Patent Publication Number: US-2005122119-A1

Title: Low noise proximity sensing system

Description:
FIELD OF THE INVENTION  
      The invention relates generally to electrically sensing proximity, and more specifically in one embodiment to a low noise proximity detection system using differential detection.  
     BACKGROUND OF THE INVENTION  
      Proximity sensing, such as detecting the presence of a finger or other object with poor conductivity, can be achieved electrically through a number of existing technologies. Optical sensors can detect a change in view, or can detect an interruption in a beam of light transmitted across a detection area. Ultrasonic transceivers can gauge the distance to an acoustically solid object, such as the distance from a car&#39;s bumper to vehicles in front of or behind the car. Electrical systems can detect the presence of a capacitive body, such as wood, glass, water, oil, or a human body part such as a finger by detecting a change in capacitance or in capacitive coupling.  
      One such system uses a capacitor having two plates in an oscillator circuit, and measures the change in capacitance when a capacitive object comes near the capacitive plates as a change in oscillation frequency. Other such systems use other methods for detecting a change in capacitance due to proximity of a capacitive object. Another type of capacitive proximity sensor uses a driven electrode that is fed a varying voltage signal and that is located physically near a sense electrode. The voltage amplitude of the signal that is detected on the sense electrode is compared to a reference amplitude that is detected when no other capacitive object is near the electrodes, so that a change in the signal level detected on the sense electrode can be attributed to capacitive coupling between the two electrodes.  
      Such systems rely upon the object being detected to cause a change in capacitance observed between the two electrodes, therefore changing the level of the signal driven to one electrode that is capacitively coupled to the sense electrode. These systems are not perfect, however, as they are somewhat susceptible to noise and electromagnetic interference from surrounding electronic devices and from electrical noise present in the object being sensed. If the driven signal level is very small, the magnitude of noise that is needed to cause unreliable operation of these proximity sensors is also very small, resulting in a high probability of noise interference. If the signal is increased to a relatively large level, the electrode and its electrical connections can act as an antenna and radiate a substantial electromagnetic signal, causing interference in other electronic components and devices.  
      It is therefore desired that a proximity switch have good electromagnetic noise immunity, addressing the problems as described above.  
     SUMMARY OF THE INVENTION  
      In one example embodiment of the invention, an electronic proximity sensing apparatus comprises at least one pair of signal pads, and each pair of signal pads comprises a first signal pad and a second signal pad. Each of the signal pads is connected to receive an electric voltage signal. At least two sensing conductors routed are routed between the first signal pads and the second signal pads of the pairs of signal pads, and a sensor detects the difference in voltage between at least two of the sensing conductors. In various other embodiments, differential sensing is applied to other types of capacitive proximity sensing circuits to reduce common-mode interference. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  shows a proximity sensing apparatus having two differential sensing conductors, consistent with an embodiment of the present invention.  
       FIG. 2  shows a proximity sensing apparatus having three differential sensing conductors, consistent with an embodiment of the present invention.  
       FIG. 3A  shows a proximity sensing apparatus having a single signal pad, consistent with an embodiment of the present invention.  
       FIG. 3B  shows an alternate configuration for a proximity sensing apparatus having a single signal pad, consistent with an embodiment of the present invention.  
       FIG. 4A  shows a proximity sensing apparatus employing a ground shield, consistent with an embodiment of the present invention.  
       FIG. 4B  shows a cross-section of a proximity sensing apparatus employing a ground shield, consistent with an embodiment of the present invention.  
       FIG. 5  illustrates a oscillator-type capacitive loading proximity sensor having differential common mode noise rejection, consistent with an embodiment of the present invention  
       FIG. 6  shows a charge transfer capacitive proximity sensor using differential common mode noise reduction, consistent with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      In the following detailed description of sample embodiments of the invention, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific sample embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims.  
      The present invention provides in various embodiments differential sensing conductors, and sensors operable to detect a voltage difference between the sensing conductors. A stimulus signal is provided via one or more single or pairs of signal pads, which are connected to receive a voltage signal. The voltage applied to the signal pads is capacitively coupled to the sensing conductors as the sensing conductors and signal pads are physically near one another, and the degree of coupling is altered by presence of a capacitive object such as a human body part. Detection of the degree of coupling by sensing changes in the voltage present in the differential sensing conductors thereby provides proximity detection of such capacitive objects.  
