6 CIM Drivetrains... What's Your Experience?

[Nathan Streeter]

Our team is nearing design completion on an off-season drivebase project... We'd decided to use a sheet metal drivebase a while back, but have been toying with different gearbox/transmission ideas. We've used Super Shifters for the past 4 years... and while we did consider trying the new Sonic Shifter and the Ball Shifter, we decided to broaden our experience by trying a single-speed 6 CIM drivetrain.

So, have you tried using 6 CIMs on your drivetrain? Do you know of other teams that have? What top speed have you geared for? Did you struggle with too much current-draw/voltage-drop in a pushing match and/or in acceleration? If so, did you use any software limits to minimize it (i.e. a voltage ramp, mentioned here (http://www.chiefdelphi.com/forums/showpost.php?p=1308774&postcount=10), or a cut-off if the robot detects a voltage drop)? Also, what did your robot weigh?

We're currently planning to gear for about 10.5 fps (about a 10.5:1 reduction), but are primarily concerned about drawing too much current in a pushing match. If we have current draw issues we can either try software limits/overrides and/or try removing the two "extra" CIMs or swap them for miniCIMs or RS550s, which each draw around 2/3 the current of a CIM.

So far, I only know of 254 using 6 CIMs in 2013 with their 2-speed+PTO gearbox and of 610 using them this year in their 1-speed+PTO gearbox. I heard though that when 610 set aside their climber mechanism they also removed the 2 "extra" CIMs... according to their website they designed for a top speed of about 9fps.

Any recommendations, cautions, or further information?

[Richard Wallace]

We used the 7:1 (84T) single speed gearbox from VexPro, on a sheet metal wide 4WD chassis with 4 inch AM performance wheels and roughtop. The only thing I would change is our choice of tread -- I think nitrile would have held up better. We ended up changing treads after every match in which we played defense. That was most of our matches because our scoring mechanism suffered from complexity (another story). The AM 1 inch wide roughtop that we used de-laminated (i.e., tread separated from woven structure) whenever we got into a good pushing contest. We ended up winning several of those contests anyway, but spending several minutes changing wheels in the pits was not fun. Especially in District competition with frequent quick-turn matches when there is no time to return to the pit at all.

TL;DR -- if you use 6 CIMs, make sure your treads can take the torque.

[SoftwareBug2.0]

We have used six CIMs in our drivetrain. However, our drivetrain never used more than four of them at full power at once which probably helped avoid any current/voltage problems. No special software was needed.

Our gearboxes were geared at 9.7:1 although the effective gear ratio from motors to groud speed varied somewhat. Either way, we probably should have gone with slightly more reduction.

We were a few pounds underweight and couldn't push worth beans, which was expected given our design. As for random tips, stability is more about how far away your points of contact are from each other than about how many there are.

[Andrew Lawrence]

We used 4 CIMs 2 Mini CIMs on our offseason bot this year and loved it. Never had any current issues at all. We ran a 6wd WCD with 4"x1.5" Colson wheels through a WCP single speed I modified to fit a 3rd motor geared for 14 ft/s on a belt drive, and it was beautiful. In the upcoming season, I'd like to run a similar setup using modified WCP shifters at 16 ft/s and 6 ft/s. We were extremely light, but were still able to push 120 lb robots without a problem and never tripped a breaker.

[magnets]

6 CIM drive trains are a great feature, but I don't know if they are worth it unless you have a power takeoff. With the power takeoff, you get 50% more power than with 4 CIMs, and you run it single speed and still have decent traction and decent pushing force.

However, if there is no power takeoff, it's easy to justify. With 2 CIMs, you can have a traction limited slow gear easily, so you won't have better pushing power. The only real benefit would be acceleration, but even then, the added weight of two motors is really, really hard to justify just for improved acceleration.

I think the main reason the GDC went to 6 CIMs this year was because of climbing. They wanted teams to have 4 CIM drives and 2 CIM + 2 miniCIM climbers or 6 CIM drive with PTO. I predict that if next year we don't see a game that involves pulling something heavy, lifting something heavy, or something that requires a ton of power, we'll be back to 4 CIMs.

[Nate Bloom]

6 CIM drive trains are a great feature, but I don't know if they are worth it unless you have a power takeoff. With the power takeoff, you get 50% more power than with 4 CIMs, and you run it single speed and still have decent traction and decent pushing force.

However, if there is no power takeoff, it's easy to justify. With 2 CIMs, you can have a traction limited slow gear easily, so you won't have better pushing power. The only real benefit would be acceleration, but even then, the added weight of two motors is really, really hard to justify just for improved acceleration.

I think the main reason the GDC went to 6 CIMs this year was because of climbing. They wanted teams to have 4 CIM drives and 2 CIM + 2 miniCIM climbers or 6 CIM drive with PTO. I predict that if next year we don't see a game that involves pulling something heavy, lifting something heavy, or something that requires a ton of power, we'll be back to 4 CIMs.

