Measuring Power Use with Complex Data Logger Duty Cycles

There is an old saying that goes: “Yesterdays solutions are today’s problems” and I think that now describes this project. You see the first year of development was focused on pretty straightforward hardware issues, and solving each one produced significant gains in performance. Now that I am consistently seeing sleep currents in the 0.1-0.2 mA range (with an SD card (~80uA) & live Adxl345 (~50uA) along for the ride), I am hunting for more elegant ways to extend the operating time while maintaining the simple three component core of the original design. With a 3xAA power pack now providing almost a year of operation for some sensor configurations, I also have the task of developing a method for testing the loggers that can discriminate between subtle code changes with relatively short runs. But artificially reducing the sleep interval between samples distorts the result enough that it’s hard to make good projections. I am slowly coming to realize that testing & calibration are the real heavy lifting when you build any new device.

The new A544 cells arrived at > 7 volts which was too high for the regulator on the Ultra. So I took them down to 5.6 volts with a Zenner that stops the discharge before it goes too far. The rare earth magnet soldered to the leads gets zapped by the heat from the iron, so you need a second little magnet to hold each battery connection securely.

Each new trick I try, like finding another place where I can put the cpu to sleep, adds complexity to code that has “once per day events” and “once per week” events, and soon there will be “only if the delta between two readings is greater than x” events. Most of these are so short that a multimeter can’t catch them but even when a friend donated an old Tektronics to help me try to get a handle on the duty cycle, I faced the challenge of displaying currents ranging from less than 0.1mA to 80mA SD writes with variable duration. To make things more interesting, some of the cheap sensor boards I have been noodling around with have components of unknown origin & dubious quality, which introduce yet another set of variables.

Even with my mediocre scope-skills the forums had convinced me that the SD card was the elephant in the room. So I tried to avoid SD use by adding an external 32k eeprom which let me buffer five or more days worth of data before having to fire up the external storage. Problem solved…or so I thought. I was quite surprised by data from the last deployment that showed using this approach to reduce SD writes by a factor of five only delivered a 5-10% improvement overall.  I had overlooked the fact that the AT24C256 eeprom pulls 3mA for five milliseconds per pagewrite. This was nearly as much current as the Rocket Ultra I was using, not to mention a significant extension of the cpu uptime for multi sensor units that were buffering up to four eeprom pages per record. All of that activity adds up.

So I took another look at buffering data in SRAM, which I flirted with at the beginning of the project. But my script was now much larger than those early versions, leaving barely 500 bytes free.  I know the real coders out there probably laugh at my use of Pstring & Ascii but that lets me add a new sensor by changing a couple of print statements, and adaptability has always been of my primary design goals. To maintain that simplicity I went searching for an Arduino with more headroom and the Moteino Mega over at LowPower Labs seemed to fit the bill with it’s 1284P offering an extravagant 16K of sram (compared to just 2K on the 328p). It also used a low drop out MCP1700 series regulator like the Ultras, and there was support for RFM transceivers.  With the Mega’s larger footprint, I decided to try them first on the larger dry cave platforms:

Rocket Ultra (left) VS Moteino Mega (right) based data loggers with pin powered RTCs and 2×4.7MΩ voltage dividers monitoring both the power supply voltage and the rtc backup battery.  I break out LED, I2C and one-wire with Deans micro connectors, and you can see a 4.7K one-wire pull-up above the main divider on the Mega. A 32K eeprom is tucked away under the rtc breakout, which I flipped over on the Moteino build to make it easier to change the CR2032.

For a standardized test, I set both loggers buffering 96 records (= one day @ 15min intervals) in drip sensor configuration. I added the I2C eeprom to the Moteino logger to make the builds as similar as possible, but it does not get used. Instead I store the raw sensor data in integer arrays. So there is no Pstring/ascii use on the Mega logger until I write the data to the SD cards.  With matched acclerometers & cards, both loggers sleep at 0.18 mA so the the only difference between them should be the data handling.  One thing I did not catch from the LowPowerLab specifications was that the 16mhz Mega draws ~12 mA (while awake) in this configuration as compared to the Ultra builds which perk along at just over 4mA. I figured that with SRAM storage the mcu up time would be so much shorter that it would not matter.

With a parallel bank of super caps to buffer SD events, you can drive the loggers with small batteries that have high series resistance. Rare earth magnets let you connect without a holder and you can build multi-layer magnet/button cell stacks to create low power options at different voltages. Those are 5v 1farad supercaps so I don’t bother to balance them as they should be able to handle any leakage asymmetry when the battery input is only 5.6 volts. The graphs below had no low volt blips at all, so this series/parallel arrangement of 4 of them was probably more capacity than I needed.

