Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

A unique, one of a kind, never before seen drive system. We have two flying inverse differentials(gyro-encabulators) that each power two swerve wheels for a complete robot. Stay posted for video!

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Besides the coolness factor (which, btw, is off the charts) what is the point of designing a drivetrain like this? It looks more complicated and more motors than a normal swerve drive (which is an accomplishment in and of itself).

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Actually it uses the same allotment of motors as a regular swerve so we don't lose anything there. as far as the point of doing it, it's multi faceted. with normal drive systems and the systems that are used for FRC, when you want a motor to go slowely or just a little bit, it often lacks the power to move at those lower values, so you have to give it a higher value than desired to get it to even move. with this design. you can move at 10% speed but have as much power as possible and make micro movements for very precise aiming without overshoot. if we run the reference motors at 100% all out, and the other one at 90% in the reverse direction. the total output will be 10%... at 90% of the motors power. At worst it will perform like a normal swerve when they spin the same direction, but when they spin opposite directions you get that major benefit at having lots of power at low speed so it offers a lot more versatility.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

That's really cool! I love the engineering and fabrication that went into this. The only thing I would suggest is to consider a way to lock your differentials if necessary (either with a limited-slip design, or a pneumatic clutch). Right now, if your robot loses traction on any corner (going over a field obstacle, or being lifted in defense), the unloaded wheel will spin freely, sapping all power from that side of the bot. (unless I'm misunderstanding the design)

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Oh, I am misunderstanding. I thought the differential split the power between wheels, but it splits the power between two motors to make an infinitely variable transmission. Neat!

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

indeed you are. this is an inverse differential not a normal differential. the wheels are locked together in speed so it doesn't matter if any of them lose traction. you can see the chain routing, it's impossible for any of the paired wheels to move at a different speed than the wheel it's paired to.

Here is a bonus pic, almost like a kit. https://i.imgur.com/QohIBXS.jpg

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

yup exactly. all without any additional motors or actuators than what would be required for a normal swerve. More complex? yes. Although it's less complex than a shifting one would be. Worth it? We shall see.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

I know I'm focusing on the "wrong part" but those Jags that appear to be nearly floating amuse me.

Looking forward to the video.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

ye we were short 2 SRXs. We have 1 coming as part of first choice whenever that comes in and we'll order 1 more. but won't let that stop us from advancing the project xD I have come to have a major dislike of jags. the SRXs are a beut though.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Anyone who saw 1658's robot from last year knows they are an up and coming powerhouse of the St. Louis area. This off-season prototype is crazy awesome, can't wait to see some video of it running!

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

I had a similar idea over the summer, but I never had time to design it. Looks awesome!

Are there any apparent problems? Does the differential work as planned?

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

This is amazing. That is about as close to the definition of inspiring as I know of. Fantastic. Wow.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

I'm sure it's not optimized for weight yet, but what does it weigh?

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

This is the coolest swerve drive (or drive, really) I have ever seen!
As far as this versus a shifting swerve goes, an IVT only gets a maximum torque equal to the maximum torque at the lowest gear ratio, so lowering the speed does not increase your torque unless the motors are drawing more current (not sure on that last part). I really want to see efficiency/ acceleration specs on this beast.
You also won't be able to do turn + drive maneuvers without scrubbing, it looks like, but you hardly need those anyway.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

What algorithm are you using to choose the motor speeds? Lets say you want to speed up the right side. You could speed up the top motor, slow down (or reverse) the bottom motor, or some combination of the two. How do you avoid running either motor at an inefficient part of its range (such as stalling one while the other runs flat out)? Just curious.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

This is pretty wild. Really cool idea and thanks for sharing!
What was your inspiration?

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

we already did a test with half a robots worth of parts and it looked very promising so we decided to continue and make the whole bot. we already know it WILL work, it's just a matter of how well as an overall design

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

in that configuration around 48 lbs however we can easily shave off over 5 lbs since a lot of the stuff on here was just thrown on because we had it and would work rather than trying to make it as low weight as possible. we will be well under 45 lbs on an actual game implementation.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

actually you can turn+drive without much issue. it just wont necessarily rotate around it's center depending on the driver/rotation relationship

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

still working on it but with the previous test we did my approach was as such:

the top cims are the reference motors, they always run just about full out but still PID speed controlled(so lets say 1k rpms) but that's completely up to us and arbitrary depending how much total power we want available. i will try to remove this though and just have them be full forward or full reverse without needing the extra control here. As you said there are a variety of ways to control this and we are still researching. Not sure how well that will work but will see. then the opposite side is variable speed to give us the speed we want from the output. with some simple math(just the average of the two inputs) we can get any output speed we want. so currently each cim is being speed(or position) PID controlled. We made half a bot work and currently in the process of making the whole bot work. Whole point of doing it is to see just how well and effective this design can be, it may or may not be favorable and that's why we're doing it, to find out!

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

a few years ago we had the idea of using an IVT on an FRC bot. There are enough advantages where it seemed like it would give us a very competitive edge in the competitions. looking online we found nothing though so we started playing with ideas. It took a while but we eventually came to realize that a differential could be used in such a fashion, but the obvious drawback is that it requires two motors per transmission and you would have to run them in active feedback speed control. You simply can't use that many motors and at that time we didn't have the capabilities to pull something like this of so it was put in the back of our minds.

