Monitoring an old electricity meter (rotating disc) - Page 1
Forum section: electronics / projects

author: JJalling
author_rank: Unknown
timestamp: January 08, 2016, 07:59:33 am
content: Hi all,In my house the electricity consumption is measured by an old electricity meter with a rotating disc (same principle as this:https://en.wikipedia.org/wiki/Electricity_meter#/media/File:Hydro_quebec_meter.JPG). The edge of the disc is blank polished with a black mark.So in order to monitor the consumption, I want to count each rotation of the disc, as well as the time for each rotation (RPM).I have made simple test with a phototransistor and a IR LED and it works ok (I use a CNY70).My problem is, that besides the intended black mark on the disk, there is a thin vertical black line as well. When my power consumption is high enough (above 750W) the thin line isn't detected, because the disc is rotating too fast, but at night, when the consumption is low, I get false readings because of the thin line.Do you have any ideas on how I can improve my setup? Either by making sure to detect the thin line at all speeds, or by making sure that I never detects it.BR Jonas

author: daqq
author_rank: Super Contributor
timestamp: January 08, 2016, 08:47:45 am
content: You could try detecting not the disc, but rather one of the dials.Or you could scan the disc in two places, far enough from one another that there will be a certainty that only the thick black spot creates a signal on both, and AND the output from both.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 08, 2016, 08:48:17 am
content: Greetings:When I worked in Energy Management, we often wanted to track each turn of the disk. The Electric company seemed more than willing to swap out the meter for one with a "dry contact" 1PPR output for us to use, and the calibration was marked on the data plate (I think it is the Kh Number). There may have been a one-time nominal charge, but I'm not sure. It was the perfect solution.Otherwise, you might circumferentially broaden the source/detector field-of-view to reduce sensitivity to narrow marks, or use a second detector, closer to centre (ISTR, the wide mark was on the periphery, and the narrow line extended further in), and use anti-coincidence gating.HTH.Cheers,Dave

author: coppice
author_rank: Super Contributor
timestamp: January 08, 2016, 08:57:16 am
content: The test benches used to calibrate utility meters at the factory count the turns of the disk by monitoring red light (it always seems to be red) reflected off its surface.

author: JJalling
author_rank: Unknown
timestamp: January 08, 2016, 09:23:50 am
content: Quote from: daqq on January 08, 2016, 08:47:45 amGreat idea!! Thanks.BR Jonas

author: JJalling
author_rank: Unknown
timestamp: January 08, 2016, 09:25:58 am
content: Quote from: CaveMannDave on January 08, 2016, 08:48:17 amHi Dave,Yes, thats indeed an option. Normally I would do it, but all electric meters in Denmark will be replaced with smartmeters within the next few years, so I don't want to spend a lot of money to get the meter replaced.BR Jonas

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 09, 2016, 03:54:35 am
content: Greetings:The new meter is probably available with 1PPR equivalent output, too, and they often have no disc to read.I hesitate to modify (or add to) meters, myself, because it could be construed by some bureaucrat as tampering, and that's not a fight you want.Cheers,Dave

author: jh15
author_rank: Frequent Contributor
timestamp: January 09, 2016, 05:52:17 pm
content: Be careful what you put around your meter:

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 09, 2016, 06:09:07 pm
content: So use another sensor for the AMBIENT light level and automatically adjust the sensitivity of the CNY70 accordingly. Although I am amazed that you can get any kind of reliable signal out of a sensor like that from such a distance.I agree that putting anything on your power meter is a significant risk. The people that come out and inspect meters aren't the sharpest knives in the drawer. They are trained to automatically assume that ANYTHING is an attempt at stealing electricity by tampering with the meter.Is there some reason you don't simply put current transformers around the line and measure current flow passively like everyone else? There is a very good reason that method is so popular.

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 09, 2016, 10:54:12 pm
content: Quote from: Richard Crowley on January 09, 2016, 06:09:07 pmGreetings:I like Rogowski Coils [https://en.wikipedia.org/wiki/Rogowski_coil] w/integrators because of many reasons: low cost, install on intact conductors, small, light, safe (no danger on open circuit secondary), etc. Put them in your indoor panel-board, and just use a microP with A/D converters to totalize consumption.AND, they are easy to home-build: lots of plans on the interwebz.HTHCheers,Dave

author: CaveMannDave
author_rank: Regular Contributor
timestamp: January 10, 2016, 12:46:28 am
content: Additional info:Analog Devices ap notes are where I discovered Rogowski coils, check these parts (column 4 tells sensor type):http://www.analog.com/en/products/analog-to-digital-converters/integrated-special-purpose-converters/energy-metering-ics.htmlA good paper:http://www.analog.com/media/en/technical-documentation/technical-articles/16174506155607IIC_Paper.pdfSome info from DigiKey:http://www.digikey.com/en/articles/techzone/2013/oct/efficient-measurement-of-electrical-power-on-the-smart-gridOther stuff:http://forum.arduino.cc/index.php?topic=54672.0http://www.datasheetarchive.com/rogowski%20coil%20analog%20integrator-datasheet.htmlhttp://www.wago.com/infomaterial/pdf/51285476.pdfI hope you get something out of this.Cheers,Dave

author: coppice
author_rank: Super Contributor
timestamp: January 10, 2016, 09:46:45 am
content: Quote from: CaveMannDave on January 09, 2016, 10:54:12 pmRogowski coils have some good qualities, but some really bad ones too. They work well for large currents, and are popular for measuring thousands of amps. They are cheap, simple and have no core to saturate at these high currents. For low current use, however, they are very troublesome if you need high accuracy. They are rather susceptible to external fields, bits of metal in the viscinity, temperature change, and vibration. This leads to low current Rogowski coils being largely limited to protection system applications, where a few percent influence doesn't invalidate the usefulness of the device. For precision measurement, like utility energy meters, lots of people have experimented with Rogowski coils, but few put them into production. I think Sensus is the only current major manufacturer of consumer energy meters based on Rogowski coils. Their coils required a lot of development to tame the influence issues to the point where an accurate meter was at all possible, but I am unclear how well they work in practice.

author: JJalling
author_rank: Unknown
timestamp: January 10, 2016, 03:55:34 pm
content: Quote from: Richard Crowley on January 09, 2016, 06:09:07 pmI get plenty of signal, and I'm already taking ambient light into account, as well as auto adjusting the emitter to obtain the largest S/N ratio. The distance is about 10mm, so no problem there.I have no problem in putting electronics around my meter. The meter is placed inside my house, so I can remove whatever electronics around the meter, if the electrics company comes around.You are right, I could put a CT around each of the three phases, and use that, but that requires more electronics than just a PIC micro - I make smartmeters for a living, so I could also just borrow a sample meter from work and install that.BR Jonas

author: Kalvin
author_rank: Super Contributor
timestamp: January 10, 2016, 04:49:53 pm
content: - You could add a led to give a constant ambient light.- You could measure the analog voltage/current of the detector with the microcontroller and do some simple  filtering before detection.- Like others has suggested, try the red light instead of the infrared if you could get better results.

author: JJalling
author_rank: Unknown
timestamp: January 10, 2016, 04:56:13 pm
content: Quote from: Kalvin on January 10, 2016, 04:49:53 pmBut the transistor isn't that sensitive to visible light, so a red led wouldn't do much. I have added another CNY70 next to the first one, to be able to measure at two points at the same time - it works great.BR Jonas

author: Kalvin
author_rank: Super Contributor
timestamp: January 10, 2016, 04:57:52 pm
content: Quote from: JJalling on January 10, 2016, 04:56:13 pmExcellent!