Abstract:
Manufacturing processes monitor forces or pressures within a machine. Metal within machines affect wireless communications within the machines for reporting monitored data. An embodiment of the present invention is a sensor that provides wireless communications unaffected by metals and with less electrical noise than slip rings. An embodiment can monitor manufacturing processes, such as by employing a piezoelectric transducer to measure forces or pressures in a machine and generate an electrical signal representing, for example, forces measured by the piezoelectric transducer. A threshold modulator circuit converts the electrical signal into a series of electrical pulses, which can be transmitted as a corresponding series of magnetic field pulses to a wireless receiver. The receiver reconstructs the original electrical signal, thereby enabling a receiver system to determine physical activities in the machine. The embodiment may be self-powered through use of power generated by the piezoelectric transducer.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/508,390, filed on Jul. 15, 2011. The entire teachings of the above application are incorporated herein by reference. 
     
    
     GOVERNMENT SUPPORT 
       [0002]    This invention was made with government support under grant # CMMI-0936075 awarded by the National Science Foundation. The government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    For improved performance of manufacturing operations, such as forming, cutting, and grinding processes, it is useful to understand and compensate for dynamic interactions between the processes and machines. Validity and utility of structural analysis based on Finite Element models are dependent on boundary conditions assumed when running simulations. Maintaining process parameters becomes difficult without closed-loop control, especially under high loads and working stresses. 
         [0004]    Manufacturing processes often employ monitoring of forces and pressures to ensure that resulting articles of manufacture meet certain specifications of quality. Many of these manufacturing processes do not have adequate physical space where sensors can be placed without interfering with the manufacturing processes. Alteration of the process to accommodate sensor wiring can be expensive and time consuming. Likewise, wires for either power or communications, running to and from the sensors, can disrupt the manufacturing processes by occupying limited free space. In addition, wires have a potential to get tangled with moving or non-moving parts, further disrupting the manufacturing processes. 
         [0005]    Sensors also generally cannot communicate, in a robust manner, wirelessly through radio frequency (RF) within manufacturing processes because most manufacturing machines employ steel or other RF-blocking metals, rendering RF communications unreliable or ineffective. 
         [0006]    Some manufacturing processes use mechanical slip rings to communicate information wirelessly across rotating or translating mechanical interfaces and transfer power. Slip rings, in their current form, use metal contacts that rotate or translate relative to each other that tend to add electrical noise to signals. In addition, slip rings wear over time, adding more electrical noise or loss-of-signal errors, thereby requiring replacement and stoppage of the corresponding manufacturing process. Likewise slip rings, by a natural wearing process, produce a fine particulate dust which can be disruptive in clean working environments. 
       SUMMARY OF THE INVENTION 
       [0007]    An embodiment of the present invention includes a method of transmitting information via a wireless medium. This method comprises converting an electrical signal representing information into a series of electrical pulses and generating a series of magnetic field pulses corresponding to the series of electrical pulses to transmit a representation of the information via a wireless medium. This embodiment can further comprise receiving the series of magnetic field pulses via the wireless medium and reconstructing the information based on the series of magnetic field pulses received. 
         [0008]    Another embodiment of the present invention includes a circuit comprising an input port and an output port, a resistor having a first lead and a second lead, a silicon bidirectional switch electrically coupled in series with the input port and first lead of the resistor, and a thyristor having its voltage control input coupled in series with the second lead of the resistor and its current flow path coupled in series with the input port and output port. 
         [0009]    Another embodiment of the present invention includes a communications system comprising a transducer configured to produce an electrical signal representative of an applied force or pressure, a threshold modulator circuit coupled to the transducer and configured to convert the electrical signal to a series of electrical pulses, and a transmitter coupled to the threshold modulator circuit and configured to convert the series of electrical pulses to a series of magnetic field pulses and transmit the series of magnetic field pulses to a wireless medium. 
         [0010]    Another embodiment of the present invention includes a communications system comprising means for converting an electrical signal into a series of electrical pulses, means for generating a corresponding series of magnetic pulses, and means for transmitting the series of magnetic field pulses to a wireless medium. 
