PLC elevator control
On modern elevators
the control logic is PLC based, probably because relays are more expensive than electronic components.
Besides a PLC makes simple the electrical connections and reduces the chance of failure.
On old elevators
of the car is controlled by selectors mounted in the shaft, one at each floor. On modern elevators there may be one impulse
switch only, mounted on the top of the car. When the elevator moves the impulse switch detects a sequence of magnets placed
in the shaft, then sends the signals to the PLC.
The pdf link below is a ladder logic of a 4 floors, double speed and automatic door elevator. The program has been simulated
but not tested on a real elevator model. The direction of the car is controlled by one selector impulse switch (S0). On real
elevators, for a better precision, there are more selector switches. The dump switches at the first and top floor (S1-S2) are
supposed to reset the program once the PLC is supplied. In that condition none of the selector merkers are activated.
In other words the PLC does not know where the elevator is. Therefore the direction instructions (rung 17 and 18) would be both
on. For that reason I used the delay instruction T40. The image is a simplified schematic of the direction instructions. Note
that S7 PLC software does not allow two instructions within one rung.
Ladder logic (pdf)