The Architecture of Electronic Data Carriers - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 296

Table 10.4 Sensors that can be used in passive and active transponders (mm = micromechanic)

Sensor Integratable Passive

transponder

Active
transponder

Single chip
transponder

Temperature Yes Yes Yes Yes

Moisture Yes Yes Yes Yes

P r e s s u r e mm Ye s Ye s Ye s

Shock mm Yes Yes

Acceleration mm Yes

L i gh t Ye s Ye s Ye s Ye s

Flow Yes Yes

PH value Yes Yes

Gases Yes Yes

Conductivity Yes Yes Yes

v

a

Direction of propagation

Reader

v

Microwave
transponder

d

Figure 10.37 Distance and speed measurements can be performed by exploiting 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.
If the electromagnetic wave is reflected back to the transmitter from an object
that has moved, then the received wave contains twice the frequency shift. There is
almost always an angle α between the direction of propagation of the microwaves