Month: February 2014

Our smallholding is an island site with no power and no broadband. No power means things like a Raspberry Pi are marginal – we’re looking at a current drain of about 500mA for a Model A. For a 85AH leisure battery of which you only want to use half the capacity for good service life that’s about 80 hours. So it’s time to look at the system architecture of a remote sensing network to try and reduce the power used at the remote site. The aim is to offload the processing for graphing to a site with mains power (home), so the system has sensors, a gateway that collates all the sensor data and sends it via SMS in my case, and a data processor.

Remote sensing network system architecture

At a site with mains power the gateway and data processor can be the same thing. The architecture of such a system is therefore two-level –

• many sensors out in the field
• a gateway/data processor

At home I have a Raspberry Pi that collects data from my sensors using a RF board, Ciseco’s Slice of Radio. The same Raspberry Pi runs RRDtool to do the graphing and SFTP to put the graphs onto the web.

At the farm, however, because of the power constraint I need to use a gateway to transfer the data onto the mobile network using SMS.

That sort of site typically consists of three levels –

• many sensors, out in the field doing the data collection,
• a gateway, that collects all the sensor data
• a data processor – that consolidates the data from the sensors, graphs it and saves it, perhaps uploading to the web

So the next stage down from a Pi is something like an Arduino

The Raspberry Pi can be at home so its power consumption isn’t an issue any more. It takes an SMS message of various sensor readings and munges this via Python and some scripts into a RRD database which then goes with RRDgraph. Which gives the single-sensor plot as shown below. The battery voltage chart shows that something rather nasty is happening to the main battery – it looks like the charge controller is not disconnecting the solar panels from the battery when the battery voltage is high enough – if so this is the second Kemo charge controller I’ve had from Maplin that has failed in service.

What, no IoT platform?

I’ve gone off third-party IOT services, because the RPi lets me run RRDtool, and shifts the conversion from simple CSV text files to home. I get to control what goes on, I am not subject to the whim of third-party changes, cessations of service, charging for what was free and all the other hurt that goes with relying on people you don’t pay. I found the Xively Arduino stack memory leaky and buggy despite my initial enthusiasm, and now I can run a Raspberry Pi at home I can insource the job, and get the Ethernet stack off the Arduino.

In that way the data gets processed more as it is staged along the signal path as the processing power of my devices and electrical power available to them increases. Electrical power is shortest at the sensor, which draws an average of about 1mA. It gets consolidated at the OKG gateway, which is powered by a 12V leisure battery and draws about 60mA, handling all the sensors. Once it gets to the Raspberry Pi at home that is mains powered I can live with the 500mA-700mA@5V and that does the collating, data transforming into a RRD database, graphing and uploading to the Web.

Sensor design

For my system architecture I have pinched ideas from the design of industrial process control system – historically these used wired sensors, firstly using 4-20mA analogue signalling using current (this independent of wire resistance). With zero response the sensor would draw 4mA and at full scale it would draw 20mA. These fed into a console for display.

However, I don’t want to run wires all over the farm, so I will use Ciseco’s LLAP serial data format over radio. This replaces the naalogue current loops and wiring and lets me reduce cost and power at the sensor, which can sleep for most of the time, only waking every 10 minutes to send a 12-byte packet on the radio network to be received at the gateway. That’s just as well, since losing the wiring means the sensors each have to be autonomously powered.

Ciseco already make some sensors using the processor on the XRF radio – you simply upload a different firmware to the XRF – there’s an example in the picture, and this is powered from a CR2032 coin cell on the board just under the XRF.

These make a neat small sensor, and great for measuring the temperature in a shed where sunlight doesn’t fall directly on the black box (generating large readings unrepresentative of the air temperature) but they aren’t great for soil temperature measurements.

The blue line is the LLAP sensor in a box – placed next to the soil sensor – they are physically very close, but the box on the soil experiences a much wider temperature range!

Not only is there the sunlight problem, but being on the soil keeps the radio low which minimises range, hence the choppy blue line. However, it wouldn’t be hard to mod one of these with a jack switched socket to use an internal thermistor unless an external one was plugged in, and they’re quick and easy to deploy, which is great. They can also be run off two NiMH AA cells instead of the Li battery, which opens up the possibility of using solar power for unattended operation (the CR2032 battery is good for 6 months at least at a 10min update rate).

To get more out of the limited data rate on the SMS gateway, I’ve also got a LLAP sensor design with two sensors and a PIC microcontroller than encodes two temperatures onto one LLAP packet. I use two of the the Dallas18B20 digital temperature sensors for that.

The Gateway

Ciseco do a Arduino Uno based gateway PCB that has an UNO and sockets for their XRF modules, called the OpenKontrol Gateway. This also has space for a Real Time clock which is nice. I only needed the Arduino, the RTC and the XRF hence the gobs of unused space on the LHS. I wired this via the serial port to a Sainsmart TC35 used as a SMS gateway, mounted on the bottom of the box. Unfortunately the XRF ends up between two ground planes, which doesn’t do wonders for the RF sensitivity. However, I was ready for that, bringing out the serial connections to a DIN socket, so I can mount the XRF remotely and up high if necessary, taking just power and 9600-baud RS232 back to the box.

So far I have learned a lot from this deployment – it’s proved the principle, but I need to improve the RF performance of the OKG/SMS gateway with a remote XRF receiver to be in with a good chance of covering a significant part of the smallholding.