Designing Wireless Sensor Nodes

Designing Wireless Sensor Nodes

Marcos A.M. Vieira1, Adriano B. da Cunha2, and Diogenes C. da Silva Jr.2

1 Dept. of Computer Science, Federal University of Minas Gerais,2 Dept. of Electrical Engineering, Federal University of Minas Gerais,

Av. Antonio Carlos, 6627, Belo Horizonte, MG, [email protected], {adborges, diogenes}@cpdee.ufmg.br

Abstract. Wireless sensor networks are networks of large quantities ofcompact microsensors with wireless communication capability. Emerg-ing applications of data gathering range from the environmental to themilitary. Architectural challenges are posed for designers such as com-putational power, energy consumption, energy sources, communicationchannels and sensing capabilities. This work presents the current state-of-the-art for wireless sensor nodes, investigating and analyzing thesechallenges. We discuss the characteristics and requirements for a sen-sor node. A comprehensive comparative study of sensor node platforms,energy management techniques, off-the-shelf microcontrollers, batterytypes and radio devices is presented.

1 Introduction

A wireless sensor network (WSN) is composed of many autonomous and compactdevices called sensor nodes. The objective of this network is to collect data.The availability of integrated low-power sensing devices, embedded processors,wireless communication kits, and power equipment are enabling the design ofsensor nodes.

Wireless Sensor Network has the potential for many applications and somealready exists, for example in a large metropolis to monitor traffic density androad conditions; in engineering to monitor bridges and buildings structures; in aforest for fire detection, in other environments like oceans and air resources; inprecision agriculture; in disaster recovery service; in condition based maintenancedevices like powerplants; in biomedicine. Other applications include managingcomplex physical systems like airplane wings and complex ecosystems.

A sensor node is composed of a power unit, processing unit, sensing unit, andcommunication unit. The power unit has the purpose to supply the energy to thenode. The processing unit collects and processes signals captured from sensorsand transmit them to the network. Sensors devices are devices that produce ameasurable response to a change in a physical condition like temperature andpressure. The wireless communication channel enables a medium to transfersignals from sensors to exterior world (provided by a gateway), and also aninternal mechanism of communication to establish and maintain of WSN.

S. Vassiliadis et al. (Eds.): SAMOS 2006, LNCS 4017, pp. 99–108, 2006.c© Springer-Verlag Berlin Heidelberg 2006

100 M.A.M. Vieira, A.B. da Cunha, and D.C. da Silva Jr.

Power consumption is and will be the primary metric to design a sensor node.While there is the Moore’s Law, that predicts doubling the number of transistorsof microelectronic chips every 18-month, and Gilder’s Law, which theorizes thatthe total communication bandwidth triples every year, there is no equivalentforecast for battery technology.

The objective of this work is to present the design considerations and discussthe component choices for a device for WSN. We present the state-of-the-art forsensor node architectures, investigating and analyzing some of the architecturalchallenges posed by these devices, including a survey of sensor node platformsand energy management techniques. A comparative study of component-off-the-shelf (COTS) such as microcontrollers, battery types, and radio devices, whichare very important for system design, is presented. The design focus on individualcomponents and not in subsystem level details. We also discuss some architectureissues and design trade-offs.

2 WSN Architecture

This section gives an overview of the WSN architecture. WSNs are networkscomposed of a large number of sensor nodes. The objective of these networks isto collect data. Sensor nodes are usually deployed over a desired area, then theywake-up, self-test and establish dynamic communications among them, compos-ing a network [1].

Fig. 1 illustrates a WSN. Each dot represents a sensor node. Each device sensesthe environment, processes andusually transmits the data to gatewaynodes. Gate-way nodes transmit their data to an external observer called base station. Gatewaynodes are ordinary sensor nodes or more complex devices, having more computa-tional capabilities like greater radio range and more computational power. Thediscussion of gateway nodes is outside the scope of this work.

In a conventional network, such as cellular phone or local wireless networks,communication between computational elements is done through radio basestations, which represent a communication infrastructure. WSNs usually donot have such a communication infrastructure. That is why a wireless sensor

Sensor Node

Gateway Node

Base Station

Fig. 1. Wireless sensor network architecture

Designing Wireless Sensor Nodes 101

network is considered as a special type of ad-hoc network, since its topology isdynamic, due to the fact that sensor nodes can wake-up joining the network,or go to sleep, leaving the WSN. An important characteristic is that the flowof data is typically unidirectional. The information flows from source nodes togateway nodes.

