Olin Lathrop wrote:

> Padu wrote:
>> The distance from the steering column axis to the point in the
>> handlebar where I took the measurements (I tried to apply force to
>> the tangent of that point) is 0.34m, so I take that the torque
>> necessary to steer my robot is 43.35Nm. Is that correct?
>>
>> There is also the issue of how fast I want to turn it from lock to
>> lock, and I believe 250ms would be appropriate (60rpm). From that
>> I've calculated that the motor needs to have a power rating of no
>> less than 273.4W (roughly 1/3HP).
> 
> I get the same values given your input values.
> 
>> I don't know why but I was expecting less power.

Look at the graph in this data sheet:
http://www.casaferreira.com.br/pdf/CEP453042.pdf

It shows the typical relationship between torque, power, rotational speed
and current for an electric servo motor. It also shows where the rated
numbers typically are located on the graph.

And don't forget that this is not a constant movement; you'll have an
acceleration phase, you may or not have a phase with constant maximum
speed, you'll have a deceleration phase and you'll have a "move in on the
target" phase where your position controller is active and the torque goes
up and down around 0.

Maybe this helps you for the controller:
http://www.compumotor.com/whitepages/ServoFundamentals.pdf

> Must it really be able to slam the steering from full right to full left in
> 1/4 second?  That sounds fast.  Even so, you will need a substantial motor.
> To actually get 1/4 second you need more than your 273W at startup and to
> overcome inertia.  The energy you put into the inertia gets wasted and is
> not recoverable.

It could be, probably, partially, but the effort is not worth it :)

In any case, it's this inertia that you need to accelerate and decelerate. 


Re linear actuator: for these calculations, it doesn't matter much whether
you use a gear motor that has a rotating output or a linear actuator that
is a motor with a screw gear that then gets translated back into a rotating
movement. The torque, force, power calculations are pretty much the same;
the only things that change are the friction losses and that the
translation back into a rotating movement usually is not linear -- both
often negligible effects.

A linear actuator needs to do exactly the same a rotating actuator needs to
do: accelerate, move along, decelerate, move in on the target, and provide
the same torque/rotational speed (at the steering column) in the various
phases.

Gerhard

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