Direct drive minibot - output diameter?

[Ether]

Originally Posted by dsmoker

As for the output diameter, the students have been experimenting with diameters ranging from 1/4" to 3/8".

For sure, experimenting is good! But I think one example of what John might have had in mind is has anyone taught the students how to read the motor curve for the Tetrix motor and do the calculations? It's not that hard to do and can be very rewarding and inspiring for a student.

[DonRotolo]

Originally Posted by pfreivald

There are three lathes in the shop in which we work, but we aren't allowed to use any of them due to

OK, so is ANYone allowed to use the lathe(s)? If yes, have HER (or him) drill the hole for you. end of story.

Originally Posted by dsmoker

Wow. Harsh.

Not really. Just a reminder that copying is good, but learning and making it even better is, well, better. That's Engineering.

[Ether]

Originally Posted by boomergeek

We don't recommend direct drive on a heavy robot- it did not work for us on a 4 lb'er.

Could you provide a bit more detail? What exactly didn't work?

[Paul Copioli]

I can't find it now but I made a post a long time ago on this forum (maybe 2004 or 2005) titled "Gear ratio doesn't matter." The point of that post was that gear ratio alone doesn't matter, you have to take wheel diameter into consideration, too. In addition, weight has nothing to do with the optimal gear ratio either. Higher mass means that the wheel diameter for a given gear ratio (in this case a gear ratio of 1) must be smaller to generate enough force to counteract the weight (mass * g) and other acceleration of the mass. To simply show the relationship let's use simple F = ma. F in this case is T/Rw (torque divided by wheel radius) and a is g + your desired acceleration at max power. As you decrease your wheel radius you will increase your force which is needed for higher mass.

The bottom line is that the ability to climb the pole at all really only has to do with the motors and mass (in its simplest form). Motors represent the max available power. You just have to find the right combination to lift the mass.

[Bill_B]

Originally Posted by Jeffy

Then, put tape over the holes in the motor and connect it to a power source and turn down the brass gear to ~.175" ID.

.175 OD, right?

[Ether]

Originally Posted by Paul Copioli

The bottom line is that the ability to climb the pole at all really only has to do with the motors and mass (in its simplest form). Motors represent the max available power. You just have to find the right combination to lift the mass.

Like this?

[Gdeaver]

A direct drive mini-bot can give a minimalist design and most likely the fastest times. After looking at 118's design on the forum, we played with direct drive. After, some experimenting with direct drive I pushed our team away from direct drive. For us direct drive is a bad choice. Why? With direct drive any binding or accidents can lead to a smoked motor. We already have 8 carcasses laying around and this budget drain had to stop. With a little more metal we were able to design a friction drive using surgical tubing. Now if something goes wrong we shred a little piece of surgical tubing. 2 minutes per side to replace the tubing and the bot is going again. This also allows easy experimentation with the width of the rod wheels and pole tension. Even better it does not require precision machining. Our mini-bot is sub 2 seconds and weight is about 2.5 Lbs. The mini-bot turned out to be a beast of a problem and can be tamed in many ways. Now alignment and deployment is another beast of a problem. I hope teams are watching deployment offensive and defensive as the game evolves in the coming weeks.

[boomergeek]

Originally Posted by Ether

Could you provide a bit more detail? What exactly didn't work?

Our 4 lb robot just destroyed both latex tubing and shrink tubing for small diameter shafts and 4 lbs would stall for larger diameter shafts that gave enough surface area under enough normal force. It is possible that someone can find a surgical tubing that can hold the pole with the very little surface area of smaller diameter shafts without getting destroyed.
(But energy is better spent on lightening the minibot to allow a larger diameter shaft).

The related issue is the normal force on the wheel needs to be proportional to the weight of the minibot. There was another thread that did some experiments to determine optimum normal force.

So in effect. the stress on the tread was geometrically reduced by the reduction of the weight of the robot.

[buchanan]

The shaft diameter calculation based on peak power should be valid for a machine already moving at the speed at which peak power is developed. There are a couple of other considerations, though.

1) You have to check that the torque is available to accelerate the mass of the machine from a standing start at the chosen drive ratio (diameter). Otherwise your bot just sits on the launch pad and smokes. This calculation should be fairly easy, though.

2) The rate of acceleration must be taken into consideration if you want to (and you should) optimize elapsed time rather than simply a top speed there may not be time/distance to reach. Improved acceleration from a smaller shaft trades off against reductions in top speed. This is a somewhat harder problem, but certainly solvable, though I'm personally too lazy to do it. As a practical matter other factors argue for a far more conservative value than this optimization would give anyway.

[Grim Tuesday]

Originally Posted by Ether

Like this?

We tried with a .625 diameter shaft, and the minibot didn't work for us, though our design involves two motors on one shaft. We had to settle for a .25 shaft wrapped in surgical tubing. Still clocks in under 2 seconds, though.

[Ether]

1) You have to check that the torque is available to accelerate the mass of the machine from a standing start at the chosen drive ratio (diameter). Otherwise your bot just sits on the launch pad and smokes. This calculation should be fairly easy, though.

The highest torque output is at stall, not at max power. So if you have enough torque to maintain a speed of V at max power, the torque only gets higher as the speed decreases. So it's not a question of enough torque. It's a question of will the motor overheat. And you can't calculate that from the published motor data.

2) The rate of acceleration must be taken into consideration if you want to (and you should) optimize elapsed time rather than simply a top speed there may not be time/distance to reach. Improved acceleration from a smaller shaft trades off against reductions in top speed. This is a somewhat harder problem, but certainly solvable

You read my mind. It's on my to-do list. Just haven't gotten to it yet. I suspect however that the accuracy of such a calculation will be driven largely by unknown factors like friction, motor tolerances, tire slip ratio (especially with compliant material such as surgical tubing), battery condition, etc.

As a practical matter other factors argue for a far more conservative value than this optimization would give anyway.

Other factors like those listed above, or did you have others in mind?

[pfreivald]

Originally Posted by DonRotolo

OK, so is ANYone allowed to use the lathe(s)? If yes, have HER (or him) drill the hole for you. end of story.

Nope. No one on the team... And the tech teachers are singularly uncooperative. (We're working on a way to end-around them, but it's taking longer than I'd like.)

And for the record, a sub-2s minibot doesn't require direct-drive with a tiny output shaft. Our custom wheels are over an inch in diameter, and we're still using one stage from the gearbox, and with deployment we're at 1.8s.

[Ether]

Originally Posted by Grim Tuesday

We tried with a .625 diameter shaft, and the minibot didn't work for us, though our design involves two motors on one shaft. We had to settle for a .25 shaft wrapped in surgical tubing. Still clocks in under 2 seconds, though.

How much does it weigh?

[Ether]

Originally Posted by pfreivald

Our custom wheels are over an inch in diameter, and we're still using one stage from the gearbox, and with deployment we're at 1.8s.

May I ask, what is your gear ratio and how much does the bot weigh? I assume you are using 2 motors?

[JVN]

Originally Posted by dsmoker

Wow. Harsh. They simply wanted to hear what others' experiences had been in order to confirm or dispel their own conclusions.

Glad to hear. I hope everyone else is following the same order of operations. I suspect this isn't true. Kudos to you and your team.

-John