Figure 1
International Journal of Scientific & Engineering Research, Volume 3, Issue 10, October-2012 1
ISSN 2229-5518
Design and Prototyping of PSoC based Pulse
Oximeter
Mahima Rathore
This paper attempts to understand the designing of a pulse oximeter and the application of PSoC in the same. Later it highlights the applications of both to produce a more efficient medical device.
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system architecture. It depicts the basic module of PSoC, which is a reconfigurable mixed signal system on chip. It
ulse oximeter has gained wide spread clinical
acceptance in the latter part of the 21st century. It is a medical device used for monitoring the oxygen content
of the arterial blood. It measures the absorption of light
passing through the body, which varies as blood pulses into the tissue with each beat of the heart. It made its first appearance in 1995 but gained recognition in 2000 [1].
PSoC based Pulse oximeter is built on a single chip i.e. Programmable system on chip, hence increasing its efficiency as well as its portability. PSoC features digital and analog programmable blocks, which themselves allow implementation of large number of peripherals. It combines the programmable digital and analog components, indicating that it is able to provide the signal amplifier, analog-to-digital converter/digital to analog converter, filter and PWMs required for pulse oximeter. PSoC was commercialised by Cypress Semiconductors in 2002 [2].
Our present technology using Atmel’s Atmega 16 microcontroller has an analog environment, separate from a digital core whereas the pulse oximeter on PSoC is a single chip mixed signal design. Pulse oximetry is a useful method of monitoring patients in many circumstances, in the face of limited resources and allows for the assessment of several different patient parameters. It is a standard method for monitoring preoperative and noninvasive conditions [3]. Due to its simplicity and speed, pulse oximeter is of critical importance in emergency medicine [4]. Moreover due to the low prices, an economical product can be manufactured which would benefit the poor.
In this section, we will illustrate the design of our proposed
PSoC based Pulse Oximeter. Figure 1 shows our overall
comprises of two parts – configurable digital blocks and configurable analog blocks. These blocks enable implementation and placement of predefined user configurable modules [5].
The digital blocks are state machines that are configured using the blocks registers. There are two types of digital blocks, Digital Building Blocks (DBBxx) and Digital Communication Blocks (DCBxx). Only the communication blocks can contain serial I/O user modules, such as SPI, UART etc. Each digital block is considered 8-bit resources that designers can configure using pre-built digital functions or user modules (UM), or, by combining blocks, turn them into 16-, 24-, or 32-bit resources. Concatenating UMs together is how 16bit PWMs and timers are created. The digital block here comprises of PWM, timer, counter and UART.[6]
PWM- Pulse Width Modulator is a device which controls the power of DC devices which draw up to few amperes of current.
UART- Universal Asynchronous Receiver/Transmitter is an individual (or part of an) integrated circuit used
for serial communications over a computer or peripheral device serial port.
In our system it is responsible for the wireless
transmission of data via IEEE 802.15.4 (zigbee - specification for a suite of high level communication protocols).
One more module used here is SPO2 logic block. It
receives its input in form of PPG signals
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International Journal of Scientific & Engineering Research, Volume 3, Issue 10, October-2012 2
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(photoplethysmographic signal) which is amplified, filtered and then fed to an ADC. It is coded with the logic of finding the SPO2 content of the blood [7].
Figure 1
There are two types of analog blocks: Continuous time (CT) blocks and Switch cap (SC) blocks. The continuous time (CT) blocks are composed of an op-amp circuit and designated as ACBxx where xx is 00-03. The other type is the switch cap (SC) blocks, which allow complex analog signal flows and are designated by ASCxy where x is the row and y is the column of the analog block. Designers can modify and personalize each module to any design [8]. The analog block here comprises of ADCs, DACs, filters, cap sense and amplifiers.
ADC- Analog to Digital Converter is a device that converts a continuous quantity to a discrete time digital representation. Here we have used a 12 bit resolution ADC-
because we need the best accuracy possible in order to give correct and accurate results. DAC - digital-to-analog converter (DAC or D-to-A) is a device that converts a digital (usually binary) code to an analog signal. Filters are the basic building blocks of signal processing used for separation of signals depending on definite criteria. TIA – amplifier-is a device for increasing the power of a signal by use of an external energy source. Cap sense- capacitive sensing is a technology based on capacitive coupling used in different types of sensors.
Figure 2 depicts the few basic modules of PSoC.
