[mactec54]

Yes I did spot the date but figured anyone that is unclear on the subject is not likely sticking to any standard and if they are and they don't cross reference anything they wish to take from it with the standards they wish to adhere to then they have to be pretty dumb. I mostly referenced for the quote about why not to earth both ends of a screened wire and I'm pretty damn certain the issue explained hasn't miraculously been made redundant by any more recent standard.

Yes that is redundant if we are talking about VFD Drives

I did not look at to many things in there, but if it is about Shields being grounded at both ends that is normal practice for a VFD drives output motor to VFD should be Grounded at Both Ends

Most Manufactures of VFD drives recommend that it's done this way also

All other types of shielded wire applications should always be Grounded at the source end only, ( one end )

So it just depends what's being talked about, normal wiring or VFD wiring

[DonKes]

I say FIX IT and GO ON TO SOMETHING ELSE!

Don

[A_Camera]

No one was referring to you, as old school, your philosophy is the correct way,that any good engineer should follow,(Some have what it takes some do not ) there are a lot of engineers that do old school hacks and think this is ok, also known as a Band-Aid fix, Old school does not mean that you have to be old, a young kid could try this as well, just to see the outcome, this would still be called old school, it is never a fix to do somthing like this, it just creates a bigger problem

Read my other posts, I was supporting what you had said

Don't worry, my question was a joke.

[wizard]

While you aren't wrong sometimes a capacitor can solve a lot of problems until a good solution is implemented. While I doubt it is an issues here, I've seen problems caused by mechanical switches on machines with intense mechanical vibration and impulse disturbances. Mind you these are a different sort of machine than a router, where the die weighed tons and the whole machine shook each cycle.

In any event in this case I'm nit too certain that grounding is the issue. Still reading the thread so we will see how this turned out. But my understanding is this problem happens machine running or not.

That is a totally wrong approach. If you have noise you should find the cause of the noise and eliminate it. By adding a capacitor across the switch you make an attempt in HIDING the cause of the problem, not solving it. Other than that, you are creating a delay circuit, which is TOTALLY wrong in limit switches. The point of a limit switch is that it should stop the machine as soon as it has been hit and the signal should NOT be delayed or debounced at all. Limit switches should be normally closed circuits, meaning that if you get a cable fault (broken limit switch cabling) your machine should stop automatically, even if the limit switch was not hit. This can be a case of a lose connection, for example, which should be fixed immediately. Vibrations may trip the limit switch if you have bad connection, and the RC circuit you introduced by adding a capacitor across the switch may prevent this to happen.

In short, if you NEED to add capacitors across switches to make your machine work then there is something wrong with your design.

[Jon.N.CNC]

Yes that is redundant if we are talking about VFD Drives

I did not look at to many things in there, but if it is about Shields being grounded at both ends that is normal practice for a VFD drives output motor to VFD should be Grounded at Both Ends

Most Manufactures of VFD drives recommend that it's done this way also

All other types of shielded wire applications should always be Grounded at the source end only, ( one end )

So it just depends what's being talked about, normal wiring or VFD wiring

Why screen vfd wiring? They are not particularly noise sensitive. And if you do your just introducing alot of ac eddy currents into your grounding circuit doing more harm than good. Screen everything thats sensitive and vfd noise is not an issue.

[Jon.N.CNC]

While you are wrong sometimes a capacitor can solve a lot of problems until a good solution is implemented. While I doubt it is an issues here, I've seen problems caused by mechanical switches on machines with intense mechanical vibration and impulse disturbances. Mind you these are a different sort of machine than a router, where the die weighed tons and the whole machine shook each cycle.

In any event in this case I'm nit too certain that grounding is the issue. Still reading the thread so we will see how this turned out. But my understanding is this problem happens machine running or not.

I agree, sounds more like hes used a ground where it should be a negative.

[wizard]

Actually if you search around you can find some very interesting write ups with respect to equipotential grounding. There are good theoretical reasons to take this approach but even then there are exceptions to the rule. Also in a large installation there is a need to have the grounding solution planned at the building design stage.

