Drill Motor + Gearbox + Breaker Problems

Technical Electrical
1

Team 1189 is cursed when it comes to drill motors.

Last year (our first), we made the mistake of using two drill motors hooked up via direct drive to the wheels (don’t blame me, I’m the programmer). This never caused a real problem until our first competition, when we thought the radio was crapping out on us. Tests revealed that it was the 40A breakers tripping that resulted in the jerky movement of the robot. Countless airholes and fans added on the spot did nothing to relieve the problem. Obviously, those motors were drawing WAY too much current.

So this year, we set out to be different. We would use the gearbox designed and put in the kit by FIRST! Isn’t that a wonderful idea? Unfortunatly, we couldn’t seem to get it lined up straight: axles chafed, drill motors stunk, and gears bound. Eventually, this was worked out (or so we think).

But now, when we drive the robot around the shop, the familiar jerking control and clicking of the breakers has returned! It makes all of us in the shop (except the drive train person) scream for use of CIM’s, if only for their extreme simplicity from the outside! I mean, so many things can go wrong with the drill motors: Are the pins okay? Is the clutch engaged? High/Low gear? Brushes bent? Solder joints weak? Those thick black Korean cylinders look mighty tempting when all you have to decide is ‘broken, or not?’.

Also being the electronics dude, I checked all of our wiring. Everything is straight to FIRST specification, and I’ve got the current sensors all built and ready to be read by the RC (that’s another issue: figuring out what the ~500 value I get from the sensor really is, and what’s the correct formula to convert it to amps). Neither motor draws more than the assigned voltage, as tested by a multimeter, but I don’t think that really means much.

This is sort of my plea for help in the final week of building. We have a very small team (~25) with even fewer members who have a clue as to what they’re doing (~10) and not a single engineer. Our teacher assistance has been wonderful, as well as parent, but this is a situation where we need someone to step in and tell us what we’re doing wrong, how to fix it, and how not to do it again. It’s killer to one’s moral when there are 3 kids left in the shop, two of them electronics, one chassis, and none of us knowing what to do.

Thank you SO much in advance for any help we may recieve. If we get this resolved quickly, it’ll be much calmer in the shop as I replace 12 24VDC coils on additional SMC double solenoids with 12VDC ones (couldn’t put the right ones on in the factory, now could we?) and pop the submounts on (those too hard to package up as well?).

2

This is just a list of things I would check:

Are the 40 amp breakers are tripping?

Is the clutch in the drill motor locked into high or low gear?

Is possibly gears slipping and not making contact?

Are the drills set to the same gear on both sides (ie… high/high or low/low)?

Is cooling efficient? Are the drill motors over heating at all? They tend to like to do that.

3

One big problem is alinement and binding. In you wheels are binding then your motors have to work really hard to do anything. Tryrunning with wheels off the ground and see what happens. If your are still popping breakers then there is a deffinate problem with friction somewhere.

4

There’s a few obvious double checks you need to do:

You need to use the spacers they give you to keep you gear aligned, they work well if you use them properly. Without them, the helical gears will be a nightmare. With proper spacers, you can assume almost zero losses. Ours out of the box were very fluid. If you need to make some, purchase a pair of digital calipers ($20 off eBay) and get them accurate relative to the gear box. Poorly meshed gears can make or break a drivetrain.

The other obvious gearing troubles:

Do you have your helical gears made such that you’re slowing down through the gear box (as opposed to speeding up?) If this isn’t the case, you may need too much current from your drill motors.

It’d be easiest if I had a picture of your robot, perhaps you could IM or email me so we can talk more. I’m extremely sympathetic to getting drivetrain issues resolved for student run teams, and am more than willing to talk you through some solutions.

Good luck!

Matt

5

Efficiency is a huge thing when it comes to drive trains. You have to make sure that your holes where the shafts mount are drilled straight, and that everything aligns properly. If it does not, it is definitely worth the time to redo whatever part that may be and fix the problem. This may not be the easiest thing to do, but you will be glad you did it in the long run. Make sure you are locked in the right position, make sure you are aligned, right tension in chain, no wheel slippage. Also, if you have the current sensors installed, then use them to the full advantage! Run a dashboard and see what kind of numbers you are pulling when the breakers trip, that way, you can setup your gear ratios to match that current output, and won’t trip the breakers. I am an electronics guy myself, so this is why i suggest this route. If you are tripping because of the wrong gear ratios used in your drive, see where the breaker trips, and then calculate the gear ratio such that you cannot pull that much current and thus not trip your breaker.

Good luck!!

