78 PID Control tuning consists of adjusting these three variables in such a man- ner that the process variable stays at its set point under process operating conditions. The PID velocity (incremental) algorithm can be expressed as: PID Controller Implementation The above discussion describing the PID controller illustrates the operation of a parallel PID con- troller in which each control algorithm is performed indepen- dently and subsequently summed. Therefore, there is no interac- tion between the three tuning parameters; that is, changing the value of one parameter has no effect on the other two. PID controller implementations utilize control algorithms that create interaction between the tuning parameters to simplify loop tuning. In addition, the algorithms may be performed either in series or in parallel due to electronic or computational limitations. The user should be advised that this can cause dif- ferent controller output responses from two nonidentical con- trollers with identical tuning parameter values. In addition, identical controllers can exhibit the same symptoms when they are constructed with components that are subject to drift, such as those components used in mechanical, pneumatic, and ana- log electronic controllers.