What's the math for a traction limited drive? Accounting for the fact that the CIMs are run through motor controllers with a 40A max (probably closer to 20-30A in competition), I have trouble imagining a traction limited drive with blue nitrile wheels.

[Richard Wallace]

What's the math for a traction limited drive? Accounting for the fact that the CIMs are run through motor controllers with a 40A max (probably closer to 20-30A in competition), I have trouble imagining a traction limited drive with blue nitrile wheels.
Try your math again using more current draw -- FRC breakers will handle well over 40A for short durations. 60A draw for a few seconds is not uncommon.

Joe Ross linked the breaker data sheet in another thread (http://www.chiefdelphi.com/forums/showthread.php?p=1215913#post1215913) earlier this year.

[cbale2000]

While we have never used a 6 CIM drive before, we did once try a 4 CIM + 2 Large CIMs (FP CIMs?) drive system back in 2006. As a whole the system worked well but wasn't really anything special to talk about as far as performance (the drive was a fairly low geared tank tread system).

[Nate Bloom]

Try your math again using more current draw -- FRC breakers will handle well over 40A for short durations. 60A draw for a few seconds is not uncommon.

Joe Ross linked the breaker data sheet in another thread (http://www.chiefdelphi.com/forums/showthread.php?p=1215913#post1215913) earlier this year.

During a pushing match wouldn't you be limited to 20A per CIM with 6 CIMs or 30A per CIM with 4 CIMs due to the 120A breaker?

[Jared]

What's the math for a traction limited drive? Accounting for the fact that the CIMs are run through motor controllers with a 40A max (probably closer to 20-30A in competition), I have trouble imagining a traction limited drive with blue nitrile wheels.

A CIM motor can draw more than 40 amps. The victors are rated for 60 amps continuous draw and are rated for 120 amps surge current.

During a pushing match, your CIM motor will draw much more than 40 amps. You can draw 80 amps for 1.5-4.9 seconds and you can draw 70 amps for around 8 seconds on the snap action breaker before they start clicking.

You should probably read the datasheet for the breakers.

http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf

If you would like to do the math, search for JVN's mechanical design calculator, but many teams base their low gear speed at the point where added more torque would just make the wheels spin. This is why nobody puts together 10 motors on a gearbox and just pushes everybody around.

[Jared]

During a pushing match wouldn't you be limited to 20A per CIM with 6 CIMs or 30A per CIM with 4 CIMs due to the 120A breaker?

Read the datasheet

http://team358.org/files/electrical/120aMainBreaker.pdf

You can draw 240 amps for 30 seconds.

[pmangels17]

We had 6 CIMs this year, in a two speed configuration. The drive was awesome and we really enjoyed the pushing power, speed, and acceleration of the extra motors. However, we did a lot of pushing this year, and the constant strain on the batteries is resulting in them not being able to hold much of a charge, to the point that we cannot re-use this year's batteries next year.

[Nate Bloom]

Read the datasheet

http://team358.org/files/electrical/120aMainBreaker.pdf

You can draw 240 amps for 30 seconds.

Oh I didn't know that. Even so, you're still limited to 40 amps per CIM (assuming there are 6 CIMs), so I still think the drive can't be slip limited.

Edit - at least not at any reasonable gearing ratio

[magnets]

Oh I didn't know that. Even so, you're still limited to 40 amps per CIM (assuming there are 6 CIMs), so I still think the drive can't be slip limited.

I'm not really much of an electrical guy, but my interpretation of the data sheet shows that you could draw 480 amps for 5 seconds. However, I will guarantee that with 4 CIMs geared at 5.5 fps, you WILL slip way before the motor is even close to stalling, regardless of which wheels you use, blue nitrile, AM high grip, or any other.

[Nate Bloom]

I'm not really much of an electrical guy, but my interpretation of the data sheet shows that you could draw 480 amps for 5 seconds. However, I will guarantee that with 4 CIMs geared at 5.5 fps, you WILL slip way before the motor is even close to stalling, regardless of which wheels you use, blue nitrile, AM high grip, or any other.

I think that's a legitimate reading of the graph - and if you can pull that much power then yeah you would be slip limited at that gearing - at least for 5 seconds...

[Mark McLeod]

Even so, you're still limited to 40 amps per CIM...
The Snap-Action 40a Breaker Spec. (http://www.team358.org/files/electrical/MX5%20Spec%20Sheet.pdf) overload ratings:

150 % overload = 3.9 - 47 seconds
175 % overload = 2.2 - 9.2 second
200 % overload = 1.5 - 3.9 seconds
250 % overload = 0.8 - 1.8 seconds
300 % overload = 0.5 - 1.1 seconds
400 % overload = 0.3 - 0.6 seconds
500 % overload = 0.2 - 0.3 seconds

[Ether]

What's the math for a traction limited drive?

