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Unread 01-09-2010, 21:18
Unsung FIRST Hero
Al Skierkiewicz Al Skierkiewicz is offline
Broadcast Eng/Chief Robot Inspector
AKA: Big Al WFFA 2005
FRC #0111 (WildStang)
Team Role: Engineer
 
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Re: Speed Controller Design

Geek,
I know it is hard to see but one FET driver only controls one of the pair. The other FET is controlled by the second FET driver. Note where the motor is connected in the schematic you referenced. (Don't look at the driver sheet, you have to look at the jaguar schematic) In this way it is possible to get the control needed for speed control of the brushed CD motor.
In the original design, the FETs used could handle about 40 amps each for short durations due to the internal heating of the FET. With three in parallel, the current capabilities almost met the stall current on a CIM (129Amps). In addition, the original design had a Fairchild driver circuit that in addition to being discontinued by Fairchild also had been designed for 7 amps of gate current. For the most part, the series resistance of most robot designs limited current so that stall on the CIM was less than 120 amps. For some teams that choose #10 or larger wire and short runs could cause significant currents to flow through the Jaguar. There is also a small resistor in the Jaguar that is used for current sense. The voltage across this resistor feeds into the controller for over current protection and makes it also available on the CAN buss for feedback. It was the Fairchild part that seemed to be the cause of a large number of the failures experienced last year. The failure where a motor could only be controlled in one direction was the failure of one of the gate drivers.
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.