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Capacitance sensor with sensor capacitance compensation

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Capacitance sensor with sensor capacitance compensation


A capacitance sensing circuit may include a switching circuit configured to generate a sensor current by charging and discharging a capacitive sensor electrode, and a current mirror that generates a mirror current based on the sensor current. Based on the mirror current, a measurement circuit generates an output signal representative of a capacitance of the capacitive sensor electrode.

Browse recent Cypress Semiconductor Corporation patents - San Jose, CA, US
Inventors: Andriy Maharyta, Andriy Ryshtun
USPTO Applicaton #: #20120286800 - Class: 324603 (USPTO) - 11/15/12 - Class 324 


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The Patent Description & Claims data below is from USPTO Patent Application 20120286800, Capacitance sensor with sensor capacitance compensation.

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CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application No. 61/067,540, filed Feb. 27, 2008.

TECHNICAL FIELD

This disclosure relates to the field of user interface devices and, in particular, to capacitive sensor devices.

BACKGROUND

Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse X/Y movement by using two defined axes which contain a collection of sensor elements that detect the position of a conductive object, such as a finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.

One type of touchpad operates by way of capacitance sensing utilizing capacitance sensors. The capacitance, detected by a capacitance sensor, changes as a function of the proximity of a conductive object to the sensor. The conductive object can be, for example, a stylus or a user\'s finger. In a touch-sensor device, a change in capacitance detected by each sensor in the X and Y dimensions of the sensor array due to the proximity or movement of a conductive object can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitive sensor is processed by a processing device, which in turn produces electrical or optical signals representative of the position of the conductive object in relation to the touch-sensor pad in the X and Y dimensions. A touch-sensor strip, slider, or button operates on the same capacitance-sensing principle.

A first type of conventional touchpad is composed of a matrix of rows and columns. Within each row or column, there are multiple sensor elements. However, all sensor pads within each row or column are coupled together and operate as one long sensor element. A second type of conventional touchpad is composed of an XY array of independent sense elements, where each sensor element in a row or column is separately sensed. Here, each row and column is composed of multiple sensing elements, each capable of independent detection of a capacitive presence and magnitude. These may then be used to detect any number of substantially simultaneous touches.

The capacitive sensing systems used in interface devices such as touchpads generally operate by detecting changes in the capacitances of the capacitive sensors resulting from proximity or contact of an object with the sensor, however the ability to resolve changes in capacitance may be impaired if the changes in capacitance to be detected by the sensor are small relative to the capacitance of the sensor. For instance, a capacitive sensor element that is configured to detect an input, such as proximity or contact with a finger or other object, may have a capacitance CP between the sensor element and ground when no input is present. The capacitance CP is known as the parasitic capacitance of the sensor. For capacitive sensors having multiple sense elements, a mutual capacitance CM may also be present between two or more sense elements. An input detected by the sensor may cause a change in capacitance CF that is much smaller than CP or CM. Accordingly, where the sensor capacitance is represented as a digital code, the parasitic or mutual capacitances may be represented by a larger proportion of the discrete capacitance levels resolvable by the digital code, while the capacitance change CF is represented by fewer of these discrete levels. In such cases, the capacitance change CF due to an input may not be resolvable to a high degree of resolution.

Additionally, the design of some capacitive sensors also results in a high susceptibility to noise due to electromagnetic interference (EMI). For example, a capacitive touchpad or slider device may include an array of capacitive sensor elements, each of which may include a conductive trace having a substantial length. Such conductive traces may couple noise into a capacitance measurement circuit and reduce the ability of the measurement circuit to measure capacitance levels accurately and precisely.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings.

