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What engine to use with this big outdoor robot?


Dear Sirs, Im sorry for asking such rookie questions, but i still havent been able too locate an internet resource which can answer my questions. PM me or answer here if my question is not clear enough..

I was hoping you could help me determine a range of DC gearmotor specifications needed to run our robot.
 It’s an all-terrain (mainly dirt and grass) outdoor robot with a total weight of no more than 200lbs (probably 150lbs) that has to run at a top speed of 15mph. The robot is a ’tank-drive’ design (2 sets of wheel and DC Permanent-Magnet gearmotor combinations and 1 front caster wheel) running on two 12V batteries.  The robot will be roughly 4 feet long, 2 feet wide and 4 feet tall with most of the weight (battery and motors) in the back and bottom of the bot. The tires will probably be no less than 12 inches in diameter.
If possible, could you please fill in the chart below:
Shaft Speed:   At least      and no more than      rpms
Continous Torque:   At least      and no more than      in-lbs
Continous Current:   At least      and no more than      amps
Continous Output Power:   At least      and no more than      hp
Overhang load:   At least      and no more than      lbs

How would the specs change if we needed to run the robot at greater or lesser max speeds (10mph, 12mph, 20mph)?
How does the total weight of the robot affect the specs or performance the motors?
Would the cost of a motor differ significantly if I wanted a robot that was 5mph faster or one that weighed 50lbs more?
How does the wheel diameter affect the specs?
We’ve already researched a number of motor manufacturers, do you have any suggestions on what models we should use?

What about speed controller suggestions?
Do you suggest using 1 or 2 front caster wheels?
What material and gauge/thickness would you use to build your frame? It’s not a battle-bot, so it should not need to withstand heavy beatings.

shaft speed:
the shaft will turn the same number of times as the wheel it is connected to so you can just calculate the rpm of the wheel and you have the rpm of the shaft (look below for how to calculate the shaft speed)

calculate the circumferance of the wheel (2xPIxRadius). then run the robot for like 50-100m and record the time (lets say it took 10 seconds over 100m) then divide the length (100m) by the time it took (10sec) and this gives us the distance travelled every second (10m/sec) and multiply that by 60 seconds to change it to minutes and you get 600m/min. now divide that by the circumferance of the wheel and you have the RPM.

man what did you do in maths class, this is soooooo basic.

i would help with the others but i dont know what they are, can someone explain what continous torque, continous current, continus output power, overhang load?

Thanks for the fast reply.

 I've calculated that I need about 407 RPMs output speed to push the robot in 15 mph with 12" wheels, we are going for 2 PMDC motors. Does that mean that we need  ~4000 RPM engines with a 10:1 gear ratio?  We do not have the engines yet and what my questions really are about are what range og engines are capable of sending a ~155-195 lbs robot at this speed? How much torque do we need and how do you calculate how the loadweight affects the performance of the motor? i know this is abc for a lot of you guys, so if anyone can point me in the right direction it would be greatly appreciated.

I got just what you need:

Basically it explains how to calculate all of your questions. If you just want answers fast, search for the excel sheet on that page. Just plug in values and it will calculate what motors would work best for you.

--- Quote ---What about speed controller suggestions?
--- End quote ---
this would very much depend on current draw, which is dependent on many other factors. i would recommend going for the speed controllers meant for battlebots.

--- Quote ---Do you suggest using 1 or 2 front caster wheels?
--- End quote ---
one caster is better since its less friction, but if your robot is going 5pm and is top heavy, then it might flip over on sharp turns. if you use two casters, then each would only need to handle half the robot weight (possibly a good thing). i would say go with two to play it safe, but it depends on many factors . . .

--- Quote ---What material and gauge/thickness would you use to build your frame? It’s not a battle-bot, so it should not need to withstand heavy beatings.
--- End quote ---
complicated question . . . steel is the strongest, but would also greatly increase robot weight. aluminum is the lightest, but the most expensive. the shape of the frame is also very important. if you really arent sure, and dont know how to calculate for bending stresses, just go with the strongest material you can work with your tools. i like to use plastic because its really cheap and lightweight, but if you dont design it right, it'll just bend and wont work . . .

my best advice is to do everything you can to keep the robot weight as low as possible. robot complexity and cost increases exponentially with a linear increase in weight.

Thanks! Great stuff


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