RFID sensor tag and system for small output transducers, and related methods
A battery-free wireless sensor system enables high resolution and very accurate measurements of strain or other transducers with small signal outputs. The system may comprise a sensor tag, associated readers and data processing. The sensor tag may be based on passive RFID, or other passive devices that are powered by RF energy and that communicate with the powering source by backscatter, inductive coupling or reflected energy. The system may include a reader that powers the sensor tags with RF energy and that also wirelessly reads data from and writes data to the sensor tag. The reader software or host software may perform operations on the data that enable more accurate and useable information from the sensor tag. The invention may also include methods for data processes that improve the functionality and performance of the sensor system.
1. Method of passively monitoring a strain exerted on an object, the method comprising:
with an antenna provided to an RFID sensor tag, receiving an interrogation signal transmitted by a RFID reader;
creating a stable supply voltage that is input to a transducer associated with the RFID sensor tag from an induced voltage created as a result of a current induced by the interrogation signal;
with an RF filter, blocking from the supply voltage a component of voltages created by currents induced in circuit traces by a RF field generated by the RFID reader;
with the transducer, sensing the strain and outputting a signal on the order of 10's of microvolts indicative of the strain while energized by the induced voltage;
with an amplifier, amplifying the signal from the transducer and filtering RF noise out of the signal to produce a desired signal; and
transmitting, from the antenna, the desired signal indicative of the strain that is received by the RFID reader.
2. The method of claim 1 , further comprising:
receiving, from a temperature sensor, a signal indicative of a temperature adjacent to a location of the RFID sensor tag, and providing for transducer temperature compensation.
3. The method of claim 2 , wherein the transducer temperature compensation is performed by utilizing a calibration constant determined during a temperature calibration procedure, said method further comprising:
transmitting the calibration constant that is received by the RFID reader using the antenna in response to said receiving the interrogation signal.
4. The method of claim 1 , further comprising:
storing an initial reading and subsequent measurement history in the RFID sensor tag.
5. The method of claim 1 further comprising filtering a component of a voltage resulting from a current induced in a circuit trace by the interrogation signal transmitted by the RFID reader to power the amplifier.
6. The method of claim 1 further comprising:
after installation of the RFID sensor tag on the object that is monitored, sensing a preliminary strain exerted on the object; and storing the preliminary strain in a non-volatile memory provided to the RFID sensor tag, wherein said desired signal indicative of the strain transmitted from the antenna is indicative of a deviation of the strain relative to the preliminary strain stored in the non-volatile memory.
7. The method of claim 6 further comprising storing time information indicative of a time when the preliminary strain was sensed in the non-volatile memory, and transmitting the time information in response to said receiving the interrogation signal.
8. An RFID sensor tag comprising:
an RFID IC with sensor input;
a precision voltage reference that generates a supply voltage from an induced voltage generated by an RF field created by an RFID reader operated to interrogate the RFID IC;
an RF filter that blocks, from the supply voltage, a component of voltages that are created by the RFID reader RF field inducing currents into circuit traces;
a resistive transducer that outputs a strain signal on the order of 10's of microvolts indicative of strain to which an object associated with the RFID sensor tag is subjected, wherein the RF filter is disposed electrically between the precision voltage reference and the resistive transducer to supply the resistive transducer with the supply voltage;
an amplifier arranged to generate an amplified signal by amplifying the strain signal emitted by the resistive transducer; and
a second RF filter that blocks voltages that are created by the RFID reader RF field inducing currents into the circuit traces from a signal powering the consumption amplifier.