FIG. 1 illustrates a block diagram of one embodiment of an electronic system in which a capacitance sensor is used;

FIG. 2 is a circuit diagram illustrating a circuit for measuring capacitance of a capacitive sensor, according to one embodiment;

FIG. 3A is a block diagram illustrating a connection between a capacitive sensor and an integrated circuit chip, according to one embodiment;

FIG. 3B is a circuit diagram illustrating equivalent impedances of a capacitance measurement circuit in an integrated circuit chip, according to one embodiment;

FIG. 4 is a circuit diagram illustrating one embodiment of a capacitance sensing circuit including a current mirror;

FIG. 5 illustrates one embodiment of a capacitance measurement circuit implemented in an integrated circuit chip;

FIG. 6 illustrates one embodiment of a capacitance measuring circuit implemented in an integrated circuit chip;

FIG. 7A is a graph illustrating signals associated with the operation of a capacitance sensing circuit, according to one embodiment;

FIG. 7B is a graph illustrating signals associated with the operation of a capacitive sensing circuit, according to one embodiment;

FIG. 8 is a flow diagram illustrating a process for sensing capacitance of a capacitive sensor, according to one embodiment; and

FIG. 9 is a flow diagram illustrating a process using a mirror current for measuring the capacitance of a capacitive sensor, according to one embodiment.

DETAILED DESCRIPTION

Described herein are embodiments of a method and apparatus for measuring a capacitance of a capacitive sensor while compensating for a baseline capacitance of the sensor and maintaining a low input impedance for increasing noise immunity. The following description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present invention. It will be apparent to one skilled in the art, however, that at least some embodiments of the present invention may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in simple block diagram format in order to avoid unnecessarily obscuring the present invention. Thus, the specific details set forth are merely exemplary. Particular implementations may vary from these exemplary details and still be contemplated to be within the spirit and scope of the present invention.

Embodiments of a method and apparatus for measuring capacitance of a capacitive sensor are described. In one embodiment, a capacitance sensing circuit detects an input at the capacitive sensor by detecting a change in the capacitance of a sensor element. For example, a finger placed near the capacitive sensor may cause an increase CF in the capacitance of the sensor. The magnitude of CF may be detected and converted to a voltage level or a digital code (by a capacitance-to-code conversion circuit) that can be processed by a computer or other circuit.

One embodiment of a capacitive sensor includes a positive electrode connected to a voltage source and a negative electrode connected to ground. The capacitance of such a capacitive sensor can be measured by repeatedly charging the positive electrode of the sensor from a voltage source and discharging the positive electrode to ground, causing a sensor current to flow between the voltage source and ground. Since the flow of the sensor current depends on the capacitance of the sensor, the sensor current can be measured to determine the capacitance of the sensor.

A portion of the sensor current may be attributable to a baseline capacitance CB, which represents a total capacitance of the capacitive sensor when no input (i.e., a finger touch) is present. The baseline capacitance may include the mutual capacitance CM between the positive and the negative electrodes, and/or the parasitic capacitance CP between each of the electrodes and other conductors (e.g., printed circuit board (PCB) traces or electrodes of other capacitive sensors).

The ability of a digital code to resolve many levels of CF (which represents a change in capacitance of the capacitive sensor resulting from an input such as a finger touch) may be degraded if the baseline capacitance CB is large in comparison to CF. Therefore, the dynamic range of a capacitance-to-code conversion circuit may not be used effectively, since CF is measured in the presence of the large base value of CB. Thus, in one embodiment, the effects of the baseline capacitance CB are compensated using a current source to cancel the portion of the sensor current attributable to the baseline capacitance. This allows the digital code to resolve CF using a greater number of discrete levels, increasing the dynamic range utilization of the capacitance sensing circuit.

In one embodiment, a current mirror generates a mirror current based on the sensor current. A measurement circuit measures the mirror current rather than measuring the sensor current directly. The presence of the current mirror decreases the input impedance seen by the capacitive sensor. The decreased input impedance increases immunity of the measurement circuit to low frequency noise that is coupled to the system through the capacitive sensor or other conductors, such as PCB traces. Additionally, the current mirror isolates the measurement circuit from the voltage supply used to charge the capacitive sensor so that the voltage supply is not limited by the input requirements of the measurement circuit.