A key resource of a WSN is the stored energy. Each sensor node is composed of asmall battery, with limited capacity. It is almost unfeasible to recharge all batterysince WSN can be composed of thousands of sensor nodes. Therefore, the WSNproject focus, from hardware design to network protocols, is to save energy. Othersensor node restrictions include memory capacity and processing power.

A WSN tends to be application-dependent, in other words, the hardwareand software requirements and the operation modes vary according to theapplication.

3 Characteristics, Requirements and Components

In this section we discuss some characteristics and requirements of a sensornode. The first question the designer should answer is if the sensor node willbe a real-life sensor node or a prototype. While in a real product size and costare essential requirements, a prototype focus the design in a system ease toexpand with a number of sensors, robust and easy to reprogram. Following isthe design considerations, characteristics and requirements when designing aprototype sensor node:

Energy-Efficiency. Sensor nodes must be energy efficient. Sensor nodes havea limited amount of energy that determines their lifetime. Hence, energy is thekey resource, being the primary metric for analysis. Sensor node project shouldfocus on energy-efficient COTS.

Low-Cost. It is desirable that sensor nodes be cheap since WSN may havehundreds or thousands of sensor nodes.

Wireless Communication. The sensor node needs to be wireless. In many ap-plications, the environment being monitored does not have installed infrastruc-ture for communications. Laying wires may be too difficult or expensive. Thedata rate is low, and a short range transceiver in a license free band is sufficient.The sensor node should receive and transmit, needing a bidirectional communi-cation channel.

Processing. Each sensor node should be able to process local data, using filter-ing and data fusion algorithms to collect data from environment and aggregatethis data.

Programmability. Since this node will be a test prototyping, it will be oftenreprogrammed for development of communication protocols and applications forWSN. Hence, the programming should be easy.


Expansibility. The hardware design must be expandable with a number ofsensors to support a variety of applications. Some projects defined a genericsensor bus for future expansion.

Size. For demonstration purposes the devices should be reasonable small. Butsize is of less importance in our project since it does not need to be as small asa real-life wireless sensor nodes.

Power-Aware. The hardware should be able to estimate how much energy isavailable in order to allow algorithms to adapt themselves to the available power.

4 Sensor Node Functional Components

A sensor node, as shown in Fig. 2, is composed of four major blocks: powersupply, communication, processing unit, and sensors. The power supply blockhas the purpose to power the node and usually consists of a battery and adc-dc converter. The communication block consists of a bidirectional wirelesscommunication channel. Most platforms use a short-range radio. Other solu-tions include laser and infrared media. The processing unit is composed of in-ternal memory to store data and applications programs, a microcontroller toprocess data and an Analog-to-Digital Converter to receive signal from the sens-ing block. The sensing unit block links the sensor node to the physical worldand has a group of sensors and actuators that depends on the application of theWSN.

Sensors

AD

C

Memory

MCU

Communication Processing unit Sensing

Radio

Bat

tery

DC

-DC

Power supply

PowerManagement

NetworkProtocols

OperatingSystem &Algorithms

Filtering &

Data fusion

Fig. 2. Sensor node block diagram

Sensor nodes may also have a storage unit or a debugging unit. The storageunit is an external memory device that works as a secondary memory, keeping adata log. The debugging interface is used to program and test the sensor node, forexample, programming interface, LEDs, serial interface, JTAG (IEEE 1149.1).This block can be omitted in a final sensor node product.


5 Processing Unit

Since the sensor node is expected to communicate, process and gather sensordata, sensor nodes must have processing units. The central processing unit of asensor node determines to a large degree both the energy consumption as well asthe computational capabilities of a sensor node. Many different types of CPUscan be integrated into a sensor node and they are discussed in this work. Thereare a large number of commercially available microcontrollers, microprocessorsand programmable logic, which allows great flexibility for CPU implementations.

In general, microcontrollers are microprocessor with additional peripheral orsupport devices embedded in a single chip [2]. Microcontrollers include not onlymemory and the processor unit, but also non-volatile memory and interfaces suchas UART, USB, SPI and I2C, and peripherals such as A/D Converters (ADCs),counters and timers. Therefore, a single chip microcontroller can interface todigital and analog sensors and to communication devices, such as a short-rangeradio module, to compose a sensor node.

