High noise immunity sensing methods and apparatus for a capacitive touch device

The present invention is related generally to a capacitive touch device and, more particularly, to sensing methods and apparatus for a capacitive touch device.

A capacitive touch device detects a touch point by sensing the capacitive variation of a capacitive touch sensor. For example, referring to FIG. 1 , a two-dimension touch sensor 10 has X-direction traces X 1 -X 9 and Y-direction traces Y 1 -Y 8 , wherein the traces X 1 -X 9 and Y 1 -Y 9 are not electrically connected to each other. In addition to having their self capacitances, the traces have mutual capacitances therebetween. When contacted by a finger 12 , the trace has both of the self capacitance and the mutual capacitance it senses varying. Currently, a sensor structure for the capacitive touch device may use either all point scan or projection scan. The former one is measuring the variations of the mutual capacitances at all trace intersections during a sensing period of a frame, while the latter one is measuring the variations of the self capacitances of all traces during a sensing period of a frame and generating intersecting points by projecting the intersections of the two directions. In sensing the variations of the self capacitances, taking a trace Y 7 as an example, an excitation signal Tx is applied to the trace Y 7 , and a sensed signal Rx fed back by the same trace Y 7 is received. The sensed value generated from the sensed signal Rx represents the self capacitance of the trace Y 7 . When a finger 12 contacts the trace Y 7 , the ground capacitance of the finger 12 is connected to the self capacitance of the trace Y 7 in parallel, so the value now sensed at trace Y 7 is different from that obtained when the trace Y 7 is untouched. This is therefore a basis for determining whether the trace Y 7 is touched. In sensing the mutual capacitance, taking traces X 8 and Y 7 for example, an excitation signal Tx is applied to the trace Y 7 (or X 8 ), and a sensed signal Rx fed back by the corresponding trace X 8 (or Y 7 ) is received. The sensed value generated from the sensed signal Rx represents the mutual capacitance between the traces X 8 and Y 7 . When the finger 12 contacts the intersection between the traces X 8 and Y 7 , the ground capacitance of the finger 12 reduces the sensed mutual capacitance, which is a proof of that the intersection of the traces X 8 and Y 7 are touched.

In sensing process of a capacitive touch sensor, noise interference can bring errors to the sensed value, causing an untouched point to be mistaken as a touched point, or causing the reported coordinates inaccurate, which in turn leads to misoperation. For example, referring to FIG. 1 , when a finger 12 touches at a point between the traces X 7 , X 8 and Y 7 , Y 8 , the mutual capacitance between the traces X 7 , X 8 and Y 7 , Y 8 or the self capacitance of the trace X 7 , X 8 and Y 7 , Y 8 can be used to calculate the coordinates representing where the finger 12 is. However, if the sensed value is error due to noise interference, the calculated coordinates are deviated from the actual position of the finger 12 . If the sensed value of the mutual capacitance between the trace X 3 and Y 7 or the sensed values of the self capacitances of the traces X 3 and Y 7 varies over the threshold set by the sensing circuit due to noise interference, the sensing circuit will identify the intersection 14 of the traces X 3 and Y 7 as another touched point. For eliminating mistakes caused by noise interference, an approach is to use a median filter for filtering signals, yet this can reduce the sensitivity. Another approach is to raise a threshold of the algorithm, but this can make the capacitive touch sensor less supportive to hardware. Yet another approach is to sense the self capacitance or mutual capacitance for several successive times in a frame, and then average the sensed values. For example, referring to the system block diagram of FIG. 2 , a capacitive touch panel 16 has a capacitive touch sensor (not shown) connected to a sensing apparatus 18 . When a finger or other electrically conductive object touches the capacitive touch panel 16 , the capacitance of the capacitive touch sensor changes and this change is sensed by the sensing apparatus 18 and reflected in an output signal So. In the sensing apparatus 18 , a control unit 20 controls an excitation unit 22 to apply an excitation signal Tx to the capacitive touch sensor, a sampling unit 24 samples the sensed signal Rx fed back by the capacitive touch sensor, an analog-to-digital converter (ADC) 26 converts the sampled signal Rx into a sensed value Sd, and an averaging unit 28 averages sensed values Sd obtained in a given time period to generate an output signal So. In sensing the same self capacitance or mutual capacitance, referring to FIG. 3 , according to one scan frequency, the excitation signal Tx is applied successively for several times and the sensed signal Rx is sampled successively for several times, and one sensed value Sd is obtained in each sensing cycle Ts, while the average of such sensed values Sd is the output signal So. The more times of repeatedly sensing the same self capacitance or mutual capacitance, the smaller the interference caused by noise of a certain frequency to the output signal is, yet the lower the frame rate is, making the response of the capacitive touch device slower and bringing about users' unsmooth operation.

Conventional solutions all compromise other parameters, and therefore it is desired a sensing method and apparatus for suppressing noise interference without compromising other parameters.