         [0011]    Another embodiment of the present invention includes a non-contact slip ring, comprising an interior member and an exterior member. The interior member communicates with the exterior member (or vice versa) without coming into physical contact with each other. The interior member comprises a transducer configured to produce an electrical signal representation of measured data, a threshold modulator circuit coupled to the transducer configured to convert the electrical signal to a series of electrical pulses, and a transmitter coupled to the threshold modulator circuit configured to convert the series of electrical pulses to a corresponding series of magnetic field pulses and transmit the series of magnetic field pulses to a wireless medium. The exterior member comprises a receiver configured to receive the series of magnetic field pulses from the transmitter via the wireless medium and a signal reconstruction circuit configured to determine the force or pressure as a function of the series of magnetic field pulses received. The interior member does not transfer power to the exterior member and the exterior member does not transfer power to the interior member. 
         [0012]    Some embodiments of the present invention employ a threshold modulator circuit to convert the electrical signal. In some embodiments, the threshold modulator circuit includes a silicon bidirectional switch, thyristor, and resistor. In one embodiment, the threshold modulator circuit receives an electrical signal at the input port and converts the electrical signal to a series of electrical pulses. In another embodiment, the resistor can be selectively programmed. In yet another embodiment, the threshold modulator circuit can be programmed to control the strength, and therefore the range, of the magnetic field transmission. The threshold modulator circuit can also be programmed to control the resolution of the transmitted signal. 
         [0013]    Some embodiments of the present invention include a transmitter component coupled to the output port configured to generate a series of magnetic field pulses corresponding to the series of electrical pulses. Some embodiments include a receiver component configured to receive the series of magnetic field pulses via a wireless medium from the transmitter component. Some embodiments include a signal reconstruction circuit configured to determine the force or pressure from the series of magnetic field pulses. Some embodiments include a hardware- or software-based signal reconstruction module configured to approximate the original electrical signal from the series of magnetic pulses it receives. 
         [0014]    In some embodiments, the electrical signal represents a physical force or pressure. In some embodiments, the electrical signal is an analog waveform. In some embodiments, the electrical signal is a digital waveform that represents information. In some embodiments, the series of magnetic field pulses is a digital waveform. 
         [0015]    Some embodiments of the present invention resolves the electrical pulses using n-levels, wherein n is based on a largest expected amplitude divided by a minimum amplitude detection level of the electrical signal. In one embodiment, the threshold modulator circuit allows for automated control to modify the range and resolution of the magnetic pulses by receiving wireless or wired instructions. 
         [0016]    Some embodiments of the present invention employ a piezoelectric (or other ceramic formulation) transducer or a force-to-electrical transducer to produce the electrical signal. In some embodiments, the force-to-electrical transducer is coupled to the input port and provides an electrical signal to the threshold modulator circuit as a function of force or pressure. In some embodiments, the force-to-electrical transducer, threshold modulator circuit, and transmitter form a current loop around which current flow is bidirectional. In some embodiments, the force-to-electrical transducer is a piezoelectric transducer or a piezoelectric stack. In some embodiments, the piezoelectric transducer may include several layers that are arranged in an electrically parallel and mechanically serial configuration. 
         [0017]    Some embodiments of the present invention operate in a self-powered manner. For example, embodiments may be powered by energy generated by the transducer from the application of a time varying force or pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0019]      FIGS. 1A and 1B  are mechanical schematic diagrams illustrating a design and component listing of an embodiment of the invention. 
           [0020]      FIG. 2  is an electrical schematic diagram of the threshold modulator circuit according to one embodiment of the invention. 
           [0021]      FIG. 3  are example graphs illustrating experimental results of a force reconstruction. 
           [0022]      FIG. 4  is a diagram illustrating one embodiment of a functional unit without packaging. 
           [0023]      FIG. 5  is a diagram illustrating two embodiments within their respective packaging. 
           [0024]      FIGS. 6A and 6B  are diagrams illustrating an example measurement station for characterizing sensor performance. 
           [0025]      FIG. 6C  is a chart diagram that displays a reconstructed force and a reference force at an example measurement station. 
           [0026]      FIG. 7  is a plot illustrating example received pulses by a receiver according to an embodiment of the invention. 
           [0027]      FIG. 8  is a diagram illustrating an automated test rig for characterizing sensor performance. 
           [0028]      FIG. 9  is a plot illustrating a relationship between wireless signal range and resolution of a wireless signal transmitted between a transmitter and a receiver of an embodiment of the invention. 
           [0029]      FIG. 10  is a graph illustrating a voltage-current characteristic of a silicon bidirectional switch (SBS) within a threshold modulator circuit. 
           [0030]      FIG. 11  are graphs illustrating example test results of a silicon bidirectional switch (SBS) within an embodiment of the present invention. 