       FIG. 1  illustrates an example embodiment of such a proximity sensor apparatus. A pair of signal pads is formed by first signal pad  101  and second signal pad  102 . Between the signal pads are located a first sensing conductor  103  and a second sensing conductor  104 , which are coupled to a sensor  105 . In this embodiment, the sensor  105  is an amplifier that senses a differential voltage between sensing conductor  103  and sensing conductor  104 , and provides an amplified signal representing this sensed difference as output  106 . This output signal is evaluated by a control module  107 , which receives the sensed difference signal  106  and determines from this signal whether a capacitive object is relatively proximate to signal pads  101  and  102 .  
      In operation, a voltage signal  108  is applied to the first signal pad  101 , and a voltage signal  109  that is the inverse of voltage signal  108  is applied to the second signal pad  102 . These voltage signals cause a voltage differential to form between sensing conductors  103  and  104  due to capacitive coupling between the signal pads and the sensing conductors, resulting in a measurable voltage difference signal  106  when the changing voltage signals  108  and  109  are applied. If a capacitive object, such as glass, water, oil, or a human body part such as a finger comes into relative proximity to signal pads  101  and  102 , the capacitive nature of the body contributes to capacitive coupling of sensing pads  01  and  102  to the sensing conductors  103  and  104 , resulting in a measurable difference in the voltage difference signal output at  106 . The control module  107  can then compare the received voltage difference output signal against an expected voltage output signal measured without a capacitive object near the signal pads to detect the presence or proximity of such an object.  
      When a voltage signal is provided to the signal pads  101  and  102 , the signals from each of the signal pads is conducted more strongly to its nearest sensing conductor. Therefore, sensing conductor  103  receives a relatively strong signal from signal pad  101 , and sensing conductor  104  receives a relatively strong signal from signal pad  102 . When a capacitive object is present, the capacitive object couples more signal to the sensing conductor opposite each pad, and promotes some mixing or cancellation of signals within the capacitive object, resulting in a lesser voltage difference induced between sensing conductors  103  and  104 . The capacitive object may further cause absorption or dispersion of some induced electrical energy, further reducing the induced voltage difference between sensing conductors. Sensing the drop in voltage difference between the sensing conductors  103  and  104  when a signal is presented to signal pads  101  and  102  can therefore be used to establish proximity of a capacitive object, providing proximity sensor functionality of such a system.  
      Electromagnetic interference affects each sensing conductor to approximately a similar degree, and can be greatly reduced by sensing the difference between the conductors rather than the absolute value of a single sensing conductor. Good common-mode interference rejection remains even when a finger or other object comes near the sensor, as the interference induced by the finger is induced approximately equally to both sensing conductors and is again easily reduced by differential sensing.  
      Some embodiments of the invention utilize a staggered signal pad timing system in which the signals provided to opposite signal pads such as  201  and  202  are not presented at the same time. This facilitates determination of whether a proximate capacitive object is off-axis to one side or the other relative to the sensing strips, and to what degree the object is off-center. Such a system is particularly useful in systems such as where several proximity sensing modules such as that shown in  FIG. 1  are located side-by-side to form a two-dimensional array of proximity sensors, such as on a touchpad or touchscreen.  
      The control module is coupled to the voltage signals  108  and  109  in a further embodiment of the invention, and coordinates proximity sensing with the voltage signals supplied to multiple pairs of signal pads. For example, the voltage signal source of one embodiment first provides a voltage signal having a changing voltage or voltage pulse to pads  101  and  102 . After this, a similar voltage signal is provided to the signal pads in signal pad pair  110 , and then is supplied in sequence to pad pairs  111 ,  112 , and  113 . This enables the sensor  105  and the control module  107  to monitor several pairs of signal pads by knowing which pair of pads created the signal detected by sensor  105  and control module  107 .  
