The control valve’s stem position is the process variable (PV) for the positioner, just as the command signal is the positioner’s setpoint (SP). Thus, a cascade control system consists of two feedback control loops, one nested inside the other:Ī very common example of cascade control is a valve positioner, which receives a command signal from a regular process controller, and in turn works to ensure the valve stem position precisely matches that command signal. The first controller (called the primary, or master) essentially “gives orders” to the second controller (called the secondary or slave) via a remote setpoint signal. To cascade controllers means to connect the output signal of one controller to the setpoint of another controller, with each controller sensing a different aspect of the same process. This flow of information is collectively referred to as a feedback control loop. The controller’s task is to inject the proper amount of negative feedback such that the process variable stabilizes over time. control valve), influencing the process which is sensed again by the measuring device. transmitter) goes to the controller, then to the final control device (e.g. Information from the measuring device (e.g. In a typical M2M scenario, an anomaly is detected by the sensor then sends to a monitoring middleware an alert that gives information to a business operation software, produces a command for an actuator, and to an operator off the loop, it sends an alarm.A simple control system drawn in block diagram form looks like this: The use of machine-to-machine applications (M2M) has been increasing. Researchers are looking into using IP technology to be used in wireless networks to provide deterministic control loops. In a system where a high number of users exist, Wireless Sensor Network simulations done using the IEEE 802.15.4 standard showed a considerable outperformance of the original standards by these mechanisms. The above can be by-passed by having an application use the wireless medium associating every packet with a delay requirement. In situations or processes that change slowly, the above is used where the process can be brought back easily, is not within its limit or terminated. An open loop system, on the other hand, returns analyzed data with changes to a central controller, ordered in minutes or even seconds. Data delays lead to negative reinforcement, hence instead of a process being kept within close limits, it runs away. For a closed loop control system, this presents a major challenge since actuators influencing the system are controlled by the sensor’s data. The network’s quality is a trivial part of the functioning of the overall system when implementing a closed-loop control system using a common communication system.Įarlier short range wireless networks pose a problem since delay deadlines are not used in packet consideration and regardless of these requirements, the packets are treated the same. Below is a Microchip’s MRF24WB Wi-Fi module.įor sharing a common medium of communication, sensors and controllers are required by a control system that is adopting wireless communication. For tight control loops, devices supporting the IEEE802.15.4 ZigBee standard can be used in supporting the recent protocols. Though embedded modules can be used be used with Wi-Fi, the aim of recent protocols is providing wireless networks with more focused support for control loops.
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