Victor Burnouts

[ConKbot of Doom]

Originally Posted by Sparks333

Al,
That sounds reasonable. My only point here is that some teams really gear their motors down, so it is sorta kinda maybe possible that by backdriving they are spinning the motors much faster than the wheels. I'm drawing at straws here, aren't I. What I saw was a team pushing their robot in a parking lot, then complaining of a burning smell. On closer examination, the victors wouldn't turn on. Since it was another team's, I couldn't get a close look at the vics, but a couple mentors blamed it on backdriving. I have avoided it ever since.

On our robot, driving the arm at any decent speed, (its powered by a geared down Van Door Motor) will provide enough juice to turn on the RC, and our LEDs will continue to blink running off the 7.2v battery after the motor stops providing power. I've even seen the fans on the victors spin up from pushing the bot on the floor. Not the big fans though... I think the major source of problems is if there is somewhere for the current generated to go or not. Because if the current cant flow, the voltage generated increases. That is what could possibly cause problems.

If this were the problem, it would be very possible to provide something to sink the current and keep voltage spikes down. A 120mm fan or two would do the job, or maybe some sort of lighting. Just make sure that the load is wired so that it is on when the main breaker is on. Can I say for sure that this is the problem? No. Is it a possibility? yes. Look at the schematic symbol for some FETs and you see a reverse biased diode, with the H-bridge, that would act as a rectifier, and power up the bot.

[Andy A.]

I'm with Al here on the backdriving. I've seen some LED's light up, and the fans twitch, but there can't be anywhere near enough current flowing to mess up the fets.

Or is there?

Could it be that with dynamic braking on, the current on some victors just ends up as heat as opposed to getting sent onto the rest of the harness?

I'm just wondering that maybe if dynamic braking is not on, all that backdriven current gets disapatied through all the victors, RC, etc., causing little if any problems. If braking is on, that current would 'stay' in the victor, right? Or am I just missing something key about how braking works on a victor?

Just grasping at some more straws.

-Andy A.

[Al Skierkiewicz]

Andy,
I am guessing on this, but I don't think the jumper keeps the brake on when the controller is not powered. If that was the case, you wouldn't be able to push the robot with the wheels on the floor. Also, as I remember, IFI started including a varistor across the output. This was something we previously had to add to the output terminals. The device is designed to eat up spikes from brush noise and back drive so I think it limits the voltage to 40 or 50 volts. I am going to have to look back at the specs on the devices we had to add in the past.

[Sparks333]

Al,
I can't remember the team number, but I will see if anyone else does.

Sparks

PS: Are FETs sensitive to polarization? I think they are, but I don't want to make false assertions. IFI Robotics has a notice on the vics page that says:

Do not reverse the input polarity on a Victor. Doing this will permanently damage the unit and void the warranty.

Is it such a jump to say that using an output as an input would have much the same affect?

[ConKbot of Doom]

Like I was saying, the body diodes in the FETs rectify the power from the motor and, which then puts it back through the circuit breaker, and makes it available for everything else. There is a diode formed in the process of making the FET, but some diodes may even have an additional diode on the die for extra protection.
But if you hook up the victor backwards, on the inputs, the body diodes on the FETs all of a sudden point from the line with +12v on it to the gnd line. So you then have a dead short through 2 diodes. (the top and bottom halves of the H-bridge)

The attached schematic is a typical H-bridge with 3 FETs in parallel for each "quadrant" When you power up a victor with the inputs reversed, a massive amount of current can flow, and you could burn up the FETs. That isn't even worrying about the control circuitry. Reverse polarity on that could just kill a chip and the victor would be nuked, simple as that.

But the rectification provided by the body diodes, directs the power put in on the output to the correct side on the input. So if you hooked up a victor backwards, (inputs as an output, output as an input) when you turn on the circuit breaker, you would get +12v on the +12v connector, regardless of if the battery was hooked to the m+ or m-.

As for braking while powered down. The victors don't even brake when they are powered up, have the jumper set to brake, and they aren't receiving a signal. I know this for a fact. On our bot this year, you can move the arm around with one hand when the bot is off, and when you have no link or it is disabled. But if you have a link and it is on, that arm is not moving (not with one hand anyway...) IFI says that dynamic braking still works while it is not receiving a signal, but not from what Ive seen. Even if it did, most of the power would be dissipated in the motor windings, and not the FETs. Looking at the datasheet for the FET that Mark McLeod linked to, the on resistance is 14 mOhms . With 2 banks of 3 FETs in parallel, in series, that would be 9.33 mOhms. That would be almost insignificant compared to the resistance of the motor windings. So we know that braking can't really hurt the FETs while it is on.

Also, looking at the datasheet, the D-S diode has a maximum continuous current of 60A. Remember, there is 3 FETs in parallel. I really don't think you are going to turn your motor fast enough to make 180A for an extended period of time. I seem to remember that the maximum discharge for the ES18 batteries that we use is 230+ A so it would be possible to fry the victor in the manner that I mentioned earlier, reversed polarity in the inputs. But I'm sure the diodes wont exactly share evenly either, making a cascading failure.

Sorry to make this so long, but looking at the electronics, I would have to say that backdriving a motor hooked correctly to a victor in any state ( on, off, signal, no signal...) could not fry it.

[Al Skierkiewicz]

Originally Posted by Sparks333

Al,
I can't remember the team number, but I will see if anyone else does.

Sparks

PS: Are FETs sensitive to polarization? I think they are, but I don't want to make false assertions. IFI Robotics has a notice on the vics page that says:

Is it such a jump to say that using an output as an input would have much the same affect?

