Differences Between PLC and DCS
DCS features a comprehensive infrastructure designed to conduct advanced regulation management across the entire plant.
Slower processes typically require coordination among different production units.
Meanwhile, the superior speed of PLCs makes them a more suitable alternative for applications involving rapid production startups and the use of various I/Os.
Additionally, they offer a range of maintainability and I/O granularity.
Furthermore, DCS integrates PLC and PC to control certain functions and provide covering services. To differentiate between PLC and DCS, five key factors need to be considered:
1) Response Time:
The high speed of PLCs makes them ideal controllers for executing time-sensitive operations such as Emergency Shutdown (ESD) or ignition control. When response time is critical, DCS takes longer to process information, thus preventing it from achieving the correct resolution.
2) Scalability:
While PLCs handle thousands of I/O points, DCS also handles thousands of I/O points and adapts to new instruments, process enhancements, and data integration. DCS is preferred when advanced process control is required and must be depicted across a large geographical area with thousands of I/O points.
3) Complexity:
DCS requires advanced process control capabilities to handle the complexities of many continuous production processes such as oil and natural gas, water treatment, and chemical processes.
4) Frequent Process Changes:
PLCs are used for processes that are generally unmodifiable. However, when the process is complex, requires frequent changes, or necessitates the analysis of large amounts of data, DCS is the better solution.
5) Vendor Support:
DCS vendors require user engagement to provide integrated services and implement method changes. System integrators perform similar functions for PLC-based systems. It’s becoming commonplace for PLC vendors to offer support services through their system measurement partner networks.
In summary, DCS is commonly used in large, continuous process factories where high reliability and safety are critical, and where the facilities are not geographically distant.
PLC vs DCS
| PLC | DCS |
| 4. Provisions must be available to integrate functions/ products into an integrated architecture | 1. Custom logic created from existing function blocks |
| 1. High level programming languages are available for creating custom logic | 2. Customized routines are usually required |
| 6. Analog Control: Simple PID only | 3. Standard application libraries are expected (function blocks and faceplates) |
| 3. Standard libraries are considered nice features | 2. Many algorithms（i.e. PID）are complex and do not vary among application |
| 5. Redundancy may not be cost justified | 5. System redundancy is often required |
| 6. Analog Control: Simple to advanced PID control up to Advanced Process Control | 4. Entire system is expected to function as a Complete Solution |
| 7. Diagnostics to tell you when something is broken | 7. Asset Management alerts you to what might break before it does |
| 8. System designed to be flexible | 1. High-level programming languages are available for creating custom logic |
