Robot doesn't turn in high gear

[Al Skierkiewicz]

Originally Posted by newton418

Could you further explain what the Rockwell blocks can actually handle?

As for the resistance in the circuitry, we kept the wire lengths to a minimum, and will definitely be more conscious of it in the future; however, I do not understand how this would fully explain our problem. At 40 amps, a small CIM outputs ~.8 N*m (please correct me if my calculations are wrong), and IF we could run them significantly beyond this, say, near stall current, then even if you take the ~2.2 N*m stall current then the wire resistance would have to be somewhere around 76 WF. Though I am not the most familiar with our electronics system, I am confident it does not have this much resistance. Now, I did make several assumptions when calculating the torque needed to turn in 2nd gear, so in actuality .8 N*m of torque might not have been enough; but, assuming that the difference is negligible, how do you explain the problems we faced?

Also, I have just talked to our electronics/code mentor and he suggested that we bypass the Rockwell block to see if it is (at least in part) the problem. He also suggested we monitor the voltage on both sides of the Rockwell block using a USB DAQ, so when we get those results I will make sure to follow up on this.

OK,
The Rockwell blocks are simply a piece of copper with a wire clamp at each end embedded in a plastic housing. The rated current is the safe limit when the block has continuous current (measured in days) passing through it when mounted in a closed electrical enclosure. (i.e. no cooling air flowing around it) The safe limit is based on the temperature rise of the block that would cause damage to the wire and block. Since we are only using this block for a few minutes at a time, the heat rise is so low that several hundred amps of current flow will not significantly raise the temperature. However, a high resistance connection (loose, small wire size, improper crimp) will cause significant temperature rise as current flows. I would expect you to find the voltage drop across the Rockwell block to be in the millivolt range.
I did not do an actual calculation but your numbers look like they are in the ballpark. Remember that the actual performance of the motor is skewed down the curves by the series resistance of your system. The motor is tested with a power supply that has no output resistance whereas the battery is .011 ohms. The resistance limits/reduces the available current to the motor and so for a given design point, your speed and torque are reduced. Don't forget to include in your calculations the efficiency of the transmission you are using. As you add up mechanical losses and try to push the motor to gain more speed, the current begins to skyrocket. I think that if you back into the operating point by measuring the wheel speed of your robot in a turn you might be surprised to find that you have moved far to the right on the curve where current and torque is very high, but efficiency has started to drop off. (max power is at 2655RPM but current is at almost 70 amps) Without seeing your robot design and watching it perform it is hard to tell exactly where the problem might be. So a few ideas...check the wheels by hand with the power off and no brake on the Victors. Do both sides move freely and with the same force? Are drive motors on both sides at about the same temperature after a match? Is there a similar amount of wire in each side of your robot? i.e. does one side have a longer length of wire to the block and fuse panel than the other? Do you bring the black wires back to the Rockwell block (as required) or do you instead attach them to the small fuse panel? A picture could tell us a lot.

[Al Skierkiewicz]

Now that I have had some sleep, it is important to also know the diameter of the wheels you are using.

[Jared Russell]

Having seen 1647's drivetrain this weekend, I feel obligated to reply.

Their machine has a 6WD configuration with four custom 8x3" wheels with roughtop tread in the middle and rear, and unpowered omniwheels in the front for support.

I can't comment on the ratios they are using, but I WILL say that I noticed a few things when they were running. One, their right gearbox had a big bend in one of the plates (from what I can't guess). I pointed this out to a team member but I don't think they did anything to fix it.

Two, their machine makes a terrible noise when turning. I think that this is a byproduct of the aforementioned dent and possibly grease/chain tension issues.

But all in all, their middle and rear wheels are VERY close together and I really don't see why they would have issues turning. I don't think that changing the motors will make a huge difference - I'm willing to bet that the problem is in the gearbox, greasing, and chain tension.

That said, I loved 1647's machine, and they were a fierce competitor in the playoffs!

[Beta Version]

Originally Posted by Al Skierkiewicz

This is another one of the myths of electrical design.

Thank you. Thank you. Thank you. I can almost *almost* guarantee that your problem is in no way electronic, the cims are getting all the power you need. The problem is in your torque, moment, and traction. Put a long bar/axel through your wheel, weight it with a known weight, then take a fish scale and pull the wheel sideways and see how much force it takes to drag the wheel sideways. Find your k of friction, and calculate how much torque your needing, then see if you need less traction on your wheels. Even if you've lowered your center wheel, your weight needs to be balanced for that to work.

[Lil' Lavery]

Originally Posted by JohnMorenski

its definatlly the fact that we have the 1 big and 1 small cim, were going to have to swap over to 2 small cims

That's a start. It's going to give you more power and less traction (as small CIMs will save a good chunk of weight). But seeing as the description of your gearing, you will probably have to reduce your speed to get better results (it's too late to adjust your wheel base most likely). Get some bigger sprockets and/or smaller wheels.