Coach - need some basic physics help

Hi everyone, first post.

I am NOT a team member, but a mentor/coach. We decided to start a vex club for some 14yolds, and have had some fun. One of the things I noticed this season was that we were suffering from lack of physicsing. My young members are surpassed in lack of physics knowledge only by myself who hasn’t taken science since I was 15.

We’ve found that drive-train is very forgiving of our bumbling, but when we try to build anything meaningful - lifting arm, launcher, etc our trial and error doesn’t work. Motors stalling sporadically, too many redesigns, etc. I’ve looked at the VEX Curriculum and this forum for some tips on calculations and design, but it’s a bit overwhelming and sometimes contradictory. My math is just fine, I can follow the formulas and calculations, but can’t see how it all comes together specifically for VEX motors. Some of the formulas seem to also be to calculate ‘holding’ an object vs. lifting it or ‘throwing it’.

I was wondering how teams went about figuring out gears / number of motors for their design. To be a bit more practical… say a lifting arm from this season which we had to abandon. (They went from throwing the star, to lifting the star, to being a pusher bot, lol.) Just some broad strokes would be helpful to guide my research into the details.

With our robotics classroom, we usually take cornell notes and get tested of all things about this website which shows practically all equations an information when creating the most efficient and reliable robot:
http://curriculum.vexrobotics.com/curriculum
This website is directly from the VEX community, and it is one of the main sources of practically most equations.
Whenever it goes to programming with robotc and easyc(I think as well) we use cs2n.org
Hopefully this helps :slight_smile:

I second the http://curriculum.vexrobotics.com/ as a resource. There’s also a “super clawbot” trainer here: https://content.vexrobotics.com/docs/Super_Clawbot_Web_110416.pdf

And consider joining the VEX World Coaches Association of facebook. Many ideas/tips are exchanged among coaches/mentors.

To be honest, we really don’t do that many calculations in my club. How many motors/what gear ratio we use is typically the result of trial and error, but more experience with the mechanisms. There are a few tips that apply to most mechanisms though:

  1. Minimize friction. The gears in a gearbox should be able to move will extremely little effort when the arm and motors are not attached.
  2. Don’t use steel on an arm. Seriously. IIRC the vex steel is 3x heavier than the aluminum, and of course that is amplified more and more the longer the arm is.
  3. RUBBER BANDS! Whenever you have any mechanism that has a limited degree of freedom and only needs significant torque in one direction, add rubber bands to it. This year especially, >= half of the energy required to lift stuff should be supplied by rubber bands.
  4. When holding an arm still, be sure to never apply more than ~15 motor power or so, otherwise the PTCs will overheat and the motors will be temporarily inoperable.

I’ll have to give the curriculum another shot. I found that while I could understand it while reading, and took a few notes regarding formulas, I was still at a loss as to how to turn that into something that would help to “lift”. Perhaps just apply a fraction.

@sazrocks: Thanks. Those are practical tips. I’m hoping that I can find something a little more mathematical. 4 is a good tip. I don’t think the team was really limiting power much. They just tied it to the channel. That’s probably why they had as many problems as they did.

To be honest, it’s always a viable strategy to clone a strong robot early in the season and then tweak an already working robot to find what works best for you

For us, we have a level of intuition for how much a VEX motor (or a certain quantity) will lift. Of course, that also depends on arm length (our tower is only 13.5 inches tall to facilitate a very short arm that can lift lots of objects insanely fast).

I find it very helpful to have an understanding of the 393 motor torque-speed curve. Of course, properties like system friction and robot inertia will affect actual overall performance A LOT, but having an appreciation of these curves helps understand the tradeoffs of any design. I’m always stunned by the number of students who seem to know/care nothing about the torque-speed properties of the motors.
https://vexforum.com/index.php/attachment/56464d7f91a13_torque_speed_393.jpg

It’s from a post done by the venerable jpearman, here: https://vexforum.com/t/motor-torque-speed-curves-rev2/21868/1

Specifically for lifts, the equation you’re looking for is torque = radius x force. Torque of one motor is given by vex, so plugging that in looks like (torque of 1 motor)(number of motors)(gear ratio)/2 for non-stall torque = arm length * load.

From here we solve for the gear ratio and choose the closest possible gear ratio. Sometimes we will do this in reverse using an already set gear ratio to calculate how much we can theoretically lift.