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1344 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 7, JULY 2008

USB Sensor Network for Industrial ApplicationsAlessandro Depari, Member, IEEE, Alessandra Flammini, Member, IEEE,

Daniele Marioli, Member, IEEE, and Andrea Taroni, Member, IEEE

AbstractThis paper proposes a universal serial bus (USB)solution for sensor networking. First, network architecture hasbeen presented to have, as its primary objective, its integrationwith existing infrastructure. For this reason, a USB-to-Ethernetgateway has been introduced. Then, many aspects of industrialapplications have been considered to realize a suitable solution.Insulation problems and low-cost implementation have beentackled. Working prototypes of each network component havebeen defined and realized: USB host + gateway, USB insulator,and USB hub. Several commercial USB devices can be used assensors. Finally, some experiments have been carried out: Timingperformances, network activities, and power consumption havebeen tested.

Index TermsEthernet, galvanic insulation, smart sensor,universal serial bus (USB).

I. INTRODUCTION

THE UNIVERSAL serial bus (USB) [1] is one of the most

widespread technical innovations in personal computer

and home consumer applications over the past few years: Key-

boards, mouse devices, printers, webcams, and several other

computer peripherals are available with this type of connection.

In addition, USB is also employed in several measurement

applications [2], [3].

This success is due to some bus characteristics, such as

simplicity, plug & play features, hot plug support, and,particularly, the possibility of supplying power to the devices.

In industrial application projects, as well as consumer projects,

cabling costs and maintenance are important factors. Reducing

the need to carry power supply to peripherals can yield several

advantages. Other advantages are attained by the USB transmis-

sion type, which is differential and therefore offers good noise

immunity. In addition, the quick diffusion of USB has led to a

drastic fall in component prices. All these characteristics make

USB suitable to be used as a simple sensor network in industrial

environments. In fact, if compared to fieldbuses (PROFIBUS-

DP, DeviceNet, etc.), it is quite prevalent in every personal

computer (PC; desktop, portable, industrial, PC board, etc.).

Moreover, with respect to widespread Ethernet, it can supply

power to sensors and actuators if their current consumption

does not exceed 500 mA (as defined by USB specifications),

Manuscript received March 14, 2006; revised November 30, 2007.A. Depari, A. Flammini, and D. Marioli are with the Department of Elec-

tronics for Automation, University of Brescia, 25123 Brescia, Italy (e-mail:alessandro.depari@ing.unibs.it).

A. Taroni is with Carlo Cattaneo University, LIUC, 21053 Castellanza (VA),Italy.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2008.915487

and up to now, it is more diffused and of low cost with respect

to new Ethernet 802.3af.

Today, available USB sensors [4] are generally designed

for home or consumer applications, whereas fieldbuses are

normally used in interface sensors.

A possible employment of USB as a fieldbus could allow

clear advantages in terms of cost, standardization, and wiringreduction in simple and small systems. Nowadays, this objec-

tive has still not been realized because of the requirements

imposed by industrial applications (protection, insulation, andpackaging).

The objective of this paper is mainly to propose a USB-based

sensor network architecture. Specifically, the most critical prob-lem about safety and insulation is experimentally approached

by the authors to demonstrate the effectiveness for an industrialenvironment.

II. PROPOSED ARCHITECTURE

The proposed sensor network with USB connectivity is de-

picted in Fig. 1. The host manages communication with USB

devices and links the network to a higher level unit. Therefore,the host must have appropriate interfaces with, for example,

fieldbus- or Ethernet-based factory networks.

USB specifications define the typical USB network topologyas a tiered-star architecture. To create this kind of architecture,

USB hubs are needed. These components are easily available,both as commercial products and as ad hoc solutions. In fact, we

can find several commercial off-the-shelf chips that implement

all USB hub functionalities and allow the creation of thedesired hub with few external components. USB devices are

the terminal users of the USB network. They can be specific

USB transducers or USB smart sensors that are compliant withIEEE 1451 [5], which we call the USB Smart Transducer

Interface Module (U-STIM) [6].In industrial applications, it is necessary to take any pre-

caution to avoid damages due to the particularly hostile en-

vironment. USB was not developed to operate under theseworking conditions. Hence, it has no safety measures against

failures. For instance, a short circuit in a network node could

seriously damage others nodes. Usually, USB devices mostlysuffer from a hostile environment, whereas the USB host is

generally located in a protected area. For this reason, network

devices should be isolated from each other by means of agalvanic insulator (hereinafter called the USB insulator).