6

we are also a second year team, we tried the same direct drive that you are talking about, but we got some time to test it and we also tried it on carpet and up the ramp and got that jerky motion. We then found out it was our 40 AMP breakers. Once we had to problem narrowed down, we found out that the shafts were perfectly alligned. Things to make sure, all shafts are straight, the more perfect the better. Also they shafts on motors should be secured by some bearings, block bearins if you ask me. Than your wheel shafts should be perfectly alligned, with a mechnaism that allows you to tenion yor chain.

So basically,
Shafts should be alligned perfectly, the chains should be right tension not too much, but also not too little and also i would never recommend gearing up, always gear down.

Now the gear boxes you talk about, if they are not custom than you may not run into that much problems but if they are custom makes sure to have accurate measuring equipment that allows you to be precise to atleast 1/200 of an inch. Also have the spacers ready, make sure no component in the gear box is allowed to have any play(move).

Last but not least make sure the gears dont bind, the best way of checking this is, put a piece of paper and mesh your gears if the peice of papers slids through easily tht means you have a clearence of 4/1000 inch which is optimum for helicul gears. Also make sure you dont have that much of a slip, the lesser the better. Slip means that you have a little bit of movement between your gears, this can cause additional friction you may not want.

Also did you work out the speed you want to be going on and the RPM you want before you designed the gearbox?.

Also if you dont understand sitll, PM me and i’ll spend some time on MSN with you snd walk you through all the parts and we might find out what is causing the poblem.

7

Well you have a lot of good suggestions so far. I would add a few more.

  1. Are you using tank steering? (i.e. steering is accomplished solely through the drive motors and no tires turn to steer.) If so can you drive straight up and back without tripping the breakers? Turning produces incredible friction on the drive train and any friction is translated into high current.

  2. Have you removed the drill motors and tried turning the drive system by hand? If you can’t do this then the parts are not aligned as well as you think. Make sure the shafts, pillow blocks, and bearings are all running true. The right angle helical drive must be at perfect right angles for you to minimize friction there. My suggestion is to rebuild the drive starting at the wheels. Get the wheels turning freely before adding the next drive component, get that in alignment and add the next componenet one at a time until you get the drill and transmission mounted up.

  3. Are you asking the drive to go too fast? With big wheels on the robot you may need to slow the speed down in software to compensate for the large wheel. Another alternative is to select the low speed on the transmission. I would trade speed for reliable operation any day. You can’t play if you can’t play!

8

Well, now I’m excited to go into the shop again!

I’m going to show all of your replies to the other guys, and possibly PM one of you guys if we can’t figure it out.

Only 45 minutes until I can mess with that pretty robot again…

9

Back in the shop, we’ve done some more testing.

Up on a table, with no load on the wheels, the breakers do not trip. Both gearboxes seem to be okay, and the few people here reassure me that everything is alined. We are using the spacers, we can turn it by hand, and nothing seems to be binding.

As soon as you grab a wheel and try and move, breakers start clicking. Another thing we’re not entirely sure about is when the breakers trip, should the entire system go off? We get dimming of the relay LEDs while running with load, and Battery LED on the RC clicking to orange every few seconds.

I stress again that the wiring is not at fault, we’re up to spec with FIRST’s diagram.

Putting the robot on the ground fares no better. Driving in a straight line on a smooth shop floor is a fatal task for the robot. After straining to travel 5ft at a ridiculously slow speed, it totally stops and just clicks.

They just came in and told me that loosening things up made it a little better, but I’m going to go see if that’s really true.

10

What gear ratios are you using?

Could you post a picture (or PM me one?)

This would help out a bunch.

Matt

11

Hopefully I can help you out with the current sensors, at least. Some other sucker on our team is responsible for the drive train :wink:

I also read about 500 counts from the Get_Analog_Value(rc_ana_inXX) function call in the software, when the motors aren’t moving. When you think about this for a minute it makes sense. Get_Analog_Value() returns an unsigned int (16 bits), but only the 10 least significant bits represent the voltage on the analog input. The biggest number you can make with 10 bits is 1023. We’re reading about half of that, around 500. The analog input on the robot controller can read voltages between 0 and 5 volts. 2.5 volts output from the current sensors corresponds to 0 amps (no current, motor stopped). Maybe we’re onto something!

Since we’re digitizing the input voltage using 10 bits, we have to scale the number that comes out of Get_Analog_Value() appropriately:

input_voltage = Get_Analog_Value(rc_ana_inXX) * 5 / 1023;

The current sensors put out a voltage between 0 volts and 5 volts. 4 volts corresponds to 75 amps (fast forward), and 1 volt corresponds to -75 amps (fast backwards). It’s pretty linear in between. Do a y = mx + b on this and rearrange to solve for current in terms of voltage. Try it yourself, and you will find that drive_current = 50 * input_voltage - 125. This will only give you an integer value for the current but at least it gets you started. There are some tricks you can play with averaging and bit-shifting in order to get more resolution, but I’d concentrate on making the sucker drive first.

