"Testing" motor via Jag output terminals

[Danny Diaz]

This is more a "is it safe" versus a "is this legal" kind of question about performing electrical tests on the robot prior to the code team being complete with their work. Often our electrical team wires up PDB->Jags/Victors->Motors and immediately want to know if the motors work correctly (but are too lazy to remove the connections to the Jags/Victors). So, they unplug the robot battery and grab some alligator clips and momentarily connect the battery directly to the M+/M- terminals on the Jags/victors (via the alligator clips) so that they can see that the motors are running and in the correct direction. Sometimes they do this with a 7.2V battery instead. And, of course, they usually also leave the breakers in place so the battery voltage temporarily powers the robot.

Even though they've never damaged any robot components in any noticeable way (so that I could point out the danger in what they're doing) by doing this, I still yell at them when I catch them doing this. My question is, "Have we just been getting lucky that they haven't fried something?"

-Danny

[v2oxviesta]

Yes. This has the potential to break the jags as well as the motors (And if you are really unlucky the IO and the cRIO). There is probably a safety limit on the jags but I would not push it.

[Bochek]

While in theory, the Jags should have protection for this, i would not recommend it as a standard practice.

You have to remember that when you push your robot, the motor that you'r spinning is creating a voltage the same as what you are doing with the alligator clips and battery.

I have accentually done this once before, and the Jag survived no problem. But again, I do not recommend doing that.

[Danny Diaz]

Originally Posted by Bochek

You have to remember that when you push your robot, the motor that you're spinning is creating a voltage the same as what you are doing with the alligator clips and battery.

Yeah, that's the standard rationale for why it's okay. I just don't like that rationale.

-Danny

[EricH]

The voltage when backdriven may be the same as if you applied the batter. I'd be VERY surprised if the current is the same, though! What's the old saying, "current kills"? (High voltage can kill, but it's often accompanied by high current when it does.)

[cgmv123]

If you put quick-disconnects in between your motors and jaguars, you can connect your alligator clips to the quick disconnect instead of the jaguar, protecting the jaguar.

[Danny Diaz]

Originally Posted by cgmv123

If you put quick-disconnects in between your motors and jaguars, you can connect your alligator clips to the quick disconnect instead of the jaguar, protecting the jaguar.

Yup, that's usually what I do - but I personally don't like quick-disconnects due to the fact that they can, well, disconnect. Sure, sure - stress-relief strategies, layout strategies, wrapping in electrical tape; lots of ways to help prevent them from pulling apart. I was trying to (mostly) get away from press-on quick disconnects - anyone know of any GOOD disconnects that actually have positive reinforcement (other than a friction hold)? Another thing we sometimes do is just drop some solder on the connections (if they're unshielded connects) to ensure they won't come out during competition, but I was trying to prevent from having to do that too (I already crimp AND solder all connections between wires and connectors).

-Danny

[WizenedEE]

I've heard that you can plug in some normal RC car receiver PWMs and get the jaguars to power stuff.

Also, why don't they just test the motors before they wire them up to the jaguars? Or just disconnect them from the jaguars? Or just have the programming team spend the five minutes making a new project that just runs a jaguar based off of a joystick? On our team, the mechanics get cranky whenever the programming team don't get their motors moving within five minutes of mounting (no pun intended).

But yeah, there's lots of ways to easily run the motors without having the jaguars along for the ride. And if they're too lazy to disconnect the motor from the jaguar, get them some spade connectors instead of ring connectors for the jaguars - then it only takes a half turn with a screwdriver to get the wire off (and it's highly recommended for the gray jags you use them anyway).

[Al Skierkiewicz]

Danny,
Yes you have been lucky, very lucky, for a variety of reasons. One, the safety issue of having battery voltage present on clips allows the potential for shorting the clips together with no fuse/breaker and a battery capable of welding the clips together if they should touch. At that point, the wire on the clips turns incandescent as 600 amps begins to flow. If you are lucky to be able to drop them before injury occurs, the smell of burning plastic will permeate the room, your clothes and other materials close by.
While it is true that pushing the robot does back feed voltage through the output devices, it is not a good idea. When pushing the robot, you have to get the robot moving at least as fast as it travels at full throttle to match the battery voltage you are trying to apply. Using a 7.2 volt battery helps some. However, think what might happen if someone clips the battery in while the main robot power is still applied and gets it backwards.
A good rule of thumb is to test a device as it will be used. In this case with battery connected and commanded by your DS. We have the students check each other's work as the robot is wired, then check it again when complete, then a mentor checks it. Then we apply power with all breakers removed and start adding them one by one looking for problems, no power indication on the device under test or smoke. With this method if something is wired wrong, only one device will give it's life for it's brothers and sisters.

[kramarczyk]

How is this different than accidentally wiring the jags backwards? Last time I checked that was a process that destroyed hardware.

[kamocat]

Originally Posted by Danny Diaz

Yup, that's usually what I do - but I personally don't like quick-disconnects due to the fact that they can, well, disconnect. Sure, sure - stress-relief strategies, layout strategies, wrapping in electrical tape; lots of ways to help prevent them from pulling apart. I was trying to (mostly) get away from press-on quick disconnects - anyone know of any GOOD disconnects that actually have positive reinforcement (other than a friction hold)? Another thing we sometimes do is just drop some solder on the connections (if they're unshielded connects) to ensure they won't come out during competition, but I was trying to prevent from having to do that too (I already crimp AND solder all connections between wires and connectors).

-Danny

Yes, there is a good connector for this. They're called "Anderson PowerPoles" (APP), and are similar to the connector you use between your battery and PD board (an SB-50).
http://www.powerwerx.com/anderson-po...ings-contacts/

My team uses red/black for the input side of motor controllers, and white/green for the output side.
They also make an excellent way of adapting the battery chargers to an SB-50.

[Al Skierkiewicz]

Danny,
There is also an "L" shaped terminal that has a hole for mounting on the Jag or Victor. They are available from Digikey. Check my electrical presentation on the WPI site for pictures. This is similar to the old fuse block outputs. You use push on terminals to mount on the tabs you add to the controller. We use them on both input and output. It makes changing a dead controller into a two minute job.
Mark, you are not pushing the robot at a fast enough speed to force 12 volts backwards into either controller. A battery so connected not only forces 12 volts into that path but currents may exceed the output FETs specifications. Remember the current flows through the diodes internal to the output FETs.

[kramarczyk]

Originally Posted by Danny Diaz

... So, they unplug the robot battery and grab some alligator clips and momentarily connect the battery directly to the M+/M- terminals on the Jags/victors (via the alligator clips) so that they can see that the motors are running and in the correct direction.
...
My question is, "Have we just been getting lucky that they haven't fried something?"

Al, this is the part I was referring to. I was not worried about pushing the robot.