III. NETWORK COMPONENT

A USB network is composed of several parts that must

be adapted for industrial needs. In the following, a detailed

0018-9456/$25.00 2008 IEEE

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DEPARI et al.: USB SENSOR NETWORK FOR INDUSTRIAL APPLICATIONS 1345

Fig. 1. Proposed industrial USB network.

Fig. 2. Scheme of the proposed USB host with Ethernet LAN interface.

description of each component is presented according to the

proposed architecture.

A. USB Host

In an industrial integrated environment, the USB host mustbe provided with a high-level network interface. Ethernet can be

a valid solution because of its great diffusion, even in industrial

plants. Supported protocols could be the well-known Transmis-

sion Control Protocol/Internet Protocol (TCP/IP) or emerging

industrial protocols, such as PROFINET [7] or Ethernet/IP [8].

The use of a PC system to perform the functionalities of the

USB host and the gateway does not seem to be a good solution

since the primary objective is a low-cost architecture.

The proposed USB host solution (Fig. 2) consists of a

microcontroller-based system. A microchip 8-bit microcon-

troller (PIC 18F452) has been used to manage both the interface

with the sensor network and the bridging with the high-levelnetwork. The USB bus control is achieved by means of a host

controller (Cypress SL811HS), whereas the LAN interface is

remitted to a 10-BaseT Ethernet controller (Crystal CS8900A).

The interconnections between the microcontroller and these

devices have been attained by means of the implementation

of parallel buses using the microcontroller ports and a little

external glue logic. All these components are of low cost and

are easily available, so the global USB host is a cheap and

compact adapter.

The microcontroller software consists of two different parts:

One manages the USB dialog, and the other one is related to the

communication with LAN. Particularly, a well-tailored TCP/IP

stack has been implemented to allow data exchange betweenthe users on the LAN [9].

The USB host software handles communication with USB

devices, so it must do several things. Detect if a new device

is connected and consequently configure it with appropriate

parameters, recognize if a unit is detached, and provide or

receive data from the USB network. For these purposes, acomplete implementation of the USB device driver is not

needed. In fact, the host does not directly manage data, which

must be exchanged with the high-level LAN (e.g., Modbus

over TCP/IP). In the proposed host, a dual-port memory mech-

anism has been used for data exchange. The host simply

extracts data from incoming TCP packets or inserts data into

outgoing packets, without performing any operation on data

meaning.

B. USB Insulator

It is currently very difficult to find commercial low-cost USB

insulators, because USB is not commonly used for applica-tions requiring a particular degree of protection. Anyway, the

implementation of a USB galvanic insulator is compulsory to

protect USB devices, and our proposed solution is depicted

in Fig. 3.

Signal line insulation is achieved by means of optoelectronic

devices (optobarrier). A programmable logic device (PLD)

manages the bidirectional signal flow. In fact, USB signal lines

are bidirectional, and a logic arbiter is necessary to avoid

collisions.

The PLD must handle the signals from source to destination

in real time, because the USB insulator propagation delay must

be limited. The decisional logic always listens to both the hostand device sides. When one starts the packet transmission, the

communication in the opposite way is automatically temporar-

ily turned off, and data coming from the transmitting unit are

replicated on the other side. In the realized prototype, a 48-MHz

clock allows input line oversampling. Therefore, the length of

bit time is preserved at low speed (1.5 MBd) as well as full

speed (12 MBd). This arbiter was developed using a Cypress

CPLD (CY37256V).

The insulator must appear at the USB host side like a device

and at the device side like a USB host. When no devices are

attached, the insulator operates like it is disconnected from the

bus. When a device is attached, the insulator recognizes the

link speed and configures its host interface to emulate a deviceconnection with the same speed.

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Fig. 3. Scheme of the proposed optical USB insulator with both dcdc converter and external 5-V power supply.

Fig. 4. Pull-up and pull-down resistors in full-speed and low-speed connections.

The host-side transceiver is a MAX3343 from Maxim. It has

an integrated pull-up resistor that can be connected to the D+

or D lines, as requested by USB specifications for the speed

selection (Fig. 4). This is achieved by means of a signal driven

to the desired value by the PLD, according to the connected

device. An additional line allows the resistor to be disconnected

from both signal lines, which happens when no devices are

attached to the bus.

The device-side transceiver is a 1T11A from Fairchild. It has

no integrated pull-up resistor for the speed selection, but in this

application, it must be configured to act as a host transceiver by

means of two external pull-down resistors (Fig. 4).

