Patent Publication Number: US-8539830-B2

Title: High precision sensing for parameter measurement of bone density

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 12/825,852 filed on Jun. 29, 2010 the content of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present invention pertains generally to measurement of physical parameters, and particularly to, but not exclusively, ultrasonic electronic devices for high precision sensing. 
     BACKGROUND 
     The skeletal system of a mammal is subject to variations among species. Further changes can occur due to environmental factors, degradation through use, and aging. An orthopedic joint of the skeletal system typically comprises two or more bones that move in relation to one another. Movement is enabled by muscle tissue and tendons attached to the skeletal system of the joint. Ligaments hold and stabilize the one or more joint bones positionally. Cartilage is a wear surface that prevents bone-to-bone contact, distributes load, and lowers friction. 
     There has been substantial growth in the repair of the human skeletal system. In general, orthopedic joints have evolved using information from simulations, mechanical prototypes, and patient data that is collected and used to initiate improved designs. Similarly, the tools being used for orthopedic surgery have been refined over the years but have not changed substantially. Thus, the basic procedure for replacement of an orthopedic joint has been standardized to meet the general needs of a wide distribution of the population. Although the tools, procedure, and artificial joint meet a general need, each replacement procedure is subject to significant variation from patient to patient. The correction of these individual variations relies on the skill of the surgeon to adapt and fit the replacement joint using the available tools to the specific circumstance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features of the system are set forth with particularity in the appended claims. The embodiments herein, can be understood by reference to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an illustration of a sensor placed in contact between a femur and a tibia for measuring a parameter in accordance with an exemplary embodiment; 
         FIG. 2  is a block diagram of an zero-crossing receiver in accordance with one embodiment; 
         FIG. 3  illustrates a block diagram of the integrated zero-crossing receiver coupled to a sensing assembly in accordance with an exemplary embodiment; 
         FIG. 4  is an exemplary propagation tuned oscillator (PTO) incorporating a zero-crossing receiver or an edge detect receiver to maintain positive closed-loop feedback in accordance with one embodiment. 
         FIG. 5  is a sensor interface diagram incorporating the zero-crossing receiver in a continuous wave multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment; 
         FIG. 6  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the integrated zero-crossing receiver for operation in continuous wave mode; 
         FIG. 7  is a sensor interface diagram incorporating the integrated zero-crossing receiver in a pulse multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment; 
         FIG. 8  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the integrated zero-crossing receiver for operation in pulse mode in accordance with one embodiment; 
         FIG. 9  illustrates a block diagram of an edge-detect receiver circuit in accordance with an exemplary embodiment; 
         FIG. 10  illustrates a block diagram of the edge-detect receiver circuit coupled to a sensing assembly; 
         FIG. 11  is a sensor interface diagram incorporating the edge-detect receiver circuit in a pulse-echo multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment; 
         FIG. 12  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the edge-detect receiver circuit for operation in pulse echo mode; 
         FIG. 13  is a simplified cross-sectional view of a sensing module in accordance with an exemplary embodiment; 
         FIG. 14  is an exemplary assemblage for illustrating reflectance and unidirectional modes of operation in accordance with an exemplary embodiment; 
         FIG. 15  is an exemplary assemblage that illustrates propagation of ultrasound waves within a waveguide in the bi-directional mode of operation of this assemblage; 
         FIG. 16  is an exemplary cross-sectional view of a sensor element to illustrate changes in the propagation of ultrasound waves with changes in the length of a waveguide; and 
         FIG. 17  is a simplified flow chart of method steps for high precision processing and measurement data in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are broadly directed to measurement of physical parameters, and more particularly, to fast-response circuitry for detecting specific features of the energy waves or pulses. 
     The following description of exemplary embodiment(s) is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. 
     Processes, techniques, apparatus, and materials as known by one of ordinary skill in the art may not be discussed in detail but are intended to be part of the enabling description where appropriate. For example specific computer code may not be listed for achieving each of the steps discussed, however one of ordinary skill would be able, without undo experimentation, to write such code given the enabling disclosure herein. Such code is intended to fall within the scope of at least one exemplary embodiment. 
     Additionally, the sizes of structures used in exemplary embodiments are not limited by any discussion herein (e.g., the sizes of structures can be macro (centimeter, meter, and larger sizes), micro (micrometer), and nanometer size and smaller). 
     Notice that similar reference numerals and letters refer to similar items in the following figures, and thus once an item is defined in one figure, it may not be discussed or further defined in the following figures. 
     In a first embodiment, an ultrasonic measurement system comprises one or more ultrasonic transducers, an ultrasonic waveguide, and a propagation tuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonic measurement system in this embodiment employs a continuous mode (CM) of operation to evaluate propagation characteristics of continuous ultrasonic waves in the waveguide by way of closed-loop feedback to determine levels of applied forces on the waveguide. 
     In a second embodiment, an ultrasonic measurement system comprises one or more ultrasonic transducers, an ultrasonic waveguide, and a propagation tuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonic measurement system in this embodiment employs a pulse mode (PM) of operation to evaluate propagation characteristics of pulsed ultrasonic waves in the waveguide by way of closed-loop feedback to determine levels of applied forces on the waveguide. 
     In a third embodiment, an ultrasonic measurement system comprises one or more ultrasonic transducers, an ultrasonic waveguide, and a propagation tuned oscillator (PTO) or Phase Locked Loop (PLL). The ultrasonic measurement system in this embodiment employs a pulse echo mode (PE) of operation to evaluate propagation characteristics of ultrasonic echo reflections in the waveguide by way of closed-loop feedback to determine levels of applied forces on the waveguide. 
       FIG. 1  is an illustration of a sensor  100  placed in contact between a femur  102  and a tibia  108  for measuring a parameter in accordance with an exemplary embodiment. In general, a sensor  100  is placed in contact with or in proximity to the muscular-skeletal system to measure a parameter. In a non-limiting example, sensor  100  is used to measure a parameter of a muscular-skeletal system during a procedure such as an installation of an artificial joint. Embodiments of sensor  100  are broadly directed to measurement of physical parameters, and more particularly, to evaluating changes in the transit time of a pulsed energy wave propagating through a medium. In-situ measurements during orthopedic joint implant surgery would be of substantial benefit to verify an implant is in balance and under appropriate loading or tension. In one embodiment, the instrument is similar to and operates familiarly with other instruments currently used by surgeons. This will increase acceptance and reduce the adoption cycle for a new technology. The measurements will allow the surgeon to ensure that the implanted components are installed within predetermined ranges that maximize the working life of the joint prosthesis and reduce costly revisions. Providing quantitative measurement and assessment of the procedure using real-time data will produce results that are more consistent. A further issue is that there is little or no implant data generated from the implant surgery, post-operatively, and long term. Sensor  100  can provide implant status data to the orthopedic manufacturers and surgeons. Moreover, data generated by direct measurement of the implanted joint itself would greatly improve the knowledge of implanted joint operation and joint wear thereby leading to improved design and materials. 
     In at least one exemplary embodiment, an energy pulse is directed within one or more waveguides in sensor  100  by way of pulse mode operations and pulse shaping. The waveguide is a conduit that directs the energy pulse in a predetermined direction. The energy pulse is typically confined within the waveguide. In one embodiment, the waveguide comprises a polymer material. For example, urethane or polyethylene are polymers suitable for forming a waveguide. The polymer waveguide can be compressed and has little or no hysteresis in the system. Alternatively, the energy pulse can be directed through the muscular-skeletal system. In one embodiment, the energy pulse is directed through bone of the muscular-skeletal system to measure bone density. A transit time of an energy pulse is related to the material properties of a medium through which it traverses. This relationship is used to generate accurate measurements of parameters such as distance, weight, strain, pressure, wear, vibration, viscosity, and density to name but a few. 
     Sensor  100  can be size constrained by form factor requirements of fitting within a region the muscular-skeletal system or a component such as a tool, equipment, or artificial joint. In a non-limiting example, sensor  100  is used to measure load and balance of an installed artificial knee joint. A knee prosthesis comprises a femoral prosthetic component  104 , an insert, and a tibial prosthetic component  106 . A distal end of femur  102  is prepared and receives femoral prosthetic component  104 . Femoral prosthetic component  104  typically has two condyle surfaces that mimic a natural femur. As shown, femoral prosthetic component  104  has single condyle surface being coupled to femur  102 . Femoral prosthetic component  104  is typically made of a metal or metal alloy. 
     A proximal end of tibia  108  is prepared to receive tibial prosthetic component  106 . Tibial prosthetic component  106  is a support structure that is fastened to the proximal end of the tibia and is usually made of a metal or metal alloy. The tibial prosthetic component  106  also retains the insert in a fixed position with respect to tibia  108 . The insert is fitted between femoral prosthetic component  104  and tibial prosthetic component  106 . The insert has at least one bearing surface that is in contact with at least condyle surface of femoral prosthetic component  104 . The condyle surface can move in relation to the bearing surface of the insert such that the lower leg can rotate under load. The insert is typically made of a high wear plastic material that minimizes friction. 
     In a knee joint replacement process, the surgeon affixes femoral prosthetic component  104  to the femur  102  and tibial prosthetic component  106  to tibia  108 . The tibial prosthetic component  106  can include a tray or plate affixed to the planarized proximal end of the tibia  108 . Sensor  100  is placed between a condyle surface of femoral prosthetic component  104  and a major surface of tibial prosthetic component  106 . The condyle surface contacts a major surface of sensor  100 . The major surface of sensor  100  approximates a surface of the insert. Tibial prosthetic component  106  can include a cavity or tray on the major surface that receives and retains sensor  100  during a measurement process. Tibial prosthetic component  106  and sensor  100  has a combined thickness that represents a combined thickness of tibial prosthetic component  106  and a final (or chronic) insert of the knee joint. 
     In one embodiment, two sensors  100  are fitted into two separate cavities, the cavities are within a trial insert (that may also be referred to as the tibial insert, rather than the tibial component itself) that is held in position by tibial component  106 . One or two sensors  100  may be inserted between femoral prosthetic component  104  and tibial prosthetic component  106 . Each sensor is independent and each measures a respective condyle of femur  102 . Separate sensors also accommodate a situation where a single condyle is repaired and only a single sensor is used. Alternatively, the electronics can be shared between two sensors to lower cost and complexity of the system. The shared electronics can multiplex between each sensor module to take measurements when appropriate. Measurements taken by sensor  100  aid the surgeon in modifying the absolute loading on each condyle and the balance between condyles. Although shown for a knee implant, sensor  100  can be used to measure other orthopedic joints such as the spine, hip, shoulder, elbow, ankle, wrist, interphalangeal joint, metatarsophalangeal joint, metacarpophalangeal joints, and others. Alternatively, sensor  100  can also be adapted to orthopedic tools to provide measurements. 
