Designing a test safety cell !

[passwordg]

Hello everyone,

I am trying to build a test setup on my bench to train myself but hit a roadblock.

The setup is a compactlogix plc and HMI, SMC solenoid valve pack, small SMC pneumatic cylinder, vacuum generator and assortment of sensors, push buttons.
There is a 120Vac to 24VDC supply and air regulator etc.

I was able to make all of these work together very well. Now I want to integrate a pair of light curtains and EStop buttons.

I have a few safety relays of multiple kinds but I need a good bit of guidance on how to wire this.
Along with knowing how to wire this, I also wanted to know the intended action, say if EStop is pressed, does it cut power to the PLC or just the IO cards, valve pack etc ?
Thanks for any help or direction I can get.

 

[parky]

First of all, not a good idea to cut power to the PLC, apart from many faults on electronics are on power loss/ power up, so not turning off the power reduces the risk, also power up needs to ensure that it does not automatically start anything. Also if e-stop (assume more than one e-stop button, how would you know if you drop power to the PLC & possibly the HMI (also note the panelviews I used a few years ago used to fail quite often as the machine was built in the US, this seems to be the way in the US drop power to everything), this seemed to be a bit of a weird thing as the system was programmed to display alarms on e-stops & which button ?.
Generally, leave power on any thing that will not cause a danger in other words only needs to remove power from PLC outputs, contactors, VFD's possibly etc.
It depends on the safety category, but for a reasonable safety aspect use dual channel safety relay, manual reset & tie in a main power contactor (for all parts you need to remove power from + all contactors N/C in reset loop).
The idea is on an e-stop, the main + motor contactors should drop out, the system is then disabled until a rest is pressed, also if the main contactor or one of the others sticks in the reset loop is broken so cannot be reset. This gives an enhanced protection i.e. in the event of one or more contactors welds in then there is very little possibility of being able to re-start or any drive starting, also feed back the SR relay status to the PLC so that in your logic no outputs are energised and also needs an operator intervention to re-start.
I suggest you check out Pilz website there are plenty of typical circuits for single/dual channel configurations.

 

[plvlce]

Along with knowing how to wire this, I also wanted to know the intended action, say if EStop is pressed, does it cut power to the PLC or just the IO cards, valve pack etc ?

In general, I don't see any good reason for the Estop to cut power to the PLC itself or to inputs. Whether to cut power to outputs depends on what those outputs control and whether being energized or de-energized renders things more safe. An output card used only for status lights likely doesn't need to have supply power cut, but one controlling moving parts may.

Exactly what should happen is generally tailored to the cell -- if you have a heavy load that will drop if power is removed, you need to ensure power won't be removed while there's any chance someone is near it, for example. If your valves use detents, then cutting power may not be sufficient to prevent unsafe motion. Etc, etc.

For wiring I would start by looking up example circuits from a safety relay manufacturer.

 

[passwordg]

I did not think that part through. Its pretty obvious now you would not cut PLC power.

However, would you cut power to the PLC Output card itself, or cut the power to a motor or pneumatic actuator like the SMC valve ?

 

[willxfmr]

Wiring the relays is pretty straight forward, just check the manual and it should give you a diagram of what goes where. "Intended action" is where things get interesting. As stated above dropping power to the PLC is almost never a good idea with an estop, so I'd skip that. Very often power to the output card is dropped, sometimes the inputs as well, but what if you need an output to energise in an emergency? There are some applications where dropping power to the outputs will in fact cause much bigger problems, and that needs to be avoided. This is where test rigs really come in handy. Invent a scenario where you need all outputs disabled. Then trow a twist into the mix and only disable one output card, or just half of one if one is all you have. Keep shuffling the deck, and see what it takes to meet a slightly different requirement. I love to tinker with our shop PLC just to see what I can learn today. Hopefully you'll enjoy it too.