The optobarrier is realized using HCPL2430 optocouplers

from Agilent. These components have been adopted due to the

high-speed characteristics (20 MBd) needed for the full-speed

connection.

The power supply insulation is realized using a dcdc

converter with insulated input and output. A high-efficiency

converter is required to transfer the requested power to the

devices. Alternatively, an external power supply could be used

if the insulator cannot receive from the USB host the sufficient

current to supply itself and the attached devices. The modelused in this prototype is a TEL 3 0511 from Traco Power, which

Fig. 5. Scheme of the insulated USB hub.

is a 5-V-to-5-V dcdc converter with insulated input and output

and efficiency on the order of 70%.

C. USB Hub

USB hubs are requested by USB specifications to allow the

connection of more than one USB device. They can also be

used to extend the maximum link length and, therefore, the size

of the network. In fact, a single USB cable length is limited

to 5 m, and using the hubs, it is possible to reach distances of

20 m between the host and a device (USB specifications limitthe number of cascade hubs to three).

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Fig. 6. Scheme of the proposed IEEE1451.2 smart sensor with USB connec-tivity (U-STIM).

In the proposed architecture, the USB hub can be a com-

mercial, as well as a self-made, system. The latter solution is

allowed by the availability of low-cost and flexible integrated

circuits that implement all the USB hub functionalities. In

addition, with this solution, it is possible to realize an insulated

USB hub by placing the insulation blocks on the same unit of

the hub (Fig. 5). Therefore, the network is simpler, because

there are fewer units, and the devices can be directly connectedto the USB hub.

The realized USB hub employs a commercial ISP1122 by

Philips. It can operate in self-powered or bus-powered mode

and supports full-speed and low-speed devices (USB 1.1).

The hub controller manages all operations, and only a few

external components are needed, mainly for the regulation of

power to the devices. In the realized hub, four downstream ports

are available.

D. USB Device

Up to now, USB transducers for industrial applicationsare not diffused, and USB connectivity could be useful for

PC peripherals, such as webcams and printers. In the past,

some sensors with USB connectivity have been developed for

home automation and Heat Ventilation and Air Conditioning

(HVAC) applications. Therefore, temperature, humidity, pres-

sure, and other sensors are utilizable in accomplishing specific

tasks.

Moreover, an IEEE1451 smart sensor with USB connectivity

(U-STIM) has been developed [6] and its structure is shown in

Fig. 6.

A USB Device controller and IEEE1451.2 [5] structures have

been realized using a hardware description language; in this

way it is possible to implement the system in an application-specific integrated circuit (ASIC). Only the STIM and the

transducer electronic data sheet (TEDS) elements of IEEE 1451

smart sensor have been developed, while the network capable

application processor (NCAP) level has been neglected.

The communication with the smart sensor is accomplished

by an emulated transducer independent interface (TII). Its com-

mands are sent using a simple protocol over USB.

IV. RESULT

In the following, some experimental results concerning de-

vice resources, timing, and power dissipation are reported foreach network component.

A. Required Resource

With regard to the USB host, the microcontroller software

takes about 70% of the total code memory space (40 kB).

Most of this space is taken by the Ethernet-side operations

(60%), whereas only 10% concerns the USB management. The

USB protocol is easier to implement than the TCP/IP stack

commonly used to exchange data through Ethernet.The VHSIC hardware description language (VHDL) pro-

gram for the USB Insulator PLD takes about 80 macrocells

of the 256 available in the employed device (CY37256V);

implementation in a smaller device, such as CY37128V, is

therefore possible.

Different devices can be included in the proposed network.

To have an idea of the USB interface complexity, a U-STIM

occupancy is on the order of 1000 macrocells.

B. Timing

To experimentally evaluate delays introduced by the realized

USB host, a point-to-point connection has been established

with commercial low-speed and full-speed computer peripher-

als. An Agilent 1692A logic analyzer has been employed to

measure time intervals. Experimental results show that time

delay tsoft between the software transmission command (issued

to the analyzer through a dedicated line driven by the micro-

controller) and the effective packet transmission on the USB

bus is on the order of 50 s, as shown in Fig. 7. This delay,

which, in this particular case, is estimated when connecting

a low-speed device, is due to both the software and intrinsic

delay of the SL811HS host controller and is independent of the

device speed.

Ethernet communication timing has been evaluated usingthe Internet Control Message Protocol network service ping.

