5.1 Projected capacitive: electric fields, electrodes and touch maps

Slide 29 of 157 · Projected capacitive: electric fields, electrodes and touch maps

Welcome to chapter 5: Projected capacitive: electric fields, electrodes and touch maps. I'm Diggy Breiling with FPD.DEV, The Future of Display and Systems Development. Our goal is to explain self and mutual capacitance, scanning, multi-contact extraction and signal margin.

Slide 30 of 157 · Self-capacitance

Self-capacitance measures an electrode relative to the controller's electrical reference. A finger changes the effective coupling. The return path includes the person, equipment and surroundings; the finger does not need a wired connection to chassis ground. A controller establishes a no-touch baseline and detects changes from it. Sensitivity depends on geometry, cover dielectric and thickness, signal processing and interference. Separate row and column measurements can create ambiguous pairings when several contacts are present. This is a limitation of that measurement arrangement, not a universal ban on self-capacitive multitouch. The useful engineering quantity is touch signal relative to disturbances. A larger reported coordinate range cannot recover a signal buried in noise.

Slide 31 of 157 · Mutual-capacitance scanning

Mutual sensing measures coupling between a transmit electrode and a receive electrode. Their crossing regions form an array of measurement nodes. A finger perturbs the field and changes the measured coupling. The sign and interpretation of the reported delta depend on the circuit and firmware convention. The controller scans, removes baseline effects, identifies regions of change, estimates contact centroids and associates them between frames. A finger spans several nodes; interpolation allows coordinate increments finer than electrode pitch. It does not make the physical uncertainty vanish. Real controllers may combine self and mutual measurements for different tasks. Our animated grid is a qualitative map, not a field solver or a vendor scan sequence.

Slide 32 of 157 · The real stack decides

Thicker cover material and gloves change coupling; water may create additional paths, especially when it bridges to grounded structures. A controller tuned to detect weak glove signals may also become more vulnerable to disturbances. Some products use specialized sensing modes and classification to balance these needs. Test the final cover, adhesive, display, metal bezel, heater, power supply and cable together. Change one variable at a time during troubleshooting. Compare idle and touched raw measurements across brightness levels and display patterns, then confirm application behavior. Acceptance should specify gloves by construction and condition, water by composition and application, and success by missed touches, false touches and coordinate error.

Slide 33 of 157 · Apply the idea

Why can a PCAP panel work on a bench and fail after installation behind thicker glass? Pause here and consider your answer. The installed dielectric stack changes signal coupling; bezel and display coupling may also change. Retune and validate the assembled configuration rather than relying on the bare-panel demonstration.

Slide 34 of 157 · Chapter 5 complete

Complete the chapter exercise and review any open questions. Next is Chapter 6: Infrared beam grids: locating an interrupted path. I'm Diggy Breiling with FPD.DEV, The Future of Display and Systems Development. Thank you for joining me.

The installed dielectric stack changes signal coupling; bezel and display coupling may also change. Retune and validate the assembled configuration rather than relying on the bare-panel demonstration.

Accuracy

Closeness of a reported coordinate to a defined reference position.

Precision

Repeatability or spread of results under specified conditions.

Resolution

Smallest coordinate increment or representable change; not equivalent to accuracy.

Latency

Time between explicitly defined input and output events.

PCAP

Projected capacitive touch; patterned electrodes sense electrical coupling changes.

Self-capacitance

Measurement of an electrode relative to an electrical reference.

Mutual capacitance

Measurement of coupling between transmit and receive electrodes.

Baseline

Reference measurement used to identify changes associated with touch.

SNR

Signal-to-noise ratio; specify the signal, noise measure and conditions.

SAW

Surface acoustic wave sensing through controlled propagation on a surface.

APR

Acoustic pulse recognition; a touch-generated mechanical response is used to locate contact.

DST

Dispersive signal technology; a bending-wave sensing implementation.

EMR

Electromagnetic resonance, used by compatible pen digitizers.

Haptics

Tactile feedback produced by the system.

HID

Human Interface Device class/protocol family; the descriptor defines report interpretation.

Report descriptor

Data describing the structure and meaning of HID reports.

Contact ID

Identifier used to distinguish and track a contact.

I2C

Two-wire synchronous bus; raw I2C communication does not itself implement HID.

SPI

Synchronous serial peripheral interface, with device-specific protocol behavior.

UART

Asynchronous serial hardware; electrical levels and framing must be specified.

Optical bonding

Joining optical layers with a transparent material to manage the interface.

Parallax

Apparent relative position shift caused by viewing separated planes from an angle.

NVIS

Night Vision Imaging System; pronounce 'en-vis' in this course.

Qualification

Evidence against a defined set of requirements for an identified configuration.

Validation

Evidence that the system satisfies its intended use.

Verification

Evidence of conformance to specified requirements.

Ghost touch

Reported contact that does not correspond to an intended physical touch.

Palm rejection

Classification or suppression of unintended hand contact.

On-cell / in-cell

Touch structures at a display-cell surface / integrated within the display structure; verify supplier construction.

Touch plane

Geometric region in which the sensing system detects an object.