Page 722 - Mechatronics with Experiments

708 MECHATRONICS
3. The discrete input channel state is represented by two vertical lines like a contact.
4. The discrete output channel state is represented as a circular symbol.
5. Timers, counters, and other function blocks are represented by a rectangular shape
with appropriate input source and output lines in the rung.
6. Logic AND is implemented by putting two contacts in series, and logic OR is
implemented by putting them in parallel (Figure 9.8).
One of the standard logics in a ladder logic diagram (LLD) is a seal-in circuit. The
idea is to turn ON an output channel (relay output) and keep it ON based on a momentary
input (START switch). Then keep it ON until another momentary input (STOP Switch) is
pressed. This is shown in Figure 9.8.
The following software concepts are commonly used in PLC programs: shift register,
sequencer, and drums. Shift register and sequencer are both registers. The individual bit
locations in the register are used to indicate the location of a part in a multi station assembly
line or the number of the operation currently performed from a sequence. A bit is associated
with the current product or operation. As the bit is shifted in the register, its location on
the assembly sequence is advanced. Similarly, drum is the software analog of a mechanical
cam. There is a master shaft and multiple cams on it. Each cam actuates (turns ON and
OFF) an output line as a function of the input cam shaft position. This functionality is
duplicated in software with the electronic drum concept. The benefit of the software drum
compared to the mechanical drum is that it is programmable.

9.3.1 Hard-wired Seal-in Circuit

In original hard-wired relay circuits, all of the control logic is physically wired into the elec-
trical connections of input, output, and intermediate control devices. When programmable
logic controllers replaced the hardwired relay logic, the logic between the inputs/outputs is
defined in software (programmed) in the PLC. Very little of the control logic is hard-wired.
Still, some of the safety related functions are hard-wired in case the software fails. Most
emergency shut-down and cycle start functions are hard-wired. One of the most frequently
hard-wired logics is the seal-in circuit used between START and STOP push buttons. The
same diagram is used as the one shown in Figure 9.9. The only difference is that the logic

1 2 5 6
CR

3 4
CR
(a)
Start Stop
1 2 5 6

4 FIGURE 9.9: Hardwired seal-in circuit in
a ladder logic diagram. (a) Ladder logic
3
diagram of connections, (b) component
connections between START button,
(b) STOP button, and the RELAY.