DESIGN OF A WIRELESS PULSE OXIMETER USING A MESH

ZIGBEE SENSOR NETWORK

José M. Castillo, Joaquín Olivares and José M. Palomares

Department of Computer Architecture, University of Córdoba, Córdoba, Spain

Keywords: Pulse Oximeter, Wireless, ZigBee, Mesh, RCM4510W, Xbee, Wireless Sensor Networks, Biotelemetry.

Abstract: This work describes a prototype to convert a conventional pulse oximeter into a wireless device. Samples

have been taken from a commercial Nellcor DS-100A probe using an FPGA. This programmable device is

integrated with an XBee wireless modem; using ZigBee protocol, data is sent through the wireless network

to a central server. The system generates alarms to the medical staff when vital signals are critical. Data can

be saved in a database allowing the review of monitored data furthermore, with these data, future research

on medicine can be started.

1 INTRODUCTION

In hospital, life constants of patients are monitored

by several systems, most of them use wires to

connect sensors to the monitoring device. These

systems are not comfortable and their mobility is

reduced, the hindering the medical work. Most of

these devices do not record the history of the

monitored constants.

This work presents the adaptation of a

conventional pulse oximeter (Tremper et al., 1989)

(Kamat, 2002) (Baran, 2006) to suppress its wire,

sending data in real time through a wireless sensor

network. Data can be stored in a PC, allowing the

review of monitored data; furthermore, with these

data future medical research can be started.

Pulse oximeter was the first device adapted,

future works will comprise other devices as non-

invasive blood pressure, temperature, fetal heart rate,

uterine contractions on labor, and, WHO partogram.

Telemetry wireless (Jovanov et al., 2000)

(Hameed, 2003) monitoring will allow the patient

mobility. By using wireless monitoring, both patient

satisfaction and working conditions for the staff is

expected to improve.

There are Bluetooth wireless pulse oximeters in

the market (Morón et al., 2005) (Nonin Inc., 2010).

In this work, ZigBee is the selected communication

protocol used in the wireless sensor network.

Bluetooth requires higher power consumption than

ZigBee. Another difference is that Bluetooth devices

use a point to point directive whereas ZigBee

devices constitute a mesh network enabling flexible

connection of new sensors.

2 ROUTING IN ZIGBEE MESH

NETWORKS

Pulse oximeter is a critical device and, therefore, it

needs real time communication, low latency, and the

possibility of alternative routes in case of connection

problems in remote monitoring. These requirements

make the mesh topology as the most appropriate.

The end-devices are Reduced Function Devices

(RFD) that can be in sleep mode periodically. They

cannot route and always send their data to their

parent node (a router or a coordinator). In the

association stage, the end-device selects its parent

and receives a notification with a short 16bit

address. If the end-device is restarted, it is declared

as an orphan and immediately, its router resends its

short 16bit address in order to enter into the network

again. If this process fails, the end-device will make

the association process again, until it can be

associated to another parent.

The main mission of a router consists in

forwarding data through the network and managing

the information of its end-devices as a buffer until

they wake up. The end-devices sleep periodically

and send a “Data Request” frame when they wake

up. The parent can send data stored in its buffer

401

Castillo J., Olivares J. and Palomares J..

DESIGN OF A WIRELESS PULSE OXIMETER USING A MESH ZIGBEE SENSOR NETWORK.

DOI: 10.5220/0003170104010404

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2011), pages 401-404

ISBN: 978-989-8425-37-9

 2011 SCITEPRESS (Science and Technology Publications, Lda.)



Figure 1: Global system and network architecture.

to the end-devices.

In mesh networks, routers and coordinators

discover the route to the destination point using

messages at network level as ”Route Request” and

”Route Reply”. If there is no direct communication,

messages will pass through the routers until they

reach their destination. It is possible to detect if the

route fails, thanks to the acknowledgment (ACK)

procedure held by 802.15.4. The router will search

for a new route in order to be able to send data to the

destination. In a full mesh network with n nodes,

each node has a link with the others n-1 nodes, with

a total of n(n-1) links. These topologies (partial-

mesh and full-mesh) can be set up by adding new

router nodes, with the subsequent increase of energy

consumption.

3 SYSTEM DESIGN

This work attempts to implement a wireless pulse

oximeter using ZigBee mesh network. Figure 1

shows the architecture of the network.

To design a Wireless Pulse Oximeter, the

following components have been used: Rabbit

RCM4510W, XBee USB, FPGA Spartan, Pulse

Oximeter Nellcor, and PC Windows XP.

3.1 Pulse Oximeter

Nellcor DS-100A is a popular pulse oximeter used in

hospital, and therefore it has been selected to use in

this work.

In order to couple the Pulse Oximeter with the

FPGA is necessary to know the timing diagram of

Nellcor (Webster, 1997), shown in Figure 2. The

Nellcor pulse oximeter system uses a four state

Figure 2: Timing Nellcor Diagram.

clock, or has a duty-cycle of 1/4, as compared to the

Ohmeda (Webster, 1997) system, where the duty

cycle is 1/3.

In the first quarter, the IR LED is on, whilst the

R LED is on in the third quarter. In the second and

the fourth quarters, these LEDs are off. During these

periods, ambient light measurements are done. The

gate pulses are the sampling pulses applied to the

input signal to separate out the R and the IR

components from the input signal. The sample pulse

during the OFF period of the respective LED is used

to sample the ambient. The gradual rise or fall of the

pulses is due to the transients, which are smoothed

out using low-pass filters. The ambient light

component is larger in the four quarter, compared to

the value in second. Using suitable values for the

gain in the programmable DC offset amplifiers, this

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ambient light component can be eliminated. The AC

plus the DC components of the R and IR signals are

digitized in the FPGA in order to send this data to

the Rabbit RCM4510W.

