Robot doesn't turn in high gear

[JohnMorenski]

well i know that this has been posted before but i also know that other teams are having this problem and im sure they want all the info they can get. it seems that we are not getting the full potential out of our andy mark shifters not in low gear, low gear works great, but in high gear. we cannot turn in high gear, the battery voltage on the O/I drops and the motors seem to stall, we are using 1 big cim and 1 little cim on each gearbox any input????

would it be a smarter choice to switch to 2 small cims on each gearbox??

thanks

[Lil' Lavery]

Originally Posted by JohnMorenski

well i know that this has been posted before but i also know that other teams are having this problem and im sure they want all the info they can get. it seems that we are not getting the full potential out of our andy mark shifters not in low gear, low gear works great, but in high gear. we cannot turn in high gear, the battery voltage on the O/I drops and the motors seem to stall, we are using 1 big cim and 1 little cim on each gearbox any input????

would it be a smarter choice to switch to 2 small cims on each gearbox??

You're generating more torque in low gear, which means your robot will experience more torque when trying to turn, resulting in better turning. There are a number of ways to solve this problem. Increasing the torque in your shifters (by switching to 2 small CIMs) is one option. Another would be reducing the ratio between your shifters and wheels, and/or using smaller wheels (both of those would reduce your speed though). You may also want to consider lowering the traction of your wheels, or even using a pair of omni wheels to improve turning.

[Andy Baker]

Originally Posted by JohnMorenski

well i know that this has been posted before but i also know that other teams are having this problem and im sure they want all the info they can get. it seems that we are not getting the full potential out of our andy mark shifters not in low gear, low gear works great, but in high gear. we cannot turn in high gear, the battery voltage on the O/I drops and the motors seem to stall, we are using 1 big cim and 1 little cim on each gearbox any input????

would it be a smarter choice to switch to 2 small cims on each gearbox??

thanks

How fast are you going while in low gear? It sounds like your after-gearbox ratio may be too low... you may need bigger sprockets on your wheels to slow down a bit.

Andy B.

[MrForbes]

Also it will help us help you if you give a complete description of your drivetrain/chassis....ratios, width, wheelbase, number of wheels, tread type, whether the center wheels are lower than the end wheels and by how much, etc.

Long, narrow, 4 wheel robots with sticky tread, and not much final drive reduction, just don't turn easily. Wide, short, 6 wheel bots with lowered center wheels, not so aggressive tread, and more final reduction turn just fine in high gear.

[kramarczyk]

Originally Posted by squirrel

Also it will help us help you if you give a complete description of your drivetrain/chassis....ratios, width, wheelbase, number of wheels, tread type, whether the center wheels are lower than the end wheels and by how much, etc.

Long, narrow, 4 wheel robots with sticky tread, and not much final drive reduction, just don't turn easily. Wide, short, 6 wheel bots with lowered center wheels, not so aggressive tread, and more final reduction turn just fine in high gear.

I'm guessing a 4x4 with a long wheelbase... take a look at this white paper. <edit> Titled: Drive Train Basics
(How to Be Sure Your Robot Will Turn)</edit>
http://www.chiefdelphi.com/media/papers/1443

[Al Skierkiewicz]

Originally Posted by JohnMorenski

low gear works great, but in high gear. we cannot turn in high gear, the battery voltage on the O/I drops and the motors seem to stall,

This is a classic symptom of too much friction in turns. As other have pointed out, your drivetrain and final gear ratio couple to produce near stall current on the drive motors. The small Chalupas stall at 129 amps while the larger ones stall at 96 amps. Add that up and you are likely drawing something in the neighborhood of 300+ amps even if your wheels are turning. With a fully charged battery with an internal resistance of .011 ohms, you are already dropping 3.3 volts at the battery, plus whatever series resistance you have in the wires. Remember that the RC will go to backup mode when the input voltage falls below 8 volts and all outputs are disabled. As you draw current from the battery, that voltage drop takes you seriously close to the magic 4 volt drop and RC shutdown.

[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
thanks guys

[sanddrag]

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
thanks guys

This will help some, but I still think either your wheels are too grippy, your wheel base is too long, your track width is not wide enough, and/or you do not have enough reduction between the gearbox and the wheels, with the latter being most likely.

How many teeth are on the sprocket on the wheel? How big are the wheels?

[Brandon Holley]

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
thanks guys

Im sure a big cim and small cim will work just fine, but i think the easiest fix would be to get some bigger sprockets on those wheels.