     The prosthesis incorporating sensor  100  emulates the function of a natural knee joint. Sensor  100  can measure loads or other parameters at various points throughout the range of motion. Data from sensor  100  is transmitted to a receiving station  110  via wired or wireless communications. In a first embodiment, sensor  100  is a disposable system. Sensor  100  can be disposed of after using sensor  100  to optimally fit the joint implant. Sensor  100  is a low cost disposable system that reduces capital costs, operating costs, facilitates rapid adoption of quantitative measurement, and initiates evidentiary based orthopedic medicine. In a second embodiment, a methodology can be put in place to clean and sterilize sensor  100  for reuse. In a third embodiment, sensor  100  can be incorporated in a tool instead of being a component of the replacement joint. The tool can be disposable or be cleaned and sterilized for reuse. In a fourth embodiment, sensor  100  can be a permanent component of the replacement joint. Sensor  100  can be used to provide both short term and long term post-operative data on the implanted joint. In a fifth embodiment, sensor  100  can be coupled to the muscular-skeletal system. In all of the embodiments, receiving station  110  can include data processing, storage, or display, or combination thereof and provide real time graphical representation of the level and distribution of the load. Receiving station  110  can record and provide accounting information of sensor  100  to an appropriate authority. 
     In an intra-operative example, sensor  100  can measure forces (Fx, Fy, Fz) with corresponding locations and torques (e.g. Tx, Ty, and Tz) on the femoral prosthetic component  104  and the tibial prosthetic component  106 . The measured force and torque data is transmitted to receiving station  110  to provide real-time visualization for assisting the surgeon in identifying any adjustments needed to achieve optimal joint pressure and balancing. The data has substantial value in determining ranges of load and alignment tolerances required to minimize rework and maximize patient function and longevity of the joint. 
     As mentioned previously, sensor  100  can be used for other joint surgeries; it is not limited to knee replacement implant or implants. Moreover, sensor  100  is not limited to trial measurements. Sensor  100  can be incorporated into the final joint system to provide data post-operatively to determine if the implanted joint is functioning correctly. Early determination of a problem using sensor  100  can reduce catastrophic failure of the joint by bringing awareness to a problem that the patient cannot detect. The problem can often be rectified with a minimal invasive procedure at lower cost and stress to the patient. Similarly, longer term monitoring of the joint can determine wear or misalignment that if detected early can be adjusted for optimal life or replacement of a wear surface with minimal surgery thereby extending the life of the implant. In general, sensor  100  can be shaped such that it can be placed or engaged or affixed to or within load bearing surfaces used in many orthopedic applications (or used in any orthopedic application) related to the musculoskeletal system, joints, and tools associated therewith. Sensor  100  can provide information on a combination of one or more performance parameters of interest such as wear, stress, kinematics, kinetics, fixation strength, ligament balance, anatomical fit and balance. 
       FIG. 2  is a block diagram of a zero-crossing receiver  200  in accordance with one embodiment. In a first embodiment, the zero-crossing receiver  200  is provided to detect transition states of energy waves, such as the transition of each energy wave through a mid-point of a symmetrical or cyclical waveform. This enables capturing of parameters including, but not limited to, transit time, phase, or frequency of the energy waves. The receiver rapidly responds to a signal transition and outputs a digital pulse that is consistent with the energy wave transition characteristics and with minimal delay. The zero-crossing receiver  200  further discriminates between noise and the energy waves of interest, including very low level waves by way of adjustable levels of noise reduction. A noise reduction section  218  comprises a filtering stage and an offset adjustment stage to perform noise suppression accurately over a wide range of amplitudes including low level waves. 
     In a second embodiment, a zero-crossing receiver is provided to convert an incoming symmetrical, cyclical, or sine wave to a square or rectangular digital pulse sequence with superior performance for very low level input signals. The digital pulse sequence represents pulse timing intervals that are consistent with the energy wave transition times. The zero-crossing receiver is coupled with a sensing assembly to generate the digital pulse sequence responsive to evaluating transitions of the incoming sine wave. This digital pulse sequence conveys timing information related to parameters of interest, such as applied forces, associated with the physical changes in the sensing assembly. 
     In a third embodiment, the integrated zero-crossing receiver is incorporated within a propagation tuned oscillator (PTO) to maintain positive closed-loop feedback when operating in a continuous wave mode or pulse-loop mode. The integrated edge zero-crossing receiver is electrically integrated with the PTO by multiplexing input and output circuitry to achieve ultra low-power and small compact size. Electrical components of the PTO are integrated with components of the zero-crossing receiver to assure adequate sensitivity to low-level signals. 
     In one embodiment, low power zero-crossing receiver  200  can be integrated with other circuitry of the propagation tuned oscillator to further improve performance at low signal levels. The zero-crossing receiver  200  comprises a preamplifier  206 , a filter  208 , an offset adjustment circuitry  210 , a comparator  212 , and a digital pulse circuit  214 . The filter  208  and offset adjustment circuitry  210  constitute a noise reduction section  218  as will be explained ahead. The zero-crossing receiver  200  can be implemented in discrete analog components, digital components or combination thereof. The integrated zero-crossing receiver  200  practices measurement methods that detect the midpoint of energy waves at specified locations, and under specified conditions, to enable capturing parameters including, but not limited to, transit time, phase, or frequency of energy waves. A brief description of the method of operation is as follows. 
     An incoming energy wave  202  is coupled from an electrical connection, antenna, or transducer to an input  204  of zero-crossing receiver  200 . Input  204  of zero-crossing receiver  200  is coupled to pre-amplifier  206  to amplify the incoming energy wave  202 . The amplified signal is filtered by filter  208 . Filter  208  is coupled to an output of pre-amplifier  206  and an input of offset adjustment circuitry  210 . In one configuration, filter  208  is a low-pass filter to remove high frequency components above the incoming energy wave  202  bandwidth. In another arrangement, the filter is a band-pass filter with a pass-band corresponding to the bandwidth of the incoming energy wave  202 . It is not however limited to either arrangement. The offset of the filtered amplified wave is adjusted by offset adjustment circuitry  210 . An input of comparator  212  is coupled to an output of offset adjustment circuitry  210 . Comparator  212  monitors the amplified waveforms and triggers digital pulse circuitry  214  whenever the preset trigger level is detected. Digital pulse circuit  214  has an input coupled to the output of comparator  212  and an output for providing digital pulse  216 . The digital pulse  216  can be further coupled to signal processing circuitry, as will be explained ahead. 
     In a preferred embodiment, the electronic components are operatively coupled together as blocks of integrated circuits. As will be shown ahead, this integrated arrangement performs its specific functions efficiently with a minimum number of components. This is because the circuit components are partitioned between structures within an integrated circuit and discrete components, as well as innovative partitioning of analog and digital functions, to achieve the required performance with a minimum number of components and minimum power consumption. 
       FIG. 3  illustrates a block diagram of the integrated zero-crossing receiver  200  coupled to a sensing assembly  300  in accordance with an exemplary embodiment. The pre-amplifier  206  and the digital pulse circuit  214  are shown for reference and discussion. In one embodiment, sensing assembly  300  comprises a transmitter transducer  302 , an energy propagating structure (or medium)  304 , and a receiver transducer  306 . As will be explained further hereinbelow, the sensing assembly  300  in one embodiment is part of a sensory device that measures a parameter such as force, pressure, or load. In a non-limiting example, an external parameter such as an applied force  308  affects the sensing assembly  200 . As shown, applied force  308  modifies propagating structure  304  dimensionally. In general, the sensing assembly  300  conveys one or more parameters of interest such as distance, force, weight, strain, pressure, wear, vibration, viscosity, density, direction, and displacement related to a change in energy propagating structure  304 . An example is measuring loading applied by a joint of the muscular-skeletal system as disclosed above using sensing assembly  300  between the bones of the joint. 
     A transducer driver circuit (not shown) drives the transmitter transducer  302  of the sensing assembly  300  to produce energy waves  310  that are directed into the energy propagating structure  304 . Changes in the energy propagating medium  304  due to an applied parameter such as applied forces  308  change the frequency, phase, and transit time of energy waves  310  (or pulses). In one embodiment, applied forces  308  affect the length of propagating structure  304  in a direction of a path of propagation of energy waves  310 . The zero-crossing receiver  200  is coupled to the receiver transducer  306  to detect zero-crossings of the reproduced energy wave  202 . Upon detecting a zero-crossing digital pulse circuit  214  is triggered to output a pulse  216 . The timing of the digital pulse  216  conveys the parameters of interest (e.g., distance, force weight, strain, pressure, wear, vibration, viscosity, density, direction, displacement, etc.). 
     Measurement methods that rely on such propagation of energy waves  310  or pulses of energy waves are required to achieve highly accurate and controlled detection of energy waves or pulses. Moreover, pulses of energy waves may contain multiple energy waves with complex waveforms therein leading to potential ambiguity of detection. In particular, directing energy waves  310  into the energy propagating structure  304  can generate interference patterns caused by nulls and resonances of the waveguide, as well as characteristics of the generated energy waves  310 . These interference patterns can multiply excited waveforms that result in distortion of the edges of the original energy wave. 
     Briefly referring back to  FIG. 2 , to reliably detect the arrival of a pulse of energy waves, the zero-crossing receiver  200  leverages noise reduction section  218  that incorporates two forms of noise reduction. Frequencies above the operating frequencies for physical measurements of the parameters of interest are attenuated with the filter  208 . In addition, the offset level of the incoming waveform is adjusted by the offset adjustment  210  to optimize the voltage level at which the comparator  212  triggers an output pulse. This is more reliable than amplifying the incoming waveform because it does not add additional amplification of noise present on the input. The combination of rapid response to the arrival of incoming symmetrical, cyclical, or sine waves with adjustable levels of noise reduction achieves reliable zero-crossing detection by way of the ultra low power zero-crossing receiver  200  with superior performance for very low level signals. 