           [0031]      FIGS. 12 and 13  are block diagrams of non-contact slip rings according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    A description of example embodiments of the present invention follows. 
         [0033]    An embodiment of the present invention is a self-energized wireless force sensing apparatus and corresponding method that employs magnetic field communications. In embodiments in which no batteries or cable connections are present or available for sensor operations, in one application, the force sensing apparatus can be directly embedded within a machine tooling structure for remote measurement in real-time, where the force sensing apparatus includes a transducer and constituent components for converting transducer output signals into conditioned electrical signals. 
         [0034]    A comprehensive model describing the sensor transfer function under electro-mechanical loading and the behavior of the embedded electronics for wireless data transmission may be derived for particular specifications of transducers and electrical components, or, in an embodiment providing electronics only, a model of just the electronics may be derived. Simulations performed indicate that an embodiment of the wireless sensor can accurately measure dynamic forcing events that last longer than 2 msec. 
         [0035]    An embodiment of the sensor can measure forces up to 5 kN, with a resolution of 40 N. An embodiment of the sensor can be packaged as a cylindrical unit of 5 cm in height, with a possible maximum diameter being 2 cm in some configurations and more or less in others. Other ranges of time, force, and physical length are also to be understood within the scope of the present invention. 
         [0036]      FIG. 1A  shows a mechanical schematic diagram of a sensor  100  that illustrates a design of an embodiment of the invention. Optional compact size and ability to measure large forces allow embodiments of the sensor  100  to be integrated into a wide variety of machine structures for direct measurement of process-machine interactions and improved process controls. The sensor  100  includes an impact cap  102 , threaded moving end  104 , piezostack  106 , main casing  108 , countersunk piezostack bolt  110 , threshold modulator circuit  112 , circuit holding screws  114 , transmitter holding spring  116 , bottom cap  118 , and transmitter  120 . 
         [0037]      FIG. 1B  is a diagram  150  illustrating a three-dimensional cutaway view of an embodiment of the invention. The sensor of  FIG. 1B  includes the features of the sensor  100  in  FIG. 1A . 
         [0038]      FIG. 2  is a block diagram  200  illustrating a threshold modulator component  204  of an embodiment of the sensor. The main components in the threshold modulator component  204  are a silicon bidirectional switch (SBS)  208  and the thyristor (THY)  212 . Examples of commercial available model of silicon bidirectional switches can be found using the following model numbers NTE 6403, MBS 4991-4993, or BS080; and an example of a commercial available thyristor includes an LX8003. These model numbers may be employed in several embodiments of the invention, and other silicon bidirectional switches and thyristors can alternatively be integrated into the threshold modulator component  204 . 
         [0039]    The SBS  208  has a high internal impedance until the voltage across it reaches a certain threshold voltage, at which instant the SBS  208  switches into a low impedance state. This indicates that, theoretically, the SBS  208  acts alone as the threshold modulator; however, the applications of this embodiment can be limited. 
         [0040]    The switch-off function of the SBS  208  is current controlled. The SBS  208  switches into an “off” mode when the voltage drops under the threshold voltage. This happens when the throughput current is lower than a certain holding current I h . Tests have identified that the current generated by an electronic transducer, which in one embodiment can be a piezoelectric transducer or a piezostack (e.g., input from piezostack  202 ), often exceeds I h . This indicates the SBS  208  gets “stuck” in an “on” mode and does not generate pulses. For this reason, the thyristor  212  is employed within the threshold modulator component  204  where the thyristor  202  has a higher current handling capacity than the SBS  208 . The SBS  208  then only switches the thyristor  212  ON and OFF, and the load current is handled by the thyristor  212 . The resistor R  210  limits the current through the SBS  208 . 
         [0041]    The main components in the circuit are the SBS  208  and a thyristor  212 . The SBS  208  has a high internal impedance. When the voltage across it reaches a predetermined threshold voltage value, it instantly switches into a low impedance state. 
         [0042]      FIG. 3  are plots illustrating an example of the sensor operation. A first plot  300  shows the force applied to an embodiment of the invention by a hammer hit. In this example, the piezostack experiences a force of 800 N during a 1.5 millisecond time period. 
         [0043]    A second plot  310  shows the voltage generated by an electronic transducer, such as a piezostack. In this embodiment, while the applied force is increasing, the piezostack generates charge in a positive direction, thus increasing its internal voltage. In this embodiment, during a period of decreasing force, the electronic transducer or piezostack generates charge in the negative direction. 