FIG. 1 illustrates a block diagram of one embodiment of an electronic system in which a capacitance sensor with baseline capacitance compensation circuit and current mirror can be implemented. Electronic system 100 includes processing device 110, touch-sensor pad 120, touch-sensor slider 130, touch-sensor buttons 140, host processor 150, embedded controller 160, and non-capacitance sensor elements 170. The processing device 110 may include analog and/or digital general purpose input/output (“GPIO”) ports 107. GPIO ports 107 may be programmable. GPIO ports 107 may be coupled to a Programmable Interconnect and Logic (“PIL”), which acts as an interconnect between GPIO ports 107 and a digital block array of the processing device 110 (not illustrated). The digital block array may be configured to implement a variety of digital logic circuits (e.g., DACs, digital filters, or digital control systems) using, in one embodiment, configurable user modules (“UMs”). The digital block array may be coupled to a system bus. Processing device 110 may also include memory, such as random access memory (RAM) 105 and program flash 104. RAM 105 may be static RAM (SRAM), and program flash 104 may be a non-volatile storage, which may be used to store firmware (e.g., control algorithms executable by processing core 102 to implement operations described herein). Processing device 110 may also include a memory controller unit (MCU) 103 coupled to memory and the processing core 102.

The processing device 110 may also include an analog block array (not illustrated). The analog block array is also coupled to the system bus. Analog block array also may be configured to implement a variety of analog circuits (e.g., ADCs or analog filters) using, in one embodiment, configurable UMs. The analog block array may also be coupled to the GPIO 107.

As illustrated, capacitance sensing circuit 101 may be integrated into processing device 110. Capacitance sensing circuit 101 may include analog I/O for coupling to an external component, such as touch-sensor pad 120, touch-sensor slider 130, touch-sensor buttons 140, and/or other devices. Capacitance sensing circuit 101 and processing device 102 are described in more detail below.

The embodiments described herein are not limited to touch-sensor pads for notebook implementations, but can be used in other capacitive sensing implementations, for example, the sensing device may be a touch screen, a touch-sensor slider 130, or touch-sensor buttons 140 (e.g., capacitance sensing buttons). In one embodiment, these sensing devices may include one or more capacitive sensors. It should also be noted that the embodiments described herein may be implemented in other sensing technologies than capacitive sensing, such as resistive, optical imaging, surface wave, infrared, dispersive signal, and strain gauge technologies. Similarly, the operations described herein are not limited to notebook pointer operations, but can include other operations, such as lighting control (dimmer), volume control, graphic equalizer control, speed control, or other control operations requiring gradual or discrete adjustments. It should also be noted that these embodiments of capacitive sensing implementations may be used in conjunction with non-capacitive sensing elements, including but not limited to pick buttons, sliders (ex. display brightness and contrast), scroll-wheels, multi-media control (ex. volume, track advance, etc) handwriting recognition and numeric keypad operation.

In one embodiment, the electronic system 100 includes a touch-sensor pad 120 coupled to the processing device 110 via bus 121. Touch-sensor pad 120 may include a multi-dimension sensor array. The multi-dimension sensor array includes multiple sensor elements, organized as rows and columns. In another embodiment, the electronic system 100 includes a touch-sensor slider 130 coupled to the processing device 110 via bus 131. Touch-sensor slider 130 may include a single-dimension sensor array. The single-dimension sensor array includes multiple sensor elements, organized as rows, or alternatively, as columns. In another embodiment, the electronic system 100 includes touch-sensor buttons 140 coupled to the processing device 110 via bus 141. Touch-sensor buttons 140 may include a single-dimension or multi-dimension sensor array. The single- or multi-dimension sensor array may include multiple sensor elements. For a touch-sensor button, the sensor elements may be coupled together to detect a presence of a conductive object over the entire surface of the sensing device. Alternatively, the touch-sensor buttons 140 may have a single sensor element to detect the presence of the conductive object. In one embodiment, touch-sensor buttons 140 may include a capacitive sensor element. Capacitive sensor elements may be used as non-contact sensor elements. These sensor elements, when protected by an insulating layer, offer resistance to severe environments.

