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Building a power meter application with an 8-bit MCU

5/15/2011 05:10 AM EDT
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Luis Sanchez
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re: Building a power meter application with an 8-bit MCU
Luis Sanchez   5/16/2011 8:05:14 PM
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This is a great article. I wasn't sure that we could make power measurement with a small device and no mechanical elements. Now I see it's possible... I'm called to build one myself. I'd like to know how much power is sucking each one of my devices in my house. Measuring the stand-by consumption and so on. Now that we are all concerned with the environment and green technologies, it's an important tool to have at home. Great article! Thanks!

Muhammad.Yaqoob
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Muhammad.Yaqoob   5/17/2011 6:49:58 AM
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This is very useful design article for power measurements.

grehgrth
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grehgrth   5/19/2011 10:05:44 AM
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Sheetal.Pandey
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Sheetal.Pandey   5/17/2011 8:14:28 AM
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Electronic meters have a number of advantages over their electro-mechanical predecessors. Namely, the mechanical construction is more cost-effective and is simpler due to the fact that there are no moving parts. Is there some error in the second line, instead of mechanical it should be electonics.

jcabetas
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jcabetas   5/17/2011 11:47:09 AM
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In Spain, because of a European Directive, all meters will have to meet requirements of Open Meter. That implies a communication layer and a more need of processing power

AB345
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AB345   5/17/2011 12:01:16 PM
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Where is Fig 1. ?

fdunn0
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fdunn0   5/17/2011 4:47:15 PM
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Figure 1. is on the second page.

Haldor
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Haldor   5/17/2011 8:34:31 PM
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Figure 2 is a mess (rendered with IE V7). Does anyone else see it correctly (not all jumbled up)?

Haldor
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Haldor   5/17/2011 8:37:32 PM
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Figure 1 doesn't make much sense. Drawing power from the mains neutral? And isolating the high side voltage and current measurement from mains power is also not a trivial task. Did the author actually build this power meter? How about a link to a schematic?

RadomirKozub
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RadomirKozub   5/23/2011 8:23:04 PM
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In meters it is usual to have Live wire as system ground, which enables you to use shunt measurement on Live to avoid disconnected neutral tampering technique.

Jerry.Brittingham
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Jerry.Brittingham   5/18/2011 2:03:46 AM
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Where is the schematic?

Efried
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Efried   5/18/2011 8:13:26 AM
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I don't know why WLAN routers/switches don't have embedded web servers with S0 and 1wire interface. Would ease home automation so much.

elektryk321
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elektryk321   6/20/2012 9:16:24 AM
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1 wire is custom interface, hard to make "own" devices. Serial port port more usable, but still will require to put some kind of application inside router. You can always buy some low power PC with 2 ethernet and 1 wifi interface and made own router with using Linux.

Laxman.Karandikar
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Laxman.Karandikar   5/18/2011 8:57:05 AM
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The equation of ENOB does not give stated result.

apummer945
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apummer945   5/18/2011 9:39:40 AM
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it would be very practical to have the e-mail address of Radomir Kozub around that article

RadomirKozub
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RadomirKozub   5/23/2011 8:23:49 PM
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radomir.kozub@freescale.com

kendallcp
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kendallcp   5/18/2011 5:12:19 PM
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The relationship between meter reading error and input power for any given ADC is very subtle, and a broad-brush statement of an ENOB requirement completely fails to capture it. If the ADC's specific linearity behaviour is not good enough, averaging will also not help. You just get the same wrong number with less variability. When trying to persuade meter manufacturers to accept a general purpose micro rather than using a dedicated design that's characterized for this specific role, you need to show a great deal of proof that the impairments in the signal path produce predictable and bounded errors in the power measurements. Most of these errors could be calibrated out, but no meter manufacturer wants a calibration process that takes many minutes to run. Single-point is the way to go, and that can't compensate for any gain linearity errors, either in the ADC or in the current sensing element.

