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Clive S
01-03-2017, 09:24 AM
There has been a lot of discussion on the accuracy of sensors and I came across this vid. https://www.youtube.com/watch?v=il9bNWn66BY that I thought might interest some.

JAZZCNC
01-03-2017, 08:29 PM
There has been a lot of discussion on the accuracy of sensors and I came across this vid. https://www.youtube.com/watch?v=il9bNWn66BY that I thought might interest some.

No Clive he must be wrong because every one knows the cheap Inductive sensors don't sense accurately enough with huge errors.!!! . . . . Well every one who lives in spain anyway. .:smiley_simmons:

Neale
01-03-2017, 09:18 PM
My only complaint with my cheap (around 2 each) inductive sensors is that mine don't seem to have any significant separation between on and off points. Not generally a problem, except that if I home all three axes and then jog in X or Y, the vibration tends to trip the Z limit switch as the sensor is just at its trigger point. Difficult fix, though - jog the Z down a millimetre or so first. Repeatability seems to be spot on, at least for my woodworking purposes. Must be our cold northern climate that means they work better here?

Clive S
01-03-2017, 10:26 PM
My only complaint with my cheap (around 2 each) inductive sensors is that mine don't seem to have any significant separation between on and off points. Not generally a problem, except that if I home all three axes and then jog in X or Y, the vibration tends to trip the Z limit switch as the sensor is just at its trigger point. Difficult fix, though - jog the Z down a millimetre or so first. Repeatability seems to be spot on, at least for my woodworking purposes. Must be our cold northern climate that means they work better here?

Neale You should be able to set the home position to about 1mm off the switch. Do you have the software set to back off the switch after it is triggered?

Neale
02-03-2017, 12:00 AM
I'm using Mach3, which seems to run the axis towards the switch until it triggers, then slowly backs off until the switch drops out. I haven't actually seen this behaviour described but it's the way it appears to work. Not like LinuxCNC where you have some control over how the homing works, but in principle it's a reasonable method of homing. In my case, the Z axis (actually, all three axes) overshoots on approach to the switch, which is fine, but particularly for Z, when the axis backs off until the switch trips again, the on/off switch points are not distinct. So, axis backs off, switch trips, home position is established, no problem. Mach3 now considers the switch to be a limit switch rather than home switch. Because the slightest movement will cause the switch to come on again, this is now seen as a limit event. During homing, this is OK but when rapid jogging, the tiny movement (vibration?) in Z trips this limit. I suspect that a better quality proximity switch with on/off hysteresis would avoid all this - and according to everything that I have read, they all have some small amount of hysteresis. Mine don't, though...

I'm still fairly new to Mach3; I haven't found a way to set the limit switch position to be, say, +1mm and then automatically jog down to 0 after homing. I have been playing with the "Ref All Axes" script behind the button - so I can home X and Y simultaneously, for example - but I was thinking that I might modify the script to drop Z 1mm after homing and before homing X and Y. I've done things like set SafeZ and tool change position to Z=-1, so generally I can work around the problem as long as I remember to drop Z a tiny amount after homing.

It is noticeable that as I wind an axis by hand and watch the switch's built-in LED indicator, there is a significant distance over which the LED changes brightness. There is no absolutely unambiguous switching point. I notice this particularly because I use master/slave X motors via a CSMIO IP/M which does not support independent master/slave homing. I ref all axes, then kill the motors so that they are free to turn, and wind the slave axis by hand until the LED indicator trips. The switching ambiguity is probably no more than a full step (you can feel the motor cogging) which is 25micron max and I'm reasonably happy to accept that degree of error at the end of a 1m gantry! Motors back on and I'm ready for a machining session - I generally only do the master-slave thing at the start of a session.

Boyan Silyavski
02-03-2017, 07:01 PM
No Clive he must be wrong because every one knows the cheap Inductive sensors don't sense accurately enough with huge errors.!!! . . . . Well every one who lives in spain anyway. .:smiley_simmons:

As he said, heat introduces huge errors in sensors. As you know its hot here in Spain :hysterical:

JAZZCNC
02-03-2017, 09:18 PM
As he said, heat introduces huge errors in sensors. As you know its hot here in Spain :hysterical:

Touche.!!. . :yahoo:

JAZZCNC
02-03-2017, 09:26 PM
I'm still fairly new to Mach3; I haven't found a way to set the limit switch position to be, say, +1mm and then automatically jog down to 0 after homing.

Neale because your using the IP-M you can use the "Home Off" setting in "Homing and Limits". The machine will home until it see the switch then back off the distance entered then Set Zero. This will stop your problem with false trips.

This doesn't work for None Cslabs controllers (unless supported by plug-in) or Mach3 with parallel port.

Neale
02-03-2017, 09:45 PM
That sounds like just what's needed - thanks very much for the pointer. I read manuals carefully when setting up something like this but if you don't know that a feature is present, it's easy to miss while you are looking for something else. Like the M31 macro (or at least the code inside it), it's a CSLabs extension that doesn't exist in vanilla Mach3 and they're not easy to spot.

Might go out to the garage shortly and give that a try. I did some quick tests last night looking at Z axis homing repeatability with my inductive prox switches. Doing a "Ref Z" followed by an auto tool height zero on a touchplate, I was seeing around 10micron variation. However, I had not clamped the block down (it was sitting on the wood blank I was about to machine and I know that there's a tiny amount of give there) and I have found that using a tool like that does leave a microscopic dent in the touchplate surface so that the zero point gradually sinks. We are talking microsteps here, though - it's really not significant. I'll try with a clamped-in-place dial gauge and see what I get. And in this context, 10micron ain't worth worrying about anyway. But in the interests of science (and as long as it doesn't get in the way of doing the job)...

JAZZCNC
02-03-2017, 11:56 PM
That sounds like just what's needed - thanks very much for the pointer. I read manuals carefully when setting up something like this but if you don't know that a feature is present, it's easy to miss while you are looking for something else. Like the M31 macro (or at least the code inside it), it's a CSLabs extension that doesn't exist in vanilla Mach3 and they're not easy to spot.

Yes there's quite few "hidden" aspects to the cslabs controllers which are good but not very well or at all documented. Like fact you can talk directly to controller using modbus and bypass mach3, which is exactly what the G31 macros does. Gives that bit more control and faster reactions times for I/O.!! . .. So long as you speak the lingo so to speak.!!!