Abstract:
A transducer is implemented to operate with transmitted frequency signals. These transmitted signals provide a bias potential to the transducer and enable the transducer to transmit a transducer output signal to a remote location. The transducer has an antenna for receiving transmitted signals. The antenna is coupled to a rectifier circuit; the rectifier circuit is operative to provide a DC voltage in response to said received transmitted signals. This DC voltage is applied to the piezoresistive array via a switching arrangement coupled between the array and the rectifier circuit. An interface circuit controls the switching arrangement to enable DC bias to be applied to the array in a first mode and to remove the bias in a second mode.

Description:
CLAIM FOR PRIORITY  
       [0001]     This application claims priority from U.S. provisional patent application Ser. No. 60/535,966 filed on Jan. 12, 2004, the subject matter thereof being incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to transducers in general and, more particularly, to a transducer which employs wireless transmissions for sending and receiving signals. The received signals operate to provide a bias to the transducer assembly, therefore eliminating the need for batteries or an external power supply.  
       BACKGROUND OF THE INVENTION  
       [0003]     Transducers are widely employed for monitoring various environmental conditions. A very popular type of transducer is the pressure transducer. Such pressure transducers are well known and conventionally employed piezoresistive devices arranged in a Wheatstone bridge configuration which is coupled to an operating biasing source, such as a voltage supply or a battery. The transducers are usually hard-wired into a circuit, whereby the output of the pressure transducer is monitored to provide a voltage indicative of a pressure applied thereto. Many of the transducers employed utilize piezoresistive sensors or piezoresistive devices, which produce an output voltage according to an applied pressure. As indicated, these are well known and the assignee herein, Kulite Semiconductor Products, Inc., commercially sells such devices in a plurality of different configurations.  
         [0004]     It is extremely desirable to provide a transducer which would be connected to a monitoring station using a wireless link. In this manner, the monitoring station can transmit a signal to the sensor. The signal can be used by the sensor to provide an operating potential. The sensor can also transmit an output signal to the monitoring station indicative of a measured pressure. In this manner, one eliminates the need for a sensor or a transducer which is hard wired into a circuit and further eliminates the need for a sensor which requires the use of a battery or an operating potential.  
       SUMMARY OF THE INVENTION  
       [0005]     A transducer is implemented to operate with transmitted frequency signals. These transmitted signals provide a bias potential to the transducer and enable the transducer to transmit a transducer output signal to a remote location. The transducer has an antenna for receiving transmitted signals. The antenna is coupled to a rectifier circuit; the rectifier circuit is operative to provide a DC voltage in response to said received transmitted signals. This DC voltage is applied to the piezoresistive array via a switching arrangement coupled between the array and the rectifier circuit. An interface circuit controls the switching arrangement to enable DC bias to be applied to the array in a first mode and to remove the bias in a second mode. The interface circuit further includes a converter circuit for converting the bridge output into an RF signal for transmission through the same antenna back to a remote location. In this manner the transducer requires no operating voltage as it receives the operating voltage from the transmitted signals. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]      FIG. 1  is a simple block diagram of a wireless transducer according to this invention.  
         [0007]      FIG. 2  is an embodiment of a transducer unit according to this invention.  
         [0008]      FIG. 3  is an alternate embodiment of a transducer unit according to this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]     Referring to  FIG. 1 , there is shown a basic block diagram of a system according to an embodiment of the present invention. A monitoring station  10  transmits a radio signal which may be in the UHF band. The signal can be transmitted at a relatively low level. The choice of the UHF frequency band is desirable because it is in one of the unregulated frequency bands around 400 MHZ or 900 MHZ. This choice was made because the antennas for this range are very small and signal propagation is not affected by rain or humidity in the atmosphere. As shown, the monitoring station  10  includes an antenna  11 . As is well known, the antenna  11  can conventionally operate in a transmit or receive mode.  
         [0010]     A transducer unit comprising a tuned antenna  12 , bridge  15 , and interface  14  is in operative communication with monitoring station. In one configuration, tuned antenna  12  associated with the transducer unit comprises a printed circuit coil and capacitor. These form a resonant circuit at the operating frequency. The tuned antenna  12  is coupled to wireless electronic interface  14 , which as illustrated, is coupled to piezoresistive bridge  15 . The bridge  15  is a Wheatstone bridge configuration and includes piezoresistive devices, which basically change their resistance according to an applied pressure.  
         [0011]     In operation, the monitoring station transmits a frequency which is received by the tuned antenna  12 . The tuned antenna  12  is coupled to a wireless electronic interface, whereby the signal transmitted is rectified to produce a DC voltage, which DC voltage operates to bias the circuitry, as well as the bridge  15 . The bridge  15 , when biased, will measure an applied pressure. The applied pressure measured by the bridge is then converted into voltage, which voltage may be converted into a digital signal, which signal is again transmitted by the tuned antenna  12  back to the monitoring station which receives the signal via its antenna  11 .  
