Spindle Limit Switch Interference

**Macca05** · 06-27-2018, 07:34 AM

Hey all,
recently I have put together a new cnc made from aluminium and have gone about it the best to my knowledge and internet wisdom. Im having a problem that when the spindle is started it trips the limit switches thus shutting it all down. I have used a special spindle cable that is shielded and has 4 core cable inside and I have also used a 4 core cable (not using one core) for the limit switches. It seems to only have trouble with my y and z axis as they are the ones running through the cable drag chain along side the spindle cable.
What can I do to solve this problem.
One thing Im uncertain of is the shielding on all cables. Does it have to be grounded at each end?
I did a quick test with my multimeter and found that when i activate my spindle i get up to 30v through my limit switch sensor cable but none of the lights activate.

Thanks for your help
Merlin

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**john-100** · 06-27-2018, 08:40 AM

what type of limit switch do you have ?

what breakout board do you have ????

are you using a PC's parallel printer port or a motion controller ??

to ground one or both ends of the cable screen depends on the machines grounding

if both ends are at equipotential you can ground both ends of the screen
otherwise you will have earth currents flowing along the screen

with some equipment, they get around the problem by using a balanced system
that only sees the difference between two antiphase signal wires
like RS485 data ports or balanced XLR3 audio used by radio & TV broadcasters

an unbalanced signal between ground and a signal wire as usually used in CNC machines are always susceptible

having the limit switch wires and motor cable in the same chain does not help !!

using 24V logic levels can help but not many breakout boards are designed for that

John

**Macca05** · 06-27-2018, 07:45 PM

Thanks for the reply.

im using a gecko g540 board with parallel port.
My limit switches are the NO NPN inductive proximity sensors from eBay.
Ive been doing a bit of reading and have noted that I have not grounded my machine yet so I think it’s best I get that done before I do anymore work on it.
If all else fails I may need to remove my spindle cable from the drag chain and possibly hang it above the machine but I would prefer it to be inside the drag chain.

thanks

**BrendaEM** · 06-28-2018, 11:18 PM

30v is a lot of induced current. Could you instead have a ground loop if some kind?
Is the limit switch cables shielded?

**john-100** · 06-29-2018, 06:21 AM

which points did you measure to get the 30V ?

what is the supply voltage to the G540 ?

the G540 has an internal 12V regulator that powers internal logic circuits and the LED's inside the input opto-isolators
so you should have just under 12V between an open circuit input terminal & the power supply negative terminal

how are you powering the N/O NPN inductive proximity sensors ,
what is the supply voltage have you used ? - a link to the data sheet will be helpful

with normally open limit switches
the wire connecting the switch to the G540 input is at a high impedance and will easily act as an aerial picking up stray voltages
( N/C switches have the advantage of grounding the signal wire and failing safe if the wire breaks )

a problem with Chinese 3 Phase spindle motors is even when they have a 4 pin connector
they usually don't have the 4th pin connected internally to ground the spindle motor !

John

**mactec54** · 06-29-2018, 09:11 AM

Originally Posted by Macca05

Hey all,
recently I have put together a new cnc made from aluminium and have gone about it the best to my knowledge and internet wisdom. Im having a problem that when the spindle is started it trips the limit switches thus shutting it all down. I have used a special spindle cable that is shielded and has 4 core cable inside and I have also used a 4 core cable (not using one core) for the limit switches. It seems to only have trouble with my y and z axis as they are the ones running through the cable drag chain along side the spindle cable.
What can I do to solve this problem.
One thing Im uncertain of is the shielding on all cables. Does it have to be grounded at each end?
I did a quick test with my multimeter and found that when i activate my spindle i get up to 30v through my limit switch sensor cable but none of the lights activate.

Thanks for your help
Merlin

The first thing to check is the Ground pin # 4 in the spindle plug so disconnect the plug and check the continuity of the 4th pin to the spindle body, most are not connected, and this needs to be corrected, if you have a water cooled spindle, blow the water out before you take the top off

Your cable from the VFD to the Spindle needs to have the Shield Grounded at both ends, the shield is ineffective if not, this also needs to be terminated correctly

A main power supply filter before the VFD is also something worth adding