Jaguars, what can we use in FIRST 2010?

[Joe Johnson]

Will someone please have mercy on an old Mechanical Engineer? I have searched ChiefDelphi.com. I have done my best to look at the Luminary Micro site. In desperation, I've even tried to read the FIRST Manual!!! But all to no avail.

I have what I think is a simple question: Among the bewildering array of functions that the Jaguars can perform, which ones are legal and available to FIRST teams in 2010?

I understand that we can use CAN. Fair enough. But I don't know if all the features that are available via CAN are available to us. I am sure that this is available somewhere, but I'll be dag nabbed if I can find it. I was like Alice in Wonderland, chasing rabbits down holes trying to find it. I just kept seeing a cat's smile floating in space (was the cat a Jaguar? Who can say).

Seriously though, I just want to know if I can access the motor current feature. I would love to have that information without having to do anything other than asking the Jaguar what the current is (or rather, just listening as it tells me).

The reason I ask is that I want my controller guys to monitor motor speed for one of the actuators I am designing. I don't want to have to bother with an encoder if I don't have to. If I can get motor current for free (basically) then I can do a simple calculation (involving the current battery voltage, the PWM value of the Jaguar and the stall & free current of the motor I am driving) to get a pretty darn good estimate of the motor speed.

And it is essentially free (software is free, all ME's know that ;-)

But, is it legal and available to FIRST teams in 2010?

Please help.

Joe J.

P.S. If there isn't a white paper with the following title, there should be: "How to gain a competitive advantage by using the cool features of a Jaguar on a 2010 FIRST robot"

By the way, that paper is worth something in the high six figures in added sales to Luminary Micro. If they were smart they'd hire me to write it for them ;-)

[EricVanWyk]

My Understanding Of The Rules (TM) is this:

If you are using CAN, have at it, use everything (current included).

If you are using PWM, it is just a big Victor. No limit switches, encoders, pots, etc are allowed to be plugged into the jaguar.




Edit per Al's comment.

[Al Skierkiewicz]

Erik,
So as not to confuse, the Victors do not have any sensor inputs or outputs. You can use whatever you would like to monitor motor current, position, speed with whatever sensor you can think of that fits all the other rules, as long as it is external to the Victor.
Joe, another Joe, Joe Ross, is a good reference for the new control system.
As a reminder, the power wiring does contain some resistance that you should add to your calculations. #10 is about .001 ohms/ft., both leads please. (red and black) #12 is .0016/ft. To put it in perspective, a one foot length of #10 wire at 100 amps drops 0.1 volt. This is my definition of the "wire foot" of WF. Four good connections equals one WF, a Jaguar series resistance is 3.3 WF including the current sense resistor (which is in the return lead of the low side FETs). If you use the voltage sense in the Jaguar, you will be more accruate and can forget the resistance between the PD and Jaguar. You will need to add in the resistance between the Jag and motor. Some motors have internal thermal limits that may add some resistance as well as changing resistance with brush wear in. I suggest you mount the Jag close enough to the motor you are controlling that you can use the wiring that is attached to connect directly to the Jag (CIM motor) or as small a connection as possible for the other motors.

[Joe Johnson]

Originally Posted by Al Skierkiewicz

<snip>
As a reminder, the power wiring does contain some resistance that you should add to your calculations.
<snip>

Yes you are right, the voltage on the motor will not be as simple to calculate as it might be (e.g. battery voltage X PWM duty cycle) because there is some wiring resistance to factor in. But even with this, given the current, it is still pretty straight forward as compared to the pain and suffering associated with getting a sensor out on the end of an arm or some other inconvenient spot.

As to the particulars, it is a pretty simple thing to calibrate: command a known duty cycle, read the current from the Jaguar, read the battery voltage, read the motor terminal voltage, calc a wiring voltage drop, use V=IR to estimate the resistance of the wiring.

For extra credit, repeat this at several duty cycles & loads while approximating (or measuring if you can) the actual speed of the motor. A simple curve fit later and you can know motor speed for that motor in real time without having any other sensor than the Jaguar. Pretty nifty.

For double bonus you can even try to model motor temperature...

...but I don't suppose it's worth it. After all, I am just trying to keep my ball possessor rollers going at a constant speed. Let's not get crazy here... ;-)

Cheers,
Joe J.

[Al Skierkiewicz]

Joe,
Your response triggered something else I had forgotten. The Jaguars have a much higher switching frequency than the Victors (15kHz vs 150 Hz for the Victors). As such the inductance of the motors play into the rise times of the pulses supplied to the motor. This effect is particularly nasty at low throttle values but the effect is a more linear response of throttle command to motor speed than in the Victors. Due to the effects of the inductance, small motor pulses cannot reach full battery voltage during the pulse width.

[Joe Johnson]

Originally Posted by Al Skierkiewicz

Joe,
Your response triggered something else I had forgotten. The Jaguars have a much higher switching frequency than the Victors (15kHz vs 15o Hz for the Victors). As such the inductance of the motors play into the rise times of the pulses supplied to the motor. This effect is particularly nasty at low throttle values but the effect is a more linear response of throttle command to motor speed than in the Victors. Due to the effects of the inductance, small motor pulses cannot reach full battery voltage during the pulse width.

I've seen this a number of times in my automotive experience. We get a door working at 100 or 400Hz PWM frequency, then in the middle of a program, someone decides that that hum has just got to go... ...so we switch the PWM frequency to 8kHz or 10kHz or in one case 20kHz, but the system inevitably has some flaky behavior. The culprit is usually the effect you discuss.

Essentially, changing to the higher PWM frequency screws up all the motor calibrations for low duty cycles. A 10% duty cycle at 100Hz is a 10msec pulse which is FOREVER for these small PM motors so they get up to "full speed" in less a single pulse. Not true for a 10% duty cycle at 10kHz (= a .01msec pulse).

On the ups side, velocity control at low loads usually improves (for the same reasons only in reverse).

It's all good...

Joe J.