We had our robot on a ramp for the first time tonight and the traction of our rear-wheel gear drive 4" Tetrix wheels is not what we are wanting. We wanted to use 4" Hi-Grip AndyMark wheels but we ran into difficulties mounting them in a satisfactory way so we went to the Tetrix wheels. Now we are having to rethink.
Reading various forums, it sounds like teams attach (glue?) strips of non-skid to their wheels. I know sheet materials are legal (and several non-skid sheet materials have been posed and approved in the rules forum) and glue/adhesives are legal… but how does increasing the diameter of the 4" wheel come into play and aren’t we modifying a standard part by doing something like this?
I’m posting here before I ask in the rules forum because: (a) I’m a total FTC novice and I’m not even sure I’m asking the questions correctly, and (b) I get great answers fast here on Chief Delphi!
As long as you add the padding and did not purchase it pre-attached, it should be OK (we do something similar with special Lego tires on the 4 inch wheels). For an official answer post on the official FTC forums.
The 4" limit only applies to purchased non-Tetrix/Matrix/Lego wheels. Teams can modify or build wheels to be > 4" as desired, subject to the allowed materials, field damage, and 18" size constraints. There are several official forum post dealing with this issue and it might be a good idea to have printouts handy as the above posts make it sound like some teams have been wrongly failed at their tech inspection.
All that said, if you have a quick way to make your robot 4 wheel drive (chains or gears?) you will likely get HUGE improvements in traction and drivability.
We definitely need to improve our drive system. We are too slow and we have unpredictable traction around and on the Ramp. Should we go for driving all four wheels? Do we need to go for four motors or is two motors on a chain/gear drive enough?
In my experience working with the ten Eagan Robotics FTC teams this year, either of the setups you mentioned would be just fine, especially considering your main problem is “unpredictable traction”-- the main solution for which is making sure your wheels have the same or similar traction and are getting the same or similar power from your motors.
If I were you, I would ask the question of which fits into your existing robot with the least amount of work.
That being said, under nearly any normal circumstances, a 4-motor drive will beat out a 2-motor drive in terms of pushing power. If your only goal is behaving predictably around the ramp and you have a better use for two DC motors, by all means a 2-motor chain drive should behave just fine.
Unfortunately I’m not at our workspace right now, but I’d be happy to get you pictures of our various drive setups if you’d like.
On another note, good job qualifying for (I presume) your super-regional tournament, and good luck when you get there!
EDIT: You can also check out some of the pictures in the FTC link in my signature for photos, but most of them don’t focus on the drive trains.
Without knowing all the details of your robot, I think the best advice I can give is to experiment and find what works. We had a very successful 2 motor, 6 wheel drive bot two years ago. It was slow but our design was such that we didn’t need much speed. That said, if we hadn’t spent our allotment of motors elsewhere, we definitely would have added them to the drive train.
Having a good drive base is really where every robot needs to start. TOO many times does our team see rookies with only two wheels motorized, usually direct driving them. Direct driving the wheels is really bad on the motors. Ypu really need to either gear or chain drive every wheel you have.
To give you an idea on a good drivebase, we use 6 wheels, all geared together, using the medium 80tooth gears on the motors, and the 120 tooth gears on the tires. Reason being for 6 wheels, one falls off, you can STILL drive. Not stuck. Along with all the wheels being geared, we run to motors per side of the robot, wired into the same motor port. 2 Motors left, 2 motors right. Alot of pushing power as well! Any robot that tries to play defence against us, just gets pushed out of the way. If you want to see our bot in action, look on youtube, Valley X Robotics, or here, www.valleyx2844.com <— pictures of it on the website.
We currently have two 40:80 gear driven 4" AM Hi-Grip wheels with two dual omni wheels on the other end. It works but…
Thinking through Saturdays events, it became obvious a lot of our weaknesses are traceable to inadequate traction, being too slow, and not being able to smoothly and reliably transition over the ramp edges.
I think having four driven wheels would be a big step to better.
We only own one motor controller so that is what we started with. Later I was able to borrow a motor controller for our lift motor and flag motor. For regionals I think I can borrow another motor controller.
