Section: D.1 Factory automation – motion control D.1.0 General
Content: Factory automation requires communications for closed-loop control applications. Examples for such applications are motion control of robots, machine tools, as well as packaging and printing machines. All other factory automation applications are addressed in Annex D.2. The corresponding industrial communication solutions are referred to as fieldbuses. The pertinent standard suite is IEC 61158. Note that clock synchronization is an integral part of fieldbuses that support motion control use cases. In motion control applications, a controller interacts with a large number of sensors and actuators (e.g. up to 100) that are integrated in a manufacturing unit. The resulting sensor/actuator density is often very high (up to 1 m-3). Many such manufacturing units have to be supported within close proximity within a factory (e.g. up to 100 in automobile assembly line production). In a closed-loop control application, the controller periodically submits instructions to a set of sensor/actuator devices, which return a response within a so-called cycle time. The messages, which are referred to as telegrams, are typically small (≤ 56 bytes). The cycle time can be as low as 2 ms, setting stringent end-to-end latency constraints on telegram forwarding (≤ 1 ms). Additional constraints on isochronous telegram delivery add tight constraints on the lateness (1 s), and the communication service has also to be highly available (99,9999%). Multi-robot cooperation is a case in closed-loop control, where a group of robots collaborate to conduct an action, for example, symmetrical welding of a car body to minimize deformation. This requires isochronous operation between all robots. For multi-robot cooperation, the lateness (1 µs) is to be interpreted as the lateness among the command messages of a control event to the group robots. To meet the stringent requirements of closed-loop factory automation, the following considerations have to be taken: - Limitation to short-range communications. - Use of direct device connection between the controller and actuators. - Allocation of licensed spectrum. Licensed spectrum can further be used as a complement to unlicensed spectrum, e.g. to enhance reliability. - Reservation of dedicated radio interface resources for each link. - Combination of multiple diversity techniques to approach the high reliability target within stringent end-to-end latency constraints (example: frequency, antenna and various forms of spatial diversity, e.g. via relaying) - Utilizing OTA time synchronization to satisfy latency-variation constraints for isochronous operation. A typical industrial closed-loop motion control application is based on individual control events. Each closed-loop control event consists of a downlink transaction followed by a synchronous uplink transaction, both of which are executed within a cycle time. Control events within a manufacturing unit might need to occur isochronously. Factory automation considers application layer transaction cycles between controller devices and sensor/actuator devices. Each transaction cycle consists of (1) a command sent by the controller to the sensor/actuator (downlink), (2) application-layer processing on the sensor/actuator device, and (3) a subsequent response by the sensor/actuator to the controller (uplink). Cycle time includes the entire transaction from the transmission of a command by the controller to the reception of a response by the controller. It includes all lower layer processes and latencies on the radio interface as well the application-layer processing time on the sensor/actuator. Figure D.1.0-1: Communication path for isochronous control cycles within factory units. Step 1 (red): controller requests sensor data (or an actuator to conduct actuation) from the sensor/actuator (S/A). Step 2 (blue): sensor sends measurement information (or acknowledges actuation) to controller. Figure D.1.0-1 depicts how communication can occur in factory automation. In this use case, communication is confined to local controller-to-sensor/actuator interaction within each manufacturing unit. Repeaters can provide spatial diversity to enhance reliability.