Ready Steady Eddy

[Jonathan]

Please forgive my ignorance but could someone explain how this will work in basic terms? Why is Eddy's first circuit not fail safe and how does the next circuit achieve that?

The transistor is used as a switch. When positive current is applied to the base of an NPN it switches on. In the original circuit, the transistor is simply connected in series with the relay, so when the current is applied the transistor switches on and so does the relay. The problem is, if the signal to the base is broken (e.g. wire accidently cut), the transistor and thus the relay will switch on. We want the relay to be off in this situation, so another transistor is used to invert the signal. When the current is applied to 'new' transistor, it switches on so connects the base of the second transistor to ground, which in turn switches it off - hence the signal is inverted.

[cncJim]

The transistor is used as a switch. When positive current is applied to the base of an NPN it switches on. In the original circuit, the transistor is simply connected in series with the relay, so when the current is applied the transistor switches on and so does the relay. The problem is, if the signal to the base is broken (e.g. wire accidently cut), the transistor and thus the relay will switch on. We want the relay to be off in this situation, so another transistor is used to invert the signal. When the current is applied to 'new' transistor, it switches on so connects the base of the second transistor to ground, which in turn switches it off - hence the signal is inverted.

Thanks for taking the time to reply Jonathan. Very helpful. I was also having hard time understanding/visualising the "active low" part of the puzzle.

This also helped me understand:-

"If the input signal is high there will flow current through R2 and the transistor's base-emitter junction (base, not gate). This current will be amplified, and the collector current through R1 will cause a voltage drop so that the output will be low. Input high, output low.
If the input signal is low there won't be any base current, and no collector current. No current through R1 means no voltage drop, so that the output will be at +V. Input low, output high."

[irving2008]

Just to clarify Jonathan's answer, the BOB ENable signal is Active Low i.e. enabled = 0v, disabled = 5v. In Eddy's original circuit the relay would be OFF when the BOB says Enabled and ON when disabled. But a BOB failure, a supply voltage failure, or burn out of the transistor would all turn the relay OFF, an enabled state, so not fail safe. As J says, the addition of the second transistor inverts the logic so relay is ON only when system is enabled and voltages are present.

J's later solution achieves the same result by using a PNP transistor to invert the logic.

[edit] typed this ages ago but forgot to hit submit.... :roll: and now its out of date lol

The transistor is used as a switch. When positive current is applied to the base of an NPN it switches on. In the original circuit, the transistor is simply connected in series with the relay, so when the current is applied the transistor switches on and so does the relay. The problem is, if the signal to the base is broken (e.g. wire accidently cut), the transistor and thus the relay will switch on. We want the relay to be off in this situation, so another transistor is used to invert the signal. When the current is applied to 'new' transistor, it switches on so connects the base of the second transistor to ground, which in turn switches it off - hence the signal is inverted.

[EddyCurrent]

BD679 is fine as output transistor if a bit of an overkill.

Thank's for info on component sizes.
That is correct, it's just that I have some and they have a high gain because I need to keep the load on ENA as low as possible as it's also supplying 4 x stepper drivers.

[EddyCurrent]

The breakout board I'm using is this one SYSTEM1
I emailed Roy regarding the charge pump relay and he obviously suggested a transistor buffer but more interestingly the board has 'spare' 12v and 5v outputs and his suggestion was to connect a small 5v reed type relay between +5v and ENA. This has the advantage that when Mach3 is healthy ENA is at 0v therefore a potential of 5v exists between +5v and ENA making it fail safe plus it would share the load better with the 4 connected AM882's. This might be the way to go other than the reed relay does not sound too appealing although in practice one of those encapsulated type that look like a chip with about 8 legs have proved to be reliable in industrial situations.

[m_c]

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!

[cncJim]

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.

[m_c]

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.

[EddyCurrent]

@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.

http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/b98cfaed98af7397c12573ad00471f6a/$File/2CDC110004C0205_03.pdf

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

[EddyCurrent]

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.