Our team has blown 3 talon srx modules. We have been watching very carefully to try to understand how they got fried. The common element seems to be that they stop working when we move the motor connected to the talon manually. My theory is that the motor is generating a voltage that is backfeeding the talon and frying it. We believe at least one of the talons was fried when there was no 12v power on the talon.
We did verify that there is no short to ground or other kind of short. Each time the talon fried it happened on a different motor.
The motors are generating voltage when moved, and if there it’s no load presented by the talon then the voltage can be very high.
Had anyone else seen this problem?
We did send the first two units back to crosstheroads electronics but have not heard back from them on the analyses. They did replace the first two.
We will send the third talon back for analyses as well.
The purpose of this thread us to try to understand why we are blowing talons, and to see if anyone else is having a similar problem.
Manually rotating the motors does generate current and can fry motor controllers. We have fried many Victors and Jaguars this way. We have adopted the following “rule” because of this:
Do not push the robot. Do not manually turn mechanisms. Pick up the robot or use a cart to move it.
Also, we’ve started adding connectors between the motors and the controllers. Among many other benefits, one is being able to disconnect the motor if we need to manually rotate the motor.
We have used Victors for years and we have never had one fry because we moved the robot without using a cart.
You can actually see the status lights lite when we move the bot.
Can either of you offer an explanation as to why numerous teams have pushed numerous robots great distances with power on, or off, and have never suffered a blown motor controller? What about mechanisms (like the drive-train) that create the same back-current as they coast down?
If your theory cannot explain all present evidence, you may want to revise your theory.
In all my years in FRC I have never blown a Victor. Jaguars, sure, never Victors though.
Back in the IFI controller days, I used to jump start the RC to get it to power itself off the backup battery by pushing the robot so I could download code with out a battery. I’ve since learned this is a bad idea and in general we try to avoid pushing the robot quickly, though we haven’t broken a speed controller because of this that I know of.
Pushing a robot turns motors which generate voltage, and that voltage is passed through the motor controller’s MOSFETs to the battery terminal which subsequently can power the robot. The current path from M+/M- to V+/V- in the motor controller is done through the MOSFET’s reverse body diode and they act as a rectifier.
You don’t want to pass current through these diodes since they’re not intended to pass significant current (they’re parasitic), and when they do pass current through them, the power dropped across them is the forward diode voltage times the current. This is why the Black Jaguar, Talon SR, and Talon SRX use synchronous rectification (SR) when driving the motor. SR turns the low side of the H-Bridge on during the “off” phase of the drive. Unfortunately, when pushing the robot motor controllers are not doing SR.
When testing Black Jaguars, the team used a test bench with two CIMs, the output shafts coupled to each other through an AM ToughBox. One CIM was driven from the motor controller and the other used as a generator with the leads connected across a switchable bank of 1 ohm power resistors. You’d be surprised at how much power you can draw out of a CIM when the shaft is turned.
Turning a motor when it is unpowered does generate a voltage. That voltage is no different in principle from the back EMF that the motor generates when it is powered.
Unless you’re backdriving the motor much faster than its free speed, I don’t think you’re going to create a situation where the speed controller is in any danger.
Back driving a motor on a Talon SRX, Victor SP or Talon SR will not cause it any harm. The back EMF that gets generate is no more harmful than the forward voltage that is applied during rectification.
I REALLY want this to be understood so teams do not needlessly adopt measures to prevent something that is not even a problem.
After discussing this with Mike at CrossTheRoadsElectronics, he and I have concluded that we probably damaged these Talon SRX drives with ESD (ElectroStatic Discharge) when we were assembling them.
Mike took apart the first two that were destroyed and reported that multiple chips inside the device had been damaged, and the only mechanism that he thinks fits the evidence is ESD. This is clearly evidence against my previous theory.
We are now up to 4 blown talon SRX devices. The last Talon SRX to die was under power and moving a bag motor, so it is somewhat different than the other 3 that died. It is clearly evidence against my previous theory.
We think that a combination of extremely dry conditions (i.e. no humidity) and assembling in a carpeted room may have contributed.
We will now start start following standard ESD handling rules when assembling these devices. This will include humidifer, grounded anti-static mats, and getting the kids to wear anti-static wristbands when wiring the robot. We may also spray anti-static spray on the carpet where we do our assembly.
I do want to give CrossTheRoadsElectronics Kudos for being extremely supportive and willing to extensively discuss this issue with us. I feel that they are a very good company.
What do you mean by assembly? Just putting on connectors/pwm?
If so, I have a really hard time believing that ESD was the problem. Our team has used hundreds of Victors over the years, and has never heard of an ESD problem. We have used absolutely no ESD protection ever with the components, and many teams do the same. If the new controllers were super ESD sensitive, then many more people would complain.
If you are skeptical of this, then given the evidence, what is your theory?
I am certainly open to other theories, as my main goal is to figure this out and stop the bleeding!
Here is a quick summary:
2 blown talon SRX devices when working on the robot, may or may not have been moving the motor (we just didn’t notice exactly when they stopped working). These 2 were on different motors, and did not blow at the same time. When we replaced the talon we did not change any wiring or any motor, and the talons that were replaced are still working fine. The Talon SRX devices have no LED after they are blown, i.e. the CPU and the regulator were fried. These two were looked at by CrossTheRoadsElectronics and found multiple chips were fried (just nonworking and shorted, the tops were not blown off the chips).
1 more blown Talon SRX, this one apparently went out while we were moving the motors manually, and it may have been powered or not powered on by the battery, not sure. Again the sympton is the LED does not come on at all. Changed the Talon, did not change anything else, replacement Talon is still functioning. This one has not yet been returned for post mortem.
Final Talon SRX blown when moving a bag motor. I will verify the symptoms (i.e. no LED) this evening, as I haven’t seen it in person yet.
This one has not yet been returned for post mortem.
Note that we are using the PD board with circuit breakers, and we are using a standard FRC battery. This robot has been very carefully assembled, we did not connect up the Talon SRX modules backwards or get the battery polarity backwards. All connections were done with solder, covered by heatshrink (although we are switching back to high amperage connectors now because we suspect we may have to change out Talons in between matches if we can’t solve this problem)
Dry air, carpet, and electronics are not a good combination. I think your best remediation will be using an anti-static mat to cover the carpet. Wrist straps will help a lot, but I am confident that they will not always be used when appropriate.
Were the chips that CTRE found to be damaged connected to “the outside world” or were they only connected “internally”? The ESD would have been amazingly powerful to cause damage past the first device it hits. ESD can cause mis-operation of a device when injected into the system while it is running but I doubt that you would be touching those connections while running your robot.
Perhaps CTRE may want to check their supply chain for counterfeit parts. We have had several instances at work where counterfeit parts sort of work but caused failures where there was no “abuse” of the system.
We have had two Talon SRX controllers drop from the Can network after working for a day or longer each and then complete turn up dead…no signal lights … Not on the network…: just dead with after being fully functional. I have not contacted CTRE yet but will.