Hopefully this helps with the current sensing. Let me know if you have any questions at all.

{edit} There’s a far easier way to do all of this. In your message you mentioned converting the voltage reading to current, and I tried to answer along those lines. But you don’t have to convert to current in order to do simple current limiting. All you have to do is figure out what voltage from the sensor corresponds to your threshold current where you want to limit the motor, and then figure out the corresponding number you’d get from Get_Analog_Value() for that voltage. That’s your threshold. Note that there are actually two threshold voltages if you drive the motor backwards, because the current is negative if you drive backwards. {edit}

12

Here are the pics

DSC01333.JPG
DSC01328.JPG
DSC01329.JPG
DSC01330.JPG
DSC01331.JPG


DSC01333.JPG
DSC01328.JPG
DSC01329.JPG
DSC01330.JPG
DSC01331.JPG

13

Alright,
Where are you located? Are you close to Chicago?

14

no, we’re outside of detroit

15

Troubleshooting this problem will be much easier if you can get you hands on a clamp- on ammeter. Check with your sponsors and mentors or find a local electrician that could help in finding one.

Put the robot on the bench with the wheel suspended. Use the ammeter to monitor the current of each drive motor. The meter should be placed between the circuit breaker and the speed controller.

Compare both sides running at the same speeds. The current should be close for both sides. There is a slight difference between forward and reverse for the drill motors.

If the current is high (> 10-15 amps) then you may have too much friction in the drive system. If the current is low on both sides, place the robot on the ground and monitor the current for each side while the robot is running straight. IF the robot is drawing much higher current when on the ground vs. the bench, look for things that might add friction when weight is added to the wheels (i.e. wheel axles).

One of these two steeps will tell you where the potential problem is located.

Tim Tedrow

16

What are you using for the non drive wheels?

17

I won’t go so far as to say what our problem was, but I’ll warn other teams to CHECK THE BATTERY POWER before you take out the gearbox!

(We’ll check around for an ampmeter, but a fresh battery seems to have fixed the problem for now!)

Thanks for all of your help.

18

just looking at your pictures i think i can tell why you are having your problems. with the direct output of your gearbox going all the way to the wheel your axle is acting like a huge lever and missaligning your gear box. while it may seem like your wheels might have a flex of only a milimeter it will affect your gearbox. what i might sugest is to separate that axle and conect it with a flex coupler to account for any mis-alignment that may occur. if you have a second set of the black plastic couplers that first gives you, you could cut your shaft attach the hex gear to either side and use the black coupler as your flex coupler…hope this helps feel free to PM me if you have any questions

btw have you made sure that the clutch is in the drill setting aswell…try this first

19

Ok,
I can’t run over there this afternoon, but you have a lot of teams close by that could give you some help if they were to see this in action. You have removed the locking pawl from the drill transmission, haven’t you?
My advice again is remove the drill motor and transmission and the driving shaft. The remaining shaft with wheel on should turn freely. If that is OK then, put the robot on the floor and try to push it. It should just keep going by itself, in a straight line. If it doesn’t keep going but goes straight then the bearing on the wheel shafts are not in alignment. If it rolls, but not straight then you do not have the wheels parallel.
If all of that works OK then put the driving shaft back in and check everything again. Remember that the driving shaft and driven shaft must be perpindicular else there is side friction on the gear faces. Check that the gears are in correct mesh alignment. This is a hard one. Visualize a line perpindicular to each shaft (as if drawn on the face of the gear.) Those lines should meet perfectly. If the tangent of the driven gear is not aligned with the center line of the driving gear, additional friction is built up. If all of that is OK, you should be able to set the wheel in motion by turning the hex coupler on the driving shaft and the wheel should continue to turn by itself for a few seconds. It make take a little trial and error to get everything to the point where it can make this test. When it is all moving freely, put it back on the floor and test again. There should be a significant improvement. Once that is OK, put the drill and transmission back in and try powering the motors again off the floor. Again the drive should free wheel after you remove the driving power if you have the speed controller set to “coast”. If all of this is OK then put the robot back on the floor and try to drive. If everything was OK up until now and the breakers still trip, you may have to add a bearing to the wheel shaft. The weight of the robot may be putting too great a load on the bearing you are using. Let me know what you find. I will check in again in an hour.

20

If the problems start up again and you can’t get through them, PM me with your contact info, I may be able to either talk you through the process or stop by (though our machine is not running yet, so you know where my mind will likely be).

Joe J.