Section: D.2 Factory automation – other use cases D.2.0 General
Content: Factory automation encompasses all types of production that result in discrete products: cars, chocolate bars, etc. Automation that addresses the control of flows and chemical reactions is referred to as process automation (see clause D.3). Discrete automation requires communications for supervisory and open-loop control applications, as well as process monitoring and tracking of operations inside an industrial plant. In these applications, a large number of sensors, which are distributed over the plant, forward measurement data to process controllers on a periodic or event-driven base. Traditionally, wireline field bus technologies have been used to interconnect sensors and control equipment. Due to the sizable extension of a plant (up to10 km2), the large number of sensors, rotary joints, and the high deployment complexity of wired infrastructure, wireless solutions have made inroads into industrial process automation. The related use cases require support of a large number of sensor devices per plant, as well as high communication service availability (99,99%). Furthermore, power consumption is relevant since some sensor devices are battery-powered with a targeted battery lifetime of several years (while providing measurement updates every few seconds). Range also becomes a critical factor due to the low transmit power levels of the sensors, the large size of the plant, and the high-reliability requirements on transport. End-to-end latency requirements typically range between 10 ms and 1 s. User-experienced data rates can be rather low since each transaction typically comprises less than 256 bytes. However, there has been a shift from field busses featuring somewhat modest data rates (~ 2 Mbit/s) to those with higher data rates (~ 10 Mbit/s) due to the increasing number of distributed applications and "data-hungry" applications. An example for the latter is the visual control of production processes. For this application, the user experienced data rate is typically around 10 Mbit/s and the transmitted packets are much larger than what was stated earlier. Existing wireless technologies for factory automation rely on unlicensed bands. Communication is therefore vulnerable to interference caused by other technologies (e.g. WLAN). With the stringent requirements on transport reliability, such interference is detrimental to proper operation. The use of licensed spectrum could overcome the vulnerability to same-band interference and therefore enable higher reliability. Utilization of licensed spectrum can be confined to those events where high interference bursts in unlicensed bands jeopardizes communication service availability and end-to-end latency constraints. This allows sharing the licensed spectrum between process automation and conventional mobile services. Multi-hop topologies can provide range extension and mesh topologies can increase reliability through path redundancy. Clock synchronization will be highly beneficial since it enables more power-efficient sensor operation and mesh forwarding. The corresponding industrial communication solutions are referred to as fieldbuses. The related standard suite is IEC 61158. A typical discrete automation application supports downstream and upstream data flows between process controllers and sensors/actuators. The communication consists of individual transactions. The process controller resides in the plant network. This network interconnects via base stations to the wireless (mesh-) network which hosts the sensor/actuator devices. Typically, each transaction uses less than 256 bytes. An example of a controller-initiated transaction service flow is: 1. The process controller requests sensor data (or an actuator to conduct actuation). The request is forwarded via the plant network and the wireless network to the sensors/actuators. 2. The sensors/actuators process the request and send a replay in upstream direction to the controller. This reply can contain an acknowledgement or a measurement reading. An example of a sensor/actuator device-initiated transaction service flow: 1. The sensor sends a measurement reading to the process controller. The request is forwarded via the wireless (mesh) network and the plant network. 2. The process controller can send an acknowledgement in opposite direction. For both controller- and sensor/actuator-initiated service flows, upstream and downstream transactions can occur asynchronously. Figure D.2.0-1 depicts how communication can occur in discrete automation. In this use case, communication runs between process controller and sensor/actuator device via the plant network and the wireless (mesh) network. The wireless (mesh) network can also support access for handheld devices for supervisory control or process monitoring purposes. Figure D.2.0-1: Communication path for service flows between process controllers and sensor/actuator devices. Left-hand side: Step 1 (red) – the sensor/actuator (S/A) sends measurement report autonomously, Step 2 (blue) controller acknowledges. Right-hand side: Step 1 (red) - controller requests sensor data (or an actuator to conduct actuation), Step 2 (blue): S/A sends measurement information (or acknowledges actuation) to controller.