Thread: Ready Steady Eddy
Alternatively make Vs = 5v
Rb for Vs = 5v is 1k
Rb for Vs = 24v is 4k7
1/4w resistors will be fine
Anything that switches arcs.
Technically a contactor is just another name for a relay, however the term contactor has been adopted to describe a specific type of relay.
Contactors main benefit is they can handle far higher currents, and normally have a pretty large airgap to ensure a well isolated disconnection. There's no real reason to use a contactor unless the application dictates it, especially if you can get away with a far cheaper relay.
I'm pretty sure you can get contactor set-ups that will detect a welded contact, which essentially gives you the equivalent of a positive guided relay. All you need to do with a contactor is monitor the contact plate travel, so it's just a case of actuating a suitable switch to detect that it's not returned fully. However if you've just welded a contactor shut, then there's a major design flaw somewhere!
Thanks for the info m_c!
So am I missing something?? (more than likely!) - We have a fail safe safety relay which will be switching non-fail safe relays? (I'm thinking its possible for a contact to weld?)
Isn't this kinda defeating the purpose?
Apologies if I have miss-read your circuit diagram EddyCurrent.
The key to your answer lies in the fact you have a single relay controlling several others. If one of the several fails, chances are it's not going to cause a major issue as everything else will still stop, but if the single one fails it's more likely to cause major issues.
However a correctly designed system, the relays should be suitably sized that contact welding is not an issue, and suitably fused. Safety relays aren't entirely fail safe, they've just got extra contacts so you can check to see if they're working correctly.
It's all a case of risk management and cost.
Ideally safety systems should be tested reguarly, to ensure they are functioning as designed, and any problems found before they cause problems in the case of an emergency.
However, if you have something that needs to run 24/7, or where regular testing could be a problem, then you want to minimise the amount of testing that needs done, and look at using more failsafe methods.
In the context of a homebrew machine running Mach/Linux, you want a couple things to happen during an e-stop. First you want to cut power, and second you want to the tell the computer things have stopped. Should the power fail to cut, then the computer should still know things have gone wrong and stop generating pulses. That way should the main power not die, the computer should still stop commanding any movement. Plus you always have the completely failsafe method of unplugging it.
Industrial machinery does get a bit more involved, but there are no hard and fast rules on implementation, just that things should stop in a controlled and quick manner. There are stipulations on time taken to stop things like exposed cutters, but there is no requirement for power to be cut, just for movement to stop. However generally anything that can be stopped faster under power, will use a time delay system, which when activated, will issue a stop command, then after a short delay, cut power.
One thing to remember, nothing electrical is ever truly failsafe, and is why you should still be able to manually kill all power should things go totally wrong.
@cncJim more reading regarding the 4 safety categories, once the risk assessment identifies the required safety category only then can the correct hardware be selected.
It's correct when m_c says the components are selected to minimise faults such as contact welding, also if a failure does occur then a restart should not be possible. For example if a contactor was used to remove power form the VFD then it should have a normally closed auxilliary contact in series with the safety relay reset button to ensure the contactor is de-energised or not stuck closed by contact welding and I could do that with my K1 for the stepper driver power.
As I said in a earlier post the drawings I have put on here are for my machine and anyone else needs to carry out their own risk assessment for their machine. At the end of the day as JAZZCNC pointed out none of this is strictly required for a DIY machine for use at home because the legislation does not apply to domestic situations but my theory is that for the little extra involved we might as well go the extra mile and make it as good as it needs to be. Because the VFD I'm using has the STO feature means I have decided not to use a contactor to cut power once the safety relay times out, if it did not have this feature then I would have used a contactor.
So the correct procedure is to risk assess the machine to identify the required safety category then design the safety system to meet that category both by choice of appropriate components and how those components are connected together.
I would be very surprised if the type of breakout boards we are using would meet the requirements of a safety system but this is the only means of telling the computer that things have stopped (as far as I know) but if other things come into play first such as cutting power to moving parts or making moving parts safe then it might not matter what the computer is trying to do as it's commands are falling on deaf ears so to speak.
Plus I realise all this procrastination on my part is just putting off the mechanical build part of my project
Last edited by EddyCurrent; 20-11-2013 at 09:55 PM.
