Proportional-Integral-Derivative (PID) control is the most common control  algorithm used in industry and has been universally accepted in industrial  control. The popularity of PID controllers can be attributed partly to their  robust performance in a wide range of operating conditions and partly to their  functional simplicity, which allows engineers to operate them in a simple,  straightforward manner.
As the name suggests, PID algorithm consists of  three basic coefficients; proportional, integral and derivative which are varied  to get optimal response. Closed loop systems, the theory of classical PID and  the effects of tuning a closed loop control system are discussed in this paper.  The PID toolset in LabVIEW and the ease of use of these VIs is also discussed.
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http://zone.ni.com/devzone/cda/tut/p/id/3782