The electronic system 100 may include any combination of one or more of the touch-sensor pad 120, touch-sensor slider 130, and/or touch-sensor button 140. In another embodiment, the electronic system 100 may also include non-capacitance sensor elements 170 coupled to the processing device 110 via bus 171. The non-capacitance sensor elements 170 may include buttons, light emitting diodes (LEDs), and other user interface devices, such as a mouse, a keyboard, or other functional keys that do not require capacitance sensing. In one embodiment, buses 171, 141, 131, and 121 may be a single bus. Alternatively, these buses may be configured into any combination of one or more separate buses.

Processing device 110 may include internal oscillator/clocks 106 and communication block 108. The oscillator/clocks block 106 provides clock signals to one or more of the components of processing device 110. Communication block 108 may be used to communicate with an external component, such as a host processor 150, via host interface (I/F) line 151. Alternatively, processing block 110 may also be coupled to embedded controller 160 to communicate with the external components, such as host 150. In one embodiment, the processing device 110 is configured to communicate with the embedded controller 160 or the host 150 to send and/or receive data.

Processing device 110 may reside on a common carrier substrate such as, for example, an integrated circuit (IC) die substrate, a multi-chip module substrate, or the like. Alternatively, the components of processing device 110 may be one or more separate integrated circuits and/or discrete components. In one exemplary embodiment, processing device 110 may be a Programmable System on a Chip (PSoC™) processing device, manufactured by Cypress Semiconductor Corporation, San Jose, Calif. Alternatively, processing device 110 may be one or more other processing devices known by those of ordinary skill in the art, such as a microprocessor or central processing unit, a controller, special-purpose processor, digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or the like.

It should also be noted that the embodiments described herein are not limited to having a configuration of a processing device coupled to a host, but may include a system that measures the capacitance on the sensing device and sends the raw data to a host computer where it is analyzed by an application. In effect the processing that is done by processing device 110 may also be done in the host.

Capacitance sensing circuit 101 may be integrated into the IC of the processing device 110, or alternatively, in a separate IC. Alternatively, descriptions of capacitance sensing circuit 101 may be generated and compiled for incorporation into other integrated circuits. For example, behavioral level code describing capacitance sensing circuit 101, or portions thereof, may be generated using a hardware descriptive language, such as VHDL or Verilog, and stored to a machine-accessible medium (e.g., CD-ROM, hard disk, floppy disk, etc.). Furthermore, the behavioral level code can be compiled into register transfer level (“RTL”) code, a netlist, or even a circuit layout and stored to a machine-accessible medium. The behavioral level code, the RTL code, the netlist, and the circuit layout all represent various levels of abstraction to describe capacitance sensing circuit 101.

It should be noted that the components of electronic system 100 may include all the components described above. Alternatively, electronic system 100 may include only some of the components described above.

In one embodiment, electronic system 100 may be used in a notebook computer. Alternatively, the electronic device may be used in other applications, such as a mobile handset, a personal data assistant (PDA), a keyboard, a television, a remote control, a monitor, a handheld multi-media device, a handheld video player, a handheld gaming device, or a control panel.

FIG. 2 is a circuit diagram illustrating a circuit for measuring capacitance of a capacitive sensor, according to one embodiment. Capacitance sensing circuit 200 may be included in an electronic system, such as electronic system 100.

Capacitive sensing circuit 200 includes capacitive sensor 221, switching circuit 220, compensation circuit 240, current mirror 260, and measurement circuit 280. Switching circuit 220 includes switches 223 and 222, through which sensor current IS 224 flows. Compensation circuit 240 includes current digital-to-analog converter (IDAC) 241 that outputs a compensation current IC 242 that flows into node 225. Current mirror 260 includes transistors 264 and 265, which are connected through resistors R1 262 and R2 263, respectively, to supply voltage VCC. Compensated sensor current ID 261 flows out of transistor 264 and mirror current IM flows out of transistor 265. Measurement circuit 280 includes an integration capacitor CINT connected to an input of a comparator 284, a timer 285 connected to the output of the comparator 284, and an oscillator 286 connected to the timer 285. The output of comparator 284 is also connected to a discharge switch 281 that discharges CINT 282.

