Page 267 - Electric Drives and Electromechanical Systems

264 Electric Drives and Electromechanical Systems

speed demand to the motor-drive or for the control of the heating element. As discussed
in Chapter 1, Electromechanical Systems, and subsequently in Chapter 11, Cyber
Physical Systems, the importance of inter-machine and factory-wide communication
has increased in recent years, providing ample opportunities for interconnecting PLC
applications. The need to pass information between PLCs and other devices within a
manufacturing plant has ensured that all but the simplest PLCs are provided with a
communications port. The main uses for PLC communications are:

Allowing the remote display of operational data and alarms using either printers or
visual display units.
Data logging for the archiving or quality control.
Passing program changes, either process parameters or the resident program.
Linking the PLC into a computer hierarchy, which may contain many other PLCs
and computers, to form a computer-integrated-manufacturing (CIM) facility. The
programming of a communications channel is effectively no different from the
programming of the logic sequences discussed in the next sections.
The programming unit of a PLC can be provided as a separate removable unit, whose
functions are dependent on the size and complexity of the PLC. For small PLCs a simple
keyboard will suffice, while larger systems can use a separate personal computer to
provide simulation and other supporting software.
In early versions of PLCs, the execution of the program cycle was initiated at the peak
of the PLC’s supply waveform. Therefore, in practice, a PLC effectively remains idle for a
considerable portion of time. In theory this may lead to errors in the logic if the output of
a logic step is used as an input to a logical relationship earlier in the ladder sequence.
With careful programming this effect can be minimised; and, in any case, the logic will
be re-executed within 20 ms on a 50 Hz system. This will not cause any problems in most
applications. However, if the logic becomes highly interdependent, the possibility of
catastrophic failure will increase; hence such applications will need to be considered
with care, and in extreme cases the PLC should be replaced with other control systems,
and the programming should be undertaken with considerable rigour. In many current
PLCs, a different approach is taken where the reading of the inputs and writing to the
outputs is undertaken as a separate activity prior to processing the PLC programme. One
point to note is than many modern PLCs can undertake wide range of digital and
analogue process, hence the length of the process cycle could become excessive, in many
cases limits are placed on the process cycle length.

10.4.1 Combinational logic programming
There are two basic approaches to programming a PLC, a combinational-logic approach
or a sequential approach. The former can be demonstrated by considering the ladder
rung shown in Fig. 10.15, where the output Y1 will become active only if the input X1 is
on, and the input X2 is off; the symbols used are identical to those used in conventional