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Unread 03-09-2010, 09:01
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Al Skierkiewicz Al Skierkiewicz is offline
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AKA: Big Al WFFA 2005
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Re: Speed Controller Design

Ether,
The switching of the FETs was explained to me by a member of the Luminary team at the end of 2009. I have no documentation that either of the Jags are locked anti-phase devices in normal control although it is my understanding that all controllers are capable of this action. There is no mention of locked anti-phase in either of the manuals for Jags. The brake mode on both Jags is described in the literature as acting the same, a simple dynamic short across the motor with both low side FETs turned ON. In the description for both Jags, there is a caveat that while this mode does provide braking at zero speed it should not be considered a hard brake that prevents movement.
While there is minimal current if the duty cycle is low in locked anti-phase, a condition where the duty cycle is 50% (common use), even in the Jag the current has a chance to rise to near full level due to the inductance of the motor.
As I said in an earlier post, the Fairchild device is reportedly out of production. This had a lot to do with the change in FET drivers in addition to the decision to move to 24 volts.

Geek,
PWM signals are used in two ways in the speed controllers. The input PWM for hobby interface is a defined standard that is used to send control signals to servos and speed controllers for both direction and speed. The output of the speed controllers is also PWM but in no way is it similar to the input signal. For the motor side, 50% duty cycle means the motor is supplied current for 50% of the time. That is, a 3.3mSec pulse for the Victor or a 33 microSec pulse for the Jags. For direction the output current actually changes polarity.

For all,
It is my understanding that both Jags leave the low side FETs ON during all throttle conditions except zero (subject to which direction pair is selected) and during locked anti-phase (where the direction is changing at a 50% duty cycle rate). I think it has to do with the need for charging of the gate drive bootstrap capacitor. The Victor designers chose to open both FETs during the OFF period. In early designs of the Victor, the switching frequency was 2kHz. Engineers at IFI chose to move to 150Hz to give maximum low throttle torque for the motors we were using at the time. 150Hz is less affected by the inductance of the motors. It does however, give the impression that low speed linearity suffers and it does cause some acoustic output for the motors it is controlling. 15kHz switching does interact with the inductance of the motors and is sufficiently high to cause no discernible acoustic noise except in the smaller motors.
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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.

Last edited by Al Skierkiewicz : 03-09-2010 at 09:06.