Measuring Physical Variables
Transponder with Sensor Functions
Battery-operated telemetry transmitters in the frequency range 27.125 or 433 MHz are normally used for the detection of sensor data. The fields of application of these systems are very limited, however, and are restricted by their size and the lifetime of the battery.

Specially developed RFID transponders incorporating an additional A/D converter on the ASIC chip facilitate the measurement of physical variables. In principle, any sensor can be used, in which the resistance alters in proportion to physical variables. Due to the availability of miniaturised temperature sensors (NTC), this type of system was first developed for temperature measurement.

Temperature sensor, transponder ASIC, transponder coil and backup capacitors are located in a glass capsule, like those used in animal identification systems. (Ruppert, 1994). The passive RFID technology with no battery guarantees the lifelong functioning of the transponder and is also environmentally friendly.

The measured value of the A/D converter can be read by a special reader command. In readonly transponders the measured value can also be appended to a periodically emitted identification number (serial number).

Nowadays, the main field of application for transponders with sensor functions is wireless temperature measurement in animal keeping. In this application the body temperatures of domestic and working animals are measured for health monitoring and breeding and birth control. The measurement can be performed automatically at feed and watering points or manually using a portable reader (Ruppert, 1994).

In industrial usage, transponders with a sensor function may be used anywhere where physical variables need to be measured in rotating or moving parts where cable connections are impossible.

In addition to the classical temperature sensors a large number of sensors can already be integrated. Due to their power consumption, however, only certain sensors are suitable for passive (battery-free) transponders. Table 10.4 (B¨ogel et al., 1998) shows an overview of sensors that can be used in active or passive transponders. Solutions that can be realised as a single chip are cheaper.

Measurements Using Microwave Transponders
Industry-standard microwave transponders can also be used to measure speed and distance by the analysis of the Doppler effect and signal travelling times.

The Doppler effect occurs in all electromagnetic waves and is particularly easy to measure in microwaves. If there is a relative movement between the transmitter and a receiver, then the receiver detects a different frequency than the one emitted by the transmitter. If the receiver moves closer to the transmitter, then the wavelength will be shortened by the distance that the receiver has covered during one oscillation. The receiver thus detects a higher frequency.

The Doppler frequency fd is the difference between the transmitted frequency fTX and the received frequency fRX. The relative speed of the object is v · cos α, c is the speed of light, 3 × 108 m/s.

A transmission frequency of 2.45 GHz yields the Doppler frequencies shown in Table 10.5 at different speeds.

To measure the distance d of a transponder, we analyse the travelling time td of a microwave pulse reflected by a transponder.

The measurement of the speed or distance of a transponder is still possible if the transponder is already a long way outside the normal interrogation zone of the reader, because this operation does not require communication between reader and transponder.

Sensor Effect in Surface Wave Transponders
Surface wave transponders are excellently suited to the measurement of temperature or mechanical quantities such as stretching, compression, bending or acceleration. The influence of these quantities leads to changes in the velocity v of the surface wave on the piezocrystal. This leads to a linear change of the phase difference between the response pulses of the transponder. Since only the differences of phase position between the response pulses are evaluated, the measuring result is fully independent of the distance between transponder and reader.

The working range of surface wave transponders extends to low temperatures of −196 ◦C (liquid nitrogen) and in a vacuum it even extends to very low temperatures.1

The normal surface wave crystals have only limited suitability for high temperatures. For example, in lithium niobate segregation occurs at a temperature of just 300 ◦C; in quartz there is a phase transition at 573 ◦C. Moreover, at temperatures above 400 ◦C the aluminium structure of the interdigital transducer is damaged.

However, if we use a crystal that is suitable for high temperatures such as langasite with platinum electrodes, surface wave sensors up to temperatures as high as around 1000 ◦C can be used (Reindl et al., 1998c).