How much torque is needed to drive a 30lb combat robot?

Question

Im building my first combat robot. I know plenty about computers and robots, but im stumped as to how much torque my robot will need. I cant continue without knowing this as it will determine my motor needs, which will determine my battery needs, defining my budget and weight remaining for everything else.

the robot will be under 30lbs, and the floor i believe is steel(though if prefer not to go into friction)

Thanks in advanced for your help, it is greatly appreciated!

Answer 1

Well, it will depend on what characteristics you want to give your robot. For example, it says that you don't mind too much on rpm's then i guess you are using most of the torque on this one. Every motor include the hp's it produces. This will help you a lot when you make your calculations. I'll give you an example of what you should do and you do the math with the conditions you want to have.

Let's say i build a robot that weighs 30lb with the motor included. I could say you want it to accelerate at 1ft/s^2 but let's say you want it to climb a hill with a 20º angle.

In this case you need to know the smallest torque you need for your motor to make it go uphill instead of downhill. For this you need to make a free body diagram that tells you the forces the motor has to overcome. Let's say we live in an ideal world where there is only friction between the wheels but no friction between the axis of the robot.

you would need 10.26lb in order to make your robot with a 0 acceleration. This is the smallest force you need.

Now you calculate your torque. This one will have to do with the radius of your wheels. Let's say you have 2 in radius wheels, then the torque you need would be

T=F*r = 10.26lb*2in = 20.52 lb*in.

if you are connecting directly the wheels to the motor, then this is the torque you need. Now, if you are using gears of pullies with a band, then you have to make extra calculations.

Let's say the axis of your wheels is connected to a 1in of radius gear and this one to a 1/2 ing gear on the motor.

you know then that

T1/r1 = T2/r2

where T1 is the torque you want your wheels to have, r1 is the radius of the gear attached to your wheels, T2 is the torque of the second gear connected to the motor and thus the torque you want your motor to have and r2 is the radius of the gear attached to the motor.

solving for T2

T2 = T1*r2/r1

T2 = (20.52lb*in)*(0.5in)/(1in) = 10.26lb*in

as you see this way you will need less torque and a smaller motor.

The same would be with two pulleys and a band.

now comes the issue of the speed you want.

let's say you want your robot to go 1in/s

now again you need the radius of your wheels. So

V = wr

solving for w

w = V/r = (1in/s)/2in = 0.5 rad/s

rad stands for radians and it's a measure of angle.

ir your wheels are directly connected to the motor, then that's the angular velocity you need. Now you can get the power of the motor you want.

P=Tw

P = (20.52lb*in)(0.5rad/s) = 10.26 lb*in/s = 1.555*10^-3 hp

lol, this is kind of low but you understand i'm just taking random quantities. Now, if you use gears, you have to know that

W1r1 = W2r2



solving for W2

W2 =W1*r1/r2 = (0.5rad/s)(1in)/(0.5in ) = 1rad/s

then the power you need will be (10.26lb*in)(1rad/s) = 1.555 *10^-3 hp like the other one.

The thing is that motors generaly specify the hp's it has, so you can play with numbers and with the equations i just gave you to know what the approximate behavour of your robot will be according to the motor you get and then try to get the qualities you want it to have. Good luck with your robot and hope this helps.

Answer 2

  Can't tell you until you come up with acceleration  F = MA remember.  In this case F is torque.   Somebody may have a rule of thumb, but you will probably be going through belts or gears, which complicates matters.

Answer 3

Im about to start buying for my first combat robot.  I would suggest getting cheap 20 dollar drills.  Then use the motors and batteries inside.  That way you don't need to figure this out.