Interesting. So instead of having a nice big wire grounding your metalwork which would take most of the current in your frame your sharing it down all of your screen wires and right up to the Bob? I'd say that's okay if all your grounds and screens are a good wieght. A mains short down your alpha cable screens could get a bit iffy couldn't it?

The thing here is that the entire machine and all components get bonded together in such a way that all parts of the machine are at ground potential. Proper bonding requires that all bonding straps be rated for potential fault currents. When you bond the screens you are doing so to what is hopefully a well bonded machine.

Or I suppose on the basis current will take the easiest route ie thickest cable to ground you would mainly just be equalising the voltage so long as your metalwork ground wire is big enough. Worst case scenario is a mains short, if the ground wire were to get damaged and disconnected from ground, in the "old" method the current may get through the insulation and damage your cables. In al mans method it would be fried cables and possibly Bob for sure I would have thought.

That could happen if bonding fails.

I may be looking at this wrong but finding it hard to see that method applying to cable screening. Metalwork grounding certainly, but the two combined not so sure.

The idea is to minimize the impedance to ground level.

[wizard]

In another thread there is a "mechanical engineer" selling drawings of extremely poorly designed CNC... in this one, somebody claiming to be a Bachelor of Science, Electrical (or Electronics) Engineering and doesn't seem to understand the purpose of limit switches. A properly designed input should not need any capacitor across the switch. There is NO WAY any noise can have ANY effect on limit switches.

Having read this thread up to his point I really don't think anybody is implying that the caps have an impact on the switch. The idea is to have an impact on the noise.

In any event it is pretty common practice to have caps across PLC inputs.

There should be a simple pull-up resistor on the input and the input is constantly kept low by connection to GND through the normally closed switch. Once the switch is hit, the circuit is broken and the input is pulled high through the pull-up resistor and a "limit switch hit" condition is triggered. That situation should IMMEDIATELY trigger the input and stop the machine from moving and even if there would be ANY electrical noise, causing the input to be pulled low again (or due to switch bouncing) it would make ZERO difference since the input is already triggered and the machine is stopped.

I don't think there are too many disagreeing with this either.

Immediately stopping the move is the VERY purpose of limit switches.

True but absolutely nothing happens instantly in a machine control system. Even if the switch input is handled entirely in hardware outside of computer processing the reaction to the switch input is not instant.

By putting a capacitor across the switch a delay is introduced without any reason or useful purpose, and is in fact contradictory to the purpose of limit switch. Very strange that somebody claiming to be BSEE can not see that and even claims that "There is NO shame in putting good time constants in a system." Indeed there is shame to do it if it has no purpose and useful reason, even more so if it is proposed by somebody claiming to be BSEE.

The useful reason is to prevent the control from reacting to noise on the line. Seriously have you taken a PLC input module apart lately? Running field wiring into a raw digital input isn't something that happens often.

At this point I'm not sure what is going on with the original posters machine. What I find perplexing is your belief that signal conditioning doesn't happen in real world industrial electronics.

[Jon.N.CNC]

Actually if you search around you can find some very interesting write ups with respect to equipotential grounding. There are good theoretical reasons to take this approach but even then there are exceptions to the rule. Also in a large installation there is a need to have the grounding solution planned at the building design stage.


The thing here is that the entire machine and all components get bonded together in such a way that all parts of the machine are at ground potential. Proper bonding requires that all bonding straps be rated for potential fault currents. When you bond the screens you are doing so to what is hopefully a well bonded machine.

That could happen if bonding fails.



The idea is to minimize the impedance to ground level.

