Re[4]: ATM A Cage as Secondary Holder

[Dwight Elvey <elvey@hal.com>]

Dominic-Luc Webb molmed <domweb@ki.se> wrote:
> 
> I don't recall this coming up recently, no..... but I know about
> this trick.... but what exactly happens to conventional steppers 
> when you reduce the current? Will they still retain the same 
> torque, etc?
> 
> Pardon my ignorance, but the energy is drawn from the total 
> current is it not? and if this is reduced, I don't see how 
> the motor can sustain the high torque and inertia we are 
> interested in. In some earlier discussions, it has come up that
> we are (most likely) after optimizing the performance of smaller
> motors for the sake of weight and size considerations for 
> portability....
> 

Hi
 First, Mel mentioned that microstepping is one way
to reduce vibration. This is more complicated than
the simple driver but could easily be done with
something like a PIC processor and PWM output signal.
 There are two basic stepper motors out there. One type
has magnets in it. You can easily tell this type because
it has a cogging effect when turned with the power off.
The second will turn smoothly with the power off. The
first kind is the more common for disk drives and such
because once positioned, one can reduce current and hold
position. The first type tends to make more vibration.
The vibration is generated any time one accelerates
or decelerates the steppers rotor. Reducing this can
be done in the simple stepper setup by changing the current.
In the case of the one with the magnets, the least
vibration is when the acceleration is equal to the deceleration.
This may be more or less current than you are currently driving.
This is usually less but only experimenting will tell you.
For the case of the second type, reducing the current will
reduce vibration. In both cases, you need to make sure that
you have sufficient current to move the stepper. One
other thing that can be done is to profile the drive
current. This is similar to mirco stepping and in most cases
just as complicated.
 In the case of using the small light weight motors from
surplus, you'll never know if you are overdriving the motor
unless you try reducing the current. Decreasing the drive
current and experimenting is usually simple enough that
one should try it. If with lower current, you have enough
drive torque, why would you want to use more than needed?
Like I said, the telescopes load is a small part of the
steppers effective load. Most of the load is motor inertia,
gear inertia and lubricant friction.
Dwight