Re: how to calculate req'd motor strength ?


"Curt Welch" <curt@xxxxxxxx> wrote in message
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"Harry Rosroth" <hrosroth@xxxxxxxxxxx> wrote:
"Matt Timmermans" <mt0000@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
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"Pogo" <replytothegroup@xxxxxxxxxxxxxxxx> wrote in message
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Finally started trying to get smart about this motor selection thing
6. Multiply the force by the wheel diameter (watching units again) to
get the needed motor torque.

Shouldn't that be wheel radius? (I've never tried to do any calculations
like this so I'm just asking questions and thinking out loud).

Either one as long as the units and conversion constants are managed.
I like to use diameter since its easier to measure accurately and wheels
are usually specified by diameter. Torque is force times moment arm
and that would be radius or diameter/2. I was trying to show the general
procedure and the dependence on variables, in this case wheel size.

You might have to factor in energy loss to bearings and energy loss for
the
wheel depending on the wheel and the surface you are riding on.

That's all accounted for in the motor output specs.

And what about the power needed to accelerate the wheel itself?
True, but in all my robots, and all the hobby robots I've seen, the
moment of inertia of the wheel is small and well within all the other errors
Besides, after doing the calculations I always overspecify by 50 to 100 %


Had to omit some details but that's the basic procedure for deriving
motor specs on a level playing field.

Suggestions, comments, and corrections are appreciated.

I suspect in practice you will never get accurate enough data about the
motors and the rest of your system to correctly calculate the performance
before hand. The calculations will probably only verify that you are in
the correct ballpark.

In my experience this works. The motor RPMs are easy to verify, the
torque is harder but the numbers given by the manufacturers are quite good
and although the resellers only give a handful of specs, those that do have
accurate numbes.

I think for example the max RPMs quoted are what you get with nothing
attached to the motor. Driving a robot, the max RPM will always be lower.
I don't know by how much.
I mentioned that. You are referring to the no load RPM Recall I said
"The min specs needed are no load RPM, and the RPM at some specified
torque".
Those are enough to find any other pair of (RPM, torque) points.

And torque ratings I think are normally stall torque (how much it puts out
right before stopping). The faster it spins, the less torque it has. At
maximum speed (with nothing attached to the motor), it's torque I think is
0.
Right. Stall torque is not usually given. Again, on wants an operating
point,
the one usually given is at max efficiency, which is very roughly at about
80% of the no load RPM and 20% of the stall torque.

So, to do the type of calculations I think you are trying to do (max speed
of the robot and time or distance to accelerate to some speed), you need
accurate torque vs speed curves for the motors. Maybe the shape of these
curves are typical enough for most DC motors that you can estimate them
from normal RPM and torque specs but I think the calculations you are
trying to do are a lot more complex than what you are making out above
since the torque will not be constant.

That's taken into account. I didn't post the equations. If you want to
some
thing similar look in the books I quoted, especially Mobil Robots. I think
they
don't go far enough in making clear how to pick surplus motors from
performance requirements.


.

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