Instrumentation and Process Control

[From:来自网络] [Author:admin] [Date:12-07-09] [Hit:]

Instrumentation and Process Control

Distributed control system or a well known as DCS is the process automation devices that do the monitoring and control of the process system. In the past when the process controller is still using a pneumatic system, all process control was done centralized. Nowadays the pneumatic control system has been replaced by the digital process controller which can be distributed in the process plant such as distributed per unit area etc. Then the distributed system is integrated into a integral system through the digital communications. That’s why it is called Distributed Control System (DCS).

In the very basic configuration, the DCS is consisting of the power module, the controller module, the I/O card module, and the backplane. The power module is the module that fed the entire module in the backplane. The controller module is the brain of the DCS that think and decide what the final actuator element must do based on the input of the field transmitter. The I/O card module is the module that converts the analogue field signal into digital data that can be processed by the controller module. The backplane is the supporting system for the above module that provide the buses for power distribution and also communication system. The backplane is also the place to mount the modules.

See below schematic to understand the basic system of the DCS.

 

From the above drawing, we can understand that the power module is receiving a 24 VDC or an AC voltage in typical from the electrical systems. This voltage input is then converted into 5 VDC to power the module and some of them are not converted (if the fed power is 24 VDC) and goes to the module directly to power the field devices that need external power from the DCS. By powering up all the modules, then the DCS is ready for operation.

First of all, the analogue signal from the field instrument such as pressure transmitter, temperature transmitter, 24 VDC input signal from manual call point etc, is converted into the digital signal by the I/O card module. From the I/O card module, the digital signal is then sent to the controller module through the communication buses in the backplane. The digital signal is then processed by the controller to decide what the final actuator element must do, or what kind of alarm will be shown in the operator stations. Normally this basic DCS configuration can be extended into another unit area by using an additional communication module to integrate the two modules.

As explain above, this configuration is only the very basic DCS configuration. There are some complex configuration that involving a communication module and also a third party control system that might be integrated together as a sub unit but right now this article is not the place to discuss it into the detail. Hopefully it can be discussed further in the near future.

 

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