If:

- the wheels are all the same type and diameter and are chained (or belted) together on each side, and

- you are powering each side with a single gearbox with N CIMs, and

- you assume that traction force = normal force * coefficient of static friction (where coefficient is a constant), and

- the center of mass lies along the vehicle longitudinal axis,

then:

A = [(4*D/G*Istall*mu*W)/(eff*Tstall)]/N

where:

A = amps per CIM required to break traction and spin the wheels

W = weight (lbs) of vehicle

mu = coefficient of static friction

D = diameter of wheel in inches

G = the total speed reduction gear ratio from motor to wheel (including sprockets/pulleys)

eff = drivetrain torque efficiency fraction

Tstall = CIM spec stall torque (oz_in)

Istall = CIM spec stall amps

[Nate Bloom]

Just out of curiosity, how do you estimate the drive train torque efficiency?

[Richard Wallace]

Just out of curiosity, how do you estimate the drive train torque efficiency?
An equally pertinent question might be, what steps can a team take to minimize drivetrain mechanical power losses? There are many factors to consider, including alignment, lubrication, fits/clearances, materials, etc.; however, the concept is simple. If it rolls straight and easily, the losses are low. Many of us learned this as Cub Scouts building pinewood race cars. :)

80% to 85% is a figure often "plugged in" when using drivetrain calculators (e.g., JVN) -- however, some teams build drive trains that are better than that, and quite a few build them worse.

One good test to try is measuring the free current of the completed drivetrain (wheels up).

[Ether]

Just out of curiosity, how do you estimate the drive train torque efficiency?

That's the 64 Thousand Dollar Question (http://answers.yahoo.com/question/index?qid=20100603035835AAjlhon).

[Ether]

One good test to try is measuring the free current of the completed drivetrain (wheels up).

That is probably a good test to determine relative efficiency (e.g when trying to tune the alignment, chain/belt tension, etc of a drivetrain), but how would you tease the absolute efficiency from just that one piece of data?

[Richard Wallace]

That is probably a good test to determine relative efficiency (e.g when trying to tune the alignment, chain/belt tension, etc of a drivetrain), but how would you tease the absolute efficiency from just that one piece of data?


I don't think you can do that. The free current test is useful for determining if 'tweaks' are moving in the right direction. It cannot capture the effects of losses that depend on loading, particularly those that become more significant at higher loads.

[AllenGregoryIV]

We ran a 6 CIM drive this year and it was okay but we geared it pretty high to be able to keep up with other robots. (5.33:1 than 26:22) with 8 versawheels. We could hold our own in pushing matches but we were always on the edge of brown/black outs. We blacked out once at Razorback and browned out in our first match at IRI other than that our driver kept it with in it's limits.

It was a fun experience and it helped us win a few matches but we will almost certainly be using 4 CIMs and Ball shifters this year instead. It wasn't worth the fear of possibly popping the main breaker in any given match.

[philso]

We have used six CIMs in our drivetrain. However, our drivetrain never used more than four of them at full power at once which probably helped avoid any current/voltage problems. No special software was needed.


What do you mean by this?

Are you delaying the start up of one of one, or more, of the motors on each side to spread out the current peaks?

[philso]

capture the effects of losses that depend on loading, particularly those that become more significant at higher loads.

This would require a calibrated "rolling road" dynamometer and instruments to measure the instantaneous battery voltage and current and the load that the dynamometer was applying. The effect of wheel spin would could be approximated if the dynamometer rollers were wrapped with the proper type of carpet.

Maybe your local race shop might be able to help out :D

[Ben Martin]

We ran 6 CIM motors through this VEXpro gearbox (http://www.vexrobotics.com/vexpro/gearboxes/single-speed-single-reduction.html) at 6:1 reduction, the standard KOP belt system, and 4" wheels. Worked great for us, no issues whatsoever all year. When we increased the weight up to 120 + 20 for bumpers + battery and installed traction wheels, the motors gave us that little bit of oomph that we needed to power through to the feeder station. If we were in more sustained pushing matches, we would have found ourselves more at risk of breaker tripping.

[SoftwareBug2.0]

What do you mean by this?

Are you delaying the start up of one of one, or more, of the motors on each side to spread out the current peaks?

We didn't have any delays to spread out current peaks. We used six CIMs and each of our wheels was powered by two CIMs, so we had three wheels. Our drivetrain would have just been very hard to control if we had turned all the motors on full at once because it would have wanted to spin very quickly.

http://www.youtube.com/watch?v=oMBTmfBnjsc

I was wondering whether people would read closely enough to see that what I had to say didn't make sense for a typical 6-wheel drive. :)

[Oblarg]

That's the 64 Thousand Dollar Question (http://answers.yahoo.com/question/index?qid=20100603035835AAjlhon).




Find a surface on which you know the CoF of the wheels to reasonable accuracy, get a high-speed camera (or accelerometer), and measure the acceleration/current draw of the robot running at full power pushing various objects whose dynamic friction on that surface has been measured?

Not perfect, obviously, and rather time-consuming, but it should give you something to compare against theory.

[drwisley]

That is probably a good test to determine relative efficiency (e.g when trying to tune the alignment, chain/belt tension, etc of a drivetrain), but how would you tease the absolute efficiency from just that one piece of data?




