Non-Intrusive Monitoring: Types, Applications, and Benefits

Introduction to Non-Intrusive Monitoring

Non-intrusive monitoring was developed by George Hart, Ed Kern, and Fred Schweppe in the 1980s at the Massachusetts Institute of Technology. It is commonly discussed in terms of non-intrusive load monitoring (NILM), a means of monitoring an electrical circuit that encompasses a number of appliances, all capable of turning on and off independently of one another. Rather than attaching a separate monitor to each appliance, non-intrusive monitoring uses electric meters to determine the different patterns of power use within a given home. Similarly, nonintrusive appliance load monitoring (NALM) operates through "a sophisticated analysis of the current and voltage waveforms of the total load; the NALM estimates the number and nature of the individual loads, their individual energy consumption, and other relevant statistics such as time-of-day variations" (Hart). Non-intrusive monitoring is particularly valuable because it can measure voltage and current without requiring access to individual components or appliances within a home or other entity under assessment. The resulting data can be extremely useful to public policymakers, energy auditors, consumers, and appliance manufacturers, providing an accurate snapshot of energy consumed over time (Hart). This can highlight deficiencies, irregularities, or issues within energy consumption patterns.

Within non-intrusive load monitoring, power meter readings offer a clear identification of the loads created by specific appliances, generating a reliable way to verify load sheds in homes and buildings (Bergman et al., 2011). A clear example of this is within a home during a two-hour period. Over those two hours, the activity might show energy use from only a heater and a refrigerator: the refrigerator turns on and off three times, the heater twice as often (Hart). This process can easily illuminate the total energy used by these appliances and their individual expenditures: "By also considering measurements of the total reactive power or harmonic current, along with the real power shown, changes in the resulting vector function of time would reveal even more information about the particular appliances" (Hart).

The key principle of non-intrusive monitoring is that one uses information available from the normal operation of an item, placing no extra requirements on the system (Thornton & Sanghera, 2011). This type of monitoring can be applied to all forms of energy, including sound — this is where acoustic and vibration non-intrusive monitoring becomes relevant. Acoustic and vibration monitoring can detect signals or vibrations caused by the movement of material. "This movement causes impacts and frictional contact with a containing face, for example the inside of a pipe. The sensor is fastened to the outside of the structure, and its high-frequency detection picks up these signals, which are often undetectable to the human ear" (pulsar-pm.com). This type of detection relies on tools that function effectively even in environments with a tremendous amount of machinery noise, because they use advanced technology to detect changes or disruptions in acoustic emissions from equipment or machinery during operation (pulsar-pm.com).

This non-intrusiveness is one of the defining attributes of the approach: materials and processes are examined while engaged in their normal functioning. With acoustic and vibration non-intrusive monitoring, a sensor examines acoustic discharges from machinery during normal operation, and these sensors are sensitive to the smallest changes in conditions. This type of monitoring is used in industrial scenarios such as mills and plants, in the assessment of automobiles and trucks to determine engine efficiency, and even in the field of medicine to evaluate the success of joint and limb replacements or therapies. The possibilities with these technologies are genuinely broad.

Different Types of Non-Intrusive Monitoring

Several different types of non-intrusive monitoring have already been alluded to above. Non-intrusive appliance load monitoring gives energy auditors, homeowners, building owners, manufacturers, and other interested parties a clear picture of the combined energy that their appliances use as well as each appliance separately. It is a way of measuring energy output that yields a more detailed picture of which electrical loads are contributing to the largest or smallest consumption figures.

One of the most significant disadvantages of this type of monitoring involves real and immediate privacy concerns for the individual. These concerns arise most directly when the energy use of a home or other residential structure is being examined. Fundamentally, the way that individuals or families use energy reveals patterns of behavior. Patterns of behavior are directly related to privacy rights: individuals have a right to keep private what they do at home, when they are home, when they are showering, and other personal or potentially sensitive activities. The concern is compounded when individuals are unaware that their energy use is being monitored. It is a genuine issue of balance, as this type of monitoring provides valuable information to society at large, yet no one wishes to sacrifice the rights or needs of the individual.

Another type of non-intrusive monitoring is biological in nature. This is seen in technologies such as non-invasive blood glucose monitors for diabetics, sensors to detect intoxicated drivers, and other tools focused on biological functioning. For example, non-invasive glucose blood level monitoring uses a sensor placed closely against the skin: "a special camera, called a Raman spectrometer, inside the sensor uses light to identify and analyze glucose molecules under the skin, via interstitial fluid. Each glucose molecule has a special 'signature' the sensor identifies, and from there, analyzes and extrapolates a glucose value, which is transmitted via Bluetooth to a handheld device, like an iPhone or Android, or to a computer" (Allison, 2011). A Raman spectrometer is a device that also measures vibrations within a system, illustrating how this tool represents both biological and vibrational non-intrusive monitoring. The significant advantage of this technology is its capacity to transmit results immediately to a phone, meaning that the user's family members or even paramedics can be alerted if glucose levels reach a dangerous point. This demonstrates how sensors are making life safer and more manageable for people with chronic illnesses. The non-intrusive nature of this monitoring is apparent: the diabetic no longer needs to prick their skin to monitor glucose levels, greatly improving quality of life.

The same principle applies to non-intrusive biological sensors capable of monitoring the level of intoxication in a driver. While this technology is still developing, the possibilities are substantial. According to Mothers Against Drunk Driving, one in three people will be involved in an alcohol-related crash in their lifetime, and almost every 90 seconds a person is killed in a drunk-driving accident: "In 2011, 9,878 people died in drunk driving crashes — one every 53 minutes" (Madd.org, 2012). Biological sensors could have a tremendous impact on eliminating or greatly reducing the number of these accidents. These systems are designed to assess the "biological condition of a driver and issue warnings during instances of drowsiness. Moreover, many researchers have reported that biological signals, such as brain waves, pulsation waves, and heart rate, are different between people who have and have not consumed alcohol. Currently, we are developing a noninvasive system to detect individuals driving under the influence of alcohol by measuring biological signals" (Murata et al., 2011). The advantages of making such technology a permanent feature in all automobiles are significant for human safety and for society as a whole. Some critics argue that such measures infringe on personal freedom, but the life-saving potential of the technology is difficult to dispute.

Acoustic and vibrational forms of non-intrusive monitoring, which measure emissions from machines and other entities in order to assess their functioning, also offer clear environmental benefits. For example, passive, non-intrusive acoustic monitoring can observe the valve movement of bivalve mollusks (Di Iorio et al., 2012). Though this may not sound significant at first, bivalve mollusks are vital ecological and economic components of coastal ecosystems: "The formation and size of growth increments delimited by striae are affected by environmental stressors. The mechanisms linking shell growth and striae deposition in relation to environmental variations are poorly understood but are likely associated with the animal's valve-movement behavior" (Di Iorio et al., 2012). Non-invasive acoustic monitoring can illuminate these issues in ways previously not possible. Similarly, non-invasive acoustic and vibrational monitoring has been used by physicians to better assess in vivo hip conditions and improve understanding of total hip arthroplasty (Glaser et al., 2010).