     There are a wide range of applications for compact measurement modules or devices having ultra low power circuitry that enables the design and construction of highly performing measurement modules or devices that can be tailored to fit a wide range of nonmedical and medical applications. Applications for highly compact measurement modules or devices may include, but are not limited to, disposable modules or devices as well as reusable modules or devices and modules or devices for long term use. In addition to nonmedical applications, examples of a wide range of potential medical applications may include, but are not limited to, implantable devices, modules within implantable devices, intra-operative implants or modules within intra-operative implants or trial inserts, modules within inserted or ingested devices, modules within wearable devices, modules within handheld devices, modules within instruments, appliances, equipment, or accessories of all of these, or disposables within implants, trial inserts, inserted or ingested devices, wearable devices, handheld devices, instruments, appliances, equipment, or accessories to these devices, instruments, appliances, or equipment. 
       FIG. 4  is an exemplary block diagram  400  of a propagation tuned oscillator (PTO)  404  to maintain positive closed-loop feedback in accordance with an exemplary embodiment. The measurement system includes a sensing assemblage  401  and propagation tuned oscillator (PTO)  404  that detects energy waves  402  in one or more waveguides  403  of the sensing assemblage  401 . In one embodiment, energy waves  402  are ultrasound waves. A pulse  411  is generated in response to the detection of energy waves  402  to initiate a propagation of a new energy wave in waveguide  403 . It should be noted that ultrasound energy pulses or waves, the emission of ultrasound pulses or waves by ultrasound resonators or transducers, transmitted through ultrasound waveguides, and detected by ultrasound resonators or transducers are used merely as examples of energy pulses, waves, and propagation structures and media. Other embodiments herein contemplated can utilize other wave forms, such as, light. 
     The sensing assemblage  401  comprises transducer  405 , transducer  406 , and a waveguide  403  (or energy propagating structure). In a non-limiting example, sensing assemblage  401  is affixed to load bearing or contacting surfaces  408 . External forces applied to the contacting surfaces  408  compress the waveguide  403  and change the length of the waveguide  403 . Under compression, transducers  405  and  406  will also be move closer together. The change in distance affects the transit time  407  of energy waves  402  transmitted and received between transducers  405  and  406 . The propagation tuned oscillator  404  in response to these physical changes will detect each energy wave sooner (e.g. shorter transit time) and initiate the propagation of new energy waves associated with the shorter transit time. As will be explained below, this is accomplished by way of PTO  404  in conjunction with the pulse generator  410 , the mode control  412 , and the phase detector  414 . 
     Notably, changes in the waveguide  403  (energy propagating structure or structures) alter the propagation properties of the medium of propagation (e.g. transit time  407 ). The energy wave can be a continuous wave or a pulsed energy wave. A pulsed energy wave approach reduces power dissipation allowing for a temporary power source such as a battery or capacitor to power the system during the course of operation. In at least one exemplary embodiment, a continuous wave energy wave or a pulsed energy wave is provided by transducer  405  to a first surface of waveguide  403 . Transducer  405  generates energy waves  402  that are coupled into waveguide  403 . In a non-limiting example, transducer  405  is a piezo-electric device capable of transmitting and receiving acoustic signals in the ultrasonic frequency range. 
     Transducer  406  is coupled to a second surface of waveguide  403  to receive the propagated pulsed signal and generates a corresponding electrical signal. The electrical signal output by transducer  406  is coupled to phase detector  414 . In general, phase detector  414  is a detection circuit that compares the timing of a selected point on the waveform of the detected energy wave with respect to the timing of the same point on the waveform of other propagated energy waves. In a first embodiment, phase detector  414  can be a zero-crossing receiver. In a second embodiment, phase detector  414  can be an edge-detect receiver. In a third embodiment, phase detector  414  can be a phase locked loop. In the example where sensing assemblage  401  is compressed, the detection of the propagated energy waves  402  occurs earlier (due to the length/distance reduction of waveguide  403 ) than a signal prior to external forces being applied to contacting surfaces. Pulse generator  410  generates a new pulse in response to detection of the propagated energy waves  402  by phase detector  414 . The new pulse is provided to transducer  405  to initiate a new energy wave sequence. Thus, each energy wave sequence is an individual event of energy wave propagation, energy wave detection, and energy wave emission that maintains energy waves  402  propagating in waveguide  403 . 
     The transit time  407  of a propagated energy wave is the time it takes an energy wave to propagate from the first surface of waveguide  403  to the second surface. There is delay associated with each circuit described above. Typically, the total delay of the circuitry is significantly less than the propagation time of an energy wave through waveguide  403 . In addition, under equilibrium conditions variations in circuit delay are minimal. Multiple pulse to pulse timings can be used to generate an average time period when change in external forces occur relatively slowly in relation to the pulsed signal propagation time such as in a physiologic or mechanical system. The digital counter  420  in conjunction with electronic components counts the number of propagated energy waves to determine a corresponding change in the length of the waveguide  403 . These changes in length change in direct proportion to the external force thus enabling the conversion of changes in parameter or parameters of interest into electrical signals. 
     The block diagram  400  further includes counting and timing circuitry. More specifically, the timing, counting, and clock circuitry comprises a digital timer  420 , a digital timer  422 , a digital clock  426 , and a data register  424 . The digital clock  426  provides a clock signal to digital counter  420  and digital timer  422  during a measurement sequence. The digital counter  420  is coupled to the propagation tuned oscillator  404 . Digital timer  422  is coupled to data register  424 . Digital timer  420 , digital timer,  422 , digital clock  426  and data register  424  capture transit time  407  of energy waves  402  emitted by ultrasound resonator or transducer  405 , propagated through waveguide  403 , and detected by or ultrasound resonator or transducer  405  or  406  depending on the mode of the measurement of the physical parameters of interest applied to surfaces  408 . The operation of the timing and counting circuitry is disclosed in more detail hereinbelow. 
     The measurement data can be analyzed to achieve accurate, repeatable, high precision and high resolution measurements. This method enables the setting of the level of precision or resolution of captured data to optimize trade-offs between measurement resolution versus frequency, including the bandwidth of the sensing and data processing operations, thus enabling a sensing module or device to operate at its optimal operating point without compromising resolution of the measurements. This is achieved by the accumulation of multiple cycles of excitation and transit time instead of averaging transit time of multiple individual excitation and transit cycles. The result is accurate, repeatable, high precision and high resolution measurements of parameters of interest in physical systems. 
     In at least one exemplary embodiment, propagation tuned oscillator  404  in conjunction with one or more sensing assemblages  401  are used to take measurements on a muscular-skeletal system. In a non-limiting example, sensing assemblage  401  is placed between a femoral prosthetic component and tibial prosthetic component to provide measured load information that aids in the installation of an artificial knee joint. Sensing assemblage  401  can also be a permanent component or a muscular-skeletal joint or artificial muscular-skeletal joint to monitor joint function. The measurements can be made in extension and in flexion. In the example, assemblage  401  is used to measure the condyle loading to determine if it falls within a predetermined range and location. Based on the measurement, the surgeon can select the thickness of the insert such that the measured loading and incidence with the final insert in place will fall within the predetermined range. Soft tissue tensioning can be used by a surgeon to further optimize the force or pressure. Similarly, two assemblages  401  can be used to measure both condyles simultaneously or multiplexed. The difference in loading (e.g. balance) between condyles can be measured. Soft tissue tensioning can be used to reduce the force on the condyle having the higher measured loading to reduce the measured pressure difference between condyles. 
     One method of operation holds the number of energy waves propagating through waveguide  403  as a constant integer number. A time period of an energy wave corresponds to energy wave periodicity. A stable time period is one in which the time period changes very little over a number of energy waves. This occurs when conditions that affect sensing assemblage  401  stay consistent or constant. Holding the number of energy waves propagating through waveguide  403  to an integer number is a constraint that forces a change in the time between pulses when the length of waveguide  403  changes. The resulting change in time period of each energy wave corresponds to a change in aggregate energy wave time period that is captured using digital counter  420  as a measurement of changes in external forces or conditions applied to contacting surfaces  408 . 
     A further method of operation according to one embodiment is described hereinbelow for energy waves  402  propagating from transducer  405  and received by transducer  406 . In at least one exemplary embodiment, energy waves  402  are an ultrasonic energy wave. Transducers  405  and  406  are piezo-electric resonator transducers. Although not described, wave propagation can occur in the opposite direction being initiated by transducer  406  and received by transducer  405 . Furthermore, detecting ultrasound resonator transducer  406  can be a separate ultrasound resonator as shown or transducer  405  can be used solely depending on the selected mode of propagation (e.g. reflective sensing). Changes in external forces or conditions applied to contacting surfaces  408  affect the propagation characteristics of waveguide  403  and alter transit time  407 . As mentioned previously, propagation tuned oscillator  404  holds constant an integer number of energy waves  402  propagating through waveguide  403  (e.g. an integer number of pulsed energy wave time periods) thereby controlling the repetition rate. As noted above, once PTO  404  stabilizes, the digital counter  420  digitizes the repetition rate of pulsed energy waves, for example, by way of edge-detection, as will be explained hereinbelow in more detail. 
     In an alternate embodiment, the repetition rate of pulsed energy waves  402  emitted by transducer  405  can be controlled by pulse generator  410 . The operation remains similar where the parameter to be measured corresponds to the measurement of the transit time  407  of pulsed energy waves  402  within waveguide  403 . It should be noted that an individual ultrasonic pulse can comprise one or more energy waves with a damping wave shape. The energy wave shape is determined by the electrical and mechanical parameters of pulse generator  410 , interface material or materials, where required, and ultrasound resonator or transducer  405 . The frequency of the energy waves within individual pulses is determined by the response of the emitting ultrasound resonator  404  to excitation by an electrical pulse  411 . The mode of the propagation of the pulsed energy waves  402  through waveguide  403  is controlled by mode control circuitry  412  (e.g., reflectance or uni-directional). The detecting ultrasound resonator or transducer may either be a separate ultrasound resonator or transducer  406  or the emitting resonator or transducer  405  depending on the selected mode of propagation (reflectance or unidirectional). 
     In general, accurate measurement of physical parameters is achieved at an equilibrium point having the property that an integer number of pulses are propagating through the energy propagating structure at any point in time. Measurement of changes in the “time-of-flight” or transit time of ultrasound energy waves within a waveguide of known length can be achieved by modulating the repetition rate of the ultrasound energy waves as a function of changes in distance or velocity through the medium of propagation, or a combination of changes in distance and velocity, caused by changes in the parameter or parameters of interest. 
