Fundamentals of electrostatics & capacitance for capacitive touch sensors

Capacitive sensing can be overwhelming for a newcomer. The nature of the underlying electromagnetic phenomena is complex. That’s why many newcomers tend to cut corners. They tend to skip the part of understanding the physics involved and jump head-first to projects. Naturally, they soon feel lost and resort to trial and error.

This chapter contains all the knowledge we wish we had when we first started working on capacitive sensing. It will teach you the basics of electrostatics, conductors, and capacitance tailored to the need of a capacitive touch sensor designer.

Then you’ll familiarize yourself with one of the most useful concepts in capacitive sensing: the fundamental case of the parallel plate capacitor. Understanding this case can help you unlock the mysteries of capacitive sensing!

Finally, you’ll get an intuitive understanding of how certain parameters, like electrode geometry, dielectrics, and traces affects the performance of a capacitive touch sensor.

Thanks to Benjamin Franklin’s pioneering experiments, that took place almost three centuries ago, almost everyone knows today that electric charges can travel through materials causing flow of electric current.

The tendency of a material to allow the flow of electric current through its body is described by conductivity, σ. On the contrary, the difficulty that a material poses to electric charge conduction can be quantified by resistivity, ρ.

We must keep in mind that conductivity, therefore also resistivity, are inherent properties of the material, so they are independent of the size and shape of the sample. Based on its electrical conductivity, almost every material found on earth can be classified as a conductor or insulator. Conductors, like metals, have very high conductivity values (σ > 105 S/m) and consequently very low resistivity (ρ < 10-5 Ohm⋅m). So the electric charges can travel through them very easily, with a high speed and in large amounts. On the other hand, insulators (or dielectrics), like plastics and polymers, have low conductivity values (σ < 10-8 S/m), as they inhibit the motion of electric charges, allowing just a low current flow in the presence of high electric potential. Materials with intermediate conductivity values are called semiconductors, and generally bridge the conductivity gap between conductors and dielectrics.

Considering the materials that are most commonly used in capacitive touch sensors, PET, FR-4 and glass are insulators, whereas silver, copper, and ITO (Indium Tin Oxide), are all metals, that is, conductors, so they have very high conductivity values.

Rotary Encoders are another use case for inductive sensing.  This section discusses the design of inductive rotary encoder.  The construction of rotary encoder involves two sensor coils and N number of targets placed on the rotating platform as shown in the figure below.

Construction of Inductive Rotary Encoder

The number of targets (N) decides the angular resolution as shown in:

Angular resolution = 360/(N*4)

The sensor coils need to be separated at an angle twice the angular resolution. For example, if the number of targets N = 8, then the achievable angular resolution is 11.25° and the required spacing between sensor coils is 22.5°. The direction of rotation can be determined using the previous value of coil ‘A’ and the current value of coil ‘B’ as shown in Table 2. If the current value of coil ‘B’ and previous value of coil ‘A’ are same, it means the direction of rotation is anti-clockwise direction; if the values are different, then is the direction of rotation is clockwise. 

Direction of Rotation

To find out more on MoT designs, please see the Inductive Sensing Design Guide.

Metal over Touch (MoT) involves detecting the deflection of a metal overlay upon a touch. MoT uses a metal overlay separated from the sensor using a thin spacer or etched surfaces of metal overlay as shown in the figure below. When you touch the metal, the metal deflects. This deflection is detected by the inductive sensor.  An example of front and back (with etched cavities) of a metal overlay is also shown below. The sensitivity of touch detection depends on the following parameters.

• Amount of metal deflection
• Sensor dimensions
• Spacer thickness or depth of etched cavity
• Applied force

Note that the amount of metal deflection for the applied force depends on metal material, thickness, and flexural rigidity.

Metal over Touch Arrangement

Example Metal Overlay

To find out more on MoT designs, please see the Inductive Sensing Design Guide.

Liquid-Level Sensing (LLS) detects the presence and level of liquid in a container without any physical contact. There are various types of liquid-level sensors such as capacitive, mechanical float, inductive, magnetic, Hall effect, optical, acoustic density, and ultrasonic; each has advantages and disadvantages. Capacitive liquid-level sensing has become popular due to its low cost, high reliability, low power, sleek aesthetics, and seamless integration with existing control architectures.

Cypress’s PSoC MCUs support liquid-level sensing with resolution down to 1 mm. Capacitive liquid-level sensing is provided through the use of the CapSense Component available in the free PSoC Creator™ Integrated Development Environment (IDE). The CapSense Component configures the on-chip CapSense peripheral hardware and provides required firmware for operation on PSoC MCUs. The following key liquid-level sensing benefits are provided using CapSense:

• Non-contact measurement avoids contamination and cleaning problems.
• Sensors located on the exterior of a non-conductive liquid container simplify industrial design and improve product user experience.
• Optimized resolution and accuracy to support varying price points with a single, low-cost, base system
• Sensors may be constructed out of low-cost materials such as plastic substrates and conductive ink.

A capacitive LLS system comprises of two key design elements:

• Capacitive sensor pattern to sense the liquid level
• PSoC MCU with CapSense Component to measure the sensors and calculate the liquid level

Cypress provides the CY8CKIT-022 Liquid-Level Sensing Shield Kit, which shows the simplicity of a PSoC MCU based LLS design.

Liquid-Level Sensing Block Diagram with PSoC 4 MCUs

We recommend that a later generation PSoC MCUs, such as PSoC 4 and PSoC 6 be used for optimal LLS performance. Development kits for PSoC 4 and PSoC 6 MCUs can be found here.

CapSense Basics

Capacitive liquid-level sensors are conductive pads or traces laid on a non-conductive material such as a PCB, plastic, or glass. The intrinsic capacitance of the PSB trace, pads, and other sensor connections is called the sensor parasitic (C_P). when a target object such as water approaches the sensor, a small amount of liquid capacitance (C_L) is added to the C_P. LLS involves measuring the increased capacitance when a liquid is near the sensor.

                                                 Capacitance and Electric Field of a Capacitive LLS                          Added Capacitance (C_L) when Liquid Approaches a Capacitive Sensor

The CapSense Component measures the capacitance by injecting a current into the sensor with a current Digital to Analog Converter (IDAC). A timer measures how long it takes the IDAC to charge the sensor’s voltage to a reference voltage using a comparator. When the conversion is complete, the timer count value that measured the IDAC charge time is used as the raw sensor value used in calculations and is commonly referred to as the sensor count.

To find out more about CapSense Liquid-Level Sensing, please refer to the Capacitive Liquid-Level Sensing AN and the PSoC 4 and PSoC 6 MCU CapSense Design Guide.