PROBLEMS AND CAUSES

Dead time is the leading cause of control loop problems either directly or by accentuating other causes. A large dead time makes a control loop more vulnerable to nonlinearities, periodic upsets, and poor tuning.

Dead time anywhere in the control loop prevents the proportional-integral-derivative (PID) control algorithm from either seeing or reacting to an upset. The best a PID controller can do is to limit the peak error to how far the process variable deviatesin 110% of the dead time if the controller is tuned for quarter-amplitude response (each succeeding peak is 1/4 the amplitude of the previous peak), as shown in Fig. 1. However, such aggressive tuning is not practical because a 25% or more increase in total dead time or open-loop gain or a 25% or more decrease in the largest time constant can cause instability. The loop is too close to its stability limit. Also, any oscillation in the closed-loop response may be undesirable from a standpoint of causing process variability because of unattenuated cycles in plug flow fluid systems (e.g., pipelines, static mixers, exchangers, extruders, and desuperheators) or solid systems (yarns, sheets, and conveyors) or periodic upsets to other loops. Thus quarter amplitude is a benchmark and not a practical goal. Typically, the peak error or maximum excursion of the process variable is reached in 150% to 1500% of the total dead time for industrial process control systems. The time to...