I have to decide in what circuit to build in the Emergency Stop button.
There seems to be a choice between two positions: A) direct in the mains power supply to the CNC control box which kills everything, or B) as an input to the "breakout" board that would then give an E stop signal to the CNC software.
Any advice on the best solution to adopt is welcomed?
Alan
button.
My preference would be to have the emergency stop button drop out a contactor that carries the power to the spindle motor and the servo (or stepper) motors. It should also provide the E stop signal to the controller. If you have a spindle brake it should bring that on as well.
AWEM
Alan,
It is recognised in the industrial world to not put the EM Signal through any PLC / PC controller, the only connection here should be just to warn the system not to control it. The Em signal should as Andrew has stated be hard wired to bring everything to a halt.
Mart>> I have to decide in what circuit to build in the Emergency Stop >> button.
Alan,
What I forgot to mention is that you wire it so that the processor is kept alive so as not to loose any info i.e positions etc but you kill all power to drives etc.
Mart> Alan,
Depends whether you have to comply with any specific safety rules.
Generally, we put emergency stops on our charging equipment to stop output power and to disconnect from the battery. Our PLC controlled systems have software that monitors the ES switch and stops the programme if it goes true (switch enabled)
On machinery you have to bring it all to a halt in as short a time as ptractical, which usually means power off and brakes on as a minimum. I think a lot of hydraulically power kit also cuts motor/pump power as well.
Our PLC software is not affected by a shut down and we have no positional info to lose, but the PLC will force a complete restart after ES is pushed.
There is EU directive guidance on this, look on the net.
Peter
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If you're just building it for your own use it's up to you, but the law (the machinery directive) forbids the emergency stop from being dependent on any form of processor or even electronics for any machine that is sold. It has to act directly on the power to the moving, and hence dangerous, part of the machine. Of course it's quite ok for software to monitor the emergency stop and also take appropriate action, but the source of danger must be stopped regardless of the software. In this case the button needs to kill the power to the spindle and servo drivers via nothing more than relays.
Greg
As others have said, an emergency stop should kill all power to the spindle and stepper drives. The contacts on most emergency stop switches are rated at 240V at 10 or 15A, so it should be possible to just route all your power through the e-stop switch.
Although you can also monitor this switch with your software, it is almost inevitable that if you hit this switch you will loose positioning (asuming you're using the usual type of stepper drive). It is probably worth considering also putting a "cycle stop" button on, wired to the e stop signal on the controller. This would stop the machine movement, and the spindle, but would maintain positional accuracy as it had been brought to a halt under control -and alow you to start up again. Just use the hard-wired emergency stop button for genuine emergencies.
Regards
Kevin
Many thanks to all for the excellent advice and observations.
I am going for the ES to kill power to the electrics directly and give a signal to the E-stop input on the software by using a double pole E-stop switch. I will have to investigate further the possible loss of positional data.
Alan
In an emergency it's the last thing you're concerned about 8-). We have a CNC router at work and one operator used to have the habit of pressing the ES button instead of the stop button on the control pendant, pure laziness and it wasted time and materials due to losing it's position, just use the right button for the situation and you don't need to worry.
Greg
In the controls I design I have the power cut by the emergancy stop directly. However as I normaly have inverters, and some other components that store energy in the system I have a secondary contact on the emergency stop which sends a signal to the PLC so it "pulls the plug" on all motors. This gives the quickest possible response to the stop button.
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