So far we have tried various drive systems and the one we had the most success with is 4 corner holonomic which we used for years. It has a lot of it's own drawbacks though so we always had ideas on new drive systems and tried out different things over the years. We had to keep the maneuverability we were used to but we had to gain in speed and power. The only solution was some type of swerve. We prototyped a shifting swerve last year(of which a lot of parts can be seen on this bot) but when we found out the game had no defense we felt it was unnecessary to give up on the holonomic just yet. Due to this fact we were able to perform like we never had before since the weaknesses of holonomic drive didn't matter in that game and we were able to win the st.louis regional since we spent all of our time scoring rather than fighting other robots.

Well we know bumpers will be required again this year, which means robot on robot contact. From previous years we know that if you have a good scoring bot(which we had in previous years but would always get shut down) the other team will dedicate 1 if not 2 of their alliance bots just to get in your way and do anything they can to stop you. Well we don't want to be stopped. so we're trying to come up with the most effective solution we can think of. One of the issues with any drive system or motor control is you lack any control in the low end due to how the power is varied to the motors. so if you want to go slow or make fine adjustments most of the time you can't unless you over gear it which isn't desirable or spend a lot of time trying to tweak in overshoot/undershoot the best you can with PID control which in the end may still not be enough. So with this design we kill multiple birds with 1 proverbial stone.

As with any design however, it has it's own advantages and disadvantages. While we are aware of some of them already, we decided to build a whole bot to try to determine most if not all of the rest of them with such an implementation.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Certainly different! I haven't seriously tried to wrap my brain around a swerve drive with pair-coupled wheel speeds and fully independent steering, but I have done a bit of thinking about the transmission.

An inverse differential is a great idea to work around the motor controller dead band, but I don't think this is the right application for it. In the no-friction approximation, a the speed of an inverse differential on the output is equal to the average of the input speeds, and (if frictionless) the torque is equal to the sum of the input torques. If you're driving the wheels in opposite directions, this implies that the speed is equal to half of the difference between the magnitudes of the input speeds, and the output torque is equal to the difference of the magnitudes of the input torques.

If I've done my mental algebra correctly, this implies that if the two motors are driven in opposite directions, the net output power of the inverse differential is never greater than you could have obtained with a single motor at that speed (leaving motor failure out of consideration for the moment). If so, putting two CIMs into such a drive system is just a better way to generate heat than putting two CIMs into an ordinary gearbox; they have enough thermal mass to handle that bit of back driving when you need low speed, low torque (single motor dead band) maneuvers.

It seems to me that a better application for the inverse differential would be for a manipulator arm driven by low-thermal mass, fan cooled motors. You could run them at (for example) 51% and -49% of free speed to get a 1% output speed, while providing plenty of cooling air to both. If I'm figuring things correctly, when under an appropriate load the reverse motor would be running nearly free at around -6V, and the forward motor would be running around 12V (at the top of the power curve), so you could get nearly half of one motor's stall torque at very low (or even zero) speed for a prolonged duration, making it great for holding up elevators and arms without brakes and without releasing the magic smoke.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Update on this point: I haven't done any formal knematics with this yet, but if you're going to couple wheel speeds while having independent swerve steering, there does not appear to be significant benefit to having two different drive trains each powering two wheels compared to having one system that serves all wheels. A single drive train system with wheels in a rectangular or equilateral triangular configuration can translate in any direction or spin around its geometric center with no slippage. It can be steered in directed motion similarly to an automobile (or even a fire truck). It can also rotate around any point which is not too near a wheel with minor wheel slippage. It is also amenable to use with a 3-wheel drive system, which would reduce cost, weight, and number of parts compared to a 4 wheel system. The 3 wheel system would also have fewer places to avoid trying to rotate about, and wider avenues to transition the center of rotation from infinity to the center.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

http://www.chiefdelphi.com/forums/showthread.php?t=2548

217 did something similar back in the day before Vex, AndyMark, etc...

Very impressive.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

I remember 217's IVT (and designed a couple of my own) but I did not know they put it on a swerve drive! How well did it perform?

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

Cool prototype. The long unsupported chain spans may become problematic. Chains in that config like to derail. IVTs are very cool. Be mindful of 0 out put speed as some designs end up throwing a lot of power into the gearbox and cause damage. I am not exactly sure how your set up is configured, but would love to see a diagram.

Re: Team 1658 prototype flying inverse differential(gyro-encabulator) bi-swerve
 

we already added one set of chain stabilizers/guides on the long runs which seem to have addressed floppy chain there but may need additional support on the diagonal runs. We will address issues as they arise. There will be several modes of operation with this design. A "direct" mode where the motors are always driven in the same direction and we just vary the pwm/speed(depending on control mode) going to the coupled motors. This mode won't give us any advantage of the IVT but good to have as a fallback. An "efficient" mode where we run the motors at their peak efficiency instead of all out in opposite directions in an attempt to conserve power, and a "power" mode where we go full tilt and will indeed dump all available power into it. in 0 output we just completely turn the motors off though. It's certainly still in the developmental and non field tested but we're doing what we can to get there. Hopefully before the new season starts so we can see if it's even worth implementing on a real competition bot.