Patent Application
20090135031
This invention relates to capacitive keypad entry devices and more particularly relates to an armored keypad apparatus and method for data entry into a protected device.
Keypads and keyboards are familiar devices used to input numeric or alphanumeric data into computers and other electronic devices. The technology used for most common types of keys, such as a computer keyboard, is mechanical, where a spring-loaded key is depressed by a finger and electrical contact is made when the key is fully depressed. It is basically a variation of the momentary contact switch. A second technology is the flexible membrane keypad, commonly found on many microwave ovens. It uses a flexible insulator membrane bubble that has a conductive coating. When pressed, the conductor on the flexible bubble makes contact with the conductor on the substrate below, completing the circuit. A third technology uses a piezo-electric material, which, when pressed, generates a voltage that can be sensed by the underlying circuit board. A fourth technology, the one of interest here, is the capacitive keypad. When a finger is brought in close proximity to an electrode structure, a change in the electric field occurs, inducing a change in current flow which is sensed by electronics on an underlying or nearby circuit board. The capacitive keypad is known in the art and has the advantage of no moving parts. It doesn't suffer from contact resistance or corrosion, and can be placed behind a non-conductive dielectric to protect the electrodes from wear.
For high security purposes, there is a need to restrict access to buildings, offices, and laboratories or to lock boxes containing access keys or confidential information. The buildings might be limited access government facilities. The lock boxes might contain keys to automobiles on a car dealer's lot or cash and receipts from a bank or casino. Because the keypad may be located on the outside of a building and accessible to the general population, it needs to be mechanically protected to be tamper proof or tamper resistant, especially to deliberate attack from a chisel or an electric drill. Furthermore, it is also desirable to have it partially electrically shielded as well, to discourage electronic tampering and to isolate it, at least to some extent, from external electromagnetic interference (EMI).
There have been a number of solutions proposed for vandal proof or EMI shielded keypad applications. For example:
U.S. Pat. No. 7,002,084 (Cox, et al) discloses the use of a touch sensitive capacitive keypad located behind a rigid clear plastic panel and adhesively sealed to provide a watertight enclosure. The device is intended to be employed in the meat packing industry and is made to withstand being washed down with water under high pressure. It is not intended to be electrically shielded or drill proof;
U.S. Pat. No. 5,557,079 (Jackson et al.) discloses an electrically shielded keypad for a two way portable radio. It employs mechanical keys in the keypad, but is not intended to be vandal or tamper proof;
U.S. Pat. No. 5,513,078 (Komrska et al.) discloses an electrically shielded keypad for a radiotelephone that shields the internal electronics from electrical interference from the transmitting antenna. It employs bubble membrane switches in the keypad but is not intended to be tamperproof; and
U.S. Pat. No. 4,716,262 (Morse) discloses a vandal resistant telephone keypad switch, using mechanical keys.
The above references provide for electrical shielding or some type of vandal protection. Only one has a capacitive keyboard and it is neither electrically shielded nor very well protected because of its plastic cover. In the past, keypads with non-capacitive mechanical buttons have often been addressed with a pencil or sharp stylus object, damaging the keypad. It can be appreciated that there would be significant benefits to a keypad that had no moving parts and could be protected mechanically against intrusion from vandals or a determined attack and provide a measure of electrical shielding as well. Such a keypad could be mounted on the exterior of a building, and be sufficiently resistant to the weather and human tampering as not to require a surveillance camera, yet provide robust and reliable sensing of finger touch. Pencils or other sharp objects that can damage a keypad, not having the high dielectric constant of human tissue, would not work reliably, thus discouraging their use.
The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.
The Basic Capacitor
The present invention uses the capacitance of an insulating spacer, the air and the human body in the process of sensing a “key press”. In order to better understand the operation of the present invention, it is first useful to review the elements of a capacitor.
C=keA/D
where k is the dielectric constant of the insulator between the plates, e is the permittivity of free space (the dielectric constant of air), A is the area of the electrode, and D is the separation distance between electrodes. In a capacitive keypad switch, bringing a finger near the key effectively introduces another electrode and reduces D, changing the capacitance. This action results in a net flow of charge which is sensed by the electronics. The value e represents the contribution of the air between the finger and the other two electrodes. The finger need not touch the surface to be sensed.
Capacitive Keypad
Bringing a finger in close proximity or touching the insulating panel 17, changes the electric field and therefore in the charge flow, as shown in
Methods other than that of Quantum of implementing detection of a finger are also in the literature. The Sensor magazine article “Playing the E-Field: Capacitance Sensors in Action”, P. Sieh and M. Steffen, September 2006 asserts that a sine wave is better than a square wave because signal interference is less of a problem. The authors, from Freescale Semiconductor Inc., also presume that the protective layer is a dielectric. They point out that larger electrodes have greater range and sensitivity, but are more susceptible to interference, electrical noise, and stray electric field paths.
Other general guidelines to keypad electrode design are noted in the Quantum Research Group Application note, 2004 where they recommend that the electrode area be as large as practical, but that the size of a fingertip forms a natural limit. The electrode can exceed the size of the fingertip by about 3 mm, because the electric fields drop off near the edges anyway.
A robust and reliable keypad for data entry, resistant to vandalism and deliberate attack.
Conventional wisdom says that replacing the protective dielectric insulating panel 17 of
As shown in
The electronic driver circuit 15 allows for recalibration due to drift in material properties over time. To detect the altered signal when using a metal plate, the driver electronics needs to be calibrated, per the manufacturer's instructions, but with the metal plate in place to provide a robust detection signal. This calibration allows for the effect of the metal plate on the detected signal.
The metal plate 23 will also provide some attenuation of external electromagnetic fields, since these fields will produce eddy currents in the metal and dissipate some of the energy. The metal plate would typically be prevented from shorting the electrodes 11, 12 by using an insulating layer 22 such as a thin insulating spacer or air gap. The metal plate could be made of any common metal such as stainless steel or aluminum, but hardened tool steel gives excellent protection against tools such as drills, saws and chisels. Those skilled in the art may propose similar materials other than specifically mentioned here, but these are considered within the scope of this invention.
Structure of Armored Keypad
On the touch surface 25 of the metal plate, a label or numerical key identifier 24 can be placed. Typically, this label can be painted, embossed, engraved, anodized or silk-screened. Any suitable process that provides good wear resistance is acceptable.
In a case where electrical shielding is unnecessary and shatter resistance is adequate, the metal could be replaced with a durable ceramic such as zirconia, beryllia, or alumina, or any combination of those. The device would look like that shown in
Electrode Patterns
The pattern shown in the example uses an interdigitated electrode structure, a simplified version of that shown in the Quantum Application Note, to provide numerous electric field lines to reliably sense a finger coming from any direction. According to the application note, the patterns for the keys can be any shape, including round, square, rectangular, etc. The electrode width and the electrode gap together form a dimension T. T should be similar to the thickness of the plastic touch panel. Smaller dimensions will work, but they will reduce signal strength.
In the case of the metal protective plate of this invention, the thinner the plate, the better the field penetration to the finger and the better the sense signal. The metal plate thickness is somewhat of a tradeoff between robust sensing and tamper resistance but there are a range of thicknesses that will work well. Tool steel at least as thick as 2 mm is acceptable.
The invention has been described in detail but it will be understood that variations and modifications can be affected within the scope of the invention.
