Slide 23 of 157 · Surface capacitive: current paths across a sheet

Welcome to chapter 4: Surface capacitive: current paths across a sheet. I'm Diggy Breiling with FPD.DEV, The Future of Display and Systems Development. Our goal is to distinguish a surface-capacitive sheet from a projected-capacitive electrode matrix. If this raises a question about your own display, you're welcome to ask us at FPD.DEV.

Slide 24 of 157 · One sheet, several measurements

Surface capacitive systems use a conductive coating on glass with electrodes around its edge. A finger changes coupling to the environment and draws AC current through the sheet. The controller compares the measured currents to infer location. The important idea is the distribution of current, not simply detecting that capacitance exists. In an intuitive four-corner model, a touch nearer one region changes the relative contributions. Actual field shaping and calibration belong to the controller design; simple inverse-distance drawings are not a production algorithm. This is different from PCAP, which measures patterned electrodes and may resolve many local interactions.

Slide 25 of 157 · Benefits and constraints

The approach offers a glass-based interface with a relatively simple sensing structure. Conductive coupling through the user is part of the measurement, so insulating gloves and passive tools can be limiting. A basic single-touch model does not uniquely resolve arbitrary simultaneous contacts. For a legacy installation, document both the panel and controller. Replacing one with a visually similar part may change the electrical field, calibration, host reporting or driver requirements. Elo identifies its own surface-capacitive technology as obsolete; that is a lifecycle statement about its offering, not proof that every manufacturer has stopped using the principle.

Slide 26 of 157 · A legacy replacement exercise

Before replacing an installed surface-capacitive panel with PCAP, capture the existing application behavior: touch-down, drag, lift-off, long press and any mouse emulation. Then compare bezel clearance, cover thickness, cable, electrical grounding and host software. A new panel may deliver more contacts than the old application expects. The product can become less usable if a palm creates unwanted gestures or an operating system interprets a long press differently. Define the desired behavior and configure both controller and application accordingly. Run the old and new assemblies against the same task script and retain raw events. A successful desktop tap is only the starting point.

Slide 27 of 157 · Apply the idea

Why is changing a surface-capacitive controller to a PCAP controller not a simple firmware upgrade? Pause here and consider your answer. The sensor topology and acquisition electronics differ. Matching firmware cannot turn a continuous sheet into the required patterned electrode system.

Slide 28 of 157 · Chapter 4 complete

Complete the chapter exercise and review any open questions. Next is Chapter 5: Projected capacitive: electric fields, electrodes and touch maps. I'm Diggy Breiling with FPD.DEV, The Future of Display and Systems Development. Thank you for joining me.

The sensor topology and acquisition electronics differ. Matching firmware cannot turn a continuous sheet into the required patterned electrode system.

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.