Posted by arlene

Wire Replacement

This is most commonly done with point-to-point or point-to-multipoint technology, but mesh networks still provide complete transparency. The network does not know that copper has been replaced with an RF link, but the mesh network is inherently more reliable, more adaptable, and scalable.

Distributed Control

A specific opportunity for wireless, multi-hop, mesh networks is in distributed control systems. There has been a trend in recent years to place more intelligence throughout the control system. The IEEE 1451 standard Smart Transducer Interface for Sensors and Actuators is evidence of this. Distributed intelligence is naturally served better by wireless multi-hop mesh networks, which do not require a central control topology.

The control of the wireless system is distributed throughout the network, allowing intelligent peers to communicate directly to other points on the network without having to be routed through some central point. Modular distributed control systems are easier to install and maintain. Since more of the system logic is at the instrument or subsystem level, clusters of instruments can interact and make local decisions. This is often done with small PLCs, which gather information from nearby instruments or sensors and then provide processing power and decision-making for this local instrument cluster. These clusters can then be connected as a group back into the main control system. The result is a less complex installation because individual instruments and points do not have to be directly connected to the main control panel.

Is Distributed Control Cheaper to Maintain?

Proponents insist that modular control systems are easier and less costly to maintain. The rationale is that highly modular control systems enable localized decision-making, which results in faster isolation of problems within the system. These problems can usually be diagnosed back to a single instrument cluster, allowing engineers and maintenance staff to focus their attention on one area of the system. Fast problem-solving means less downtime when something goes wrong. Likewise, when the system is operational, local decision-making by intelligent instruments and small PLCs identifies problems before they impact the entire system and cause bigger problems. Finally, these modular subsystems can be replaced or upgraded without affecting the entire system. These many factors make systems much cheaper and easier to operate and maintain.

Matching multi-hop, wireless mesh communication with distributed control facilitates a whole new dimension of interactions between sensors or sensor clusters. Sensors can now communicate directly to other devices on the network. This topology allows a tank level sensor to communicate directly with nearby valves, alerting them to open or close to prevent an overflow situation. Monitoring equipment could take readings from sensors without having to directly access the sensor with wired connections. This is useful in calibration and troubleshooting.

Diagnostic Monitoring

A third area of application for wireless, multi-hop mesh networks is in the diagnostic monitoring of devices. This monitoring can occur outside the normal control loop and wireless communication can be sent to notify the system user of any abnormal operation of the device. In this control loop, an additional signal is extracted and analyzed during the course of normal operation of the sensor. As the sensor operates, the signal is monitored for abnormalities without affecting the sensor’s operation. If an abnormal signal or trend is observed, an alert is triggered.

The beauty of using a wireless link for onboard monitoring and alert is that the monitoring link remains independent of the control loop. By using a wireless, multi-hop mesh network, data can be routed dynamically to similar wireless devices. Surrounding devices can respond to the alert from the failing device, even as the alert is being sent to maintenance personnel. Another benefit of wireless is that maintenance personnel can directly connect to the diagnostic output of the sensor, without running wires. This can eliminate a huge task in the case of a tank level sensor in a large storage tank, or a temperature probe at the top of a tower stack at a chemical refinery. In a wireless, multi-hop mesh network, a user can get that data via any wireless node on the network.

By using a diagnostic device with additional processing power (e.g., a laptop computer, handheld computer, or handheld diagnostic device), maintenance personnel can check on configuration and other information about any node on the network. This information is a valuable tool for checking and verifying sensor operation when questionable data is received from a sensor through its primary control loop.

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