RadomirKozub
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RadomirKozub   5/23/2011 8:25:05 PM
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Thats true, but having only ENOB spec, you have to deal with it .. :)

UnderboatBoy
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UnderboatBoy   5/19/2011 4:15:58 PM
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PDF it or forget it... Sorry

B.V.Rao
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re: Building a power meter application with an 8-bit MCU
B.V.Rao   5/20/2011 5:53:28 AM
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I agree with about ENOB not quite capturing the meter performance. I also fail to understand how averaging can be done on samples, without reducing effective sampling rate, where phase relation and measurement of at least 3rd harmonic is very important. Flash life is not mentioned in the article, will it replace EEPROM where load survey information need to be logged? I have designed meters with controllers from 3 different manufacturers and meeting (qualified to) the IS standards (very close to IEC, in some places more stringent). My experience is as follows: 1. Even a 10bit converter with appropriate front end analog circuitry will meet the 1% requirement, needs multi-point calibration and time. 2. when using a multiplexed ADC the phase correction algo is vital for accuracies in lead/lag 3. Some (Most?) of the SOCs are found lacking the required code/data space to fully implement various comm. protocols and event logging. For this Vendors do provide solutions that are difficult to maintain. While surveying the market I have even found some meters with non-linearity (still meeting the spec. as the spec demands testing at fixed points) In my view a 16bit (min), differential input, Ī500mV, 500usec(minimum) ADC is the best bet for metering application. Any GP micro with h/w multiplier and good memory space, LCD driver, serial ports etc., would do for rest. Finally, it is BOM cost and not technology any more in this segment. You choice should depend on how much you can bargain from your chip vendor.

kendallcp
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kendallcp   5/20/2011 6:36:46 PM
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One significant trend is to go away from a single-chip solution. By separating the metrology from the user interface and comms pieces, a manufacturer can get the legally-binding stuff sorted out but still alter the product for various utilities, different AMR/AMI schemes, different logging/billing requirements and so on. It's possible to achieve spectacular performance with recent devices like the Cypress PSoC3 - I spent months deep-diving into that. In that case, BTW, I eliminated the single-ADC multiplexer delay problem with a polyphase FIR fractional interpolator. We easily beat Class 0.2 and OUML R46D specs across the board and were 6x better than the 62053-23 reactive specs, in all 4 quadrants, with a single-point cal. But as you say, it's not about the specs, but cost and also comfort factor; it costs a lot to get certification done.

RadomirKozub
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RadomirKozub   5/23/2011 8:28:40 PM
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Hi, WELMEC is coming, thats true... we will deal with this in the future.

byteme..1
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byteme..1   5/20/2011 7:06:24 PM
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Make vs buy - check out the Kill-A-Watt product sold on ebay and elsewhere - I've seen them in Home Depot. Great where "close enough" is sufficient. I think "Kill" is based on the Cypress PSOC. The Cypress site has design ap notes for an ac meter application. I don't remember/believe if the PSOC ADC is 14.5 bits but what really is? Check out "Understanding the impact of digitizer noise on oscilloscope measurements" by Jit Lim of Tektronix.

kendallcp
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kendallcp   5/21/2011 1:19:26 AM
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My colleague Dave van Ess did some cool AC power measurements by XORing the 1-bit outputs from a couple of del-sig modulators. This enables the modestly powered PSoC1 to do a usable job in such products. The work I did was on the recently introduced PSoC3 which has a lovely quiet, clean, flexible del-sig ADC. But therein lies an important concept; it's the inherent linearity of the ADC, not its SNR-calculated ENOB, that determines power measurement accuracy. Uncorrelated random noise in the current and voltage channels produces a net zero energy contribution. My simulations indicated that you could get good metrology results just taking the raw 3-bit output from our modulator. The point is that ENOB is a misleadingly irrelevant metric for this task. You need to take much more of an audio perspective. Departures from power measurement linearity at low levels are determined by small-signal non-linearity, not either large signal non-linearity or small-signal noise.

Amer Zeater
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Amer Zeater   5/21/2011 8:55:43 PM
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I was wondering about the anti-aliasing filter design problem especially in case of high value of THD at power line System .. How the did the standards describe this problem demands ( Pass band flatness , at which order of harmonics we should stop acquisition and therefore at is the best value of the cut frequency and stop frequency of the anti-aliasing filter ... I hope some expert would answer me .. thanks

kendallcp
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kendallcp   5/22/2011 6:19:36 PM
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The metrology specs don't specifically require measurement of harmonics - this is just for test and diagnostic instruments. They do define some voltage and current waveforms containing harmonics and state the accuracy that should be achieved when measuring them. It's not difficult to meet these. Aliasing of harmonics present in both current and voltage doesn't actually affect the measurement of energy, so all that's required is a modest input filter to take off significant high frequency noise, which would otherwise spoil your determinations of V**2 and I**2 which are needed for apparent power assessment.

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