MULTIPLE CAPACITANCE MEASURING CIRCUITS AND METHODS
In capacitance measurement circuits and methods that measure capacitances at multiple locations by ramping voltage signals between reference levels, the phase of the voltage signal ramping can be controlled, for example to mitigate currents flowing between measurement locations. Such capacitance measurement circuits and methods can be utilized in touch sensor devices that determine touch position based on capacitance measurements on multiple locations on a touch surface. Controlling the phase of the voltage signal ramps can include concurrently starting the voltage signal ramps, for example by effecting a delay on at least one signal channel that has reached a voltage threshold, or by regulating all the voltage ramps according to a fixed frequency.
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This patent document claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 61/017,451, entitled “Multiple Capacitance Measuring Circuits and Methods” as was filed on Dec. 28, 2007, the disclosure of which is incorporated by reference herein in its entirety.
The present invention relates generally to circuits and methods for measuring multiple capacitances, and to systems such as capacitive touch sensing systems that utilize multiple capacitance measuring circuits and methods, and in particular to mitigating current flow between capacitance measurement locations in such circuits and methods.
BACKGROUNDTouch sensitive devices allow a user to conveniently interface with electronic systems and displays by reducing or eliminating the need for mechanical buttons, keypads, keyboards, and pointing devices. For example, a user can carry out a complicated sequence of instructions by simply touching an on-display touch screen at a location identified by an icon. In many touch sensitive devices, the input is sensed when a conductive object in the sensor is capacitively coupled to a conductive touch implement such as a user's finger. Such devices measure capacitance at multiple locations due to the touch disturbance, and use the measured capacitances to determine touch position.
SUMMARY OF THE INVENTIONIn certain embodiments, the present invention provides methods (and corresponding circuitry) for use in a touch sensor device that measures capacitances at a plurality of locations on a touch surface in response to a touch object coupling to the touch surface at a touch position, the capacitances being measured by applying electrical charge to the plurality of locations to ramp respective voltage signals between a first reference level and a second reference level. Such methods include controlling the phase of the voltage signal ramps to mitigate currents flowing between the locations. In certain embodiments, controlling the phase of the voltage signal ramps includes concurrently starting the voltage signal ramps, for example by effecting a delay on at least one signal channel that has reached a voltage threshold, or by regulating all the voltage ramps according to a fixed frequency. In certain embodiments, time differentials between when respective voltage signal ramps reach a threshold level can be determined, and subsequent ramp starting times can be adjusted in response.
In certain embodiments, the present invention provides methods for use with a device that measures capacitances at a plurality of locations, each location associated with a capacitance measurement channel to ramp respective voltage signals on the respective capacitances. Such methods include (a) concurrently initiating forward-direction voltage signal ramps on multiple channels, and (b) effecting a delay on the forward-direction voltage signal ramp of at least one channel in response to reaching a voltage signal threshold. In certain embodiments, such methods can further include (c) concurrently initiating reverse-direction voltage signal ramps on the multiple channels, and (d) effecting a delay on the reverse-direction voltage signal ramp of at least one channel in response to reaching a low voltage signal threshold. Such alternate forward and reverse ramping can be repeated a desired number of times.
In certain embodiments, the present invention provides capacitive touch sensor devices for determining touch position by measuring capacitances at a plurality of locations on a touch surface in response to a touch object coupling to the touch surface at the touch position. Such devices include circuitry associated with each location to ramp respective voltage signals between a first reference level and a second reference level by applying an electrical charge, and circuitry to control the phase of the voltage signal ramps to mitigate currents flowing between the locations.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.
The present disclosure may be more completely understood and appreciated in consideration of the following detailed description of various embodiments in connection with the accompanying drawings, in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It is to be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF EMBODIMENTSIn the following description of the illustrated embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration, various embodiments in which the invention may be practiced. It is to be understood that the embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
In certain embodiments, the present disclosure is generally directed to mitigating current flow between capacitance measurement locations in multiple capacitance measurement circuits and methods. When voltage signal ramps are used to determine capacitance at multiple locations, current flow between measurement locations can be mitigated by controlling the phase of the voltage signal ramps. For example, individual voltage ramp cycles can be concurrently initiated to keep the ramps in phase. In certain embodiments, this can be accomplished by effecting a delay in measurement channels that have reached a voltage ramp threshold until other measurement channels have also reached the threshold. Alternatively or in addition, the voltage signal ramps can be regulated by a preset ramping period so that they are kept in phase. Alternatively or in addition, detected differences in ramping periods can be used to adjust the relative start times of subsequent ramping periods, for example to match the voltage levels of the ramps in the middle of the respective ramp periods to reduce the net current flow between measurement locations. These and other techniques can be used, including any suitable combinations or permutations thereof, as will be appreciated by those skilled in the art based on the descriptions provided herein.
