Markku, your idea might work on glass variables, but you'd probably have to
use laser diodes to focus a beam throught the gap. Another (easier)
possibility might be to use the same technique SteppIR uses to home the
stepper motors in their yagis. SteppIR antennas don't use any feedback or
zero positioning detection in their antennas. Instead, the firmware just
instructs each motor to turn a larger number of steps than the number
corresponding to the maximum extension of the elements. Then, when homing
the elements, they can be sure all alements are completely withdrawn to
their stop. The method uses the fact that it's not harmful for a stepper
motor shaft to be unable to rotate as the motor coils are pulsed.
Applying the idea to a vacuum variable, you'd "home" or initialize the
variable by turning it to minimum capacitance, when the bellows are fully
compressed. If it takes, say, 1000 clockwise steps of the motor to turn the
variable from maximum to minimum capacitance, then the firmware would
instruct the vatiable to turn 1001 steps, thus ensuring it would be homed no
matter where it started from. Once homed, there would be no further need to
repeat the process, since the firmware would always count steps and thus
know the position of the variable (so long as the firmware remembered the
settings when it powered off). To deal with power failures or other
problems, you could have a "home" button which re-initialized the motors as
needed. This initialization procedure has the advantage that it's done
completely in firmware and requires no external circuitry.
73.
Jim W8ZR
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Markku
Oksanen
> Sent: Sunday, April 27, 2014 2:11 AM
> To: Colin Lamb; Amps
> Subject: Re: [Amps] Auto tune conversion using stepper motors?
>
> Hi
> Thanks for the ideas.
>
> Does anybody have a good idea how to find the "zero point" on vacuum cap
without
> adding a potentiometer for position feedback? I was thinking about
reflective optical
> sensors and setting them up so that when the cap opens up totally, the gap
between the sides
> of the capacitor are detected as a drop of reflection . These sensors are
really inexpensive,
> only thing is how to position them correctly and accurately.
> Markku
>
>
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