      In a further embodiment of the invention, a number of proximity sensing strips such as that shown in  FIG. 1  enable sensing proximity in multiple regions, and may be used for purposes such as setting input parameters for an electronic circuit. One example of such a system is an electronic audio equalizer system, having a separate group of signal pads and sensing conductors forming a one-dimensional touchpad as shown in  FIG. 1  for each frequency band. The touchpad apparatus is then used to set the amplitude response for the frequency signal band corresponding to each touchpad apparatus. Further embodiments include using lights associated with the various parameter levels that can be selected via the touchpad apparatus such that the presently set level is lit, or using another similar visual indicator such as a liquid crystal display.  
      In another embodiment of the invention, a proximity sensor array such as that shown in  FIG. 1  is able to detect an object&#39;s relative proximity to each pair of signal pads rather than simply detecting which pair of signal pads is most proximate to an object, so that interpolation between adjacent signal pads can be used to determine an object&#39;s position with greater resolution than simply the number of signal pad pairs in the array.  
      Various other array configurations are within the scope of the present invention, including a two-dimensional array comprising a number of proximity sensor arrays such as is shown in  FIG. 1  positioned side-by-side, so that an object&#39;s position relative to the two-dimensional plane formed by the proximity sensor arrays can be determined. When such a system is combined with the ability to detect an object&#39;s relative proximity to more than one pair of signal pads, the proximity sensor arrays can detect an object in three dimensions, including an object&#39;s relative proximity to the two-dimensional array of sensors.  
       FIG. 2  shows another embodiment of a one-dimensional array of proximity sensors similar in many ways to that of  FIG. 1 . The most significant differences lie in that sensor pads  201  and  202  now receive the same voltage signal  208  rather than receiving signals that are the inverse of one another, and sensing conductors include three sensing conductors  203 ,  204 , and  205  instead of just two. The center conductor  204  serves as a reference to both sensors  206  and  207 , which in this diagram are differential amplifiers connected to the respective sensing conductors  203  and  205 . The outputs of both amplifiers  206  and  207  are fed into differential amplifier  209 , which outputs as a voltage signal the difference between the sensed differences between sensing conductors  203  to  204 , and between  204  to  205 . The resistors coupling center sensing conductor  204  to sensing amplifiers  206  and  207  are employed in this particular embodiment to compensate for any difference in electrical noise sensitivity between the center conductor and the adjacent sensing conductors  203  and  205 , which may shield center conductor  204  to a limited extent. These resistors can be selected based on a particular layout to improve the ability of the proximity sensor apparatus to reject common mode interference or electrical noise.  
      In operation, the signal pads  201  and  202  are provided identical voltage signals, and induce the same voltage signal to differential sensing conductors  203  and  205 , and to a lesser extent to  204  due to its greater distance from the pads and due to the shielding effect provided by sensing conductors  203  and  205 . In the presence of a capacitive object, a greater amount of signal is coupled between the signal pads and the center sensing conductor  204 , resulting in a lower sensed voltage difference between sensing conductors  203  and  204 , and between sensing conductors  204  and  205 . This reduction in sensed voltage difference indicates proximity of a capacitive object.  
      This configuration further has the advantage of increased noise reduction when the capacitively sensed object is not located directly over the sense elements, but is off-axis. Consider, for example, a finger nearer sensing conductors  203  and  204  than to  205 . A greater amount of noise will be induced into sensing conductors  203  and  204  than to  205 , but this common mode noise substantially cancels in differential sense amplifier  206 . A smaller amount of noise coupled into sensing conductor  205 , and sense amplifier  207  outputs a signal including the difference in noise between sensing conductors  204  and  205 . This output signal is provided to differential amplifier  209  which in turn cancels remaining noise common to sensing conductor pairs  203 - 204  and  204 - 205 . Therefore, the signals from the sensing conductors that have the most common mode noise will experience the greatest reduction in common noise in the first differential sensing amplifiers  206  and  207 , reducing the common mode noise significantly relative to standard non-differential or to some two-sensing conductor differential capacitive proximity sensor configurations, while the second differential amplifier  209  reduces noise common to all three.  