     Measurement methods that rely on the propagation of energy waves, or energy waves within energy pulses, may require the detection of a specific point of energy waves at specified locations, or under specified conditions, to enable capturing parameters including, but not limited to, transit time, phase, or frequency of the energy waves. Measurement of the changes in the physical length of individual ultrasound waveguides may be made in several modes. Each assemblage of one or two ultrasound resonators or transducers combined with an ultrasound waveguide may be controlled to operate in six different modes. This includes two wave shape modes: continuous wave or pulsed waves, and three propagation modes: reflectance, unidirectional, and bi-directional propagation of the ultrasound wave. The resolution of these measurements can be further enhanced by advanced processing of the measurement data to enable optimization of the trade-offs between measurement resolution versus length of the waveguide, frequency of the ultrasound waves, and the bandwidth of the sensing and data capture operations, thus achieving an optimal operating point for a sensing module or device. 
     Measurement by propagation tuned oscillator  404  and sensing assemblage  401  enables high sensitivity and high signal-to-noise ratio. The time-based measurements are largely insensitive to most sources of error that may influence voltage or current driven sensing methods and devices. The resulting changes in the transit time of operation correspond to frequency, which can be measured rapidly, and with high resolution. This achieves the required measurement accuracy and precision thus capturing changes in the physical parameters of interest and enabling analysis of their dynamic and static behavior. 
     These measurements may be implemented with an integrated wireless sensing module or device having an encapsulating structure that supports sensors and load bearing or contacting surfaces and an electronic assemblage that integrates a power supply, sensing elements, energy transducer or transducers and elastic energy propagating structure or structures, biasing spring or springs or other form of elastic members, an accelerometer, antennas and electronic circuitry that processes measurement data as well as controls all operations of ultrasound generation, propagation, and detection and wireless communications. The electronics assemblage also supports testability and calibration features that assure the quality, accuracy, and reliability of the completed wireless sensing module or device. 
     The level of accuracy and resolution achieved by the integration of energy transducers and an energy propagating structure or structures coupled with the electronic components of the propagation tuned oscillator enables the construction of, but is not limited to, compact ultra low power modules or devices for monitoring or measuring the parameters of interest. The flexibility to construct sensing modules or devices over a wide range of sizes enables sensing modules to be tailored to fit a wide range of applications such that the sensing module or device may be engaged with, or placed, attached, or affixed to, on, or within a body, instrument, appliance, vehicle, equipment, or other physical system and monitor or collect data on physical parameters of interest without disturbing the operation of the body, instrument, appliance, vehicle, equipment, or physical system. 
     Referring to  FIG. 17 , a simplified flow chart  1700  of method steps for high precision processing and measurement data is shown in accordance with an exemplary embodiment. The method  1700  can be practiced with more or less than the steps shown, and is not limited to the order of steps shown. The method steps correspond to  FIG. 4  to be practiced with the aforementioned components or any other components suitable for such processing, for example, electrical circuitry to control the emission of energy pulses or waves and to capture the repetition rate of the energy pulses or frequency of the energy waves propagating through the elastic energy propagating structure or medium. 
     In a step  1702 , the process initiates a measurement operation. In a step  1704 , a known state is established by resetting digital timer  422  and data register  424 . In a step  1706 , digital counter  420  is preset to the number of measurement cycles over which measurements will be taken and collected. In a step  1708 , the measurement cycle is initiated and a clock output of digital clock  426  is enabled. A clock signal from digital clock  426  is provided to both digital counter  420  and digital timer  422 . An elapsed time is counted by digital timer  420  based on the frequency of the clock signal output by digital clock  426 . In a step  1710 , digital timer  422  begins tracking the elapsed time. Simultaneously, digital counter  420  starts decrementing a count after each measurement sequence. In one embodiment, digital counter  420  is decremented as each energy wave propagates through waveguide  403  and is detected by transducer  406 . Digital counter  420  counts down until the preset number of measurement cycles has been completed. In a step  1712 , energy wave propagation is sustained by propagation tuned oscillator  404 , as digital counter  420  is decremented by the detection of a propagated energy wave. In a step  1714 , energy wave detection, emission, and propagation continue while the count in digital counter  420  is greater than zero. In a step  1716 , the clock input of digital timer  422  is disabled upon reaching a zero count on digital counter  420  thus preventing digital counter  420  and digital timer  422  from being clocked. In one embodiment, the preset number of measurement cycles provided to digital counter  420  is divided by the elapsed time measured by digital timer  422  to calculate a frequency of propagated energy waves. Conversely, the number can be calculated as a transit time by dividing the elapsed time from digital timer  422  by the preset number of measurement cycles. Finally, in a step  1718 , the resulting value is transferred to register  424 . The number in data register  424  can be wirelessly transmitted to a display and database. The data from data register  424  can be correlated to a parameter being measured. The parameter such as a force or load is applied to the propagation medium (e.g. waveguide  403 ) such that parameter changes also change the frequency or transit time calculation of the measurement. A relationship between the material characteristics of the propagation medium and the parameter is used with the measurement value (e.g. frequency, transit time, phase) to calculate a parameter value. 
     The method  1700  practiced by the example assemblage of  FIG. 4 , and by way of the digital counter  420 , digital timer  422 , digital clock  426  and associated electronic circuitry analyzes the digitized measurement data according to operating point conditions. In particular, these components accumulate multiple digitized data values to improve the level of resolution of measurement of changes in length or other aspect of an elastic energy propagating structure or medium that can alter the transit time of energy pulses or waves propagating within the elastic energy propagating structure or medium. The digitized data is summed by controlling the digital counter  420  to run through multiple measurement cycles, each cycle having excitation and transit phases such that there is not lag between successive measurement cycles, and capturing the total elapsed time. The counter is sized to count the total elapsed time of as many measurement cycles as required to achieve the required resolution without overflowing its accumulation capacity and without compromising the resolution of the least significant bit of the counter. The digitized measurement of the total elapsed transit time is subsequently divided by the number of measurement cycles to estimate the time of the individual measurement cycles and thus the transit time of individual cycles of excitation, propagation through the elastic energy propagating structure or medium, and detection of energy pulses or waves. Accurate estimates of changes in the transit time of the energy pulses or waves through the elastic energy propagating structure or medium are captured as elapsed times for excitation and detection of the energy pulses or waves are fixed. 
     Summing individual measurements before dividing to estimate the average measurement value data values produces superior results to averaging the same number of samples. The resolution of count data collected from a digital counter is limited by the resolution of the least-significant-bit in the counter. Capturing a series of counts and averaging them does not produce greater precision than this least-significant-bit, that is the precision of a single count. Averaging does reduce the randomness of the final estimate if there is random variation between individual measurements. Summing the counts of a large number of measurement cycles to obtain a cumulative count then calculating the average over the entire measurement period improves the precision of the measurement by interpolating the component of the measurement that is less than the least significant bit of the counter. The precision gained by this procedure is on the order of the resolution of the least-significant-bit of the counter divided by the number of measurement cycles summed. 
     The size of the digital counter and the number of measurement cycles accumulated may be greater than the required level of resolution. This not only assures performance that achieves the level of resolution required, but also averages random component within individual counts producing highly repeatable measurements that reliably meet the required level of resolution. 
     The number of measurement cycles is greater than the required level of resolution. This not only assures performance that achieves the level of resolution required, but also averages any random component within individual counts producing highly repeatable measurements that reliably meet the required level of resolution. 
       FIG. 5  is a sensor interface diagram incorporating the zero-crossing receiver  200  in a continuous wave multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment. The positive closed-loop feedback is illustrated by the bold line path. Initially, multiplexer (mux)  502  receives as input a clock signal  504 , which is passed to the transducer driver  506  to produce the drive line signal  508 . Analog multiplexer (mux)  510  receives drive line signal  508 , which is passed to the transmitter transducer  512  to generate energy waves  514 . Transducer  512  is located at a first location of an energy propagating medium. The emitted energy waves  514  propagate through the energy propagating medium. Receiver transducer  516  is located at a second location of the energy propagating medium. Receiver transducer  516  captures the energy waves  514 , which are fed to analog mux  520  and passed to the zero-crossing receiver  200 . The captured energy waves by transducer  516  are indicated by electrical waves  518  provided to mux  520 . Zero-crossing receiver  200  outputs a pulse corresponding to each zero crossing detected from captured electrical waves  518 . The zero crossings are counted and used to determine changes in the phase and frequency of the energy waves propagating through the energy propagating medium. In a non-limiting example, a parameter such as applied force is measured by relating the measured phase and frequency to a known relationship between the parameter (e.g. force) and the material properties of the energy propagating medium. In general, pulse sequence  522  corresponds to the detected signal frequency. The zero-crossing receiver  200  is in a feedback path of the propagation tuned oscillator. The pulse sequence  522  is coupled through mux  502  in a positive closed-loop feedback path. The pulse sequence  522  disables the clock signal  504  such that the path providing pulse sequence  522  is coupled to driver  506  to continue emission of energy waves into the energy propagating medium and the path of clock signal  504  to driver  506  is disabled. 
       FIG. 6  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the zero-crossing receiver  640  for operation in continuous wave mode. In particular, with respect to  FIG. 4 , it illustrates closed loop measurement of the transit time  412  of ultrasound waves  414  within the waveguide  408  by the operation of the propagation tuned oscillator  416 . This example is for operation in continuous wave mode. The system can also be operated in pulse mode and a pulse-echo mode. Pulse mode and pulsed echo-mode use a pulsed energy wave. Pulse-echo mode uses reflection to direct an energy wave within the energy propagation medium. Briefly, the digital logic circuit  646  digitizes the frequency of operation of the propagation tuned oscillator. 
     In continuous wave mode of operation a sensor comprising transducer  604 , propagating structure  602 , and transducer  606  is used to measure the parameter. In general, the parameter to be measured affects the properties of the propagating medium. For example, an external force or condition  612  is applied to propagating structure  602  that changes the length of the waveguide in a path of a propagating energy wave. A change in length corresponds to a change in transit time  608  of the propagating wave. Similarly, the length of propagating structure  602  corresponds to the applied force  612 . A length reduction corresponds to a higher force being applied to the propagating structure  602 . Conversely, a length increase corresponds to a lowering of the applied force  612  to the propagating structure  602 . The length of propagating structure  602  is measured and is converted to force by way of a known length to force relationship. 