I still had not sorted out the oscilloscope issues but I realized that I could flip the problem around: instead of struggling to display the effect of every little tweak to the duty cycle why not provide a fixed amount of power and just see how long the unit runs. It’s a data logger, so I already have a time stamp and a battery voltage reading with every record.  A couple of people suggested capacitors, but even a 1F supercap only gives you about 0.27 mAh per volt, translating into a few hours of operation for my loggers. I needed longer runs than that because the Moteino was going to loose the data in it’s sram buffer when the unit browned out (I can always dig into eeproms later for the last few records on the Ultra).  A bank big enough for multi day runs was going to be expensive, and is probably a hazard for my little bots.

Fortunately there are a host of small form factor batteries out there for things like fire alarms, medical devices, etc. Energiser’s A544 seemed to fit the bill at 6 volts & 150 mAh: promising to power the Pearls in “sleep current” mode for about 40 days. Even better, they were alkaline cells, so their discharge curve would be more like the AA’s used in real world deployments. There was some risk that these little cells would drop to the low voltage cutoff when the SD write current spikes occurred, so I added a few super caps to buffer those loads. I then set the units up on a book shelf where they would not be triggered and waited for my “baseline” load result.

This is the voltage record from the two different logger platforms, when they were powered by a single 150mAh A544:

(I stopped the test after a month, because I couldn’t take these suspense any longer. There were few sensor interrupts during the test, so this was a baseline power use comparison)

I was sure the SRAM buffering Moteino logger would come out far ahead of the Rocket build that was sandbagged by all that I2C eeprom traffic. But if you correct for the slightly higher starting voltage those two curves are so close to each other they might well have come from the same machine. So there is no longevity boost from SRAM buffering if I use an mcu that draws 3x as much current, but at least I now have a good way to test the loggers without waiting too long for results. This result also agrees with some of my earliest drip sensor results which hinted that the sampling/buffering events were consuming 2/3 of the power budget.

For the next round of tests I will put them on the calibration rigs to see how the A544’s handle the interrupts being triggered all the time. Presumably the Moteinos will draw more power there so I will need to normalize the results to match drip counts. To go beyond the conservative one day buffering I will need some way to capture data from the SRAM buffer before the units power down, so perhaps I will end up using the eeprom on those Moteino Mega builds after all. We will use a few Mega based drip sensors set for very long buffering (8-10 days?) on the next real world deployment. I also have a feeling that the DS18B20 temperature strings would benefit more from SRAM buffering than these simple drip sensors, as they poll up to 40 sensors per record. That’s a lot more data to shuffle around.

Addendum 2015-07-05

Hackaday just posted about [Majek] putting “live” data into Arduino’s flash ram (which is normally not accessible after startup) via a Optiboot hack.  This opens up another possible data buffering strategy, though I am not sure if it could handle the duty cycle of a long deployment. Or it might let you do calculations with the 328p that would otherwise run out of space.  So this is an interesting development that involves no extra hardware, which is usually good news for the power budget. I had already been wondering if calibration data could be stored in flash with Progmem, but that solution only works for data that is not changing all the time.

Addendum 2016-01-06

We finally have some data from the first field deployment of Moteino based loggers which store sensor readings in ram (array variables), rather than buffering all the data as ascii characters in an external eeprom like my 328p based loggers do.

Here is the power curve from a Moteino:

Battery (mV): 3xAA supply, 0.18 mA sleep current, 5 days of data in ram

And here is a directly comparable build using a rocket scream ultra with a slightly higher drip count (ie: number of processor waking events) over the duration of the deployment.

Battery (mV): 3xAA supply, 0.18 mA sleep, 5 days of data buffered to AT24C256 eeprom

So once again, these performance curves are so close that it makes no odds. But on the bright side, this confirms that accelerated testing with 150mAh A544 batteries does give me results that translate into real world. So this is still pretty good news even if the 1284’s did not deliver the magic performance bullet I was hoping for.

Addendum 2016-02-15

If I wanted something a bit beefier than the 150 mAh in the A544’s, I could hack my way into a 9v battery, and use half of the set of 500 mAh AAAA batteries you find inside. That would give me about 1/4 the power of the AA batteries I typically use on deployment.

Addendum 2016-08-15

I finally figured out how to view  individual logger events using an Arduino UNO as a DAQ with the serial plotter tool built into the IDE:

  I’m quite tickled about being able to replicate a task that you normally would need an oscilloscope to see.  Of course my chances of actually catching one of those big unpredictable SD card latencies (from something like  age related wear-leveling) is still pretty low, so I will continue to use this A544 method for solid longevity predictions.