The following figure represents an industrial operation of thermal processing of metals, the machine hardens the metal in the shape of a steel ring. the hardening process is done by heating the steel ring to a very high temperature, then it goes through a sudden cooling. So the piece we want to harden is heated by passing very high currents through a coil that heats the piece, then we cool it very quickly by sending cold water through the holes in each side.
This process is summarized as following:
1. When the main push button is pressed, the system is activated.
2. the piece is put in its place (on spindle)
3. pressing two push buttons (the left and the right) to start the process.
4. the system can be deactivated (off) completely and quickly by OFF push button.
5. the piece goes from down to above through an air-cylinder that works by a Solenoid valve. it comes back after the valve has been disconnected by the Spring or by the falling weight (gravity). this will not happen, of course, unless the piece exists, which is known using the sensor LSPP which detects the piece (i.e. working piece).
6. the spindle activates the switch LSU when it reached it.
7. then the heat will start for only 10 seconds.
8. after that, cooling starts and lasts only 10 seconds.
9. the piece returns to the bottom by the gravity, after the Solinoid valve has been disconnected. then the LSU returns to the normal state – normally open (NO) – after the piece has gone down.
10. when the piece goes to the buttom, the switch LSPP will be ON again.
11. the system after that will be in a RESET state.
12. then we could put a new piece and repeat the whole process.
And here is the ladder logic diagram: