Page 61 - The Mechatronics Handbook

FIGURE 4.5 Example of PLC ladder diagram: 000.xx/
010.xx—address group of inputs/outputs, TIM000—timer
delays 5 s. 000.00—normally open input contact, 000.02—
normally closed input contact.

diagram) of the program using internal (state) bit variables if any, and to update the output status.
The scan time is dependent on the complexity of the program (milliseconds or tens of msec). The next
scan operation either follows the previous one immediately (free running) or starts periodically.
Programming languages for PLCs are described in IEC-1131-3 nomenclature:
LD—ladder diagram (see Fig. 4.5)
IL—instruction list (an assembler)
SFC—sequential function chart (usually called by the proprietary name GRAFCET)
ST—structured text (similar to a high level language)
FBD—function block diagram

PLCs are programmed using cross-compiling and debugging tools running on a PC or with programming
terminals (usually using IL), both connected with a serial link. Remote operator panels can serve as a
human-to-machine interface. A new alternate concept (called SoftPLC) consists of PLC-like I/O modules
controlled by an industrial PC, built in a touch screen operator panel.

4.6 Digital Communications

Intercommunication among mechatronics subsystems plays a key role in their engagement of applica-
tions, both of fixed and flexible configuration (a car, a hi-fi system, a fixed manufacturing line versus a
flexible plant, a wireless pico-net of computer peripheral devices). It is clear that digital communication
depends on the designers demands for the amount of transferred data, the distance between the systems,
and the requirements on the degree of data reliability and security.
The signal is represented by alterations of amplitude, frequency, or phase. This is accomplished by
changes in voltage/current in metallic wires or by electromagnetic waves, both in radiotransmission and
infrared optical transmission (either “wireless” for short distances or optical fibers over fairly long
distances). Data rate or bandwidth varies from 300 b/s (teleprinter), 3.4 kHz (phone), 144 kb/s (ISDN)
to tens of Mb/s (ADSL) on a metallic wire (subscriber line), up to 100 Mb/s on a twisted pair (LAN),
about 30–100 MHz on a microwave channel, 1 GHz on a coaxial cable (trunk cable network, cable TV),
and up to tens of Gb/s on an optical cable (backbone network).
Data transmission employs complex methods of digital modulation, data compression, and data
protection against loss due to noise interference, signal distortion, and dropouts. Multilayer standard
protocols (ISO/OSI 7-layer reference model or Internet 4-layer group of protocols including well-known
TCP/IP), “partly hardware, partly software realized,” facilitate an understanding between communication
systems. They not only establish connection on a utilizable speed, check data transfer, format and
compress data, but can make communication transparent for an application. For example, no difference
can be seen between local and remote data sources. An example of a multilayer communication concept
is depicted in Fig. 4.6.

©2002 CRC Press LLC