Wireless Positioning System

Compiled by Austin StokerECE 5320 Spring 2012

Dr. Chen

I want a robot lawn mower but I don’t want to bury a wire. What sensor could I use?

GPS

• Only accurate to a few meters.• Small (1cmx1cmx.2cm) • Cheap ($3-$30)

Image from Wikipedia/GPS

Dimensions and prices http://www.semiconductorstore.com/cart/pc/viewprd.asp?idProduct=42046

Corrected GPS

• Expensive• Requires base station.

• Note: By 2020 (probably sooner) new GPS satellites will make 10cm GPS as cheap as 10m GPS is today.

Video Sensors

• Complex to implement• Not highly reliable

Acoustic GPS

• Accurate. (~1 inch)• Fairly easy to implement• Cheap

• Only works within setup area• Requires programming• Requires assembly

How APS works

Measuring the time in takes to

receive a sound signal from 3 different beacons

with known locations the sensor

can triangulate it’s

location. With only 2

bases the position can

be found if the robot is

supposed to be within the bounds

of the bases.

Base #2Base #3

Base #1

How APS works (cont.)

• The robot sends a request via RF transmitter.• The RF request is encoded for a specific base.

The base responds with a uniquely modulated audio transmission. This is repeated for each base.

How APS works (cont.)

• The robot times the response time of each signal. Calculates the distance to each base and triangulates it’s position.

Parts

• Acoustic sensor/transmitters – Murata MA40S4S/R– $10 each

Prices from www.mouser.com/

Parts

• Computation board – Microchip dsPIC 30F401 – $7

Prices from www.mouser.com

Parts

• RF transmitters/reciever – EasyRadio ~$20-$40

www.active-robots.com/

How to use

• Accuracy of 1cm or less has been shown. Plenty good for a lawn mower if it is equipped with touch sensors for curbing and such.

How to use

• Avoid interference noise of 40kHz

How to use

• Allow enough time between requesting from station 1 and station 2. At least 30ms per meter of diameter of lawn.

Calculations

http://www.ias.uwe.ac.uk/~a-winfie/Bjerknes_etal_TAROS07.pdf

Details

• Since the beacons positions are known S is known

• It is simple from the knowledge of the position of the beacons to calculate the h and y and thus pinpoint the location of the robot.

http://www.ias.uwe.ac.uk/~a-winfie/Bjerknes_etal_TAROS07.pdf

Details

Make sure the beacons respond without overlapping

http://www.ias.uwe.ac.uk/~a-winfie/Bjerknes_etal_TAROS07.pdf

How accurate?

http://www.ias.uwe.ac.uk/~a-winfie/Bjerknes_etal_TAROS07.pdf

Z accuracy in 3D

• Z axis accuracy