Speed Controller Design

[Ether]

Originally Posted by kamocat

To run at half speed forward, you have a 75% duty cycle. Backward at half-speed, you have a 25% duty cycle.

Marshal,

Have you confirmed the above experimentally?

[kamocat]

No, I haven't.
Every Jaguar that I haven't fried I've kept together. However, it seems to be what makes sense.
I believe there is a thin conformal coating on the Jaguar, so I would not be able to probe the microcontroller directly. However, I can probe the Jaguar output if you like.

If you're asking about the PWM drivers, all the MOSFETs are identical (all N-channel), and so switching the high and low should work fine.

[EricVanWyk]

Originally Posted by Ether

Marshal,

Have you confirmed the above experimentally?

I have. It is referred to as "locked anti-phase", and is usually used when you need fine control while nearly off.

EDIT:
Locked anti-phase is only applicable with high chop frequencies. Locked anti-phase could work on a Jaguar, but not on a Victor.

[Ether]

Originally Posted by EricVanWyk

I have. It is referred to as "locked anti-phase", and is usually used when you need fine control while nearly off.

EDIT:
Locked anti-phase is only applicable with high chop frequencies. Locked anti-phase could work on a Jaguar, but not on a Victor.

Is this the default operation mode of the Jaguar ?

[kamocat]

I'm pretty sure this is the only option on the Jaguar.
Even in coast mode, when I take a motor down from full speed to 10%, it goes down like it was braked. (It also draws a lot of current in doing so, tripping the crowbar on my 9A power supply, even if the motor is free-running. It performs fine on battery.)

[Ether]

Originally Posted by kamocat

Even in coast mode, when I take a motor down from full speed to 10%, it goes down like it was braked. (It also draws a lot of current in doing so...

Is this with open-loop (voltage) control, or with closed-loop speed control ?

[Gdeaver]

Black jags switch different than tan jags. The tan jags are high side switchers. The black jags are locked antiphase. Allegro makes a nice fet full bridge automotive driver chip. The A3941K. The data sheet gives a good description of the different ways the Fet bridge can be driven. May be it will help.
http://www.allegromicro.com/en/Produ.../3941/3941.pdf
Warning to understand this chip requires allot of back ground Knowledge but the diagrams and tables help with the fundamentals. This chip is used on some of Pololu Robotics motor controllers. http://www.pololu.com/
The Jags really are a deal for FIRST teams compared to many other less capable drivers.

[Ether]

Originally Posted by Gdeaver

Black jags switch different than tan jags. The tan jags are high side switchers. The black jags are locked antiphase.

Well that's certainly useful to be aware of. Maybe black Jags shipped to FRC teams should come wrapped in yellow caution tape with a warning to this effect.

[Geek 2.0]

Actually, I find that A3941K chip very intriguing... I might end up using it in the future, because it provides a lot of nice features.


Why is it that they use multiple MOSFETs per H-Bridge leg? Is it to provide more current?

If so, could I use such a configuration on another Gate Driver, such as the A3941K?

[Geek 2.0]

Okay, for the sake of education (and that I've exhausted the extent of Wikipedia's ability to have a page on every minuscule topic) could someone please give a better definition of locked antiphase? I understand that it's using a PWM where a 50% duty cycle is brake, 100% is full forward, 75% half, 25% half reverse, etc. But maybe an illustration or something to that effect? I'm kinda swimming in all these terms...

Also high chop frequency. I have only a very slight idea of what that is, and how it affects Victors and Jaguars differently.

Thanks!

[Ether]

Originally Posted by Geek 2.0

Also high chop frequency. I have only a very slight idea of what that is, and how it affects Victors and Jaguars differently

The Jag's output PWM frequency is 15,000 Hz. The Victor's is 150 Hz.

[Geek 2.0]

Originally Posted by Ether

The Jag's output PWM frequency is 15,000 Hz. The Victor's is 150 Hz.

That only makes too much sense, thanks. Where would you possibly draw the line for what works with locked antiphase?

[Joe Ross]

Since you're in control of the design, you might try designing it so that it can work with a variety of chop rates and different output methods and see if you can answer the questions in the following thread: http://www.chiefdelphi.com/forums/sh...ad.php?t=77297 and this thread: http://www.chiefdelphi.com/forums/sh...ad.php?t=83973

[Al Skierkiewicz]

OK,
Time for a little better definition here. The Jaguar and the Victor differ a little in the method of motor control but not much. The H bridge is basically the same for both, four sets of FETs. To drive in one direction a FET must be connected to the high side and another to the low side. In your drawing above, the diagonally connected FETs are turned on for this mode. Q1 and Q4 for one direction and Q2 and Q3 for the other direction. (Observe current flow through the motor when visualizing these modes.)

Where the Jaguar and Victor differ is that in the Victor, both FETs in the pair are controlled by PWM signals while in the Jaguar, only the high side FET is PWM while the low side FET is ON for direction.

The Brake Mode jumper operates in a similar manner for both types. The Brake Mode will turn on both low side FET pairs in a zero throttle condition while the Coast mode will turn off all FETs. Note that during Brake Mode, current supplied by the motor is shunted through one pair, the common power supply lead and then through the other pair. During Coast, no current flows.

The locked anti-phase that Eric refers to is a condition where the controller is supplying a 50% duty cycle of forward and reverse commands essentially locking the motor shaft in position under power. Please note that this could be a relatively large current demand as current is flowing in the motor all the time. Gary, correct me if I am wrong, but I was under the impression that both the Tan and Black Jaguars could only provide lock mode (analog control mode) under CAN control with encoder input provided at the controller or through the buss. The difference between the two is essentially the RS232 input and the change in the FETs to provide 24 volt input on the Black.

[Ether]

Originally Posted by Al Skierkiewicz

The locked anti-phase that Eric refers to is a condition where the controller is supplying a 50% duty cycle of forward and reverse commands essentially locking the motor shaft in position under power

locked anti-phase does provide some damping (resistance to sudden changes in speed), but it doesn't "lock the motor shaft in position". If there is a load on the motor, the motor will not stay locked in position.