Abstract:
A sensing device includes a Wheatstone bridge and a source of a stimulation configured to apply the stimulation across two electrodes of the Wheatstone bridge. The device also includes a timing sensitive circuit configured to detect timing of a signal appearing across one of the other electrodes of the bridge as a result of the stimulation being applied. The timing provides a way to read the sensor. The device can be powered remotely and data so read can be transmitted using the remote power. The timing sensitive circuit includes a comparator. The comparator provides a high logic signal for a time related to the reactance of one leg of the Wheatstone bridge, and that provides a reading of a differential sensor having elements in each leg of the bridge.

Description:
This application claims the benefit of U.S. Provisional application No. 60/177,364, filed Jan. 24, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to sensors. More particularly, it relates to low power differential sensors. Even more particularly, it relates to a device for low power sensing and transmitting data. 
     BACKGROUND OF THE INVENTION 
     Smart sensors are being developed for use in roads, bridges, dams, buildings, towers, and vehicles. The sensors may provide many types of information, including displacement, strain, speed, acceleration, temperature, pressure, and force. For remote sensing one challenge has been to provide sensors that consume very low power for reading the sensor and transmitting the data. 
     Available sensors have required continuous energizing either for operation or for data transmission, and have required substantial power supplies. For example, a paper, “Multichannel Strain Gauge Telemetry for Orthopaedic Implants,” by G. Bergmann, et al., J. Biomechanics Vol. 21 no. 2 pp 169-176, 1988, describes remote powering of a Wheatstone bridge with active strain gauges that require continuous power. A paper, “Remotely powered, multichannel, microprocessor based telemetry systems for smart implantable devices and smart structures, by Chrisopher Townsend, et al, describes an implantable sensor telemetry system that uses low power microprocessors integrated circuits, Wheatstone bridge signal conditioning, and a remote powering system. The Wheatstone bridge has advantage in providing temperature compensation. However, the bridge circuit also requires a continuous voltage and flow of current, so substantial energy is eventually used. Conventional Wheatstone bridge signal conditioners, such as Townsend&#39;s, require instrumentation amplifiers and analog to digital converters which increase the power demand, size, and complexity of these systems. 
     International patent WO 87/00951 shows an inductive sensor used as the feedback element in an astable multivibrator. This circuit requires a non-differential sensor, which results in poor temperature stability. In addition, the astable multivibrator requires continuous power to operate. 
     A book, “Capacitive sensors design and Applications,” by L. K. Baxter, IEEE Press, 1997, shows a microcontroller providing a train of pulses or a microcontroller providing a single interrogation pulse to excite a capacitive limit switch. However, the circuit described by Baxter does not provide a way to measure more than the two positions of the capacitor and does not compensate for changes in temperature. 
     A paper, “Microminiature, high resoluton, linear displacement sensor for peak strain detection in smart structures,” by Steven W. Arms, et al., proceedings of the SPIE 5 th  Annual International Conference on Smart Structures and Materials, San Diego, Calif., March 1-5, 1998, describes a differential method of capturing the peak displacement of a member attached to a structure without requiring any power. The paper did not describe micropower methods for remote interrogation. 
     Thus, a better system for acquiring and transmitting data is needed that uses less energy and that provides temperature compensation, and this solution is provided by the following invention. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a circuit for collecting sense data that avoids a continuous flow of current and use of power; 
     It is a further object of the present invention to lower power requirements for a sensor by providing a circuit in which a single signal, such as a single pulse, is sufficient for performing a measurement; 
     It is a further object of the present invention to combine a low power circuit for reading a sensor with a remotely powered interrogation system; 
     It is a further object of the present invention to provide a differential sensor in a Wheatstone bridge configuration with a pulse signal to provide a low power data sensing circuit; 
     It is a feature of the present invention that the Wheatstone bridge provides for a temperature compensated reading of the differential sensor; 
     It is a further feature of the present invention that the remotely powered interrogation system provides power for running the sensor; and 
     It is an advantage of the present invention that the circuit for reading a sensor uses very low power. 
