Direct drive minibot - output diameter?
 

We're redesigning our minibot after having seen how effective the direct drive minibots were at WPI, but we have a couple of questions. Have people been leaving the little gear that's on the output shaft and then press fitting something over it, or have they removed it? Second, what have people found to be effective as an output diameter wheel against the pole? We were looking at around 3/8."

Re: Direct drive minibot - output diameter?
 

It sounds like you're on all the right tracks! :-)

First of all, I'll preface this by saying that 1519's minibot has not been optimized... what we're doing are likely not the best ways of doing things!

Our minibot uses direct-drive and can climb the pole in about 1.3 seconds. To answer your questions, we use a 1/4" shaft with surgical tubing of 1/4" ID and 3/8" OD simply slid on over the shaft. It is "press fit" (snug, but by no means a true press fit!) onto the pinion gear of the Tetrix motor with an 8-32 set screw to help lock it in place on the pinion. That hasn't failed once in 1 regional plus quite a lot of practice, although making sure your set screws are tight is always important! The other end of the shaft is supported by a bearing.

We are considering increasing the diameter of the motor and trying to remove a couple ounces (we weigh in at 2.3 pounds, currently) to increase our speed slightly. Really though, it's all about reliability... the primary reason we went with this design was the magnet attachment to the tower - no clamping or grasping involved.

Feel free to ask any questions you like! Best of luck!

Re: Direct drive minibot - output diameter?
 

3/8 inch is good. Remove the spur gear . The motor shaft is 2.5 mm. A two inch long piece of 6061 drilled with a 3/32 bit can then be carefully pressed on with thread locker.

Re: Direct drive minibot - output diameter?
 

we removed the gear.

3/8" is in the ball park.

Let's consider if you are currently using a 4" * PI wheel circumference with a 52:1 gearing

4" * PI divided by the loss of 52:1 efficiency would imply about a 0.075" * PI wheel

Efficiency goes way up getting rid of the gears.

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

If any of these ideas help... consider paying it forward. If they don't help, ...sorry.

Has anyone drilled the shafts without using a lathe? I would be curious as to how to get it accurately centered enough and straight in without a lathe.

Re: Direct drive minibot - output diameter?
 

We also removed the brass gear and drilled our shaft with a #40 drill bit. A lathe was used to ensure concentricity of the hole to the shaft. We used the flatted tetrix shaft.

Re: Direct drive minibot - output diameter?
 

It will be a rough ride to the target if you don't use a lathe wih a properly aligned tailstock. You could try a drill press, drilling a hole in a block of plastic or aluminum with it securely clamped on the drill press table. This hole would be equal to the shaft diameter. Then change drills to a small center drill, and then step up in crements to your final hole size. Not perfect, but will get you close. Good luck. Isn't there a local machine shop you could appeal to?:) :)

Re: Direct drive minibot - output diameter?
 

Team 241 has a lathe that we spent time to align the tail stock- it started off about 0.07" off center.
My point is there are about 10 threads on minibot direct drive that say "drill a hole".
(I think some of the audience here on CD sees "drill a hole" and assumes a power drill or a drill press might be sufficient to try;
Or see suggestions like "take off the gear" - but have no idea what a real gear puller is and think of "hammer and pry bar" instead.

I cringe at the potential for wasted time and wasted parts from the possibility of people not having a concept of what the right tools are when someone makes a suggestion to do something.

Re: Direct drive minibot - output diameter?
 

I think you misread what was posted. I interpreted what they were asking to do as how to drill a concentric hole without a lathe. Making a jig to hold a shaft vertical on a drill press is certainly better than "eyeballing" the center. Sometimes you have to be creative to get the job done.:) :) The final word was to suggest soliciting a local machine shop to help. The days are long gone when Dad had a workshop in the garage and taught his son or daughter how to use tools, and what they were called. FIRST is helping to fill the gap between engineers and the "hands on" people who actually build the product.

Re: Direct drive minibot - output diameter?
 

It's all good. I was the person to bring up the use of the word "lathe" after the word "drilled" was used. In many cases, the person reading "drilled" in this context assumes a lathe... but many reading here on CD may not have enough machine shop experience.

Adding some detail on identifying the right tool or a reasonable equivalent is useful to many folks. I really like your idea of the potential for using a good drill press to drill a concentric hole.

Press-fitting long shafts is also tricky:
We used our heavy duty drill press to hold the shafts and raised the motor on the drill table into the pre-drilled shafts.

Re: Direct drive minibot - output diameter?
 

If you can't find a way to make perfectly center drilled shafts, you are not alone.
Here is what we did:
We used 1/4" aluminum tubing with about .172" ID.
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.
Wen you go to press on the shaft, you need to be cautious to get it lined up straight. Also, you will need to support the end of the motor AND the gear. We used a pair of steel rulers and a vice for this.
A note on wheel size:
The optimal size is different for each minibot. You will need to know your weight and estimate the amount of drag you have correctly. If you need help with the math, first try your team mentors and physics teachers. If they arent helpful, I'm not sure what they are doing, but nonetheless, then CD can help.

