Rebuilding a new smaller bot!! Calculating the amount of weight a motor can carry?
rwgast_logicdesign
Posts: 1,464
Ok so ive searched around alot before asking this question, and basically im finding scattered and fragmented or overly complicated information. I figured someone here should know how to do this..
The reason I ask is because Ive partially given up on the robot I was building. Mechanically it was a mess!! Every time I thought I was ready to solder a board or write some code the right angle gear boxes would flake out, or the wheels would slip off. Basically the original RO-BOT was only intended as a test platform to ensure my h bridge was working but I tried to reinforce it and push it to far.
Im really at the point where I just want to start learning the prop chip and writing algorithms and code, very tired of fussing with mechanics. Ive decided to transfer most of the work I had done over to a much smaller chassis and im also moving to 4 wheel drive, as the whole caster wheel thing is causing about 25% of the frustration I was having. I showed some pics of my new plan to a fellow forum member and he though I had a much better design going for my motors, which he is probably right. The thing is it would have been a bit to small for me. So I went through the robot bonanza book to get some ideas and ran down to home depot for a few 50 cent nail plates and one two hole straps to make a chassis. Its going to be a 5x6 rectangle when completed. This will give me plenty of room on each deck and will allow me to add new decks as I choose. I will be directly mounting 1 3/4 inch erector wheels to the motor shaft using a few mm of brass rod, this way I have no issues keeping wheels on worrying about hubs etc, I want to move to lossi 2 1/2inch RC wheels, but thats not going to happen until everything is working and I can just focus on mounting custom wheels.
So basically I want to do this right the first time, not have to rebuild the thing after every 10 minutes of use! One of the issues I need to solve have to do with weight, right now my battery's weigh about 2lbs, I know the motors can carry the battery effortlessly at the full 18v, but I dont intend to run at full supply all the time so I need to get a realistic estimate of what kind of weight this guy can carry before I go making any more decisions.
Heres some info that is known
wheels 1.75 inches
motor 240rpm max
speed@240 22 inches per second max
HP=.0294
Torque=120 oz/inch or .643ft/lbs
Now I cant find a data sheet on these motors so here is my HP and Torque calculations, hoping I did this right
Horse Power
18/11=1.63636363
VxIxE/746 or 18x1.64x.74/746
0294 HP
[FONT=arial, helvetica, clean, sans-serif]Torque in ft/lbs[/FONT]
[FONT=arial, helvetica, clean, sans-serif]T = [HP x 5252] / rpm[/FONT]
[FONT=arial, helvetica, clean, sans-serif](.0294x5252)/240=.643ft/lbs[/FONT]
[FONT=arial, helvetica, clean, sans-serif]used conversion calculator to get oz/in[/FONT]
[FONT=arial, helvetica, clean, sans-serif]Ok so now Ive gotten all that info and I know with a inch rod my motor can pull 120oz max. How exactly does one go about finding the amount of weight the motor can hold when on a certain size wheel? This is where my searching skills started failing, is there no simple formula for this? I read an article on society of robots but didn't understand it all, and the equations were for more than weight capacity. Then once you know how much one motor/wheel combo can support do you just multiply that that by 4, on a 4 motor chassis?[/FONT]
The reason I ask is because Ive partially given up on the robot I was building. Mechanically it was a mess!! Every time I thought I was ready to solder a board or write some code the right angle gear boxes would flake out, or the wheels would slip off. Basically the original RO-BOT was only intended as a test platform to ensure my h bridge was working but I tried to reinforce it and push it to far.
Im really at the point where I just want to start learning the prop chip and writing algorithms and code, very tired of fussing with mechanics. Ive decided to transfer most of the work I had done over to a much smaller chassis and im also moving to 4 wheel drive, as the whole caster wheel thing is causing about 25% of the frustration I was having. I showed some pics of my new plan to a fellow forum member and he though I had a much better design going for my motors, which he is probably right. The thing is it would have been a bit to small for me. So I went through the robot bonanza book to get some ideas and ran down to home depot for a few 50 cent nail plates and one two hole straps to make a chassis. Its going to be a 5x6 rectangle when completed. This will give me plenty of room on each deck and will allow me to add new decks as I choose. I will be directly mounting 1 3/4 inch erector wheels to the motor shaft using a few mm of brass rod, this way I have no issues keeping wheels on worrying about hubs etc, I want to move to lossi 2 1/2inch RC wheels, but thats not going to happen until everything is working and I can just focus on mounting custom wheels.