     These and other objects, features, and advantages of the invention are accomplished by a electronic circuit comprising a first electrode, a second electrode, a third electrode and a fourth electrode. The circuit also includes a differential sensor comprising a first variable element connected to a second variable element at the first electrode. The first variable element is also connected to the second electrode. The second variable element is also connected to the third electrode. A fixed device is connected between the second electrode and the fourth electrode. A source of a stimulation is connected to apply a stimulation across the first and the fourth electrodes. A timing sensitive circuit is configured to measure timing of a signal appearing at the second electrode that arises from the stimulation applied across the first and fourth electrodes. 
     Another aspect of the invention is accomplished by a method of reading a sensor comprising several steps. The first step is providing a differential sensor having a first variable element and a second variable element. Next, providing a comparator. Then providing a signal to the first variable element wherein the sensor produces an output depending on magnitude of the first variable element. Finally, using the comparator for providing a signal that is a measure of that magnitude. 
     Another aspect of the invention is accomplished by a method of using an electronic circuit, comprising the step of providing a circuit comprising a sensor, a circuit for reading the sensor, and a circuit for transmitting data. The next step is wirelessly providing power to the circuit from a remote source of power. Then, sensing a change in an environmental condition with the sensor. Then, reading the sensor with the circuit for reading the sensor, wherein only a single stimulation signal to the sensor is needed to read the sensor. Then, providing the reading to a transmitting circuit and transmitting the data with the transmitting circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description of the invention, as illustrated in the accompanying drawings, in which: 
     FIG. 1 is a block diagram of a sensing unit comprising a Wheatstone bridge, a comparator, interrogator, and remotely powered power supply of the present invention; 
     FIG. 2 a  is a timing trace of a pulse input to the Wheatstone bridge sensor; 
     FIGS. 2 b  and  2   c  are timing traces of the pulse of FIG. 2 a  at electrodes along a first and a second leg of the Wheatstone bridge when the bridge is balanced; 
     FIG. 2 d  is a timing trace of the output of a comparator connected to the electrodes along a first and a second leg of a Wheatstone bridge when the bridge is balanced; 
     FIG. 3 a  is a timing trace of a pulse input to the Wheatstone bridge sensor identical to FIG. 2 a;    
     FIG. 3 b  is a timing trace of the pulse of FIG. 3 a  at an electrode along a first leg of an unbalanced Wheatstone bridge having a larger capacitor than the other leg; 
     FIG. 3 c  is a timing trace of the pulse of FIG. 3 a  at an electrode along a second leg of the unbalanced Wheatstone bridge having a smaller capacitor; 
     FIG. 3 d  is a timing trace of the output of a comparator connected to the electrodes along a first and a second leg of a Wheatstone bridge when the bridge is unbalanced; 
     FIG. 4 is a block diagram of a sensing unit comprising a Wheatstone bridge, a pair of comparators, an interrogator, and remotely powered power supply of a second embodiment of the present invention; 
     FIG. 5 is a block diagram of a sensing unit comprising a Wheatstone bridge, a pair of comparators comprising a set/reset latch, an interrogator, and remotely powered power supply of a third embodiment of the present invention; 
     FIG. 6 is a block diagram of another embodiment of a sensing unit of the present invention including inductive sensors and showing additional variations in the invention; and 
     FIG. 7 is a block diagram of another embodiment of a sensing unit of the present invention including resistive sensors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors recognized that substantially less energy could be used by a sensor configured as part of a Wheatstone bridge if a single pulse signal was sufficient to provide a reading of the sensor. In that case, power in the Wheatstone bridge is only used during the time of the pulse. They then developed a circuit that could read the sensor with such a single pulse. The circuit takes advantage of the timing difference as the signal travels in parallel along each leg of the bridge. This timing difference provides a measure of the imbalance in the bridge and gives a reading of data collected by the sensor. The timing difference arises because of the difference in capacitance, inductance, or resistance on each side of the bridge which provides different RC or RL time constants on each side. The timing difference is captured by a comparator, and used by a micro controller to measure the magnitude of bridge unbalance and magnitude of change in conditions sensed by the sensor. If one comparator is used, change in one direction can be measured, and this approach is practical for many applications. Change in either direction can be measured if two comparators are used in the circuit, and the direction of the change in condition can also be determined with this configuration. The inventors also recognized that power required by this system was so low that all the power needed for the signal, to run the sensor and electronics, and for data transmission could now be provided by a wireless remote power supply. This has the advantages of allowing temporary, remote powering and allows for reading difficult to access and embedded sensors. 
