Re: Focuser Question

Martin Bonfiore

I have been thinking about the focuser functionality for a while.  What I did is to add a momentary contact switch mounted close to the focuser.  This switch, when pressed, disables the focuser motor drive (it uses two wires that run back to the STM32 board...I modified the board to allow interception of the Enable line to the focus motor driver).  When I press the button at the focuser, the motor free wheels and I can do manual focus.  Release the button and the motor drive is enabled and the holding current is asserted. 

Sounds good however, it has one unanticapted (at least on my part) short coming.  I am using a stepper motor for the focus motor.  When you de-assert the stepper motor driver enable, it free wheels....however, when you re-assert the driver, and you have moved the focus manually from where it starts when you de-asserted the driver, the motor jumps to the nearest "position".  In the case of a 200 step per revolution stepper, this "cogging"...not the typical cogging...has a resolution of 50 steps per revolution or 7.2 degrees as opposed to the inherent stepper resolution of 1.8 degrees the focus can change...depending on where you moved it manually, +/- 3.6 degrees when you switch back to STM32 control.  Basically, the stepper driver "remembers" the relative step postion and moves to satisfy that position when the enable is re-asserted.  This, I believe is inherent in using the enable to turn on and off the "free wheeling" manual focus.  In my case, I have a 1.6 pulley reduction and I am driving the fine focus knob on the scope so it is not an issue.

For fun, I have considered a more major hack where I interpose a small microcontroller (say an Arduino Nano) that monitors the Step, Direction and Enable lines from the STM32 and also a couple of UP/DOWN focuser buttons located on the focuser itself.  The microcontroller would allow Step/Direction commands through from the STM32 but would also allow the UP/DOWN buttons to interject Step/Dir pulses.  The idea being that the focuser would always be enabled; it would receive Step/Dir commands from both the STM32 as well as from the "manual" focus controls.  There are some timing and coordination stuff to consider in the SW but I think it could work and eliminate the 7.2 degree cogging.  Personally, I decided it was not worth the trouble to implement.

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