Abstract:
A radio frequency identification device having sensing elements incorporated in the clock generators of its tags, which generate a leading code and an identification code. In the interrogator, the leading code is detected and its pulse width is measured for calculating sensing values and the baud rate in receiving the identification code. No analog to digital converter is needed in digitizing sensing values and only discrete signals exist in sensing signal generation and communication. The tag device is insensitive to variations in its power supply voltage obtained from a continuous wave RF carrier.

Description:
[0001]    This present application claims priority from U.S. provisional application No. 60/763,315 having the same tile as the present invention and filed on Jan. 30, 2006. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0003]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
       [0004]    Not Applicable 
       FIELD OF THE INVENTION 
       [0005]    This invention relates to radio frequency identification (RFID) devices, and more particularly, to RFID based sensors, the data acquired from which is read by an interrogator. 
       BACKGROUND OF THE INVENTION 
       [0006]    Radio frequency identification devices generally comprise RF tags and a read-out device that is usually called interrogator or integration reader. The interrogator generates a continuous wave (CW) RF carrier that is used by the tag as a power source to modify the amplitude of the CW carrier by loading and unloading its antenna with stored digital codes. The modulated backscattering signals are then reflected back to the interrogator and demodulated, and thereby, the information stored in tags is read by the interrogator. RFIDs tags can be read through water, paint, dirt, wood, plastics, and human bodies. They are used broadly in security systems, electronic access cards, and inventory management systems. 
         [0007]    RFIDs can also be used with sensors. In this application, typically, physical or chemical properties of an object, such as temperature, humidity, pressure, speed, pH, and acceleration, are detected as analog electrical signals. Then an Analog to Digital Converter (ADC) is employed to convert the analog signals into digital signals, which are read by the interrogator during a sampling cycle. Generally, an ADC compares the analog input voltage with a reference voltage in generating digital signals. Therefore, to obtain an accurate result, a high precision and stable reference voltage source is needed, and the variation of input voltage during sampling should be minimized. However, the power supply of RFID tags is usually generated by converting CW to direct current (DC). It is not easy to obtain a precise reference voltage without using an embedded battery cell. On the other hand, the signal sensing and A/D need extra power consumption, which needs a more powerful CW or closer operation range. 
         [0008]    It is an object of the present invention to provide a RFID tag that is able to work with an interrogator to convert sensing values into digital signals without using A/D converters. Another object of the present invention is to provide a means to transmit the sensing information with identification (ID) codes. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The invention provides a sensing device based on RFID technology. In this device, sensing elements are incorporated into RFID tags, and sensing information is sent back to the interrogator with ID codes. Typically, in RFID communication, only digital signals are transferred by loading and unloading tags&#39; antenna under CW. Consequently, in using RFID with sensors, an ADC is normally employed to convert the analog signals to digital signals before they can be sent to the interrogator, since most sensors convert the physical or chemical properties of an object into analog electric signals. In the present invention, pulse width instead of analog voltage level is used for sensing the objects&#39; physical or chemical properties through sensing elements. Since only discrete signals are used, they are easily sent with digital ID codes by the interrogator, and no ADC is needed. The digitalization of the sensing pulse signals is achieved in the interrogator instead of in tags when receiving the signals, i.e. the digitalization process is incorporated in data communication process, thereby no dedicated analog to digital conversion is needed and a faster and more power economical process can be realized. 
         [0010]    In one embodiment of the present invention, the tag device includes an RC oscillator, the frequency of which is determined by a resistive sensor or a capacitive sensor. Through a logic control circuit, clock signals generated in the RC oscillator are used to trigger a data sequence including a leading code and an ID code. The width of pulses (leading pulses) in the leading code is a function of sensing values that changes with the resistance (or capacitance) of the sensor, and in the ID code, the sensor information, such as sensor type, sensor position and number, sensing baseline, and sensing range can be included for further data processing. The result code sequence is then sent out by the tag device through loading and unloading its antenna. In receiving the code, the interrogator device firstly digitizes the pulse width of the leading pulses, and then uses the results to calculate the baud rate for ID code communication. The sensing value is calculated after the communication is complete. 
