5.1 Capacitive Sensor Design

Due to their sensitivity and reliability, interleaved electrode capacitive sensors are used frequently in liquid detection applications. The sensor consists of two electrodes arranged in a comb-like structure where the fingers of one electrode are placed between the fingers of the other electrode. The electrodes are typically made of conductive materials such as copper and are usually printed or etched onto a nonconductive substrate, such as a Printed Circuit Board (PCB).

Fringing capacitance is the key principle in the operation of interleaved capacitive sensors. In a typical parallel plate capacitor, the electric field lines are mostly confined between the plates. However, in capacitive sensors with interleaved electrodes, the electric field lines extend outward from the edges of the electrodes, creating a fringing effect. The total capacitance is a combination of the direct capacitance between the electrodes and the fringing capacitance.

When the sensor is exposed to water, the dielectric constant of the medium between the electrodes changes significantly. Water has a high dielectric constant (around 80) compared to air (approximately 1) or other common materials. This change in the dielectric constant leads to a measurable change in the capacitance of the sensor.

The geometry of the interleaved electrodes, including the width, spacing, and length of the fingers, affects the fringing capacitance. The fringing capacitance becomes more significant when the electrodes are closely spaced and have a large surface area. The interleaved design increases the number of edges from which the fringing fields emanate, which enhances the sensor's sensitivity.

Fringing capacitance is generally harder to analyze and calculate than the capacitance of a parallel plate capacitor. If simulation programs are unavailable, prototyping can be a practical and effective approach to designing an interleaved electrode capacitive sensor.

When designing an interleaved capacitive liquid detection sensor, it is important to ensure that the sensor capacitance is within the range that allows accurate measurements. Ideally, the sensor’s capacitance may be in the same order of magnitude as the internal capacitance of the acquisition system. This aspect helps to achieve a good signal-to-noise ratio and ensures that the acquisition system can accurately measure changes in capacitance. In the case of MTCH9010, the internal capacitance of the system is 10 pF.

In capacitive sensing applications, series resistors can help improve the measurement accuracy by filtering out high-frequency noise and providing a more stable signal, which can be especially important when detecting small changes in capacitance due to the presence of liquids.

The combination of the series resistor and the capacitance of the electrodes form an RC circuit, which has a specific time constant. This time constant can affect the sensor response time. Selecting the appropriate resistor value is crucial to ensure the sensor responds promptly while delivering stable and accurate measurements.

To determine the appropriate series resistor value for a 20 pF capacitive sensor, given that the acquisition system has an internal capacitance of 10 pF and a minimum acquisition time of 5 µs, the combined capacitance needs to be considered and ensure that the RC time constant allows the internal capacitor to charge sufficiently within the acquisition time. The total capacitance seen by the system will be the sum between the sensor capacitance and the system’s internal capacitance.

For the system to acquire a stable voltage, the internal capacitor might charge a significant portion of its final value within the acquisition time. A common rule of thumb is to allow the capacitor to charge for at least 3 to 5-time constants to reach over 95% of its final value.

Given the minimum acquisition time of 5 µs, the time constant can be a fraction of this time. Choosing 1 µs as a reasonable time constant allows for multiple time constants within the acquisition time.

There are advantages and disadvantages of using a sensor with either a greater or lesser capacitance value, given the same series resistor and figure dimensions (length and width).

As the sensor capacitance is directly influenced by its geometry, a higher capacitance sensor is achieved by increasing the number of fingers in the comb-like structure, which, in turn increases the strength of the fringing electric field and results in greater sensitivity to changes in the dielectric constant at the surface of the sensor, improving the detection accuracy. Higher capacitance can also lead to a stronger signal and may improve the signal-to-noise ratio and make the system more robust against electrical noise. As a downside, higher capacitance increases the RC time constant, which means the sensor will take longer to charge and discharge, and the acquisition system may not be able to acquire the signal within its minimum acquisition time, leading to inaccurate readings.

A sensor with fewer fingers will have a decreased capacitance, resulting in a shorter RC time constant, allowing the sensor to charge and discharge more quickly. Such a sensor may be more compatible with the system’s acquisition time, ensuring a more accurate reading. As a downside, the sensor may be less sensitive to changes in the dielectric constant as the fringing electric field is weaker. The acquired signal may also be weaker which can reduce the signal-to-noise ratio, making the system more susceptible to electrical noise.