False Signal From Proximity Detector

[Geof]

The toolchanger on our Super MiniMill started sporadically giving alarm 115 Turret Rotate Fault but would then work fine with manual ATC FWD or REV and keep working okay for a variable period of time. The picture shows the cause; a small chip of aluminum sticking on the proximity detector for the Geneva mechanism.

[Karl_T]

Just curious,

The prox switches on my CNC machines are all magnetic. They don't detect AL. Do you have some sort of capacitance detector?

Karl

[gar]

060718-0930 EST USA

Karl_T:

I believe the prox switches which are usually labeled "magnetic prox" are probably based on a sensing coil in an oscillator circuit, and the proximity of a conductive or magnetic material will change the loading on the oscillator. This load change is sensed by a threshold circuit that provides the on-off output.

The load change that is sensed could be seen as a frequency change, or amplitude, or a cessation of oscillation. The later being the more likely technique.

This type of prox switch will be more sensitive to magnetic, than to conductive but non-magnetic materials.

If the sensor has a true magnetic field sensor, like a Hall device, then this type of unit will probably have a biasing magnet and will not be influenced by copper or aluminum.

.

[Geof]

gar gave a more complete explanation than I can. The detectors respond less to aluminum than steel, the trigger distance is closer for aluminum than steel and depends more on the size of the piece of aluminum. I did some testing with the door proximity detector. I think the intermittent nature of the toolchanger fault was because the chip was 'floating' around in the drop of coolant. I have had similar faults with axis home switches, mostly the Y axis when someone just dumps a bucke of coolant into the machine through the door and splashes chips under the way covers.

The moral is on a Haas always check the proximity detectors when the machine has a motion alarm.

[HuFlungDung]

Good to know about, thanks Geof.

[Geof]

Originally Posted by HuFlungDung Good to know about, thanks Geof.

Proximity Detectors started being used around 2002 - 2003 so I should have specified that. Earlier machines use mechanical switches.

[Al_The_Man]

Originally Posted by Geof Proximity Detectors started being used around 2002 - 2003 so I should have specified that. Earlier machines use mechanical switches.

Some machines maybe, I've been using Prox. switches on PLC's and CNC's since the '80's. So has Mazak and many Fanuc's.
Or do you just mean on HAAS?
Al.

[Geof]

Originally Posted by Al_The_Man Some machines maybe, I've been using Prox. switches on PLC's and CNC's since the '80's. So has Mazak and many Fanuc's. Or do you just mean on HAAS? Al.

Just Haas.

And I wish they still used mechanical! No problems at all (so far) with the machines I have with mechanical switches.