SPC-type DRO scale tale

Well, it's mostly done.

I broke down and bought a DRO system for my Shoptask. ( WAY too many other
projects on the go to finish my homebrew model anytime soon )

It uses three of the inexpensive scales with built-in readouts as seen on
eBay all the time.

The unit comes with an LED display unit that takes input from the reader
via the SPC output. The large LEDs are a welcome adjunct for my failing eyes
and also allows the scales to be well shielded from chips and fluids.

After struggling with the standard mounting problems of getting things parallel
and shimmed just so, I discovered a problem. While the two shorter scales
"turned on" when moved ( and off, when left to sit for a time ), the long one
required a press of the ON button to get going ) Problem is, the buttons
were now behind the mounting bracket and splash shield, making them impossible
to access.

While struggling to devise a way to press these buttons remotely, Chris Baugher
pointed out that they might turn on at power up and further pointed out pin 4
of the SPC cable supplied power. A quick check showed he was right ( MANY
thanks ). So the display unit came apart and a 1.5 volt supply stuffed inside.
( An LM317, a cap and a pair of resistors )

Tada ! Success. Powering the display also turned on the scales. I was in
business.

Right up until I turned the spindle on, at which point the readouts went nuts,
both the little LCDs on the readers as well as the LEDs on the main display,
jittering away like crazy, even with the table stationary.

Interestingly, upon turning the spindle off, both the readers and the main
display showed the same ( wild ) numbers, indicating that the communication
between them all was working correctly.

Now, some time ago, I'd added a VFD to the machine and they're notorious for
LARGE amounts of electrical noise. I shielded everything from the VFD well,
( although not the DRO leads ) but I guess there was still enough noise to be
causing problems.

Powering the whole system from batteries did not solve the problem, indicating
it was not noise sneaking in along the power line.

But putting the batteries back into the readers did fix things !

So it seemed as if noise was being induced into the power lines from the reader
to the display.

My first thought for a fit was to lower the impedence of the system and add
some filtering. To that end, I paralleled a 100 ohm resistor, a .01uf ceramic
cap and a 47uf tantalum cap. These I soldered up into as small a bundle as
I could, and then soldered this bundle to the battery leads. ( No going back
now )

And this solved the problem.

Now, which of the three items was actually responsible for the fix, I didn't
take the time to investigate. I'm just happy to have it working.

Now the problem of all these cables; three motor cables, three DRO cables,
three limit/home switch cables, three cables from the (remote) VFD to the
motors and switches.

It's starting to look like an octopus ...


Alan


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta