Which Type of Limit Switch

[manofgresley]

Hi

I have a 3 axis Milling machine which i have converted to CNC with the help from members of this forum, i have installed, a maximum travel Limit Switch (Mechanical Lever Type) to each axis, wired through the PILZ relay i am using, so complete shut down on activation. I now wish to install "HOMING" switches, but which type ? Mechanical or Optical. I am using Mach 3 software, or can this be done in Mach 3. Can anyone supply the relevant info?

Regards

Ray

[cropwell]

Don't forget to include an over-ride switch, so when you hit the limit, you have some means of getting off it. "looking for a smiley that means - Painted into a corner" :hopelessness:

[manofgresley]

Hi.

I have left the hand wheels on the motors for this purpose.

Ray

[irving2008]

Optical or proximity would work, difficulty with either, on a mill as opposed to a router, is finding somewhere they can be kept clean and dry. Also where is 'home' on a mill? nominally its the centre of the table (as opposed to one corner on a router). Mach3 homing is to detect the homing switch change of state then back off til it changes back again. a bit tricky to do for the middle of the table.

Hi

I have a 3 axis Milling machine which i have converted to CNC with the help from members of this forum, i have installed, a maximum travel Limit Switch (Mechanical Lever Type) to each axis, wired through the PILZ relay i am using, so complete shut down on activation. I now wish to install "HOMING" switches, but which type ? Mechanical or Optical. I am using Mach 3 software, or can this be done in Mach 3. Can anyone supply the relevant info?

Regards

Ray

[EddyCurrent]

Is this any use ? it might solve the issues Irving rightly brought up.

http://www.cnccookbook.com/CCMillCNCHomeSwitches.htm

[manofgresley]

Thanks for the advice, i have a ZP5A BoB, the voltage on the Bob for the HOME limits is 5 volts, i am no electrician, could you recommend a type and brand of switch i could use?
Optical or proximity would work, difficulty with either, on a mill as opposed to a router, is finding somewhere they can be kept clean and dry. Also where is 'home' on a mill? nominally its the centre of the table (as opposed to one corner on a router). Mach3 homing is to detect the homing switch change of state then back off til it changes back again. a bit tricky to do for the middle of the table.

[cropwell]

If you use a proximity switch such as the LJ12A3-4-Z/BX, you will need a supply voltage of 6 to 36v. anything below 10v won't work reliably (I found to my cost). The advantage of this proximity switch is that it is a sealed unit, you just need to keep swarf away from the sensor end.

[irving2008]

If you use a proximity switch such as the LJ12A3-4-Z/BX, you will need a supply voltage of 6 to 36v. anything below 10v won't work reliably (I found to my cost). The advantage of this proximity switch is that it is a sealed unit, you just need to keep swarf away from the sensor end.
Which is tricky on a mill, also the mill itself is a big chunk of iron so mounting proximity switches so they're not affected isn't so easy. Optical sensors as shown in the link on Eddie's post in suitably sealed boxes are a neater solution. A suitable sensor is this one: http://uk.farnell.com/omron-electronic-components/eesx398/opto-switch-transmissive/dp/1348961

[manofgresley]

I f i read it right, this unit you recommend is fixed on the mill bed and a peice of metal is attached to each axis, the metal inserts between the 2 posts of the switch and makes the contact?
Which is tricky on a mill, also the mill itself is a big chunk of iron so mounting proximity switches so they're not affected isn't so easy. Optical sensors as shown in the link on Eddie's post in suitably sealed boxes are a neater solution. A suitable sensor is this one: http://uk.farnell.com/omron-electronic-components/eesx398/opto-switch-transmissive/dp/1348961

[Clive S]

I f i read it right, this unit you recommend is fixed on the mill bed and a peice of metal is attached to each axis, the metal inserts between the 2 posts of the switch and makes the contact?
Yes its an optical switch that is to say an invisible beam of light it being shone across the gap in the sensor and then when use pass a piece of dark plastic or metal into the gap it breaks the beam and triggers. ..Clive

[manofgresley]

Many thanks for the explanation, this switch seems rather small, the spec sheet says it has a gap of only 3 mm, therefore i would need a piece of metal less than 3 mm thick to go through it, would this not be a bit flimsy for a robust milling machine? and also would this switch be ok powered by my ZP5A Breakout Board, which only gives out 5 volts? and does it wire directly to the BoB?

Ray
Yes its an optical switch that is to say an invisible beam of light it being shone across the gap in the sensor and then when use pass a piece of dark plastic or metal into the gap it breaks the beam and triggers. ..Clive

[EddyCurrent]

These are the ones I used;
http://www.ebay.co.uk/itm/2pcs-SN04-N-5mm-Approach-Sensor-6-36V-DC-Inductive-Proximity-Switch-/400385947125?pt=LH_DefaultDomain_3&hash=item5d38dcb5f5
Albeit on a router, they work well and are very repeatable. Because they are 3 wire NPN you can power them from a 24v supply but still connect the output to your 5v bob input where they either leave it open (high) or short it to ground (low).
The drawing here shows how they are connected; (bottom left corner of drawing)
http://www.mycncuk.com/threads/6565-Ready-Steady-Eddy?p=57622#post57622

[Clive S]

Eddy beat me to it but there are these as well and bit cheaper. I use this sort of thing on my router and will be fitting them to the mill. You do have to watch out for swarfe but if they are mounted under the bed or have a cover over I don't see a problem.

Normally open http://www.ebay.co.uk/itm/TL-Q5MC2-Inductive-Proximity-Sensor-Detection-Switch-NPN-DC6-36V-FST-/121366030435?pt=LH_DefaultDomain_0&hash=item1c41faa863 or

http://www.ebay.co.uk/itm/New-LJ12A3-4-Z-BX-Inductive-Proximity-Sensor-Detection-Changer-NPN-4mm-DC6-36V-/360774116069?pt=LH_DefaultDomain_2&hash=item53ffd022e5

Normally closed:- http://www.ebay.co.uk/itm/DC-6-36V-NPN-NC-2mm-Tubular-Inductive-Proximity-Sensor-Switch-LJ8A3-2-Z-AX-/281174992046?pt=LH_DefaultDomain_3&hash=item417755e4ae and

http://www.ebay.co.uk/itm/4mm-Tubular-Inductive-Proximity-Sensor-Switch-Detector-NPN-NC-DC6-36V-300mA-/290913705975?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item43bbcedff7 Also
this :- http://www.ebay.co.uk/itm/DC-10-30V-NPN-NC-Inductive-Proximity-Sensor-Switch-4mm-Detective-Distance-SN04-N-/290913639483?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item43bbcddc3b ..Clive

[manofgresley]

Many thanks, I like the look of this one,http://www.ebay.co.uk/itm/New-LJ12A3...item53ffd022e5 (http://www.ebay.co.uk/itm/New-LJ12A3-4-Z-BX-Inductive-Proximity-Sensor-Detection-Changer-NPN-4mm-DC6-36V-/360774116069?pt=LH_DefaultDomain_2&hash=item53ffd022e5) i can easily fit these to my machine, my Bob is only 5 volt so do i need a relay? and how do i wire a 3 wire switch, can you do me a schematic, i'm not an electrician.

RayQUOTE=Clive S;60092]Eddy beat me to it but there are these as well and bit cheaper. I use this sort of thing on my router and will be fitting them to the mill. You do have to watch out for swarfe but if they are mounted under the bed or have a cover over I don't see a problem.

Normally open http://www.ebay.co.uk/itm/TL-Q5MC2-Inductive-Proximity-Sensor-Detection-Switch-NPN-DC6-36V-FST-/121366030435?pt=LH_DefaultDomain_0&hash=item1c41faa863 or

http://www.ebay.co.uk/itm/New-LJ12A3-4-Z-BX-Inductive-Proximity-Sensor-Detection-Changer-NPN-4mm-DC6-36V-/360774116069?pt=LH_DefaultDomain_2&hash=item53ffd022e5

Normally closed:- http://www.ebay.co.uk/itm/DC-6-36V-NPN-NC-2mm-Tubular-Inductive-Proximity-Sensor-Switch-LJ8A3-2-Z-AX-/281174992046?pt=LH_DefaultDomain_3&hash=item417755e4ae and

http://www.ebay.co.uk/itm/4mm-Tubular-Inductive-Proximity-Sensor-Switch-Detector-NPN-NC-DC6-36V-300mA-/290913705975?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item43bbcedff7 Also
this :- http://www.ebay.co.uk/itm/DC-10-30V-NPN-NC-Inductive-Proximity-Sensor-Switch-4mm-Detective-Distance-SN04-N-/290913639483?pt=UK_BOI_Electrical_Components_Suppl ies_ET&hash=item43bbcddc3b ..Clive[/QUOTE]

[irving2008]

Wiring of these was discussed at length on many threads. Here is a schematic I did for one such thread showing all possible variants...
You don't need a relay, its overkill and unnecessary if your BOB has opto-isolated inputs.
.

12800

[manofgresley]

Hi Irving, I did say i am not an electrician, how do i know if my Bob is opto isolated i have the paperwork for it but there is no mention of this? Which do i need, NPN or PNP, i'm confused.

Regards

Ray
.

12800[/QUOTE]

[manofgresley]

These are the ones I used;
http://www.ebay.co.uk/itm/2pcs-SN04-N-5mm-Approach-Sensor-6-36V-DC-Inductive-Proximity-Switch-/400385947125?pt=LH_DefaultDomain_3&hash=item5d38dcb5f5
Albeit on a router, they work well and are very repeatable. Because they are 3 wire NPN you can power them from a 24v supply but still connect the output to your 5v bob input where they either leave it open (high) or short it to ground (low).
The drawing here shows how they are connected; (bottom left corner of drawing)
http://www.mycncuk.com/threads/6565-Ready-Steady-Eddy?p=57622#post57622
I only have a 5 volt supply will this work?

Ray

[Clive S]

I only have a 5 volt supply will this work?

Ray You really need a 12V or 24V power supply to work but they can still be connected to your BOB, power supplies are not expensive. ..Clive

[manofgresley]

You really need a 12V or 24V power supply to work but they can still be connected to your BOB, power supplies are not expensive. ..Clive
I do have both power supplies in my CNC cabinet, so can i refresh -
I need 12 to 24 vdc proximity switches, NPN, N.C. all i need know is a scheme / Wiring diagram, can you help?

Ray

[EddyCurrent]

This is the drawing. You will have to make sure the 0v terminals of both power supplies are connected together otherwise it will not work. Where it shows a 24v dc PSU you could use a 12V dc PSU instead if desired.
Colours of the proximity switch wires are for the ones I used but I think they are pretty standard.
Because these are home switches I was not overly concerned about N/O, N/C, so for example mine are 'off' and 'open' until they are activated whereupon they are 'on' and 'closed'. If you get yours wired up and they operate the wrong way you thought they would you can just 'invert' their operation in the software.

12831

[irving2008]

Hi Ray,

You need NPN NC switches. I'm fairly sure, but not 100% that your BOB doesn't have opto-isolated inputs (the manual is useless), but maybe someone can confirm this. Do you have a voltmeter/multi-meter? If so measure the voltage from limit switch input to ground (with power on the BOB). If its more than 2v then its probably not opto-isolated and if it 5v it probably has an internal pull up. In which case wire the switch as:
.
Brown: +12 or 24v power supply
Black: To BOB limit input
Blue: to Ground on BOB limit input AND to Ground on 12/24v supply.
.
Now you need to decide for X and Y if home is at one end of table travel or in the middle (Z home is always the top). Since proximity sensors work on approach of ferrous material, if Home is in the middle of the movement range, Mach has the problem of knowing which side to approach the sensor from since approaching from either side will give the same result electrically but will be offset by potentially 8mm mechanically. I don't know how you tell Mach to home from a given direction, but I'm sure it can be done...
.
edit: Eddy types faster lol. But I want to reiterate the proximity sensor accuracy previously discussed is for material approaching axially (i.e. towards the end of the sensor). For this requirement the approach will have to be radially, i.e. across the face.

[EddyCurrent]

edit: Eddy types faster lol.

Sometimes it's best to type it up into Notepad first then just paste it into a post. :glee:

[cropwell]

I don't know how you tell Mach to home from a given direction, but I'm sure it can be done...
.


In Mach3 there is a dialogue box to set the homing direction of each axis.

[irving2008]

In Mach3 there is a dialogue box to set the homing direction of each axis.
Right, not used it myself. Presumably you need to set up the nominal home location and Mach3 moves far enough to the appropriate side of that to approach from the right direction.

[Clive S]

Just a thought although all of the above is correct are we assuming that the sensor has a resistor built in of say about 10K If not would it be prudent to put a current limiting R in.. My sensors have 10K internal so not neaded.