       FIG. 3A  shows yet another embodiment of the invention, in which a single signal pad  301  is located in proximity to differential sensing conductors  302  and  303 . The differential sensing conductors are connected to sensor amplifier  304 , which is operable to detect the difference in voltage between the two sensing conductors. As with the previous examples, a voltage signal  305  is applied to the signal pad  301 , and the resulting voltage difference measured between sensing conductors  302  and  303  is compared to a reference voltage difference to determine whether a capacitive object is in proximity to the proximity sensor apparatus. An alternate configuration of the signal pad and differential sensing conductors is shown generally in  FIG. 3B , which shows how a single signal pad  301  may be located between the differential sensing conductors  302  and  303 . The simple, single button versions of the invention shown in  FIGS. 3A and 3B  are well suited to applications where a simple button is needed, such as in a hostile environment like a laboratory or manufacturing facility, or in a high-use environment where durability is desired, such as in an elevator.  
      The differential sensing conductor configuration illustrated by these examples plays an important role in decreasing the effect of electromagnetic interference on the proximity sensing apparatus. Because the differential sensing conductors are routed parallel and in proximity to one another, any interference will likely affect the sensing conductors in substantially the same way. A voltage induced in one sensing conductor by electromagnetic interference will therefore likely induce a similar voltage in neighboring, parallel sensing conductors. Determination of the difference in voltage between the sensing conductors via a sensing mechanism such as a differential amplifier will therefore result in relatively little electromagnetic interference sensed, as only the difference between voltages in the sensing conductors is measured. Although the signal pads and differential sensing conductors are parallel strips in many of the example embodiments illustrated, they can take other forms in various other embodiments of the invention, including circular signal pads or circular differential sensing pads.  
      Grounded shield strips are incorporated in various further embodiments of the present invention to improve resistance to electromagnetic interference by shielding the sensing conductors of the various embodiments of the invention from electromagnetic interference such as induced electrical noise. These grounded strips conduct the interference in their relative proximity to ground, reducing the amount of electromagnetic interference reaching the sensing conductors.  FIG. 4  shows generally how grounding strips  401  can be employed to shield a proximity sensor array such as that of  FIG. 2 . The large, grounded copper pad  401  is formed on a layer of a printed circuit board  402 , positioned directly below the sensing conductors  403 ,  404 , and  405 . In some embodiments of the invention, the grounded pad  401  is formed on a power distribution layer or ground layer of a multi-layer printed circuit board, and uses a dedicated grounded return signal path to a central grounded location to drain induced electric signals caused by electromagnetic interference.  
      The voltages induced on the differential sensing conductors are sensed by differential amplifiers such as  105  of  FIG. 1 , and provide the sensed voltage to a control module such as  107  in some embodiments of the invention. The control module receives the signal, and compares the sensed voltage signal to the expected voltage signal to estimate or determine proximity of a capacitive object. In some further embodiments, a degree of hysteresis is built in to the control module in proximity sensors having multiple pads or pairs of pads such as is shown in  FIG. 1 .  
      In operation, when an object such as a user&#39;s finger comes into proximity to one of the pairs of signal pads in  FIG. 1 , the threshold for detection of an object and determination of the object&#39;s presence is reduced, to ensure that as the object moves about the proximity sensing apparatus from one pair of pads to another, the proximity sensor remains actively tracking the object. Without a hysteresis system such as this built in to the control module, an apparatus such as that of  FIG. 1  used for a finger-actuated control would be more susceptible to losing track of a finger as it slid up and down in proximity to the differential sensing strips from one pair of pads to another. The inclusion of hysteresis therefore provides relatively smooth and reliable operation of a proximity sensor apparatus such as that of  FIGS. 1 and 2  for touch-actuated controllers. Further embodiments use hysteresis even with a single proximity sensor, to eliminate “bounce” and ensure that a single touch actuates the switch a single time.  
      Digital sampling of the received signal within the control module  107  enables performance of these methods and others within a digital processor, simplifying in some embodiments operation of the control module. Sampling the signal also enables various filtering techniques, both digital and analog, to be applied to the received signal to shape the frequency response of the sensing system and further improve noise immunity. The most recently sensed or touched level in a multi-pad or multi-pair sensing apparatus such as that of  FIGS. 1 and 2  is in some embodiments stored in a memory, so that the selected position of a controller can be held once it is no longer being touched. Such a system also enables loading a preset value or setting an initial state via the memory, so that presets or electronic control of settings can be achieved.  
