Limina.Log » xbee

Wireless Positioning & Location Awareness: An Overview

Tedb0t — Wed, 21 Dec 2011 19:56:50 +0000

For one of my major projects I’m tasked with figuring out how we can locate mobile wireless devices within a limited location, and it seems a lot of other people I’ve talked to are in the process of figuring out the same thing, so here’s an overview of what I’ve learned so far.

Wireless Technologies / Protocols

WiFi (802.11)
XBee/ZigBee (802.15.4)
BlueTooth (802.15.1)
RFID

Near Field Communication (Two-Way RFID)

These are four of the principal wireless technologies in mainstream use today. The goal is to determine within some degree of accuracy the position in 2 or 3 dimensions of a wireless node. Most, if not all, of the available approaches utilize the RSSI (Received Signal Strength Indicator) of the target node’s connections to nearby nodes.

If you have fixed routers with known locations, you can use the RSSI values of their connections to the target node to calculate the distances from the fixed nodes to the target node. However, any RSSI value will be heavily influenced by a number of factors that may be out of your control: interfering structures, interfering people (we are walking bags of water, which readily absorbs 2.4ghz radiation) and other interfering radio signals or noise.

Due to these issues, some smart folks developed a system called RADAR that uses ‘location fingerprinting’ to take a bunch of readings of signal strengths under varying conditions and associate them with known locations. There are a variety of algorithms to tabulate this information, one of which uses neural networks.

Here is a collection of extremely useful papers and links that describe these various approaches, techniques and technologies:

Resources

Wireless Projects over the Web: Digi Dia for Hackers Part 1

Tedb0t — Mon, 02 May 2011 21:18:20 +0000

Suppose you want to log data from a group of sensors, like temperatures around the house, or turn lights on and off remotely via a webpage, and you want to do this wirelessly. You could use a WiFi shield for an Arduino, but they’re notoriously hard to set up and stay reliably connected.

My preferred way uses Digi XBee radio modules with the ConnectPort gateway, which connects a network of XBee modules with a LAN over ethernet. So far, easy: build your microcontroller project that sends and receives data via an XBee ZigBee module (using “Series 2″ hardware), and the ConnectPort is your mesh’s coordinator.

Then the question becomes how to send and receive the data from the coordinator. If you had the coordinator plugged into a computer, you could write an interface however you liked. Lucky for us, the ConnectPort is basically a tiny computer that runs Python! So how do you access your project’s data via the ConnectPort over the web?There are two overall approaches: transmit to another webserver, or use the ConnectPort as a webserver itself. XBee fiend Rob Faludi and I developed a handy way to do the first approach, called the XBee Internet Gateway (XIG), and Digi mastermind Jordan Husney vastly improved it.

XIG’s one ability is simple but hugely useful: it allows your Arduino to retreive a URL from the Internet. Say you’re collecting temperatures from sensors via your microcontroller. With XIG, you can post the values to a web script on a server somewhere by sending this string to the ConnectPort: “http://my.server.com/myscript.php?val1=6&val2=10&etc=etc”. Your script can then optionally return some string for your Arduino to parse and use.

This is fast, easy, and effective! Its only downside is that you need an external webserver to get the data moving. For many of us, this is no problem, but wouldn’t it be nice to cut out the middleman?

Enter Digi’s Dia platform. Dia (Device Integration Application) is a Python program that runs on the ConnectPort that talks to your XBees and—the important part—runs its own web server so you can talk directly to the ConnectPort instead of using an external web server.

Now, Dia is a fairly new platform, and definitely designed for engineers more than hackers like ourselves. So I did a deep dive into the platform recently and have brought the results to you! Stay tuned for the next part!

Measuring XBee signal strength from the RSSI pin

Tedb0t — Thu, 08 Apr 2010 01:52:24 +0000

In my thesis I wanted to read the signal strength of incoming packets and store it in a variable on a microcontroller without using the XBee’s API Mode (In API Mode, you construct packets to command the XBees explicitly, as opposed to just sending serial data in AT or “Transparent Mode”). XBee modules have an RSSI (Received Signal Strength Indicator) pin that outputs a PWM signal representing this value. But how do you turn that PWM duty cycle into a useable integer on your microcontroller? Luckily Arduino has a function made for just this application: pulseIn.

Connect the RSSI pin (pin 6) to a digital pin on your microcontroller.
Use this line of code in your Arduino loop:
```
rssiDur = pulseIn(digitalPin, LOW, 200);
```

pulseIn returns the duration of a pulse (specified HIGH or LOW) in microseconds (µs). The 200 is a timeout value in µs—it waits this long to see if there’s going to be a pulse. Since the XBee’s RSSI PWM period is 200µs, we want to wait at most that long to see if there’s a pulse at all.

Since this function gives you a duration, it’s up to you to map that to a value relevant to your program. Since the maximum duration of the pulse would be 200µs, you could map that to your logical maximum.

Keep in mind that if you were planning on using API Mode anyway, every API Mode packet has the latest RSSI within it.

current_working_directory @ Solas

Tedb0t — Thu, 09 Jul 2009 18:28:04 +0000

Voilà! Here are three videos from my performance for Splice NYC, two of which demonstrate my wireless, beat-repeating space eggs of doom:

Postfix

She Landed Here

Can We Now

And if you missed this show, I’m playing next Wednesday (7/15) at Le Poisson Rouge for XRAY! Facebook event here: XRay

Thanks again to everyone for coming, Matt for playing with me, GP for shooting video, and Maxx for inviting me to play!

XBee Handy Command Reference

Tedb0t — Mon, 09 Mar 2009 21:51:30 +0000

Here are some common AT commands I have most frequently while working on XBee projects. Remember, ALL NUMBER VALUES ARE IN HEX!

ATID – PAN ID
ATDH/DL – Destination address high/low: which local address to send ADC data
ATMY – Local address of this unit
I/O:
- ATD0…D8 – digital pins I/O enable
- ATP0…P1 – PWM output enable
- ATIU – I/O output enable (UART) – send out through serial
- ATIA – I/O input address – address to pay attention to for serial output
- ATIT – ADC Samples before TX
- ATIR – ADC Sample rate (ms)
- ATBD – Baud Rate:
  - 3 = 9600
  - 4 = 19200
  - 5 = 38400
  - 6 = 57600
  - 7 = 115200
ATWR – Write parameter values

Remember to ATWR to save changes to parameters.

Interesting note from the manual: “The maximum sample rate that can be achieved while using one A/D line is 1 sample/ms or 1 KHz (Note that the modem will not be able to keep up with transmission when IR & IT are equal to “1”).”