WIRELESS TEMPERATURE SENSOR NETWORK

Wireless Sensor Data Transfer as Well as Processing

Miroslav Husak, Petr Oberreiter and Julius Foit

Department of Microelectronics, Czech Technical University in Prague, Technicka 2, CZ-166 27 Prague 6, Czech Republic

Keywords: Sensor, Wireless, Data transfer, Measurement, Control, Temperature.

Abstract: The realized system for wireless measurement of temperature in the many different places is described. The

paper describes a new architecture of a multisensor system for remote temperature measurement using

wireless communications. The system is composed of temperature sensors with digital output. The system

ensures RF wireless data transfer, wireless communication between control unit and sensors as well as

wireless switch unit. The integrated RF chip nRF9E5 ensures wireless communication. The control unit

controls communications, sensor data processing as well as switching of actuator unit. The control unit

contains main control microprocessor ATMEL as well as wireless communication unit. The actuator switch

unit communicates with control unit using RF wireless way. Partial control programmes were designed for

wireless unit control. There are many program functions implemented. The control software of the whole

system has been designed. The designed system, i.e. hardware and software has been realized and tested.

The system can operated in the range of 300 m in the free space. The number of sensor can be extensible.

The system can be use to operate with different type of sensor, i.e. for measurement other type of quantities.

PDA or mobile phone can be used to communication with control unit.

1 INTRODUCTION

Wireless transmission is very popular today for

sensor data transmission. Most RF chips are capable

to perform coding, decoding and transmitted data

synchronization at least partly, they can also feature

enhanced data security by adding CRC, parity bits

etc. These circuits perform these activities without

the need of programmed control, simplifying the

programmer’s work considerably. The RF chips are

frequently fabricated in the form of transceivers, i.e.

they include both a transmitter and a receiver in a

single package; then, the same chip can be used in

all instruments, bidirectional communication

presents no problems (Varadan, 2003). Many

producers also combine transceivers with other

circuits like A/D converters or microprocessors.

The ZigBee communication system fits well to

sensor data transmission. The standard makes

possible mutual communication of numerous

instruments over a distance of hundreds of meters. It

is distinguished by low requirements on hardware

and by extremely low power consumption. It is

described by the IEEE 802.15.4 specification.

ZigBee replaces the data transmission by RS232 or

RS485 series bus.

In our case we have used a RF module in the

Nordic technology. Its principle of operation is

similar to ZigBee. The chips operate at frequencies

433 MHz, 868 MHz, 915 MHz or 2.4 GHz.

Transmission speed is 50 kb/s and 250 kb/s in the

2.4 GHz band. In the 868 MHz band, Nordic has 7

channels, as compared with 1 channel in ZigBee, 83

channels in the 2.4 GHz band compared to 16. The

data frame is smaller, too, in addition to data it only

contains a 10-bit header, 32-bit address and a 16-bit

CRC. All chips are capable of bidirectional

operation.

The system is designed with temperature sensors.

With small adjustments, the above mentioned

principle can be used for quantities from other

energy domains (the pressure system, flow

multisensor system, humidity multisensor system,

etc.), (Kirianaki,N.V., 2002).

2 COMMUNICATION CIRCUITS

DESIGN

The communication circuit. The type nRF9E5 circuit

was selected for the communication circuits design.

The circuit contains a RF transceiver, an 8051

Husak M., Oberreiter P. and Foit J. (2007).

WIRELESS TEMPERATURE SENSOR NETWORK - Wireless Sensor Data Transfer as Well as Processing.

In Proceedings of the Second International Conference on Wireless Information Networks and Systems, pages 39-44

DOI: 10.5220/0002145500390044

 SciTePress

architecture compatible microprocessor and an AD

converter (nordicsemi, 2006).

The transceiver consists of a frequency

synthesizer, power amplifier, modulator and receiver

unit. It operates in the 433/868/915 MHz ISM bands

with adjustable frequency and output power. GFSK

(Gaussian Frequency Shift Keying) modulation is

used for the transmission, featuring a lower

bandwidth than conventional frequency modulation.

The modulation width is ±50 kHz. The data are

internally coded/decoded according to Manchester

code, resulting in 50 kb/s effective transmission

speed. The transceiver communicates with the built

in microprocessor through an internal SPI interface.

Three operating modes are available in the

transceiver: standby, transmission and reception.

The AD converter in this circuit has 4 inputs

and 10 bits. The inputs can be operated as

differential and the supply voltage can be measured

internally. Resolution can be adjusted in 4 steps

from 6 to 12 bits. The internal SPI interface is used

for A/D converter control just like the transceiver.