Also if you home at the middle you have gcode with -ve and +ve code I find it more convenient to have the home switch at one end then all gcode is +ve (for me being a novice it's easier to read) ..Clive

[manofgresley]

Ray

You need NPN NC switches. I'm fairly sure, but not 100% that your BOB doesn't have opto-isolated inputs (the manual is useless), but maybe someone can confirm this. Do you have a voltmeter/multi-meter? If so measure the voltage from limit switch input to ground (with power on the BOB). If its more than 2v then its probably not opto-isolated and if it 5v it probably has an internal pull up. In which case wire the switch as:
.
Brown: +12 or 24v power supply
Black: To BOB limit input
Blue: to Ground on BOB limit input AND to Ground on 12/24v supply.
.
Now you need to decide for X and Y if home is at one end of table travel or in the middle (Z home is always the top). Since proximity sensors work on approach of ferrous material, if Home is in the middle of the movement range, Mach has the problem of knowing which side to approach the sensor from since approaching from either side will give the same result electrically but will be offset by potentially 8mm mechanically. I don't know how you tell Mach to home from a given direction, but I'm sure it can be done...
.
edit: Eddy types faster lol. But I want to reiterate the proximity sensor accuracy previously discussed is for material approaching axially (i.e. towards the end of the sensor). For this requirement the approach will have to be radially, i.e. across the face

[manofgresley]

Hi Irving, i have stated already, my BoB is only 5 volts, are you now saying i can wire a 3 wire sensor direct on to the Bob?

Ray
Ray

You need NPN NC switches. I'm fairly sure, but not 100% that your BOB doesn't have opto-isolated inputs (the manual is useless), but maybe someone can confirm this. Do you have a voltmeter/multi-meter? If so measure the voltage from limit switch input to ground (with power on the BOB). If its more than 2v then its probably not opto-isolated and if it 5v it probably has an internal pull up. In which case wire the switch as:
.
Brown: +12 or 24v power supply
Black: To BOB limit input
Blue: to Ground on BOB limit input AND to Ground on 12/24v supply.
.
Now you need to decide for X and Y if home is at one end of table travel or in the middle (Z home is always the top). Since proximity sensors work on approach of ferrous material, if Home is in the middle of the movement range, Mach has the problem of knowing which side to approach the sensor from since approaching from either side will give the same result electrically but will be offset by potentially 8mm mechanically. I don't know how you tell Mach to home from a given direction, but I'm sure it can be done...
.
edit: Eddy types faster lol. But I want to reiterate the proximity sensor accuracy previously discussed is for material approaching axially (i.e. towards the end of the sensor). For this requirement the approach will have to be radially, i.e. across the face

[manofgresley]

Hi Eddy. I'm confused, why do i need a separate 5 volt psu, when my BoB is already putting out 5 volt. On the board there are terminals for 5 limits but only two wire each, + & -, Am i reading it wrong or what?

Ray
This is the drawing. You will have to make sure the 0v terminals of both power supplies are connected together otherwise it will not work. Where it shows a 24v dc PSU you could use a 12V dc PSU instead if desired.
Colours of the proximity switch wires are for the ones I used but I think they are pretty standard.
Because these are home switches I was not overly concerned about N/O, N/C, so for example mine are 'off' and 'open' until they are activated whereupon they are 'on' and 'closed'. If you get yours wired up and they operate the wrong way you thought they would you can just 'invert' their operation in the software.

12831

[cropwell]

Right, not used it myself. Presumably you need to set up the nominal home location and Mach3 moves far enough to the appropriate side of that to approach from the right direction.

All Mach3 does is have a set direction to move an axis when heading for home (the home sensor). After detecting the sensor Mach3 then backs off a little and zeroes the DRL. This, presumably, is not intended for having a home position in the centre of the table. I could give you some screenshots, but my Mach3 computer is disconnected whilst I rebuild the control box. If you had an input set to 0/1 depending on which half of the axis you were in, then I am sure some clever sod could write a macro to centre the axis.
I think you mentioned previously that proximity switches work on detecting ferrous metal, which is true, but mine also work on ali and brass. The ones I use are 4mm axial detection, but I use them in bypass mode, where the detection position is not necessarily 4mm, but it is consistent. In axial mode you have a great danger of jogging onto the switch and crushing the end, which kills it. (That is why I have the spares, bought as a lot of 8 from china.)