     Transducer  604  is an emitting device in continuous wave mode. The sensor for measuring a parameter comprises transducer  604  coupled to propagating structure  602  at a first location. A transducer  606  is coupled to propagating structure  602  at a second location. Transducer  606  is a receiving transducer for capturing propagating energy waves. In one embodiment, the captured propagated energy waves are electrical sine waves  634  that are output by transducer  606 . 
     A measurement sequence is initiated when control circuitry  618  closes switch  620  coupling oscillator output  624  of oscillator  622  to the input of amplifier  626 . One or more pulses provided to amplifier  626  initiates an action to propagate energy waves  610  having simple or complex waveforms through energy propagating structure or medium  602 . Amplifier  626  comprises a digital driver  628  and matching network  630 . In one embodiment, amplifier  626  transforms the oscillator output of oscillator  622  into sine waves of electrical waves  632  having the same repetition rate as oscillator output  624  and sufficient amplitude to excite transducer  604 . 
     Emitting transducer  604  converts the sine waves  632  into energy waves  610  of the same frequency and emits them at the first location into energy propagating structure or medium  602 . The energy waves  610  propagate through energy propagating structure or medium  602 . Upon reaching transducer  606  at the second location, energy waves  610  are captured, sensed, or detected. The captured energy waves are converted by transducer  606  into sine waves  634  that are electrical waves having the same frequency. 
     Amplifier  636  comprises a pre-amplifier  634  and zero-cross receiver  640 . Amplifier  636  converts the sine waves  634  into digital pulses  642  of sufficient duration to sustain the behavior of the closed loop circuit. Control circuitry  618  responds to digital pulses  642  from amplifier  636  by opening switch  620  and closing switch  644 . Opening switch  620  decouples oscillator output  624  from the input of amplifier  626 . Closing switch  644  creates a closed loop circuit coupling the output of amplifier  636  to the input of amplifier  626  and sustaining the emission, propagation, and detection of energy waves through energy propagating structure or medium  602 . 
     An equilibrium state is attained by maintaining unity gain around this closed loop circuit wherein sine waves  632  input into transducer  604  and sine waves  634  output by transducer  606  are in phase with a small but constant offset. Transducer  606  as disclosed above, outputs the sine waves  634  upon detecting energy waves propagating to the second location. In the equilibrium state, an integer number of energy waves  610  propagate through energy propagating structure or medium  602 . 
     Movement or changes in the physical properties of energy propagating structure or medium  602  change a transit time  608  of energy waves  610 . The transit time  608  comprises the time for an energy wave to propagate from the first location to the second location of propagating structure  602 . Thus, the change in the physical property of propagating structure  602  results in a corresponding time period change of the energy waves  610  within energy propagating structure or medium  602 . These changes in the time period of the energy waves  610  alter the equilibrium point of the closed loop circuit and frequency of operation of the closed loop circuit. The closed loop circuit adjusts such that sine waves  632  and  634  correspond to the new equilibrium point. The frequency of energy waves  610  and changes to the frequency correlate to changes in the physical attributes of energy propagating structure or medium  602 . 
     The physical changes may be imposed on energy propagating structure  602  by external forces or conditions  612  thus translating the levels and changes of the parameter or parameters of interest into signals that may be digitized for subsequent processing, storage, and display. Translation of the operating frequency into digital binary numbers facilitates communication, additional processing, storage, and display of information about the level and changes in physical parameters of interest. Similarly, the frequency of energy waves  610  during the operation of the closed loop circuit, and changes in this frequency, may be used to measure movement or changes in physical attributes of energy propagating structure or medium  602 . 
     Prior to measurement of the frequency or operation of the propagation tuned oscillator, control logic  618  loads the loop count into digital counter  650  that is stored in count register  648 . The first digital pulses  642  initiates closed loop operation within the propagation tuned oscillator and signals control circuit  618  to start measurement operations. At the start of closed loop operation, control logic  618  enables digital counter  650  and digital timer  652 . In one embodiment, digital counter  650  decrements its value on the rising edge of each digital pulse output by zero-crossing receiver  640 . Digital timer  652  increments its value on each rising edge of clock pulses  656 . When the number of digital pulses  642  has decremented, the value within digital counter  650  to zero a stop signal is output from digital counter  650 . The stop signal disables digital timer  652  and triggers control circuit  618  to output a load command to data register  654 . Data register  654  loads a binary number from digital timer  652  that is equal to the period of the energy waves or pulses times the value in counter  648  divided by clock period  656 . With a constant clock period  656 , the value in data register  654  is directly proportional to the aggregate period of the energy waves or pulses accumulated during the measurement operation. Duration of the measurement operation and the resolution of measurements may be adjusted by increasing or decreasing the value preset in the count register  648 . 
       FIG. 7  is a sensor interface diagram incorporating the integrated zero-crossing receiver  200  in a pulse multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment. In one embodiment, the circuitry other than the sensor is integrated on an application specific integrated circuit (ASIC). The positive closed-loop feedback is illustrated by the bold line path. Initially, mux  702  is enabled to couple one or more digital pulses  704  to the transducer driver  706 . Transducer driver  706  generates a pulse sequence  708  corresponding to digital pulses  704 . Analog mux  710  is enabled to couple pulse sequence  708  to the transmitter transducer  712 . Transducer  712  is coupled to a medium at a first location. Transducer  712  responds to pulse sequence  708  and generates corresponding energy pulses  714  that are emitted into the medium at the first location. The energy pulses  714  propagate through the medium. A receiver transducer  716  is located at a second location on the medium. Receiver transducer  716  captures the energy pulses  714  and generates a corresponding signal of electrical pulses  718 . Transducer  716  is coupled to a mux  720 . Mux  720  is enabled to couple to zero-cross receiver  200 . Electrical pulses  718  from transducer  716  are coupled to zero-cross receiver  200 . Zero-cross receiver  200  counts zero crossings of electrical pulses  718  to determine changes in phase and frequency of the energy pulses responsive to an applied force, as previously explained. Zero-cross receiver  200  outputs a pulse sequence  722  corresponding to the detected signal frequency. Pulse sequence  722  is coupled to mux  702 . Mux  702  is decoupled from coupling digital pulses  704  to driver  706  upon detection of pulses  722 . Conversely, mux  702  is enabled to couple pulses  722  to driver  706  upon detection of pulses  722  thereby creating a positive closed-loop feedback path. Thus, in pulse mode, zero-cross receiver  200  is part of the closed-loop feedback path that continues emission of energy pulses into the medium at the first location and detection at the second location to measure a transit time and changes in transit time of pulses through the medium. 
       FIG. 8  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the zero-crossing receiver  640  for operation in pulse mode. In particular, with respect to  FIG. 4 , it illustrates closed loop measurement of the transit time  412  of ultrasound waves  414  within the waveguide  408  by the operation of the propagation tuned oscillator  416 . This example is for operation in pulse mode. The system can also be operated in continuous wave mode and a pulse-echo mode. Continuous wave mode uses a continuous wave signal. Pulse-echo mode uses reflection to direct an energy wave within the energy propagation medium. Briefly, the digital logic circuit  646  digitizes the frequency of operation of the propagation tuned oscillator. 
     In pulse mode of operation, a sensor comprising transducer  604 , propagating structure  602 , and transducer  606  is used to measure the parameter. In general, the parameter to be measured affects the properties of the propagating medium. For example, an external force or condition  612  is applied to propagating structure  602  that changes the length of the waveguide in a path of a propagating energy wave. A change in length corresponds to a change in transit time  608  of the propagating wave. The length of propagating structure  602  is measured and is converted to force by way of a known length to force relationship. One benefit of pulse mode operation is the use of a high magnitude pulsed energy wave. In one embodiment, the magnitude of the energy wave decays as it propagates through the medium. The use of a high magnitude pulse is a power efficient method to produce a detectable signal if the energy wave has to traverse a substantial distance or is subject to a reduction in magnitude as it propagated due to the medium. 
     A measurement sequence is initiated when control circuitry  618  closes switch  620  coupling oscillator output  624  of oscillator  622  to the input of amplifier  626 . One or more pulses provided to amplifier  626  initiates an action to propagate energy waves  610  having simple or complex waveforms through energy propagating structure or medium  602 . Amplifier  626  comprises a digital driver  628  and matching network  630 . In one embodiment, amplifier  626  transforms the oscillator output of oscillator  622  into analog pulses of electrical waves  832  having the same repetition rate as oscillator output  624  and sufficient amplitude to excite transducer  604 . 
     Emitting transducer  604  converts the analog pulses  832  into energy waves  610  of the same frequency and emits them at a first location into energy propagating structure or medium  602 . The energy waves  610  propagate through energy propagating structure or medium  602 . Upon reaching transducer  606  at the second location, energy waves  610  are captured, sensed, or detected. The captured energy waves are converted by transducer  606  into analog pulses  834  that are electrical waves having the same frequency. 
     Amplifier  636  comprises a pre-amplifier  638  and zero-cross receiver  640 . Amplifier  636  converts the analog pulses  834  into digital pulses  642  of sufficient duration to sustain the behavior of the closed loop circuit. Control circuitry  618  responds to digital pulses  642  from amplifier  636  by opening switch  620  and closing switch  644 . Opening switch  620  decouples oscillator output  624  from the input of amplifier  626 . Closing switch  644  creates a closed loop circuit coupling the output of amplifier  636  to the input of amplifier  626  and sustaining the emission, propagation, and detection of energy waves through energy propagating structure or medium  602 . 
     An equilibrium state is attained by maintaining unity gain around this closed loop circuit wherein pulses  832  input into transducer  604  and pulses  834  output by transducer  606  are in phase with a small but constant offset. Transducer  606  as disclosed above, outputs the pulses  834  upon detecting energy waves propagating to the second location. In the equilibrium state, an integer number of energy waves  610  propagate through energy propagating structure or medium  602 . 
     Movement or changes in the physical properties of energy propagating structure or medium  602  change a transit time  608  of energy waves  610 . The transit time  608  comprises the time for an energy wave to propagate from the first location to the second location of propagating structure  602 . Thus, the change in the physical property of propagating structure  602  results in a corresponding time period change of the energy waves  610  within energy propagating structure or medium  602 . These changes in the time period of the energy waves  610  alter the equilibrium point of the closed loop circuit and frequency of operation of the closed loop circuit. The closed loop circuit adjusts such that pulses  832  and  834  correspond to the new equilibrium point. The frequency of energy waves  610  and changes to the frequency correlate to changes in the physical attributes of energy propagating structure or medium  602 . 
