Processcontrol Level 1331 Main Device

The main device of the process-control level is the programmable logic controller (PLC). A typical PLC consists of a central processing unit (CPU), a memory unit, analogue and digital input/ output blocks, as well as a synoptic display and an operating panel that are often combined. All these elements are usually incorporated into a single cabinet designed for rugged industrial environments. One PLC is commonly associated with one process unit (e.g. bioreactor) or a group thereof, for which it is continuously 'available'. A PLC is characterized by its computing capacities, i.e. the number of inputs and outputs and the rate (cycle time) at which these can be handled. Typical cycle-time values are in the range of 50-500 ms.

In addition to performing some operations automatically and to handling certain failures, the key function of a PLC is to control variables at user-defined set-points, via actuators, based upon measurements provided by sensors. The combination of a sensor (with a display and a recorder), a controller and an actuator constitutes a control loop, which is the basic unit in a control system (Hartnett 1994). The most common method for controlling bioprocesses is based upon feedback control with closed loops, using proportional-integral-derivative (PID) or proportional-integral (PI) algorithms (Dunn et al. 2003; Gonzalez & Herb 1984). Modern PLCs now have enhanced computing capacities and are often supported by powerful software applications at the supervision level (see Section 13.4.1). This has enabled the use of more advanced control techniques, based on multivariable, adaptive, non-linear and fuzzy logic algorithms as well as on expert systems (Dochain & Perrier 2000; Konstantinov et al. 1994b; Lenas et al. 1997; Lubbert & Simutis 1994; Schugerl 2001; Shimizu 1993). An example of a control loop for a process variable is illustrated in Figure 13.5. Other examples, for a fermentation plant, are discussed in detail in

Figure 13.5 Example of automatic control of glucose in a perfusion bioreactor. (a) Scheme of the perfusion bioreactor with the control system. Glucose is monitored on line by an external enzymatic analyser (not shown) and the computer maintains the glucose concentration at the desired set point by adjusting the feed and the harvest rates. (b) Time profile of the glucose concentration measured on line (•) and of the glucose set point (—) in a perfusion CHO cell culture. The set point was increased in a stepwise mode (2.0, 2.5, 3.0 and 3.5 g/l); standard deviations ('std') of measurements are given for each step. (Reproduced from Konstantinov et al. (1996), with permission.)

Figure 13.5 Example of automatic control of glucose in a perfusion bioreactor. (a) Scheme of the perfusion bioreactor with the control system. Glucose is monitored on line by an external enzymatic analyser (not shown) and the computer maintains the glucose concentration at the desired set point by adjusting the feed and the harvest rates. (b) Time profile of the glucose concentration measured on line (•) and of the glucose set point (—) in a perfusion CHO cell culture. The set point was increased in a stepwise mode (2.0, 2.5, 3.0 and 3.5 g/l); standard deviations ('std') of measurements are given for each step. (Reproduced from Konstantinov et al. (1996), with permission.)

Lam (1992). PLCs typically have a limited memory, used only to store the data required for the control algorithms. Storage and archiving of process data is performed by supervision tools, located on the next CIM level.

Originally, PLCs were built as standalone units, each being dedicated to its own process unit or small set of functions. Communication was limited to a human machine interface (HMI) and to other PLCs to exchange a limited amount of data. Nowadays, PLCs have much higher communication capacities, thanks to field bus systems and other local area networks. Instead of building centralized PLCs with continuously increasing performance, the tendency is now towards a 'distribution of intelligence', i.e. to keep the various control functions spread over several machines, in a decentralized way, and to use a supervision tool to coordinate the operations of the various PLCs (see Section 13.4.1). This leads to a higher availability of the control systems and more flexibility, for instance for validation (see Section 13.5) than a centralized design. Additionally, in the case of a PLC failure, only a small part of the plant will be affected.

Was this article helpful?

0 0
Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment