Computing stuff tied to the physical world

Energy monitoring

In Hardware, Software on Mar 19, 2009 at 00:01

This setup has been running here for a few months:

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A light reflectance sensor detects the revolutions of the electricity meter. It is hooked up to a JeeNode, which counts the revolutions and sends its counter value to the central JeeHub node every 10 seconds.

For basic tracking of total electricity consumption, that’s all you need. Occasional packet loss has no influence on aggregated results since the counts are tracked near the sensor. The same works for gas and water consumption.

However, I also want to get a good estimate of the consumption rates in real time. This can be done by timing the rotation period, at millisecond resolution preferably. So the first improvement a while back was to let the sensor JeeNode send packets the moment a revolution is detected, and not just every 10 seconds. That way the central node can do millisecond-resolution timing, which is easy with a stable clock and Linux.

This works reasonably well, but only if reception is nearly perfect. Any lost packet messes up the preceding and the following timing estimate. Only with two packets sent right after a counter change can the period be measured. I kept the periodic 10-second sends enabled as fail-safe, but these late packets are useless for real-time rate calculations.

The next improvement was to let the sensor node do the millisecond timing. It wasn’t obvious how, though, as sensor nodes do not have a very accurate clock: a ceramic resonator has around 0.5% accuracy and probably also some temperature sensitivity. Turns out that it doesn’t really matter: as long as these real-time estimates are not summed together, errors won’t propagate any further.

So now the sensor node sends both the counter value and the last rotation period in milliseconds. The counter value is perfect for long term energy consumption monitoring – after all, it’s exactly the same as what the power company will use to charge me later. The millisecond period is a good estimate of current electricity use, and probably fairly stable from one measurement to the next. Absolute accuracy and drift are not so important here, because the main use of the real-time estimate is to see small changes as they occur.

Here’s an example of the entries logged on the JeeHub:

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All log entries have the format “T [time] [millis] [type] …”. Where “time” is the time on the Linux module of the JeeHub in YYYYMMDD.HHMMSS format and “millis” is the current millisecond counter on the JeeNode inside the JeeHub. This might seem redundant, but keep in mind that the Linux module is not always on – it can pull data off the JeeNode when started up again. Additional timing data will come from the on-board DCF77 receiver once that is fully operational. So there are three sources of timing information inside the JeeHub: Linux, JeeNode, and the DCF77 atomic-clock reception. Each have different levels of accuracy and drift, but because all of them are logged, this can be fully determined later.

The raw text log files grow by around 0.5 Mbyte/day right now. Gzip compresses them to roughly 20% of that, so they can easily be kept around: the MMnet1001’s flash memory will hold several years of these full-detail logs.

Back to the actual log entries now.

The “HM2” packets are from the electricity & gas metering node. Packets 56, 57, 58, 60, 61, 62, 63, 0, and 1 were lost (more on that later). The electricity revolutions went from 132 to 136 in this time frame, and the gas counter stayed at 139. The rotation time for the electricity meter in packet 54 was 74*256+209 = 19,153 ms. There is a third slot of four 0’s in each packet for measuring water consumption, currently waiting for a new sensor-enabled flow meter to be installed.

Looking at some calculated results, I think this setup will be able to track real-time power consumption with a resolution of under 1 Watt. The resolution actually improves with lower energy consumption rates – precisely what you’d want.

The VOLT and BARO entries are from sensors within the JeeHub. The ALT entry is weather data from a remote sensor and comes from another system for now.

A further improvement with the energy data was to switch to the latest RF12 library code and to use acknowledgements. Every time a packet has been properly acknowledged, the sensor node stops sending data until one of the counters changes. When no ack is received, data is resent every 3 seconds. With good reception most of the time, the number of packets sent (and logged) willl be quite small. And with packet loss, the data will usually arrive 3 or 6 seconds later anyway. Note how in the above log file excerpt each counter value from 132 through 136 was recorded, but retransmits were needed to get counts 134 and 135 to the JeeHub.

Final point: the wireless communication speed was raised from 38400 to 57600 baud – and reception appears to be at least as good as before.

Packet loss – I’ve been seeing long stretches of fairly bad reception at times. Until I realized that these coincided with our wireless headphone set being turned on. I don’t know what that thing is sending out into the ether, but it’s clearly interfering with the 868 MHz communication between JeeNodes. Oh well – it’s a good test for dealing with missed packets…

  1. Great to see you progress.

    As far as your network timing goes, have you thought about adding some timing information to your ACK packets? i.e. your hub reads in your timing information reveived from the data packet then responds with and ACK packet containing an offset value (allowing for propagation and interrupt handling), the node then uses this in some form to keeps it’s clocks adjusted, e.g. node could keep a running average of this offset and use this to adjust it’s time.

    Keep up the great work!


  2. Aha, that’s a great idea. So far, the sensor nodes have free-running clocks – but now that the communication is bi-directional, synchronized clocks are feasible. I’ll keep this in mind once the hub has a stable DCF77-synced notion of time – thanks for the suggestion!

  3. Sigh…down here in Australia we don’t have a Synced time service like the DCF77 (closest service is Japan I think) so looks like I’ll try using GPS as my time base.

    Alternatively, does your JeeHub have the ability to sync it’s time via NTP? I think you mentioned that it runs linux.

    • Yes, the current JeeHub setup has an embedded Linux, including a battery-backed RTC – so that would work. The MMnet1001 I’m using is running OpenWrt and has several ntp packages: luci-app-ntpc, ntpclient, openntpd. No idea which one to pick, but I’ll keep it in mind.

  4. An alternative to DCF77 in the JeeHub could be the DS3231, see

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