         [0044]    In an example embodiment, once the internal voltage reaches a threshold voltage (e.g., 8.5V), the threshold modulator  204  transitions into the open condition (i.e., high impedance state) and (almost) instantaneously discharges the charge collected in the electronic transducer or piezostack into the transmitting inductor. Every time this discharge takes place, a magnetic field pulse is generated. This magnetic field pulse is picked up by the receiver as shown in third plot  320 . The magnetic field pulse is directional and its direction, determined by whether the pulse is positive or negative, depends on the direction of the original charge on the electronic transducer or piezostack. In one embodiment, increasing force leads to negative pulses, and decreasing force leads to positive pulses. In an alternative embodiment, increasing force leads to positive pulses, and decreasing force leads to negative pulses. In another embodiment the pulses generated corresponding to increasing and decreasing force may be distinguished based on respective component frequencies of the pulses. 
         [0045]    In one embodiment, the received pulses can be used to reconstruct the original, applied force. A fourth plot  330  shows the reconstructed force using two different reconstruction methods. 
         [0046]      FIG. 4  is a diagram  400  illustrating an example embodiment for testing or debugging a sensor. The piezostack  106  is coupled with an embodiment of the threshold modulator component  204 . This embodiment of the threshold modulator component  204  is implemented within a breadboard, however, could alternatively be implemented on a printed circuit board, integrated circuit, or other embodiment of a circuit. An example scale  402  illustrates that the testing embodiment of the threshold modulator component  204  is on the scale of ten centimeters, and the piezostack  106  is on the scale of five centimeters. However, other sizes of the piezostack  106  and threshold modulator component  204  can be implemented. 
         [0047]      FIG. 5  is a diagram  500  illustrating two example embodiments in which the components of a sensor are integrated into a single package. The package can be designed using computer aided design (CAD) methods, among other methods. The package can be prototyped using a 3D printer, among other methods. Two example materials of the packaging are ABS plastic (sensor  1   502 ) and aluminum (sensor  2   504 ). Initial testing of the prototype led to a third prototype machined in aluminum (not shown). Example scale  506  illustrates that sensor  1   502  and sensor  2   504  can be on the scale of under 10 centimeters. However, other embodiments of a sensor can be more or less than 10 centimeters. 
         [0048]      FIG. 6A  is a block diagram  600  of a testing setup  608 . The testing setup  608  includes a function generator  604  that creates a test drive signal stimulus, which is then amplified through a power amplifier  606  to drive an actuating piezostack  632 . 
         [0049]      FIG. 6B  is an overhead view of the test setup  618  of  FIG. 6A . In the test setup  618 , a piezostack actuator (e.g., an actuating piezostack  632 ) applies a force to a sensing piezostack  630  that generates power. The applied force can also be measured by an independent reference sensor  628 . The pulses generated by the threshold modulator component  204  are transmitted by a solenoid transmitter  626  in a steel block  624  and received by a solenoid receiver  622  (e.g., a solenoid coil). The pulses received by the solenoid receiver  622  are captured by a high speed digitizer, such as Oscilloscope  610 . A program may be employed in the test setup  618  to reconstruct the force applied to the sensing piezostack  630  and the reference sensor  628  from the representations of the applied force and reference force on a computer monitor or in another form and display pulses. 
         [0050]      FIG. 6C  is a chart diagram  640  that displays reconstructed force  644  and a reference force  642  from the test setup  618  of  FIGS. 6A and 6B . Notably, although transmission of a representation of the applied force from the solenoid transmitter  626  to the solenoid receiver  622  is performed using pulses, the solenoid receiver  622  is performed using pulses, the reconstructed force  644  plot is smooth and substantially matches the reference force  642 , which did not get converted into pulses. 
         [0051]      FIG. 7  is a chart diagram  700  illustrating an example embodiment of captured digital pulses such as the ones of the test setup  618  in  FIGS. 6A-C . For example, a positive digital pulse can represent an increasing force, and a negative digital pulse can represent a decreasing force. In another example, positive digital pulse can represent an decreasing force, and a negative digital pulse can represent an increasing force. The digital pulses generated corresponding to increasing and decreasing force may be distinguished based on respective component frequencies of the digital pulses. 
