Experiment #6: Additional sensor covering comparison
Some time after various sensor coverings were tested in experiment #4, some Avery Self-Adhesive Signmaking Vinyl was acquired with a view to using that as a covering for the tongue drum. It was hoped it would be suitable to use as a sensor cover too. This experiment compares a sensor covered with the vinyl with the results obtained in experiment #4 for sensors #3 (plastic card), #6 (primer + top coat), #7 (parcel tape) and the control sensor.
The new sensor was constructed in the same way as the other sensors except that a backing material was glued over the cardboard at the rear of sensor. The backing material was the backing peeled from the vinyl sheet. The vinyl itself is self-adhesive and was stuck directly over the sensor foil.
The resistance each of the underlying foil sensors was calculated as before, using an attached DuPont cable. The results were all very similar:
| Sensor | Resistance |
|---|---|
| #3 | 0.7Ω |
| #6 | 0.9Ω |
| #7 | 0.7Ω |
| #9 | 0.8Ω |
| Control | 1.9Ω |
Methodology
The new sensor (#9) was tested as in experiment #4: the right hand index finger was tapped on the sensor in a pattern of three long taps followed by three short taps. The resulting sensor readings were written to the Uno's serial port and captured to text files using CoolTerm.
Unlike in experiment #4, tests were conducted at 25 only and 50 samples per reading this time.
Circuit
The circuit used is functionally the same as that used in experiment #4, although the layout on the breadboard was slightly different:
The following photo shows the circuit in use with the new vinyl sensor #9:
Code
The code used is again GuidedSensorTouchDataLogger. This code takes repeated sensor readings using the CapacitiveSensor library and reports the results over the Uno's serial port while indicating when the user should touch the sensor by means of tones emitted though a speaker or buzzer.
Results
The contents of the text files containing results for sensor #9 were combined with relevant results from experiment #4 and analysed in a Libre Office Calc spreadsheet. Results were tabulated in two different ways. Firstly the sensor readings at each sample rate were collected together, analysed and graphed. Secondly the results for each sensor were grouped by sample rate and again analysed and graphed.
In each case the following statistics were determined, again as per experiment #4:
- The maximum reading
- The average of all readings
- The lower average, i.e. the average of the lower readings, where “lower” is taken to be values between 0 and the average of all readings, inclusive.
- The upper average, i.e. the average of the higher readings, where “upper” is taken to be values greater than the average of all readings.
To see all the data and the analysis, download the spreadsheet (zipped).
Comparison of sensors to each other
The following image carousel shows two graphs, one graph for each sample rate, that shows the relative performance of each sensor. The y-axis shows the sensor reading as raw data while the x-axis is time:
The next image carousel also contains an image for each sample rate, but this time plots the maximum, average, lower average and upper average for each sensor. The y-axis is the sensor reading while the different sensors are represented on the x-axis:
Comparison of samples per reading for each sensor
The following image carousel shows five graphs, one graph for each sensor, showing the readings at different sample rates:
Conclusions
The first thing to notice is that the results for sensor #9 compare reasonably well with the other sensors, being much better than sensor #3 and about the same or a little worse than sensors #6 & #7.
Secondly it is interesting that the results for the new sensor are relatively better, when compared to other sensors at 50 samples per reading than at 25 samples per reading. Could this be due to a poor performance of the test at 25 samples per reading?
Thirdly, there appears to be less variation in both touched and un-touched readings, at both sample rates, than for the other sensors.
Finally, there is sufficient gap between the upper average and lower average readings to make it easy to choose a trigger point that is unlikely to cause a mis-trigger.
These results seem to indicate that this vinyl covering should work well enough to be adopted as the covering to be used for the final instruments. It is suggested that future experiments should be conducted only on sensors covered in this material.