OL4.AUG–pm
Keypad technology aims to set performance/cost standard
Essentially a means of capacitive proximity sensing, Field Disturbance
Input (FDI) is a patent-pending technology that allows for a noncontact,
solid-state, through-barrier keypad for applications where low cost,
reliability, and environmental immunity are prerequisites. Introduced by
Hughey-Pisau, Ltd. (Elk Grove Village, IL), the FDI-based keypad uses
off-the-shelf components and is designed to replace membrane and
traditional capacitive devices in such applications. Prior devices based
on capacitive sensing methods are generally considered to be unstable and
expensive. In the case of FDI, four factors have allowed it to overcome
these problems: * An improved method for measuring small variations of
capacitance. * A redesigned proximity sensor cell. * The application of
off-the-shelf technology. * Improved algorithms for keystroke detection
and environmental calibration. Key to the implementation of FDI
technology is the measurement of small variations of capacitance. The
diagram describes the basic circuit elements used by Hughey-Pisau for this
measurement.
The generator injects a low-frequency, simple-geometry waveform into the
proximity variable capacitor. The output from the capacitor is fed into a
resistance, a simple single bipolar transistor signal-conditioning block.
The output is then digitized by the A/D converter, which provides the
microcontroller with a numerical representation of the capacitor's value.
This value is manipulated by proprietary algorithms to either detect a
keystroke or be filtered as noise or an environmental aberration. The
enabling factors behind this capacitance measuring method are the nearly
instantaneous measurement of voltage (using the A/D converter), and the
availability of a stable and accurate elapsed time base. In a practical
keypad implementation, the capacitor is replaced by a capacitive sensor
cell array. According to Hughey-Pisau, the circuit allows for the
measurement of capacitance variations in the subpicofarad range–compared
to a minimum value in the tens of picoFarads using conventional
techniques. This level of sensitivity, paradoxically, is essential for
environmental immunity. Current technologies are digital, meaning the user
has to actually touch the pad to get a response. The FDI-based keypad is
analog, in that it senses a disturbance in the field before the finger
actually arrives. This sensitivity allows the keypad to detect smaller
environmental changes and so compensate for them before a false trigger
occurs. The distance from finger to pad required to generate a response
depends mainly on the thickness of the material used to cover/protect the
pad. Typically, it varies from 0 to 1/2 in. To guard against false
triggering, Hughey-Pisau has generated a software algorithm that includes
debouncing, detection threshold hysteresis, asymmetrical environmental
adjusting mechanism, and filtering of environmental adjustments.
Low-cost components A reduced cost relative to traditional capacitive
touchpads has been achieved by design simplicity leading to the use of
off-the-shelf components. A typical keypad with 15 keys requires 23 active
components; a 2N2222 bipolar transistor for each sensor cell, an
off-the-shelf 8-bit microcontroller, a 7406 TTL chip, five diodes, and one
LED. The use of a Motorola controller is arbitrary. The keypad can be
completely custom designed according to the user's requirements for
temperature stability, ruggedness (bullet-proof faceplate, etc.), and
sensitivity for a manufactured cost of less than $40 each in 1,000-piece
quantities. For further information contact Brad Hughey at 708-924-5688 or
. –Patrick Mannion
CAPTION:
The proprietary circuit used by Hughey-Pisau allows for the detection of
capacitance variations in the subpicofarad range to gain, among other
advantages, environmental immunity.
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