PLCs or Programmable Logic Controllers, and RTUs, or Remote Terminal Units, are controllers used for the control of processes in industrial automation. PLCs can be used as RTUs in a Supervisory Control and Data Acquisition (SCADA) system. Moreover, PLCs are commonly used as RTUs in many SCADA installations.
In this guide, we will see the range of advantages PLCs offer when used as RTU and the difference between the two.
What is a PLC?
PLC is a controller which has analog or digital inputs and outputs linked to it, with communication protocol available in it. Software languages such as instruction lists, ladder logic, structured text, functional block diagram, and others are used inside the controller to write the logic.
Depending on the logic written, the PLC manages the outputs as per the inputs received. It utilizes communication protocols to establish connections with other control systems, such as SCADA/HMI interfaces or even other controllers, enabling seamless data exchange and coordination.
What is an RTU?
RTU is a microprocessor-based device that interfaces with actuators and sensors in the field, gathering data and controlling processes. It communicates with a central SCADA system which is accountable for controlling and managing the overall operations. PLCs are developed to execute these functions and are well-suited for integration into SCADA systems.
RTUs pass sensor information from input streams in control loops to an output stream to be sent on to centralized control in an industrial control system (ICS). RTUs abruptly negotiate links to either remote or local controls. RTUs are aspects of the ICS used in multiple processes like petrochemical and refineries, agriculture, chemical plants, food processing, pharmaceutical manufacturing, nuclear power plants, quality control, sewage treatment plants, automobile manufacturing, and water treatment plants.
RTUs perform comprehensive monitoring of both analog and digital field data. They collect information from sensors that track specific variables in industrial processes, transmitting this data to centralized monitoring and control systems. The hardware of an RTU encompasses the necessary setup software to establish connections, manage communication protocols, and facilitate troubleshooting. These devices are typically powered by AC mains with DC converters and occasionally have battery backup support.
Benefits of PLC When Used as RTU
PLCs provide a range of benefits when used as RTUs in SCADA systems:
- Robustness: PLCs are commonly designed to withstand harsh industrial conditions, making them reliable for field operations.
- Programmability: They offer the ability to program logic and control algorithms enabling personalized control and data acquisition techniques.
- Communication capabilities: PLCs provide several communication capabilities such as ethernet, serial, and fieldbus protocols making them compatible with SCADA systems’ communication needs.
- Scalability: PLCs are available in various sizes and configurations enabling scalability to include the size and complexity of the SCADA system.
- I/O capabilities: It offers a broad range of input and output (I/O) options enabling direct connection to field devices like sensors, actuators, and switches.
Difference Between PLC and RTU
- Ruggedness: RTU is immensely rugged and resistant to harsh environmental situations and attacks than PLC. RTU is developed for remote IO applications where the IOs are segregated far in the plant and data is required to be transferred wirelessly. Hence, it is more tough and rough to use and has more efficient than a PLC.
Furthermore, due to their deployment in remote locations with varying power supply availability, RTUs can operate on solar power or batteries, often surpassing the energy efficiency of PLCs. In terms of geographical coverage, RTUs have a more extensive telemetry reach compared to PLCs.
- Communication speed and data transfer: PLC’s speed of data transfer is faster as compared to RTU. Also, all the programming data and process data will be communicated in a PLC. However, in RTU, only requested information and reforms in it will be communicated. Generally, in a PLC, the programs are enforced cyclically, and in an RTU they are enforced in an event way.
- Operating voltage: A PLC functions effectively on 24V DC or 230V AC. On the other hand, an RTU functions with any process voltage.
- IO control: RTU is commonly not applied for output control such as motors, pumps, valves, and others. The foremost reason is low power consumption and poor wireless network. It hinders output operations. In this case, PLC is the most appropriate option.
- SCADA connectivity or other graphic system connectivity: RTU’s program control is much less and does not have an in-built display; a SCADA is commonly used everywhere when an RTU is applied.
However, a PLC does not mandatorily need a SCADA, as several controllers today have an in-built display, and if not, the program control is highly strong in it to prevent its necessity. The operation can adequately control the plant through hardwiring buttons.
Conclusion
By integrating PLCs as RTUs, the SCADA system can leverage the flexibility, reliability, and functionality of PLCs to control and monitor processes in real time. The PLCs gather data from the field devices, execute control strategies, and communicate the information to the SCADA system for analysis, visualization, and decision-making.
