Issue with Talons
 

Hi all, I was hoping to get some help with a strange problem with some Talons.

Tonight we decided to do driver tryouts with a simple robot consisting of a drive train with two CIMs to each side, all controlled over Talons. Normally it drives fine, but occasionally while driving straight the side going forward (the lights on the Talons are green) reverses for a very quick moment, going backwards. The lights on the Talons turn red when this happens. We are using simple Arcade Drive code, and I've checked the values being sent to the Talons in the code, and they are not reversing at all there. This happens when driving straight forward and backwards at full power (so all 4 Talons have had the issue), but not when purely turning.

We think there is an issue somewhere between (and not including) the crio and the Talons themselves, since it happens to all 4. Additionally, once when powering the robot on one side of the robot moved at a high speed for a very short amount of time, which leads us to rule out code issues.

Tomorrow we plan on replacing the Digital Module, the Digital Side Car, and the cable connecting the two one at a time to see if any of them are causing the issue. Is there anything else we should be doing or checking for that anyone can think of? I tried to give all relevant info but if anything else is needed I can provide it. Thanks!

EDIT: Also, going to try swapping victors for the Talons.

Re: Issue with Talons
 

I'd like to add that my team has experienced the same problems... No idea how to fix them.

Re: Issue with Talons
 

Here's what we have already tried and some additional info:
We tried turning the robot off and back on. Sometimes this fixed it, other times it didn't, and the problem would intermittently come back after varying amounts of time.
All 3 lights on the digital side car are on.
The observed effect is that the robot is commanded to move forward (and only forward) and it starts out doing so, then pulses a turn to the left, then continues on its way forward.
This happens only when the talon is commanded to go in the green direction. It does not occur in the red direction (or we have yet to observe it).
All 4 of the talons are in coast mode.

Re: Issue with Talons
 

A brief twitch in reverse can be a sign that the robot is getting disabled for a moment. If the PWM signal is interrupted for just the right amount of time, it is possible for the speed controller to misinterpret the interruption as a valid command. Look at the Diagnostics tab of the Driver Station and see if there are any helpful error messages.

Re: Issue with Talons
 

Have you tried calibrating the Talons?

Re: Issue with Talons
 

Make sure you do these one at a time. This way if the issue gets resolved after swapping one thing, you can trace the problem to a specific item.

Re: Issue with Talons
 

I would interpret the LED change as a sign that the PWM signal did in fact change state. However, I would like to see Mike C weigh in on this.

Re: Issue with Talons
 

Yeah, we have calibrated them.

Re: Issue with Talons
 

After printing out the voltage values being sent to the talons to the console, we found nothing that suggests the PWM is changing direction. This leads us to believe it is something other than the code and likely down stream from the cRio causing the PWM signal to flip the other way for an instant.

Re: Issue with Talons
 

Last year we had something similar happen to us.
Observing the PWM output from the Digital Sidecar with an oscilloscope revealed that the PWM signal was dropping out occasionally. Replacing the DSC fixed the problem.
My $.02.

Re: Issue with Talons
 

Our digital sidecar this year had a problem with DIO Channel 7 not working, I wonder if there was a problem with the production of the sidecars this year that is causing some of these problems.

Re: Issue with Talons
 

We have had a host of problems with the DSC, and had to replace three of them last season. I would definitely look at testing with a new DSC.

Re: Issue with Talons
 

Also take a look at the ribbon cable from the cRIO to the DSC, especially if you are using one that required repair from last year. Dodgy connections on the ribbon cable can cause strange behaviors.

Re: Issue with Talons
 

I don't have any issues with either the design or reliability of the DSC. It's just that it's such a swarf magnet. You really have to be careful working around them.

Re: Issue with Talons
 

We too seem to be having an issue like this.

The problem is noted as a drive train twitch. We're using the default drive code (tank) without any modifications.

The issue is experienced with the Talons; however, we're only using the PWM interface with the Talons. All other motor drivers are connected using the CAN bus.

No one is near the controls when this occurs. It is just a brief twitch. I'm a bit worried reading this because we haven't had a good chance to run the robot much yet.

I looked at the schematic for the Digital Sidecar, and I'm not seeing how it can be an intermittent problem with the DSC.

Link for the schematic can be found here.

The PWM signals originate directly from the NI 9403, so the DS isn't actually supplying the PWM signals. There are three 'chips' that touch the PWM signal.

1. A 10K pullup resistor used for a disconnected DSC. Resistors don't intermittently fail. However, a cold solder joint might cause an intermittent connection; however, this would only affect the system IF AND ONLY IF the 32-pin cable becomes disconnected.
2. A disable chip. There is a chip, in line, with the PWM signals. This relies on a signal defined as OUTPUT_EN. If this signal toggles while sending a NEUTRAL PWM signal to the speed controllers, this could cause the speed controllers to 'twitch.' However, the OUTPUT_EN line should remain HIGH while the robot is ENABLED.
3. A current limiting resistor. It's placed between the disable chip and the actual output. Again, resistors don't intermittently fail, and the only issue I can imagine is a bad soldering job.

Just going on a wild hunch... How many of you experiencing this issue are using the home-made DSC cable that connects your DSC to the NI 9403? It's possible that there is an intermittent or high resistance (bad connection) pin that is causing this issue. Looking at the pinout, OUTPUT_EN is DIO_21 on the NI 9403. First, try a different (real) cable. I don't see much failing in the DSC, so I'm pointing my finger at the cables right now.