         [0052]    An embodiment of the invention provides online force reconstruction. The reference force  642  and reconstructed force can be shown on the display  602  or the oscilloscope  610  of  FIG. 6A . In an embodiment, the pulses in chart diagram  700  represent the actual force increasing or decreasing by a set amount. In another embodiment, each pulse is associated with a finite and equal development of charge in the piezostack. In another embodiment, the input force can be reconstructed using a charge-to-force transfer function. In yet another embodiment, the input force can be reconstructed using polynomial approximations. 
         [0053]      FIG. 8  is a block diagram illustrating another test setup  800 , in this case a hammer drop test setup to determine how well the sensor  802  works in detecting and reporting shock impact. Force is applied by a hammer hit to the top of the rig  806 . This force is transmitted, via the reference sensor  628 , to the sensor  802  which generates magnetic field pulses. The magnetic field pulses are received by a solenoid receiver  622  external to the sensor prototype  802 . An embodiment of the invention can generate fast loading forces that can last 2-3 milliseconds and have a magnitude of 800 N. Other time periods and force magnitudes can be achieved in alternative embodiments. 
         [0054]    A sheet metal stamping process is an example application to which some embodiments of the invention may be applied. The stamping process is characterized by very fast pressure ramping rates, sometimes approximately 300 MPa/s, due to the inherent contact stiffness of metal-to-metal contact. In addition, long lasting contacts of up to 10 s may occur during the stamping process, for example in pan drawing stamping. 
         [0055]    In an embodiment, a threshold modulator circuit with high current handling capability and low leakage current meets certain design requirements for capturing process dynamics in high impact applications. In some embodiments, the threshold modulator is designed to have the following properties: 
         [0056]    Turn ON Criteria: 
         [0057]    The threshold modulator may transition from an OFF to an ON mode when the output voltage of the piezostack, or any other input to the threshold modulator, exceeds the set threshold voltage in either the positive or negative direction. As an example, if the threshold voltage is set for 10V, the threshold modulator turns ON if the voltage increases above +10V or falls below −10V. 
         [0058]    Turn OFF Criteria: 
         [0059]    Once in the ON mode, the threshold modulator turns OFF when the voltage from the piezostack is 0V. Such a switching action is represented as a functional substitute by a class of silicon bidirectional switches that changes mode from the ON mode to OFF mode, when the electrical current flowing through the switch falls under a certain threshold value, which is determined by the type of manufacturing process. This threshold current is referred to as the holding current, I h . If I h  is sufficiently low, the threshold modulator turns OFF when the voltage is close to 0 V. There may be certain disadvantages, however, if the holding current I h  is low. As an example, if the force applied to the sensor is increasing at a fast rate, the current from the piezostack “swamps” the threshold modulator circuit such that the current through the threshold modulator circuit does not decrease below the holding current, which, in one embodiment, causes the threshold modulator circuit to remain in the ON mode. 
         [0060]    Other Threshold Modulator Control Issues: 
         [0061]    An embodiment of the threshold modulator circuit has a near-infinite OFF impedance state, zero switching time, zero ON impedance state, and a negative resistance switching region. 
         [0062]    An embodiment of the threshold modulator design includes a threshold modulator circuit with properties to turn it ON and OFF at the correct times while also using powerless components. In some manufacturing systems, no constant power supply is available to the circuit. The threshold modulator design resolves a trade-off between the signal transmission range and sensing resolution by controlling the threshold voltage at which it triggers. 
         [0063]      FIG. 9  is a set of graphs that illustrate this trade-off between signal transmission range and sensing resolution. For higher threshold voltages  906  of the threshold modulator circuit, the capacitor, which is a piezoelectric stack in some embodiments, charges to a higher voltage before triggering the modulator circuit into the ON mode. Therefore, when there is a higher threshold voltage  906 , the capacitor stores a higher amount of energy when the threshold modulator circuit turns ON. This higher amount of stored energy in the capacitor causes a higher current to flow in the transmitting coil of the sensor, generating a stronger magnetic field pulse and consequently, a longer signal transmission range. The increase in transmission range is at the expense of a lower measurement resolution of the sensor because increasing the threshold voltage increases the charging time of the capacitor. As a result, with a higher threshold voltage, each pulse that the threshold modulator emits represents a larger change in the force signal acting on the piezostack. 
