general question on proximity/homing sensors

[thegreatwaldo]

Hello All
I am putting the electronics together for my cnc machine and am working on choosing proximity switches for end stops and homing. I have see that there are sensors with varying sensitivity from 1 to 4 mm. Are the 1 mm sensors more accurate than the 4 mm ones or not. I've got the new AXBB controller and was wondering which would be better npn or pnp. Any information on this subject would be gratefully accepted.
Cheers

Andrew

[cncdrive]

Because both anode and cathod (with series resistor) are wired out on the inputs of the AXBB therefor you can connect NPN and also PNP sensors.
You can see the connection diagram for both on the pages 23. and 24. of the AXBB-E manual: http://cncdrive.com/downloads/AXBB_E_manual.pdf

Yes, the shorter sensing distance sensors are usually more precise, but with short distance sensors you will have to be careful how you placing the sensor, I mean you should place it to let the axis overrun, because it is not sure that the axis can slow down and stop on a 1mm distance (depends on the velocity and acceleration/decceleration values). So you should not place the sensor when it can collapse to the axis.
We usually use 2mm or 4mm distance NPN sensors on our machines and we place the sensors in a way that the axis can travel next to it so it never collapses to the sensor, even if the sensor fails to trigger the axis will not collapse to the sensor.