[Billfred]

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
thanks guys

I'm not so fast to pin it all on the large CIM. Sure, it's got its faults (weight and relative lack of power compared to its smaller sibling being the largest), but it does have its highlights. (We ran this exact setup at Palmetto in a 6WD configuration, and it turned smooth as butter.)

I'm also inclined to pin it on a configuration issue along the lines of sanddrag's post, we can't be sure without getting more details (sprocket sizes, wheel coefficients of friction, etc.) without pictures (CD-Media is a wonderful thing) or more details.

[Chuck Glick]

I've seen their setup first hand. They have four 8"(?) custom traction wheels in back and 2 AM omni's in front. After watching them at Philly, their low gear was relatively fast, very similar to our speed, but we used the AM Single Speed Gearboxes on a 1 to 1 output to wheel ratio. So I would suggest that you increase the size of your sprockets on the wheels, decrease the size from the gearbox, or even both. That hopefully should help with the turning problem.

-Chuck

[newton418]

Team 418 used a 6 wheel drive, 4 small CIM-DeWalt transmission drive train. We lowered the center wheels 3/32", which is enough to sink the center wheels in the carpet, but still have all 6 wheels contact the ground. In 2nd gear we realized we had some difficulty turning (the RC was resetting, I believe), and after talking with the electronic guys we think we figured out why. When designing the drive train, I figured we could run each small CIM at 40 amps (since they are on a 40 amp breaker); however, I was informed that the power distribution block is rated for 85 amps (I'm not exactly sure, but around this number). This means we weren't getting the current, and thus the torque, we had planned on. Perhaps you guys are facing a similar problem.

[Al Skierkiewicz]

Originally Posted by newton418

When designing the drive train, I figured we could run each small CIM at 40 amps (since they are on a 40 amp breaker); however, I was informed that the power distribution block is rated for 85 amps (I'm not exactly sure, but around this number). This means we weren't getting the current, and thus the torque, we had planned on. Perhaps you guys are facing a similar problem.

This is another one of the myths of electrical design. The Rockwell blocks are designed for continuous current of 85 amps per block. The 40 amp breaker will function with up to 600% of it's current trip rating for several seconds. Neither of these devices limit the current to that of their ratings. But there are limiting factors in the wiring of the robot. Things to consider are the series resistance of the wire, terminals, Victors, and distribution. A simple rule of thumb I use is the "wire/foot" or WF. It is simple to equate series resistance with the resistance of a foot of #10 wire which is .001 ohms. Using this as a rule of thumb, then the battery has 11 WF internally, the #6 feed wiring has 0.5 WF per foot of wire (you must include both the postivie and negative wire) while the #12 has almost 2 WF per foot, the Victors have 4 WF of series resistance and you can have between 1 and 3 WF for every crimp if not done with proper tooling, not tight on the block or screw terminal or not fully crimped. So, how does this affect your design? At 100 amps of current (roughly the stall current of either Chalups motors or the old FP motors) each wire foot will drop 0.1 volt. Just take a look at the circuitry that feeds your motor, add up the resistance using this method and multiply by 0.1 to find the voltage drop to your motor.
Ex. 4 ft #6 wire, breaker panel, one victor, 4ft of #12 from fuse panel to Victor to motor, 20 connections= (4x0.5)+1+4+8+20=35x0.1=3.5 volts drop in all losses at stall. This translates to a loss of about 1/3 of the available current or about a loss of almost 100 oz.in. of torque and a drastic change in the efficiency.

[newton418]

Originally Posted by Al Skierkiewicz

This is another one of the myths of electrical design. The Rockwell blocks are designed for continuous current of 85 amps per block. The 40 amp breaker will function with up to 600% of it's current trip rating for several seconds. Neither of these devices limit the current to that of their ratings.

I was aware that the 40 amp breakers could be pushed past their limit for small amounts of time, but decided to design the drive train such that we wouldn't need to constantly push the breakers, since we could get sufficient performance at 40 amps. I am much less familiar with the Rockwell blocks, since they are new and I do not work on the electronics. I figured that since they are rated for 85 amps they could be pushed past this limit, but I have no idea by how much or for how long. 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.

[MrForbes]

Originally Posted by newton418

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

They can probably handle a couple hundred amps for a very short time...

what happens with electrical parts when a lot of current goes thru them, is that they get hot....every part has resistance, and resistance turns electrical power into heat energy. When the metal parts get hot enough, they melt thru and can no longer conduct electricity.

The point of having circuit breakers and fuses, is to put a "weak link" in the circuit that will fail first, and prevent the other wiring and components from overheating.

The power distribution system for the robots is a well designed system, and will do it's job if you follow the instructions carefully so you build it as designed (and make sure to tighten all the wire connections firmly)