First and foremost, I would like to make clear that I have very limited knowledge when it comes to electronics and robotics, however, I would love to change that ASAP. I’ve always had a deep passion and interest for electronics and robotics, and I would really like expand my knowledge and begin to hobby, at the very least.
I am currently grouped with a team of aspiring engineers to create a portable, mobile (on/off road), and solar powered desalinization unit to provide clean drinkable water. I am in charge of building the chassis for the mobile unit. After doing a little research online and a recent visit to a local high school robotic team’s shop, I have truly been inspired, and my interest in electronics and robotics has reached a new high. I hope I will be able to impress myself as well as well as my peers, instructors, and the crowd during presentation time.
Although many of my team members have expressed their concerns and doubts, I am convinced that I will construct a motorized, robotic chassis, and I will have a blast in the process! The frame of my chassis cannot exceed 24"x36" and I would like the chassis to be able withhold and efficiently transport at least 200 lbs. of material. Again, I am very raw when it comes to this, but my first thoughts would be to construct a 4/6 wheel, skid steer chassis able to operate on and off-road (light terrain). The chassis will eventually have to house and transport a portable water desalinization plant, and solar panel tracking unit. Still waiting on specifications from the other teams. This is currently where I am at, but I am looking to improve on the thought, design, and fabrication of this project with haste.
I have been recently referred to this forum and I am simply here to seek knowledge to help me with the process. I am NOT pleading for someone to design a chassis for me. If anyone has links to help guide me through this process or if anyone would be kind enough to talk with me, mentor me, and guide me throughout my process I would be elated. Any and all advice will be greatly appreciated.
I hope to hear from you all, and thanks in advance!!
There are plenty of whitepapers and such on CD about robot drive system design. As you look at them, be aware that your vehicle will need to support a heavier payload (200 pounds vs a game piece or two), and that FRC robots are designed for a flat floor and carpet as the primary surface, with an occasional well-defined platform, ramp, or bump. The 6-wheel skid steer most typically used in FRC has the center wheels dropped about 1/8", which is enough to shorten the wheelbase for turning - on a flat surface. Off-road, this will not be the case, and you would often find that you are unable to turn.
I also did not notice anything indicating that you were in competition with another similar device, so speed is probably not as important as getting there reliably.
I would therefore think more in terms of a pickup truck than an FRC robot in looking for candidate drive trains. I would consider rear-wheel drive, with front-wheel linkage steering. Steering might be powered through a lead screw, and the rear wheel drive system might feature a limited slip differential (better performance) or separate drive trains (simpler).
Edits:
Definitely use a larger battery than we do in FRC (18 A hr). A big deep-cycle marine battery is probably the ticket.
You’ll need a suspension. Leaf springs look simpler and more reliable than coil springs and shocks, but I haven’t designed with either.
Don’t forget to seal the gearboxes and motors against mud!
This is not a competition, and reliability is definitely more important than speed. I am noting all your suggestions and will be implementing them into my design. I will definitely keep you posted on the process and outcome.
I have not seen the trailer, nor have I ever heard of the movie, but it seems like and very inspirational and knowledgeable documentary. I hope to watch, and learn from it as soon as it is available to the public.
I am in the same state… but on da Big Island and would love to help…
What you are planning to do can easily be done with FRC based knowledge and parts. I will send you a PM with my contact information.
Looking forward to helping with this, you could even run some tests out here!
We have lots of extremely sunny lava shorelines, not a lot of sand shores over here.
My team would love to help and also have a lot of experience with underwater robotics.
Which will be very useful in working on designing a drive base that will have interactions with water.
Sounds like a great amount of fun and learning will exist with this project.
Hi Glenn:
You have what sounds like an interesting project.
Like others have said your device will be different from our competition in that it will be outside and on uneven terrain. Those are 2 huge requirements.
Like all new projects, I would start by defining relevant requirements.
For example:
How tight a turn will your robot be expected to take?
What is the obstacle height needed to traverse?
How far will it need to travel?
Payload weight and size?
Ride quality or how smooth does the ride need to be?
How firm is the ground or how much wheel / track surface area is needed to support you fully loaded bot?
I agree with Geetwo about a simple truck style steering. But based on your requirements you might find that you will need something closer to a 4 wheel powered monster truck or something closer to a Mars Robot. I would look at model RC trucks for inspiration.