The sensor node microcontroller needs to be energy-efficient, with differentoperating modes, and fast wake-up time. It does not need to have high computingpower as a 32-bit microcontroller. The MCU should have an embedded systeminterface to facilitate the programming and debugging phases. An importantfeature is the start-up time, since the MCU of a sensor node will usually go toidle mode, but this feature is not very often divulged.

Static dissipation and dynamic dissipation are the two components that es-tablish the amount of power dissipated in a CMOS (Complimentary Metal OxideSemiconductor) circuit. The most important one is the dynamic dissipation be-cause it is due to the switching transient current and the charging and discharg-ing of load capacitances. In this case, the power dissipation is proportional to theclock frequency of the device and the capacitance of the load and the SQUARE ofthe voltage margin between low to high levels. Thus, dynamic power dissipationmay be limited by reducing supply voltage, load capacitance and the frequencyat which the logic is being clocked. That is why, applications should use low clockfrequency for energy conservation and time keeping, but it also should use highclock frequency for fast reaction to events and fast burst processing. The fasterit finishes processing, the longer it remains at low-power mode. Thus, changingthe operating clock frequency is a much desired capability.

Table 1. Comparison of commercial microcontrollers

Characteristic Bits Flash Power Active

ATMEGA128L 8 128KB 5.5mA @4MHz@3VMSP430F169 16 60 KB 2mA @4MHz@3VStrongARM 1100 32 N/A 230mW @133MHzDragonBall MC9328MX1 32 N/A 120mA @96MHz


Table 1 shows a comparison of actual microcontrollers. Microcontroller Con-trol Units (MCUs) have many attributes like word size (number of bits), flashmemory capacity, memory size, number of ADCs and timers, operating voltage,current consumption and power modes. In the rest of this section we will discusssome interesting MCUs for WSN.

The Texas Instruments MSP430F169 is a 16-bit, 8 MIPS, and a ultra-lowpower CPU. It has 60Kbytes of program memory and 2Kbytes of data memory.It is equipped with a full set of analog and digital processors. It has embeddeddebugging and in-system flash programming through a standard JTAG inter-face. Texas has developed a new member of this family, the MSP430F1611 with10Kbytes of RAM.

The ARM family has floating-point computational capabilities, which makesit suitable for devices demanding more computational power, such as high per-formance gateways. The Intel SA1100 is a general-purpose, 32-bit RISC mi-croprocessor based on the ARM architecture rated as the most efficient 32-bitlow-power processor (in MIPS/Watt). The processor has three states: normal,idle and sleep that can be controlled to manage power consumption. Intel isupgrading the StrongARM SA-1110 processor-based designs to Intel PXA255processor or Intel PXA26x processor family-based designs.

The Motorola DragonBall MC9328MX1 is a 32-bit CPU with a BluetoothAccelerator radio interface, an Analog Signal Processing (ASP) Module, a Mul-timedia Accelerator (MMA), and a DPLL Clock and Power Control Module thatprovides power management capabilities.

The choice of MCU depends on application scenario. The ideal choice ofmicrocontroller is the one that matches its performance level with application’sneed. Other factors that affect the selection of the proper microcontroller besidesenergy level include word size, peripherals, memory, speed, physical size, price,availability, personal experience, and vendor support.

The majority of current projects on wireless sensor node are usingATMEGA128L or MSP430. Projects that need simple microcontroller and theknowledge of personal experience are using the ATMEGA128L. Projects thatneed low-power capability and want to study and apply the Dynamic VoltageScaling (DVS) technique and need more computing power are using MSP430and ARM family CPUs.

It is desirable to know the time when an event happens, like keeping recordwhen a sensor signal was read. Adding a real-time clock allows the sensor nodeto time and date stamp, and create a logbook. It is possible to create a real-time clock with the microcontroller (using the timer), but at the same time isalso desired to put the microcontroller in the low-power mode to save energy,which implies in turning off the timer. This solution would make the softwaremore complex. A simpler approach is to add a real-time clock device, usuallyexternal.

Many algorithms and applications require a large number of data to be stored.The amount of RAM in the microcontroller is limited. The solution to thisproblem is to add an external memory device that will work as secondary storage.


6 Power

The power supply block has the purpose to supply the energy to the node,and usually consists of a battery, but sometimes a DC-DC converter is used toboost the battery voltage. A voltage regulator can be added, whose purpose isto maintain the output voltage at a fixed value.