         [0064]    A lower threshold voltage  902  corresponds to a smallest detectable force change that can be resolved by the sensor. Decreasing the threshold voltage lowers the transmission range in exchange for higher sensing resolution. Such adaptability in the threshold modulator circuit design is useful for a broad range of energy-efficient wireless applications. An embodiment of the invention allows for automated control of the threshold modulator circuit. This automated control can include modifying the threshold voltage, effectively changing the resolution and range of the transmission. Such an automated control may be done wirelessly by sending instructions to a receiver connected to the threshold modulator circuit. 
         [0065]      FIG. 10  is a graph  1000  illustrating the voltage-current (VI) characteristic of the silicon bidirectional switch (SBS)  208  ( FIG. 2 ). The SBS is symmetric in its positive and negative voltage regions. 
         [0066]    In one embodiment, as the voltage applied to the SBS increases from 0 to V S1 , the device has a high impedance, illustrated by the linear portion of the curve in the first quadrant  1002  of the graph  1000 , with high voltage and low current values. As a result, only a small amount of current I B1  flows through the silicon bidirectional switch, which is referred to as the leakage or the blocking current. This leakage current or blocking current, in one embodiment, can be on the order of 0.08 to 1 μA DC. Having a low leakage current in the circuit design is beneficial in certain embodiments, as a low leakage current represents a slow waste of energy stored in the piezostack when signals are not being transmitted. 
         [0067]    As the applied voltage approaches the threshold voltage V S1 , the impedance of the device starts decreasing, i.e., more current starts passing through, even though the applied voltage remains largely constant. After the threshold voltage V S1 , the silicon bidirectional switch device undergoes a negative resistance region, where the current increases drastically to I H1 , and beyond, even though the voltage is decreasing. The SBS behaves in a similar manner when a negative voltage is applied, as shown in third quadrant  1006 . 
         [0068]    The SBS alone can act as the threshold modulator; however, the applications of this design may be limited. In some embodiments, the switch OFF criteria for the SBS is current controlled. In such embodiments, the SBS switches OFF only if the throughput current is under a certain holding current I h . This holding current is on the order of 0.7˜1.5 mA DC, in certain embodiments employing a commercially available SBS. An SBS can get “stuck” in the ON condition and not generate pulses. The SBS does not have any voltage-current characteristics that are displayed in the second quadrant  1004  or the fourth quadrant  1008 . 
         [0069]      FIG. 11  is a set of chart diagrams illustrating behavior of the sensor (e.g., sensor  100 ,  FIG. 1 ) in response to a force. A first plot  1110  shows an example of a force applied to the piezostack, while second plot  1120  shows an example of the corresponding voltage observed across the piezostack. The plot  1120  is not oscillating as ideally expected and illustrated in the third plot  1130 , which represents the ideal voltage behavior. A current generated by the piezostack can exceeds I h    1140 , which causes the SBS to get “stuck” in the ON mode, and, therefore, it does not generate pulses, as in the ideal case of the third plot  1130 . 
         [0070]    In some embodiments, the overcurrent condition can be avoided by using suitable electronic components. For example, the circuit of the sensor may include a thyristor into the circuit can handle the higher current. The silicon bidirectional switch then functions only to switch the thyristor ON and OFF, and the load current is handled by the thyristor. The resistor functions to limit the current through the silicon bidirectional switch; thus, the resistor&#39;s resistance value is chosen to keep the current through the silicon bidirectional switch under I h  at all times. The sensor may combine the SBS, for switching current ON and OFF, with the thyristor, for handling high current, such that the desired performance is as illustrated in the ideal behavior plot  1150 . The thyristor may also have an upper current handling limit that is specific to the thyristor device chosen. Should the current flow exceed the current handling limit of the thyristor, the overcurrent (e.g., excess current) problem described above may re-appear. 
         [0071]    In some embodiments, the range of forces that can be measured by the sensor is a function of the electrical transducer or piezostack and the silicon bidirectional switch/thyristor circuit parameters. 
         [0072]    In some embodiments, the upper limit of the sensor performance is determined by the upper current handling capability of the threshold modulator circuit, which depends on the specifications of the selected thyristor, and the maximum yield strength of the electrical transducer or piezostack (e.g., the amount of force that can be applied to the electrical transducer without creating irreversible structural damage). 
         [0073]    The lower limit of the sensor is determined by leakage current in the SBS/thyristor combination in the OFF mode, and the electrical transducer or piezostack specifications. In one example, for small forces a particular choice of piezostack may not generate enough voltage to trigger the threshold modulator. A piezostack with thicker piezoelectric discs may be selected which generate a higher voltage for the same force applied to trigger the threshold modulator for these small forces. 