The circuit also has an input for AD converter

reference voltage. Alternately it is possible to use the

internal 1.22 V reference source. It is further

possible (when the external input is left free) to

measure the circuit supply voltage. In this case,

internal reference is used and 1/3 of the supply

voltage is measured. Conversion time is 8 to 14 μs,

depending on resolution.

As further hardware the circuit contains a single

channel PWM modulator, controlled by PWMCON

and PWMDUTY registers. Further the Watchdog,

Wakeup timer and low power consumption clock

pulse source. The watchdog timer is a 16 bit counter

serving to reset the microprocessor in case of an

error or endless loop in the program. The Wakeup

timer serves for interrupt or finish of some of the

economy modes. It is a 24 bit counter with

automatic fresh value recording, calling the required

operation upon reaching.

The main microprocessor. The microprocessor is

8051 architecture compatible. It includes 4 kB

program memory, 256 bytes of data memory and

special function registers. The upper 128 bits are

accessible by indirect addressing since they are

shared with special function register addresses. The

program memory is a RAM type and the program is

recorded in it by the Bootloader after SPI from the

EEPROM memory after resetting. A header must be

present in front of the program in the memory,

containing the memory speed, crystal frequency and

user data.

An ATMEL 89S8253 type was selected as the

control processor. It is a 8051 architecture

compatible microprocessor, containing additional

2 kB data EEPROM, 12 kB FLASH program

memory, SPI interface and further hardware. Both

the program and data memories are In System

Programmable (ISP), by series programming

through the included SPI interface. Further the

circuit contains a 256 byte ARM memory, whose

upper 128 bits are accessible by indirect addressing

since they are shared with SFR special function

four register banks. Instructions are fully compatible

with the 8051 architecture and operate identically

(atmel, 2006). The instruction timing is the same as

in the preceding case.

The sensors. The DS620 sensor as example can be

used for the measurement of temperature. The

circuit contains a temperature sensor, A/D converter,

comparator and a series interface. It does not need

any external components for its operation (

DS620,

2006)

. The temperature measurement range is -55

to +125

C. The A/D converter resolution can be

adjusted from 10 to 13 bits, corresponding to 0.5

to 0.0625

C resolution. The conversion duration

depends on resolution and takes between 25 ms and

200 ms. A twin lead I

C bus is used for

communication. The SCL terminal serves for

reception of clock pulses and the SDA terminal for

data reception or transmission. The circuit also

contains an EEPROM memory to which a part of

registers can be copied and so preserve the setting

even when the power supply is disconnected.

The real time circuit. DS1302 is a real time circuit,

counting seconds, minutes, hours, day of the week

and date including the year. For its operation the

circuit needs only an external crystal with 32768 Hz

frequency and an optional battery for continuing

function even when the primary power supply is

switched off. The circuit also contains 31 bytes of

user RAM memory, supported by the secondary

supply voltage (

DS1302, 2006).

3 SENSOR SYSTEM CONCEPT

The suggested wireless thermostat consists of

several parts. The control unit is the basic part,

securing communication with the user by means of

the display and keyboard. The unit communicates

with wireless temperature sensors and wireless

switch units. It is provided with custom-set

programs “temperature intervals”, controlling the

whole system. Second part of the system is

represented by the wireless temperature sensors. The

sensors are placed in required localities. The sensors

WINSYS 2007 - International Conference on Wireless Information Networks and Systems

measure temperature periodically and convey the

measured data tot the control unit. The third part is

the wireless switch units. The units communicate

with the control unit periodically and switch relays

on or off according to the measured temperatures

and values set by the user. A block schematic

diagram of the whole system is shown in Figure 1.

The temperature sensors and switch units can be

placed at random individually and independently

according to the user’s needs. The only condition

they have to fulfill is that they must lie within the

wireless communication range. The communication

with the user is performed by means of six push

buttons and a 4 line display. Most user program

settings and settings of further parameters is dome

with the aid of a menu appearing on the display

upon pressing the appropriate button. During normal

operation, information from individual zones are

displayed periodically. They indicate the set and

measured temperature in a particular zone, sensor

and control unit battery voltage as well as an

indication of on or off state of the relay contacts.

The communication runs according the following

scheme: Before start of a transmission, each

temperature sensor or switching unit checks if

another unit is not transmitting. Then the sensor

transmits its data to the control unit and waits for

reception acknowledgement. If no acknowledgement

arrives until a certain time interval, the whole

process is repeated. When the control unit receives

data from a sensor, it first checks if it can transmit

and then sends the acknowledgement reply. The

reply to a switch unit also contains the information

on required new relay state.

