Hey guys, I am looking for a robot kit that is able to carry a payload of about 50lbs and is also able to operate on grass. I found this kit that looks extremely promising except the tracks are smooth and I fear that there would be a lot of slip on grass: https://www.robotshop.com/en/agent-390-tracked-robot-kit-motors.html
People have suggested that I stud the track or find a new one. I have not been able to find another 1.5" track that would work. If anyone has a link to anything that should work please feel free to share it!
My concern with studding the track, is that the track only has a small tolerance to the front bars that attach the pulley brackets to the chassis. Does anyone have any ideas for relocation or fixes for this, as well as what to stud the track with?
I’m sure not an expert in this, but I have a couple thoughts that may help.
First, if grass is your biggest concern you might be better off with wheels than tracks. Skid steering such a long vehicle without better mechanisms to hold the tracks on the wheels doesn’t seem like a good idea. Each end of the tracks is going to get a LOT of sideways force in a skid steer. Tracks are most useful for soft terrain where the wheels will sink into the surface.
If you do want to use the tracks perhaps some 1/8" (3mm) pop rivetes through the tracks, inserted from the inside would work. That way, the open end, which will provide more traction, is on the outside, but it isn’t so long that it gets caught on stuff as easily. I would think that would provide enough traction.
Also, keep in mind that on a flat surface the force to get moving isn’t that great, or to keep moving. Angled surfaces (hills) will present the most problems. Of course, that is going to require MUCH more power than a flat surface anyway.
Can you give us more details on the terrain the bot will be used in?
My first question is; would the rivets mess up the contact surface between the pulley and the belt? I was thinking some smaller screws that only pierce through the top of the track belt so the drive system is not messed up.
The robot should be able to travel over grass and small obstacles in a yard while carrying a payload of about 50 pounds. The robot will mostly travel in a straight line for most of its path, but it needs to stay at a pretty consistent speed so slippage is critical.
I don’t think the rivets heads would have much affect on the drive system. I wouldn’t put a lot of them and not very close together. Maybe stagger them about 1/2 across the track in a row (only one per row) and spread the rows about 3 to 4 inches apart. The track is long enough that you would still have about 3 or 4 rivets in contact with the ground but perhaps only one in contact with the drive wheel. You could always add more later if needed.
The head of those rivets is petty small and flat. If that had much affect on the drive system it wouldn’t be a very effective drive system anyway.
Again, though, I would probably use wheels. Skid steering that think with 50 pounds on it is going to create a lot of force on the front and back wheels.
@nschreiber0813 - This says: " The robots are capable of carrying 5lbs of payload", is this defined by the motor or by the chassis? I need to carry a payload of about 50lbs through grass. I am new to the robotics field so I am unsure where the payload limit is coming from, thanks!
Personally I would build a robot from scratch. I would start finding something I can hack and turn it into a robot. Maybe building a robot that can carry 50 lbs is probably very intimdating. Maybe start out small and work your way to a bigger project like that. Even I haven’t built a robot like that ever. Like what is your experience? Because I can go on to incredible details on how to do so. Good luck! @Helicopter12
Unfortunately I don’t know much about available kits. I can only offer some general guidance on what to look for to meet your needs.
50 pounds is much larger than most of the kits available can handle. Most are intended to be low cost and “cute” or educational without doing much real work. Of course the chassis to handle 50 pounds will need to be much bigger and stronger than most. And the motors will need to be bigger and stronger. Which means the electronics to control the motors needs to be bigger and stronger. And the batteries will of course have to be big enough to provide all that power for a reasonable time, which means they will be rather large and heavy.
Since all those things make a rather specialized, and considerable more expensive, kit, the manufacturer / retailer is most likely going to advertise its capacity as a selling point to justify the price. If you see one you think might work and it doesn’t list the capacity, I would contact the retailer and ask.
To answer the question you asked about the kit nschreiber0813 linked, it is probably both the chassis and the motors (and electronics.) It’s unlikely the manufacturer would spend the money to make a chassis that could handle more and skimp on the motors, or vice versa.
One thing you might want to consider is to get something like a garden cart than can handle your requirements and turn it into a robot. It sounds like your requirements are relatively simple so that wouldn’t be that hard. Some appropriate motors with linkage, motor control hardware, and some type of controller (Arduino, etc.) If you want to try that, there is lots of help available right here. I, personally, love to help people do stuff like this as long as I have time.
Unfortunately due to the overall goal of the project I don’t have the time to really make my own platform from scratch so I am stuck with trying to find a readily available kit. I know that the kit will have to be slightly modified to meet my needs, which is fine, I just don’t have the time to design everything from the ground up as there is much more electrical and programming work to be done.
I found this kit: Kit. Which is 4WD and looks promising. I am not sure if the chassis could support a payload of 50lbs, but how would I calculate if the motors could move a payload of 50lbs?
Given a rated load of 62.5 oz-in means that a lever arm of 1 inch could lift a load of 62.5oz, correct? If the wheel is the lever arm than the radius of the wheel is the length of the lever arm and thus dictates the force applied?
62.5 oz-in * 2.7 in = 168.75 oz = 10.55 lbs
If there are four motors and each can move 10.55lbs than a max payload of 42 lbs is found, correct?
Can anyone explain what I am doing right/wrong with these calculations? Thanks!
The first thing to look at is your calculation. Torque is force * distance : oz in. To find the force (ounces) you DIVIDE by distance:
62.5 oz in / 2.7 in = 21.1 oz YOu can see that clearer this way:
62.5 oz * in
-------------------- = 21.1 oz
2.7 in
To work with the units use them as variables. The “in” cancels on top and bottom, leaving "oz’
So, four of those motors could provide a max total of 4 * 21.1 = 84.4 oz or about 5 1/4 pounds of force
BUT! a 50 pound robot doesn’t (usually) need 50 pounds of force to push it on a flat surface. The 50 pound weight is from gravity, and straight down. A person or two can relatively easy push a 4000 pound car on a flat road. They couldn’t ever LIFT it. The amount of force needed to minimally move it is small, based on frction of all the moving parts (tires on ground, bearings in wheels, etc.) Once you have the minimum required the extra determines how fast it can accelerate: force = mass * acceleration, or acceleration = force/ mass (mass is NOT the same as weight, but they are related: something that weighs 50 pounds on Earth has a mass of around 22 kilograms). As a rough approximation, you would need something like 1/4 the torque you would need to LIFT it. So, for a robot carrying 50 pounds and the robot weighs say 20 pounds, (total 70) you would need something like 70/4 or 18 pounds of force. That’s a VERY rough estimate.
Somewhere on this site, maybe under tutorials, is a motor calculator. Take a look at that. Some useful material there.
Awesome, all of that is some good stuff! I see my error with the calculation, for some reason I was reading the “-” as a “/” which caused that calculation! I found the Drive Motor Sizing Tool and have been playing around with the values for the last kit I found and this is what I got:
Total Mass: 55lbs (50lb payload and about 5 for the chassis, motors, and batts estimated) Drive Motors: 4 Drive Wheel Radius: 2.7" Robot Velocity: 1.6 m/s (~3.5MPH) Max Incline: 13 deg Supply Voltage: 12VDC Desired Accel: 0.2 ft/s^2 Desired Operating Time: 60 min Total Efficiency: 65%
This gives: Angular Velocity: 222.9 rpm Torque: 211.29 ozf-in Total Power: 34.815 W Max Current: 2.9 A Battery Pack: 11.605 Ah
This means that the motors and wheels of the kit could do what I needed, but the chassis would most likely be the weak point, correct? Also the 11.6 Ah battery is a bit larger than what I am used too, but then again I have never done a project on this big of a scale before either.
I didn’t check any of those numbers. I just glanced at them. Mostly they seem reasonable except the additional weight of the robot. I don’t think 5 pounds is anywhere close. I would estimate a minimum of 10 and probably closer to 20 pounds.
A 12 Amp Hour battery is going to be a sizeable percentage of that, maybe more depending on type. Lithium types are lightest.
I reached out to the support at servo city as you suggested and this is the reply I got about the 50lb payload on the Scout: "The tires in the kit are not built to handle a 50lb load. They’re foam-filled tires which typically get used on R/C monster trucks which way 10lbs give or take. 50lbs will distort the foam and cause a lot of rolling resistance. You’ll probably want to go with a pneumatic tire for such a load. "
I also asked if there was an offroad track or some other variation of track that could replace the slick track on the Agent kit and this was his response: "The XL timing belts, used as tracks, are the only ones we offer for the agent chassis. That chassis, while stout, is also very low profile and may not provide the clearance needed in the grass when weighed down. "
It looks like I will need to take the good qualities off of each kit and piece together my own, unless anyone else has any further input?
