On October 4, the US Patent & Trademark Office published Apple’s patent application titled Force Imaging Input Device and System . Apple’s patent generally relates to electronic system input devices and, more particularly, to force imaging and location-and-force imaging mutual capacitance systems. While the invention generally relates to such input devices as the iPod and iPhone, the patent also includes such devices as computer systems and particularly a notebook.

Apple’s Patent Background

Numerous touch sensing devices are available for use in computer systems, personal digital assistants, mobile phones, game systems, music systems and the like (i.e., electronic systems). Perhaps the best known are resistive-membrane position sensors which have been used as keyboards and position indicators for a number of years. Other types of touch sensing devices include resistive tablets, surface acoustic wave devices, touch sensors based on resistance, capacitance, strain gages, electromagnetic sensors or pressure sensors, and optical sensors. Pressure sensitive position sensors have historically offered little benefit for use as a pointing device (as opposed to a data entry or writing device) because the pressure needed to make them operate inherently creates stiction between the finger and the sensor surface. Such stiction has, in large measure, prevented such devices from becoming popular.

Owing to the growing popularity of portable devices and the attendant need to integrate all input functions into a single form factor, the touch pad is now one of the most popular and widely used types of input device. Operationally, touch pads may be categorized as either “resistive” or “capacitive.” In resistive touch pads, the pad is coated with a thin metallic electrically conductive layer and resistive layer. When the pad is touched, the conductive layers come into contact through the resistive layer causing a change in resistance (typically measured as a change in current) that is used to identify where on the pad the touch event occurred. In capacitive touch pads, a first set of conductive traces run in a first direction and are insulated by a dielectric insulator from a second set of conductive traces running in a second direction (generally orthogonal to the first direction). The grid formed by the overlapping conductive traces creates an array of capacitors that can store electrical charge. When an object is brought into proximity or contact with the touch pad, the capacitance of the capacitors at that location change. This change can be used to identify the location of the touch event.

One drawback to using touch pads as input devices is that they do not generally provide pressure or force information. Force information may be used to obtain a more robust indication of how a user is manipulating a device. That is, force information may be used as another input dimension for purposes of providing command and control signals to an associated electronic device. Thus, it would be beneficial to provide a force measurement system as part of a touch pad input device.

Apple’s Summary

In one embodiment the invention provides a force sensitive touch pad that includes first and second sets of conductive traces separated by a spring membrane. When a force is applied, the spring membrane deforms moving the two sets of traces closer together. The resulting change in mutual capacitance is used to generate an image indicative of the location (relative to the surface of the touch pad) and strength or intensity of an applied force. In another embodiment, the invention provides a combined location and force sensitive touch pad that includes two sets of drive traces, one set of sense traces and a spring membrane. In operation, one of the drive traces is used in combination with the set of sense traces to generate an image of where one or more objects touch the touch pad. The second set of drive traces is used in combination with the sense traces and spring membrane to generate an image of the applied force’s strength or intensity and its location relative to the touch pad’s surface. Force touch pads and location and force touch pads in accordance with the invention may be incorporated in a variety of electronic devices to facilitate recognition of an increased array of user manipulation.

In yet another embodiment, the described force sensing architectures may be used to implement a display capable of detecting the amount of force a user applies to a display (e.g., a liquid crystal display unit). Display units in accordance with this embodiment of the invention may be used to facilitate recognition of an increased array of user input.

Force Imaging System

Referring to FIG. 3, a block diagram of force imaging system 300 utilizing force detector touch pad 100 is shown. As illustrated, force imaging system 300 comprises force detector 100 coupled to touch pad controller 305 through connectors 120 (for sense signals 310) and 145 (for drive signals 315). Touch pad controller 305, in turn, periodically sends signals to host processor 320 that represent the (spatial) distribution of force applied to detector 100. Host processor 320 may interpret the force information to perform specified command and control actions (e.g., select an object displayed on display unit 325).

Referring to FIG. 4, during operation drive circuit 400 in touch pad controller 305 sends (“drives” a current through drive signals 315 and connector 145 to each of the plurality of drive layer conductive paths 140 in turn.

Because of capacitive coupling, some of this current is carried through to each of the plurality of sense layer conductive paths 115 (see FIG. 1). Sensing circuits 405 (e.g., charge amplifiers) detect the analog signal from sense signals 310 (via connector 120) and send them to analysis circuit 410. One function of analysis circuit 410 is to convert the detected analog capacitance values to digital form (e.g., through A-to-D converters). Another function of analysis circuit is to queue up a plurality of digitized capacitance values for transmission to host processor 320 (see FIG. 3). Yet another function of analysis circuit is to control drive circuit 400 and, perhaps, to dynamically adjust operation of sense circuits 405 (e.g., such as by changing the threshold value at which a “change” in capacitance is detected). One embodiment of controller 305 suitable for use in the present invention is described in US patent application entitled “Multipoint Touch Screen Controller,” Ser. No. 10/999,999 by Steve Hotelling, Christoph Krah and Brian Huppi, filed 15 Mar. 2006 and which is hereby incorporated in its entirety.

Apple lists Steven P. Hotelling (San Jose, CA) and Brian Q. Huppi (San Francisco, CA) as the inventors of this patent. To learn about this patent, click here.

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Written and researched by Neo.