         [0011]    Another embodiment of the RFID tag device employs a LC oscillator for clock signal generation. The LC oscillator allows an inductive sensor to be used with the RFID tag device. As that in the RFID tag device using an RC oscillator, the pulse width of the leading pulses, which changes with the inductance or capacitance of the sensor, is used for determining the baud rate for the ID code communication. The sensing value is calculated by the interrogator after the ID code is read. 
         [0012]    Features and advantages of the invention will be apparent from the following description of presently preferred embodiments, given for the purpose of disclosure and taken in conjunction with the accompany drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of an RFID sensing system including a tag device and an interrogator device; 
           [0014]      FIG. 2  illustrates a schematic block diagram of a RFID tag device using an RC oscillator as clock generator; 
           [0015]      FIG. 3  is a schematic block diagram of an LC oscillator that allows an inductive sensor to be used in an RFID tag device; 
           [0016]      FIG. 4  is a timing chart for the code sequence including a leading code and an ID code; 
           [0017]      FIG. 5  shows a schematic block diagram of an RFID interrogator device; 
           [0018]      FIG. 6A  shows a realization of the pulse processing block in  FIG. 5 ; 
           [0019]      FIG. 6B  is a timing chart for the pulse processing; 
           [0020]      FIG. 7  is a flow chart of an interrupt service routine used for detecting leading pulses; 
           [0021]      FIG. 8  is a flow chart of a main routine used by an RFID interrogator device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    As depicted in  FIG. 1 , the RFID sensing system comprises an interrogator device  101  with an antenna  102  and an RFID sensor tag device  105  with an antenna  104 . The RFID sensor tag device  105  has no internal power source. It gains power from a near field or far field RF 103  generated by the interrogator device  101 . After the tag device  105  is powered, it then changes the amplitude of the RF carrier with a sequence of code stored inside the device. The change in amplitude is detected by the interrogator device  101  and therein the patterns in the amplitude change, which contain the code information, are examined. The demodulated code is used for further data processing. 
         [0023]    The block diagram of an embodiment of the RFID sensor tag device is shown in  FIG. 2 , in which, a clock generator  235 , which includes an oscillator  240 , is employed to provide a synchronous signal for a logic control block  230  to read the RFID code from a memory array  225 . The RFID code is then encoded and modulated on the RF carrier signal obtained from an antenna  205  through a modulation control block  220  and a load circuit  210 . 
         [0024]    The power supply is generated by a rectifier  215  from the carrier signals passing through the load circuit  210 . The oscillator  240  in the embodiment shown in  FIG. 2  is an RC oscillator with a capacitor  201  and a resistor  202 . Sensing elements can be included in either the capacitor  201  or the resistor  202 . For example, a capacitive humidity sensor can be used as the capacitor  201  or part of the capacitor  201  together with the resistor  202  in generating the clock signals. In addition to RC oscillators, LC oscillators can also be used for the clock generator  235 . As shown in  FIG. 3 , the LC oscillator  310  includes an inductor  301  and a capacitor  302 . The LC oscillators allow inductive sensors to be incorporated in the sensing circuit. 
         [0025]    The codes stored in the memory array  225  include two sections: the leading code, which includes at lease one pulse the width of which is determined by sensing values, and the ID code, which includes the ID of the tag. Since the clock frequency changes with sensing values, in communication, the baud rate is not constant. In the present invention, the baud rate is calculated for each communication by using the leading pulse width that is determined by sensing values. As depicted in  FIG. 4 , if NRZ-L code is used, a leading pulse  401  at least has two digits: 1 and 0. The pulse width t 1  of the digit 1 is determined by sensing values. The baud rate for the ID code  402  is 1/(2 t 1 ). When Manchester code is employed, the leading pulse  401  can be just one digit, 1. The pulse width t 2  of the digit 1 is determined by sensing values, and the baud rate of the ID code  404 , different from that in NRZ-L code, is 1/t 2 . 
