Page 713 - Mechatronics with Experiments

PROGRAMMABLE LOGIC CONTROLLERS 699
of the timer may be program control, free running, or externally triggered by an input.
Counter modules are used to count pulses. For instance, ON/OFF state transitions
from a proximity sensor which counts the number of teeth on a gear can be used
by the counter module to count the number of teeth on a gear for quality control
purposes. The pulses from an optical encoder can be input to the counter module to
measure displacement.
6. High speed counter modules are used for counting high frequency pulses and detect
very short periods of trigger signals (i.e., a high resolution encoder signal input).
For instance, this module can be used to measure position in high speed and high
resolution encoders.
7. Programmable cam switch module is used to emulate the function of a mechan-
ical cam switch set. A mechanical cam switch set turns ON/OFF a number of
outputs as a function of a master cam shaft position. In the mechanical system,
the state of outputs is determined by the shape of each cam switch shape that is
machined into the cam. In a programmable cam switch module, these functions are
programmable.
8. Thermocouple sensor interface modules (any many other special sensor interface
modules).
9. PID controller modules (i.e., closed loop temperature controller, closed loop pressure
regulator, closed loop liquid level regulator).
10. Motion control modules (servo motor, stepper motor, electrohydraulic valve control
modules) used for the closed servo motion control. The actuators may be electric
motor and drive or hydraulic valve and amplifiers. The motion control module sends
out either the desired number of position pulses as the position command to the drive
or sends a voltage command proportional to the desired speed or torque depending
upon the mode of the amplifier.
11. Most PLCs support a standard module called a ASCII/BASIC module. This module
provides a RS-232 serial interface as well as a separate processor that supports BASIC
programming language. The BASIC program is stored in the module on a battery
backed RAM. A PLC with an ASCII/BASIC module basically is a dual processor
controller. The ladder logic running in the main CPU and the BASIC program running
in the ASCII/BASIC module communicate with each other over a predefined memory
for data exchange. Complicated mathematical calculations, that may be difficult to
code in ladder logic, can be implemented in the ASCII/BASIC module.
12. Master and slave scanner modules when the PLC is used as an I/O interface station
and the control logic is implemented by a master controller (another PLC or PC).
13. Network communication modules (DeviceNet, CAN, ProfiBus, Ethernet, RS-232-
Cm etc.). PLCs are increasingly part of a larger networked control system. Many
so-called fieldbus communication protocols are available.
14. Other special function modules such as fuzzy logic modules. New special function
modules are being added to PLCs.

Network communication protocols suitable for real-time control systems, such as
DeviceNet, have been changing the hardware configuration of PLC controlled systems in
recent years (Figure 9.3). More and more I/O devices (individual sensors, motor starters,
closed loop controllers) are being made available with a network interface (i.e., a proximity
sensor or a motor starter with DeviceNet interface). Therefore, the I/O devices do not need
to be wired into the modules on the PLC’s I/O rack. Instead, each I/O device connects to a
common communication bus using a T-type connector. This reduces the amount of wiring