The realized Ethernet gateway has been connected to a PC,

which sends several ping requests to the device under test

and estimates the ping reply delay. In a not-so-busy 10-BaseT

network, the average ping time, with a data payload of 32 bytes,

is less than 2 ms. Additional tests have been performed to

estimate the protocol stack execution time, which has been

measured on the order of 2 ms. It should be noticed that

performances required by sensor networking are generally on

the order of tens of milliseconds.

Simulation tests that are related to the USB insulator show

a delay between the input and output signals of about 75 ns,as shown in Fig. 8. This delay is related to the PLD logic only

since it has been evaluated using the simulator included in the

PLD application development tool (Warp from Cypress), which

does not include models to simulate the transceivers and the

optocouplers.

The global delay, including the PLD logic arbiter, the USB

transceivers, and the optobarrier contributions, is shown in

Fig. 9, which reports a host transmission in a full-speed link.

The delay has been estimated using the Agilent 1692A logic

analyzer, directly detecting the USB signals from the insulator

USB connectors. It is clear that data coming from the host are

repeated to the device without any manipulation.

The overall delay is on the order of 160 ns, which representsabout twice the bit time in a full-speed transmission and about

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Fig. 7. Delay between the software transmission command and the effective packet transmission on the USB bus.

Fig. 8. Delay between the input and output signals of the PLD logic (obtained by timing simulation).

Fig. 9. Delay between the input and output signals of the USB insulator.

a quarter of bit time in a low-speed link. In fact, the delay is

independent of the speed and transmission direction.

The hub delay depends on the data rate and is on the order

of 50 ns for the full-speed link and about 300 ns for low-speed

connections.

C. Power Consumption

During a normal communication between the Ethernet and

the USB sides, the USB host/LAN gateway shows an averagecurrent consumption on the order of 120 mA at 3.3 V. Half

of it is drained by the Ethernet controller, whereas the block

concerning the USB controller takes only 20 mA.

Preliminary measurements concerning the realized USB in-

sulator show a current request on the order of 280 mA at 5 V.

Most of this load is due to the optocouplers and the 48-MHz

local oscillator. This value has been measured without any

device connected to the USB insulator, so that it can be

considered as the average no-load consumption. When a de-

vice is connected, the current required by the insulator in-

creases by a value that is greater than the current required

by the device, because the USB insulator must power the

bus through the dcdc converter, which does not have 100%efficiency.

If the system is limited to the low-speed data rate, low-

power optocouplers and a low-frequency oscillator could be

employed, limiting the power consumption. Alternatively, other

galvanic insulating components, such as magnetic isolators,

could be used. They require a current that is ten times less than

that for optocouplers (at the same speed). In addition, a dcdc

converter with better efficiency and a low-power PLD could

be utilized. This way, the global current consumption might be

easily limited to about 150 mA.

The current absorbed by the USB hub depends on the power

requested by the devices connected to the hub. In fact, it must

provide power supply to the bus-powered devices connected to

its downstream ports. It absorbs about 100 mA at 5 V for normal

operation.

The maximum current that the USB host can furnish is

about 500 mA, as indicated in USB specifications. If the global

current requested by the hub is greater than this limit, the USB

hub must be set to work in self-powered mode by means of an

external power supply.

V. CONCLUSION

USB has been widely accepted as a serial bus for homeapplications. In an industrial context, USB is not diffused since

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it lacks transducers and some safety options. In this paper,

a complete USB system for industrial applications has been

proposed, discussed, and implemented. An embedded USB

host with integrated Ethernet gateway capability is defined,

and a low-cost USB insulator for optical insulation of USB

network segments is introduced. Prototypes of several other

network components have been realized, and the whole networkhas been tested. Experimental results show that the timing

characteristics are comparable with the timing performances

of commonly used fieldbuses (few milliseconds), even with the

lower cost of the required components.

With regard to power consumption, a bus-powered archi-

tecture seems to be suitable only if low-speed USB devices

are employed. Over this network architecture, a data exchange

protocol must be implemented to guarantee the data security

and reliability needed in industrial applications.

REFERENCES

[1] USB Specifications rev 1.1. [Online]. Available: http://www.usb.org[2] T. Twardowski, Will be USB a standard of PCInstrumentation com-munication? in Proc. VXII IMEKO World Congr., Dubrovnik, Croatia,Jun. 2227, 2003.