It is not the FETs that are damaged by hooking up the power incorrectly, the drive circuitry is not protected from reverse polarity. These FETs do have protection zener diodes in the construction (as pointed out above) so reversing the power to the FETs will cause the diodes to conduct with a 1.2 volt drop. When the Victor is off and the motors are turned, the diodes conduct and the only path is through the rest of the driving circuit and back through ground. Since they are in a bridge configuration, the direction of the motor doesn't matter. Theoretically, there is nothing to limit the voltage produced by the motor, but with two diode in series, there is a 2.4 volt drop before anything else comes into play. I agree that it is unlikely that damage will result from a backdriven motor in power off.
I agree that the brake will not be applied unless the controller is enabled now that I think about it. All PWM outputs are disabled unless the OI and RC are talking and enabled. The brake should only cut in during a PWM 127 +/- the deadband right?

[ConKbot of Doom]

Originally Posted by Al Skierkiewicz

The brake should only cut in during a PWM 127 +/- the deadband right?

Yup, and if your OI is on, and you actually power up the RC, they get the link, and give the victor the signal, it will short out the motor, which happens to be its power source at the time, so for the 5 seconds or so the backup battery powers the RC, it will brake.

I could actually see having a fun time getting freshmen to push the robot in this condition. They push it, it brakes, they stop to see what is up, and by that time, it has turned back off. They continue to push...

[Sparks333]

All,
Okay, I'm convinced. Backdriving doesn't hurt a vic. Unfortuantely, this makes this mystery even harder to solve...

Sparks

[WizardOfAz]

We'd like to build a test setup for the Victors. Anybody have a suggestion for a cheap fixed resistance dummy load? I don't want to use a motor because the motor is not fixed resistance and it's hard to measure motor power output. Would like to have the ability to set the resistance to about 0.32 ohms to get a 40 A max current test, and also to something like 2.5 ohms for a 5 A max current test. Would like to use a fixed resistor that would spread the heat dissapation over a large area so it doesn't get dangerously hot (500+ watts for the 40 A test).

I can put a big resistor in a can of oil like we do in RF power testing. Anybody got a better idea?

Bill

[Mark McLeod]

I passed your query on to one of our electrical sub-system mentors.
Our electrical group mostly use the ribbon-style power resistor with a fan blowing on it. The ribbon style are easier to get, cheaper and take a lot of abuse. Between Ebay and online surplus vendors, this can be done on the cheap.
http://www.milwaukeeresistor.com/pdf/ribwound.pdf

There is a chassis mount style which one of our electrical mentors has used at work as well. These are aluminum cased and can be mounted to a finned or water cooled plate, but are more expensive.

[Al Skierkiewicz]

Originally Posted by WizardOfAz

We'd like to build a test setup for the Victors. Anybody have a suggestion for a cheap fixed resistance dummy load? I don't want to use a motor because the motor is not fixed resistance and it's hard to measure motor power output. Would like to have the ability to set the resistance to about 0.32 ohms to get a 40 A max current test, and also to something like 2.5 ohms for a 5 A max current test. Would like to use a fixed resistor that would spread the heat dissapation over a large area so it doesn't get dangerously hot (500+ watts for the 40 A test).

I can put a big resistor in a can of oil like we do in RF power testing. Anybody got a better idea?

Bill

How about a 500' roll of #10. An RF load would not get you to the current you want to draw but you could parallel 10 resistors, 5 ohm at 10 watt and drop them in the oil for cooling and be OK I bet.

[BrianBSL]

Originally Posted by WizardOfAz

We'd like to build a test setup for the Victors. Anybody have a suggestion for a cheap fixed resistance dummy load? I don't want to use a motor because the motor is not fixed resistance and it's hard to measure motor power output. Would like to have the ability to set the resistance to about 0.32 ohms to get a 40 A max current test, and also to something like 2.5 ohms for a 5 A max current test. Would like to use a fixed resistor that would spread the heat dissapation over a large area so it doesn't get dangerously hot (500+ watts for the 40 A test).

I can put a big resistor in a can of oil like we do in RF power testing. Anybody got a better idea?

Bill

For discharging our Prius battery packs for our electric airplane project, we use 6 electric water heater elements in a trashcan full of water. Unfortunately, I think their resistance is somewhere around 10-15 ohms, which wouldn't be very doable to achieve .32 ohms.

[Mike Betts]

A long time ago, we tested fuel cells by placing two electrical plates into a small swimming pool filled with water and then added rock salt until the resistance was what we wanted. I'm sure that a small garbage can could be used in this case. Some experimentation is required and must be done in a ventilated area (possibility of electrolysis).

Mike

[WizardOfAz]

This sure provoked some interesting suggestions! My favorite is the salt water swimming pool...

At lunch I went over to the electronics store and bought 10 3.6 ohm 25 watt resistors. We'll wire them in parallel, in two banks of 5 each, so we can test with either (approx) 450 or 225 watts load at full power (12.8 volts). This should give a max current through the Victor of about 36A, a reasonably safe 10% below max rating for it. These are rectangular concrete resistors, so we can cool them with a slab of steel laid across all of them that will have pretty good surface contact, and keep the full power tests reasonably brief, since at max the resistors will be dissipating about 46 watts of heat each, substantially over their 25 watt rated limit. The steel will also protect from the shrapnel if they blow up....

Thanks again for the ideas.

Bill

[gburlison]

I remember seeing a DIY battery tester that used steel banding for the load. A suitable length of steel banding that is used to secure crates was attached to a piece of plywood in a zigzag pattern. The resistance might ba a little low for this project though.