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I am creating a simple control loop for a safety door for a machine. When the door is opened, the machine turns off. When its closed, the machine can operate. The way I have it set up now is that we are looking for the door to be open, and if its open the sensor will tell the controller and the controller will flip a switch and the machine will turn off. I feel like the output just does not make sense. I think I am maybe confusing what the set point should be, but I am not sure. This is just a little project I am doing to better understand safety/security control loops, but I think I may be over thinking this one a bit. Any help appreciated.

Here's a link to my current design.

Block diagram

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    \$\begingroup\$ Schematics would help. What sensor are you using to tell if the door is open? How is the controller "flipping" the switch? The more detail you put in to your questions the easier it is for others to help you. \$\endgroup\$ – JCollins Oct 9 '17 at 5:57
  • \$\begingroup\$ Sorry, I forgot to add the picture. Its just a general situation, no specific parts or sensors. \$\endgroup\$ – Cheeseman Oct 9 '17 at 14:03
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    \$\begingroup\$ "Control loop" is normally used for continuously variable systems, whereas your system is just an on/off "dead mans switch". \$\endgroup\$ – pjc50 Oct 9 '17 at 14:39
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    \$\begingroup\$ I don't see why this is a feedback loop. \$\endgroup\$ – Scott Seidman Oct 9 '17 at 14:55
  • \$\begingroup\$ @Cheeseman " Its just a general situation, no specific parts or sensors. ". It's extremely crucial for a circuit diagram when everything is crystal clear for you, but not crystal clear for everybody else. No matter how simple it is. It's unnecessary to be vague. \$\endgroup\$ – Harry Svensson Oct 9 '17 at 15:06
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Safety systems like this generally need to be independent. Making the door sensor just another input to the controller is a bad idea.

There should be independent circuitry and possibly mechanics that ensures the device is safe when the door is open. The door sensor could be connected to a main power relay, for example. In some cases you might also want to release mechanical safety systems, like a brake.

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The door status is the whole control itself, no need to add any logic behind. Just a mechanical switch suffice, unless you want to add extra protective devices, such as delay, debouncers, soft-start, etc...but you didn't state anything about your motor working; as posed, seems like just an 'on-off' control.

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