Abstract:
A radio frequency identification device having sensing elements incorporated in its tags, and the sensing value determines the width of a sensing pulse in its RFID code. Through a pulse-processing unit, during communication, digital sensing values are obtained by measuring the width of the sensing pulse. Since only discrete signals exist, the tag device is insensitive to the fluctuation in its power supply voltage, which is derived from a continuous wave RF carrier.

Description:
This present application claims priority from U.S. provisional application No. 60/786,193 having the same tile as the present invention and filed on Mar. 27, 2006. 
    
    
     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. 
     FIELD OF THE INVENTION 
     Background of the Invention 
     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 therein, 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. 
     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. Since an ADC compares the analog input voltage with a reference voltage in generating digital signals, 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 steady and precise reference voltage. Additionally, sensing signal conditioning and analog to digital signal conversion need extra power consumption. As a result, a more powerful CW or closer operation range is required. 
     It is an object of the present invention to provide a RFID tag that is able to work with an interrogator in converting sensing values obtained from a sensor into digital signals without using ADCs, so that the signal acquisition is not sensitive to variations in power supply. 
     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 
     The present invention provides a sensing device based on RFID technology. In this device, sensing elements are incorporated in RFID tags, and sensing information is sent back to the interrogator with ID codes. Different from other devices converting voltage level obtained from a sensor into digital signals, in the present invention, pulse width is used in sensing the objects&#39; physical or chemical properties. Pulse signals that change with the sensing values are digitized in the interrogator during communication rather than being converted into digital signals in tags, thereby no dedicated ADC and its complex peripheral circuits are needed, and a faster and more power economical process is enabled. 
     In one embodiment of the present invention, the tag device has a monostable multivibrator. Triggered by a synchronous signal, the monostable multivibrator generates a pulse, the width of which changes with the sensing values obtained from a resistive sensor or a capacitive sensor. This sensing pulse is then concatenated with an ID code sequence generated with a memory array. The ID code can be either leading the sensing pulse or behind it (in a more complex circuit, the sensing pulse can be inserted in ID code), and in the ID code, the sensor array information, such as sensor type, sensor location, sensing baseline, and sensing range, can be included. The result code sequence is then modulated and transmitted by the tag device by loading and unloading its antenna. 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 
         FIG. 1  is a block diagram of an RFID sensing system including a tag device and an interrogator device; 
         FIG. 2  illustrates a schematic block diagram of an RFID tag device with sensor elements included; 
         FIG. 3  is a timing chart for illustrating the generation of an RFID code sequence including a sensing pulse and an ID code; 
         FIG. 4  shows a schematic block diagram of an RFID interrogator device; 
         FIG. 5  shows a realization of the pulse processing block in  FIG. 4 ; 
         FIG. 6  is a timing chart for the pulse processing; 
         FIG. 7  is a flow chart of an interrupt service routine used for detecting sensing pulses; 
         FIG. 8  is a flow chart of a main routine used by the RFID interrogator device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 the pattern of the amplitude change, which contains the code information, is examined therein. The demodulated code is used for further data processing. 
     Referring to  FIG. 2 , in an embodiment of the RFID sensor tag device, a clock generator  201  is employed to provide a synchronous signal for a logic control block  203  to read the RFID code from a memory array  204 . The synchronous signal is also used for generating a trigger signal for a monostable multivibrator  205  through a frequency divider  202 . At rising edge or falling edge of the trigger signal, the monostable multivibrator  205  generates a pulse with its width determined by a resistor  206  and a capacitor  207 , either of which could have sensor elements included. The pulse signal from the monostable multivibrator  205  is then concatenated with the RFID code signal provided by the memory array  204  in a signal generator  209 , which in this embodiment is an OR gate  210 . The result signal is then modulated on the RF carrier signal obtained from an antenna  212  through a modulation control block  211  and a load circuit  213 . The power supply for the RFID tag is generated by a rectifier  214  from the carrier signals passing through the load circuit  213 . 
     The RFID code stored in the memory array  204  include two sections: leading code, which comprises a series of zeros, and ID code, which includes the ID of the tag. If the RFID code has 2 n  bits, then the frequency divider  202  should have n registers (frequency is divided by 2 n ), where n is an integral. When a capacitive sensor is included in the capacitor  207  (or a resistive sensor is included in the resistor  206 ), the resistor  206  (or capacitor  207 ) should be selected to make the width of the pulse generated by the monostable multivibrator shorter than that of the leading code. For example, if the pulse width t is a function of the values of the resistor  206  (R) and the capacitor  207  (C):
 
