Fault Tolerant Remote Terminal Units (RTUs) in SCADA Systems

Scada System Architecture

A SCADA system consists of a Master Terminal Unit (MTU), communication equipment, and geographically distributed Remote Terminal Units (RTUs), as in Figure 1. The RTUs are linked with MTU via communication link as shown below:

Figure 1.

Typical SCADA system with master unit connected with RTU

The RTUs collect data and send it to the SCADA’s host computers located at some central position. Generally, a SCADA system may be associated with several of I/O points for data collection. An operator can decide polling rate for field data collection. The polling rate is influenced by several parameters such as number of sites, the required amount of data at each site, the available maximum bandwidth of the communication channel and the minimum required display and control time (Scadalink, 2010).

Arguably, it can be stated that the SCADA system’s rationale and objective depend upon the proper functioning of a remote terminal unit. The failure of one or more RTUs is therefore detrimental. In this chapter we have investigated a mechanism which will guarantee the service availability of RTU despite its failures. The Reliability and Availability analysis of the proposed scheme is discussed and the discussion of results is presented later in the chapter.

SMART SENSORS

The sensors normally work under the control of a microprocessor or microcontroller. The host processor is responsible for initiating the data sampling and collecting the digital output of the physical parameter the sensor is sensing. In smart sensors, the intelligence is delegated to the sensor unit itself. A smart sensor contains a Microcontroller (µC), a signal conditioning unit, an Analog-to-Digital Controller (A/D) and the interface (Alba, 1988). The microcontroller makes the sensor capable of distributed processing, compensation, self calibrating and so on. Figure 2 shows the elements of smart sensor.

Figure 2.

Smart sensor

The sensed data is signal conditioned and converted into digital form under the control of a microcontroller. The interface unit inside the smart sensor places the converted data onto the communication bus. Controller Area Network (CAN) communication protocol may be used to send the sensor data the in the form of CAN messages (Bosch, 1991). Transmitted sensor data encapsulated in CAN message format can be read by any CAN-enabled processing node connected to the bus. The application of smart sensors will mitigate the host processor from directly controlling the sensors.