Simulations with Fault-Tolerant Controller Software of a Digital Valve

Model-based design approach is widely used to optimize controllers of distributed digital valve systems, leading to controllers simultaneously minimizing power consumption and tracking error. According to this approach an optimal controller (OC) is designed by comprehensive modelling of the relationship between parallel connected on/off-valves and the cylinder actuator in steady-state conditions. Unfortunately this relationship is highly non-linear. The design complexity of the OC is high. For this reason existing verification tools do not allow ensuring absence of design errors in this controller. These errors can cause incorrect valve control signal or result in absence of this signal leading to potentially hazardous physical processes in the hydraulic system. This requires designing the controller as a fault tolerant safety-critical hard real-time system. It is important to ensure that the system will work in a reasonable manner despite of possible design errors in the OC. This paper presents a method to resolve the problem by introducing an acceptance test (AT) to verify output signals of the OC. A safe controller (SC) – a simplified version of the OC for which the design can be verified is proposed. Control signal from SC is submitted to valves if AT detects incorrect output signals of OC. Simulation study shows that the SC gives good enough control performance even though the control resolution is not as good as with the OC.