Proximity Sensor Recommendations

[broered]

I'm building a 3-axis CNC router with PMDX-424 and tandem X axis motors.  I had a single motor, prior to the tandem x-xis upgrade.  In anticipation of gantry squaring in the PMDX-424 plug-in, I've wired my X-axis motors to "Motor 3a & 3b".  I assume the gantry squaring process would work better with something passing across the face of a sensor vs. approaching it.  I originally used Micro-switches that are positioned poorly for this.

I'm considering re-doing the x-axis with 4 proximity sensors - probably the PMDX-PROX-01.  To make this work, I'd need to attach a plate to each side of the gantry.  The plate will be positioned to pass in front of the face of each sensor - at or near the end of X-axis travel.

I'm hoping that the gantry squaring software will use an initial setup process where offset parameters, for each end of the gantry, can be measured an input as constants.  It would be difficult and tedious to adjust the sensor/plates so that they simultaneously trip - only when the gantry is square.

Will the use of proximity sensors, such as the PMDX-PROX-01, provide the repeatability necessary to square the gantry?

I'm also hoping you can comment on the availability of Gantry Squaring.

Thanks.

[Steve Stallings]

It is normally best to have the target passing across the sensors, mainly to avoid
physical damage if the machine does not stop as expected.

Accuracy will be similar for either passing by or end on approach. Best results
come from ferrous targets, but aluminum and brass will work in a pinch.

Accuracy will be affected by temperature, but in a normal "office" environment
you can expect about plus or minus 0.002" repeatability.

For gantry squaring you must have the sensors for each end of the gantry on
different signals. You can combine the gantry sensors with other axes if
needed, just not with the other sensor on the gantry.

[Bob at PMDX]

I'm also hoping you can comment on the availability of Gantry Squaring.

Gantry squaring is still a little ways out.  There are a couple of features currently in the works that need to be released first.  When we do get to gantry squaring, the first versions to support it will likely rely on the home switches being positioned such that they define "square" (or "square enough").  The ability to enter different offsets from each home switch may come later.  Or not :)  We won't know until we get to designing/writing the code for this.

Bob

[broered]

Thanks Steve. 
My previous limit switch design was such that over-travel would result in crashing into the switch.  Retrofitting with proximity sensors gave me a chance to fix this situation.  Just received 4 of the PMDX-Prox-01's.  Looks like very high quality parts. 
I'm guessing that +/- .002 repeatability over the 50-55" gantry length should give great results.  I'd almost bet that standard 4x8 plywood sheets are somewhere within this range.  It's a good starting point anyway.
My revised PMDX-424 solution now uses all 8 available inputs:
1&2 are microswitch limits for the +/-Z axis
3&4 are microswitch limits for the +/-Y axis
5&6 are Proximity sensors on both -X ends of the gantry (for squaring)
7 has Proximity sensors on both +X ends of the gantry (wired in parallel)
8 has a connection for a Zero Z axis touch-plate.

Bob - thanks for commenting on Gantry Squaring!
I urge you to have manually entered offsets in your initial release of gantry squaring.  I can think of two reasons:
(1) Can't think of a better way to distinguish the PMDX solution.
(2) If the repeatability of the sensors is +/- .002 it would be extremely hard to tweak sensor/target mounting to this level without additional manually input offsets.

Thanks again for all you help and support

[Steve Stallings]

Do keep in mind that with a large table such as this, the wires are long
and there is much opportunity for electrical noise to couple into them.

It is essential that the power cable from the spindle driver (VFD) to the
spindle motor be shielded and that the shield is connected to the safety
ground at the driver (VFD).

Where possible the sensor cables should also be shielded. It is best to
separate these from the spindle cable. If you must run a sensor cable
along the same path as the spindle motor cable, consider shielding of
the sensor cable essential.