     Micropower voltage comparator  20  is used to measure the balance of Wheatstone bridge  24  across electrodes  26 ,  28 , as shown in FIG.  1 . Comparator  20  has output  29  that switches from 0 to 1 if signal provided to the + input is higher than signal provided to the − input. By contrast output  29  of comparator  20  stays at 0 or switches to 0 if signal at the + input is equal to or less than signal at the − input. 
     Practical comparators, as provided by manufacturers, vary in their switching voltage. This variation in switching voltage is called offset. The present invention is more workable with practical comparators that have such offset. In that case output  29  of comparator  20  switches to a high logic level  1  when the voltage on the + input rises more than the offset value above the voltage on the − input. The comparator switches back to 0 when the voltage on the + input falls below the offset value above the voltage on the − input. This offset eliminates switching of comparator output  29  caused by noise at its inputs. Additional resistors can also be used to adjust the input bias on the comparator or to add hysteresis to the comparator&#39;s response, as is well known in the art. 
     Wheatstone bridge  24  comprises differential sensor  30  and two identical bridge completion resistors  32   a,    32   b.  Differential sensor  30  includes variable capacitors  36   a,    36   b  with center electrode  38  there between at the top of Wheatstone bridge  24 . Wheatstone bridge also includes ground electrode  39  between completion resistors  32   a,    32   b.    
     Interrogator  40  generates an alternating current in excitation coil  42  to produce magnetic field  44 . Field  44  induces an alternating current in receive circuit  46 , which is rectified and filtered in power supply circuit  48 . Excitation pulse generator  50  uses power from power supply  48  to generate a pulse sometime after Vcc voltage from power supply  48  has stabilized. The excitation pulse is then applied to center electrode  38  at the top of Wheatstone bridge  24 . Excitation pulse generator  50  may be controlled by controller/ID generator  52 . The signal applied to center electrode  38  can also be a step function or any other kind of signal. A pulse having a sharp leading edge is preferred since the energy used by the sensor is least and the sharp leading edge provides a timing reference from which to measure the RC time constant. 
     Wheatstone bridge  24  is balanced when variable capacitors  36   a,    36   b  are equal. An excitation pulse is applied between electrode  38  and ground electrode  39 . In this case, voltage measured between electrode  26  and ground  39  should be identical to voltage measured between electrode  28  and ground at all times since the RC is the same on both sides of the bridge and capacitor charging curves on each side of bridge  24  are therefore identical. Although exponentially varying voltages will appear at electrodes  26 ,  28  and on + and − inputs of comparator  20 , as shown in FIGS. 2 a,    2   b,  the voltages at electrodes  26 ,  28  and at + input and − input to comparator  20  should be identical (within the offset tolerance) at every moment in time. With signal on each side of comparator  20  always identical, output  29  of comparator  20  remains fixed at 0 for a balanced Wheatstone bridge. 
     But Wheatstone bridge  24  is unbalanced when a change in environmental conditions causes a change in sensor  30 , and this causes a change in variable capacitors  36   a,    36   b,  making them unequal. For example capacitive, differential sensor  30 , shown in FIG. 1, could be a well known type of pressure sensor which typically has a conductive diaphragm with conductive plates on each side of the diaphragm. A change in pressure across the diaphragm will move the diaphragm toward one plate and away from the other plate, increasing the capacitance between the diaphragm and the first plate and decreasing the capacitance between the diaphragm and the second plate. 
     Another example of capacitive, differential sensor  30  is a linear displacement sensor in which two lower plates are connected to electrodes  26  and  28  of bridge  24  as shown in FIG. 1. A single common top plate is connected to top electrode  38 , and this single top plate extends partly over both lower plates. Movement of the top plate relative to the other two causes an increase in overlap area for one plate and a decrease in overlap area for the other plate, increasing one capacitance and decreasing the other. 