I use the no load losses from test stands, which we can derive from the current, and add 1% per gear mesh under load. I have some powertrain industry experience to correlate this practice from. This is a good starting point for shift points. Although the 6 CIM rimpull calc's I have put together don't require shifting.

[Ether]

Find a surface on which you know the CoF of the wheels to reasonable accuracy, get a high-speed camera (or accelerometer), and measure the acceleration/current draw of the robot running at full power pushing various objects whose dynamic friction on that surface has been measured?

Not perfect, obviously, and rather time-consuming, but it should give you something to compare against theory.

Here's the theory to compare it to (http://www.chiefdelphi.com/media/papers/2868).

Adjust Kro, Krv, and Kf until the model matches the test data.

[thefro526]

We ran a 6 CIM Drivetrain in 2013, and other than a couple of hiccups, it worked really well for us.

We were geared a bit on the higher side of things, 6:1 for an effective max speed of ~12.5-13fps. Compared to a 4 CIM Drive with Similar Gearing or a 4 CIM 2-Speed Drive, it seemed like our acceleration performance and/or general "Zippy-ness" of the drive was on par with most of the front-runners in the field.

We did experience current draw problems a handful of times over the course of the season, most of which were triggered by defense being played by, or on us. It's also worth noting that we had 5 other motors on our robot, (4) of which were RS550's, and one of which was an AM-9015, so if any of them were running (especially being pulsated or stalled) it didn't exactly help the problem. In either case, even though we thought that the drive should have popped the PD Breakers (40A) prior to popping the 120A main, this was rarely, if ever, the case.

After the season, we did some digging into the problem, and found that the cause was due to a misunderstanding (or assumption) made about the PD and Main Breakers. As Mark posted above, the 40A Snap action breakers are capable of handling some amount of current in excess of 40A for some finite period of time - which basically means that a drive can draw more than enough current to pop the 120A Main breaker well before any of the 40A PD Breakers pop.

If you reference the data sheets for the 120 (http://www.waytekwire.com/datasheet/18X_CIRCUITBREAKER.pdf) and the 40A Snap Action breaker (http://www.snapaction.net/pdf/MX5%20Spec%20Sheet.pdf) you'll be able to fairly accurately estimate when your breakers will trip, if ever, during a match. In the case of an aggressively geared robot (like ours) the drive motors can pull somewhere between 60 and 70a each, for a total of between 360A and 420A. If you reference the curves for the snap action breakers, you'll see that at 150% rated current (60A for a 40A) the breaker can last between 3.9 and 47 seconds, but the Main (loaded at 300%) should pop in 5 to 15 seconds. The numbers are even worse at 420A, the 40A PD Breakers will only hold for 2.2 - 9.2 seconds, and the main will last for 2.5 - 8 seconds. It's also worth nothing that these calculations don't take any other current draws into account, so if were running a 6 CIM Drive and had some sort of RS550 powered load pulsating (stop/start) or a compressor kick on, you'd more than likely spend the rest of the match with a robot turned off.

The easiest way to get around the current draw problem seems to be to gear the drive more conservatively, use a wheel with a lower CoF, or to avoid pushing matches... But none of those are really that 'fun', so the other method would be/could be to use a 2-speed transmission along with a 6 Motor Setup. If you were to gear for ~40A Current Draw at maximum load in low gear, you should see very few problems with extreme current draw - if anything, you may have issues with voltage drops and minor brown outs - but those are still better than looking at your robot while it sits dead on the field.

FWIW, circuit breakers are also very, very susceptible to temperature, so keep that in mind. If the venue is abnormally hot, or abnormally cold, or your robot is still warm from the previous match, the breaker performance can change. More often than not, a hot breaker will be able to hold less current, and a cold breaker is able to hold more. As we monitored breaker performance throughout the off season, I noticed that the main breaker was 'warm' to the touch after the majority of our elimination matches.... Anyway, should you decide that monitoring the temperature of the breakers is something you'd like to do, it's best done with proper supervision and safety equipment (Always) - and non-contact tools (think IR Thermometer) when possible.

[MichaelBick]

We didn't use a 6 CIM drive, but did use a 6 motor drive(4 CIM 2 550). Regardless, we ran into a couple problems. We were geared really high(19fps at 100% efficiency) with high traction wheels(2" wide vex traction). We never blew the main breaker in a pushing match, but going from full speed forward to full speed backwards bl;ew it a couple times in practice and competition. We ended up inserting a 1/8 second ramp up on big velocity changes like this and never encountered the problem again.

[philso]

We ended up inserting a 1/8 second ramp up on big velocity changes like this and never encountered the problem again.

I think half or more of the 3-phase motor drives we build at work are used to reduce or eliminate the starting surge of the motor like you have with your ramp up. It also reduces the mechanical stresses in your drivetrain.

[Jonathan Norris]

So far, I only know of 254 using 6 CIMs in 2013 with their 2-speed+PTO gearbox and of 610 using them this year in their 1-speed+PTO gearbox. I heard though that when 610 set aside their climber mechanism they also removed the 2 "extra" CIMs... according to their website they designed for a top speed of about 9fps.