Yeah i do get the concept of sharing voltages and equal impedance and it does make sense. The more i read into it, good paper on vfd equipotential bonding here: http://www.iceweb.com.au/ElectricalW...0Drives%20.pdf , Its pretty clear that its not as simple as just connecting both ends of a screen wire. Improving impedance to ground but at the same time introducing more electromagnetic coupling and lf and hf noise, and the work around to reduce the noise is to keep the ground loops you are effectively making small, well i have 7 or 8 8 meter screened cables in and out of machine or the so if i was to do it id have the equivalent of a 56-64 meter ground loop! I'll be able to hear spanish radio from my motors! and they are as short as they can be. Maybe there's some circuitry required. Personally i think if you dont understand electronics to degree level. You could very easily give yourself some big issues doing it this way if you were to do this to all your wires.

Ground impedance was part of the pat test my machine underwent late last year and it passed without issue with single point earthing. If you are building and selling machines to set standards it may be something that would require consideration for certain aspects, but with these dc & ac systems it would be worth consulting an engineer before doing so because im pretty sure some isolation is required.

[wizard]

Your points aren't really helping your argument.

We don't miss your point
No one said Caps don't work, it is not the correct method to fix something like this, if the switch is used as a Home switch ( which most are )

Could be an axis limit switch.

the cap will delay the signal when the machine is homed,

There are all sorts of delays in a control system, this point really has no value to me. Even if the home signal gets handled completely in hardware, zeroing a counter or whatever, there are still delays in the system. The only thing in homing that is really important is repeatability which indeed can be a problem with some inputs if there is too much capacitance on the input.

allowing the machine to be not in the correct place it should be when homed

That is nonsense. As long as the trigger is repeatable you have accomplished home referencing. That only applies on systems where the home switch is the ultimate indications of home. Most servo based systems just use that as a starting point to look for the encoder home pulse.

No not all have the skill to figure this out, but a simple $10 EMF meter from Amazon can find the noise source in a second or two, not much to figure out from there

I don't disagree with you when you say that you need to control the noise problem at it source, however when you say it is easy to fix the problem I have to disagree with you completely. It can take a very long time to find root causes especially on machines that have been running for a good long time.

I can remember an issue I ran into years ago where a machine would loose a micron on position maybe every 15 cycles of the machine. These where diamond turning machines expected to hold a couple of microns in a production environment. For some unknown reason a packaged drive, a crude one at that, decided to cause problems. We traced the problem down to a relay mounted on a circuit board which didn't have a snubbing diode installed. A quick installation of a diode cleared up the problem. Finding that issue was not easy at all and I certainly didn't expect to have to pull a drive apart to correct the issue.

Now for the time constant obsessed what impact does a snubbing diode have on relay operation? Slows it down a bit or in other terms changes the time constant of the circuit. The fact is controlling noise generators often involves changes that impact the time constants in the machine. Even with the best of efforts you will have electrical noise in a machine controller this is why it is common to do signal conditioning on inputs.

[wizard]

Yeah i do get the concept of sharing voltages and equal impedance and it does make sense. The more i read into it, good paper on vfd equipotential bonding here: http://www.iceweb.com.au/ElectricalW...0Drives%20.pdf , Its pretty clear that its not as simple as just connecting both ends of a screen wire. Improving impedance to ground but at the same time introducing more electromagnetic coupling and lf and hf noise, and the work around to reduce the noise is to keep the ground loops you are effectively making small,

Some of that strikes me as perfect world, ideal conditions solutions.

well i have 7 or 8 8 meter screened cables in and out of machine or the so if i was to do it id have the equivalent of a 56-64 meter ground loop! I'll be able to hear spanish radio from my motors! and they are as short as they can be. Maybe there's some circuitry required. Personally i think if you dont understand electronics to degree level. You could very easily give yourself some big issues doing it this way if you were to do this to all your wires.

There are simply too many variables to say it has to be the solution.

I'm not sure where these ideas come from, but I only have to look at Eurpean standards and their love of safety relays. They may or may not serve a function in a machine but safety relays greatly reduce your overall machine reliability. The failure rate on safety relays is shocking and after replacing dozens on machines built to Eurpean standards I really have to wonder if the regulations where written by the safety relay manufactures to assure a market for their crap.

At least with grounding it is easier to understand what they are trying to accomplish with equipotential grounding. If every part of the machine is at ground level you can't have ground loops.

