks will make your motors able to runs somewhat higher current in steady state, but FIRST is not really steady state. Our uses come in bursts. Heat transfer takes time (especially conducting heat out of the windings of the armature, through the air or the shaft to the outer can to the thermal grease to the cooling fins). By that time, your windings are toast.
Air blowing across the armature – now THAT is cooling. Keep your current down and your armature speed up (1/4 of stall torque) and you’ll be fine.
…you mileage may vary…
Joe J.
I thought is was the other way around. (I have gotten this goofed up before though.) What may be confusing is that the input PWM is 120 Hz as is the internal loop reading the input.
The difference between the probes is pretty good. Still if you want to have accurate current data you must use another method. i.e. if you want to correlate current and breaker heating.
After a diligent search of the archives, I did find this…
It lists the 883 at 2kHz. However this sheet compares the 883 and 884 as having different deadband characteristics and the 884 being programmable as to the deadband.
http://web.archive.org/web/20020805205656/www.innovationfirst.com/FIRSTRobotics/Victor_884.htm
OK, I’m confused. 
We found this:
But I’m still not sure what that means. Is it talking about a code loop rate, victor input sample rate, or victor output pulse rep rate? I think somebody’s gonna have to hook up a 'scope.
I believe that it’s the Victor sample rate. It states that you cannot send it data any faster than it’s refreesh rate, which is 8ms. Thoug I believe that a pulse width is longer than that anyway.
Victor 883 - 2000 Hz
Victor 884 - 120 Hz
Direct from the designers.
-JV
Thanks,
Now I just have to figure a way to tuck that into my brain for next year when the question comes upt again.
OK, that’s pretty authoritative! Thanks, John.