3.2 FPGA

The FPGA used in this work is a Spartan 3E

equipped a XC3S500E chip. In this FPGA, a

MicroBlaze processor has been designed, with a

Delta Sigma AD converter IP Core included.

Xilinx Platform Studio (XPS) software was used

in this work. This software has been developed to

manage the probe and to obtain the received

information in the photodiode. These data are

processed in order to obtain the values of pulse and

SpO

. These values are sent to the parallel port of

the RCM4510W.

The information is processed in the FPGA and it

is sent to the Rabbit via parallel port. Rabbit

decrypts this information and prepares the packet in

order to send it to its destination. Both, pulse and

oxygen values are sent on real time to the Database

Server (DBS).

3.3 Rabbit RCM4510W

RCM4510W RabbitCore is the ZigBee hardware

used to transmit the pulse oximeter data to the

coordinator. It is equipped with an on-board

ZigBee/802.15.4 modem for wireless connectivity.

Features also include 512K flash memory and

SRAM, 40 general-purpose I/O, and up to 9 general-

purpose I/O, 4 of which can be set up as analog

inputs via the ZigBee module.

The RCM4510W has a Rabbit 4000 processor at

29.49MHz. Other features include hardware DMA,

auxiliary I/O, quadrature decoder, input capture,

GPIO lines shared with up to five serial ports, and

four levels of alternate pin functions that include

variable phase PWM.

3.4 Xbee USB Coordinator

The coordinator is the most important element of the

network and therefore there should be one in a

network. The main function is to monitor the state of

the network and establish the routes that devices

have to use in order to send the information.

Java library Xbee-api.lib has been used to extract

the information from the coordinator. XBee class is

the main communication interface between a module

XBee and gateway to the wireless network. This

class coordinates the sent and received data by

packages. After the instantiation of the Xbee class,

the system will start to manage the information of

the network. The coordinator XBee USB is

connected to the COM3 port at 9600 data rate.

The software for managing the network has been

successfully developed to receive data from the

probe, and, to send it to the DBS.

3.5 Database Server

The DBS is implemented using MySQL Server over

a Linux Machine. This database stores the following

information about patients:

• Patient name

• Sex

• Age

• Historical Pulse

• Historical % SpO

• Drug treatment

Future researches will allow further processing

of the information about patients. Thus, the system is

able to determine real time if any patient suffers

from a cardiac or respiratory problem. In this case,

an alarm will be generated to inform to the staff.

Other possibilities are to compare multiple histories

to study population responses; responses after

different drug treatment, etc.

4 RESULTS

The reliability of the network and the developed

pulse oximeter was tested with these experiments:

4.1 Packet Tests

To check the reliability in the communications, it

was developed a program to count the number of

packets received in the coordinator. The test was

carried out using 5 Routers, and 3 End devices.

Electromagnetic contamination with Bluetooth and

Wifi signals was introduced.

The first test starts with the previously presented

configuration and the number of transmitted packets

reached 100%.

The second test tries to eliminate the connection

between the End device associated to the pulse

oximeter and its parent node. When the route

crashes, the End device has to re-associate to another

router. This operation delays for approximately 2

seconds (Casilari, 2010). In this time, the node

cannot transmit to the coordinator, however it stores

data in a buffer to avoid the loss of data. When the

node re-associates to another parent, it sends the

DESIGN OF A WIRELESS PULSE OXIMETER USING A MESH ZIGBEE SENSOR NETWORK

403

Figure 3: FPGA, RCM4510W and Nellcor Probe.

stored data in the buffer and continues normally.

During these 2 seconds, the display at the

coordinator shows the last received pulse and SpO

values. When the end device sends the new data, it is

processed and printed on the coordinator display and

stored in DBS.

If all routers crash, the End device tries to detect

if a direct connection to the coordinator is available.

As the network is redundant, when the End device

looses the connection to its parent, it reconnects

directly to another router transparently to the user.

4.2 Measurement Tests

In order to test the correctness of the data obtained

with the pulse oximeter integrated with the FPGA, a

comparison between these data, and those acquired

by a professional ISSO certified pulse oximeter used

in Hospitals, was studied.

The experiment used two probes, one of them

connected to the ISSO certified pulse oximeter and

the other one, connected to the FPGA pulse

oximeter. The values of pulse and %SpO

were

identical in both systems.

5 CONCLUSIONS

A prototype to adapt a standard pulse oximeter into a

wireless device has been proposed. Samples have

been taken from a commercial probe using an

FPGA. This programmable device packs the

information and, as it has been integrated with an

XBee wireless modem, it sends data through the

wireless network to the central server. In future,

FPGA and XBee will be integrated in a small

wireless device supplied with batteries.

Results in terms of the reliability and

technological viability have been proved to be

successful. Several tests were applied to check the

robustness of the system.

ZigBee provides a flexible network, which is

ideal for environments were multiple sensors can be

added or suppressed on-the-fly in any moment.

Zigbee networks are able to operate in densely Wifi

and Bluetooth populated environments. In front of

Bluetooth or Wifi, ZigBee consumption is notably

lower and XBee modems are cheaper. Besides, the

configuration of ZigBee devices is simpler since

Bluetooth has a more complex protocol.

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