To Ray:- You have to regard a proximity switch as an off/on (ie normally open) push button switch, but it needs 6 to 36v to make it work. The ground connection is common to the switch and the electronics in there. 5v won't cause it to switch, either not at all or not reliably.
PM me with your phone number if you want to talk it through. I have got 5 LJ12A3-4-Z/BX switches and some 12v power supplies, which are intended to be used in my next build.

[Clive S]

Hi Eddy. I'm confused, why do i need a separate 5 volt psu, when my BoB is already putting out 5 volt. On the board there are terminals for 5 limits but only two wire each, + & -, Am i reading it wrong or what?

Ray

I don't know your BOB but you have to power it from somewhere I and others are assuming it is powered from a 5V source if it is powered from 12V then you can use that to drive your sensors.
Can you confirm what the BOB is powered from? ..Clive

[manofgresley]

My BoB is powered by a seperate Transformer which has a duel out put of 5 v0lt and 36 volt
I don't know your BOB but you have to power it from somewhere I and others are assuming it is powered from a 5V source if it is powered from 12V then you can use that to drive your sensors.
Can you confirm what the BOB is powered from? ..Clive

[cropwell]

That BoB, I believe is powered either from the USB lead or a separate 5v input. There will be a switch on the board or a jumper to select which. The BoB doesn't put 5v out AFAIK.

[manofgresley]

You are correct, it is powered by a seperat Transformer, does this mean i can use sensors with higher than 5 volt?

Ray
That BoB, I believe is powered either from the USB lead or a separate 5v input. There will be a switch on the board or a jumper to select which. The BoB doesn't put 5v out AFAIK.

[irving2008]

Yes, as long as they switch to ground and don't have an internal pullup. You can check this by measuring the voltage between the black and blue wires when they are powered up. If it's always zero or there abouts then you can without any other parts. If there is volts then you need a resistor between the black wire and the Bob.

[EddyCurrent]

Ray,
Take a deep breath, chill out, and read the info posted so far.


You really need a 12V or 24V power supply to work but they can still be connected to your BOB, power supplies are not expensive. ..Clive

I do have both power supplies in my CNC cabinet, so can i refresh

Just wire them like the drawing I posted and that's the job done.

Edit: posted after supping 3 bottles of Grassmoor Dark Ale.

[JAZZCNC]

Here you go ray this might help. (Provided Switch can use 36V)
12832

Don't put the switches in middle because if your on wrong side of switch when you click home then mach will never see the switch.
Mach only homes in the direction set in it's parameters IE Home NEG or Home Positive direction. So lets say you always home Negative but your already on the negative side of the switch then Mach will never see the switch and crash the limits. If you position the switches at the ends of travel then Mach will always see the switch.
Now you can still have your ZERO home position in the centre of table just you use MACH's HOME OFF(set) feature and define ZERO a set distance away from the switch. Mach will enter this amount into it's DRO so now your MACHINE coordinates will display a Negative number and when you send to Zero the table will go to the centre of travel which is MACHINE ZERO.
MACHINE ZERO won't be at the ends where table stops but the MACHINE ZERO coordinate will be in the centre of travel and all WORK coordiantes will be Referenced from this position so it works just like has if the switch was in centre.

[irving2008]

Ray,

Eddy, Jazz and myself have all said the same thing, and any supply to the switch between 10 and 36 volts will work.

The only reason I'm being slightly pedantic and suggesting you do some basic voltage checks is to avoid any issues or disappointment. Chinese supplied products can vary in subtle ways especially stuff on ebay. That switch comes in 2 variants. One is truly just a switch to ground and that's what you need. The other version has an internal pull-up resistor to the +v supply and if it's that version then there is a chance you could damage the BOB if you don't have a resistor between switch and input. All I'm saying is do the electrical equivalent of 'measure twice, cut once' :)

[Clive S]

Ray This is why I posted
Just a thought although all of the above is correct are we assuming that the sensor has a resistor built in of say about 10K If not would it be prudent to put a current limiting R in.. My sensors have 10K internal so not needed. It truly is not that difficult just buy the switches and play with one of them not connected to the BOB and see it working. You need to mount them as has been said so that the target slides across the top and not head on so to speak. ..Clive

[EddyCurrent]

Ray,
I got these from your earlier threads;

12837 12838 12839

It looks like your power supply is this one;
http://www.zappautomation.co.uk/electrical-products/power-supplies-43/ps408-5-linear-power-supply-7.html

You are using the 36v to power the steppers and the 5v to power the breakout board, you also have a 24v supply in there for the Pilz ?
Because the 36v is used for the steppers I would suggest not using that for the proximity switches as it could contain a lot of electrical noise, instead I would use the 24v supply provided it is DC and not AC.