     The physical changes may be imposed on energy propagating structure  602  by external forces or conditions  612  thus translating the levels and changes of the parameter or parameters of interest into signals that may be digitized for subsequent processing, storage, and display. Translation of the operating frequency into digital binary numbers facilitates communication, additional processing, storage, and display of information about the level and changes in physical parameters of interest as disclosed in more detail hereinabove. Similarly, the frequency of energy waves  610  during the operation of the closed loop circuit, and changes in this frequency, may be used to measure movement or changes in physical attributes of energy propagating structure or medium  602 . 
       FIG. 9  illustrates a block diagram of an edge-detect receiver circuit  900  in accordance with an exemplary embodiment. In a first embodiment, edge-detect receiver  900  is provided to detect wave fronts of pulses of energy waves. This enables capturing of parameters including, but not limited to, transit time, phase, or frequency of the energy waves. Circuitry of the integrated edge-detect receiver  900  provides rapid on-set detection and quickly responds to the arrival of an energy pulse. It reliably triggers thereafter a digital output pulse at a same point on the initial wave front of each captured energy pulse or pulsed energy wave. The digital pulse can be optimally configured to output with minimal and constant delay. The edge-detect receiver  900  can isolate and precisely detect the specified point on the initial energy wave or the wave front in the presence of interference and distortion signals thereby overcoming problems commonly associated with detecting one of multiple generated complex signals in energy propagating mediums. The edge-detect receiver  900  performs these functions accurately over a wide range of amplitudes including very low-level energy pulses. 
     In a second embodiment, the edge-detect receiver  900  is incorporated within a propagation tuned oscillator (PTO) to maintain positive closed-loop feedback when operating in a pulse or pulse-echo mode. The edge-detect receiver  900  can be integrated with other circuitry of the PTO by multiplexing input and output circuitry to achieve ultra low-power and small compact size. Integration of the circuitry of the PTO with the edge-detect receiver provides the benefit of increasing sensitivity to low-level signals. 
     The block diagram illustrates one embodiment of a low power edge-detect receiver circuit  900  with superior performance at low signal levels. The edge-detect receiver  900  comprises a preamplifier  912 , a differentiator  914 , a digital pulse circuit  916  and a deblank circuit  918 . The edge-detect receiver circuit  900  can be implemented in discrete analog components, digital components or combination thereof. In one embodiment, edge-detect receiver  900  is integrated into an ASIC as part of a sensor system described hereinbelow. The edge-detect receiver circuit  900  practices measurement methods that detect energy pulses or pulsed energy waves at specified locations and under specified conditions to enable capturing parameters including, but not limited to, transit time, phase, frequency, or amplitude of energy pulses. A brief description of the method of operation is as follows. In a non-limiting example, a pre-amplifier triggers a comparator circuit responsive to small changes in the slope of an input signal. The comparator and other edge-detect circuitry responds rapidly with minimum delay. Detection of small changes in the input signal assures rapid detection of the arrival of a pulse of energy waves. The minimum phase design reduces extraneous delay thereby introducing less variation into the measurement of the transit time, phase, frequency, or amplitude of the incoming energy pulses. 
     An input  920  of edge-detect receiver  900  is coupled to pre-amplifier  912 . As an example, the incoming wave  910  to the edge-detect receiver circuit  900  can be received from an electrical connection, antenna, or transducer. The incoming wave  910  is amplified by pre-amplifier  912 , which assures adequate sensitivity to small signals. Differentiator circuitry  914  monitors the output of pre-amplifier  912  and triggers digital pulse circuitry  916  whenever a signal change corresponding to a pulsed energy wave is detected. For example, a signal change that identifies the pulsed energy wave is the initial wave front or the leading edge of the pulsed energy wave. In one arrangement, differentiator  914  detects current flow, and more specifically changes in the slope of the energy wave  910  by detecting small changes in current flow instead of measuring changes in voltage level to achieve rapid detection of slope. Alternatively, differentiator  914  can be implemented to trigger on changes in voltage. Together, preamplifier  912  and differentiator  916  monitor the quiescent input currents for the arrival of wave front of energy wave(s)  910 . Preamplifier  912  and differentiator  916  detect the arrival of low level pulses of energy waves as well as larger pulses of energy waves. This detection methodology achieves superior performance for very low level signals. Differentiator circuitry  912  triggers digital pulse circuitry  916  whenever current flow driven by the initial signal ramp of the incoming wave  910  is detected. The digital pulse is coupled to deblank circuit  918  that desensitizes pre-amplifier  912 . For example, the desensitization of pre-amplifier  912  can comprise a reduction in gain, decoupling of input  920  from energy wave  910 , or changing the frequency response. The deblank circuit  918  also disregards voltage or current levels for a specified or predetermined duration of time to effectively skip over the interference sections or distorted portions of the energy wave  910 . In general, energy wave  910  can comprise more than one change in slope and is typically a damped wave form. Additional signals or waves of the pulsed energy wave on the input  920  of pre-amplifier  912  are not processed during the preset blanking period. In this example, the digital output pulse  928  can then be coupled to signal processing circuitry as explained hereinbelow. In one embodiment, the electronic components are operatively coupled as blocks within an integrated circuit. As will be shown ahead, this integration arrangement performs its specific functions efficiently with a minimum number of components. This is because the circuit components are partitioned between structures within an integrated circuit and discrete components, as well as innovative partitioning of analog and digital functions, to achieve the required performance with a minimum number of components and minimum power consumption. 
       FIG. 10  illustrates a block diagram of the edge-detect receiver circuit  900  coupled to a sensing assembly  1000 . The pre-amplifier  912  and the digital pulse circuit  916  are shown for reference and discussion. The sensing assembly  1000  comprises a transmitter transducer  1002 , an energy propagating medium  1004 , and a receiver transducer  1006 . The transmitter transducer  1002  is coupled to propagating medium  1004  at a first location. The receiver transducer  1006  is coupled to energy propagating medium  1004  at a second location. Alternatively, a reflecting surface can replace receiver transducer  1006 . The reflecting surface reflects an energy wave back towards the first location. Transducer  1006  can be enabled to be a transmitting transducer and a receiving transducer thereby saving the cost of a transducer. As will be explained ahead in further detail, the sensing assembly  1000  in one embodiment is part of a sensory device that assess loading, in particular, the externally applied forces  1008  on the sensing assembly  1000 . A transducer driver circuit (not shown) drives the transmitter transducer  1002  of the sensing assembly  1000  to produce energy waves  1010  that are directed into the energy propagating medium  1004 . In the non-limiting example, changes in the energy propagating medium  1004  due to the externally applied forces  1008  change the frequency, phase, and transit time  1012  of energy waves  1010  propagating from the first location to the second location of energy propagating medium  1004 . The integrated edge-detect receiver circuit  900  is coupled to the receiver transducer  1006  to detect edges of the reproduced energy wave  910  and trigger the digital pulse  928 . In general, the timing of the digital pulse  928  conveys the parameters of interest (e.g., distance, force weight, strain, pressure, wear, vibration, viscosity, density, direction, displacement, etc.) related to the change in energy propagating structure  1004  due to an external parameter. For example, sensing assembly  1000  placed in a knee joint as described hereinabove. 
     Measurement methods that rely on the propagation of energy pulses require the detection of energy pulses at specified locations or under specified conditions to enable capturing parameters including, but not limited to, transit time, phase, frequency, or amplitude of the energy pulses. Measurement methods that rely on such propagation of energy waves  1010  or pulses of energy waves are required to achieve highly accurate and controlled detection of energy waves or pulses. Moreover, pulses of energy waves may contain multiple energy waves with complex waveforms therein leading to potential ambiguity of detection. In particular, directing energy waves  1010  into the energy propagating structure  1004  can generate interference patterns caused by nulls and resonances of the waveguide, as well as characteristics of the generated energy wave  1010 . These interference patterns can generate multiply excited waveforms that result in distortion of the edges of the original energy wave. To reliably detect the arrival of a pulse of energy waves, the edge-detect receiver  900  only responds to the leading edge of the first energy wave within each pulse. This is achieved in part by blanking the edge-detect circuitry  900  for the duration of each energy pulse. As an example, the deblank circuit  918  disregards voltage or current levels for a specified duration of time to effectively skip over the interference sections or distorted portions of the waveform. 
       FIG. 11  is a sensor interface diagram incorporating the edge-detect receiver circuit  900  in a pulse-echo multiplexing arrangement for maintaining positive closed-loop feedback in accordance with one embodiment. The positive closed-loop feedback is illustrated by the bold line path. Initially, multiplexer (mux)  1102  receives as input a digital pulse  1104 , which is passed to the transducer driver  1106  to produce the pulse sequence  1108 . Analog multiplexer (mux)  1110  receives pulse sequence  1108 , which is passed to the transducer  1112  to generate energy pulses  1114 . Energy pulses  1114  are emitted into a first location of a medium and propagate through the medium. In the pulse-echo example, energy pulses  1114  are reflected off a surface  1116  at a second location of the medium, for example, the end of a waveguide or reflector, and echoed back to the transducer  1112 . The transducer  1112  proceeds to then capture the reflected pulse echo. In pulsed echo mode, the transducer  1112  performs as both a transmitter and a receiver. As disclosed above, transducer  1112  toggles back and forth between emitting and receiving energy waves. Transducer  1112  captures the reflected echo pulses, which are coupled to analog mux  1110  and directed to the edge-detect receiver  900 . The captured reflected echo pulses is indicated by electrical waves  1120 . Edge-detect receiver  900  locks on pulse edges corresponding to the wave front of a propagated energy wave to determine changes in phase and frequency of the energy pulses  1114  responsive to an applied force, as previously explained. Among other parameters, it generates a pulse sequence  1118  corresponding to the detected signal frequency. The pulse sequence  1118  is coupled to mux  1102  and directed to driver  1106  to initiate one or more energy waves being emitted into the medium by transducer  1112 . Pulse  1104  is decoupled from being provided to driver  1106 . Thus, a positive closed loop feedback is formed that repeatably emits energy waves into the medium until mux  1102  prevents a signal from being provided to driver  1106 . The edge-detect receiver  900  is coupled to a second location of the medium and is in the feedback path. The edge-detect receiver  900  initiates a pulsed energy wave being provided at the first location of the medium upon detecting a wave front at the second location when the feedback path is closed. 