[3] C.-P. Young, M. J. Devaney, and S.-C. Wang, Universal serial busenhances virtual instrument-based distributed power monitoring, IEEETrans. I nstrum. Meas., vol. 50, no. 6, pp. 16921697, Dec. 2001.

[4] Inside Out Network Sensors. [Online]. Available: http://www.ionetworks.com

[5] Inst. Electrical Electron. Eng., IEEE Standard for a Smart Transducer In-terface for Sensors and ActuatorsTransducer to Microprocessor Commu-nication Protocols and Transducer Electronic Data Sheet (TEDS) Formats,IEEE Std. 1451.2-1997, 1997.

[6] A. Depari, P. Ferrari, A. Flammini, D. Marioli, E. Sisinni, and A. Taroni,IEEE1451 smart sensors supporting USB connectivity, in Proc. SIcon,New Orleans, LA, Jan. 2729, 2004, pp. 177182.

[7] PROFINET Specifications. [Online]. Available: http://www.profibus.com

[8] Ethernet/IP Specifications. [Online]. Available: http://www.ethernet ip.org[9] A. Flammini, P. Ferrari, D. Marioli, E. Sisinni, and A. Taroni, Sensor inte-

gration in industrial environment: From fieldbus to web-sensors, Comput.Stand. Interfaces, vol. 25, no. 2, pp. 183194, May 2003.

Alessandro Depari (S07M08) was born inBreno, Italy, in 1976. He received the M.S.degree in electronic engineering and the Ph.D.degree in electronic instrumentation from the Uni-versity of Brescia, Brescia, Italy, in 2002 and 2006,respectively.

Since 2007, he has been an Assistant Professor(Researcher) with the Department of Electronicsfor Automation, University of Brescia. His researchinterests are signal conditioning and processing forchemical sensors, in particular, resonant and resistive

sensors for electronic noses, the development of sensor networks for distributedmeasurement, and the design of methods and digital electronic circuits fornumeric measurement instrumentation.

Alessandra Flammini (M99) was born in Brescia,Italy, in 1960. She received the M.S. degree inphysics (with honors) from the University of Rome,Rome, Italy, in 1985.

From 1985 to 1995, she worked on industrialresearch and development on digital drive control.From 1995 to 2002, she was a Researcher with theDepartment of Electronics for Automation, Univer-

sity of Brescia, where she has been an AssociateProfessor since 2002. She teaches several courseson measurements in industrial environments, digital

electronics, and microprocessor-based systems. Her research interests includethe design of methods and digital electronic circuits for numeric measurementinstrumentation, sensor signal processing, smart sensor networking, and field-bus applications.

Daniele Marioli (M04) was born in Brescia, Italy,on January 21, 1946. He received the M.S. degree inelectrical engineering from the University of Pavia,Pavia, Italy, in 1969.

Since then, he has been working on research and

educational activities with the Polytechnic Univer-sity of Milan, Milan, Italy, and the University ofBrescia, where he has been a Full Professor ofelectronics since 1990. In addition, he is the Chiefof the Department of Electronics for Automation,University of Brescia. He is the author and coauthor

of more than 200 scientific papers published in international and national journals and conference proceedings. He is the holder of four patents. Hisresearch interests are the design, realization, and test of sensors, electronicinstrumentation, and signal processing electronic circuits. His activities in thesefields are related to the realization of innovative sensors in thick-film tech-nology, based on the piezoelectric, pyroelectric, and piezoresistive behaviorsof screen printable pastes, and in MEMS technology, for the detection ofphysical quantities (e.g., acceleration, force, pressure, and mass); the realizationof high-resolution electronic instrumentation for capacitive measurements; thedesign and realization of integrated electronic circuits as the front end ofpiezoresistive-based sensors; the development of new linearization techniquesbased on neural networks; and the development of web sensors and wirelesssensors.

Andrea Taroni (A04M04) was born in Cotignola,Ravenna, Italy, in 1942. He received the M.S. degreein physics from the University of Bologna, Bologna,Italy, in 1966.

From 1971 to 1986, he was an Associate Professorwith the University of Modena, Modena, Italy. Since1986, he has been a Full Professor of electricalmeasurements with the Department of Electronicsfor Automation, University of Brescia, Brescia, Italy.Since 1993, he has been the Dean of the Facultyof Engineering, University of Brescia. Since 2007,

he has been rector of the Carlo Cattaneo University, LIUC, Castellanza(VA), Italy. He has done extensive research in the field of physical quantitiessensors and electronic instrumentation, both in developing original devices andpractical applications.