ozr2222
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I need to rotate a heavy mass in a controlled manner - Do I need ramps?

Thu Oct 03, 2019 1:51 pm

I would be glad if somebody could check if my thinking process is right. Never done or learned this so....
I have a mass of 1,5 kg with its center of mass 15 cm away from the rotations center. That gives me 2,55 Nm.

My Nema 23 motor gives me 70 N.cm at 300 rpm.

I think I go for a 1:5 transmission, because that gives me 3,5 Nm at 60 rpm.
At 60 rpm my angle speed is 94cm/s, which gives me a 10% increase for the 2,55 Nm = 2,80
So im pretty much 20% (0,7Nm) above what I need. Is this enough to be sure the stepper doesnt lose steps upon stopping/starting?
When Ill be going for that configuration would it be wise to get ramps for the starting/stopping of the motor or will the transmissions slippage in itself compensate for the sudden jerks?
My gears are 3D printed from PLA.

jamesh
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Re: I need to rotate a heavy mass in a controlled manner - Do I need ramps?

Thu Oct 03, 2019 2:09 pm

Is this in any way related to Raspberry Pi?
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ozr2222
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Joined: Sat Jul 27, 2019 1:51 pm

Re: I need to rotate a heavy mass in a controlled manner - Do I need ramps?

Thu Oct 03, 2019 2:23 pm

Well... Its driven by a raspberry PI and I would need to code the ramps in Python. Does that qualify?
Edit: Im sorry but I dont know any other forum thats mechatronics related, but the Pi is in itself a good candidate for realizing those kind of projects so I thought this would be fitting

pfletch101
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Re: I need to rotate a heavy mass in a controlled manner - Do I need ramps?

Thu Oct 03, 2019 8:28 pm

It is a long time since I studied the physics, but I am not quite sure that I understand the basis for your calculations. If your rotating mass is symmetrical about its axis of rotation or rotating in the horizontal plane, you need torque only to accelerate it, not (assuming perfect bearings) to keep it rotating, so the torque you need depends not only on the angular inertia of the mass (m * r^2), but also on how fast you need to accelerate it. You certainly won't get it up to speed instantly by applying any finite torque, but you are likely to get it up to speed fastest if you use the maximum drive current from the start. Only if you want it to accelerate more slowly than it will 'naturally' do when the motor is driven by its maximum current will you need to ramp this. If the mass you are driving is not symmetrical and is rotating in anything other than the horizontal plane, life gets very complicated, and the torque calculations would be above my pay grade.

ejolson
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Re: I need to rotate a heavy mass in a controlled manner - Do I need ramps?

Fri Oct 04, 2019 4:00 am

pfletch101 wrote:
Thu Oct 03, 2019 8:28 pm
You certainly won't get it up to speed instantly by applying any finite torque, but you are likely to get it up to speed fastest if you use the maximum drive current from the start.
What you say sounds logical, but I think a stepper motor driven by a Raspberry Pi works differently. Also there is need to rotate the object in a controlled manner.

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Burngate
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Re: I need to rotate a heavy mass in a controlled manner - Do I need ramps?

Fri Oct 04, 2019 10:03 am

ozr2222 wrote:
Thu Oct 03, 2019 1:51 pm
I have a mass of 1,5 kg with its center of mass 15 cm away from the rotations center. That gives me 2,55 Nm.
That doesn't make a lot of sense, to me, so maybe there's something you're not telling us.

Just as, in a linear system, force / mass gives acceleration, so in a rotating system, torque, moment of inertia and angular acceleration are related.
Torque is measured in Nm; moment of inertia in kg m^2; and angular acceleration in radians/s^2

With 1.5kg and 0.15m as inputs, my calculator refuses to output any value for torque unless I give it an angular acceleration as well - unless I try putting the system in earth's gravity. Even then, 2.55Nm doesn't pop out.

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