      One example of an application in which such a system is desirable is to provide control of settings on an electronic music keyboard or synthesizer. Various parameters such as attack, decay, envelope, oscillator frequency, waveform control, volume level and such are used to define the nature of the sound being synthesized, and can be readily set via presets from memory, and actively controlled by a user using proximity sensing apparatus such as that of  FIGS. 1 and 2  to alter the characteristics of the synthesized sound. The present invention is easily adapted to a wide variety of other such systems as a parameter control or selection module.  
       FIG. 5  illustrates a oscillator-driven capacitive loading proximity sensor having differential common mode noise rejection, consistent with an embodiment of the present invention. An oscillator  501  is connected via a resistor R 1  shown at  502  to a first sense pad  503 . An amplifier  504  is connected across the resistor  502 , providing an output signal proportional to the voltage drop across the resistor  502 . Prior art versions of this type of proximity sensor typically contain no more elements than these elements  501 - 504 , and do nor provide differential sensing. In this example embodiment of the present invention, a second sense pad  505  is located in physical proximity to the first sense pad  503 , and is similarly configured. The second sense pad is coupled to ground via a resistor R 2 , shown at  506 . An amplifier  507  is coupled across the resistor  506 , and its output varies with the voltage drop across resistor R 2 . The outputs of amplifiers  504  and  507  are coupled via resistors  508  and  509  to an output  510 , which provides a signal that varies with proximity of a capacitively coupled object to the sense pads  503  and  505 , but which uses the differential sensing circuit shown here to cancel noise common to both sense pads  503  and  505  from the output signal  510 .  
      The sense pads are in some embodiments of the invention configured to sense certain capacitive objects, such as a human finger. Because noise immunity is greater in some variations when the pads are configured physically near each other and are physically similar in size, shape, and orientation, the pads are further configured to be similar and near to one another. When sensing a finger, for example, it is desired that the distance between sense pads not be significantly greater than the width of a finger, but is instead desired to be only somewhat bigger than or smaller than the width of a finger. This physical proximity increases the likelihood that the sense pads will be equally subject to the same noise and interference signal, increasing the effectiveness of the differential common mode noise elimination of the present invention.  
      In operation, the oscillator  501  provides a signal, such as a 100 kHz sine wave, through R 1  to the sense pad  503 . The sense pad  503  capacitively couples to a nearby capacitive proximate object when such an object is near, thereby introducing a capacitive load to the oscillator  501 . This can be measured as an increased voltage drop across resistor  502 , which is sensed by amplifier  504 . Although this is sufficient to detect proximity of such an object, the present invention further uses a sense pad  505  to detect common mode noise, which is sensed via amplifier  507  as a voltage difference across resistor  506 , which is in some embodiments similar in resistance to the resistor  502  or to the impedance as seen by the sense pad  503  such as may be formed by resistor  502 , oscillator  501 , and amplifier  504 . Because the amplifier  507  amplifies noise present on pad  505  opposite in polarity to amplifier  504 &#39;s amplification of noise sensed in sense pad  503 , the signals from amplifiers  504  and  507  can be combined to substantially eliminate noise that is common to both sense pads  503  and  505 . In the example circuit of  FIG. 5 , this is done by coupling both amplifier outputs via resistors  508  and  509  to the output  510 . In a further embodiment of the invention, the gain of at least one of amplifier  504  and  507  is adjustable or is pre-configured to substantially eliminate common mode noise sensed by sense pads  503  and  505 .  
       FIG. 6  shows a charge transfer capacitive proximity sensor using differential common mode noise reduction, consistent with an embodiment of the present invention. To initiate a proximity sensing sequence, switches  601  and  602  are closed and then re-opened, to ensure that capacitors  603  and  604  are fully discharged. The switches in some embodiments of the invention will be transistors, such as FET transistors, that are momentarily brought into a conducting state from a nonconducting state. The capacitors are in some embodiments preferred to be capacitors with a low dielectric absorption, such as polypropylene, mylar, polystyrene, or teflon dielectric, with a film and foil or metallized film construction. The voltage at  605  and the inverse voltage at  606  are supported by bypass capacitors  607  and  608 , which serve to minimize local voltage fluctuations. These voltages are applied to virtual capacitances Cx by switching switches  609  and  610  to a closed position for a period of time sufficient to charge the virtual capacitances Cx, formed by proximity of a capacitive object to capacitive proximity sense pads  611  and  612 .  