 t=f ( R,C ),
 
then the maximum pulse width t max  in sensing range should be shorter than the width of the leading code t c ;
 
 t   c   =m/f   c,  
 
where m is the number of bits in the leading code and f c  is the clock frequency. The signal waveforms in  FIG. 2  are depicted in  FIG. 3 . Triggered by the synchronous signal B, the frequency of which is f c /2 n , the signal C generated by the monostable multivibrator  205  includes a sensing pulse  301 . Its pulse width is f(R,C). Synchronized by the clock signal A, the RFID code signal D is generated through the memory array  204 . The low level leading code signal  302  lasts for t c  seconds, and
 
 t   c   &gt;f ( R,C ),
 
while the overall time of the ID code signal  303  is (2n−m)/f c . In the signal generator  209 , the RFID code signal D and the sensing pulse signal C are concatenated in the OR gate  210 . The result signal E has a pulse  304  and an ID code signal  305 . The width of the pulse  304  changes with the values of the sensing elements in the RFID tag,
 
     The signals generated by the RFID tag are then received by an interrogator. As shown in  FIG. 4 , in the interrogator, signals acquired from an antenna  401  are sent to an envelope detector  407 , where the code signals are separated from the carrier. The output signals from the envelope detector  407  pass through a filter and amplifier circuit  408 . The result signals  410  are processed in a pulse-processing block  409 , where the pulse width of the sensing pulse is digitized. A microcontroller  405  reads ID code and calculates the sensing value, while a circuit  406  is used for the communication between the microcontroller  405  and a host computer (not shown in the figure). The clock pulses for the microcontroller  405  and the pulse processing circuit  409  are provided by an oscillator  404  through a frequency divider  411 . RF carrier in the interrogator is generated by the oscillator  404  through a frequency divider  403  and a driver  402 . 
     An example of the pulse-processing block  409  in the interrogator is shown in  FIG. 5 , where it is realized by a counter  501 . In the circuit, the “Clear” signal is provided by the microcontroller  405 . The Pulse Sequence is the “Signal to Modulation Control” E ( FIG. 3 ), and the “Clock” signal is generated by the oscillator  404  through a divider  411 . The output signals Q 0  to Q r  of the counter  501  are sent to the microcontroller  405 . Referring to the timing chart, which is shown in  FIG. 6 , before the sensing pulse  304  appears in the signal E ( FIG. 3 ), the Clear signal is at low level, and the counting value is set to 0. When a sensing pulse is received, the high level signal enables the counter, which keeps counting up until a falling edge of the sensing pulse appears. Then, an interrupt is trigged for the microcontroller  405  and the microcontroller reads the counting value in its interrupt service routine and clears the counter for the next code reading. Since the counter only counts during the sensing pulse period, the counting value is a measure of the pulse width. 
     The flow chart for an interrupt service routine example is depicted in  FIG. 7 . When the interrupt service program starts, it reads the counting value. Before the interrupt service routine ends, the counter is cleared and disabled by setting the Clear signal to 1, and the sensing pulse interrupt service is disabled (this interrupt service will be enabled in the main routine after the communication process is complete), so that it will not be triggered by the ID code pulses. In addition to a dedicated counter, the pulse processing can also be realized by using the microcontroller  405  directly based on timer interrupts. Some standard pulse measuring routines can be employed for digitizing the pulse width. 
     The flow chart of a main routine example is shown in  FIG. 8 . During initialization, the sensing pulse interrupt service is enabled, and then the program waits for a sensing pulse to be detected by examining if the interrupt service is disabled (this interrupt service is disabled after a sensing pulse is detected). When a sensing pulse is received, a communication process for detecting ID code starts. The ID code can be read using a standard serial communication program. After the ID code communication is complete, the program sets the Clear signal ( FIG. 5 ) to 0 for clearing the pulse-processing counter  501 , and the sensing value is calculated during data processing. Before the program ends, the sensing pulse interrupt service is enabled for the next interrogation.