With the soaring cost and scarcity of resources, accurate metering is becoming increasingly important for both utility companies and householders to monitor usage and plan for demand.
UK technology consultant Sagentia has developed a system based on mathematical principles that allows a common type of water meter to be read remotely for the first time.
Automatic Meter Reading (AMR) technology is an essential part of utility companies’ armoury, automatically gathering data from metering systems and transmitting it to a central database.
As water meters need to remain in the ground for a number of decades, by law in some parts of the world there must be a mechanical means of logging the amount of water used, as batteries could go flat or electric systems could fail.
This is usually done in a register, the top part of the water meter, with a combination of odometer-style wheels and a dial. Encoding is a way of electronically reading the mechanical position of the wheels, which allows AMR to be carried out.
The reading can be collected using a handheld data logger, a low-powered radio transmitter in the meter picked up by a reading van, or the meter can be connected via a modem to the telephone line, which also powers it.
There are two principles of domestic water metering throughout the world. One is positive displacement metering, which works like a piston in a car, measuring out a fixed quantity of water every time it goes round. With these meters, there is a lot of energy available from the water to drive the register round, with enough left for a system to detect the physical location of the wheels.
The other, multi-jet meters, work like a turbine, with an impeller that sits in the flow of water. As it is resistant to blockage, it is used extensively in regions of the world prone to particulates. Available energy is limited, and any small amount of friction on the register that might be caused by a detection system could cause it to under-read significantly.
Cambridge company Sagentiaworked with client Master Meter to develop AccuLinx, a contact-free water meter register that can be used with multi-jet meters without affecting the reading. Master Meter is a US company owned by an Israeli corporation, with markets in the US, Canada, Europe and South America.
Robin Lee, head of science and technology at Sagentia, said: ‘At the very beginning of AMR in this industry, they used to put little gold fingers and switches on all the wheels that went round and they could use a system of switches to encode the wheel positions. You can’t do that with multi-jet meters as the tiny bits of friction you get from the wipers and the switches is way too much.
‘There has been a need in the industry to have an encoding register which is very low friction, which in general has meant for AMR you mustn’t touch the wheels — you need to have a non-contact encoding technology that doesn’t introduce any friction.’
Sagentia’s solution, which lies behind AccuLinx, is dubbed Gray Wheels, a robust and power-efficient way of encoding the position of the odometer wheels without touching them. The power used comes from the meter reading technology.
‘At the time of reading, the meter reader comes along with a data logger. When they power it up to read the meter, the register steals a small amount from their data logger. It powers up for a second, reads the position of all the wheels then turns off again,’ said Lee. ‘The electronics only gets turned on for that moment every month, or whenever they read the meter.’
AccuLinx can be used with multi-jet meters without affecting the reading
Gray Wheels uses a switch technology called a gray code — a binary way of encoding position that can encode position around a circle. This means a binary code series of ones and zeroes can be used around a register wheel.
‘We tried to embody that with a magnetic principle of eddy current sensing,’ said Lee. ‘The trick was to find a gray code pattern than could be fitted around one of these wheels that was not a power of two like they normally are, and was magnetically balanced — in other words it has an equal number of ones and zeroes as you go round the circle.’
In a register using Gray Wheels, each odometer wheel carries a small conductive charge that can interact with a pattern of coils placed between each of the wheels. But eddy currents could be set up due to their proximity, unbalancing the arrangement and causing the receive coil to pick up a signal.
‘The insight we had was that gray codes exist that have equal numbers of ones and zeroes that go round in circles and form gray code patterns that didn’t have a power of two,’ said Lee. ‘That combination of things forms a very special spatial pattern, which was the secret to getting Gray Wheels working.’
Another problem Sagentia had to overcome was the fact that there are a number of different meter bodies the register needed to fit on, from small, domestic 6in diameter types to street mains. It also had to be usable in many countries, that record usage in a variety of units, including US gallons, cubic metres or cubic feet.
‘This means you need to have hundreds of different variants of gearbox on the same metre body so the register on top can measure all these different units, each within a certain standard accuracy,’ said Lee. ‘Traditionally, people designed a new gearbox for every variant they needed, which is expensive in terms of plastic injection moulding and tooling.
‘We realised that using a particular number of teeth on the gears and employing number theory developed by my colleague, Dr Ross Jones, we could make a general purpose gearbox where we could build the gearboxes up to any gear ratio we liked within reason, to the accuracy we needed, using only 17 gears.’
Sagentia can now build hundreds of different gearboxes using a ‘Lego set’ of gears which has dramatically reduced the amount of tooling needed for the gearbox.
Another change Sagentia introduced was to move away from the standard six-wheel encoder to eight wheels, to help Master Meter compete equally with a key competitor that offers eight wheel encoders. Utility companies often specify eight wheels in tenders.
Sagentia overcame two key challenges to deliver AccuLinx, said Lee. ‘There was the technical problem of initially finding the right gray code spatial pattern and optimising it in a way that’s going to be robust enough for a project like this. Then we had to make it fit with all the standards and requirements, but at a lower cost than the competitor solutions.’
Gray Wheels could be used on any kind of rotary encoder. ‘It comes into its own as a multiple turn rotary encoder, such as where you have a big wheel and a handle that gets turned through many revolutions to open and close a sluice gate. These get encoded so the companies know the position of their sluice gates,’ said Lee. ‘A steering wheel encoder is something else people are looking at, though steer by wire could be a way off.
‘The key advantage which makes it good for the water meter application is that it’s a non-contact method that doesn’t introduce any friction. Automotive companies like their encoders to be non-contact for other reasons, as any sensor is prone to wear, and if you have any vibrations the contacts can jump apart. We’re hopeful we can find some other niche applications for Gray Wheels, too.’