3.4.1 Conclusion And Choice For Capacitive Sensing
As discussed in Section ‘1.2 Objectives’ the key objectives in regards to the capacitive level sensing circuitry are to seek to optimise the following:
Accuracy Disturbance rejection Noise rejection Sampling rate Economical operations Construction cost
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3.4.1.1 Accuracy
The charge transfer QTouch system was selected because; out of all the researched capacitive sensing circuits it satisfied the most objectives. As the QTouch system is designed for detecting finger proximity it has typically not been concerned with measuring actual capacitance values, such as a relaxation oscillator used in a digital multimeter. Leakage (that is measured in charge per second or amps) that exists within all electrical components from dielectrics to transistors/MOSFETs will undermine the accuracy of the QTouch system, as it samples accumulated charge. In short, the alternative proposed designs, such as the oscillators are better at detecting actual capacitance; however, the QTouch system is very good at detecting changes in capacitance. For the purpose of developing a level sensor, the detection of changes in capacitance within a set range aligns with this project.
3.4.1.1.1 Law of Large Numbers
The Law of Large Numbers is typically a justification for assuming statistical normality. It says that any average set of independent random variables will converge to the mean as the number of samples increase. For instance, when flipping a coin four times and the results may yield 75% heads and 25% tales; however, if the number of coin flips were to approach infinity the results will always converge towards its true mean of 50-50. Moreover the mean of heads will approach 0.5. [14]
3.4.1.2 Noise And Disturbance Rejection
For the application of detecting the liquid level within a tank, the types of noises or disturbances that are undesirable are small fluctuations caused by electromagnetic interference (EMI) or slower
fluctuation caused by rippling waves on the water’s surface [15]. It is therefore desirable that a system that can filter out ‘sudden pulses’. The procedure behind the QTouch system is that for each ‘capacitive sense operation’ there is a small amount of corresponding charge stored into the larger capacitor. A voltage threshold for the larger capacitor is set so that when it is met, the microcontroller counts the number of cycles required. The microcontroller then uses this information to perform an averaging calculation. The threshold voltage indicates the total charge stored and ‘the number of cycles’ indicates how many ‘sets of charge’ were required. This information allows the microcontroller to perform a ‘summation of charge’ divided by the ‘sets of charge’ averaging function, that will reduce undesired noise and disturbances being sensed. Note that one ‘set of charge’ can be considered equivalent to the ‘Count’ variable in both Figure 23 and Figure 12 pseudo code.
3.4.1.3 Economic And Construction Costs
The bulk of the materials cost of the QTouch system is in the microcontroller. Arduino-clone microcontrollers can be priced as low as $10 on eBay. The rest of the circuitry is just the cost of the associated wires, two 150 ohm resistors and the reference capacitor. The circuitry can be adapted to increase sampling rate at the expense of increased power consumption, or the power consumption can be decreased along with the sampling rate. Methods of decreasing power consumption and methods to increase sampling rate are explored later in section ‘7.1 Future Work’. The QTouch system may
potentially offer a simple configuration where capacitive sampling is undertaken per each cyclical charge.
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The QTouch proximity sensor, patented in 2011 was invented by both Harold Phillips and Kevin Snoad [16]. It is a capacitive charge transfer based proximity sensor that includes a sensing element and a known reference capacitor. This invention exclusively relates to proximity sensing, which is an high demand function as human interfaces are increasingly leaning towards capacitive over mechanical touch sensing. In the interfaces of everyday appliances such as phones, MP3 players and some modern panels it is typical to find plastic panels or glass with a capacitive touch control system behind. [16]
The current QTouch system is designed for detecting objects such as fingers. Detecting the presence of a finger is not as simple as typically there is only a very small capacitance in the order of a few picoFarads ( Farads). Furthermore this infinitesimal change in capacitance, for many systems, needs to be detected upon an existing background capacitance in the order of tens of nanoFarads. This is where the technology of the QTouch charge transfer excels. QTouch has been shown to sense in the most
challenging environments as it mandates a high signal to noise ratio (SNR). [17]
3.4.3 Further QTouch Development Requirements
The QTouch proximity sensor circuit as it stands infers the capacitance through counting the amount of completed charge cycles. The design does not directly measure capacitance. For a capacitor level sensor it is preferable to measure the capacitance directly; because from Equation 3 in Section ’2.2.4
Characteristics Of Parallel Plated Capacitors’, the capacitance will be directly proportional to the water level. In this equation, the area and distance between the plates remains constant while the dielectric ratio of ‘air to water’ varies; thus causing the capacitance to be directly proportional to the ‘dielectric ratio’ and through it the water level. Therefore, further changes to the QTouch system are required to create a capacitive meter.
The solution may rest in a physics problem from a textbook [3]. There is an example problem where two capacitors connected in series, 6 µF and 12 µF, each initially uncharged experience a 12 V voltage difference induced by a battery, refer to Figure 18.
Figure 18, Series Capacitors, Charge and Voltage
[Image created by Author]
Vs
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The voltage between the capacitors is calculated by noting that the charge that goes through the first capacitor also must pass through the second. Also a traditional KVL voltage loop is used to identify that the voltage across the first capacitor and the second must equal the supply voltage. Therefore the equations that describe Figure 18 are the following:
(4)
Equation 4, KVL Series Capacitors
Letting C1 and C2 be the capacitance across V1 and V2 respectively. Also by using Equation 1, and noting
that the charge across each capacitor must be equal:
(5)
Equation 5, Equivalent Charge
By using both the above equations, the voltage between the two capacitors may be solved. Furthermore suppose that the voltage between the capacitors could be measured. In this scenario it is possible that if the voltage at each node is known and one of the capacitor values the remaining unknown capacitor may be solved for. In effect this circuit may operate as a capacitance meter that works by using a reference capacitor. The following chapters will explore how this circuit can be used to further develop the QTouch circuit into a capacitance meter. To achieve this certain questions will require answers, such as ‘what happens to the charge across each capacitor if the first capacitor is holding an initial voltage?’ Simulations and real world testing will be used to identify these relationships in the preceding chapters.
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Chapter 4 Assumptions
4.1 Chapter Overview
This chapter highlights some of the common assumptions made in electronic design, all of which fall under the “ideal circuit” concept, such as perfect operation of circuit elements and absence of parasitic effects. The effect of assuming an infinite impedance state for input pins in the QTouch circuit is also discussed. These assumptions are then assessed and compensatory measures discussed.