Re: Direct drive minibot - output diameter?
 

We have a ~1.8 second minibot that is very consistent, and we don't have access to a lathe (due to some stupid crap... There are three lathes in the shop in which we work, but we aren't allowed to use any of them due to silly school politics that six years of effort have not circumvented.)

We modified and kept the gearbox, but removed two stages for a total 5:1 reduction. We then added custom wheels that are machined to match the radius of curvature of the pole.

It is very consistent and pretty fast, but not as fast as, say the Thunder Chickens' press-fit 1/4" (or maybe it's 3/8") direct-drive minibot.

Re: Direct drive minibot - output diameter?
 

Obviously after the first 2 weeks of competition the secrets are all out.

If you're going to use a design that is "heavily inspired" from one you saw on the field, at least take the time to iterate the design and figure out the details yourself.

You can make this into a positive design experience for your team with some methodical experimentation...

Re: Direct drive minibot - output diameter?
 

An alternative to press-fitting may be to make a shaft coupling from latex tubing. If you get the proper size, it seems it could stretch over the helical motor pinion. Perhaps a zip-tie would help secure it.

Re: Direct drive minibot - output diameter?
 

Wow. Harsh. We asked one specific question to confirm some of our preliminary calculations. The students never asked for anyone's complete minibot design, nor do I anticipate that their final design will be "heavily inspired" by any one particular minibot that they saw perform at WPI. They simply felt that, after seeing all of the minibots in action, they wanted to try their hand at a direct-drive system to avoid the mechanical loss that naturally occurs through gearing. The drive train is but one aspect, obviously, of minibot design. As for the output diameter, the students have been experimenting with diameters ranging from 1/4" to 3/8". They simply wanted to hear what others' experiences had been in order to confirm or dispel their own conclusions. I posted their question for them because I thought that that's exactly what this forum was for.

For those who offered constructive advice on this thread, thank you!:)

Re: Direct drive minibot - output diameter?
 

Let's try to assume John was not intentionally implying anything negative in the way he responded. After all, he works for a robotics company that wants everyone to use their products- so why would he ever imply anything negative about any of his customers? I'll have to assume we must be misreading anything negative.

I offer thoughts and encouragements in order to get teams to try things. Anyone who has tried to build a direct drive minibot knows that significant amounts of trials and experimentation are necessary to be successful.
Does every team need to reinvent the physics description on their own?

I think the minibot challenge this year dramatically increased the amount of time and energy that teams would spend on engineering.
I know that is true for our team. So overall, it is good for the primary goal of FIRST. It causes the most innovative teams to lose some of their advantage over the "incrementing" teams as the season wears on.
But to me, it's a no-brainer which has better impact on getting more students working on engineering problems for longer periods of time.

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

OK, so is ANYone allowed to use the lathe(s)? If yes, have HER (or him) drill the hole for you. end of story.
Not really. Just a reminder that copying is good, but learning and making it even better is, well, better. That's Engineering.

Re: Direct drive minibot - output diameter?
 

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

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

.175 OD, right?

Re: Direct drive minibot - output diameter?
 

Like this?

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

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.

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.


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

Re: Direct drive minibot - output diameter?
 

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.

Re: Direct drive minibot - output diameter?
 

How much does it weigh?

Re: Direct drive minibot - output diameter?
 

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

Re: Direct drive minibot - output diameter?
 

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

Re: Direct drive minibot - output diameter?
 

We did some calculations to find what our desired shaft size was, built a few shafts with diameters a bit over and under that number and tried them all. It turns out in our particular case the base shaft diameter isn't as important as making sure there isn't too much wear on the tread. After it's put it about 10-20 times our tread material becomes noticeably worn and we see drastic changes in the speed.

Re: Direct drive minibot - output diameter?
 

Two motors, gear ratio out of the modified box is 5:1, and the weight I am not 100% sure of, but she's not much more than motors, mounts (with lightening holes drilled), two switches and some polycarb (more lightening holes).

Attached is an Inventor drawing. Ain't she cute?

Re: Direct drive minibot - output diameter?
 

For sure. But how do you deploy it? Is it a press-fit onto the pole, or is it articulated?

Re: Direct drive minibot - output diameter?
 

The polycarb acts as a spring to space out the wheels so they fit around the pole. It's held in place with a bracket on the HOSTBOT which is punched out by a plunger on the MINIBOT when it hits the pole (this plunger also turns on the wheels, and another one turns them back off when it hits the plate).

So the MINIBOT is 'squeezed open' and held in place by a post on the deployment mechanism, then snaps in place and turns itself on using the HOSTBOT-imparted lateral kinetic energy to pop the plunger. It then drives off of the post.

Edit: Attached is a closeup shot (a bit blurry) where you can see the wheels and plunger, as well as an Inventor drawing of our deployment mechanism. (The carriage is deliberately a bit massive. This is not a subtle device!)

Re: Direct drive minibot - output diameter?
 