Without loss of generality, and for the purpose of efficient illustration, it is useful to describe various aspects of the present invention in terms of touch sensor system environments. It will be recognized, however, that such descriptions are merely exemplary and not limiting, and that aspects of the present invention can be suitably implemented in many applications where multiple capacitances are measured, and relative magnitudes or ratios of measured capacitances are calculated. Examples include instruments, pressure gauges, and measurement of small distances, areas, and moisture.
As indicated, system 10 in
As indicated, system 20 in
For ease of illustration, and without loss of generality, various aspects of the present invention can be understood through discussions that focus on the case of four capacitance measurement channels, such as may exist when using a 4-wire analog capacitive sensor. With that in mind,
The timed-slope converters are somewhat similar to dual-slope converters, each converter configured to generate forward (+) and reverse (−) ramp signals by alternately injecting forward direction and reverse direction currents from current sources into capacitances Cx1 through Cx4. For example, timed-slope converter 61 includes current sources IS1+ and IS1− (and, although not indicated, by consistent nomenclature, timed-slope converter 62 includes current sources IS2+ and IS2−, timed-slope converter 63 includes current sources IS3+ and IS3−, timed-slope converter 64 includes current sources IS4+ and IS4−, with IS+ and IS− being used herein to indicate any or all of the current sources as indicated by context). In exemplary embodiments, the current sources are equal in magnitude so that IS1+=IS1−=IS2+=IS2−=IS3+=IS3−=IS4+=IS4−. Time-slope converter 61 also includes a comparator A1 that provides a trigger Trig1 to control logic 79 when a high threshold is reached during a voltage ramp using IS1+, or when a low threshold is reached during a voltage ramp using IS1−. Similarly, timed-slope converter 62 includes a comparator A2 that provides a trigger Trig2, and so forth.
Assuming that the measured capacitances are also equal, that is Cx1=Cx2=Cx3=Cx4, then the voltage signals V1, V2, V3, and V4 will have ramps of equal slope. For analog capacitive touch panel applications, Cx1 through Cx4 are typically close in value (for example, within about 30% of one another). A touch input to the sensor generally will have the effect of increasing one (or more) of the capacitances relative to the others, resulting in a slower ramp on the voltage signal of the channel having the greater capacitance. Differences in slopes result in differences in time required to ramp to a threshold voltage level (for example up from a low reference level such as ⅓ Vcc, or down from a high reference level such as ⅔ Vcc). The accumulated ramp times for the voltage signals V1 through V4 are measured concurrently over an integration period, and the measured differences between the accumulated ramp times are used to indicate the differences in capacitance among Cx1 through Cx4. For time-slope converter 61, the counter 71 (also denoted Ctr1) increments a count for every main clock cycle (MClk), thus accumulating the ramping time. The low and high voltage thresholds (denoted herein −Vth and +Vth) are the switching points of hysteresis comparators (Schmitt triggers) A1 through A4 (only A1 shown in
Ramp signals V1 through V4 are generated by alternately turning on forward and reverse current generators, such as IS1+ and IS1−, at a desired rate. With reference to converter 61, when IS1+ is on, a constant current flows into Cx1, generating an increasing voltage signal ramp. Unless prematurely terminated, the V1 signal will ramp up until comparator A1 triggers at +Vth. At that point, IS1+ turns off. A voltage signal ramp down then occurs when source IS1− turns on, and can continue until comparator A1 is triggered at threshold −Vth. The ramp up, ramp down cycle is repeated a desired number of times depending on required measurement resolution, response times, and so forth. Each of the timed-slope converters is connected to circuitry 65 that can include the integration counter or other counters that control all of the channels (such as a clip counter as exemplified in
Because Cx1 is larger that the other capacitances, the voltage signal ramp of V1 lags behind the voltage signal ramps of V2 through V4. The main clock MClk frequency can be any suitable frequency that provides multiple clock cycles over an expected range of ramping times, for example the MClk frequency may be about 10 MHz to 30 MHz. The periods of the ramp signals V1 through V4 (i.e., one full ramp up, ramp down cycle) have frequencies controlled by the magnitudes of currents from sources IS+ and IS−, and the capacitances Cx. In analog capacitive touch sensor examples, the frequencies of the voltage ramps might be in a range of about 20 KHz to about 200 KHz.