Ground impedance was part of the pat test my machine underwent late last year and it passed without issue with single point earthing. If you are building and selling machines to set standards it may be something that would require consideration for certain aspects, but with these dc & ac systems it would be worth consulting an engineer before doing so because im pretty sure some isolation is required.

In instrumentation sometimes you want the transducer to remain ungrounded for whatever reason. In at least one of the papers on equipotential grounding I read there was an exception made for control/instrumentation circuits. I will likely never find that paper again 😟😟.

[Jon.N.CNC]

Some of that strikes me as perfect world, ideal conditions solutions.

There are simply too many variables to say it has to be the solution.

I'm not sure where these ideas come from, but I only have to look at Eurpean standards and their love of safety relays. They may or may not serve a function in a machine but safety relays greatly reduce your overall machine reliability. The failure rate on safety relays is shocking and after replacing dozens on machines built to Eurpean standards I really have to wonder if the regulations where written by the safety relay manufactures to assure a market for their crap.

At least with grounding it is easier to understand what they are trying to accomplish with equipotential grounding. If every part of the machine is at ground level you can't have ground loops.


In instrumentation sometimes you want the transducer to remain ungrounded for whatever reason. In at least one of the papers on equipotential grounding I read there was an exception made for control/instrumentation circuits. I will likely never find that paper again ����.

Indeed.. Its health & safety regs showing its ugly expensive face, And how else are the university lecturers supposed to deviate from the curriculum and add their 2 pence worth. Suppose Lightning proof could be a good sales point :s

[mactec54]

Why screen vfd wiring? They are not particularly noise sensitive. And if you do your just introducing alot of ac eddy currents into your grounding circuit doing more harm than good. Screen everything thats sensitive and vfd noise is not an issue.

Wow you guys are really behind the times

Your thoughts are why people have problems with noise

In some countries it is a code violation if a VFD Drive is not installed without Shielded cable just some photos and information from manufactures of VFD Drives

[Jon.N.CNC]

Wow you guys are really behind the times

Your thoughts are why people have problems with noise

In some countries it is a code violation if a VFD Drive is not installed without Shielded cable just some photos and information from manufactures of VFD Drives

Yeah shed loads of standards if your manufacturing vfd's. For a pat test here in the uk it's not a requirement tho.

And what purpose does it serve if everything else is shielded?

[mactec54]

here in the uk it's not a requirement tho.

It would pay to check your UK requirement, as it is even tougher than the US, most European countries, have very strict wiring codes for things like VFD Etc

This is your UK requirement

Figure 4: is a rough diagram of a UK requirement

Installation requirements
The use of a VSD inevitably involves the installer in providing some special EMC features, in particular the correct mounting of the drive module in relation to its input filter and screened motor cable. The manufacturer carries responsibility for ensuring that clear instructions are given to the installer.

United Kingdom. Electricity Association. EMC Requirements
Further information on drives EMC, harmonics, and many other drives-related issues is given in the recently published Control Techniques Handbook

what purpose does it serve if everything else is shielded?

I don't think this even needs an answer

[Jon.N.CNC]

It would pay to check your UK requirement, as it is even tougher than the US, most European countries, have very strict wiring codes for things like VFD Etc

This is your UK requirement

Figure 4: is a rough diagram of a UK requirement

Installation requirements
The use of a VSD inevitably involves the installer in providing some special EMC features, in particular the correct mounting of the drive module in relation to its input filter and screened motor cable. The manufacturer carries responsibility for ensuring that clear instructions are given to the installer.

United Kingdom. Electricity Association. EMC Requirements
Further information on drives EMC, harmonics, and many other drives-related issues is given in the recently published Control Techniques Handbook



I don't think this even needs an answer

Yes more manufacturing and installation service guidelines. Not pat test requirements as let's face it all machines older the 2000 would fail to new manufacting regs.

my point was simply this, if all your control gear, inputs, motor wires etc are screened correctly. Vfd to spindle wire screening will have zero effect on how your machine will run.

