Intelligent sensors for smart cities

Smart cities’ is a term that is bandied around a great deal these days. Urban planners and infrastructure engineers believe that inserting intelligence into utilities distribution — electricity, gas and water — and into the crucial elements of city structures, such as bridges and buildings, will help to optimise the flow of resources, minimise energy use, provide accurate billing for domestic and business consumers and schedule structural maintenance.

However, for all the possible advantages, there are technical challenges. One of the most obvious is in how to gather all the information needed to operate such a widespread system. It’s a problem of sensors — they must be robust, cheap, easy to maintain and reliable, as their readings will be used to determine billing.

‘Traditionally, gas, electricity and water meters have been mechanical,’ said Nick Collier, head of science and technology at UK development consultancy Sagentia. ‘They’re low cost, they meet the accuracy requirements and they’re really well trusted. But with the advent of smart metering, you need to have remote sensing of meters, which means that you have to think about electronics and power.’

Sagentia works in sensor systems for both medical and domestic/industrial applications and has a specialism in finding low-cost ways to find sensing solutions for its clients. ‘My group has PhD-qualified physicists, mathematicians, chemists and materials scientists who have a very broad understanding of the types of things that can be measured; we apply that to a lot of customer problems,’ Collier said.

‘We’re particularly strong in non-contact position sensors, which work by electrical induction; we also have optical sensors that detect fluorescence and colour, image sensors that use low-cost cameras and acoustic sensors for ultrasonic flow metering.’

Position sensors are useful for rendering mechanical meters machine readable. One example was a project with Mastermeter, which makes water meters that work on the odometer principle, with a brass body containing a turbine wheel that spins in the water flow, operating a mechanical odometer with a series of numbered wheels labelled from 0 to 9, exactly as you’d see in an older car. ‘The industry still likes those, because they’re easy to read and they work even if the electricity goes down, but they needed to be computer readable,’ Collier said.

The solution used non-contact position sensors, he added. ‘We mounted a very small, round printed circuit board [PCB] into the side of each wheel, which is metallised. Next to that, we mounted another round PCB with a series of tracks on it, which act as antennas. We pulse those at several megahertz and look for a reflection from the metal on the wheel-mounted PCB. That tells us what position the wheel is in, and looking at the signal from all of the wheels gives us the reading.’

The industry still likes mechanical odometers because they’re easy to read and they work even if the electricity goes down

Nick Collier, Sagentia

One advantage of this technology is that the sensor itself requires no battery. ‘It’s powered by the act of reading it: the reader carries a powered wand that sends the pulse signals into the static PCB,’ Collier said. ‘The meter could be down a pit in quite a nasty environment, but the pit lid would have a contact pad on it that the reader could touch, and that would power up the sensor to give a reading. That’s good for the smart metering world.’

Sensors that require no battery need much less maintenance, he said, and can be deployed in large networks. ‘You have to think about how to power the output and how it’s going to communicate with the outside world,’ added Collier. ‘And the challenge is actually more on the radio-frequency [RF] side — if you look at the power distribution between meter reading and communication, it’s the communication that takes more of the power.’

Sensors that require no battery need much less maintenance and can be used in large networks

One way around this is to power the system using energy harvested from the environment. Sagentia designed a system for district heating systems that use steam pipes. ‘We’ve developed a system that uses the differential between the high-temperature steam and the lower ambient temperature to generate enough electricity to run the sensor and to transmit its reading,’ said Collier. The system uses arrays of thermocouples in parallel to generate a current flow.

Other solutions have been developed by companies such as EnOcean, a spin-off from Siemens. These use a piezo-electric material that contracts on heating, generating a voltage that sends a spark across a gap. This generates a radio signal that can be picked up by a reciever base station. ‘What’s clever is that the act of sensing a change of temperature generates its own signal — you only communicate when the temperature changes,’ Collier added.

Other approaches use a small transformer to step up the few millivolts generated from photovoltaics or vibration-powered energy-harvesting systems to about 3V and using that to constantly trickle-charge a small battery.

‘We’re now seeing all the elements coming together for commercially viable systems of independently powered RF sensors that don’t need to be hardwired into a network, and the challenge for companies like ourselves is to act as system integrators to find the right market for them,’ Collier said. ‘They aren’t lower cost than a battery — the advantage is in the increased functionality and reduced installation costs. Factory automation is probably going to be the biggest initial market for these systems, with smart cities following on later, but I’d hope to see such systems in the domestic or office environment within a few years.’