Figure 2
Pulse Oximetry is based on spectrophometric measurements of changes in blood color. The optical property of blood in the visible (between 500 and 700nm) and near-infrared (between 700 and 1000nm) spectral regions depends strongly on the amount of oxygen carried by the blood [9]. For this purpose a Pulse Oximeter consists of two LEDs: 660nm (Red LED) and 820nm (Infrared LED) and a photodiode chips mounted on a single substrate. Four LED chips for each wavelength are placed at equal distance around the photodiode to enhance the signal level as well as to minimize inhomogeneous effects of tissue [10]. Figure 3 shows a finger held wearable pulse oximeter.
PSoC based Pulse oximeter is itself a very efficient device but to enable ambulatory monitoring we need to add a triaxial accelerometer. It is designed with three orthogonal internal sensing elements to enable simultaneous multi-axis measurements (x, y, and z-axis). Its exact application will be explained in the later section (sec 4).
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Figure 4
The Electronic architecture of the PSoC based Pulse Oximeter is based on the Programmable System on Chip which helps in reducing the number of discrete components. In health care technology, it is important to miniaturize the devices to make them convenient to carry or wear and thus suitable for ambulatory monitoring applications.
PSoC’s user configurable modules and their API (“Application Programming Interface”) routines facilitate faster system development and programming, this is one of the main reasons for using PSoC . Indeed, it can carry out all the steps from signal conditioning to transmission of formatted data, including analog to digital conversion and data processing, all within a single part component. This increases the functionality and portability of the device at low cost thus making it available to common people.
The RF (radio frequency) module of the PSoC is a very useful low cost device which can increase its functionality to a large extent, helping in wireless communication [11].
A PSoC based Pulse Oximeter has the potential to revolutionise the field of automated healthcare. This device can further be developed to monitor the ambulatory motions, using a triaxial accelerometer sensor and a wireless sensor node. This could be called as ambulatory PSoC based pulse oximeter.
The proposed ambulatory PSoC based pulse oximeter monitoring system consists of two parts: wearable sensor devices part and server part. The wearable sensor devices are wearable sensor node with ECG and accelerometer sensor and a PPG module (the PSoC pulse oximeter); these are connected to a base-station and measure physiological data from human body [12]. The measured physiological data are transmitted to a base station via IEEE 802.15.4 using a Zigbee. The output waveforms of accelerometer and PPG sensor are shown at the server side.
Figure 4 shows overall system architecture of the ambulatory PSoC based pulse oximeter for ubiquitous- healthcare monitoring system [13].
Ambulatory data is helpful in determining the various physiological states of the human body. For example the data obtained for a man while he is sleeping or resting is different from the data obtained while he is walking, standing or performing any other action. This data, hence obtained through the PSoC based Pulse Oximeter can be used to monitor people who are highly susceptible to various health disorders. For example old people or people with serious ailments like AIDS or cancer.
Figure 5
Figure 5 shows the screen capture of the SpO2 monitoring program. This program was designed with Lab VIEW. It monitors the PPG signal by drawing the pulse waveform at server PC connected base-station as soon as it receives the SpO2 value (%) and heart rate from the
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ambulatory pulse oximeter. This system acts as a continuous event recorder, which can be used to follow up patients at home. ECG, acceleration and SpO2 value can be measured on the wearable sensors, and all measured data are saved in server wirelessly. It can make correct diagnosis even under situations where the patient is unconscious and does not have the ability to carry out daily activity and hence we can detect these conditions and inform the concerned or related people of the patient.
This application can also provide continuous breathing rate information by examining how a PPG signal is used by a pulse oximeter to determine arterial oxygen saturation and heart rate [14]. This can help combat medics and field commanders to better predict a soldier’s health condition and enhance their ability to perform remote triage procedures. By combining non-invasive physiological sensors with innovative microelectronics, mathematical models and equations, a wireless communication link, and global positioning, we can extract breathing rate information of the soldiers [15].
The PSoC based Pulse Oximeter is one of those modern medical devices which have reduced dimensions, increased durability and efficiency, and are less expensive as the electronic technology evolves to maturity. Further the development of its ambulatory system can help in advancement of the Ubiquitous Healthcare Monitoring system.
A comparative analysis between Microcontroller based Pulse Oximeter and PSoC based Pulse Oximeter definitely approves the latter. It owes to the fact that with PSoC the system results in a much smaller footprint and has more efficient functionality. PSoC enables wide range of integrated analog and digital circuits in a single chip.
The author would like to express her deepest gratitude towards Amit Badlani for guiding her throughout the course of the paper. The author is also grateful to her seniors Chirag agarwal and Sukant Dewangan.
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Author is currently pursuing B.E. in electrical &electronics engineering in Birla Institute of Technology and Sciences ,Pilani (BITS-Pilani), India. Email: maimarathore1@gmail.com
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