The Following User Says Thank You to EddyCurrent For This Useful Post:
Drawing updated, (added a link missing on E/Stop relay, added K1 feedback contact and E/Stop relay N/C contacts into RESET circuit). This will be the last update of drawings until the project is finished and I shall post the 'as built' version.
Instead on implementing a load more buttons on the panel itself I'm thinking about getting one of these,
Cheap High quality 2.4G CNC 3 axis/ 4 axis Mach3 wireless Handle Wheel, Mach3 engraving machine, LCD, cnc wireless channel-in Woodworking Machinery Parts from Industry & Business on Aliexpress.com
I was going to get the cheaper version, wired and without a jog wheel but it didn't have a pause function (though it might have programmable ?) but for the little extra cash this seems to have it all and it's wireless.
USB should not be an issue because I'm using an Ethernet Smooth Stepper.
Last edited by EddyCurrent; 24-11-2013 at 11:02 AM.
Thanks silyavski they look good products, the truth is because I knew nothing regarding cnc at the start I did not research enough products before deciding what to buy. If I was starting again it would probably be a bit different.
I have however reached a point where I am happy with how it is going.
One problem with not creating a proper specification is that you don't know when the job is finished, however I'm sure I've dicked about with this panel far too long so I'm saying this is the 'as built' version.
I started a thread about using VBscript but was directed by Jazzcnc to look at Brains so I did and was pleased with the results.
Mach3 Brains have been used for the control panel pushbuttons and the control panel red led for alarm indication.
I used 'output 1' from Mach3 which is configured to 'pin 14' of the breakout board and constructed a relay using one 2k2 resistor in series with the base of a BD679 (overkill but I already had some). The Mach3 Brain logic then controls output to this solid state relay which in turn controls the red led.
I also revisited the Emergency stop circuit and the Stop / Pause ' Reset functions, they now operate as described below.
Attached are the schematics and in the Zip file are the Mach3 Brains for pushbuttons and alarm indication also a spreadsheet showing the logic used.
Emergency Stop circuit
consists of 2 emergency stop buttons, one machine mounted, one control panel mounted, machine limit switches, VFD fault relay.
Upon activation of the E/stop;
1. The VFD uses special emergency stop ramp (faster than normal stop ramp) and after 6 seconds it's STO function is activated via the PILZ relay and K5
2. K1 is de-energised removing power to the AM882 drivers
3. E/Stop signal sent to Mach3, charge pump stops and breakout board uses 'enable' output to disable all outputs to AM882's and spindle.
Stops operation of machine by de-energising K3, sending Mach3 a 'stop' signal, also stops the VFD by removing the RUN signal.
Can be activated by;
1. using control panel red push button
2. any AM882 going into alarm
Pauses operation by de-energising K4, sending a 'FeedHold' signal to Mach3
can be activated by;
1. using control panel blue push button
2. spindle over temperature switch.
3. Any AM882 going into alarm will de-energise K4 because it's going to stop the machine in any case. The Mach3 Brain uses K3,K4 logic for alarm indication.
If the spindle is required to keep turning then a link can be connected as shown in the schematic diagram.
Resets the PILZ emergency stop relay also K3 and K4 via green push button.
1. If the machine has just been switched on the green pushbutton will reset the PILZ relay and K3, K4, sends Mach3 a 'reset' signal
2. If the machine has been 'paused' the green push button will reset K4 and send Mach3 a 'Cycle Start' signal.
3. If the machine has been 'stopped' the green push button will reset K3 and wait for user intervention to restart via Mach3 screen.
green led (power on)
indicates the PILZ emergency stop relay is energised
red led (alarm/fault)
provides indication by means of flashing at different rates for;
E/Stop was pressed
'Stop' initiated from Mach3
'FeedHold' initiated from Mach3
'Stop' initiated from Control Panel
'FeedHold' initiated from Control Panel
AM882 Driver in fault
Last edited by EddyCurrent; 13-12-2013 at 12:31 PM.
It's looking good Eddy and Massive OTT for DIY thou I'm struggling to find a Limit override.? . . . You'll need this to back off the Limits when tripped.!
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