Motorised focuser (Bartels system)

[kinetic (Steve)]

There are many ideas on how to homebrew a motorised focuser and even a few in the
IIS archives. Here is my unit I have been making over the last few weeks.

It is a modification of a GSO 10:1 Crayford style focuser with a mini stepper motor which
interfaces to the focus option on the Bartels system.
I used a little belt reduction unit salvaged from an old bubblejet printer as the source of
stepper, belt and nylon toothed wheel. Pic 1
First I removed the black 10:1 knob from the GSO focuser which attaches with a M4
grub screw.Pic 2
I then had to turn a new knob with a shaft for the blue toothed wheel to sit
on and spin freely. Pic 3
I also turned the blue nylon wheel down so all that remained was the toothed section.
I also made a second knob which would be the pressure plate adjustment to allow
the motor to drive the focuser but slip if it came to an end travel.
All the bits shown in Pic 4 I used a slightly larger toothed belt with
the same tooth pitch to get the right motor-wheel spacing.

Pressure is applied to the clutch by turning the two adjustment screws at the end
of the knurled knob and a white nylon washer pushes on the side of the blue
nylon wheel.
Pic 5/6 Manual focus can still be done by just turning the 10:1 knurled wheel.

The electronics: I initially tried a TA7774P Stepper driver IC as the link between Mel's
SCOPE program and the motor but it was not an exact swap for the real IC needed,
the MC3479.
The MC3479 drives it much better with step and direction control direct from parallel
port pins 16 and 17.

Finally, PIC 7 is a load test with the DSI and homemade filterwheel attached.
Lifts it nicely.

Steve

[bojan]

Steve, this really looks very good :-)

I like your clutch idea...

Some time ago I posted my clutch solution in this section - I mounted the motor in eccentric barrel, so I can adjust the belt pressure/slippage (there is no teeth on focus wheel, just a groove for belt) by rotation of the motor assembly (it is coreless DC motor, with gearbox). However, engaging the clutch moves the focus, so I think your solution is somewhat more complicated, but better :-)

My focuser is independent of Bartel's system - sometimes I just do not feel like powering-up the whole thing.. It has its own controller box (with dynamic breaking and powered by internal 9V battery, it is there for 5 years already and still working :-)

[kinetic (Steve)]

Thanks Bojan,

I did see your idea a while back while churning through IIS archives
and remember making a note of the Mosfet DC changeover
circuit.

The clutch works well by the way.
I wanted a setup where I could manually focus, but also have
slippage if I accidentally left the lock on the crayford or it
fouled on an end stop.
This seems to do the trick quite well.

Steve

[hikerbob (Bob)]

Steve beautiful work once again. Thanks for the notes, something I will look at once I finish off my mount.

Bob

[kinetic (Steve)]

I made 2 PCBs to add the functions to the drive.

First was a standalone Optocoupler board for basic
isolation. Second was a board with purely the MC3479
fitted. Many other applications could use this board.
A filter wheel motor rotator for example from logic outputs.

Then I decided to make a board that joined the two so that
it could be hacksawed in half if I only wanted one of the
two functions later.

Steve