Any recommendations, cautions, or further information?

I wasn't directly involved in designing the drivetrain last year, but the whole thinking around the drive system was to minimize the time it took to get from the loading station to the pyramid. Surprisingly when you crunch the numbers it doesn't mean building a 2-speed gearbox with a 22ft/s high gear, actually a single speed drive system with 6 CIMs geared at ~10-11ft/s is the fastest way to get from the loading station to the pyramid.

The main reason for this is acceleration, having 6 CIMs geared at a 'slower' top speed gave us incredible acceleration, actually allowing us to reach that top speed quickly. It also allowed our driver to be more creative and precise trying to get around defence, our drive team figured out quickly that we could accelerate away from anyone. So when teams tried to play D on us, it would almost seem like they just bounced off us, because when we got into a collision we could spin our robot around them and get away quickly. I believe the drive system was one of our keys to success last year.

I highly recommend teams analyze the game and use that analysis to figure out your optimal drive system. 610 has been doing this analysis for years, there is no 'perfect' drive system that works every year, the best drive system is the one thats optimized for your team's game strategy.

(pro-hint: the acceleration difference between a 6 CIM and 4 CIM drive geared at 10ft/s isn't that much different. Start thinking more about acceleration/agility/control vs. top speed, its a real tradeoff, you need an amazing driver to control a robot above ~15ft/s)

[Ether]

Surprisingly when you crunch the numbers...

Could you please elaborate a little on what method you used to crunch the numbers?

the acceleration difference between a 6 CIM and 4 CIM drive geared at 10ft/s isn't that much different.

I'm assuming you are referring to acceleration at low speeds, where you are traction limited, correct? Once you going fast enough to be torque limited, there should be a significant difference in acceleration.

[gpetilli]

(pro-hint: the acceleration difference between a 6 CIM and 4 CIM drive geared at 10ft/s isn't that much different. Start thinking more about acceleration/agility/control vs. top speed, its a real tradeoff, you need an amazing driver to control a robot above ~15ft/s)

I totally agree. I don't understand why some teams that are going to 6 CIMs AND a two (or three) speed gearbox.

In order to take advantage of the extra torque, do you need some form of traction control? Are you using a PID loop to control the voltage ramp? Are you imposing an open loop ramp on the driver or motor commands? Do you monitor motor currents for stall? Do you simply let the wheels spin and degenerate to the kinetic CoF for a small time?

[AdamHeard]

I totally agree. I don't understand why some teams that are going to 6 CIMs AND a two (or three) speed gearbox.

In order to take advantage of the extra torque, do you need some form of traction control? Are you using a PID loop to control the voltage ramp? Are you imposing an open loop ramp on the driver or motor commands? Do you monitor motor currents for stall? Do you simply let the wheels spin and degenerate to the kinetic CoF for a small time?

Mathematically probable increases in acceleration and usable speed (even without any fancy effects). Is the increase worth the cost to most teams? Probably not. Certainly there are some teams where it is worth the cost however.

[JamesTerm]

We didn't have any delays to spread out current peaks. We used six CIMs and each of our wheels was powered by two CIMs, so we had three wheels. Our drivetrain would have just been very hard to control if we had turned all the motors on full at once because it would have wanted to spin very quickly.


You said... would have been hard to control... so how did you solve it to where it didn't turn all the motors on full (or where the wheels didn't spin) so quickly? Was this a mechanical or software solution? If software... please elaborate... thanks.

BTW... this is the first video I've seen where 1114 successfully climbs... thanks for including it... I've been wanting to see that for quite some time. :)

[Andrew Lawrence]

(pro-hint: the acceleration difference between a 6 CIM and 4 CIM drive geared at 10ft/s isn't that much different. Start thinking more about acceleration/agility/control vs. top speed, its a real tradeoff, you need an amazing driver to control a robot above ~15ft/s)

Do the extra motors help with control? I never thought of that as an effect of a 6wd vs a 4wd.

[Chris is me]

Do the extra motors help with control? I never thought of that as an effect of a 6wd vs a 4wd.

He's implying a lower top speed helps with control. If a 10 FPS 4 CIM drivetrain gets you there as fast as a 15 FPS drive, then you gain controllability by going down in speed.

[Andrew Lawrence]

He's implying a lower top speed helps with control. If a 10 FPS 4 CIM drivetrain gets you there as fast as a 15 FPS drive, then you gain controllability by going down in speed.

Ah, that makes sense.

[SoftwareBug2.0]

You said... would have been hard to control... so how did you solve it to where it didn't turn all the motors on full (or where the wheels didn't spin) so quickly? Was this a mechanical or software solution? If software... please elaborate... thanks.

BTW... this is the first video I've seen where 1114 successfully climbs... thanks for including it... I've been wanting to see that for quite some time. :)

Having all six motors on for even one second would probably make you spin faster than you would want to. If you build a three-wheel holonomic drive you will find very quickly that it likes to turn. We had a gyro-based feedback loop keeping us from turning too much.