Also, off topic, have you cut any of those nuts yet ? what tooling do you intend to use for a 9 gauge string slot ?

[cropwell]

This is maybe a daft question, but do not see a 24v supply for the PILZ unless it is that box to the left of it on the DIN rail.
.
What do these safety relays do that makes them cost £360? I can see a 4 pole contactor, which is presumably 3 phases and aux and I also presume there is some sort of RCCB and E-stop interface. If you are not running 3-phase, then it seems an overkill ('scuse the ironic pun).
.
An RCCB a break switch and NVR switch in series, would seem all that is needed for a good level of safety on single phase 240v.

[Clive S]

This is maybe a daft question, but do not see a 24v supply for the PILZ unless it is that box to the left of it on the DIN rail.
.
What do these safety relays do that makes them cost £360? I can see a 4 pole contactor, which is presumably 3 phases and aux and I also presume there is some sort of RCCB and E-stop interface. If you are not running 3-phase, then it seems an overkill ('scuse the ironic pun).
.
An RCCB a break switch and NVR switch in series, would seem all that is needed for a good level of safety on single phase 240v.
I must be going blind as I don't see a 4 pole contactor the schematic is showing 240V I have picked up those pilz for less than £10 on ebay. ..Clive

[JAZZCNC]

What do these safety relays do that makes them cost £360?

They empty your wallet sharpish. .. Lol

The Narrow white box next to Pilz relay is a 24V Balluf PSU. There is no Contacter in there that I can see.?

[manofgresley]

Yes it is a 24 volt Transformer, i fitted the Pilz on good advice off this site, cost £36.00. Ray
This is maybe a daft question, but do not see a 24v supply for the PILZ unless it is that box to the left of it on the DIN rail.
.
What do these safety relays do that makes them cost £360? I can see a 4 pole contactor, which is presumably 3 phases and aux and I also presume there is some sort of RCCB and E-stop interface. If you are not running 3-phase, then it seems an overkill ('scuse the ironic pun).
.
An RCCB a break switch and NVR switch in series, would seem all that is needed for a good level of safety on single phase 240v.

[manofgresley]

Now sorted, Many thanks, once again to all who have helped me with there advice and wisdom.

Ray

[cropwell]

The contactor I was referring to seems to be built into the safety relay. Then there is also an e-stop input and presumably other safety measures built in. I looked at the PILZ website for some information, but just got boggled with detail. They quote £360 for the PNOZ. I'd want it in gold for that money !:whistle:

[EddyCurrent]

The contactor I was referring to seems to be built into the safety relay. Then there is also an e-stop input and presumably other safety measures built in. I looked at the PILZ website for some information, but just got boggled with detail. They quote £360 for the PNOZ. I'd want it in gold for that money !:whistle:

They are designed for industrial use, nobody like us pays full price, here's one for example £25;

http://www.ebay.co.uk/itm/PILZ-safety-relay-PNOZ1-3S-1O-BNIB-2-/201124740193?pt=UK_BOI_Industrial_Automation_Contr ol_ET&hash=item2ed3f7fc61

[cropwell]

I understand the pricing policy. If you are selling to a large industry customer you have to provide support and have product liability. They have to be at such a price that they can just swap out any problematic unit. Also the employer has to have employee liability and that would probably preclude buying from an 'unreliable ' source.

I wouldn't mind if anyone could tell me the modular functionality of such a unit, and what qualifies it to have the title 'safety relay'.

I'll probably include one in my next build.

[Neale]

Looking at the one I have, the key thing about it seems to be duplication. There are effectively two relays which can be wired in parallel or, ideally, driven via a double-pole e-stop switch. Then the relay n/o contacts are in series so either relay dropping out will open the circuit. It's a bit more complicated than that, of course, depending on which model relay you get, but that's the heart of it and how it can reasonably claim to be a "safety" relay. I guess that build quality is better than a cheap relay from Maplin as well.