The present invention is directed to high noise immunity sensing methods and apparatus for a capacitive touch device.

According to the present invention, a high noise immunity sensing method for a capacitive touch device uses different scan frequencies to sense self capacitances of different traces in the capacitive touch device in a frame.

According to the present invention, a high noise immunity sensing apparatus for a capacitive touch device includes an excitation unit, a sampling unit and a control unit. The control unit controls the excitation unit and the sampling unit to use different scan frequencies to sense self capacitances of different traces in the capacitive touch device in a frame.

According to the present invention, a high noise immunity sensing method for a capacitive touch device uses different scan frequencies to sense the capacitive touch device for a self capacitance and a mutual capacitance in a frame.

According to the present invention, a high noise immunity sensing apparatus for a capacitive touch device includes an excitation unit, a sampling unit and a control unit. The control unit controls the excitation unit and the sampling unit to use different scan frequencies to sense the capacitive touch device for a self capacitance and a mutual capacitance in a frame.

According to the present invention, a high noise immunity sensing method for a capacitive touch device uses different scan frequencies to sense a self capacitance of a trace in the capacitive touch device in a frame.

According to the present invention, a high noise immunity sensing apparatus for a capacitive touch device includes an excitation unit, a sampling unit and a control unit. The control unit controls the excitation unit and the sampling unit to use different scan frequencies to sense a self capacitance of a trace in the capacitive touch device in a frame.

According to the present invention, a high noise immunity sensing method for a capacitive touch device uses different scan frequencies to sense a mutual capacitance between two traces in the capacitive touch device in a frame.

According to the present invention, a high noise immunity sensing apparatus for a capacitive touch device includes an excitation unit, a sampling unit and a control unit. The control unit controls the excitation unit and the sampling unit to use different scan frequencies to sense a mutual capacitance between two traces in the capacitive touch device in a frame.

By combining time domain and space domain noise-eliminating approaches, theses sensing methods and apparatus can reduce probabilities of noise interference, without compromising other parameters.

These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic drawing of a capacitive touch sensor;

FIG. 2 is a system block diagram of a capacitive touch device;

FIG. 3 is a schematic drawing illustrating repeatedly sensing a single self capacitance or mutual capacitance and averaging the sensed values;

FIG. 4 is a schematic drawing of a first embodiment of the present invention;

FIG. 5 is a schematic drawing of a second embodiment of the present invention;

FIG. 6 is a schematic drawing of a third embodiment of the present invention;

FIG. 7 is a schematic drawing of a fourth embodiment of the present invention; and

FIG. 8 is a schematic drawing illustrating using different scan frequencies to sense a single self capacitance or mutual capacitance for several times, averaging the sensed values for suppressing noise interference.

The following embodiments may use the sensing apparatus 12 as shown in FIG. 2 , but have the control unit 20 to change the scan frequency, so as to control the excitation unit 22 and the sampling unit 24 to apply the excitation signal Tx to the capacitive touch sensor and to sample the sensed signal Rx and to control ADC conversion in various scan frequencies. Thereby, the physical circuit of the sensing apparatus 12 can remain unchanged, and the only affected part is the firmware or software used in the control unit 20 , so that the present invention can be applied to capacitive touch devices with various needs by easily changing the instruction set of the control unit 20 .

In the first embodiment of the present invention, different scan frequencies are used to sense the traces in different directions. For example, referring to FIG. 4 , in sensing a frame, a scan frequency fx is used to sense the self capacitances of X-direction traces X 1 -X 9 , and a scan frequency fy different from fx is used to sense the self capacitances of Y-direction traces Y 1 -Y 8 . To specify, the system uses the scan frequency fx to apply the excitation signal Tx to the X-direction traces X 1 -X 9 , and uses the same scan frequency fx to sample the sensed signal Rx fed back by the same X-direction traces X 1 -X 9 and perform ADC conversion. The same is applied to the Y-direction trace Y 1 -Y 8 . The system uses another scan frequency fy to apply the excitation signal Tx to the Y-direction trace Y 1 -Y 8 , and uses the same scan frequency fy to sample the sensed signal Rx fed back by the same Y-direction trace Y 1 -Y 8 and perform ADC conversion. Thereby, all the self capacitances in a frame can be obtained. Since different scan frequencies fx and fy are used to sensing the self capacitances of the X-direction traces and the self capacitances of the Y-direction traces, noise interference of a certain frequency can be suppressed. Preferably, the present invention further comprises sensing the mutual capacitances between the X-direction traces and Y-direction traces, but using a scan frequency fm different from fx and fy. Taking the case of sensing the mutual capacitance between the traces X 1 and Y 1 as an example, the system applies the excitation signal Tx to the trace X 1 (or Y 1 ) according to the scan frequency fm, and uses the same scan frequency fm to sample the sensed signal Rx fed back by the trace Y 1 (or X 1 ) and to perform ADC conversion. In an alternative embodiment, for sensing a frame, the used scan frequencies fx, fy and fm include only two different frequencies, e.g. fx=fm≠fy or fx≠fy=fm. Preferably, after all the self capacitances in a frame is obtained, the self capacitances are used to determine the touched point, and then the mutual capacitance between the traces where the touched point is on is sensed.