In exemplary embodiments, a measurement sequence starts on the rising edge of an MClk cycle, shown at t0 in
In certain embodiments, when the voltage threshold is reached on a signal channel, the ramping reverses direction (independent of other ramps and independent of MClk). For example, when the +Vth threshold is reached at time t1 during a forward ramp of voltage signal V2, the output Trig2 of comparator A2 switches to high, turning off the IS2+ current source and simultaneously turning on the IS2− source. In
In
Continuing with the ramping cycles as shown in
This process (i.e., cycling the signal ramps between threshold levels and counting the clock cycles needed for each channel to complete a ramp) continues until all the channels have gone through N full cycles, called the integration period, which can be determined by an integration counter. The integration period can be a predetermined number of cycles, or can vary based on application and conditions (for example, capacitance measurement accuracy and resolution may be enhanced by increasing the integration period, and response time may place an upper limit on the integration period). In the example of
During the integration period, counter Ctr1 cumulatively increments fifty-six counts and counters Ctr2 through Ctr4 each cumulatively increment forty-eight counts. When the integration period is completed, an interrupt request (indicated by IRQ in
As discussed, if a channel reaches the voltage signal threshold before one or more of the other channels, the voltage level of that channel is held near the threshold level until one or more of the other channels also reach the threshold. As such, rather than starting a reverse direction ramp upon reaching the threshold, a delay is effected so that the reverse direction ramp can be started concurrently on multiple channels. Effecting delays on one or more channels so that the next ramp can be initiated concurrently on multiple channels allows the ramp cycles of the channels to be approximately in phase, assuming the capacitances being measured are similar in magnitude. Keeping the ramp cycles approximately in phase (i.e., all positive ramps occurring during the same time frame, and all negative ramps occurring during the same time frame) can have the effect of mitigating inter-channel currents (i.e., currents flowing between capacitance measurement locations) by keeping any differences between the voltage signals on the various channels at any given time relatively low. Reducing inter-channel currents can be particularly desirable when channels are connected to a 4-wire capacitive sensor where channels may be connected through a resistance. When individual channels are connected to electrodes in a matrix sensor, in-phase signal ramps may also be desired to minimize currents flowing through mutual capacitances of electrodes.
It will be appreciated that, although circuit operations are described and shown in
In addition to forming signals V1 through V4 into clipped sawtooths, signals V1 through V4 may also be held to a fixed frequency, for example as shown in
As discussed, synchronizing the starts of the voltage signal ramps can be used to mitigate current flow between the measurement locations that can be caused by large voltage differences. Further refinement of current flow mitigation can be achieved in certain embodiments, as illustrated with respect to
Each ramp's duration is measured by respective counters Ctr1 through Ctr4, as with
Adjusting the relative start times of the ramps in relation to measured ramp differentials can further mitigate the effects of corner to corner currents. As shown in
In certain embodiments, the process of measuring differential counts ΔCt and re-adjusting ramp start times is iterative and continuous, so that differentials measured during one ramp cycle are adjusted for in a subsequent ramp cycle, or alternatively, differentials measured during an integration cycle (for example over several ramp cycles) can be adjusted for in subsequent integration cycles. Refined phase alignment such as illustrated in
The level of current required to generate voltage signal ramps as illustrated in
In certain embodiments, some of the parameters may be programmable, for example the integration cycles (see
Certain embodiments of the present invention measure elapsed voltage signal ramp times using accumulators. Accumulators can be used in the embodiments described above to measure time in a digital counter, in an analog integrator, or in a combination of both. Analog integrators start and stop quickly, thus being capable of measuring with high resolution. Digital counters have higher dynamic range, but temporal resolution may be limited by the clock frequency (e.g., MClk). Thus, in certain embodiments the circuit shown in
As described in this document, current flow between capacitance measurement locations can be mitigated by controlling the phases of the drive signals. It will be appreciated that, for common capacitance-to-ground measurements systems such as the touch sensor systems shown in
The foregoing description of the various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. For example, the detection methodologies described herein may be used in connection with a wide variety of touch implements, including tethered implements and implements that house a battery or other power source. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.
Claims
1. For use in a touch sensor device that measures capacitances at a plurality of locations on a touch surface in response to a touch object coupling to the touch surface at a touch position, the capacitances being measured by applying electrical charge to the plurality of locations to ramp respective voltage signals between a first reference level and a second reference level, the method comprising:
- controlling the phase of the voltage signal ramps to mitigate currents flowing between the locations.