Figure 1: Architecture of wireless sensor system.

3.1 Wireless Sensor Unit

Every sensor unit consists of a nRF9E wireless chip,

program memory and a symmetrical output element

- Figure 2. The input block is the temperature sensor.

The sensor used contains an AD converter and a

series interface. The communication with the

microprocessor takes place over an I

C bus. The

second block is the microprocessor with integrated

transceiver, processing the sensor data and

communicating with the control unit. The program

25320 EEPROM memory serves to store the

microprocessor program. The memory is connected

through the SPI series bus. The external antenna is

connected through a symmetrizing transformer

(balun).

Figure 2: Wireless sensor unit.

3.2 Wireless Switch Unit

The base of the wireless switch unit is a wireless

sensor unit, followed by the power switch part. A

block diagram is shown in Figure 3. The core is a

microprocessor with a transceiver, with program

memory connected through SPI and an antenna

connected through a balun like in the temperature

sensor. The output block is a power switch unit

controlled by two signals from the microprocessor.

The power switch device is a polarized bistable

relay. The relay is advantageous especially due to its

low power consumption (battery supply).

Power

switch unit

Microprocessor

Transceiver

Symmetrizing

transformer

(balun)

Software

memmory

EEPROM

nRF9E5

SPI

Figure 3: Wireless switch unit.

Its contacts remain in the “on” or “off” state without

the need for any power. Power is consumed in the

form of short pulses only, needed to change the relay

state. Contact switching is performed by connecting

a voltage of proper polarity to the relay. The relay

WIRELESS TEMPERATURE SENSOR NETWORK - Wireless Sensor Data Transfer as Well as Processing

selected is type G6CU-2117P-US made by OMRON

(omron, 2006). Nominal coil voltage is 3 V and the

maker guarantees correct function at 70% of

nominal voltage. The coil power consumption is

about 200 mW and minimum pulse duration for state

changeover is 200 ms.

3.3 Wireless Control Unit

The block diagram of the wireless control unit is in

Figure 4. A keyboard and a display serve for

communication with the user. The keyboard uses

matrix connection and is connected to the

microprocessor through five conductors. The display

is connected by means of a buffer circuit. Four data

and four control signals serve for communication

(one microprocessor port). The main part is the

control microprocessor, t which all remaining parts

of the system are connected. The real time circuit

communicates by means of a serial bus. The bus

consists of the SCLK signal for clock frequency

transmission, the I/O signal for data and chip select

signal transmission. The transmitting and receiving

parts are very similar to those of the wireless

temperature sensor and the switch unit. The

microprocessor, memory and antenna connections

are the same, the SPI bus is used for connection with

the controlling microprocessor (MISO, MOSI, SCK

signals). The slave select signal is software on one

of the microprocessor ports. Two voltages are

needed for control unit supply, namely 3.3 V and 5

V. The 5 V supply is used for of the LCD display

and its buffer circuit. All other circuits are supplied

by the 3.3 V. A type HD4478U controller, common

with character type LCDs is used to control the

display (alldatasheet, 2006). A type AT89S8253

circuit is the core of the control unit. Since the

nRF9E5 circuit has the SPI hardware as master only,

the control processor must be operated in

slave mode.

A simple converter serves for communication

with a PC over RS232 line. It is based on a type

MAC3232 circuit. The circuit contains a doubler and

voltage inverter in addition to the RS232 line

drivers/converters (maxim, 2006).

Figure 4: Wireless control unit.

4 CONTROL SOFTWARE

Wireless temperature sensor unit control program.

The wireless sensor unit control program is based on

an endless loop, repeatedly performing the required

operations, value measurement by the sensor,

communication with the control unit, and low power

consumption waiting mode. The operation sequence

is shown in Figure 5. The circuit is waked up from

this mode by means of the Wakeup timer, set during

initialization, running in the economy mode

constantly. The wakeup period is given by the

Wakeup timer setting, in this case approximately 30

seconds. After the wakeup from the economy mode

the program run on by the next instruction

(following the instruction that caused the set on of

the economy mode), namely jumping to the start of

an endless loop and the whole process keeps

repeating. So the system is operating in the waiting

mode most of the time, with current consumption

just tens of µA, the consumption rises to

approximately 8 mA for just several hundred ms and

to about 30 mA during maximum RF power

transmission.