[MichaelBick]

Having all six motors on for even one second would probably make you spin faster than you would want to. If you build a three-wheel holonomic drive you will find very quickly that it likes to turn. We had a gyro-based feedback loop keeping us from turning too much.

Do you really have any data to back this up? 254, 610, 973 among others(including us) all had 6 motors and were extremely controllable.

[ErvinI]

Do you really have any data to back this up? 254, 610, 973 among others(including us) all had 6 motors and were extremely controllable.
I believe he's talking about a three-wheel holonomic, not a skid-steer.

[JamesTerm]

Having all six motors on for even one second would probably make you spin faster than you would want to. If you build a three-wheel holonomic drive you will find very quickly that it likes to turn. We had a gyro-based feedback loop keeping us from turning too much.

Ah ok, thats what I thought you did (or something along these lines)... did you make some trapezoidal motion profiling on the anguar velocity to do this? Or was it a fixed angular velocity?

[SoftwareBug2.0]

Ah ok, thats what I thought you did (or something along these lines)... did you make some trapezoidal motion profiling on the anguar velocity to do this? Or was it a fixed angular velocity?

Either of those sound like good ideas, but here's the whole of how it worked:
twist = -( twist - m_gyro->GetRate() / 200. );

[Ether]

...here's the whole of how it worked:
twist = -( twist - m_gyro->GetRate() / 200. );

What is "twist"?

[SoftwareBug2.0]

What is "twist"?

The value of the joystick axis that controls rotation.

[wireties]

The value of the joystick axis that controls rotation.

You can read "twist" directly from some joysticks, they report X, Y and twist.

[JimBowey]

Team 955 used a pair of VexPro 3 CIM gear boxes for Ultimate Ascent. We were dashing about at near 18 fps which helped a lot with scoring cycles.

Since then, we designed and built much better quality 3 CIM gearboxes. Neither the VexPro or our own design depleted well cared for batteries, so not to worry.

[JamesTerm]

We were dashing about at near 18 fps which helped a lot with scoring cycles.

That is amazing! I wonder if there is any video we can see of that, please send me a link if there is. Thanks.

[Bryce2471]

Yes, it was amazing. They were the captains of our alliance for the Portland regional. Watching them run circles around us was one of the reasons I decided that I don't like mecanums, and started pursuing swerve drives. I think they were the fastest robot I saw last year.

[JamesTerm]

Yes, it was amazing. They were the captains of our alliance for the Portland regional. Watching them run circles around us was one of the reasons I decided that I don't like mecanums, and started pursuing swerve drives. I think they were the fastest robot I saw last year.

Ah, I think this is the match you are referring to: https://www.youtube.com/watch?v=bIGNlT0VVTk
That was a cool match and the video speaks for itself. I am also a big fan of swerve drives (and a fan of the rare breed of teams that actually can pull off using them in competition). :)

[andypanda6796]

Hi guys,

I was just wondering if any teams who have had experience using a 6 CIM drive train could post about their experience. My team is considering going this route this year, specifically using the new Vex Pro 3 CIM Ball Shifters with a tank drive setup.

One of the specific questions we have is whether you've had any problems with drastic voltage drops when going from a standstill to full throttle? Or going from full speed to a forced stop i.e. hitting another bot/entering pushing match? Has it effected anything else i.e. radio power?

Any feedback would be great, thanks everyone!

[magnets]

Hi guys,

I was just wondering if any teams who have had experience using a 6 CIM drive train could post about their experience. My team is considering going this route this year, specifically using the new Vex Pro 3 CIM Ball Shifters with a tank drive setup.

One of the specific questions we have is whether you've had any problems with drastic voltage drops when going from a standstill to full throttle? Or going from full speed to a forced stop i.e. hitting another bot/entering pushing match? Has it effected anything else i.e. radio power?

Any feedback would be great, thanks everyone!

A person on my team played around with one on the off season, and he said it was a pretty good experience. He said that if you geared the robot for a moderate top speed, acceleration was very quick, and was very maneuverable. He also had some power draw issues. Full forward to full reverse caused the main robot breaker to trip, and that one doesn't reset, so the robot would be dead until somebody hit the switch again.

They solved this by ramping the motors (see here (http://en.wikipedia.org/wiki/Slew_rate)), and when they switched to 2 CIMs plus a miniCIM on each side, they couldn't get the breaker to trip.

[Canon reeves]

My Team is using 6 CIMS this season with a 5.95:1 ratio. it was relatively cheap and we figured we could go faster than most and push the basic bot and hit hard. When it come to a pushing battle, we have enough experience with tank to know that no matter what gear ratio or amount of CIMs, No robot can ever push us sideways, so if we can assist and get across the field quickly, and effectively stop anyone because we can go back and forth extremely quickly, so anyone faster than us, we can push, slower than us, we can block. Thats just what we choose to do from what our experiences are. I like the idea of a fast robot because you can cover more, if you're a rookie team or newer team, and you build the six motor drivetrain, and something to pick up the ball, and drove well and fed well, than you would be a great alliance pick. 6 CIMs with a good driver is a great way to set yourself apart and a great back up if something fails on your robot.

