Hi. I've been reading about steppers and i cant find the complete guidelines of choosing the right torque. I'm making making a small cart robot which will carry about 7 kg, 4 cm wheel radius. Can you suggest me stepper that will work fine.
Is lifting 5 kg (y, z coordinate) with a lever arm of 4 cm just be the same as carrying 5 kg (x, y coordinate) using wheels?
reycreator17:
Is lifting 5 kg (y, z coordinate) with a lever arm of 4 cm just be the same as carrying 5 kg (x, y coordinate) using wheels?
No. There is no comparison between the two.
Lifting something involves fighting gravity.
Moving something sideways requires fighting friction - and the force necessary to accelerate from zero to the chosen speed. Think how one person can move a 14 tonne boat on water but could not possibly lift it even 1 millimetre.
You can work out the acceleration forces with the formula F = MA where M is the mass. But you will need to measure the friction force.
There is a simple suggestion for measuring torque in Stepper Motor Basics. It could be adapted to measure a linear force.
...R
Great. Thanks
One way to characterize the torque for traction is to specify a maximum slope you want to handle,
such as 1 in 10 or 1 in 20. Then you can do the calculations, whereas frictional losses are much less
tractable. If the friction is so bad that the thing cannot descend the design slope under gravity (with
motors removed), your friction needs addressing anyway!
You can also consider minimum desired acceleration - that (with the total mass) gives you a
minimum required total torque summed over all the wheels.
Whether friction, gravity or momentum is the dominant factor depends, so calculate/estimate/measure
all three if possible.
Friction calculations can be made (approximated) with the following F=coefficient of friction X force. So if the cart is on a level surface the friction force between the surface and wheel would be F =coefficient of friction X weight. The coefficient of friction for the material the wheels are made of from the web.
The force will not be very high for wheels, in fact from an engineering first approximation probably not significant when compared to overcoming momentum (F=MA).
No, that's not the friction we are talking about.
We are interested in the dynamic friction inside the motor, gears and drivetrain - that is what loses power
and torque.
The friction between the wheels and the ground is irrelevant, so long as its large enough to stop the
wheels skidding, since its static, not dynamic friction (no movement, no loss)
Ok,
Sorry, miss understood, since you were talking about the physical force to over come gravity and momentum to get the cart moving, I assumed you were also discussing the frictional forces acting on the cart.
you can select one of this... http://www.mvelectronica.com.mx/motores/ gear motor...
Back to the plot you need to get a grasp on the likely torque and speed requirements, then go looking for a motor.
With steppers its much more complicated as the torque/speed graphs depend on supply voltage, microstep
setting and mechanical damping in the system.
But the whole question is an xy-problem, steppers are not suitable for traction, they are far too power hungry
and inefficient, you want DC gear motor and wheel encoders if necessary. You will probably get more than an
order of magnitude battery endurance using DC motors over steppers for traction.
Hi again Thanks everyone. Yeah. I kinda realize that, it is the friction between the gears inside the motors is what stop the whole thing or we call it as static friction. I just dont know how to compute for it. For my robot, i want it pretty slow maybe 0.05 m/s - 0.1 m/s and the acceleration would be maybe 0.025 m/s^2 plus may slope of 20 degrees maximum. I want it to be able to carry 5 kg on it.
@MarkT about the power consumption, I was thinking of using this source since it has 4A max drawing power for 12V.
https://www.amazon.com/Intocircuit-26000mAh-Capacity-Portable-External/dp/B00BB5VQCE
And this stepper motor.
Any suggestions if this is overkill or good enough Thanks
This may be way out in left field, and if I am please chime in. But I think if I understand the issue, what I would do to get an order of magnitude approximation of the static friction of the motor is this. Affix a 1 inch lever to the shaft and start adding small weights until the shaft moves. This should give you an approximate idea what torque it takes to overcome the static friction of the motor.
Again, just an idea.
reycreator17:
Hi again Thanks everyone. Yeah. I kinda realize that, it is the friction between the gears inside the motors is what stop the whole thing or we call it as static friction. I just dont know how to compute for it. For my robot, i want it pretty slow maybe 0.05 m/s - 0.1 m/s and the acceleration would be maybe 0.025 m/s^2 plus may slope of 20 degrees maximum. I want it to be able to carry 5 kg on it.@MarkT about the power consumption, I was thinking of using this source since it has 4A max drawing power for 12V.
https://www.amazon.com/Intocircuit-26000mAh-Capacity-Portable-External/dp/B00BB5VQCEAnd this stepper motor.
https://www.sparkfun.com/products/9238Any suggestions if this is overkill or good enough Thanks
You aren't listening?
No to stepper,
First determine the mechnical requirements, then the gears and motor, then the motor driver, then you can
think about power source (the endurance depends on the pattern of use and actual losses, you haven't anyway to determine that yet.
that yet).
You have some figures: 20 degrees, 5kg (total mass?), 0.1m/s
So total force to wheels is sin(20) * 5 * 9.8 = 17N
power = 17 x 0.1 = 1.7W
total torque = 17 x 0.04 = 0.7Nm
Now allow a good factor of 2 for losses, and guess that 2 wheels are driven,
so thats 1.7W and 0.7Nm to each wheel at 25rpm.
A 100:1 gearbox for each motor would mean 2500rpm motors with 7mNm of torque.
Assume 3.4W mechanical from the motors, 5W electrical in, so for 5 hour endurance uphill all the
way thats 25Wh (2.2Ah for 12V supply, say). Note thats about 2km run with 600m of ascent!!!
For more modest use the average power would be correspondingly less.
Thanks
I will go now with gear motors with encode. Actually I found out a calculator for motor.
http://www.robotshop.com/blog/en/drive-motor-sizing-tool-9698
Anyways, Thanks a lot!