It might be possible to extend lifetime of a sensor node by extracting energyfrom the environment, such as light, vibration and RF. Continuum Control Corp.[3] has launched the iPower energy harvesters. These devices extract electricenergy from mechanical vibrations, motion, or impact. Amirtharajah et al. [4]have demonstrated a MEMS system that extracts electric energy from vibrations.

There are two major power saving schemas, Dynamic Power Management(DPM) [5] and Dynamic Voltage Scaling (DVS) [6]. These techniques can be clas-sified into static and dynamic. Static techniques are applied at design time, suchas compilation and synthesis for low power. Dynamic techniques are applied at runtime based on the variations in workloads. These techniques are called DPM.

The basic idea behind DPM is to shutdown the devices when not neededand get them back when needed. Turning off some components providing energysavings, but in many cases, it is not known beforehand when to turn on or offa particular device. A solution is a stochastic analysis to predict future events.An embedded operating system that is able to support DPM is also needed. Forthis approach, the devices should have, at least, the states: active, sleep and idle.Some CPUs offer several levels of active states with varying degrees of compu-tational power and energy consumption, leading to richer power managementpossibilities. However, it is important to consider that moving between theseoperating modes involves power and latency overheads.

The main idea behind DVS is to change the power to match the workload,avoiding idle cycles. DVS reduces the power consumed by a processor by lower-ing its operating voltage and frequency. By varying the voltage along with thefrequency, it is possible to obtain a quadratic reduction in power consumption.The problem is the fact that workloads are non-deterministic. For this approach,the microcontroller should permit to change its voltage supply and clock. Someapproaches uses the StrongARM SA-1100 MCU since it can vary voltage andfrequency from 59MHz/0.79V to 251MHz/1.65V.

7 Communication

Sensor nodes must communicate among themselves and also to a base stationusing a wireless communication channel. We explore optical and radio frequency(RF) channels. The sensor node communication channel needs to be bidirectionalto support different operating modes, to be energy-efficient, allows setting theoutput power, and have relatively slow date rate. The range can vary from tensto about a hundred meters magnitude.

Optical Communication. Two technologies for optical communications arethe laser and infrared. Laser communication has some advantages. It spends less


energy than radio over larger range; it is secure, since there is no broadcast andif a channel is intercepted it would interrupt the signal; and there is no need forantennas. There are some disadvantages. It needs line of sight and the laser beammust be lined up with the receiver. Lasers are sensitive to atmospheric conditionsand, finally, since the communication is directional and due to the fact that sen-sor nodes will be deployed randomly, this makes them not an attractive solution.Infrared communication is also directional. An interesting solution adopted byPushPin project [7] is to use an optical diffuser made of sandblasted polycar-bonate tubing to create a more omnidirectional communication range within aplane. But, the node still needs to be aligned within that plane. PushPin projectadopted the IrDA protocol. Its disadvantage is a short-range of about 1m. Theadvantage of infrared is no need for antennas.

Radio-Frequency. RF communication is based on electromagnetic waves. Oneof the most important challenges in RF communications devices is the antennasize. RF communication advantages are its ease of use, integrality, and well es-tablished in the commercial marketplace, which make it an ideal testing platformfor sensor nodes. Several aspects affect the power consumption of a radio includ-ing the type of modulation, data rate, and transmission power. In general, radioscan operate in three distinct modes of operation: transmit, receive, idle. Mostradios operating on idle mode results in high power consumption, almost equalto receive mode, thus, it is important to shutdown the radio.

8 Sensing Devices

The sensing unit is composed of a group of sensors, which are devices thatproduce electrical signals to a change in a physical condition. Sensors can beclassified as either analog or digital devices depending on the type of output theyproduce. This work does not intend to enumerate all sensor types, but to studytheir design trade-offs since many types of sensor exists, such as: magnetometer,accelerometer, light, temperature, pressure, and humidity. Given the diversity ofsensors, there is no typical power consumption. The type of sensor to be used ina sensor node will depend on the application.

Besides energy consumption, another important feature of a sensor is itsstartup time, in other words, the minimum time after turned on to producecorrect sample data. It is desirable that the startup time be as small as possiblebecause it is required to turn off the sensors to reduce energy when they are notbeing used.

