## 3 Concept for a Smart Control Service
### SICS Base Model  
A SICS base model must take into account both the aspects of control engineering and the web technology features.  
From a control engineering point of view, a SICS controller based on a traditional PLC consists of the following components (Figure 4):  
* SISC program: IEC61131-3 control program in the PLCopen XML notation. It includes only the program and the variables, but not the I/O configuration.  
Figure 3: C33 controller.  
Figure 2: Evolution of a PLC as a CPS component Industrial Control. (**a**) PLC with program components CPb and CPc (**b**) PLC only with CPc component (**c**) The PLC hardware contains only sensors and actuators  
* SICS runtime: Execution environment for the SICS program. It can be cycle controlled or event-based.
* SICS router: Device or I/O configuration for a SICS controller; i.e., it is determined which CPS components (which automation devices) are connected to the controller. Since the real devices are already virtualized in an Internet-based environment via the CPS component, a SICS configuration only includes the assignment between the absolute IN/OUT addresses in the SICS program and their assignment (routing) to the IN/OUTs of the CPS components. A SICS device configuration is, therefore, also referred to as a SICS router.  
By separating of SICS program, SICS runtime and SICS router in a SICS controller, and distributing the components across an IP network using cloud technologies, it is possible to change, in real time, the control program (control algorithm). This also applies to the device configuration (e.g., for replacements of modules or plug and work). A SICS program and SICS router can be exchanged on-the-fly during one program cycle.  
For the identification of a viable SICS basic model, it is also necessary to show possible solution variants for a SICS controller, starting with the basic principles on the web, and then to reflect those in the available web technologies. As explained in detail in [10], if the control technology based SICS structure is adapted to the web-based functional systems, four general SICS base models are obtained:  
1. Server mode (SM): The SICS router is linked to fixed CPS components in a configuration process. After the SICS runtime has been started via the client, the SICS router automatically connects to the associated CPS component via the IP network and the SICS runtime executes the SICS program (Figure 5a).
2. Server-based mixed mode (SMM): Before starting the SICS runtime, a SICS router is loaded from the server to the client. After the SICS runtime is started, this router dynamically connects the CPS component with the SICS runtime on the server. All process date from the automation device are now routed to the server via the client (Figure 5b).
3. Client-based mixed mode (CMM): The control program runs in the SICS runtime on the client, but the communication to the CPS component runs over a configurable SICS router in the server (Figure 6a).
4. Client Mode (CM): SICS runtime and SICS routers are executed as one instance on the client (web browser). The client is an inherent part of the SICS control system and is necessarily required for executing the control program. The server is no longer required at the runtime (Figure 6b).  
Figures 5 and 6 illustrate the four basic models of a SICS control system.  
Figure 4: General structure of a smart industrial control service (SICS) controller.  
During the execution of the control program, the automation device, as a CPS component, has to be connected with the SISC runtime engine on the server/client by as IP network with as low latency and as much reliably as possible (see also the marked communication paths in the Figures 5 and 6).