[cadandcookies]

No robot can ever push us sideways

Famous last words.

[Canon reeves]

Famous last words.

Actually, mine would probably be "hey yall, watch this!". I could never imagine an FRC robot being able to push a robot with tank drive that could only go forwards and back sideways? If one exists I hope I'm on their alliance, not the other way around!

[jman4747]

Has anyone adapted one of the gearbox/drive train calculators to account for a combination of different motors in a gearbox? With the miniCIM and Vex/Andymak having gearboxes for other motors with outputs made to mount where a CIM would, I'm wondering how to enter that. Or do I just write "2 2/3" for the number of motors if I have 2 CIMs & a miniCIM in one box?

[Richard Wallace]

Has anyone adapted one of the gearbox/drive train calculators to account for a combination of different motors in a gearbox? With the miniCIM and Vex/Andymak having gearboxes for other motors with outputs made to mount where a CIM would, I'm wondering how to enter that. Or do I just write "2 2/3" for the number of motors if I have 2 CIMs & a miniCIM in one box?I don't recall seeing any of the calculators adapted; however, I do recall two things on this topic (1) Paul Copioli designed the mini-CIM to be used in combination at 1:1 gearing with a CIM in a drivetrain, and selected its free-speed and stall torque with that in mind, and (2) Ether did some analysis last year aimed at answering your question above. Maybe he or Paul will see this thread.

Mini-CIMs have a higher peak efficiency than CIMs because their commutators are the same size, while the mini-CIM has a shorter armature. This difference allows the mini-CIM to operate in combination with a CIM without overheating, unless the loading is very high and prolonged.

[Knufire]

Has anyone adapted one of the gearbox/drive train calculators to account for a combination of different motors in a gearbox? With the miniCIM and Vex/Andymak having gearboxes for other motors with outputs made to mount where a CIM would, I'm wondering how to enter that. Or do I just write "2 2/3" for the number of motors if I have 2 CIMs & a miniCIM in one box?

See the presentation and associated spreadsheet by Paul Coipoli: http://www.fightingpi.org/Resources/Business/MSC%20Seminars/2013%20State%20Seminars.shtml

[vgdude999]

Last year my team used 6 CIMs and it was pretty awesome. We had huge torque and a pretty decent speed. Throughout the entire regional and an off-season event this past November we didn't lose any pushing fights (possibly one at the regional, can't remember).

However, use caution when having such high torque. In the last match we played at the off-season event I shattered our middle wheel on one side when I hit another robot, knocking the chain off and leaving our poor robot limping from side to side. We didn't realize until we disassembled it before this build season that all of the six wheels were cracked; that one wheel just gave first.

tl;dr: It's a load of fun to push around whoever you want, but be aware that it is in fact possible to push too far.

[Abhishek R]

Actually, mine would probably be "hey yall, watch this!". I could never imagine an FRC robot being able to push a robot with tank drive that could only go forwards and back sideways? If one exists I hope I'm on their alliance, not the other way around!

Speaking from experience, because a tank drive is inherently designed not to go sideways, you pretty much end up in a deadlock with no way to escape if you get shoved into from the side (I think it's referred to as a t-bone, but not sure). It's difficult to break until the defender lets up a bit either unintentionally or to avoid a penalty/ref call.

[Ether]

(1) Paul Copioli designed the mini-CIM to be used in combination at 1:1 gearing with a CIM in a drivetrain, and selected its free-speed and stall torque with that in mind, and (2) Ether did some analysis last year aimed at answering your question above. Maybe he or Paul will see this thread.

Both my analysis and Paul's are in this (http://www.chiefdelphi.com/forums/showthread.php?p=1314719) thread. See posts 1, 5, 13, 14, 15, 18, 19, 22, and 24

[Canon reeves]

Speaking from experience, because a tank drive is inherently designed not to go sideways, you pretty much end up in a deadlock with no way to escape if you get shoved into from the side (I think it's referred to as a t-bone, but not sure). It's difficult to break until the defender lets up a bit either unintentionally or to avoid a penalty/ref call.
And that is exactly what we want, if that one person is a great shooter, and have the only ball?? sounds like a win to me? But thats me and our team doesn't have experience in much else so if we can do that then it works for me!

[Canon reeves]

Last year my team used 6 CIMs and it was pretty awesome. We had huge torque and a pretty decent speed. Throughout the entire regional and an off-season event this past November we didn't lose any pushing fights (possibly one at the regional, can't remember).

However, use caution when having such high torque. In the last match we played at the off-season event I shattered our middle wheel on one side when I hit another robot, knocking the chain off and leaving our poor robot limping from side to side. We didn't realize until we disassembled it before this build season that all of the six wheels were cracked; that one wheel just gave first.

tl;dr: It's a load of fun to push around whoever you want, but be aware that it is in fact possible to push too far.
This may apply alot less this year because there are no obstacles to go over? last year running over those Frisbees at high speed wasn't great for wheels.