Analog sensors need an Analog-to-Digital Converter (ADC). In general, mi-crocontrollers have embedded peripherals that include ADCs and sensor boardsdo not need dedicated ADCs. The MSP430 family is capable of 200,000 sam-ples per seconds (ksps) of 12 bits divided in eight channels. For complex sensorboards that need higher sample rates or a larger channel number, the solutionis to add ADCs to the sensor boards.

Depending on the sensor type, it can change the sensor node design. Forexample, an image sensor would need a very high communication bandwidth,


which would require a communication block redesign. When designing a sensornode, it is desirable to build a sensor block that is easily expandable to supporta variety of applications. Most of the sensor node prototypes define a sensor busthat works as an expansion connector.

9 The Bean Wireless Sensor Node

BEAN (Brazilian Energy-Efficient Architectural Node) [8] is a sensor node plat-form designed at the Federal University of Minas Gerais, Brazil. BEAN is thefirst sensor node that allows the measurement of power consumption of eacharchitectural block. To our knowledge BEAN is the first sensor node designed inBrazil. Fig. 3 shows a picture of a BEAN prototype.

BEAN uses the MSP430F169 microcontroller. An external memory devicewas added to work as secondary storage. It is a 4 Mbit (512K x 8) serial flashmemory and can be switched to a low power mode when it is not used. The com-munication channel uses the Chipcon CC1000 radio transceiver. It was desirableto design a sensor node that could be expandable to support a variety of appli-cations. BEAN defines a sensor bus that provides a user interface for additionalsensor boards. Each sensor node has a unique electronic identification. BEANprovides support for real-time application using an external real-time clock chip.Finally, for debugging objectives, BEAN has four LEDs, and a JTAG interfaceused to program and debug the microprocessor. Since BEAN is a prototype,

JTAGInterface

PowerSupply

SensorBus

CPU

ExternalMemory

Real-Time ClockLEDs

RadioInterface

Fig. 3. BEAN prototype


it uses an external power supply, and an internal voltage regulator was added.The power budget for BEAN is about 10.3 µA at the power down state. Duringtransmission power consumption is about 17 mA.

BEAN also includes software components and provides an Application Pro-gramming Interface (API). The API is a set of functionalities to control andconfigure and also provides services of the hardware components. The API com-municates with an upper layer that is usually a WSN dedicated microkernel.

10 Conclusions

Wireless sensor networks present fascinating challenges for the application ofdistributed signal processing and distributed control. These systems challengethe applications of appropriate techniques to construct cheap processing unitswith sensing nodes considering energy constraints.

We presented the design considerations and components choices, investigat-ing and analyzing some of the architectural challenges posed by these deviceslike computational power, energy consumption, energy sources, communicationchannels and sensing capabilities.

This work is a guideline for developers, who want to design a microcontrollerbased sensor node platform. We also discussed some architecture issues, designtrade-offs, wireless communication channels, and impact on the WSN protocolstack.

References

1. L. B. Ruiz, J.M.N., Loureiro, A.A.F.: Manna: A management architecture for wire-less sensor networks. IEEE Communication Magazine 41 (2003) 116–125

2. Berger, A.S.: Embedded Systems Design, An Introduction to Processes, Tools andTechniques. (2002)

3. Continuum Control Corporation: ipower. Available on: http://www.powerofmotion.com (2005)

4. R. Amirtharajah, S.M.: A micropower programmable dsp powered using a mems-based vibration-to-electric energy converter. In: IEEE Intl. Solid-State CircuitsConference. (2000) 362–369

5. Wang, A., Chandrakasan, A.: Energy efficient system partitioning for distributedwireless sensor networks. In: IEEE International Conference on Acoustics, Speechand Signal Processing. (2001) 905–908

6. Pillai, P., Shin, K.G.: Real-time dynamic voltage scaling for low-power embeddedoperating systems. In: ACM Symposium on Operating Systems Principles. (2001)99–102

7. Lifton, J., Seetharam, D., Broxton, M., Paradiso, J.: Pushpin computing systemoverview: a platform for distributed, embedded, ubiquitous sensor networks. In:Pervasive Computing Conference - LNCS 2414. (2002) 139–151

8. Vieira, M.A.M.: BEAN: A computational platform for wireless sensor networks (inportuguese). Master’s thesis, C. S. Dept. - Federal University of Minas Gerais, BeloHorizonte, Brazil (2004)

Designing Wireless Sensor Nodes

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