      Once the virtual capacitances Cx of the capacitive proximity sense pads are charged to a known voltage, switches  609  and  610  are opened, and switches  613  and  614  are momentarily closed. The switches  613  and  614  are closed long enough for the voltage at the virtual capacitances Cx and the sense capacitors  603  and  604  to become substantially similar. The capacitors  603  and  604  are therefore preferably significantly larger in capacitance than the expected maximum size of the virtual capacitance Cx.  
      Next, switches  613  and  614  are opened, and the voltage at capacitors  603  and  604  are measured. Because capacitors  603  and  604  are of known capacitance and are known to be charged to the same voltage as were virtual capacitances Cx at the time switches  613  and  614  were closed, the voltages measured across capacitors  603  and  604  will be approximately proportional to the capacitance of virtual capacitors Cx. This occurs because although Cx was charged to a known voltage earlier, the actual charge it received was dependent both on the applied voltage and its capacitance, as governed by the formula Q=CV, where Q=charge in coulombs, C=capacitance, and v=applied voltage. The final voltage that appears on capacitors  603  and  604  is therefore dependent on both the known applied voltage and the unknown capacitance of Cx, Detection of a higher-than-expected voltage across capacitors  603  and  604  therefore indicates a higher than expected virtual capacitance Cx, indicating the proximity of a capacitive object to capacitive proximity sensing pads  611  and  612 .  
      The sensed voltages are fed into amplifiers  615  and  616 , which in some embodiments of the invention have a high input impedance to avoid rapidly draining the charge of capacitors  603  and  604 . The outputs from amplifiers  615  and  616 , which form differential capacitive proximity sensing circuits, are fed into differential amplifier  617 , which serves to eliminate common-mode noise sensed by both capacitive sense pads  611  and  612 . Its output  618  therefore provides a voltage signal indicating relative proximity of a capacitive object, but with improved immunity to common mode noise than would a non-differential circuit such as simply the top half of the circuit of  FIG. 6 .  
      In some further embodiments, amplifiers  615  and  616  can be eliminated from the circuit, and a single amplifier  617  having a high input impedance is employed. Further, as only either the top half or the bottom half of the circuit of  FIG. 6  is needed to sense proximity, the opposite half can in some embodiments not include a voltage source, bypass capacitor  607  or  608 , and switch  609  or  610 . As long as the sense portion of the circuit is configured to convey common mode noise from the sense pad  611  or  612  in substantially the same way as in the other half of the circuit, its purpose of sensing common mode noise for differential cancellation can be achieved.  
      The circuit of  FIG. 6  is provided with input signals that are opposite in voltage and sensed signals that are of the same phase, but various embodiments of the present invention will operate significantly better when the phase of one or more of the sensed signals is inverted before being provided to the one or more differential sensing amplifiers. An example of such is illustrated and explained in greater detail in conjunction with  FIG. 2 . The inversion provides the intended result of a greater difference signal between the two sensors representing the capacitive sensed proximity signal, and a common-mode signal representing noise common to the one or more sensed signals that can be greatly reduced by differential sensing circuitry.  
      These examples illustrate further ways in which the present invention can be applied to capacitive proximity sensing, using differential sensing to reduce common mode noise in the sensed proximity signal. A variety of systems, including linear differential sensing strips, linear parallel pads, charge transfer sensors, and oscillator-driven sense pads may be employed to practice the present invention, but the invention is not so limited. Various other formats are contemplated, including but not limited to variations in differential sensing conductor and signal pad configurations, embodiments where signal pads and differential sensing conductors are placed on different levels of a circuit board, and within mediums other than a circuit board, such as implementation as transparent conductors overlaying a screen of a Personal Digital Assistant, cellular telephone, video or computer monitor, or other such device.  
      Although specific embodiments of proximity sensors having differential sensing conductors have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the invention. It is intended that this invention be limited only by the claims, and the full scope of equivalents thereof.