       FIG. 12  is an exemplary block diagram of a propagation tuned oscillator (PTO) incorporating the edge-detect receiver circuit  900  for operation in pulse echo mode. In particular, with respect to  FIG. 4 , it illustrates closed loop measurement of the transit time  412  of ultrasound waves  414  within the waveguide  408  by the operation of the propagation tuned oscillator  416 . This example is for operation in a pulse echo mode. The system can also be operated in pulse mode and a continuous wave mode. Pulse mode does not use a reflected signal. Continuous wave mode uses a continuous signal. Briefly, the digital logic circuit  1246  digitizes the frequency of operation of the propagation tuned oscillator. 
     In pulse-echo mode of operation a sensor comprising transducer  1204 , propagating structure  1202 , and reflecting surface  1206  is used to measure the parameter. In general, the parameter to be measured affects the properties of the propagating medium. For example, an external force or condition  1212  is applied to propagating structure  1202  that changes the length of the waveguide in a path of a propagating energy wave. A change in length corresponds to a change in transit time of the propagating wave. Similarly, the length of propagating structure  1202  corresponds to the applied force  1212 . A length reduction corresponds to a higher force being applied to the propagating structure  1202 . Conversely, a length increase corresponds to a lowering of the applied force  1212  to the propagating structure  1202 . The length of propagating structure  1202  is measured and is converted to force by way of a known length to force relationship. 
     Transducer  1204  is both an emitting device and a receiving device in pulse-echo mode. The sensor for measuring a parameter comprises transducer  1204  coupled to propagating structure  1202  at a first location. A reflecting surface is coupled to propagating structure  1202  at a second location. Transducer  1204  has two modes of operation comprising an emitting mode and receiving mode. Transducer  1204  emits an energy wave into the propagating structure  1202  at the first location in the emitting mode. The energy wave propagates to a second location and is reflected by reflecting surface  1206 . The reflected energy wave is reflected towards the first location and transducer  1204  subsequently generates a signal in the receiving mode corresponding to the reflected energy wave. 
     A measurement sequence in pulse echo mode is initiated when control circuitry  1218  closes switch  1220  coupling digital output  1224  of oscillator  1222  to the input of amplifier  1226 . One or more pulses provided to amplifier  1226  starts a process to emit one or more energy waves  1210  having simple or complex waveforms into energy propagating structure or medium  1202 . Amplifier  1226  comprises a digital driver  1228  and matching network  1230 . In one embodiment, amplifier  1226  transforms the digital output of oscillator  1222  into pulses of electrical waves  1232  having the same repetition rate as digital output  1224  and sufficient amplitude to excite transducer  1204 . 
     Transducer  1204  converts the pulses of electrical waves  1232  into pulses of energy waves  1210  of the same repetition rate and emits them into energy propagating structure or medium  1202 . The pulses of energy waves  1210  propagate through energy propagating structure or medium  1202  as shown by arrow  1214  towards reflecting surface  1206 . Upon reaching reflecting surface  1206 , energy waves  1210  are reflected by reflecting surface  1206 . Reflected energy waves propagate towards transducer  1204  as shown by arrow  1216 . The reflected energy waves are detected by transducer  1204  and converted into pulses of electrical waves  1234  having the same repetition rate. 
     Amplifier  1236  comprises a pre-amplifier  1234  and edge-detect receiver  1240 . Amplifier  1236  converts the pulses of electrical waves  1234  into digital pulses  1242  of sufficient duration to sustain the pulse behavior of the closed loop circuit. Control circuitry  1218  responds to digital output pulses  1242  from amplifier  1236  by opening switch  1220  and closing switch  1244 . Opening switch  1220  decouples oscillator output  1224  from the input of amplifier  1226 . Closing switch  1244  creates a closed loop circuit coupling the output of amplifier  1236  to the input of amplifier  1226  and sustaining the emission, propagation, and detection of energy pulses through energy propagating structure or medium  1202 . 
     An equilibrium state is attained by maintaining unity gain around this closed loop circuit wherein electrical waves  1232  input into transducer  1204  and electrical waves  1234  output by transducer  1204  are in phase with a small but constant offset. Transducer  1204  as disclosed above, outputs the electrical waves  1234  upon detecting reflected energy waves reflected from reflecting surface  1206 . In the equilibrium state, an integer number of pulses of energy waves  1210  propagate through energy propagating structure or medium  1202 . 
     Movement or changes in the physical properties of energy propagating structure or medium  1202  change a transit time  1208  of energy waves  1210 . The transit time  1208  comprises the time for an energy wave to propagate from the first location to the second location of propagating structure  1202  and the time for the reflected energy wave to propagate from the second location to the first location of propagating structure  1202 . Thus, the change in the physical property of propagating structure  1202  results in a corresponding time period change of the energy waves  1210  within energy propagating structure or medium  1202 . These changes in the time period of the repetition rate of the energy pulses  1210  alter the equilibrium point of the closed loop circuit and repetition rate of operation of the closed loop circuit. The closed loop circuit adjusts such that electrical waves  1232  and  1234  correspond to the new equilibrium point. The repetition rate of energy waves  1210  and changes to the repetition rate correlate to changes in the physical attributes of energy propagating structure or medium  1202 . 
     The physical changes may be imposed on energy propagating structure  1202  by external forces or conditions  1212  thus translating the levels and changes of the parameter or parameters of interest into signals that may be digitized for subsequent processing, storage, and display. Translation of the operating frequency into digital binary numbers facilitates communication, additional processing, storage, and display of information about the level and changes in physical parameters of interest. Similarly, the frequency of energy waves  1210  during the operation of the closed loop circuit, and changes in this frequency, may be used to measure movement or changes in physical attributes of energy propagating structure or medium  1202 . 
     Prior to measurement of the frequency or operation of the propagation tuned oscillator, control logic  1218  loads the loop count into digital counter  1250  that is stored in count register  1248 . The first digital pulses  1242  initiates closed loop operation within the propagation tuned oscillator and signals control circuit  1218  to start measurement operations. At the start of closed loop operation, control logic  1218  enables digital counter  1250  and digital timer  1252 . In one embodiment, digital counter  1250  decrements its value on the rising edge of each digital pulse output by edge-detect receiver  1240 . Digital timer  1252  increments its value on each rising edge of clock pulses  1256 . When the number of digital pulses  1242  has decremented, the value within digital counter  1250  to zero a stop signal is output from digital counter  1250 . The stop signal disables digital timer  1252  and triggers control circuit  1218  to output a load command to data register  1254 . Data register  1254  loads a binary number from digital timer  1252  that is equal to the period of the energy waves or pulses times the value in counter  1248  divided by clock period  1256 . With a constant clock period  1256 , the value in data register  1254  is directly proportional to the aggregate period of the energy waves or pulses accumulated during the measurement operation. Duration of the measurement operation and the resolution of measurements may be adjusted by increasing or decreasing the value preset in the count register  1248 . 
       FIG. 13  is a simplified cross-sectional view of a sensing module  1301  in accordance with an exemplary embodiment. The sensing module (or assemblage) is an electro-mechanical assembly comprising electrical components and mechanical components that when configured and operated in accordance with a sensing mode performs as a positive feedback closed-loop measurement system. The measurement system can precisely measure applied forces, such as loading, on the electro-mechanical assembly. The sensing mode can be a continuous mode, a pulse mode, or a pulse echo-mode. 
     In one embodiment, the electrical components can include ultrasound resonators or transducers  405  and  406 , ultrasound waveguides  403 , and signal processing electronics  1310 , but are not limited to these. The mechanical components can include biasing springs  1332 , spring retainers and posts, and load platforms  1306 , but are not limited to these. The electrical components and mechanical components can be inter-assembled (or integrated) onto a printed circuit board  1336  to operate as a coherent ultrasonic measurement system within sensing module  1301  and according to the sensing mode. As will be explained ahead in more detail, the signal processing electronics incorporate a propagation tuned oscillator (PTO) or a phase locked loop (PLL) to control the operating frequency of the ultrasound resonators or transducers for providing high precision sensing. Furthermore, the signal processing electronics incorporate detect circuitry that consistently detects an energy wave after it has propagated through a medium. The detection initiates the generation of a new energy wave by an ultrasound resonator or transducer that is coupled to the medium for propagation therethrough. A change in transit time of an energy wave through the medium is measured and correlates to a change in material property of the medium due to one or more parameters applied thereto. 
     Sensing module  1301  comprises one or more assemblages  401  each comprised one or more ultrasound resonators  405  and  406 . As illustrated, waveguide  403  is coupled between transducers ( 405 ,  406 ) and affixed to load bearing or contacting surfaces  408 . In one exemplary embodiment, an ultrasound signal is coupled for propagation through waveguide  403 . The sensing module  1301  is placed, attached to, or affixed to, or within a body, instrument, or other physical system  1318  having a member or members  1316  in contact with the load bearing or contacting surfaces  408  of the sensing module  401 . This arrangement facilitates translating the parameters of interest into changes in the length or compression or extension of the waveguide or waveguides  403  within the sensing module  1301  and converting these changes in length into electrical signals. This facilitates capturing data, measuring parameters of interest and digitizing that data, and then subsequently communicating that data through antenna  1334  to external equipment with minimal disturbance to the operation of the body, instrument, appliance, vehicle, equipment, or physical system  1318  for a wide range of applications. 
     The sensing module  401  supports three modes of operation of energy wave propagation and measurement: reflectance, unidirectional, and bi-directional. These modes can be used as appropriate for each individual application. In unidirectional and bi-directional modes, a chosen ultrasound resonator or transducer is controlled to emit pulses of ultrasound waves into the ultrasound waveguide and one or more other ultrasound resonators or transducers are controlled to detect the propagation of the pulses of ultrasound waves at a specified location or locations within the ultrasound waveguide. In reflectance or pulse-echo mode, a single ultrasound or transducer emits pulses of ultrasound waves into waveguide  403  and subsequently detects pulses of echo waves after reflection from a selected feature or termination of the waveguide. In pulse-echo mode, echoes of the pulses can be detected by controlling the actions of the emitting ultrasound resonator or transducer to alternate between emitting and detecting modes of operation. Pulse and pulse-echo modes of operation may require operation with more than one pulsed energy wave propagating within the waveguide at equilibrium. 
     Many parameters of interest within physical systems or bodies can be measured by evaluating changes in the transit time of energy pulses. The frequency, as defined by the reciprocal of the average period of a continuous or discontinuous signal, and type of the energy pulse is determined by factors such as distance of measurement, medium in which the signal travels, accuracy required by the measurement, precision required by the measurement, form factor of that will function with the system, power constraints, and cost. The physical parameter or parameters of interest can include, but are not limited to, measurement of load, force, pressure, displacement, density, viscosity, localized temperature. These parameters can be evaluated by measuring changes in the propagation time of energy pulses or waves relative to orientation, alignment, direction, or position as well as movement, rotation, or acceleration along an axis or combination of axes by wireless sensing modules or devices positioned on or within a body, instrument, appliance, vehicle, equipment, or other physical system. 