2. The method of claim 1, wherein the voltage signal ramps are alternated between ramping up from the first reference level to the second reference level and ramping down from the second reference level to the first reference level, and wherein controlling the phase comprises concurrently ramping up the voltage levels and concurrently ramping down the voltage levels.
3. The method of claim 1, wherein the voltage signals are ramped in the same direction, from the first reference level to the second reference level, with a reset back to the first reference level between each ramp, and wherein controlling the phase comprises concurrently ramping the voltage levels and concurrently resetting the voltage levels.
4. The method of claim 1, wherein applying electrical charge to the plurality of locations includes applying a continuous current, applying a pulsed current, or applying a voltage through an impedance.
5. The method of claim 1, wherein controlling the phase of the voltage signal ramps comprises concurrently starting the voltage signal ramps.
6. The method of claim 5, wherein concurrently starting the voltage signal ramps comprises effecting a delay on at least one voltage signal ramp that has reached a threshold level prior to other voltage signal ramps reaching the threshold level.
7. The method of claim 6, wherein the delay is a predetermined delay started upon the at least one voltage signal reaching the threshold level.
8. The method of claim 6, further comprising adjusting the delay to minimize average differences among the voltage signal ramps.
9. The method of claim 5, wherein the voltage signal ramps are regulated by a period timer that starts the voltage signal ramps at the same time according to a fixed frequency.
10. The method of claim 1, wherein controlling the phase of the voltage signal ramps comprises determining time differentials between when respective voltage signal ramps reach a threshold level, and using the determined time differentials to adjust subsequent ramp starting times.
11. The method of claim 1, further comprising measuring capacitances associated with each location by measuring ramp times for respective voltage signal ramps.
12. The method of claim 11, further comprising using the measured capacitances to determine the touch position.
13. A touch sensor device that measures capacitances at a plurality of locations on a touch surface in response to a touch object coupling to the touch surface at a touch position, the capacitances being measured by applying electrical charge to the plurality of locations to ramp respective voltage signals between a first reference level and a second reference level, the device comprising:
- a signal control circuit that controls the phase of the voltage signal ramps to mitigate currents flowing between the locations.
14. The touch sensor device of claim 13, wherein the signal control circuit controls the phase by concurrently starting the voltage signal ramps.
15. The touch sensor device of claim 14, wherein the signal control circuit concurrently starts the voltage signal ramps by effecting a delay on at least one voltage signal ramp that has reached a threshold level prior to other voltage signal ramps reaching the threshold level.
16. The touch sensor device of claim 15, wherein the delay is a predetermined delay started upon the at least one voltage signal reaching the threshold level.
17. The touch sensor device of claim 15, wherein the signal control circuit adjusts the delay to minimize average differences among the voltage signal ramps.
18. The touch sensor device of claim 14, wherein the signal control circuit regulates the voltage signal ramps using a period timer that starts each voltage signal ramp at the same time according to a fixed frequency.
19. The touch sensor device of claim 13, wherein the signal control circuit controls the phase of the voltage signal ramps by determining time differentials between when respective voltage signal ramps reach a threshold level, and using the determined time differentials to adjust subsequent ramp starting times.
20. For use with a device that measures capacitances at a plurality of locations, each location associated with a capacitance measurement channel to ramp respective voltage signals on the respective capacitances, a method comprising:
- (a) concurrently initiating forward-direction voltage signal ramps on multiple channels; and
- (b) effecting a delay on the forward-direction voltage signal ramp of at least one channel in response to reaching a high voltage signal threshold.
21. The method of claim 20, further comprising:
- (c) concurrently initiating reverse-direction voltage signal ramps on the multiple channels; and
- (d) effecting a delay on the reverse-direction voltage signal ramp of at least one channel in response to reaching a low voltage signal threshold.
22. The method of claim 21, further comprising repeating steps (a) through (c) after step (d).
23. The method of claim 20, wherein the delay is adjusted to minimize differences among forward-direction and reverse direction voltages.
24. The method of claim 20, further comprising counting clock cycles for the multiple channels during the voltage signal ramps and correlating the number of clock cycles for a given channel with the capacitance being measured by the given channel.
25. The method of claim 24, wherein voltage signal ramp initiation steps are synchronized with a clock cycle.
26. The method of claim 20, wherein the delay ends upon all of the multiple channels having reached the high voltage signal threshold.
27. The method of claim 20, wherein the delay ends after a predetermined time.
Type: Application
Filed: Dec 16, 2008
Publication Date: Jul 2, 2009
Applicant:
Inventor: Bernard O. GEAGHAN (Salem, NH)
Application Number: 12/335,685
International Classification: G06F 3/045 (20060101);