## 4 Case Study of Industrial Control Systems  
### Application Scenarios and Security Design Implementation  
Programmable Logic Controllers (PLCs) are critical components of ICSs, widely deployed in sectors such as manufacturing, energy, and critical infrastructure. In the context of CPSs, PLCs serve as essential nodes that bridge the cyber layer and the physical process layer. They receive input signals from field sensors, execute control logic in real time, and issue output commands to actuators, enabling continuous closed-loop interaction between computation and physical dynamics. Following the design principles outlined in Section 3.3 on the diversity-redundancy security architecture, this study develops a prototype PLC system referred to as the diversity-redundancy security PLC. Technically, the system is implemented using a heterogeneous multi-core microcontroller unit (MCU), which integrates both architectural and operational redundancy.  
The custom-designed MCU includes three heterogeneous CPU cores, a lightweight polymorphic scheduler, a bus matrix, and various peripheral components. Unlike conventional single-core or homogeneous multi-core MCUs, this design introduces substantial differences among the three CPU cores. These differences span communication interfaces and memory architectures, interrupt handling mechanisms, and address mapping schemes. These cores are also supported by dedicated software toolchains to enable co-design between hardware and software. The heterogeneous CPU cores are based on three distinct instruction set architectures: ARM Cortex-M3, RISC-V Open-E906, and MIPS MicroAptiv UC. The MCU system architecture is illustrated in Figure 9, and a photograph of the physical device is shown in Figure 10. Under this heterogeneous architecture, the PLC system can perform security defense, malicious code cleansing, and resilient recovery, which are orchestrated through coordinated scheduling and runtime verification.  
The primary design goal of this system is to enhance cyber resilience for PLCs as core components of industrial CPS infrastructure scenarios. However, designing for resilience presents several practical challenges:  
* Trade-offs between performance overhead and security effectiveness in real-time control environments;
* The complexity of coordinating heterogeneous components to achieve fault tolerance and attack resistance;
* Lack of quantitative methods to evaluate how architectural diversity and redundancy translate into resilience benefits under dynamic cyber-physical conditions.  
These challenges underscore the necessity of a theoretically grounded evaluation framework. Without a rigorous modeling and assessment mechanism, it is difficult to validate whether design interventions (e.g., heterogeneity and dynamic scheduling) truly improve the system's ability to withstand and recover from cyber disturbances.  
In this context, the modeling approach proposed in Section 3 provides an essential theoretical foundation. By capturing multi-state degradation, dynamic recovery, and cyber-physical interactions, this framework enables structured evaluation of PLC resilience under various threat scenarios. The following subsection illustrates how this approach is applied to the proposed PLC system to guide and verify design effectiveness.