1 Discrete Event Control Concept Representation DEC controller design DEC controller implementation

2 Hard wired systems  inflexible  software systems  computer-based system Electric Ladder Logic Systems or Diagrams Soft Ladder Logic Systems or Diagrams High level logic systems and programming codes

3 Electrical Ladder Logic Diagrams Using switch to control a light L: L=1 (light on) L=0 (light off) S:S=1 (switch on) S=0 (switch off) Electric Ladder Logical Diagram is as follow (Fig.1): Figure 1

4 Electrical Ladder Logic Diagrams (ELLD) The structure of ELLD is: - left rail with power -> power rail - right rail with neutral -> neutral rail - one path called “rung” It is clear with the notation on Fig.1, when L=1 if and only if S=1 and when L=0 if and only if S=0 So this ELLD is a physical representation of the Boolean logic equation L=S

5 Electrical Ladder Logic Diagrams Fig.2 shows a multiple rungs (two rungs). The problem: - Two lights, L1 and L2. - Three switches, S1, S2, S3 Fig.2 physically represents: L1=S1+S2 (Rung 1) L2=S2S3 (Rung 2)  When S1 or S2 is on, L1 is on  When both S2 and S3 are on, L2 is on.

6 Electrical Ladder Logic Diagrams Control Relays: Instead of using one switch to control one light or object, a generic component of hardwired control implementation is a control relay, see Fig.3. Features of a control relay: 1.Coil, normally closed (n.c.) contacts, and normally open contacts (n.o.). 2.If there is a current in the coil, the coil is energized, then n.c. will open and n.o. will close.

7 Electrical Ladder Logic Diagrams CR is represented by a circle, as an object. n.o. contactn.c. contact Fig. 3

8 Electrical Ladder Logic Diagrams Figure 4 shows an example. - when S is open, L1 is off, L2 is on - when S is closed, L1 is on, L2 is off. Figure 4

9 Electrical Ladder Logic Diagrams The Boolean logic equations for the rungs in the ladder diagram are: Rung 1:CR=S Rung 2:L1=CR Rung 3:L2=CR Note here that nc is denoted by CR Combining the equations for the rungs yields L1=S L2=S

10 Electrical Ladder Logic Diagrams Example 1: one motor with two pushbuttons: start and stop State variables: PB1(for start), PB2(for stop), M (for motor) Figure 5 (control relays are used) Figure 5

11 Electrical Ladder Logic Diagrams From Figure 5, we know: PB1 is on -> CR1 energized, no1 is closed -> M=1 PB2 is on -> CR2 energized, nc2 is open -> M=0 Rung 1: CR1=(PB1+CR1) CR2 Rung 2: CR2=(PB2) Rung 3: M=CR1 CR2

12 Electrical Ladder Logic Diagrams Finally, M=(PB1+CR1)(PB2)(PB2)=(PB1+CR1)PB2 Contact CR1 may be omitted here if the switch here is “permanent”. When the switch is momentary (i.e., the switch will momentarily close and then be back to unclose state), we need CR1. This is because otherwise, there will be no power on rung 1 shortly after PB1 is pressed. The role of CR1 is to keep the power through on the rung even though PB1 is shortly back to unclose state. Such a role of CR1 is called “latching”.

13 Software Ladder Logic Diagrams General idea: Electric Ladder Diagram Switch Button Coil Relay Two states: on, off Examine for on Examine for off

14 Software Ladder Logic Diagrams Examine for on Examine for off An input device Viewpoint

15 Software Ladder Logic Diagrams Examine for on On is true Examine for off Off is false Equivalence