Machinery Condition Monitoring Using Wireless Self-Powered Sensor Nodes

There is rapid growth in the development of ultra-low power processors, radios, memory, and architectures that form the basis for wireless sensor nodes. Smart distributed wireless sensor nodes, originally developed for military applications over 10 years ago, are finding increased use for wireless machinery condition monitoring for industrial and commercial applications. The major drawback to low-cost wireless sensor nodes is the need to regularly replace batteries. This paper describes a prototype self-powered wireless sensor system deployed in a shipboard application. The sensor node scavenges energy from machinery vibration and uses this energy to power an embedded processor, sensors, and radio. Vibration data and energy harvesting efficiency data is periodically transmitted to a data collection server on the ship. Important issues are presented including the efficient design of the energy harvesting module, power conversion and power storage, communications, power management, embedded diagnostic algorithms, site survey and wireless sensor node installation, and the need for a systems-level approach for self-powered wireless sensor nodes. Extensions to this core work are presented that include adaptive power scavenging and self-powered sensor nodes embedded in machinery. Introduction There is a growing proliferation of wireless devices for many industrial and commercial applications such as for security, safety, surveillance, and machinery condition monitoring. Although the wiring costs for communications and power cables are eliminated with wireless devices, these costs are replaced with the ongoing cost of battery maintenance (e.g. battery cost, manpower, logistics, environmentally conscious disposal, protection from potential leakage, and potential equipment downtime due to unexpected battery failure). A critical technology is the ability to scavenge energy from the environment and use this energy to power remote, distributed sensors, radios, actuators, processors, memories, and display elements. This technology is enabling and permits power-scavenging sensor nodes to never require servicing and to be deployed in inaccessible locations or embedded into machinery. Rockwell Automation, in collaboration with BP’s Chief Technology Office in the U.K., has developed and deployed two self-powered wireless sensor nodes on the BP tanker Loch Rannoch. These sensor nodes were deployed as part of a technology evaluation program to establish the viability of wireless sensor nodes operating in a harsh shipboard environment for machinery condition monitoring. The shipboard trial of self-powered sensor nodes was done in parallel with a larger scope test of wireless sensor nodes on the tanker. Background There are important technology changes occurring that promise to change the character of machinery monitoring. These changes will affect future machinery condition monitoring, safety, control, re-configuration, and security. New developments in diagnostic and prognostics algorithms, emerging CBM architectures and standards (e.g. OSA-CBM, MIMOSA Open O&M, and OPC/ISA), and new, low-power wireless communications standards and components (e.g. Bluetooth and IEEE 802.15.4) enable the deployment of many low-cost distributed sensors. An array of distributed sensors can provide superior capabilities for mission-critical applications and can directly reduce maintenance cost and total life-cycle cost. Wireless sensor nodes often employ energy efficient processors, memory, radios, and energy-management logic. These systems have been under development for over 10 years and are targeted for applications such as surveillance, target acquisition, and machinery monitoring [1][2]. In spite of their energy efficiency, the need for a reliable, long-term energy source remains a roadblock to the broadscale deployment of thousands of distributed sensor nodes. For many distributed sensor applications it is not practical to provide wire-line power due to factors such as cost, weight, reliability, safety, or environmental hazards (e.g. explosive environments). For example, in many industrial applications the cost of wiring may be $40 USD per foot or more and often exceeds the cost of the remote sensor. The need for costly wiring can be eliminated if the distributed sensor node is self-contained and is self-powered. Options for self-powering sensor nodes include batteries, micro-fuel cells, micro-generators, and power-scavenging technologies. A self-powered sensor node utilizing micro-generators or fuel cells provide limited benefit and is currently not considered a viable solution. Utilizing storage batteries for remote sensors is also not considered a generally effective solution since the batteries required to power each sensor adds significant cost, weight, reliability and maintenance problems, particularly when the sensors are located in difficult to reach areas. These problems are multiplied when hundreds of thousands of battery-powered sensor nodes are deployed. Furthermore, batteries have an uncertain life and it is difficult to accurately predict the battery charge depletion profile. Batteries encapsulate caustic chemicals and heavy metals that may, in time, leak causing damage to equipment, injury to workers, and may affect other nearby equipment and the environment. Batteries also suffer from low power density and rapid deterioration and aging especially in hostile environments. For these reasons batteries are not considered a generally viable method to power remote wireless sensor nodes. Batteries may be effective for targeted applications requiring low duty cycle and low power requirements in readily accessible locations. Harvesting power from stray energy for remote sensors is well-suited for CBM sensor applications. CBM systems often require periodic sampling from sensors which may be distributed across a machine, vessel, or facility. Remote sensor processing can typically be performed periodically and at a frequency consistent with the rate of local power generation. Newer materials such as certain piezo-electric materials exhibit high coupling efficiencies and can operate at elevated temperatures (>550°F). These materials are already being used for vibration and ultrasonic sensing and are well-suited for power scavenging devices for CBM applications. A remote self-powered sensing device may be composed of four core elements not including the machine or communications (Figure 1). The first element is a sensing element such as temperature, pressure, vibration, or magnetic field. The second element is a parasitic energy transducer capable of transforming stray energy into an electrical potential. The third element is a power storage device such as a rechargeable battery or a capacitor bank. And the fourth element is a processor and algorithms for interpreting the sensor data. There are often additional elements such as local memory, digital output, or communications such as a wireless data link. The communications link may not be required if the self-powered sensor node is used to locally store information and operate as a “black-box” system or if only a local display such as blinking LED is used to signal a machine fault. Figure 1 – Components of a Self-powered Sensor Node A more complete configuration may include multiple power storage elements with different electrical characteristics, multiple sensor elements, multiple energy harvesting modules with different electrical and mechanical characteristics, actuator elements, and adaptive hardware and software elements to optimize system performance. The core elements described above can provide valuable machinery condition monitoring capabilities. The ability to perform machinery condition monitoring without the cost of wiring and the burden of periodic battery replacement provides attractive benefits for many industrial, commercial, and marine monitoring applications. Established power scavenging techniques such as piezo-electric devices and photovoltaic cells can provide the power needed for machinery condition monitoring for reasonable environmental conditions and provide periodic machinery monitoring. Regular machinery monitoring is particularly valuable for shipboard systems. Routing power and signal cables on ships is frequently very difficult due the presence of thick compartment walls, limited free space for cable trays and conduits, and watertight compartment requirements. Similarly, manually capturing data at machines below deck is time consuming for the ship’s crew and can be dangerous during high seas or in the presence of water, power cables, or petrochemical fluids or gases. A program was defined to evaluate the capability of existing energy harvesting technologies and to establish the deployment issues and operational benefits afforded by wireless, self-powered machinery condition monitoring. A set of preliminary specifications were developed and a shipboard target application was established. BP Shipping has supported this project by providing technical information and access to the FPSO tanker Loch Rannoch (Figure 2 BP Ship Loch Rannoch) [3][4].