Jaguars...is this ok?
 

Hey,

Last year when we used the victors, we were able to throw our bot from forward to reverse pretty quickly. This year, the jaguars lock up and start blinking red if we try that. They stay like that for 3 seconds, but then quickly go back to normal. Is this just a safety feature or a victim of our code?

Re: Jaguars...is this ok?
 

try putting the default code on there and see if it still does it.

Re: Jaguars...is this ok?
 

That sounds like the condition discussed in the 7th post here: http://www.chiefdelphi.com/forums/sh...ad.php?t=71303

Re: Jaguars...is this ok?
 

It sounds very much like you're hitting the overcurrent fault in the jags. Whipping a motor from full forward to full reverse is going to pull an enormous amount of current, especially if there's any inertia attached to it. A CIM motor at stall pulls 133A. A CIM motor suddenly whipped into reverse while running full forward will pull darn near twice stall current until it actually stops and turns around. The jags have a two second, 60A current limit. But I'd bet it's significantly shorter is you're pouring 200A through the poor thing.

So, for the benefit of your jaguars and your entire drivetrain, you should probably either coach your drivers to move a bit slower, or put an acceleration limit on your pwm outputs.

Re: Jaguars...is this ok?
 

Kevin,
I do not believe the motor would draw more than stall current in this configuration so I think there is something else taking place. As with any electrical device, they are subject to both input power and load variations. I suspect that the sudden load may be drawing down the power supply to the controller. This is possible with loose electrical connections, under size wiring and the possibility that the final gear ratios are too high. Teams don't expect that breakers are tripping but they may be and so are interupting the power input to the Jaguar.
From Luminary Micro...
Fault Conditions
A slow flashing Red LED indicates that the MDL-BDC detected one of the following fault conditions:
�� Power supply under-voltage
�� Over temperature
�� Over current
�� Limit switch activated in the current direction of motion
When a fault condition occurs, the motor shuts down and the LED indicates a fault state during the fault condition and for 3 seconds after the fault cause is cleared (except for the limit switch fault, which is cleared instantly). A slow flashing Yellow LED indicates that the MDL-BDC is not receiving a valid control signal.

Any of the first three could be occuring in your robot.

Re: Jaguars...is this ok?
 

I think that Kevin's right regarding double stall current when undertaking a full reversal. For example, if you apply +12V to the motor and get it running at full speed with little torque, the "motor voltage" will be +12V. If you suddenly apply -12V to the motor, there will actually be 24V across the winding - 0V on one motor lead and 12V on the other motor lead but an "internal" voltage of -12V. Specifically, I'm thinking of the standard motor model of resistor + inductor + "motor EMF".

If the motor and load have a large rotational inertia, it could take a while for the motor EMF to reverse direction.

It would be wise to add a bit of filtering to the PWM command to slow down the changes.

For what it's worth, the Jaguar and Victor power stage and heat transfer designs appear to be very similar (ie similar FETs, mounting, arrangement and airflow). Why would Jaguars be more sensitive to full reversal? I'm not sure but I can think of one thing - I wouldn't be surprised if the Jaguar is more efficient. They may be driving their FETs harder and therefore more fully on/off and, as a result, the actual currents to the motor could be closer to the ideal 130A stall currents. However, if they're more efficient, wouldn't that mean less power dissipation? Maybe, but high instantaneous pulse currents can be just as damaging as high power (ie heat output) levels.

Considering the slippery floors, it may be much easier to get to full motor EMFs (not much torque output).

It wouldn't hurt to filter the PWM command to 1) reduce full fast reversal effects and 2) full fast reversal is really "impossible" anyways since you're guaranteed to lose traction.

Russ

Re: Jaguars...is this ok?
 

Russ,
In using your analysis and looking at a spinning motor as an equivalent voltage source this was my thought...
During that time when the motor is supplying current but the controller has turned on the alternate set of FETs, any additional back EMF current should return through the diodes in the FETs on the off side of the bridge. The effect of the H bridge, I think produces a complex load on the motor until the point at which the motor current supplied back to the bridge falls to a point at which the diodes stop conducting. I would suspect that during that period and until the motor starts moving in the opposite direction, the motor would then draw about stall current (or a little higher) but for a slightly longer time (msec) than normal motor start.
In looking at the two FET types (Jaguar vs Victor) it would appear that the Jaguar FETs have about half the ON resistance (6mohm vs 12mohm), a little more maximum current but the current is limited by package (note 1 on the data sheet.) The FET diode on the Victor is capable of higher current though.

Re: Jaguars...is this ok?
 

Al and Russ,

Thank you for your insight. Even though this probably won't be a common occurrence on the plastic floor, if we return to carpet next year I could see this becoming a big annoyance with the Jaguars.

Re: Jaguars...is this ok?
 

Next year if the can option is available, this could be handled in software. Also the manufacture may have to modify thier firm ware to handle these cases. First teams will have to adjust to protected speed controlers.

Re: Jaguars...is this ok?
 

Next year, you can specify a ramp rate for the Jaguar unit so this will be taken care of automatically, if you want.

Re: Jaguars...is this ok?
 

We've seen the same thing with our Jags, predominant when the battery gets low for some reason? Maybe the Jags reset???

Victors do not do that. One three second delay could be a super cell in your trailer.

-SB

Re: Jaguars...is this ok?
 

This problem is real and I saw it happen last week The good news is that our programmer and a mentor were working on it when I went to the work session today. Well, they were not working on the current surge but were trying to control the acceleration to limit slip. This is where Labview really shows it's promise. Our programmer found several pre-built VI's that can take the joystick input and filter it to give a nice acceleration profile that will limit slip and also as a plus control the current spikes to the Jags. The VI's are tuneable so you can play with them to get the feel the driver is happy with. Thank you LABVIEW. This would have been a mess to do with procedural code last year. I know that some teams are trying to get a velocity measurement from a wheel and encode on the floor and then use the encoders on the gear boxes to have a traction control system. It's a complex solution. Acceleration control is simpler but not as good.

Re: Jaguars...is this ok?
 

My team is having the same problem this year (2 cims per side) with the gray jaguars. We'd prefer an electrical solution so that we can do fast reversals; should we go back to the old victors? Are the new black jaguars any better?

Re: Jaguars...is this ok?
 

Mark,
Limiting speed in software or changing the gear ratios to something lower should help this problem. I believe this is an issue with the CIM motors running much higher than 40 amps so that when stall current is reached, it is for a much longer time interval than a transmission with a lower ratio.

Re: Jaguars...is this ok?
 

I understand that using a lower gear ratio or limiting the acceleration would fix the problem, but we'd prefer to move and accelerate as fast as possible. We've switched in the victors and are happy with the results, no more delays.

Re: Jaguars...is this ok?
 

Mark,
If you had problems with the jaguars then you are stressing the Victors as well. They just don't have over current trips but the current remains the same. If you hear clicking on the robot, that is the breakers tripping. That is a real bad sign. Make sure your students don't try touching the breakers after you shut down. They are likely to be HOT. Hot breakers will trip at an increasing rate since they are a temperature sensitive device. Should this be occurring, plan on having replacements available.