Infrastructure Sensors Improve Home Monitoring

by George Lawton

A University of Washington researcher is developing a system to make it easier for individuals and companies monitor "home" activity by using strategically placed sensors on air, water, gas, and electrical infrastructure. Assistant professor Shwetak Patel calls his approach infrastructure-mediated sensing (IMS).

Patel expects IMS to be more cost effective and less labor intensive than other activity-sensing platforms. Most of these have relied on a distributed, direct-sensing approach with many sensors distributed throughout a home or office. While the materials costs for each sensor might be relatively inexpensive, the installation and maintenance costs can be quite high.

In contrast, IMS uses a single sensor in a strategic place to measure pressure signals in air vents and waterlines as well as electrical signals in power lines. Machine-learning technologies correlate physical events such as turning on a light, flushing a toilet, or opening a door with different signals. According to Patel, IMS's primary goal is to reduce the economic, aesthetic, installation, and maintenance barriers to home monitoring by reducing the cost and complexity of deploying and maintaining and activity-sensing infrastructure.Patel said that in large volumes the water, pressure, and power sensors will cost as little as US$50 each.

Under the Hood: Event Detectors

The IMS Powerline Event Detection (PED) system uses an oscilloscope plugged into any outlet within a home to monitor the signals of devices turned on or off throughout the home. It can detect electrical events with 85–90 percent accuracy, using filters to classify the signals generated when devices are turned on or off and when they're running. Each home device generates a unique signal based on its electromagnetic properties and its location on the home network. The system uses the different locations of identical devices on the circuit to distinguish between them.

This approach is based on the observation of broadband transient noise that abruptly switched electrical loads produce and continuous electrical noise. Three classes of noise sources in homes are resistive loads such as light and heaters, inductive loads such as motors, and solid-state loads such as computer power supplies. Devices such as electrical switches generate transient noise when switched on and off but no continuous noise. Others, such as solid-state loads, generate continuous noise. The PED classifies these signals into events, which it logs onto a computer.

The IMS HydroSense is a pressure sensor that screws onto any unused water outlet. It consists of a standard faucet adaptor that's closed on one end. It can identify activity and flow through any home fixture such as toilets, faucets, and showers with 97.9 percent aggregate accuracy. The HydroSense tracks the pressure wave, called a waterhammer, generated when faucets are opened or closed. This wave travels throughout the home's water pipes and can be measured at any water outlet.

Patel has tested a third system for monitoring pressure gradients in a heating, ventilation, and air conditioning (HVAC) system, The system can detect the opening and closing of doors with 75–80 percent accuracy. It consists of a set of five pressure sensors attached to the surface of a single screen on the HVAC intake tube. As people move between rooms and doors open or close, they generate a unique signal across the five sensors.

Challenges

Calibration and compound events pose the biggest challenge for all of these systems.

An installer must generate each event to be detected and record its time so that the system can calibrate the event with its unique signal. In an electrical system, this would involve turning on and off every light, fan, and appliance in the home. Calibrating the water system involves flushing every toilet and running every faucet. The HVAC system would need to track walking through and opening and closing all of the doors.

One limit of calibration is that it picks up only a particular use case. Mobile devices, such as laptops or vacuum cleaners, generate a different electrical signal when they’re plugged into different sockets. Faucets generate a slightly different water pressure signal when opened slowly or part way.

When multiple events affect the same infrastructure simultaneously, they generate a compound event in which the signals overlap and become more difficult to correlate with the individual constituents. This might happen, for example, when a light turns on at the same time as the refrigerator motor or a faucet starts running at the same time as a toilet is flushing. This is less with electrical systems because events occur over a much shorter time frame, but it could be a limitation in larger installations that attempt to track many people's activities.

A Granular Future

This technology promises to help drive both improve home automation and better electrical monitoring. Bill Ablondi, director of Home Systems and Energy Research at Parks Associates, said that consumers can reduce energy usage by 10–15 percent through better monitoring technologies. Parks Associates' surveys have found that 85 percent of consumers are willing to pay for energy monitors if they could help reduce their energy bills. But wide deployment would depend on such systems costing less than $200 for the whole house. 

There are many questions about whether monitoring systems would be sold directly to consumers or to utilities, which would give them away. Ablondi said that broadband Internet providers like Verizon are even looking at providing power monitoring and energy as a service. 

"As we migrate to the next generation of energy management solutions, the more granular data you can provide, the better. So technology like PED is likely to be beneficial," said Mareca Hatler, industry analyst with ON World. ON World predicts that home-area networks will increase worldwide from less than a million homes in 2009 to 20 million in 2013. Energy-management technology companies such as EnergyHub, Google, Greenbox, PowerMand, and Tendril are driving the market growth.

George Lawton is a freelance technology writer based in Monte Rio, California. Contact him at glawton@glawton.com.