Centroid Community CNC Support Forum

hi All,

as described here: https://centroidcncforum.com/viewtopic. ... 338#p85338

i am right now busy with building a lathe from scratch. The main functions are working, i am impressed of the funcionality of Acorn so far.

but last week i add some linear glass scales.

reasons for it: i like to improve the accuracy of the system ( by the way the 750W Servos are very accurate ). the limiting factor for that are the inductive homing switches, i have a deviation of around 3-4/100mm, so nearly 0,1mm diameter tolerance after homing, and this is way to much.

so i bought some aikron linears scales with RS422 interface and Z reference Points every 50mm and integrated them into the system. with an adaptor i changed the pinout fitting to Acorn requirements and the scales itself are working gread. setup is easy, accuracy is great, everything is good. but after that i wanted to setup the ZRI Homing.

i found a video in youtube from marty, he is showing how to connect the encoder ports to Acorn to use the ZRI input for homing:
but this is not working for me. the input never closes.

i also tried to use the JMC Servo Encoder for homing, but then realises, that JMC is not supporting Z Homing, since they have no Z-Pulse transferred from Encoder to the Servo-Drive.

so now my question is, how to handle the ZRI Homing with the linear scales ? how to connect them to an input or can i use the encoderport itself to read the Z-Marker ?

thank you in advance.

Michael

If your scales have only RS422 differential output, then you will not be able to connect the Z channel directly to an Acorn PLC input. They are not electronically compatible.

The example picture you posted from the DMM DYN4 video shows the input connected to a special photocoupler output, provided by the DMM servo drive. You can see that it is separate from the differential Z channel on pins 4 and 8 of the same connector.

Do your scales have an supplemental photocoupler outputs? It is unlikely, scales usually do not have such outputs. But you should check.

If they do not, then you will need to build a circuit using an RS422 receiver chip, to translate the differential signal into an open-collector signal. You might also need a photocoupler, or at least a transistor, to allow the output of the receiver chip to sink the 24VDC from the PLC input.

Okay, thank you for that Information.

My last try is to read the variable 25102 with a macro and Do the Homing with that. But not sure if it will work.

Best regards
Michael

hi all, after trying one week to implement ZRI Homing with the X-Axis linear scale, i have no idea, why it will not work.

in the meantime i understood, that i cannot control an Acorn Input with the differential Z-Pulse from the scale.

but Uwe (suntravel) gave me a hint, that there are system variables 25101-25108, which are getting written by the encoder counts of the index pulse signal. so i tried to read these variables, but there are always -1, what means, there was no pulse detected.
i checked also the linear scale itself with an oszylloscope, they are working fine, a clear 5V pulse every 50mm.

what is the logic behind the variables 25101-25108 and why do they not recognize the index pulse of my scales ?

thank you
Michael

can you see a index pulse flash in the PID screen?

post a fresh report.zip so we can see your setup and home program etc.

hi Keith,

yes, i can see the flash for the X-Axis. For the Z-Axis not. Maybe the reason for that is, that i changed the polarity for Z-pulse only for X-Axis to check, if that could be the reason. for the Z-Axis the star is always visible. you can see it in this video:



attached is also a fresh report.

i checked also the variables again, #25101 and #25102 are both -1.

by the way, is it possible to create a status bar for creating the report ? It took some minutes to create and in the meantime CNC12 is not usable. I open always the folder and check, when the size of the report data is not changing anymore to know, when the report is done.

best regards,
Michael

If the asterisk is always visible while you move the axis up and down the scale, then yes: you should try swapping the polarity of the index channel wires.

cncsnw wrote: ↑Mon Apr 08, 2024 12:43 pm If the asterisk is always visible while you move the axis up and down the scale, then yes: you should try swapping the polarity of the index channel wires.

I will try this of course. But for x axis the polarity is fine, but the variable cannot be used for Homing, because it is always - 1

Home programm is still the standard one. I began to write a new Homing macro but then realized, that it will not work, because the Z pulse is not usable. For debugging i use these two lines of Code:

Update:

I changed the polarity of Z pulse for the Z axis also and now the Flash in the PID Encoder settings is working for X and Z Axis.

But the variables from 25101 to 25108 are all still set to - 1. I have no idea, why it is not working. Is there a bug or is there a Parameter setting missing?

Cheers
Michael