Directed Sequencer
    In the sequencer examples previously the control system ensured that the sequence of events happened at the correct time and for the correct duration regardless of the consequences. What if there was no water available for the washing machine ? What if there was no traffic on one road and heavy traffic on the other ? Are we really going to insist that the heavy traffic stop at a red light just to make the other green for its allocated duration? Obviously not. The one thing that is missing with pure sequence control is inputs. In the example illustrated, the sequencer for the washing machine could have an "input" from a water sensor that detected the presence of water in the drum. If no water was present it could be made to change or just stop its sequence and perhaps sound an alarm. In this way we would have directed the sequence to another path because of the status of an input.
   With directed sequencers control starts to have the feel of machine intelligence about it. No longer is the machine slavishly obeying commands, but rather taking account of its environment and making decisions about what to do next. The directed sequencer is probably the most common form of automation found both in the domestic and industrial environments. Typical examples include
 

   
 

 

 
     
     
     
     
     
     
     
     
   
  • Traffic lights that use sensors in the road
     

  • Car washes that sense the position of the vehicle before commencing and control the re-circulation of the water bys sensing levels in tanks
     

  • Lifts/Elevators that decide on the most efficient stop sequence based on button push requests on each floor.
     

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