Tuesday, 27 November 2012

Low cost solar traffic monitoring unit construction

As part of our work on traffic monitoring systems, we're aiming to demonstrate that self-contained solar powered traffic monitoring devices can be constructed for less than $100 per unit. Our most recent prototype cost approximately 450,000 UGX ($180) for all parts and fabrication.

For comparison, the US Department of Transport has an interesting page listing the unit costs of a number of congestion monitoring technologies. A set of inductive loop sensors at a single intersection cost 8.6-15.3k USD. Traffic CCTV installations (for the camera, electrical services, cabinet and installation) are estimated at 9.0-19.0k USD per camera, while a machine vision sensor at an intersection is estimated at 16.0-25.5k USD per installation. So we're looking at reducing the cost by two orders of magnitude.

The basic idea of our units is to house a battery, charging regulator and cameraphone in a steel box, as shown here:


The box is locked and mounted underneath a solar panel like this:


The charge in the phone is maintained by a 14W, 22V solar panel above the unit, and excess charge tops up a 7.2Ah battery pack via a charging regulator, in case of several consecutive overcast days. An arm extending from the solar panel allows the unit to be bolted to a wall or post, and the camera can be rotated through two axes.

A steel box is necessary for security but has the issue that it acts a Faraday cage, cutting out reception for the phone. In order to be able to upload images, we have to connect a wire from the phone's internal antennae to a wire coil outside the case.


The prototype unit has been pretty stable for the last month or so, and uploads a set of three images 0.5 seconds apart every two minutes. It's positioned at the main gate of Makerere, and although the positioning isn't great (as we had to make do with the lampposts available to mount it on), we're starting to get a picture of a day in the life of Kampala's traffic...


   


6:30am, not much activity
A little after 7am the morning rush is full swing
Evening rush at 6:15pm
Still slow traffic an hour later. It settles down by 9pm or so. The characteristics of the vision problem obviously change a lot at night, and we haven't looked into this a great deal yet.

Given all this data, the focus is now on evaluating the accuracy of the software assessing the flow speeds on the road. We also have permission for installation of another two units at different exits of Makerere, so this will be interesting to be able to start looking at the correlation of traffic speeds at different places.