         [0026]    Sensing values are obtained simultaneously in calculating the baud rate. As an example, if an RC oscillator is used ( FIG. 2 ), the clock frequency is a function of the values of the resistor R and the capacitor C, f(R,C). When a capacitive humidity sensor is used as the capacitor, by measuring the width of the leading pulse, the capacitance and then the humidity value can be calculated using the equation f(R,C)=1/T, where T is the period time of the clock; T=t 1  if NRZ-L code is used, and T=2 t 2  when Manchester code is used. For example, if a linear RC oscillator is used, i.e., f(R, C)=kRC, where k is a coefficient in determining the frequency, then the capacitance C is calculated using C=1/(kRT). Multi-leading pulses can be used for calculating average sensing values and the baud rate. 
         [0027]    As shown in  FIG. 5 , in the interrogator of the RFID system, signals acquired from an antenna  501  are sent to an envelope detector  507 , where the code signals are separated from the carrier. The output signals from the envelope detector  507  pass through a filter and amplifier circuit  508 . The result signals  510  are processed in a pulse-processing block  509 , where the width of the leading pulse is digitized. A microcontroller  505  reads ID code based on baud rate calculated using the width of the leading pulse, and calculates the sensing value, while a circuit  506  is used for the communication between the microcontroller  505  and a host computer (not shown in the figure). The clock pulses for the microcontroller  505  and the pulse processing circuit  509  are provided by an oscillator  504  through a divider  511 . RF carrier in the interrogator is generated by the oscillator  504  through a frequency divider  503  and a driver  502 . 
         [0028]    An example of the pulse-processing block  509  in the interrogator is shown in  FIG. 6A , where it is realized by a counter  601 . In the circuit, the Clear signal is provided by the microcontroller  505 . The Pulse Sequence Input is the signal  510 , and the Clock signal is generated by the oscillator  504  through a divider  511 . The output signals Q 0  to Qn of the counter  601  are sent to the microcontroller  505 . Referring to the timing chart, which is shown in  FIG. 6B , before pulses appear in the signal  510 , the Clear signal is at low level. When a leading pulse is received, the high level signal enables the counter and the counting value at the falling edge of the leading pulse is its pulse width. An interrupt is trigged for the microcontroller  505  at the falling edge of the leading pulse when the counter is disabled. The microcontroller reads the counter value in the interrupt service routine and clears the counter for the next code reading. The flow chart for an interrupt service routine example is depicted in  FIG. 7 . When the interrupt service program starts, it first reads the counting value. Then according to the sensing range, the program judges if the counting value is in normal range. If it is within normal range, then a data valid flag is set and the counting value is used for calculating the sensing value and setting the baud rate for ID code communication. If the reading is out of normal range, then an invalid flag is set. The invalid flag will disable further communication until a counting value in normal range is detected. Before the interrupt service routine ends, the counter is cleared and disabled by setting the Clear signal to 1, and the leading pulse interrupt service is disabled (the leading pulse interrupt service will be enabled in the main routine when the interrogator is ready to receive another leading pulse or after the communication process is complete), so that it will not be triggered by the ID code pulses. In addition to an independent counter, the pulse processing can also be realized by using the microcontroller  505  directly based on timer interrupts. Some standard pulse measuring routines can be employed for digitizing the leading pulse width. 
         [0029]    The ID code can be read through a standard serial communication program that uses a timer interrupt. 
         [0030]    The flow chart of a main routine example, in which only one pulse is included in the leading code, is shown in  FIG. 8 . During initialization, the leading pulse interrupt service is enabled, and then the program waits for a leading pulse to be detected by examining if the interrupt service is disabled (the interrupt service is disabled after a leading pulse is detected). When a leading pulse is detected and a data valid flag is set, the baud rate then is calculated based on the width of the leading pulse and an ID code communication starts, otherwise, if a leading pulse and a data invalid flag are detected, after a delay, the program will enable the pulse-processing counter  601  by setting the Clear ( FIG. 6A ) to 0, and enables the leading pulse interrupt service for next communication. After the ID code communication is complete, the program sets the Clear ( FIG. 6A ) to 0 to enable the pulse-processing counter  601 , and the sensing value is calculated during data processing. Before the program ends, the leading pulse interrupt service is enabled for next communication.