In one embodiment, the capacitive sensor 221 is a sensor having a capacitance that is affected by the proximity of a conductive object, such as a finger. The capacitive sensor 221 is repeatedly charged and discharged by switching circuit 220. In one embodiment, switches 222 and 223 are operated in a non-overlapping manner (so that both switches are not simultaneously closed) to charge and discharge capacitive sensor 221. For example, switch 222 connects sensor 221 to a positive voltage supply VCC, charging sensor 221. Switch 222 opens and switch 223 closes, discharging sensor 221 to ground. The repeated charging and discharging of the sensor 221 results in the flow of sensor current IS 224 from node 225 to ground. Thus, the combination of capacitive sensor 221 and switching circuit 220 can be represented as an equivalent resistance R5 that conducts sensor current IS 224 between node 225 and ground.

Compensation circuit 240 compensates for a baseline capacitance CB of the capacitive sensor 221 by supplying a compensation current IC 242 to the capacitive sensor 221. Specifically, the compensation current supplies IC 242 into node 225 connected to the sensor 221 through switch 222. In one embodiment, IC 242 is approximately equal to the amount of current attributable to the baseline capacitance of sensor 221. For example, IC 242 may be chosen so that the net current flowing into node 225 is zero when no input is present at capacitive sensor 221. In one embodiment, IC 242 is supplied from a current digital-to-analog converter (IDAC) 241, which is programmed to output a desired compensation current IC 242.

In addition to the switching circuit 220 and the compensation circuit 240, node 225 is also connected to current mirror 260. Compensated sensor current ID 261, which is the difference between IC 242 and IS 224, flows from transistor 264 of current mirror 260 into node 225. The magnitude of ID 261 indicates the magnitude of a change in capacitance CF at capacitive sensor 221. For example, when no input is present at capacitive sensor 221, IS 224 is equal to IC 242 so that ID 261 is zero. When the capacitance at capacitive sensor 221 is increased by an input at capacitive sensor 221, the amount of charge stored in sensor 221 and then discharged to ground increases with the capacitance. Thus, the sensor current IS 224 and the compensated sensor current ID 261 also increase.

The compensated sensor current ID 261 flows out of the transistor 264 and is mirrored by transistor 265 to generate a mirror current IM 266. In one embodiment, IM 266 is approximately equal to ID. In alternative embodiments, the current mirror 260 amplifies ID so that IM is proportionally greater or less than ID by a desired amplification factor. For example, a high voltage may be used to charge sensor 221, resulting in a large value of ID 261 that is outside the operating range of measurement circuit 280. The current mirror 260 can generate a reduced mirror current IM 266, corresponding to ID 261, that is within the operating range of the measurement circuit 280.

The measurement circuit 280 receives the mirror current IM 266 as an input. IM 266 is used to charge integration capacitor CINT 282 so that the voltage at CINT increases over time. The voltage of CINT is applied to an input of comparator 284. Comparator 284 compares the voltage of CINT with a reference voltage VREF 283. When the CINT voltage exceeds VREF 283, the comparator outputs a signal to timer 285. The output of comparator 284 is also connected to discharge switch 281 so that discharge switch 281 discharges CINT when the voltage of CINT exceeds VREF 283, preparing CINT for the next charge cycle.

Comparator 284 thus outputs a series of pulses as the voltage of CINT exceeds VREF 283 and subsequently drops below VREF 283 as CINT is discharged. Timer 285 detects the time between these pulses and outputs a count value that corresponds to the capacitance of sensor 221. For example, the duration between pulses output by the comparator 284 decreases with an increase in the mirror current IM 266 because a higher IM charges CINT more quickly. In one embodiment, timer 285 counts the number of oscillations from oscillator 286 between pulses from the comparator 284.



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stats Patent Info
Application #
US 20120286800 A1
Publish Date
11/15/2012
Document #
13443718
File Date
04/10/2012
USPTO Class
324603
Other USPTO Classes
International Class
01R27/26
Drawings
10



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