Finkenzeller K.RFID handbook.2003

Figure 10.35 FRAM cell structure (1 bit) and hysteresis loop of the ferroelectric capacitor

Table 10.3 Comparison between FRAM and EEPROM (Panasonic, n.d.)

hysteresis loop. After the removal of the electric field the FRAM cell falls into state D, which recreates the originally stored state A (Haberland, 1996).

Writing a ‘1’ or ‘0’ to the FRAM cell is achieved simply by the application of an external voltage −UCC or +UCC. After the voltage is removed the FRAM cell returns to the corresponding residual state A or C.

10.3.4 Performance comparison FRAM – EEPROM

Unlike EEPROM cells, the write operation of a FRAM cell occurs at a high speed. Typical write times lie in the region of 0.1 µs. FRAM memories can therefore be written in ‘real time’, i.e. in the bus cycle time of a microprocessor or the cycle time of a state machine.

FRAMs also beat EEPROMs in terms of power consumption by orders of magnitude. FRAM memory was therefore predestined for use in RFID systems. However, problems in combining CMOS processors (microprocessor) and analogue circuits (HF interface) with FRAM cells on a single chip still prevent the rapid spread of this technology (Table 10.3).

10.4Measuring Physical Variables

10.4.1 Transponder with sensor functions

Battery operated telemetry transmitters in the frequency range 27.125 MHz or 433 MHz are normally used for the detection of sensor data. The fields of application of these

Address and security logic

HF interface A/D

converter ϑ

Chip

Figure 10.36 Inductively coupled transponder with additional temperature sensor

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 (Figure 10.36).

Temperature sensor, transponder ASIC, transponder coil and backup capacitors are located in a glass capsule, like those used in animal identification systems (see Section 13.6.1). (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 read-only 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 (Bogel¨ 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.

10.4.2 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 (Figure 10.37).

Figure 10.38 Influence of quantities on the velocity v of the surface wave in piezocrystal are shear, tension, compression and temperature. Even chemical quantities can be detected if the surface of the crystal is suitably coated (reproduced by permission of Technische Universitat¨ Wien, Institut fur¨ allgemeine Elektrotechnik und Elektronik)

Figure 10.39 Arrangement for measuring the temperature and torque of a drive shaft using surface wave transponders. The antenna of the transponder for the frequency range 2.45 GHz is visible on the picture (reproduced by permission of Siemens AG, ZT KM, Munich)

Figure 10.40 A surface wave transponder is used as a pressure sensor in the valve shaft of a car tyre valve for the wireless measurement of tyre pressure in a moving vehicle (reproduced by permission of Siemens AG, ZT KM, Munich

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., 1998b).

Figures 10.39 and 10.40 show transponders used for measuring physical properties.

1 At very low temperatures, however, the sensitivity S of a SAW transponder ultimately tends towards zero.

RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, Second Edition

Klaus Finkenzeller Copyright 2003 John Wiley & Sons, Ltd.

ISBN: 0-470-84402-7

11

Readers

11.1Data Flow in an Application

A software application that is designed to read data from a contactless data carrier (transponder) or write data to a contactless data carrier, requires a contactless reader as an interface. From the point of view of the application software, access to the data carrier should be as transparent as possible. In other words, the read and write operations should differ as little as possible from the process of accessing comparable data carriers (smart card with contacts, serial EEPROM).

Write and read operations involving a contactless data carrier are performed on the basis of the master–slave principle (Figure 11.1). This means that all reader and transponder activities are initiated by the application software. In a hierarchical system structure the application software represents the master, while the reader, as the slave, is only activated when write/read commands are received from the application software.

To execute a command from the application software, the reader first enters into communication with a transponder. The reader now plays the role of the master in relation to the transponder. The transponder therefore only responds to commands from the reader and is never active independently (except for the simplest read-only transponders. See Chapter 10).

A simple read command from the application software to the reader can initiate a series of communication steps between the reader and a transponder. In the example in Table 11.1, a read command first leads to the activation of a transponder, followed by the execution of the authentication sequence and finally the transmission of the requested data.

The reader’s main functions are therefore to activate the data carrier (transponder), structure the communication sequence with the data carrier, and transfer data between the application software and a contactless data carrier. All features of the contactless communication, i.e. making the connection, and performing anticollision and authentication procedures, are handled entirely by the reader.

11.2Components of a Reader

A number of contactless transmission procedures have already been described in the preceding chapters. Despite the fundamental differences in the type of coupling

Figure 11.2 Block diagram of a reader consisting of control system and HF interface. The entire system is controlled by an external application via control commands

Figure 11.3 Example of a reader. The two functional blocks, HF interface and control system, can be clearly differentiated (MIFARE reader, reproduced by permission of Philips Electronics N.V.)

11.2.1 HF interface

The reader’s HF interface performs the following functions:

•generation of high frequency transmission power to activate the transponder and supply it with power;

•modulation of the transmission signal to send data to the transponder;

•reception and demodulation of HF signals transmitted by a transponder.

The HF interface contains two separate signal paths to correspond with the two directions of data flow from and to the transponder (Figure 11.4). Data transmitted to