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Timgander
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« on: January 09, 2017, 11:37:41 AM »

Hi Guys, sort of newbie here. Have run a Pacer Cadet for some years with antiquated control and driver software (Pacers own) on a W95 (yes W95) dedicated PC. I'm an engineer (mechanical and electronic) by trade so decided to make use of the excellent mechanical construction of the unit and put all new drivers, controls and software in. All is going OK and I've answered lots of my questions by trawling through this excellent forum but I am concerned about the accuracy/repeatability of the homing sensors. Mine are two-wire inductive but any sensor, microswitch, opto, capacitive etc. can't be considered as 'absolute' position switches as lots of outside factors (temperature, vibration, dust, electrical noise etc.) can affect the trip point. These switches are fine as ultimate 'limit' switches to stop carriages mashing into things but not for accurate origin setting AFAIK... I've worked on a lot of precision industrial machines and we would normally use a Renishaw (or similar) absolute position sensor to determine a repeatable and accurate origin for toolset.

As Mach3 (which I am running) goes I understand that it takes the home position (all three axes) as the point from which to calculate the 0,0,0 location?? Is that so? If so how can I guarantee that 0,0,0 will be in the same place after each 'homing'??

Crazy thing is I've used this machine for 5 years and never questioned it, just run the machine, and it's only now that I am into the rebuild that the question has arisen.

I'm sure you guys will have an answer!

Regards

Tim
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Davek0974
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« Reply #1 on: January 09, 2017, 12:51:29 PM »

Presumably the sensors were the originals? If yes then they should be ok as it worked before Smiley

There are some tricks you can do with homing - I have seen encoder disks fitted to the motor shafts, the sensor tells the system to look for a pulse from the encoder and this is then taken as home position - this can be very accurate and is very similar to index homing on servo drive systems.
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Timgander
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« Reply #2 on: January 09, 2017, 02:50:04 PM »

Hi Davek,

Yes they are the original sensors but as they connect directly to Pacers own driver/interface board which is heading to the scrap bin I have to interface them to my breakout board which doesn't accept non-polarity conscious, 2 wire inductive sensors, so I have to add a dry-contact relay switching board which will of course add an extra element of error as there is a finite time for the relay contacts to respond and tell Mach3 it has arrived. Ironically it has 200's worth of exceptional sensors which with a relay in the circuit may just as well be a 2 microswitch!

I suppose my biggest question is that with the amount of machines out there under Mach3, with microswitches, there is an awful lot of inaccuracy going on and I'm puzzled why this forum (or any of the others) isn't/aren't awash with questions about how it can be overcome??
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Davek0974
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« Reply #3 on: January 09, 2017, 03:14:13 PM »

Probably because for a lot of applications, a microswitch is accurate enough - a good quality one is pretty repeatable.

I have not used 2-wire inductive sensors, i guess they are hall-effect maybe or capacitive (doubtful) either way i would not fit relays because as you stated it adds slack to the circuit.

I have a microswitch on the plate-sense probe of my plasma cutter and its accurate to 0.1mm repeatable, i used three-wire inductive sensors on my mini-mill and they are repeatable to <0.1mm, the big mill uses large lever switches but has index homing.
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Timgander
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« Reply #4 on: January 12, 2017, 05:24:54 AM »

Hi Dave,

The inductive sensors have a coil fed from an oscillator circuit. When the coil is brought near a metallic surface there is a change in the absorbed emf from the coil which can be detected. Basically at 0.1mm repeatability it means that each time the machine head is homed the tool could end up up to 0.2mm from the previous position in all three axes. Not a tolerance that is any good for precise machining operations Sad

I think I need to design/make a precise tool origin widget Cheesy

Regards

Tim
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RICH
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« Reply #5 on: January 12, 2017, 08:16:17 AM »

Tim,

Accuracy and repeatability will only be as good as the machines "system".
So the "system" is made up of each mechanical and electronic item.
Variations of system items add up to some general capability / tolerance.
Keep the above in mind.


So your question.......
If so how can I guarantee that 0,0,0 will be in the same place after each 'homing'??

The only way would be to test, but,be carefull about what your testing since one needs to isolate a system part. That can be difficult.


May want to try the follwing:

Probe ( single probe touch off at a very slow feed rate ) the same tool to the same reference point / surfaces from a manually created  home position while in machine coordinates and no work offset.  ( your not going to use the switches) Do it ten times and use the mean square average as the probing tolerance.

Now probe using the switches similar to the above.The setter used in touch off should be roughly located similar in distance and location as was used when not using the switches. Find tolerance.

When you compare the above tolerences that gives an good idea of what you can
expect relative to the any switch.  

Just a thought,

RICH

BTW, 1" square gage block ( can find them for cheap!) provides 5 probing surfaes and can be easily mounted to suite your needs.  
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joeaverage
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« Reply #6 on: January 15, 2017, 03:25:23 AM »

Hi Timgander,
I  was concerned about repeatability as well. In the end I used top quality roller plunger microswitches (Omron
from memory) and have achieved repeatability of about 0.03 mm which I was pretty happy with.

It is possible to use 'index homing' where the machine homes to a switch per normal but then the motor rotates until
the index signal is seen. That should in theory get you within one step. Not sure that its readily available or supported in
Mach3 but its all the rage in Mach4 if supported by the motion controller plugin which is to say most of them. The 'sim' motion
controller doesn't have it so you would need a motion controller plugin that does support it. The ESS certainly does but to view
the plugin requires communication with the device, ie you can't see and experiment without buying an ESS. I believe however that
the PMDX plugin can be viewed in absence of the device and you could experiment to your hearts content without having to buy
anything.

Craig

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Timgander
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« Reply #7 on: January 17, 2017, 03:12:28 PM »

Thanks guys, all good suggestions. I guess the proof of the pudding etcetera will be when the refurb is finished and I am actually moving things and can see if what I am worrying about is actually an issue.

Regs
Tim
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