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Charge transfer capacitance meter development for capacitive level sensor

Anderson, Robert (2013) Charge transfer capacitance meter development for capacitive level sensor. Other thesis, Murdoch University.

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An innovative technique has been recently developed to measure capacitance and capacitive touch. This technique labeled ‘QTouch’ was patented to Atmel in 2011, [1] but invented by a firm called ‘Quantum Research Group’. The heart of this thesis focuses on two different types of capacitive sensing circuits, adapted from the ‘QTouch’ technique.

The first circuit involves a ‘charge divider’ approach, which behaves similar to resistive voltage divider circuitry, where a voltage is retrieved between a pair of capacitors to compute the unknown capacitance. The key advantage of this circuitry is that it allows a microcontroller to narrow the analogue reference (AREF), which focuses a maximum analogue input range on a linear region. It also has the advantage of obtaining a differential capacitance measurement through using a fixed/known reference capacitor. And finally, it offers a measurement for each charge cycle. In the second prototype circuit, the recently patented ‘QTouch’ charge transfer technique, used specifically in digital touch sensors with large signal-to-noise ratios (SNR), is combined with the charge divider approach. Both these combined techniques form a hybrid QTouch (Analogue QTouch) that is redeveloped into a capacitance sensor that retains all of the noise rejection advantages of the traditional QTouch. The key advantage of this circuitry is that it offers easy disturbance rejection, detects average changes in level and forms a single ended sensor by using a fixed known capacitor.

The Analogue QTouch circuit makes capacitance measurements that are conveyed via an output DAC breakout board, to an analogue voltage that is then fed through an operational amplifier to create a 4-20mA signal. In this project, the QTouch circuit is applied to capacitor probes in a water tank to form a capacitive level sensor, which measures the water level by computing the proportional relationship between water level and the measured capacitance. An LCD display screen is used to display real-time data, such as the capacitance and a corresponding level.

Key findings of this thesis are that the second prototype circuit that employs the ‘QTouch’ charge transfer technique (Analogue QTouch) has been demonstrated to be at least as accurate as some of the advanced capacitive measuring devices on the market such as the ‘Digital Multimeter Q1156’. It is also able to detect capacitance changes and switch to a moving average equation (Equation 12) to improve transient measurement accuracy. Because this system samples ‘accumulated charge’ it does not require continuous sampling, as whenever the accumulated charge is sampled, a type of average accumulation per iteration calculation is performed. Moreover in mathematical terms a sampling of the summation of data points is performed rather than the data points themselves. This in turn allows the user to sample the dataset at any time to perform an average calculation and according to the ‘law of large numbers', the larger the dataset, the more likely the sample average will converge to the true average.

An additional feature of the circuits developed in this thesis is that they operate with any DC voltage range. The benefit of this is that no external circuitry or expensive oscillators are required to perform capacitive measurements. The highly accurate ‘QTouch charge transfer technique’ does not measure frequencies or transient responses against time as other capacitive measurement systems do. Rather it offers both a software and iteration based alternative that can be controlled by a microcontroller.

Publication Type: Thesis (Other)
Murdoch Affiliation: School of Engineering and Information Technology
Supervisor: Lee, Gareth
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