Load Cell
The load cell is a crucial element of the system. The entire accuracy of the system is contingent on the ability of device to quantify the amount of waste in any given bin. Since the load cell is the device that transforms physical weight quantities into an electrical signal it is paramount that the device is able to do so repeatedly and accurately. To test the accuracy of the load cell, a predetermined weight, in this case a 0.5 kg weight was placed on the cell to determine a calibration factor. Then a series of weights were placed on the scale and their measured weights were noted and compared to their known weights.
| Known Weight | Measured Weight |
| 100g | 99g |
| 200g | 199g |
| 500g | 498g |
As shown in the table above, the system is able to read the accuracy of a weight within 2g of the known weight which is greater than an order of magnitude greater accuracy than required for the system design requirement of acquiring accuracy within the 84g tolerance.
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Figure 20 – Load cell testing with 200g weight |
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Additional testing was planned to determine the temperature dependence of the load cell, but due to the time restrictions of the project, this testing was ultimately left unfinished. According to the data sheet the load cell should have certain temperature dependence so commercial applications of the system should provide temperature compensation to the programming of the system.
- RFID
Testing showed that the reader could identify tags at a maximum distance around 9 cm but the distance of the read was heavily reliant on the orientation of the card relative to the reader (i.e. parallel, perpendicular, angled), as well as displacement from the reader.
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| Figure 22 – RFID testing using a perpendicular tag orientation
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Figure 23 – RFID testing using a parallel tag orientation
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The experimental set up consisted of a marked field with the RFID reader at a set location affixed to an upright prop, and a tag that was attached to a mobile upright prop. The tag was then moved toward the reader along a testing path according to the test, until the reader signaled that it read the card. The distance from the reader was then marked and each orientation of the experiment was repeated five times.
| In front | In front (5cm displacement left) | In front (5cm displacement right) | |||||
| Trial | Distance (cm) | Trial | Distance (cm) | Trial | Distance (cm) | ||
| 1 | 9.1 | 1 | 7.25 | 1 | 4.8 | ||
| 2 | 9.2 | 2 | 7.4 | 2 | 4.6 | ||
| 3 | 9.1 | 3 | 7.5 | 3 | 4.6 | ||
| 4 | 9.2 | 4 | 7.3 | 4 | 4.7 | ||
| 5 | 9.5 | 5 | 7.4 | 5 | 4.7 | ||
| Average | 9.22 | Average | 7.37 | Average | 4.68 | ||
| Above (5cm displacement right) | Onside (perpendicular) | |||
| Trial | Distance (cm) | Trial | Distance (cm) | |
| 1 | 8.1 | 1 | 3.6 | |
| 2 | 8.1 | 2 | 3.65 | |
| 3 | 8.1 | 3 | 3.7 | |
| 4 | 7.9 | 4 | 3.8 | |
| 5 | 7.9 | 5 | 3.8 | |
| Average | 8.02 | Average | 3.71 | |
These findings show that the RFID reader must be as close as possible to the waste bin. Likely the reader will need to placed so that it can even come into contact with the waste bin, so a protective element may be necessary.
GPS
In addition to providing an auxiliary method by which to establish a customers’ identity, location services are also the backbone of the large data analytics portion of the household waste tracking system. Due to the limited resources of this project, precise scientific analysis of the GPS components used to construct the household waste tracking system was infeasible. However, using google maps to verify coordinates, a system that has been known to have its own inaccuracies, the system was able produce locations that were within 15 meters of the reference location, as shown below in the figure.
Location test revealing a location 12.27 meters from actual location
The US government verifies that GPS locations are overwhelmingly found to be accurate to 3.5 meters, and that in a worst case scenario, a GPS location with provide accuracy within 7.8 meters with 95% accuracy. [5]
GPRS
In the event of GPS failure, the system is to rely on the location provided through by GSM towers. This location is provided by calculating the distance to the closest accessible cell towers. Unfortunately, even when the system is able to communicate with multiple cell towers, giving the system the most reliable location, results are often inaccurate. The GPRS was tested in the same fashion as the GPS module, by comparing the system given location to the pinpointed location on google maps. Although this method is less that spectacularly scientific, it does give insight into the relative accuracy of the location data. Shown below is a figure of a common GPRS result.
GPRS location test results in comparison to actual location 204 meters away
Although the data is not accurate enough to use to verify the identity of a bin collection, this result could still be relevant in assessing waste trends.
SD Data Saving
SD storage is the key to keeping data secure even when GSM transmission fails. In the beginning of implementing SD data storage device there arose some concern over the viability of the method, as the break out board seemed to cut out accessibility every so often. With some experimentation it was found that the system simply needed some pull-up resistors and pull-down resistors to ensure that the board would function correctly all of the time.
Cellular Data
Data transmission off the device during collection is completely dependent on the system’s ability to make http request through the GSM network, so the feature must be very reliable in the household waste tracking system. Although occasional errors occur when attempting to use the GSM features on the board, these error have been overwhelmingly due to the AT style communication between the Arduino and the chip on the GSM board. The household waste tracking system has attempted to irradiate these errors by flushing the serial communication after every communication between the Arduino and GSM board, but occasionally the error occurs. Simply resetting the Arduino using the “reset” button fixes this problem. When GSM is available the system has no problem sending information through http requests, but in the event that the system does not have access to such a network the data will still be saved to the SD memory.
Web Display
Although the web display of the household waste tracking system accomplishes its goal of receiving and displaying waste data from the device, it would be infeasible to use the current system in a commercial setting. Due to the structure of Dweet.io, data can only be saved on their servers for a day using the free version of the service, and only a month on the subscription based version of their platform. Furthermore, Freeboard.io is limited to only displaying data that it receives during the span that the window is open. These applications work wonderfully as a proof of concept for the project but would not allow a waste management company or government to analyze the data over a long period of time, and would be difficult to use to correlate waste data to service fees. Ultimately if the system were to be implemented in a large scale commercial fashion, dedicated servers and databases would need to be established to display and store the data generated by the system.