         [0074]    The susceptibility of the transmitted signal to environmental noise and interference can be addressed on a per-application basis. The transmitter-receiver pair can be optimized for best signal reception depending on the application. 
         [0075]    In one embodiment, a simplistic and practical approach to reconstructing the original force signal from a received pulse (of the type shown in  FIG. 3 ) is to assume that each pulse corresponds to the actual force increasing or decreasing by a set amount. In another embodiment, a mathematically more rigorous method is to associate the arrival of each pulse with a finite and equal development of charge in the piezostack. The force can be accurately reconstructed by applying a charge-to-force transfer function to the input signal. Other embodiments may employ polynomial approximation or other mathematical reconstruction techniques. 
         [0076]    Slip rings have been used broadly for coupling power and/or signal transmission from one point to another. A commonly known problem associated with the use of slip rings is that they generate undesirable electrical noise and can be unreliable due to wear and tear. 
         [0077]    One embodiment of a non-contact slip ring according to an embodiment of the invention can be used to replace a conventional slip ring and provide an information coupling communications link that has less electrical noise and no mechanical parts that wear over time. In this embodiment, information can be transmitted across a rotating (or linearly moving) boundary. 
         [0078]      FIG. 12  is a block diagram illustrating an embodiment of the invention applied as a non-contact slip ring  1200 . The transmitter  1210  is coupled with a rotating member  1215 . The receiver  1205  is coupled with a stationary member  1220 . As the rotating member  1215  rotates, the transmitter  1210  can transmit wirelessly to the receiver  1205 . 
         [0079]    Similarly,  FIG. 13  is a block diagram illustrating another embodiment of the receiver applied as a non-contact slip ring  1300 . In this embodiment, the transmitter  1305  is coupled with the stationary member  1320 . The receiver  1310  is coupled with the rotating member  1315 . As the rotating member  1315  rotates, the transmitter can transmit wirelessly to the receiver  1310 . Slip ring setups  1200  and  1300  are examples of ways a non-contact slip ring embodiment of the invention could establish one or two way communication with a rotating member. 
         [0080]    Embodiments of the device can be employed to monitor high-energy manufacturing processes. The device enables remote inquiry of machines and processes through structurally integrating the device into places that are hard to access conventionally with wired devices due to restrictions in cable connections and high shock loads. 
         [0081]    Embodiments of the device can be integrated into shock absorbers to provide force feedback for better vibration and shock isolation and control in transportation vehicles and machine systems. 
         [0082]    Embodiments of the device can be used to monitor buildings, bridges, or tunnels for improved safety under high loading conditions, in one example, caused by natural or man-induced conditions. 
         [0083]    Embodiments of the device can assist in education because the wireless sensor is an excellent example of a mechatronic system. The device combines mechanical and electrical designs with piezoelectric element modeling, wireless communications, wave propagation, and signal and control theories. The device can be developed into a module that serves as an interactive teaching aid for undergraduate and graduate teaching. 
         [0084]    It should be understood that certain aspects of the invention, such as a receiver with reconstruction processes, can be implemented in the form of hardware, firmware, or software. If implemented in software, the software can be stored on any form of non-transient computer-readable media. The software may be written in any language capable of being loaded and executed by a processor and able to perform the operations disclosed herein or equivalents thereof. Likewise, the processor may be a general purpose or application-specific processor capable of performing the operations disclosed herein or equivalents thereof. 
         [0085]    As discussed, the term “information” refers to measurements of a machine. However, the term “information” can also refer to non-machine measurements, such as alpha-numeric text, data, version number, product number, product data, or any other information that can be stored in an internal memory, and such information can refer to the sensor or machine to which the sensor is applied. The information can be provided by a manufacturer or customizable by a user. The information can be stored in a memory (e.g., a RAM or ROM) associated with the sensor. Contents of the memory can be output or otherwise directed to the threshold modulator circuit by the sensor at selected times or by request from the user. In an embodiment where the sensor transmits non-machine measurements, the sensor may employ a switch (e.g., a transistor such as a MOSFET) that switches the input of the threshold modulator circuit from machine measurements to non-machine measurements and vice-versa. The switch can be toggled by the requests from the user or at the selectable times. It should be understood that the threshold modulator circuit may convert information from binary encoded representations to electrical pulse representations as described above with respect to voltages produced by the piezoelectric transducer. 
         [0086]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.