     In the non-limiting example, pulses of ultrasound energy provide accurate markers for measuring transit time of the pulses within waveguide  403 . In general, an ultrasonic signal is an acoustic signal having a frequency above the human hearing range (e.g. &gt;20 KHz) including frequencies well into the megahertz range. In one embodiment, a change in transit time of an ultrasonic energy pulse corresponds to a difference in the physical dimension of the waveguide from a previous state. For example, a force or pressure applied across the knee joint compresses waveguide  403  to a new length and changes the transit time of the energy pulse When integrated as a sensing module and inserted or coupled to a physical system or body, these changes are directly correlated to the physical changes on the system or body and can be readily measured as a pressure or a force. 
       FIG. 14  is an exemplary assemblage  1400  for illustrating reflectance and unidirectional modes of operation in accordance with an exemplary embodiment. It comprises one or more transducers  1402 ,  1404 , and  1406 , one or more waveguides  1414 , and one or more optional reflecting surfaces  1416 . The assemblage  1400  illustrates propagation of ultrasound waves  1418  within the waveguide  1414  in the reflectance and unidirectional modes of operation. Either ultrasound resonator or transducer  1402  and  1404  in combination with interfacing material or materials  1408  and  1410 , if required, can be selected to emit ultrasound waves  1418  into the waveguide  1414 . 
     In unidirectional mode, either of the ultrasound resonators or transducers for example  1402  can be enabled to emit ultrasound waves  1418  into the waveguide  1414 . The non-emitting ultrasound resonator or transducer  1404  is enabled to detect the ultrasound waves  1418  emitted by the ultrasound resonator or transducer  1402 . 
     In reflectance mode, the ultrasound waves  1418  are detected by the emitting ultrasound resonator or transducer  1402  after reflecting from a surface, interface, or body at the opposite end of the waveguide  1414 . In this mode, either of the ultrasound resonators or transducers  1402  or  1404  can be selected to emit and detect ultrasound waves. Additional reflection features  1416  can be added within the waveguide structure to reflect ultrasound waves. This can support operation in a combination of unidirectional and reflectance modes. In this mode of operation, one of the ultrasound resonators, for example resonator  1402  is controlled to emit ultrasound waves  1418  into the waveguide  1414 . Another ultrasound resonator or transducer  1406  is controlled to detect the ultrasound waves  1418  emitted by the emitting ultrasound resonator  1402  (or transducer) subsequent to their reflection by reflecting feature  1416 . 
       FIG. 15  is an exemplary assemblage  1500  that illustrates propagation of ultrasound waves  1510  within the waveguide  1506  in the bi-directional mode of operation of this assemblage. In this mode, the selection of the roles of the two individual ultrasound resonators ( 1502 ,  1504 ) or transducers affixed to interfacing material  1520  and  1522 , if required, are periodically reversed. In the bi-directional mode the transit time of ultrasound waves propagating in either direction within the waveguide  1506  can be measured. This can enable adjustment for Doppler effects in applications where the sensing module  1508  is operating while in motion  1516 . Furthermore, this mode of operation helps assure accurate measurement of the applied load, force, pressure, or displacement by capturing data for computing adjustments to offset this external motion  1516 . An advantage is provided in situations wherein the body, instrument, appliance, vehicle, equipment, or other physical system  1514 , is itself operating or moving during sensing of load, pressure, or displacement. Similarly, the capability can also correct in situation where the body, instrument, appliance, vehicle, equipment, or other physical system, is causing the portion  1512  of the body, instrument, appliance, vehicle, equipment, or other physical system being measured to be in motion  1516  during sensing of load, force, pressure, or displacement. Other adjustments to the measurement for physical changes to system  1514  are contemplated and can be compensated for in a similar fashion. For example, temperature of system  1514  can be measured and a lookup table or equation having a relationship of temperature versus transit time can be used to normalize measurements. Differential measurement techniques can also be used to cancel many types of common factors as is known in the art. 
     The use of waveguide  1506  enables the construction of low cost sensing modules and devices over a wide range of sizes, including highly compact sensing modules, disposable modules for bio-medical applications, and devices, using standard components and manufacturing processes. The flexibility to construct sensing modules and devices with very high levels of measurement accuracy, repeatability, and resolution that can scale over a wide range of sizes enables sensing modules and devices to the tailored to fit and collect data on the physical parameter or parameters of interest for a wide range of medical and non-medical applications. 
     For example, sensing modules or devices may be placed on or within, or attached or affixed to or within, a wide range of physical systems including, but not limited to instruments, appliances, vehicles, equipments, or other physical systems as well as animal and human bodies, for sensing the parameter or parameters of interest in real time without disturbing the operation of the body, instrument, appliance, vehicle, equipment, or physical system. 
     In addition to non-medical applications, examples of a wide range of potential medical applications may include, but are not limited to, implantable devices, modules within implantable devices, modules or devices within intra-operative implants or trial inserts, modules within inserted or ingested devices, modules within wearable devices, modules within handheld devices, modules within instruments, appliances, equipment, or accessories of all of these, or disposables within implants, trial inserts, inserted or ingested devices, wearable devices, handheld devices, instruments, appliances, equipment, or accessories to these devices, instruments, appliances, or equipment. Many physiological parameters within animal or human bodies may be measured including, but not limited to, loading within individual joints, bone density, movement, various parameters of interstitial fluids including, but not limited to, viscosity, pressure, and localized temperature with applications throughout the vascular, lymph, respiratory, and digestive systems, as well as within or affecting muscles, bones, joints, and soft tissue areas. For example, orthopedic applications may include, but are not limited to, load bearing prosthetic components, or provisional or trial prosthetic components for, but not limited to, surgical procedures for knees, hips, shoulders, elbows, wrists, ankles, and spines; any other orthopedic or musculoskeletal implant, or any combination of these. 
       FIG. 16  is an exemplary cross-sectional view of a sensor element  1600  to illustrate changes in the propagation of ultrasound waves  1614  with changes in the length of a waveguide  1606 . In general, the measurement of a parameter is achieved by relating displacement to the parameter. In one embodiment, the displacement required over the entire measurement range is measured in microns. For example, an external force  1608  compresses waveguide  1606  thereby changing the length of waveguide  1606 . Sensing circuitry (not shown) measures propagation characteristics of ultrasonic signals in the waveguide  1606  to determine the change in the length of the waveguide  1606 . These changes in length change in direct proportion to the parameters of interest thus enabling the conversion of changes in the parameter or parameters of interest into electrical signals. 
     As illustrated, external force  1608  compresses waveguide  1606  and moves the transducers  1602  and  1604  closer to one another by a distance  1610 . This changes the length of waveguide  1606  by distance  1612  of the waveguide propagation path between transducers  1602  and  1604 . Depending on the operating mode, the sensing circuitry measures the change in length of the waveguide  1606  by analyzing characteristics of the propagation of ultrasound waves within the waveguide. 
     One interpretation of  FIG. 16  illustrates waves emitting from transducer  1602  at one end of waveguide  1606  and propagating to transducer  1604  at the other end of the waveguide  1606 . The interpretation includes the effect of movement of waveguide  1606  and thus the velocity of waves propagating within waveguide  1606  (without changing shape or width of individual waves) and therefore the transit time between transducers  1602  and  1604  at each end of the waveguide. The interpretation further includes the opposite effect on waves propagating in the opposite direction and is evaluated to estimate the velocity of the waveguide and remove it by averaging the transit time of waves propagating in both directions. 
     Changes in the parameter or parameters of interest are measured by measuring changes in the transit time of energy pulses or waves within the propagating medium. Closed loop measurement of changes in the parameter or parameters of interest is achieved by modulating the repetition rate of energy pulses or the frequency of energy waves as a function of the propagation characteristics of the elastic energy propagating structure. 
     In a continuous wave mode of operation, a phase detector (not shown) evaluates the frequency and changes in the frequency of resonant ultrasonic waves in the waveguide  1606 . As will be described below, positive feedback closed-loop circuit operation in continuous wave (CW) mode adjusts the frequency of ultrasonic waves  1614  in the waveguide  1606  to maintain a same number or integer number of periods of ultrasonic waves in the waveguide  1606 . The CW operation persists as long as the rate of change of the length of the waveguide is not so rapid that changes of more than a quarter wavelength occur before the frequency of the Propagation Tuned Oscillator (PTO) can respond. This restriction exemplifies one advantageous difference between the performance of a PTO and a Phase Locked Loop (PLL). Assuming the transducers are producing ultrasonic waves, for example, at 2.4 MHz, the wavelength in air, assuming a velocity of 343 microns per microsecond, is about 143μ, although the wavelength within a waveguide may be longer than in unrestricted air. 
     In a pulse mode of operation, the phase detector measures a time of flight (TOF) between when an ultrasonic pulse is transmitted by transducer  1602  and received at transducer  1604 . The time of flight determines the length of the waveguide propagating path, and accordingly reveals the change in length of the waveguide  1606 . In another arrangement, differential time of flight measurements (or phase differences) can be used to determine the change in length of the waveguide  1606 . A pulse consists of a pulse of one or more waves. The waves may have equal amplitude and frequency (square wave pulse) or they may have different amplitudes, for example, decaying amplitude (trapezoidal pulse) or some other complex waveform. The PTO is holding the phase of the leading edge of the pulses propagating through the waveguide constant. In pulse mode operation the PTO detects the leading edge of the first wave of each pulse with an edge-detect receiver rather than a zero-crossing receiver circuitry as used in CW mode. 
     The present invention is applicable to a wide range of medical and nonmedical applications including, but not limited to, frequency compensation; control of, or alarms for, physical systems; or monitoring or measuring physical parameters of interest. The level of accuracy and repeatability attainable in a highly compact sensing module or device may be applicable to many medical applications monitoring or measuring physiological parameters throughout the human body including, not limited to, bone density, movement, viscosity, and pressure of various fluids, localized temperature, etc. with applications in the vascular, lymph, respiratory, digestive system, muscles, bones, and joints, other soft tissue areas, and interstitial fluids. 
     While the present invention has been described with reference to particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the invention.