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DRO Switch

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redpiperbob:
Hi All
I have a mill with a 3 axis dro setup on it  and a lathe with 3 axis dro each uses Allandale scales.
I have only one display unit what I would like is an electronic switch that would allow me to switch between the two different inputs. can anybody help with a circiut.
Thanks in advance Bob

kwackers:
Little bit of guesswork here - but what I'd do:-

There are 2 outputs from each scale for the quadrature signal and a 5v line to power the scale. The quadrature outputs are at logic levels (0 - 5v).

For a pair of scales take the 'A' quadrature lines and run each of them through a diode (cathode first), join the anodes and then connect via (say) a 10k resistor to the base of any common NPN transistor. The emitter of the transistor should be connected to ground and the collector is the output 'A' signal and should also be pulled to the 5v line via say a 4k7 resistor.

Do this for the remaining 2 'A' lines and the 3 'B' lines.

Then group the 5v power lines for the lathe and the 5v power lines for the mill. To switch between scales simply use a SPDT switch to set power to one set of scales or the other.
This'll work because you're using a simple wired 'OR' for the A and B signals, with no power the output of the scale will always be low allowing the powered scale to drive the transistor.
The transistor will invert the output but this won't matter since both A & B will be inverted and the signals are symmetrical.

Bear in mind the unit won't know that you've changed from one to the other, so it won't 'remember' position etc.

You can also do the same with logic chips, but this is simple can be built on strip board and costs buttons.

Parts:
6 * NPN general purpose transisters
6 * 10k res
6 * 4k7 res
12 * IN4148 or similar diodes
1 * SPDT (or DPDT) throw switch
Connectors to suit

If you can't visualise the circuit let me know and I'll try and find something to draw it with...

andyf:
Being a simple soul. I'd be tempted to use mechanical rather than electronic switching, with a separate Lorlin type 3 pole rotary switch for each axis, and click all three over to change the display from one machine to the other. If I were feeling particularly ingenious, crank arms on the switch shafts and a con rod would allow all three to be switched in unison.

Andy

kwackers:
One of the problems with mechanical switching is 'bounce'. Each time you flick the switch you'll get contact bounce which the console will interpret as movement.

I've never been a big fan of complex mechanical switches, invariably at some point one or more of the contacts becomes noisy which is bad for this sort of application.

If however a multipole switch is the way you fancy, you can get 'ganged' switches. Just need one with 6 poles of SPDT, lots of rotary switches can do that.

DMIOM:

--- Quote from: kwackers on January 22, 2011, 08:11:40 AM ---One of the problems with mechanical switching is 'bounce'. Each time you flick the switch you'll get contact bounce which the console will interpret as movement.

I've never been a big fan of complex mechanical switches, invariably at some point one or more of the contacts becomes noisy which is bad for this sort of application.

If however a multipole switch is the way you fancy, you can get 'ganged' switches. Just need one with 6 poles of SPDT, lots of rotary switches can do that.


--- End quote ---

Would second all that Kwackers says, the only things I would add are (a) make sure the switches are all break-before-make, and (b) add at least one extra pole to switch the power, as I'm not sure if the display would be up to driving two full sets of scales concurrently.

As an additional observation, the setup for a Mill display can be different to that for a Lathe; so if, for example, you had the display in diameter mode when using the lathe, there is a risk you might then read double the distance traversed when using it on the lathe.....

Dave

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