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Some systems like manufacture system, telecommunication network & computer, and real-time system nowadays are more complex. All of those system are dynamic and discrete. With the increased system usage, then needs a way to model the system. 

Petri net is a method for modelling a dynamic and discrete system. Petri net could be simulated using computer so it easily to analyse attitude of the system. Petri net provide a form that could be used in process of system planning. Petri net illustrates the system using graphical way so it will easier to understand.

As graphical device, petri net could be used as visual-communication assistance like flowchart, block diagram, state diagram, and networking. 

COMPONENTS

Token is showing an active place.

Transition is showing an event that can happen in the system.

Directed Arc is showing a relationship between "Place" and "Transition".

Place is showing the state of system.

EXAMPLE

START, A+, B+, B-, A-

The merged petri net,

Many of systems from input signal are totally random. An example, Elevator control system. In every floor, there is up and down button, also in the elevator, one button for every floor and open/close button. Other than that, there is limit switch on every floor and when elevator pass a floor, it will active, so the control system know the position of elevator. This method is more simple than huffman method, and it more makes sense between length of programs and the needed time.   

Example 1.

Condition,

The flow diagram,

Huffman method is classical method for planning a sequential system. This method is more complicated but the result will have a minimal relay. 

There are two kinds of state, Stable and Unstable. Stable state is where the output state is equal to input state. While unstable state is transition where the output is not same as input.

Procedures

Example

1. Build primitive flow table
   First, create a table with number of columns is 2^n, where n is number of input. Also add other column at the left for all state and at right for the output.
   
   
   Then fill each stable and unstable state also the don't care state.
   
   
2. Merged flow table row
   Need to merge because it will decrease the number of components.
   
   
3. Create state
   To avoid race, there is an important rule to ensure only one state is changed each moving state.
   
   The state flow are,
   
4. Decrease excitation
   Y1
   
   Y2
   
   And the Set Reset function are,
   
   And the output function for A+ is
   
   
5. with that output function, it will looping forever, so add a "START" to S1 and modify the output function to,
   
   so the final output function is, 
   
6. Draw the Ladder Diagram
   

   

Reference:

[1] PPT System Automation (M. Rameli & Eka Iskandar)

Cascade method is a ladder diagram design method where various steps of the sequence are divided into groups. The rule of this method is "a new group must be started the moment it becomes necessary to shut off any output signal actuated during the presently active group". 

Example 1, Suppose four cylinders, A, B, C, D,

START / A+ B+ C+ / C- A- D+ / A+ D- B- / A-

                        G1             G2              G3       G4

Each group allocated one control relay and connected as RS flip-flop. The switching functions are,

Example 2, 

START / A+ B+ / B- A-

reference:

[1] Industrial Automation: Circuit Design and Components by David W. Pessen

[2] PPT System Automation (M.Rameli & Eka Iskandar)