A DCS System is preferred over a central computer system for the following reasons. Central Computer System (CCS) exposes the plant to the risk that the system might fail. This can be minimized by having one or more back up schemes.
- A back up computer
- Analog controller
- Manual loading stations
- Various combinations of the above.
These back up methods are expensive. The DCS System also needs a backup but it distributes the risk of failure into smaller sub-sections. The failure of one node should not cause the failure of another portion. It decreases cost, kinds of redundancy and backup needed. In short, it has the capability to fail gracefully.
Another advantage is the substantial reduction of wiring costs. The data highway is generally a twisted shielded pair of wires or some type of coaxial cable which carries all the processor and computer system information required for efficient plant operation via a high speed digital data communication system. It replaces many miles of copper cable for sensor inputs and final control element outputs. Hence, a large saving in a plant with many inputs and outputs can be achieved. Routine process control is performed in the distributed micro-computers with their associated Input/Output (I/O) cards, multiplexer, ADC and DAC converters. The failure of computer at this node will leave, for example, the control valves at their last position due to hard circuitry on the output card. If backup microcomputer is installed, it will automatically take over control without a bump to the process. If no backup microcomputer is installed, the manual analog loading station may be switched in to maintain control with an operator in attendance.
Design features of TDC 3000 Architecture
Perhaps it would be better if we give an introudction as to what is TDC 3000 Architecture?
1.The TDC3000 system is designed to provide dependable service in an industrial environment. Failures are infrequent and contained. Failure effects are minimized by a fault tolerant design that also provides for optional redundancy of critical system modules. Isolation, part replacements and the return to operation of failed modules is speeded up by built- in hardware and software fault checks. One of the major design goals of the TDC 3000 has been to ensure a high level of system availability. First, by increasing the mean time between failures and second, by lowering the mean time to repair.The first objective has been met by the use of conservation design and use of quality components. The second objective is met by providing a variety of built in diagnostic tools designed to pinpoint faults to an Optimum Replacement Unit (ORU).
2.Design Benefits The TDC 3000 design philosophy of full distribution of architecture and full integration of functions give important benefits to the user. The manufacturer of TDC 3000, Honeywell, has provided expanded capabilities much more that what is available with any other
3.Incremental Expandability The functional specialization of the physical modules allows the purchaser to start with the minimal devices that match the initial requirements and the available funds. When it is required to expand, as a result of either an increase in process dimensions or the fundamental needs, it can be easily achieved by the simple addition of hardware modules, without any significant effect on the existing system, i.e. no reprogramming of the software is needed.
4.Global Database No matter, whatever is the size of your TDC 3000 system, it functions with a single database. This database is accessible to all users by way of a single access mechanism. Since multiple access mechanis’s are not required, expansion engineering costs as well as initial engineering costs are minimized.
5.Network File Management System For large systems, with more than one process or application engineer having frequent need for simultaneous access to the system in order to build and edit files, the online network file management capability is invaluable. For example, one engineer seated at a universal workstation in an office can design and implement a new control strategy in control language on an application module, while at the same time another engineer at a different universal workstation can update a graphic display for use by the operator. Also at the same time, a maintenance technician might be accessing data required to analyse a problem that has been logged by the system. While all this is taking place, the process is under control and the operators are performing their normal duties at the operator console.
6.Distributed Risk In a control system with centralized architecture, failure of a component can bring down the entire system. With the distributed architecture of the TDC 3000 graceful degradation, a long time feature of Honeywell’s Data Hiway-based systems will be in effect. A failure will, in the worst case, take the affected module with its assigned function, out of service depending on the importance of the failed module and whether or not it is redundant, the remainder of the system may continue to perform with minimum impact resulting from the failure.