So basically I want to do this right the first time, not have to rebuild the thing after every 10 minutes of use! One of the issues I need to solve have to do with weight, right now my battery's weigh about 2lbs, I know the motors can carry the battery effortlessly at the full 18v, but I dont intend to run at full supply all the time so I need to get a realistic estimate of what kind of weight this guy can carry before I go making any more decisions.
Heres some info that is known
wheels 1.75 inches
motor 240rpm max
speed@240 22 inches per second max
HP=.0294
Torque=120 oz/inch or .643ft/lbs
Now I cant find a data sheet on these motors so here is my HP and Torque calculations, hoping I did this right
Horse Power
18/11=1.63636363
VxIxE/746 or 18x1.64x.74/746
0294 HP
[FONT=arial, helvetica, clean, sans-serif]Torque in ft/lbs[/FONT]
[FONT=arial, helvetica, clean, sans-serif]T = [HP x 5252] / rpm[/FONT]
[FONT=arial, helvetica, clean, sans-serif](.0294x5252)/240=.643ft/lbs[/FONT]
[FONT=arial, helvetica, clean, sans-serif]used conversion calculator to get oz/in[/FONT]
[FONT=arial, helvetica, clean, sans-serif]Ok so now Ive gotten all that info and I know with a inch rod my motor can pull 120oz max. How exactly does one go about finding the amount of weight the motor can hold when on a certain size wheel? This is where my searching skills started failing, is there no simple formula for this? I read an article on society of robots but didn't understand it all, and the equations were for more than weight capacity. Then once you know how much one motor/wheel combo can support do you just multiply that that by 4, on a 4 motor chassis?[/FONT]
Comments
http://www.pololu.com/blog/10/force-and-torque
Torque is force times length. "oz*in" not "oz/in". For some reason many products list torque with the "/" symbol. I know I'm not the only one who is driven crazy by this.
In physics, a common why to check if a calculation could possibly produce the correct answer is to check the units. If you answer is in square inches and you need an answer in cubic inches, you know you have a problem somewhere in the equation.
With 120 oz*in or torque, and a 1.75 in wheel (0.875 in radius), your wheels should be pushing sideways on the ground with 120 oz*in/0.875 in = 137 oz (8.57 lb) of force (which agrees with your calculations).
But much of the above is useless to us. This will only tell us the theoretically possible acceleration of the robot not how much the robot can carry. (PS, I minored in physics.) The main factors are going to be (as suggested by photmankc) the mechanical strength of the motor and gearbox.
I'll say again here so other can agree or disagree with me. I think you ought to use a much smaller battery pack (I think the current one is borderline too heavy for your robot). Though those motors can produce a lot of torque they are not a good match for a large robot.
I think your idea of using a smaller robot is a good one. I think it would be helpful if you included a link to the motors your using and added some of the pictures you've taken.
Edit: As (also suggested by photmankc) the 8.57 lb figure will indicate how well the robot may do on an incline.
I agree pushing 18v to these motors is borderline, but they are sold as 3-18vdc.
http://www.goldmine-elec-products.com/prodinfo.asp?number=G16279
There is a link to them, for those of you that havent seen any of my previous posts. I have taken the right angle drive heads off to help strengthen the drive train and get maximum efficency. I havent had any issues with the gearheads at 18v yet. If i do I will back down to 12 or 9.6v.
For now the major reason to stick with 18v (besides investment) is rpms. With 1.75 imch wheels I can only go about 1.8ft/seconds. That sounds pretty slow, the wheels i plan to buy are 2.75 inches. So ill be able to get a bit more speed. Using
http://www.robotshop.com/dc-motor-selection.html?lang=en-us
the calculator mentioned above, it looks like I can pull a max of about 36lbs up a 40 degree angle while holding 1.8ft/sec speed. I have my doubts about that, but im really only shooting for a weight under 15lbs probably more like 7 to 12. This calculator shows me i definately have room to up my wheels a bit, or lower my battery voltage if need be. I dont plan on anything bore than a 20 degree incline without proper suspension.
As far as pictures i dont want to post any until im sure about the chassis iv made motor mounts and am working on a frame made from erector ill post when done. the chassis will just be a multi layer rectangle made of nail plates, sheet metal, and threaded rod. When its done I want to paint it flat rubberized black with bed liner/under coat spray. I figure this not only looks better but will be good for absorbing IR light.