     When a signal is applied to electrode  38  for the unbalanced case, voltage measured between electrode  26  and ground will differ from voltage measured between electrode  28  and ground as each capacitor charges at a different rate, as shown in FIGS. 3 b,    3   c.  This difference in voltage arises from the different RC delays for the two different capacitors,  36   a,    36   b.  Voltages appearing on + and − inputs of comparator  20  will no longer be identical during time for capacitor charging, and output  29  of comparator  20  can shift from 0 to 1. The side of bridge  24  with the larger capacitor will have the longest RC delay, so voltage across its resistor will be higher than voltage on the side with the smaller capacitor. If the electrode on the side with the larger capacitor is connected to the + side of comparator  20 , output  29  of comparator  20  will shift from 0 to 1. If connected to the minus side, output  29  of comparator  20  will remain at 0. 
     For example, FIG. 3 a  shows square wave pulse V( 38 ) applied between electrode  38  and ground electrode  39 . The rising edge of square wave pulse has a very high frequency, so on the leading edge of square wave pulse V( 38 ), each capacitor has a very low impedance at first, so no voltage appears across capacitor  36   a  or capacitor  36   b  and the voltage applied between electrode  38  and ground appears across each supporting resistor,  32   a,    32   b.  Therefore, the full voltage of square wave pulse V( 38 ) applied between electrode  38  and ground electrode  39  initially appears across resistor  32   a  and resistor  32   b  at electrodes  26  and  28 . Voltage between electrode  26  and ground (V 26 ) is shown in FIG. 3 b  starting initially at applied voltage Vcc and then decaying as capacitor  36   a  charges up with larger time constant RC a . As capacitor  36   a  fully charges voltage at electrode  26  falls from Vcc to zero. Similarly voltage between electrode  28  and ground V( 28 ) is shown in FIG. 3 c  starting initially at applied voltage Vcc and then decaying as capacitor  36   b  charges up with smaller time constant RC b . Since capacitor  36   a  is larger than capacitor  36   b  in this example, time for decay of V( 26 ) is longer than time for decay of V( 28 ). Thus, at any moment in time, voltage at electrode  26  is higher than voltage at electrode  28 . This voltage difference causes a shift in comparator  20  tied between electrodes  26  and  28  if the + input of comparator  20  is tied to electrode  26 —the electrode on the leg of Wheatstone bridge  24  with the larger capacitor—and the − input of comparator  20  is connected to electrode  28 —the electrode on the leg of Wheatstone bridge  24  with the smaller capacitor. 
     Controller/ID generator  52  can measure the time ΔT that output  29  of comparator  20  has a voltage V( 29 ) is equal to 1 (see FIG. 3 d  ), which provides a measure of the unbalance in Wheatstone bridge  24  and a measure of magnitude of the sensor data. 
     Controller/ID generator  52  then appends this sensor information time duration ΔT to an ID code which is transmitted back to interrogator  40  using energy from power supply  48  that is received by receive coil  46  via magnetic field  44  from coil  42  of interrogator  40 . 
     Variable capacitors  36   a,    36   b  can be at the top of Wheatstone bridge  24  as shown in FIG.  1 . Alternatively, as is well known to one skilled in the art, positions in Wheatstone bridge  24  can be reversed with variable capacitors  36   a,    36   b  located at the bottom of the Wheatstone bridge, while fixed resistors  32 ,  34  are located at the top. In this case, center electrode  38  would be between fixed resistors  32 ,  34  and ground connection  39  would be between variable capacitors  36   a,    36   b.    
     The embodiment of FIG. 1 works well when the sensor is providing a reading exclusively in one direction, for example, in measuring certain peak strains that go exclusively in one direction. For measuring unbalance in either direction, two micropower voltage comparators  20   a,    20   b  can be included in the circuit, as shown in FIG.  4 . In this case, depending on the direction of the change in condition and which side of bridge  24  has a larger capacitor, output  29   a  of comparator  20   a  or output  29   b  of comparator  20   b  will maintain its high logic state 1 for a time interval ΔT equal to the time for decay of voltage between electrode  26  or electrode  28  and ground. 
