hardware and software. The software design becomes, thereby, more and more the major
cost driver. In many industrial design projects, more than 50% of the manpower allocated
for the control system design and installation is scheduled for testing and debugging PLC
programs.
In addition, current PLC based control systems are not properly designed to support the
growing demand for flexibility and reconfigure ability of manufacturing systems.
PLC is well-adapted to a range of automation tasks. These are typically industrial
processes in manufacturing where the cost of developing and maintaining the automation
system is high relative to the total cost of the automation, and where changes to the system
would be expected during its operational life. PLC contains input and output devices
compatible with industrial pilot devices and controls; little electrical design is required, and
the design problem centers on expressing the desired sequence of operations. PLC
applications are typically highly customized systems so the cost of a packaged PLC is low
compared to the cost of a specific custom-built controller design. On the other hand, in the
case of mass-produced goods, customized control systems are economic due to the lower
cost of the components, which can be optimally chosen instead of a "generic" solution, and
where the non-recurring engineering charges are spread over thousands or millions of units.
For high volume or very simple fixed automation tasks, different techniques are used.
For example, a consumer dishwasher would be controlled by an electromechanical cam
timer costing only a few dollars in production quantities.
A microcontroller-based design would be appropriate where hundreds or thousands of
units will be produced and so the development cost (design of power supplies, input/output
hardware and necessary testing and certification) can be spread over many sales, and where
the end-user would not need to alter the control. Automotive applications are an example;
millions of units are built each year, and very few end-users alter the programming of these
controllers. However, some specialty vehicles such as transit busses economically use PLC
instead of custom-designed controls, because the volumes are low and the development cost
would be uneconomic.
Very complex process control, such as used in the chemical industry, may require
algorithms and performance beyond the capability of even high-performance PLC. Very
high-speed or precision controls may also require customized solutions; for example,