07/08/2017

Test 3 of the Asset Tracker went pretty much the same as Test 2 except I got almost twelve hours of continuous logging but otherwise I see the same kinds of corroborating data along with absolute position from GPS. The one big difference was power consumption. First of all twelve hours is the longest I have gotten the Asset Tracker to run on any LiPo battery, in this case an 850 mAH capacity battery fully charged. But according to the battery voltage data:

the battery was only down to 4.00 V after the 12 hour run. If I assume a linear drop off to 3.7 V this would mean about 36 hours of run time at an average current of 24 mA. Still much larger (by about 2x) than I expect but much better than the ~50 mA I was measuring when I had neglected to disable the ESP8285.

I ran another power test with a fully-charged 350 mAH LiPo battery and the Asset Tracker lasted for exactly 20 hours for an average power usage of 17.5 mA; now that is more like it!

In addition to disabling the ESP8285 I enabled stillness mode on the EM7180, which allows the EM7180 to put its sensors to sleep when it detects no motion. I verified that this works by noting on the TFT display during the test that if I picked up the Asset Tracker or nudged it a bit, the yaw, pitch and roll would respond and keep changing slightly after the nudging stopped, but after a few seconds would freeze to fixed values.

So for the car logging application I am nearly done. At 17.5 mA average power I can get 48 hours of more or less continuous logging of vehicle (or animal) motion in a small, wearable package. Car tracker, cow tracker, cat tracker...

And the CAM M8Q is doing what I asked it to do:

It is taking 5 - 10 seconds of GPS data (I am showing pressure and altitude here but my STM32L432 program only records data if there is a GPS pps pulse--a valid fix--as another low power strategy) every minute like clockwork (twenty minute scale above) with ~1 minute of satellite acquisition and ephemeris update every ten minutes.

The Asset Tracker + size-matched 850 LiPo battery is about the perfect balance of location/time resolution for fast moving objects (5 - 10 seconds per minute) and power usage (48 hours). But not logging capacity, since 48 hours is only ~3600 out of 65536 pages. Maybe I just need a 1 MByte SPI flash...

The next class of application is the Sentinel or Security Guard, if you will. In this case, the Asset Tracker needs to acquire a fix once an hour or once every eight hours and report via wifi to a server that all is well. Also, if the Asset Tracker detects that the object is moving it should alert by sending a wifi alarm. In this application 48 hours is not going to do it. If the Asset Tracker can last 48 hours with an 850 mAH LiPo battery updating its location every minute I would hope that it could last, say, 120 days reporting its position every hour and 1440 days every twelve hours, etc.

In some applications like this, say, monitoring cargo containers at sea, the device reports its location once per day and is expected to remain active for years. Here is an example of 18 months of twice-per-day tracking using an 8800 mAH battery. This is less than the longevity I am projecting with the Asset Tracker. Of course, I haven't factored in the power required by the ESP8285 to send state information/data via wifi, nor have I actually demonstrated this mode of operation, etc. Lot's to do...

I usually set up a web server to report data via wifi from a remote sensor with an embedded ESP8285 but the web service times out after 30 seconds or so of non-activity and this is not a good match to this kind of infrequent reporting anyway. Better would be an ESP8285 dongle attached to a laptop that can receive UDP messages and store them on its internal 1 MByte flash memory and/or send them on to the serial monitor for continuous observations, like might be done in a plant security control room.