     Controller/ID generator  52  then reads that time duration from the width of output signal  29   a  or  29   b  from whichever comparator  20   a  or  20   b  went high. Recognizing which of comparators  20   a  or  20   b  went high tells controller  52  the direction of bridge unbalance and the direction of change in environmental condition. Thus, two comparators allow determining which leg of bridge  24  has the larger capacitor and how big that capacitor is, giving both the direction and magnitude of the change in environmental condition. 
     In another embodiment, comparators  20   a,    20   b  are wired to provide set/reset latch circuit  80 , as shown in FIG.  5 . This set/reset latch is useful, for example, in a case where change is exclusively in one direction and an extended time is needed for controller/ID generator  52  to perform a reading. The latch provides indication of when the capacitor  36   a  has a larger capacitance value than capacitor  36   b.  Electrode  26  is connected to + input of comparator  20   a  and electrode  28  is connected to + input of comparator  20   b.  Output  29   a  of comparator  20   a  is connected to − input of comparator  20   b  and output  29   b  of comparator  20   b  is connected to − input of comparator  20   a.  Output  29   a  of comparator  20   a  is also connected to controller/ID generator  52 . If capacitor  36   a  is larger than capacitor  36   b  so electrode  26  has a higher voltage (V 26 ) than electrode  28  (V 28 ) when signal is applied to electrode  38  V( 38 ), then output  29   a  of comparator  20   a  goes to high logic level 1, and this signal is applied to − input of comparator  20   b  and to controller/ID generator  52 . This high logic level 1 applied to − input comparator  20   b  ensures a low logic level 0 output for that comparator which is fed to the − input of comparator  20   a.  That low logic level 0 input ensures that output of comparator  20   a  stays at a high logic level 1 so the high logic level 1 signal continues to be applied to controller/ID generator  52  as long as interrogator  40  is providing power to power supply circuit  48  which supplies power for comparators  20   a,    20   b.  This provides indication of when the capacitor  36   a  has a larger capacitance value than capacitor  36   b.  Controller/ID generator  52  can sample latch  80  at any time and append this single bit of information to an ID code which is transmitted back to interrogator  40 , as described herein above. 
     Capacitive sensors  36   a,    36   b  shown in FIGS. 1,  3 ,  4  can be replaced with other types of sensors, such as variable inductive sensor  100   a,    100   b,  as shown in FIG. 6, or variable resistive sensor  102 , as shown in FIG.  7 . U.S. patent application Ser. No. 09/259,615, incorporated herein by reference, describes a sensor having a differential inductive sensor that is capable of capturing the peak displacement and strain of the structure to which it is affixed without power. However, power is needed for interrogation, and the systems provided herein facilitate this with very low power required and with remote powering and communications. 
     Fixed capacitors  104   a,    104   b,  matched in value, are used in parallel with each arm of variable resistive sensor  102  to facilitate the AC component of bridge unbalance. As in the embodiment with variable capacitors  36   a,    36   b,  fixed matched bridge completion resistors  32   a,    32   b  are used in Wheatstone bridge  106  with variable inductive sensors  100   a,    100   b.  In another variation, illustrated in FIGS. 6 and 7, microcontroller  108  can directly apply excitation pulse to electrode  38 , eliminating a separate excitation pulse generator. In another variation micropower RF transmitter  110  sends the ID and sensor information back to the interrogator that now comprises receiver  112  and microcontroller ID decoder  114 . Both of these variations can also be applied in the variable capacitor embodiments shown in FIGS. 1,  4 , and  5 . 
     While several embodiments of the invention, together with modifications thereof, have been described in detail herein and illustrated in the accompanying drawings, it will be evident that various further modifications are possible without departing from the scope of the invention. Nothing in the above specification is intended to limit the invention more narrowly than the appended claims. The examples given are intended only to be illustrative rather than exclusive.