         [0012]     Referring to  FIG. 2 , there is shown a transducer embodiment which would basically include the circuitry shown within the dashed line of  FIG. 1 . As seen, the tuned antenna of  FIG. 2  consists of inductor  20  in parallel with capacitor  21 . The inductor  20  can be a printed coil. The capacitor  21  and the inductor  20  form a resonant circuit at the operating frequency. The voltage developed at the tuned antenna is rectified by means of a microwave diode  23  which charges a capacitor  24 . The capacitor  24  stores a DC voltage. This DC voltage is applied to the wireless interface circuit  28  as VCC. The wireless interface circuit  28  is a typical integrated circuit which can be provided by any number of integrated circuit suppliers/manufacturers. Essentially the wireless interface  28  can be a very simple microprocessor which operates to perform the following. As described and as explained in conjunction with  FIG. 1  and  FIG. 2  the function of the wireless interface  28  is to basically interface with the piezoresistive Wheatstone bridge  26 . In a first mode the wireless interface will receive the biasing voltage from rectifier  24  and provide an operating voltage to switch  25 . Thus the wireless interface  28  will operate the switch in a first mode whereby switch  25  will be closed and apply operating potential to the Wheatstone bridge  26 . The wireless interface also receives the bridge outputs as shown on ports IN− and IN+. These outputs are converted by the interface into a digital signal which essentially can be used to modulate an RF carrier and to transmit the bridge output signal via capacitor  22  to the antenna consisting of the inductor  20  and capacitor  21 . As on can see the wireless interface performs a relatively simple function which can be provided by many custom integrated circuits as well as special purpose integrated circuits. The DC voltage is also sufficient to operate switch  25  which closes when the interface detects a sufficient DC voltage on capacitor  24 . Switch  25  when closed supplies operating potential to the Wheatstone piezoresistive bridge array  26 . The supply to the bridge is applied to a span resistor  27  designated RSP, which provides a coarse temperature compensation of the temperature coefficient of the gage factor due to the temperature dependent bridge resistance. The voltage across resistor RSP is also digitized, as it indicates the temperature of the bridge. As one can see, the output from the bridge is applied to the wireless interface. The output of the bridge is digitized by an analog-to-digital converter, which is in the wireless interface. The analog-to-digital converter uses as the reference voltage the supply to the bridge. In this manner, variations of the bridge output due to variations in the supply voltage are eliminated.  
         [0013]     As indicated above, the voltage across RSP is also directed to the wireless interface  28 , which converts this voltage to a digital signal. The results of the analog-to-digital conversions are transmitted back to the monitoring station via the output indicated TX through capacitor  22 . In  FIG. 2  this is accomplished by modulating an RF signal at a frequency slightly different than the one transmitted by the monitoring station, but still within the resonant band of the transducer&#39;s antenna L 1 . The band width of the antenna is a function of the quality factor Q of the tuned circuit. The frequency that the transducer unit transmits to the monitoring station is slightly different than the received frequency. The receiver in the monitoring station demodulates the signal and retrieves the data. Therefore, from the received data the pressure and temperature of the sensor can then be calculated.  
         [0014]     Referring to  FIG. 3 , there is shown an alternative embodiment of a transducer unit and method of transmitting data. As shown in  FIG. 3 , a tuned circuit consists of inductor  30 , which is in parallel with capacitors  31  and  32 . The center tap between capacitors  31  and  32  is coupled to a switch  33 . Upon operation of switch  33 , the tuned circuit of inductor  30  and capacitors  31  and  32  are detuned due to the fact that the switch  33 , when operated, will short capacitor  32  to reference potential (GND) and therefore, the tuned circuit will now consist of inductor  30  and capacitor  31 .  
         [0015]     At the same time, in the monitoring station the receiver detects the back scattered signal. There is a significant difference in the back scattered signal between the tuned and the detuned states of the transducer&#39;s antenna. This difference is due to the fact that a tuned circuit absorbs much more energy than a detuned one and this is reflected in the level of the back scatter signal. By controlling the closing and opening of switch  33 , such that it reflects the data to be transmitted, the information is conveyed to the monitoring station. The entire sequence supplies voltage to the bridge and the digitized transmitted data should be very short in order not to discharge capacitor C 2  to an unacceptable low value. This cycle can be accomplished in a millisecond or less. The piezoresistive bridge, such as bridge  35 , has a high resistance resulting in low current compensation. As seen in  FIG. 3 , the same exact concept basically operates with the exception of the transmission of data. In  FIG. 3 , the tuned antenna consisting of coil  30  and capacitors  31  and  32  are operative to receive the transmitted signal from the monitoring station. The diode  36  charges capacitor  37 , whereby a suitable voltage is supplied to the wireless interface  38 . This voltage operates switch  39  applying bias to the bridge circuit  35 . The bridge circuit then produces an output voltage indicative of pressure. This output voltage is digitized and the digital signal is used to open and close switch  33 , so that the effective data is conveyed to the monitoring system.  
         [0016]     As one can ascertain, the RF signal transducer transmitted by the monitoring station, besides supplying the energy to the transducer, can also carry information needed to implement other functions. This information can be transmitted using any of the known modulation schemes. For example, AM, pulse, FM and using a corresponding demodulator at the transducer. The system described allows multiple sensors to be used in the same system. This is accomplished by modulating the RF signal transmitted by the monitoring station with the code corresponding to one individual transducer serial number in the system. All sensors in the system demodulate this code, but only one will then respond with its data. The monitoring system described above can also send a signal to the selected transducer. In return, the selected transducer will transmit its individual compensation data required to convert the voltage data to actual pressure and temperature. This compensation data can be embedded in each transducer when manufactured in a memory such as an electrically erasable programmable read only memory (EEPROM) in the form of several coefficients. The compensation coefficients can be retrieved only once when the system is turned on and then stored in the monitoring station. This function simplifies the replacement of a transducer without any changes in the monitoring station or logistic difficulties.  
         [0017]     Another function that could be implemented by the system is automatic identification of all sensors connected to the monitoring station. This can be done by the monitoring station issuing a general broadcast code, and all transducers received in this code will respond by transmitting an acknowledged signal. The monitoring station can then selectively disable the acknowledge function of some transducers; for example, those devices whose first bit of the serial number is one. By scanning this way through all bits of the serial number, the monitoring system can automatically determine which transducers are connected in the system. Those of ordinary skill in the art may recognize that many modifications and variations of the present invention may be implemented without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.