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Lecture7 Industrial Programmable Logic Controllers Plcs

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  Industrial Programmable Logic Controllers (PLCs) Eng. R. L. Nkumbwa Copperbelt University School of Technology 2010  Eng. R. L. Nkumbwa @ CBU 2010 2 Programmable Logic Controllers (PLC’s)     A programmable logic controller (PLC) is a special purpose computer aimed at implementing control solutions.  Historically PLC‟s have been used mainly for on -off or logic type applications.  However, modern PLC‟s have become increasingly sophisticated and can now cover quite complex control tasks.  Eng. R. L. Nkumbwa @ CBU 2010 3 Programmable Logic Controllers  These notes emphasize the switching capability of a PLC. However, the reader is urged to think of the dynamics of the underlying system.  For example, we recently heard of a seemingly straightforward application of a PLC to a reservoir level control problem.  The algorithm had many features but in essence conformed to the rule that if the level was too low then the pumping rate should be increased.  Eng. R. L. Nkumbwa @ CBU 2010 4 Programmable Logic Controllers  This can be seen as an approximate form of integral control.  However, pure integral control when applied to a tank (which is also an integrator) will lead to self sustained oscillations.  The reader is encouraged to verify this by drawing a root locus plot for the closed loop poles.  The moral of this story is that when implementing a controller in a PLC one needs to consider the combined dynamics of the control law and system.  In this way a PLC is no different from any other form of control implementation platform.
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