A more specific relationship than F=ma in terms of rotary acceleration is
Tau = I * Alpha
or
Alpha = Tau / I
Where
Alpha = rotational acceleration
Tau = Torque applied to the rotation
I = Moment of Inertia. Generally that's [mass, in kg] * [radius, in meters]^2

The radius^2 matters in this case. This is really because in almost all cases our sources of force are applied in the forms of torque from a motor. The generic moment of inertia works out because most of the time the mass at the radius far exceeds the mass averaged over the radius (e.g. a wheel weighs much less than the robot it pushes).

Since electric motor torque output is a function of its speed, and speed is a function of acceleration, the overall equation quickly becomes non-linear. So I've only ever put it into Excel to figure out the final numbers. Yet most of the errors I experienced went away when I swapped from trying a direct F=ma calculation to T=Ia (non-linearity induces round off error sometimes, so it's still just an estimate).

Re: Direct drive minibot - output diameter?
 

Yes. I am satisfying my CD addiction on vacation from an iPad...

Re: Direct drive minibot - output diameter?
 

I set up the differential equation and found an analytical solution. You can see the effect of acceleration, which is interesting and instructive.

I tend to agree. Compared to variations in motor performance, friction, battery voltage, and whatever else, it's probably not the major factor in determining what the real-world result will be. But still interesting and instructive.

Re: Direct drive minibot - output diameter?
 

Hm, I was thinking of the tech teachers, not someone on the team. This is an odd thing for me to hear, since our tech teachers (and supervisor, and administration, and BoE...) are all very enthusiastic about the robot team. I guess some teachers need to re-learn why they went into teaching...
That's an understatement! Our deployer is similar, riding on some Igus track and using latex tubing as the 'slingshot'. Deploy is about 0.15 second, whacking the minibot onto the pole with audible force. It definitely works.

Re: Direct drive minibot - output diameter?
 

Using a drill press and a table vice: turn the drill bit upside down and place into the chuck of the drill press, tighten the chuck and make sure it spins without wobble. Turn off the drill press. Lower the spindle so that you can position a vice and clamp the drill bit, clamp the vice into place on the table. Loosen the chuck and raise the spindle. You now have a drill bit clamped into the vice dead center of the spindle. Put the shaft into the drill chuck and tighten. Drill out center of the shaft.

The shaft spins, the drill bit remains stationary.

Take care, plan out all the steps, make sure that the shaft can be placed into the chuck without moving the vice and drill bit.

Re: Direct drive minibot - output diameter?
 

Thanks for the suggestion. Now we have two.
Your method and teched3's:
"You could try a drill press, drilling a hole in a block of plastic or aluminum with it securely clamped on the drill press table. This hole would be equal to the shaft diameter. Then change drills to a small center drill, and then step up increments to your final hole size. Not perfect, but will get you close."

Re: Direct drive minibot - output diameter?
 

This has to be one of the best fabrication tips I've heard so far. Thank you so much for sharing!

Re: Direct drive minibot - output diameter?
 

Here's a tip for those of you without a lathe.

Take the Tetrix wheel hub, remove the set screw and drill the center out to .248" Now the wheel hub will fit right over the pinion gear. Put the locating flange of the wheel hub on the motor side. Add a little Loctite and tighten down the set screw. Now take the axle hub and drill out two of the 6-32 threaded holes(opposite). now you can take two of the allen head screws in the kit and attach the axle hub to the wheel hub. Now you have a nice coupler to attach the axle. Look in the Grainger catalog at shoulder bolts and find one that will fit your specs.

Roy ;)

Re: Direct drive minibot - output diameter?
 

This is what 810 ended up with. Press fit onto the brass spur gear plus a set screw. The actual "drive" part of the shaft measures .287, which happens to be a perfect fit for the surgical tubing we're using.

Re: Direct drive minibot - output diameter?
 

We did something similar to what Roy's team did. Instead of drilling out the motor shaft hub, we hooked the motor up to a bench DC supply and held a file against the pinion gear until it fit in the motor shaft hub.

We put two of the motor shaft hubs back-to-back. We were able to find a pair of holes where the threads on the two hubs for the 6-32 screws matched up without binding on the second hub.

You can then use that to hold pieces of the gear box you have cast aside.

Phil

Re: Direct drive minibot - output diameter?
 

Does anyone know the rpm of the tetroxide motor without the gear box?

Re: Direct drive minibot - output diameter?
 

As in dinitrogen?

The Tetrix motor (without gearhead) spins at roughly 7 350 rev/min, according to the data collected in this thread.

Re: Direct drive minibot - output diameter?
 

Tristan gave an answer in the previous post, but just curious, why do you ask?

The Tetrix motor has a speed v torque curve which is more nonlinear than other motors in the FRC kit. If you linearly interpolate between free speed and stall torque you'll get answer for max power notably larger than the true value. For LogoMotion it makes more sense to use a linear interpolation between stall torque and max power (@rpm) for motor calculations.

Re: Direct drive minibot - output diameter?
 

Any reason you chose the flatted shaft? I would think the round shaft would be better for balance at high speed.

Re: Direct drive minibot - output diameter?
 

After you put surgical tubing over the tetrix shaft it becomes quite round.