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Bin sensor design to improve litter monitoring

Exploring the use of sensor technology to help with ‘smarter’ litter monitoring

An increasing number of organisations are using smart bins, and other new technologies, to maximise the efficiency of their litter and recycling bins provision.

These technologies currently use fill levels to indicate bin fullness over time. This information is provided as real-time data so that bin servicing regimes and transport routes can be tailored on the fly to maximise efficiency and cost-effectiveness.

In 2015, we explored the feasibility of producing a bin sensor that could be attached to existing bins in situ and enable a count of the items deposited into them. This would provide a means of developing a greater understanding of bin use over time and monitor how the depositing of litter changes according to local interventions such as campaign and enforcement initiatives.

The main findings of the feasibility study are outlined below. In summary, it was concluded that battery life was too short for a real time 24-7 sensor. A sensor that monitors fill-levels by bouncing a beam at defined intervals can last a lot longer. Systems like these are already available.

Given the high unit cost and limited functionality it isn’t economic to proceed with further development and deployment at this time. However, we would be keen to hear of any further developments in this area and believe the findings of this study are worth sharing and may inform future work.

Development process

Initial designs of the sensor were based on a mechanical interface that offered displacement every time litter passed a sensor. However, it became evident that this would require either flaps to be added to each opening or a flexible wand being situated in line with the rubbish ingress. This design was therefore discounted due to the requirement for the sensor to be readily useable with no alterations required to existing bins.

Subsequent designs were tested. These included:

  • Light interruption - using an LED and receiver to detect interruptions in light reception as an item of rubbish interrupted the beam. This was discounted because the emitter and receiver would need to be fixed to the bin with cabling (modification to bin).
  • Passive IR detection - similar to systems used in burglar alarms or occupancy detectors. This was discounted because most rubbish doesn’t emit an IR signature and there's the possibility of false triggering from people, sunlight and other factors
  • Audio detection - placement of a small microphone to detect objects thrown into the bin. This was discounted because small light items will not cause a sufficient audio signature to trigger the sensor.
  • Camera detection - using a camera to detect objects moving in field of view. This was discounted as the camera would false trigger on passing vehicle headlights, vibration or lightweight articles being blown about inside the bin.
  • Ultrasonic detection - using ultrasonic transmitter and receiver to detect changes in distance caused by an object passing close to the unit. This design was adopted.

The ultrasonic design was adopted because the detection software could be fine tuned, and the design could be adapted for use as a fill-level detector. It made use of a low power microcontroller kit, with the software written to detect items closer than the previous measured base level (i.e. top of rubbish in the bin). A working prototype was developed and demonstrated.

Two problems became apparent with the prototype: poor reliability and heavy power usage. The ultrasonic detector struggled at different distances with the variety of material shapes, sizes and construction. While detection sensitivity was optimised to the point where false measurements were minimised, it still didn't ‘see’ every item of rubbish.

The prototype proved to be a heavy drain on battery life because the detector circuit must be continuously powered. In a comfortable temperature range, useful battery life was estimated at 12 hours. This could be expected to fall to 6 hours when the temperature fell below zero degrees. A larger battery would make the unit much larger and obvious without greatly extending useful working life. We therefore concluded that it was not feasible to offer this unit as a continuously operating detector.

Alternatives

We believe this study has covered all applicable technologies and methodologies. The easiest way of counting individual rubbish items is still to use a mechanical interface. Something as simple as a plastic comb arrangement placed over the entrance to the internal bin container could be used with micro switches to detect an object entering the container with fairly high accuracy.However, this would need to be positioned over the internal waste receptacle every time it was replaced and it could easily be jammed with a large box or paper bag.

One further consideration was the option of placing a camera looking at the bin, with the image masked such that it would only trigger when someone is right next to the bin, indicating that rubbish was being deposited. However, this raises issues of privacy, problems around placing the camera, and accuracy.

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