Wireless switch unit control program. The program

is basically similar to the preceding temperature

sensor program. The current consumption diagram

of this circuit is similar to that of the temperature

sensor. Ten of µA are consumed, increase to 6.5 mA

for 200 ms, up to 30 mA during maximum RF power

transmission and only during the relay changeover

the consumption rises to 70 mA for 250 ms. The

WINSYS 2007 - International Conference on Wireless Information Networks and Systems

largest consumption appears during the relay

changeover, which, of course, lasts for only a short

time and the switching period is very.

Figure 5: Flow diagram of

wireless sensor unit.

Figure 6: Control

program of the wireless

transmission and

reception section of the

control unit.

Control program of the wireless transmission and

reception section of the control unit. The program

secures the control unit wireless communication

with individual sensors and switch units. Its graph is

shown in Figure 6. The program composition and its

initializing part are similar to the two preceding

programs. The transceiver is receiving most of the

time in order to be able to accept all data packets

transmitted from the other units, and only during

short intervals it transmits replies or

acknowledgements of data received from sensors or

switch units. The control unit circuit current

consumption is 13 mA and 30 mA (transmission).

Control unit program. The control program of the

Atmel processor in the control unit is the largest

program in the system. The program graph is shown

in Figure 7. The main endless loop of the program

refreshes the displayed data, evaluates and processes

data from individual zones of the memory.

Communication with the wireless part takes place

during interrupts.

Figure 7: Control unit program.

User control. There are six pushbutton provided for

the control unit handling. All stored programs can

also be listed through a PC.

5 RESULTS OF THE WORK

The programmable RF output power can be set in

four levels from -10 dBm to +10 dBm, with

corresponding communication range. The highest

WIRELESS TEMPERATURE SENSOR NETWORK - Wireless Sensor Data Transfer as Well as Processing

RF, output power +10 dBm, or 10 mW, means a free

space more than 300 m. The operating frequency can

also be adjusted. Setting channel number 117 means

a 868.2 MHz frequency. During tests, the system

worked flawlessly even in close presence of

common interference sources like TV receivers,

PCs, microwave ovens etc. The response time is

determined, first of all, by the wakeup period and

sensor and switch unit communication with the

control unit. The temperature measurement

resolution can be adjusted from 10 to 13 bits. This

influences the conversion time, corresponding to

about 200 ms in case of maximum resolution.

The example of the circuit connection of the

temperature sensor unit is shown in Figure 8. The

same way of the design was used in the other units.

Figure 8: Circuit connection of the temperature unit.

6 CONCLUSIONS

Research contribution of that work is design of the

sensor system with wireless data transfer. The

system can be used with minor modification in the

all energy domains for processing of physical as

well as biochemical quantities. In the work was used

example of the temperature domain. Where is made

design with other sensor type, it is necessary adjust

output parameters of the sensor to input parameters

of the wireless unit. Sensor with analog output must

used amplifier and AD converter. Wide amount of

sensors can be used for realization of the above

design system.

The system is designed like multisensor system to

be able to work with independent thermal sensors

and switch units. The reach of arrangement on of

free space is 300 m, the reach decreases in housing

development. The communication is two

bidirectional among parts of system, i.e. the data

transmission to the control unit as well as also to the

wireless sensors and switch units. Used circuit

includes also AD converter. Therefore is possible to

measure and data transfer after minority adjustments

of wireless sensors. Designed programme of control

unit occupies only part memoirs. That makes next

development of function and user's up-to-date

facilities. The program expansion is possible also in

wireless units. The control unit is patchable with

computer and one makes it possible extract user's

programmes. The cooperation with computer is

possible to extend using set-up, entry programmes or

transmission measured data to PC. The system can

be extended also alongside hardware.

ACKNOWLEDGEMENTS

This research has been supported by the research

program No. MSM6840770015 “Research of

Methods and Systems for Measurement of Physical

Quantities and Measured Data Processing” of the

CTU in Prague and partially by the Czech Science

Foundation project No. 102/06/1624 “Micro and

Nano Sensor Structures and Systems with

Embedded Intelligence”.

REFERENCES

Varadan,V.K. et al., RF MEMS and Their Applications,

Wiley, 2003.

Kirianaki,N.V.,et.al., Data Acquisition and Signal

Processing for Smart Sensors, Wiley, 2002.

http://www.nordicsemi.no, 2006.

http://www.atmel.com, 2006.

http://datasheets.maxim-ic.com/en/ds/DS620.pdf, 2006.

http://datasheets.maxim-ic.com/en/ds/DS1302.pdf, 2006.

http://downloadcentre.omron-industrial.com.

http://pdf1.alldatasheet.com, 2006.

http://datasheets.maxim-ic.com, 2006.

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