We are definitely going to undertake a major whale dive into drive systems during the off season.
Our team personally stays away from the tetrix Omni-wheels. They just dont do well on the wood. As you said as well, they have a hard time with even getting ONTO the wood. Yes they are easier to turn with, but you loose certain things.
Diving into different drive bases is a good idea. Try to get ahold of another DC Motor Box. It will help you later on as your team grows.
The team spent some time tonight discussing the drive question. It’s been very interesting as a teacher listening to and letting these debates play out. Someday I’m going to record a few and let our administrators here them — these are engaged passionate students!
I think the choice is between two mortors driving four wheels via a gear train or four motors, one on each wheel. The debate was intense. I vetoed chain drives, mechanum, and tank treads since we have none of the parts and no experience with them.
During the discussion the areas we were lacking information and experience were the following:
Four wheels driven independently by four motors… This would be the simplest mechanically and lowest cost approach since would just replicate the two drive wheels we have. We were not sure how you synchronize the two wheels on each side if each wheel is driven by its own motor. In the software? Would this mean we would need a total of four motor encoders as well? And PID control? If so we only have one encoder and we were never able to get PID to work with our two current motors.
Two wheels on each side driven via a gear train by one motor on each side. Minimal SW impact but this means lots more gears and axles. Will there be enough power now with one motor powering two wheels and all the gears if we gear 1:1? Seems unlikely. Robot weighs 20 lb
If we tried to drive a geared side with two motors wouldn’t they fight each other if they weren’t somehow synched using encoders and PID?
I can we us doing all kinds of cool experiments in the off season!
To synchronize the motors on one side, you can electrically wire both motors into the same motor port.
Four wheel independent driven is fine but I personally had great experiences with all wheel drives (all wheels geared together). You are correct in there will be difference in motor performance but as long as you send the same power signal to both motors on the same side; the effect is insignificant compared to most of the other issues.
Chains on the drivetrain are acceptable as long as it’s tensioned correctly and robust enough for shock loads. However it is less robust and requires a bit more maintenance than gears.
As for a 2 motor all wheel drive; it’s totally doable however if you have the extra motors, put them on. Back four to five years ago when only 4 motors were allowed total, a robot from my region had an AWD and advanced very far in elimination at the world championship with a 2 motor AWD.
Record all the discussions in the engineering journal! In the real world, it’s a critical and essential part of the industry.
Thanks Henry! I had forgotten that we could wire two motors to the same controller port. We have the motors and mounts. Lets see if I get it…
Two mirrored assemblies: Left Side Drive & Right Side Drive
Parts list for ONE Assembly:
2 wheels with bearings and axles
2 DC motors and mounts
1/2 Motor Controller
2 Wheel Gears, one per wheel with axle and hub
2 Motor Gears, one per motor with motor hub
TBD Gears as needed to connect the gears attached to the 2 motors and 2 wheels into one meshed drive train
Wire both the motors onto the SAME Motor Controller Port.
Final gear ratio is determined by wheel gear/motor gear.
Size of intermediate gears does not matter.
Make note that each intermediate gear reverses the direction.
1.Can we run all four motors off one motor controller, ganging them up two motors per port? We can borrow another motor controller. CAREFUL of the wiring!
Is it better to mount the motors closer to the ends near the wheels or put them in the center near each other and drive gears out to the wheels?
I’m assuming the fewer intermediate gears the better?
Now I see how you could have the six wheel configurations with the two motor per side. Raise the ends an 1/8, which helps transition the edge… makes much more sense now!
This would be a LOT more fun if we didn’t have deadlines LOL
You can run all four motors off one motor controller. My team did it every year we were in competition and worked great. However make sure you get the polarity correct depending on how your gearing is setup.
Doesn’t matter. Preferably minimal transition gears to reduce slop but the slop is very little to minimal. Think of motor placement more in terms of how you want the weight distributed around the robot.
Yes - to reduce slop between all the wheels and to save money. However again in the grand scheme of things, it doesn’t matter much if at all.