Chapter 3:
Representations of capacitive touch sensor systems
3.1 Introduction to touch sensor validation & circuit analysis
Introduction: How to design and validate a touch sensor
The process of designing and validating a touch sensor depends on the level of experience and expertise of the designer. However, no matter the case, a design begins from the high level (system-level) product specifications of the touch sensor. Those specifications describe the desired outcome and what is expected from the touch sensor when it is ready for production. For example, a product specification table of mutual and self-capacitive touch sensors is shown in Figure 1.
Figure 1. Typical product specifications of a touch sensor. (Source)
In this figure we can see that the capacitance touch sensor is expected to have a response time of 30ms, to be able to support 2 concurrent touches with a minimum diameter of 7mm and many more. Other high level specs can be the size of the screen, restrictions on the controller and material choices, the thickness of the protective cover lens and many others.
Once the specifications are clearly defined, the designer can depict them in an equivalent circuit, or a Target Schematic, which ideally transforms those specifications into the electrical circuit of the product.
Figure 2. Capacitive touch sensor schematic. (Source)
Finally, when the target schematic is ready, the designer is ready to produce the sensor, that is the sensor layout. The physical layout should incorporate all the product specifications and remain consistent with the target schematic in terms of the typical requirements that the product definition describes. For example, you cannot have a target schematic for a 5 inch touchscreen, but design a 5.5 inch Sensor Layout, because that better sensitivity, cheaper materials etc.
Figure 3. Layout of a capacitive touch sensor.
The most common practice of doing so is to design the layout by following generic or specific guidelines and experience. There is a very large list of guidelines for the design of a capacitive touch sensor. For example, in order to design a typical touchscreen using the common diamond double layer pattern, we learn that there is a typical row/column pitch, a typical XY separation, but also minimum and maximum values that should be considered:
Figure 4. Design rules for a capacitive touchscreen. (Source)
The next step is the product validation. The sensor layout is going to be compared against the product specifications. This can be done by performing a series of measurements and tests on the actual prototype model or by building a virtual prototype and test it against the target through a round of simulations.
Figure 5. Physical verification through prototype testing.
The generated layout must pass a series of checks in a process known as physical verification. The most common checks in this verification process are:
- Design rule checking (DRC),
- Layout versus schematic (LVS),
- Parasitic extraction,
- Electrostatic discharge test (ESD), and
- Electrical rule checking (ERC).
In the following sub-chapters we are going to talk about the schematic, the items it consists of and how a touch sensor designer can create one in order to validate the layout of the sensor, based on the product specifications. We are also going to break down some product specifications and how they can be depicted from schematic diagrams.
Key takeaways from this section:
In order to validate a touch sensor’s performance, we need to generate the schematic of the layout and then compare this new schematic with the target one
Capacitor, resistor and inductor
At first, we need to introduce the components that build an electrical system (schematic). To do that we need to focus on the basics of electrical circuits.
As described in Chapter 1, capacitance is the ability of a component to collect and store energy in the form of electrical charge. Moreover, electrical resistance of an object is a measure of its opposition to the flow of electric current.
Furthermore, electric current is the rate of flow of electric charge past a point. In order for electric current to exist and for charge to flow towards a specific direction, there is a need for free charges and something to cause this movement, that is an electric field.
Figure 6. Electric current flow (Source)
Based on the material (medium) properties, there exist ones that conduct electric current very easily due to the plethora of free charge (called conductors) and there are some that oppose electrical current and make poor conductors (called insulators). Typical conductors are the metals and typical insulators are the plastics.
A combination of conductors and insulators in different geometrical shapes and topologies can create the basic electronic components that shape the majority of the electrical circuits. The three most common ones are given below.
Α capacitor is a passive two-terminal electronic component that stores electrical energy in an electric field and releases it at the electrical circuit in the form of electric charge. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium. In capacitors the value of the capacitance,C, is the most prevalent, although they also have a small R value (only in ideal capacitors is R=0).
Figure 7. Commercial Capacitors (Source)
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electrical circuits, a resistor is used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. In resistors, the value of the resistance, R, is the most prevalent.
Figure 8. Commercial Resistors (Source)
An inductor is a passive two-terminal component that stores energy in a magnetic field when an electric current flows through it. An inductor typically consists of an insulated wire wound into a coil around a core. The value L of the inductance is the most prevalent.
Figure 9. Commercial Inductors (Source)
Those three elementary components are called passive, due to the fact that they cannot control the electric current by means of another signal source.
At this point, we are going to introduce the impedance Z. Electrical impedance is a measure of the total opposition that a circuit or a part of a circuit presents to electric current, a value that also considers frequency effects. In Cartesian form, impedance is defined as:
Z = R + jX
where the real part of impedance is the resistance R and the imaginary part is the reactance X.
Figure 10. Electrical Impedance (Source)
The resistance component arises from collisions of the current-carrying charged particles with the internal structure of the conductor. The reactance component is an additional opposition to the movement of electric charge that arises from the changing magnetic and electric fields in circuits carrying alternating current. Impedance reduces to resistance in circuits carrying steady direct current.
The impedance of a perfect resistor is ZR=R – only the real part.
The impedance of a perfect capacitor is ZC=1/jωC – only the imaginary part.
The impedance of a perfect inductor is ZL=jωL – only the imaginary part,
where ω is the angular frequency.
In an actual system all values RLC coexist, forming system’s impedance. In low frequencies, C becomes more important, whereas in high frequencies it is L that is more important. Naturally, resistance is unaffected by the frequency.
The operation frequencies of touch sensor systems are low enough, that, usually, designers neglect the influence of the inductive parts of the circuits. This means that only R and C matter.
Key takeaways from this section:
There are three elementary components in an electrical circuit, the capacitor, the resistor and the inductor.
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