[cropwell]

Thanks Neal,

I'll only take issue with one thing, I find 'cheap' and 'Maplin' don't sit easily in the same sentence. :rolleyes:

Rob

[EddyCurrent]

I understand the pricing policy. If you are selling to a large industry customer you have to provide support and have product liability. They have to be at such a price that they can just swap out any problematic unit. Also the employer has to have employee liability and that would probably preclude buying from an 'unreliable ' source.

I wouldn't mind if anyone could tell me the modular functionality of such a unit, and what qualifies it to have the title 'safety relay'.

I'll probably include one in my next build.

It's about meeting certain standards;

"PNOZ safety relays comply with international standards and are approved by BG and international certification bodies. They provide maximum safety up to and including Category 4 in accordance with EN 954-1, Performance Level "e" in accordance with prEN ISO 13849-1 and SIL 3 in accordance with IEC 62061. "

Manufacturers have to demonstrate their product meets these standards and this can be a costly and lengthy process.

These things matter under scenarios like, legislation, reliability, safety, compensation, prosecution, etc. So if these is an accident for example, you may need to demonstrate that you or your company have exercised 'Due Care' by using components that comply with the required safety category.

These devices are also called 'Safety Monitoring Relays' there are basically 4 safety categories, it's up to the owner to work out which category is applicable to their particular application, this information is used;
http://www.pilz.com/en-GB/knowhow/standards/standards/functional_safety/articles/00238

You then select a safety relay that meets the required category and also has the required electrical characteristics for the application.

Further reading would also be this, http://en.wikipedia.org/wiki/Safety_Integrity_Level

[cropwell]

It's about meeting certain standards;

"PNOZ safety relays comply with international standards and are approved by BG and international certification bodies. They provide maximum safety up to and including Category 4 in accordance with EN 954-1, Performance Level "e" in accordance with prEN ISO 13849-1 and SIL 3 in accordance with IEC 62061. "

Manufacturers have to demonstrate their product meets these standards and this can be a costly and lengthy process.

These things matter under scenarios like, legislation, reliability, safety, compensation, prosecution, etc. So if these is an accident for example, you may need to demonstrate that you or your company have exercised 'Due Care' by using components that comply with the required safety category.

These devices are also called 'Safety Monitoring Relays' there are basically 4 safety categories, it's up to the owner to work out which category is applicable to their particular application, this information is used;
http://www.pilz.com/en-GB/knowhow/standards/standards/functional_safety/articles/00238

You then select a safety relay that meets the required category and also has the required electrical characteristics for the application.

Further reading would also be this, http://en.wikipedia.org/wiki/Safety_Integrity_Level
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Thanks for the references Eddy, I had a look at them, but I gave up trying to understand those sort of documents when I retired in 1999.
They may be relevant for a corporate engineer, but I am just a hobby player.
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That's all well and good, but how do they get to be £25 on eBay (if they haven't fallen off the back of a stores shelf :whistle:).
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I'm not an employee and I don't need to have duty of care over some moron who is bound for self destruction by his own stupidity. All I need is that if I have to bang E-stop the damn machine stops PDQ, and if it can't account for all the electricity going in through the live and coming out through the neutral, I also need it to isolate itself until the leak is fixed (within 30 milliseconds, preferably a lot less, if it is more than 30 milliamps going through me). There may be other requirements yet to be determined.
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As a silly aside ;- Pilz is German for Mushroom.
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Thanks anyway for the input, but it is far too heavy for my butterfly brain.

[EddyCurrent]

Maybe the mushroom reference is to do with the red botton on an E/Stop, typically referred to as a mushroom.
The one I bought was from a place that had closed and they were clearing out the stores.
You are correct about these not being applicable to hobby use but when you consider what's inside it can work out cheaper than standard relays, timers, etc.
Also I value my health so I'm prepared to go the extra mile to preserve it.

[Neale]

Mine was cheap because it's an obsolete model and therefore probably no longer certified for new build commercial equipment. Box was damaged as well but relay looks in perfect condition. What I haven't seen cheap on eBay are the double-pole e-stop switches although I'm not too worried as I reckon that a n/c contact is more likely to work when it gets thumped in a hurry than n/o. While safety is important, I'm not building a machine that will be used by a numpty on piecework. But we all draw lines in different places...