I’ve found that its helpful to record all of your team’s assumptions, goals, requirements and design decisions as these can later be used to answer the question of why was this approach taken. If one of the team’s assumptions change, you can back up, change the related requirements and go forward with a better design. This info can also be very helpful for developing a presentation after the project has been completed.
Best of luck. Let us know what you come up with and how it works.
Any and all advice is greatly appreciated and I will definitely take into consideration the above listed information in regards to our requirements. And thanks for the helpful links!!
A concern I have is actually footprint vs. weight vs. soil (terramechanics). Think about it from a bike tire standpoint. A lot of mountain bikes operate with roughly 30 PSI tires and thus ground pressure around 30 psi. This is fine for paved, gravel road and hard pack, but soft sand and/or dirt or mud can be difficult.
Comparatively, many tractors or off road machinery tend to operate below 15 PSI for ground pressure. If you are trying to traverse Mud, you may need to go as low as 11 PSI ground pressure.
Thus for 200 pounds, you would need about 18 inches of srface area in contact with the ground.
To get down to these low levels, you typically need either balloon tires or tracks.
IE a 4 wheel solution would need over 4 in^2 per tire. 6 wheel would be 3 in^2 per tire.
A 6WD chassis with these wheels:
would be pretty reasonable to implement. They even have a base chassis at that website as well as adapters that should be close to meeting your needs. Similar to this one: http://www.andymark.com/product-p/am-2289.htm
I think you could make either of these concepts work relatively well. If you are OK with a person manipulating controls, I personally would start with the self propelled mower and double the width of the wheels as well as ad more robust axle attachments to handle the 200 lbs vs the 90 lbs it was initially design for.
With it being a walk behind, the human would also add stabalizing properties when maneuvering which I suspect will be critical (with a base of 24x26, you center of gravity will need to be below about 12" of height assuming it is relatively centered. This is pretty low for anything that heavy, and also often with solar installations, a little height is very helpful towards avoiding shadows).
I’ll bet that driving over real terrain and obstacles, and getting enough endurance from your battery(s) (affected by battery storage, motor efficiency, payload power needs, and misc) will dominate your physics problems, if you have to traverse non-trivial distances or stay on-station 24x7. But they won’t be your biggest problem (see last paragraph).
So,
Build miniature prototypes out of wood, VEX/Tetrix parts, etc.
Build a full-sized, full-weight prototype out of 2x4s and your chosen wheels/tracks (see Ike’s post).
Drive (tow, push, whatever) the prototypes outside.
Use easily-revised, modular framing for the real thing. It will need to be adjusted as you iteratively design/integrate/test
To solve any other problems, if you burrow deeply enough into the archives here, you can probably find a workable solution.
Also, it’s a good rule of thumb to estimate project size/complexity by the number of interfaces (mechanical, electrical, software) you have to design and/or implement, instead of counting the number of subsystems that exist. It’s good to pay attention to both, but it’s the interfaces that will eat you alive (in part by causing both your time spent searching for or choosing parts, and your resulting parts-count to explode).
Also, everything takes about 4 times as long as you think it will/should; and simple 10-minute tasks often balloon to take 2-4 hours by the time you are finished discussing them, setting-up for them, doing them, testing them, and cleaning up after them. The clock is your biggest enemy, not robot-physics.
Very helpful links and useful information here as I was hoping I could build a 6-wheel setup w/o suspension. And those tires are exactly what I was leaning towards. Thank you, very much, IKE!!
You may want to look at some of the robots made here as well: http://www.igvc.org/
They are relatively sow moving autonomous robots that travel over grassy fielded areas with obstacles.
Watch video to see who handles well, and you might get some good ideas.
I can tell you that you need more than one battery if you plan to have this thing run actively for more than an hour. Pushing 200 lbs around isn’t much fun and an extra 40 lbs of battery would give you a safety net.
Ours is a six wheel drop center design using wheelchair tires mounted onto wheels from Harbor Freight. A little welding and a sprocket mounts nicely to them, and they only cost $4 each. Upgrading bearings is a must, but the benefit is that they are steel and we don’t have to worry about cracks or issues with rough operation that will happen off of a traditional playing field. Ours is only two wheel drive and we have issues with spinning tires in grass. Four or Six wheel drive would be nice even if it would hurt battery life a bit more.
So, maybe what you want to do is buy one or two motorized wheel chairs and/or Rascal scooters; cut them up, weld the resulting parts into a 24"x36" frame, adapt the control and power systems to the new configuration, and call it a day?