The awser you deemed as not worthy of an awser can therefore only be a safety improvement with a reduced risk of shock. Just wondered why you would do it.

As with most if not all circuits, there is a compromise to be made, so in a bid to make the circuit safer, the screen wire itself will undergo more thermal stress and incorrectly implemented your generating additional currents and noise into your earthing network. So if anyone is likely to be causing people noise issues, it's going to be the person who states "you should screen your vfd wiring" without any explanation of the effects of doing so and guide as how to implement such circuit correctly.

[A_Camera]

Having read this thread up to his point I really don't think anybody is implying that the caps have an impact on the switch. The idea is to have an impact on the noise.

...but it HAS an impact on the switch and ZERO impact on noise. It seems that you don't understand how the proposed capacitor is connected. Maybe later today I'll make a simple drawing to show what we are discussing here.

Edit:



I made a quick drawing of how limit switches should be connected. I have added 2 switches, since at least two are normally connected to the same input. I have also added two capacitors, which is the subject of the discussion. Now, please explain what use those capacitors are if they are connected in parallel with the switches. They have ZERO use, since the switches are normally closed type, so the capacitors are normally shorted out, i.e. no impact on noise reduction whatsoever. First when the switch is hit (i.e. the axis is driven to the extreme end and MUST be stopped to avoid damage to the CNC) are the capacitors connected as they normally are if they are in use. Now, another risk you run into is if something happens to one of the capacitors (or both) and they stop working all together. Capacitors can break down in two ways, one is that they simply short the terminals and put the limit switch out of order. You will not notice this in this case until you hit that limit switch. When the limit switch is hit nothing will happen in that case, your CNC axis won't stop, it will ram into the machine wall and in the best case your stepper will stall, in the worst case your machine will be damaged. This case has not been mentioned in this thread, but it is a real possible situation. In other words, there are plenty arguments for why a capacitor across a normally closed switch is a REALLY bad idea, a pure amateur solution due to lack of understanding of electronics. The only parts necessary in that drawing to properly work is the pull-up resistor and the switches. Nothing else should be connected to the input. Of course, the number of limit switches can be any number, in my case I have all six switches and a eStop connected to the same input because I don't need the information about which one is tripped, I can see that with my own eyes.

End of edit.

I said: "There should be a simple pull-up resistor on the input and the input is constantly kept low by connection to GND through the normally closed switch. Once the switch is hit, the circuit is broken and the input is pulled high through the pull-up resistor and a "limit switch hit" condition is triggered. That situation should IMMEDIATELY trigger the input and stop the machine from moving and even if there would be ANY electrical noise, causing the input to be pulled low again (or due to switch bouncing) it would make ZERO difference since the input is already triggered and the machine is stopped."

I don't think there are too many disagreeing with this either.

No, nobody seems to disagree, yet some people are still arguing about the use of capacitor over the switch. Obviously, those who do don't really understand how limit switches are connected.

True but absolutely nothing happens instantly in a machine control system. Even if the switch input is handled entirely in hardware outside of computer processing the reaction to the switch input is not instant.

"Instantly", "immediately", "as soon as possible"... none of those are quantified terms. Never the less, if you introduce yet another delay then your system reaction will be delayed even more, so it is counter productive to the purpose of limit switch.

The useful reason is to prevent the control from reacting to noise on the line.

Like I said, you don't seem to understand how the proposed capacitor is connected.

Seriously have you taken a PLC input module apart lately?

No. But never the less, we are discussing limit switches with capacitors across the switch.

Running field wiring into a raw digital input isn't something that happens often.

What you call "raw digital input" isn't really the way you think it is.... especially not if it is designed as it supposed to be.

At this point I'm not sure what is going on with the original posters machine.

Probably lost interest in his thread or solved the issue and forgot about passing on the solution or informing about what he did.

What I find perplexing is your belief that signal conditioning doesn't happen in real world industrial electronics.

In which sentence exactly have I said that? Please don't claim anything so stupid, since that statement is nothing but insult.