[Ryan Dognaux]

We tested out our gear driven single-speed 6 CIM 8WD for the first time last night and loved it - http://youtu.be/pQ6TMzGwV6U We'll be running a similarly designed drive this season with a 2 speed shifter incorporated.

http://i.imgur.com/dB2heTNl.jpg

Note that this frame is larger because we plan to use it as a test bed robot for seasons to come and it will double as a T-shirt shooter drive base.

[gpetilli]

Note that this frame is larger because we plan to use it as a test bed robot for seasons to come and it will double as a T-shirt shooter drive base.

With 6 CIM drive, I assume you are anticipating XXXXXL T-shirts

[Ryan Dognaux]

With 6 CIM drive, I assume you are anticipating XXXXXL T-shirts

Once we throw on the 6 T-shirt barrel shooter, 2nd battery, large air tank and compressor, it starts to weigh a decent amount. This also just gave us a chance to try out 6 CIM motors in the drive as no one on the team had tried it yet.

[JesseK]

Ether, do you have a 'drag race' spreadsheet lying around (i.e. multiple d(t) scenarios on a single graph) with the proper modeling? I may have to cobble one together if not. I've poked through your acceleration models, but haven't found the time to make them more user-intuitive in a spreadsheet.

Such a spreadsheet could help a team make the decision of whether 6 CIM acceleration is worth it or not in a few different situations:
- Robot attempting to 'turn a corner' around defense (i.e. get ahead of a defensive bot by 0.8-1.0 'robot lengths' in order to be able to turn in front of the defender and zoom away towards the goal)
- Robot attempting to beat a defending robot to a specific spot with N number of seconds to spare in order to get a shot off quickly

It would also help the drivers understand the limitations & strengths of the robot so they can make smarter decisions when defending or being defended. The driver coaches could setup specific scenarios to help give the drivers better practice.

[Ether]

Ether, do you have a 'drag race' spreadsheet lying around (i.e. multiple d(t) scenarios on a single graph) with the proper modeling?
...
Such a spreadsheet could help a team make the decision of whether 6 CIM acceleration is worth it or not in a few different situations:
- Robot attempting to 'turn a corner' around defense (i.e. get ahead of a defensive bot by 0.8-1.0 'robot lengths' in order to be able to turn in front of the defender and zoom away towards the goal)
- Robot attempting to beat a defending robot to a specific spot with N number of seconds to spare in order to get a shot off quickly


Back in early December I posted a large text file which lists the "time to distance" for a range of model parameters:

http://www.chiefdelphi.com/forums/showthread.php?t=122792

Would that be useful if I updated it for 6 CIMs?



Also, I don't know if you've seen it yet, but in mid December I modified the source to get the parameters from the OS environment (instead of being hard coded) and added a "ready to run" compiled executable attachment. It's at the bottom of the list.

http://www.chiefdelphi.com/media/papers/2868?

The executable gets its input from the OS environment, which the user can edit in the script that runs the executable. The user can generate a variety of CSV files for various values of the parameters. In Excel these CSVs can be combined to get a single graph if desired.

If there's enough interest, I was thinking of integrating it with gnuplot (http://www.gnuplot.info/) so that graphing would be more seamless.

[Spatel7793]

back in my day, 4 CIMS were enough and we had enough pushing force to push at least 2 robots at a time.

[Gregor]

back in my day, 4 CIMS were enough and we had enough pushing force to push at least 2 robots at a time.

Uphill in both directions?

[waialua359]

back in my day, 4 CIMS were enough and we had enough pushing force to push at least 2 robots at a time.

Last year we had 4 CIMS and teams made threads about us pushing them.:)
In 2010, we ran 2 CIMS and 2 FP with AM Planetaries for Breakaway. It is our best pushing robot, still.
However, for this season, we will bite.....and go with a 3 motor per transmission setup.

[Mr. Van]

Uphill in both directions?

Since we're going there... I remember when only TWO drill motors were enough - and when they weren't we learned to shift the provided drill planetary transmission...

Yes. Both ways - and we LIKED it!

- Mr. Van
Coach, Robodox

[Richard.Varone]

We built a 6CIM 2 Speed with a PTO last year and the results we're amazing in terms of pushing power and acceleration. It's biggest draw back was the voltage drop, was as low as 6V in some cases, luckily we foresaw this potential problem and added current sensors on each trans to monitor and adjust for this

http://www.chiefdelphi.com/media/photos/38567

[Abhishek R]

Last year we had 4 CIMS and teams made threads about us pushing them.:)
In 2010, we ran 2 CIMS and 2 FP with AM Planetaries for Breakaway. It is our best pushing robot, still.
However, for this season, we will bite.....and go with a 3 motor per transmission setup.

Yup, in 2012, we had 2 CIMS on each side and could push two alliance partners up onto the bridge uphill at once. Definitely lived up to it's namesake, Panzer.