The second embodiment of the present invention uses different scan frequencies to sense different traces in the same direction. For example, referring to FIG. 5 , different scan frequencies fx 1 -fx 9 are used to sense the self capacitances of the traces X 1 -X 9 , respectively, so noise of a certain frequency can interfere at most one trace. In an alternative embodiment, fewer scan frequencies may be used. For example, two scan frequencies fx 1 and fx 2 are used to sense the self capacitances of the traces X 1 -X 4 and X 5 -X 9 , respectively, or two scan frequencies fx 1 and fx 2 are used to sense the self capacitances of the traces X 1 , X 3 , X 5 , X 7 , X 9 and X 2 , X 4 , X 6 , X 8 , respectively.

By applying the sensing method of the embodiment depicted in FIG. 5 to a two-dimension touch sensor, as shown in FIG. 6 , in sensing a frame, different scan frequencies are used to sense the self capacitances of the traces in the same direction and different scan frequencies are used to sense the mutual capacitances between the traces in different directions. More particularly, the different scan frequencies fx 1 -fx 9 are used to sense the self capacitances of the X-direction traces X 1 -X 9 , respectively, and the different scan frequencies fy 1 -fy 5 are used to sense the self capacitances of the Y-direction traces Y 1 -Y 8 , respectively, while the different scan frequencies fm 1,1 -fm 9,8 are used to sense the mutual capacitances at the intersections, respectively. In this embodiment, the scan frequencies fx 1 -fx 9 , fy 1 -fy 8 and fm 1,1 -fm 9,8 are all different, so noise interference of a certain frequency is minimized. In an alternative embodiment, fewer scan frequencies may be uses. For example, in sensing a frame, the different scan frequencies fx 1 -fx 9 are used to sense the self capacitances of the X-direction traces X 1 -X 9 , respectively, and a single scan frequency fy≠fx 1 -fx 9 is used to sense the self capacitances of the Y-direction traces Y 1 -Y 8 . Alternatively, two scan frequencies fx 1 and fx 2 are used to sense the self capacitances of the X-direction traces, and two scan frequencies fy 1 and fy 2 are used to sense the self capacitances of the Y-direction traces. Alternatively, different scan frequencies fm 1,1 -fm 9,8 are used to sense the mutual capacitances of the intersections, and a single scan frequency fx (or fy) is used to sense the self capacitances of the X-direction traces X 1 -X 9 (or Y-direction traces Y 1 -Y 8 ). Alternatively, the intersections are grouped into two groups, which are sensed in two scan frequencies fm 1 and fm 2 , respectively. Preferably, after all the self capacitances in a frame is obtained, the self capacitances are used to determine the touched point, and then the mutual capacitance between the traces at the touched point is sensed. Preferably, only one scan frequency fm is used to sense the mutual capacitances of the all intersections, wherein the scan frequency fm may be identical to one of the scan frequency for the X-direction traces and the scan frequency for the Y-direction traces, or may be different from the both.

The fourth embodiment of the present invention uses different scan frequencies to sense the self capacitance of a single trace or the mutual capacitance between two traces. For example, referring to FIG. 7 , each trace is repeatedly sensed for N times for its self capacitance, and each time sensing uses a different scan frequency. Then the sensed values obtained by the different scan frequencies are averaged and taken as the sensed value of this trace, thereby further reducing the incidence of noise interference. Taking the trace X 1 for example, the self capacitances of the trace X 1 are repeatedly sensed for 32 times to see its variation. Among the 32 times of sensing, 32 different scan frequencies fx 1,1 -fx 1,32 be used, respectively, or a scan frequency fx 1,1 may be used for 16 times while another scan frequency fx 1,2 may be used for the other 16 times, thereby preventing the sensed value of the 32 times of sensing for the trace X 1 getting interference from noise of the same frequency. FIG. 8 illustrates how this sensing method reduces noise interference. Since the different scan frequencies are used in 32 repeated sensing cycles, the noise affecting the first 16 times does not affect the latter 16 times of sensing. The same manner can be used to sense the mutual capacitance of an intersection, in which different scan frequencies are used to repeatedly sense the mutual capacitance of a single intersection, and then the obtained sensed values are averaged to generate the sensed value of this intersection. As described in the previous embodiments, in an alternative embodiment, fewer scan frequencies may be used.

As demonstrated in the above embodiments, by combining noise-eliminating approaches working in both time domain and space domain, the incidence of noise interference can be significantly reduced.

While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.