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.
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.
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
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.
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?
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.
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.
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.
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.
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.
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 Nm (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 Nm 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.
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)
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.
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.
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!