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  #16   Spotlight this post!  
Unread 02-14-2006, 10:43 AM
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Re: FisherPrice Problems

Quote:
Originally Posted by Al Skierkiewicz
Allen,
I question what you are using for current monitoring. If you are at anything less than full power, most current meters will not be able to accurately measure the changes in current from a PWM output.
Al - We're using a Fluke 336 clamp on ammeter to measure our DC motor currents. Am I correct in believing that it should read the average DC current in the pulsed waveform out of the Victor? Is there something better we should be using?

Bruce
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Unread 02-14-2006, 11:05 AM
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Re: FisherPrice Problems

Bruce,
The 336, as with most clamp on ammeters and VOMs are only good to 400 HZ. The output of the speed controller is a 2 kHz rep rate so the pulses are even shorter than that. That allows for a lot of error to build up at less than full throttle. I am guessing that you might be reading 50-60% of what is actually taking place. It is a good tool to check between motors and will give accurate results when the controller is producing full throttle. It is a tool many teams don't have in their tool box but should.
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Unread 02-14-2006, 11:35 AM
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Re: FisherPrice Problems

Heat sinks most likely will not solve your problem. Take a look at the gear box motor mounting. Did you block any vent holes? If so see if there is a way to reestablish the motor venting. The small can motors rely on the integral fan to move air and provide cooling. If the motors are venting then the load must be reduced.
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Unread 02-14-2006, 11:41 AM
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Re: FisherPrice Problems

Quote:
Originally Posted by Al Skierkiewicz
Bruce,
The 336, as with most clamp on ammeters and VOMs are only good to 400 HZ. The output of the speed controller is a 2 kHz rep rate so the pulses are even shorter than that. That allows for a lot of error to build up at less than full throttle. I am guessing that you might be reading 50-60% of what is actually taking place. It is a good tool to check between motors and will give accurate results when the controller is producing full throttle. It is a tool many teams don't have in their tool box but should.
Al - Thanks. I knew the output of the Victor was pulsed, but didn't realize the rep rate was that high (Note to self: Read the nice manual.) I was also thinking the meter would read the average current under the pulses, regardless of width, a holdover from my old "analog thinking."(<sigh> I miss the days of the old Simpson 260.)
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Unread 02-14-2006, 12:21 PM
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Re: FisherPrice Problems

Check the IFI specs. The 883 was about 2k output feq. The output feq of the 884 is low about 120. This was done to give control below 10%. Using a UEI digital clamp on amp meter VS an old Amprobe analog clamp on I'm getting less than 10% variation in current readings.
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Unread 02-14-2006, 03:13 PM
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Re: FisherPrice Problems

Quote:
Originally Posted by Gdeaver
Check the IFI specs. The 883 was about 2k output feq. The output feq of the 884 is low about 120. This was done to give control below 10%. Using a UEI digital clamp on amp meter VS an old Amprobe analog clamp on I'm getting less than 10% variation in current readings.
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
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Unread 02-14-2006, 04:22 PM
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Re: FisherPrice Problems

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.
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Unread 02-14-2006, 08:19 PM
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Re: FisherPrice Problems

Quote:
Originally Posted by Joe Johnson

FYI, teams I am associated with typically buy Freeze-It by the case ;-)
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...
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Unread 02-14-2006, 08:28 PM
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Re: FisherPrice Problems

Quote:
Originally Posted by Gdeaver
Heat sinks most likely will not solve your problem. Take a look at the gear box motor mounting. Did you block any vent holes? ...
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
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Unread 02-14-2006, 09:28 PM
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Re: FisherPrice Problems

Try reading the current after the breaker, going into the victor.
  #26   Spotlight this post!  
Unread 02-15-2006, 07:11 AM
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Re: FisherPrice Problems

Quote:
Originally Posted by Not2B
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...
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.
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Unread 02-15-2006, 07:15 AM
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Re: FisherPrice Problems

Quote:
Originally Posted by Gdeaver
Check the IFI specs. The 883 was about 2k output feq. The output feq of the 884 is low about 120. This was done to give control below 10%. Using a UEI digital clamp on amp meter VS an old Amprobe analog clamp on I'm getting less than 10% variation in current readings.
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...
http://web.archive.org/web/200410222...3datasheet.pdf
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/200208052...Victor_884.htm
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Last edited by Al Skierkiewicz : 02-15-2006 at 07:24 AM.
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Unread 02-15-2006, 08:37 AM
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Re: FisherPrice Problems

Quote:
Originally Posted by Al Skierkiewicz
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...
http://web.archive.org/web/200410222...3datasheet.pdf
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/200208052...Victor_884.htm
OK, I'm confused. We found this:

http://www.ifirobotics.com/forum/vie...61922d031d945b

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.
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Unread 02-15-2006, 08:53 AM
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Re: FisherPrice Problems

Quote:
Originally Posted by Bruce C.
OK, I'm confused. We found this:

http://www.ifirobotics.com/forum/viewtopic.php?t=385&view=previous&sid=f42e6c376913 1bacaf61922d031d945b

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.
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Unread 02-15-2006, 10:26 AM
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Re: FisherPrice Problems

Victor 883 - 2000 Hz
Victor 884 - 120 Hz

Direct from the designers.

-JV
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