21-07-2012, 04:40 PM
DISTRIBUTED CONTROL SYSTEMS
CONTROL SYSTEMS.pdf (Size: 83.76 KB / Downloads: 90)
Introduction
Generally, the concept of automatic control includes accomplishing two major operations; the transmission of signals (information flow) back and forth and the calculation of control actions (decision making). Carrying out these operations in real plant requires a set of hardware and instrumentation that serve as the platform for these tasks. Distributed control system (DCS) is the most modern control platform. It stands as the infrastructure not only for all advanced control strategies but also for the lowliest control system. The idea of control infrastructure is old. The next section discusses how the control platform progressed through time to follow the advancement in control algorithms and instrumentation technologies.
Historical Review
To fully appreciate and select the current status of affairs in industrial practice it is of interest to understand the historical perspective on the evolution of control systems implementation philosophy and hardware elements. The evolution concerns the heart of any control system which is how information flow and decision making advanced.
Modes of Computer control
Computer control is usually carried out in two modes: supervisory control or direct digital control. Both are shown in Figure 1. Supervisory control involves resetting the set point for a local controller according to some computer calculation. Direct digital control, by contrast, requires that all control actions be carried out by the digital computer. Both modes are in wide use in industrial applications, and both allow incorporating modern control technologies. Measurements are transmitted to computer and control signals are sent from computer to control valves at specific time interval known as sampling time. The latter should be chosen with care.
Small Computer Network
In small processes such as laboratory prototype or pilot plants, the number of control loops is relatively small. An inexpensive and straightforward way to deal with the systems is to configure a network of personal computers for data acquisition and control. An example configuration of a PC network control system is depicted in Figure 2. The network consists of a main computer linked directly to the process in two-way channels. Other local computers are linked to the main computer and are also connected to the process through one-way or two-way links. Some of these local computers can be interconnected. Each of the local computers has a video display and a specific function. For example, some local computers are dedicated for data acquisition only, some for local control only and some other for both data acquisition and local control. The main computer could have a multiple displays.
Commercial Distributed Control Systems
In more complex pilot plants and full-scale plants, the control loops are of the order of hundreds. For such large processes, the commercial distributed control system is more appropriate. There are many vendors who provide these DCS systems such as Baily, Foxboro, Honeywell, Rosemont, Yokogawa, etc. In the following only an overview of the role of DCS is outlined.
Conceptually, the DCS is similar to the simple PC network. However, there are some differences. First, the hardware and software of the DCS is made more flexible, i.e. easy to modify and configure, and to be able to handle a large number of loops. Secondly, the modern DCS are equipped with optimization, high-performance model-building and control software as options. Therefore, an imaginative engineer who has theoretical background on modern control systems can quickly configure the DCS network to implement high performance controllers.
The advantages of DCS systems
The major advantages of functional hardware distribution are flexibility in system design, ease of expansion, reliability, and ease of maintenance. A big advantage compared to a single-computer system is that the user can start out at a low level of investment. Another obvious advantage of this type of distributed architecture is that complete loss of the data highway will not cause complete loss of system capability. Often local units can continue operation with no significant loss of function over moderate or extended periods of time.
Moreover, the DCS network allows different modes of control implementation such as manual/auto/supervisory/computer operation for each local control loop. In the manual mode, the operator manipulates the final control element directly. In the auto mode, the final control element is manipulated automatically through a low-level controller usually a PID. The set point for this control loop is entered by the operator. In the supervisory mode, an advanced digital controller is placed on the top of the low-level controller (Figure 1). The advanced controller sets the set point for the low-level controller. The set point for the advanced controller can be set either by the operator or a steady state optimization. In the computer mode, the control system operates in the direct digital mode shown in Figure 1.
CONTROL SYSTEMS.pdf (Size: 83.76 KB / Downloads: 90)
Introduction
Generally, the concept of automatic control includes accomplishing two major operations; the transmission of signals (information flow) back and forth and the calculation of control actions (decision making). Carrying out these operations in real plant requires a set of hardware and instrumentation that serve as the platform for these tasks. Distributed control system (DCS) is the most modern control platform. It stands as the infrastructure not only for all advanced control strategies but also for the lowliest control system. The idea of control infrastructure is old. The next section discusses how the control platform progressed through time to follow the advancement in control algorithms and instrumentation technologies.
Historical Review
To fully appreciate and select the current status of affairs in industrial practice it is of interest to understand the historical perspective on the evolution of control systems implementation philosophy and hardware elements. The evolution concerns the heart of any control system which is how information flow and decision making advanced.
Modes of Computer control
Computer control is usually carried out in two modes: supervisory control or direct digital control. Both are shown in Figure 1. Supervisory control involves resetting the set point for a local controller according to some computer calculation. Direct digital control, by contrast, requires that all control actions be carried out by the digital computer. Both modes are in wide use in industrial applications, and both allow incorporating modern control technologies. Measurements are transmitted to computer and control signals are sent from computer to control valves at specific time interval known as sampling time. The latter should be chosen with care.
Small Computer Network
In small processes such as laboratory prototype or pilot plants, the number of control loops is relatively small. An inexpensive and straightforward way to deal with the systems is to configure a network of personal computers for data acquisition and control. An example configuration of a PC network control system is depicted in Figure 2. The network consists of a main computer linked directly to the process in two-way channels. Other local computers are linked to the main computer and are also connected to the process through one-way or two-way links. Some of these local computers can be interconnected. Each of the local computers has a video display and a specific function. For example, some local computers are dedicated for data acquisition only, some for local control only and some other for both data acquisition and local control. The main computer could have a multiple displays.
Commercial Distributed Control Systems
In more complex pilot plants and full-scale plants, the control loops are of the order of hundreds. For such large processes, the commercial distributed control system is more appropriate. There are many vendors who provide these DCS systems such as Baily, Foxboro, Honeywell, Rosemont, Yokogawa, etc. In the following only an overview of the role of DCS is outlined.
Conceptually, the DCS is similar to the simple PC network. However, there are some differences. First, the hardware and software of the DCS is made more flexible, i.e. easy to modify and configure, and to be able to handle a large number of loops. Secondly, the modern DCS are equipped with optimization, high-performance model-building and control software as options. Therefore, an imaginative engineer who has theoretical background on modern control systems can quickly configure the DCS network to implement high performance controllers.
The advantages of DCS systems
The major advantages of functional hardware distribution are flexibility in system design, ease of expansion, reliability, and ease of maintenance. A big advantage compared to a single-computer system is that the user can start out at a low level of investment. Another obvious advantage of this type of distributed architecture is that complete loss of the data highway will not cause complete loss of system capability. Often local units can continue operation with no significant loss of function over moderate or extended periods of time.
Moreover, the DCS network allows different modes of control implementation such as manual/auto/supervisory/computer operation for each local control loop. In the manual mode, the operator manipulates the final control element directly. In the auto mode, the final control element is manipulated automatically through a low-level controller usually a PID. The set point for this control loop is entered by the operator. In the supervisory mode, an advanced digital controller is placed on the top of the low-level controller (Figure 1). The advanced controller sets the set point for the low-level controller. The set point for the advanced controller can be set either by the operator or a steady state optimization. In the computer mode, the control system operates in the direct digital mode shown in Figure 1.