August 1st, 2019 by
In plant floor control, real time is defined as the required performance among traditional field bus applications (intelligent devices and I/O). Field bus is always a dilemma among users seeking to manage costs and use the control products in class. However, the onset of different field bus technologies has been challenging to product developers due to the multiple and ever changing core product variants that impact development costs making it hard to offer effective solutions to numerous customers.
In this case, performance in real time definition refer to application throughput; the reaction time for input signals to be processed by controllers with the aim of triggering an output event. In a normal control process application, this refers to the time difference from when an add ingredient signal is made to when the valve moves and permits flow of materials. in a divert-er conveyor application setting, this would be the time from when an input signal is made to indicate a product on conveyors to the time an arms moves diverting the product conveyor to the downstream conveyor. This actions differ between discrete controls and processes. However, it is assumed that most process control applications are designed to process changes within 100 ms to 1 s and 30 to 50 ms in more discrete applications.
Determinism is the guarantee that an event will occur within a specified duration; not slower and not faster. This is because, too fast or too slow can be problematic. However, most people prefer too fast as opposed to too slow. Technically, Ethernet network is non-deterministic. But, the latest Ethernet switch technology can be used through network routers or physically to separate control architectures from office and plant wide networks. In such a case, determinism is assured. With the help of RTOS, hardware chips, stacks and commercial shelf technologies, update times of at least 1 ms can be achieved to make Ethernet suitable for discrete control applications. According to device designers, the critical hard real time determinism aspect lies on how the Ethernet communication interface interacts with the controller of the device control features. With the right Ethernet hardware, stack and RTOS can offer suitable performance. However, optimal performance may not be achieved where device designers fail to manage hardware design and resources effectively. There must be a balance between meeting the communication network needs and device requirements.
For hard real time determinism to be achieved in plant floor control, control system designers must ensure that they comprehend the products’ optimal performance features as well as how it behaves within different system architectures. This entails knowing a device’s application throughput, network response duration, and controller reaction periods through control program and hardware interface.