There will be two motors connected to the device. They will need to...
What spec motors does this require? (voltage, wattage, etc.)
Thank you all very much.
note: in terms of price range, we will pay whatever we need to fit the job, but preferably they could be less than $50 each
First you need to understand mechnics a bit more. Motors have a max torque rating, we need to
figure out the torque, power and gear ratio required to perform the task.
First figure out the torque at the wheels - this depends on the wheel radius, max acceleration needed,
friction losses in the drivetrain/bearings/tyres, and the steepest slope that is needed to be climbed.
Given torque and speed you multiply them together to get power (use SI units everywhere, otherwise
its very easy to make conversion mistakes). Torque is newton-metres, angular velocity is radians/second,
power is watts.
Temperature isn't an issue for the motor, it definitely is an issue for the batteries.
The voltage is completely independent of the mechanical ratings, lower voltage means higher current
so in practice for high power you want larger voltages to prevent the leads being impractically thick.
The same basic motor is often available with different voltage windings, but identical mechanical specs.
Ok, here are my calculations:
Unknowns:
f=force
t=torque
r=wheel radius
p=power
Knowns:
m=max mass (it will lose weight as it runs due to liquid dispension)=45.359 kg
a=max acceleration=0.119 m/s^2
r=wheel radius=0.102 meters
s=max speed=1.341 m/s
f = m * a
f = 5.398
t = f * r
t = 0.551
p = t * s
p = 0.738 watts (assuming there is no friction in the wheels)
Two motors means theoretically 0.369 watts each. This is of course very low and I feel like that is because I didn't factor in the friction. But what is a way to calculate the friction? Is there a standard way of estimating it using the weight of the device?
Thanks
Friction for wheels is complex but you can make a few simplifications. First think about the difference between driving on concrete or on soft grass. The wheels sink into the soft grass, making little depressions or holes. So any time you drive anywhere, you have to drive up the slope of the depressions that your wheels are in.
So driving on soft grass is like driving up a concrete slope at 20-30 degrees angle. If you calculate for that slope without friction then you have a good chance of being able to drive at the desired speed on grass.
It will be driving on ice. I think that would make the depression unnoticeable. Is there a way to approximate the friction of ball bearings based on their size and the weight that is loaded onto them?
PureStress:
It will be driving on ice. I think that would make the depression unnoticeable. Is there a way to approximate the friction of ball bearings based on their size and the weight that is loaded onto them?
IF they are new bearings, they are factory lubricated and you can ignore the friction.
Paul
I found this motor:
http://anaheimautomation.com/products/brushless/brushless-integrated-item.php?sID=393&pt=i&tID=97&cID=48
Is this necessary or could it be cheaper? Does anyone have a link to a cheaper alternative?
I should think the lack of friction on ice is a problem.