Designing sense electrodes for non-uniform and curved 3D touchpad surfaces
Over the last few years, touchpads have become a standard input method for next-generation user interfaces for everyday applications, from white goods to smart phones, computer peripherals, and remote controls. Touchpad design requires a thorough understanding of capacitive sensing, from electrical to mechanical stack, as the entire device and the user interaction with it need to be understood, especially for hand-held or battery-operated devices. It can be challenging to add touchpads to designs that are not flat or are made with three-dimensional shapes and multiple parts.
Even vs. uneven surfaces
A touchpad is a collection of individual touch buttons arranged in rows and columns, called channels. As the user moves his or her finger over these channels, a delta is sensed in the measured count values and can be used to determine the XY touch co-ordinate.
Traditionally, touchpads are flat with a uniform overlay thickness. One of the main reasons for this is that, with a uniform pattern and a uniform thickness, the touch strength when the user’s finger touches the surface will be uniform over the entire touchpad. If, however, the overlay is not of uniform thickness, the mutual capacitance and therefore the touch deltas will no longer be uniform.
This article explains the most important design choices that need to be made when designing a touchpad for a 3D surface.
Selecting the substrate
The choice of substrate is no longer limited only to flexible circuit boards (FPCs). Depending on the overlay shape and the interconnects to the main board, the designer also has the option of an FR4 PCB. The high sensitivity of the Azoteq Trackpad ICs allows the designer to place the touchpad or slider on the main PCB. This would lower the cost of the solution significantly, as the additional FPC and connector would no longer be needed. However, there are a few factors that must be considered by the designer when selecting the substrate, and these are described in the following sections.
Table 1 below is a list of dielectric constants/permittivity for materials that are commonly used for touchpads.
Table 1: Material properties (estimated for reference purposes only – actual values to be inserted by the designer)
Equation 1 below shows the parallel plate capacitance equation, where A is the area of the pad, ε0 is the permittivity of the air, εr is the relative permittivity of the overlay material, and d is the thickness of the overlay.
Overlay considerations. The overlay, which is on top of the sensor pattern, is the user interaction area. It is a critical component of touchpad design and determines the design direction. Overlays can be of various shapes such as round, square, rectangular, etc., and can have thicknesses ranging from 0.2mm to 5mm.
Touchpad and overlay types include:
- Multi-tier touchpads and touchpads with finger guides
- Multi-tier touchpads with either one or both tiers moving
- Curve in one direction – cylindrical or cone-shaped designs
- Curves in both x and y for a 3D shape – concave or convex designs
Mechanical manufacturing limits need to be taken into consideration for overlay design. In the case of plastic injection molding, there is a limit to the thickness that can be manufactured without resulting in uneven shrinkage. Warping is caused by uneven shrinkage due to the variations in thickness of the part, resulting in cooling at different rates. This warping could lead to an uneven surface between the touchpad and the underside of the overlay, resulting in air gaps between the two layers. Air, with its low permittivity, would reduce the sensitivity significantly in these areas. Determining shrinkage ratios for specific plastics is beyond the scope of this article.
Channel pitch. The pitch [distance between channels or the channel size] of the pattern used must be larger than the overlay thickness, otherwise the E-fields would tend to blur and it would not be possible to determine the touch peaks accurately. This would lead to an increased linearity error.
CHECK FORMULAS IN NEXT PARAGRAPHS
Mutual and parasitic capacitance (CM and CP)
The mutual capacitance between the sensing (Rx) and driving (Tx) electrode is the sum of all the individual components (Equation 2). When the user touches the touchpad, the CM_touch component is lowered and approaches 0 when saturated by the touch.
A simplified single channel is shown in Figures 1 and 2.
A simplified equivalent circuit is shown in Figure 3 , with the transmitter (Tx) driving the mutual capacitance and a receiver (Rx) sensing the mutual capacitance.