N. Madhu Mohan

Virtual Instrument for the Measurement of Haemo-dynamic Parameters Using Photoplethysmograph

This paper presents the design and development of a virtual instrument for the measurement of haemodynamic parameters namely, pulse rate and oxygen saturation in arterial blood based on the popular photoplethysmographic (PPG) principle. A clip-on sensor, housing red and infrared (IR) light emitting diodes and suitable photo detectors is developed. The sensor is interfaced to a PC utilizing the audio channel of the sound card, thus dispensing with expensive analog to digital converter hardware. Since the frequency response of the audio channel is not suitable for the PPG waveforms of red and IR, FM modulation and demodulation are employed. An empirical relationship is developed for the computation of the oxygen saturation in arterial blood using the red and IR PPG data and the well-known and well-established extinction coefficients of haemoglobin with and without oxygen. Data acquisition and processing are accomplished under LabVIEW virtual environment

Linearising dual slope digital converter suitable for a thermistor

To measure temperature using a thermistor as the sensing element, linearization to compensate for the inverse exponential nature of the resistance-temperature characteristic of the thermistor is required. A linearizing dual-slope digital converter (LDSDC) that accepts a thermistor sensor as input and provides a digital output that is directly proportional to the temperature being sensed is presented here. A logarithmic amplifier at the input of the LDSDC compensates for the exponential characteristics. The conversion logic of the underlying dual-slope converter is suitably modified to implement the required inversion and offset correction and thus obtain linearization over a wide range of input temperature. The efficacy of the proposed LDSDC is established through simulation studies and its practicality demonstrated with experimental results obtained on a prototype unit built and tested. Analysis of the proffered method to identify possible sources of errors is also presented.

Digital Converter for Differential Capacitive Sensors

A capacitance to digital converter that directly translates the changes in a differential type capacitive sensor to a proportional digital value is described in this work. The proposed method employs a suitably modified, dual-slope, analog to digital conversion technique. Error analysis and results obtained from a prototype built around a micro-controller establish the efficacy of the proposed method.

A Linear Variable Differential Capacitive Transducer for Sensing Planar Angles

A linear variable differential capacitive transducer for the measurement of planar angles from 0deg to 360deg is presented in this paper. The sensor part of the transducer consists of parallel plates of standard and easy to fabricate shapes and the signal conditioning electronics is made of a couple of simple relaxation oscillators. The output of the transducer is dictated only by a couple of dc reference voltages and hence high accuracy and linearity over the entire 0deg to 360deg are obtained easily by the use of precision dc reference voltages. Error analysis indicates that the sensitivity of the transducer for parameter variation is minimal. Test results obtained from a prototype indicated that the worst-case error was less than 0.1 %

Direct digital converter for a single active element resistive sensor

A direct digital converter that provides a digital output, proportional to the measurand being sensed by a single active element resistive sensor is presented in this paper. To accomplish this task, the structure and the switching sequence of a conventional dual slope, analog to digital converter (DSADC) is appropriately altered so that the altered DSADC accepts the resistance of the sensor as an integral part and provides a digital output that is linearly proportional to the physical quantity being sensed by the resistive sensor. Since the output of the dual slope resistance to digital converter (DSRDC) is dictated only by the magnitudes of a pair of DC reference voltages, a fixed value resistor and the transformation constant of the sensor, the error in the output is minimal. Hence, a high level of linearity and accuracy is achieved. Simulation studies establish the efficacy of the proposed scheme.

Contact-less, Multi-Spectral Imaging of Dermal Perfusion

Photoplethysmography (PPG) has come to be accepted as a common, non-invasive method for measuring the level of oxygen saturation in arterial blood. This paper describes a variation of the accepted PPG technique, to provide a contact-less and non-invasive method of characterizing dermal perfusion. It employs a 3-CCD, multi-spectral camera, to image the selected area. The acquired images are analysed by making use of the fact that the absorption co-efficient of oxygenated blood is more in the infra-red (IR) region of the spectrum, when compared to the red region. This fact is utilised to characterize the extent of dermal perfusion in the selected area of the skin.

A novel signal conditioning circuit for push-pull-type resistive transducers

A novel signal conditioning circuit suitable for push–pull-type resistive transducers is proposed. The circuit developed is capable of providing a linear output over a wide range of values of the measurand. Even when the transducer has an inverse relationship with the measurand, the circuit provides a linear output. The push–pull-type resistive transducer becomes an integral part of a relaxation oscillator, the duty cycle ratio of the output of which becomes proportional to the measurand. Since the output depends only on the relative sensitivity of the transducer and a pair of dc excitation voltages, it is possible to obtain very low errors. The various sources of error in the circuit are analysed and quantitative expressions to estimate such errors are derived. The circuit was set up in the laboratory and the results obtained from the prototype are presented. The prototype possessed an error of ±0.02% of the reading. It is seen that this circuit is not only simple but also produces errors which are much less than those from circuits currently mentioned in the literature.

Online assessment of winding deformation based on optimised excitation

On line assessment of winding deformation in a power transformer can be performed via a bushing tap excitation with optimized high frequency excitation. A benchmark layer winding is used to validate the use of the specific high frequency excitation. Axial displacement, compression and buckling of the winding are simulated and the changes identified. Measurements are performed on a voltage transformer to verify the suitability of the technique. The procedure will result in a considerably shorter time for measurement.

Design and Implementation of a Calibration - Free Pulse Oximeter

Oxygen saturation in human beings is measured as the ratio of the amount of oxygen carried by hemoglobin to the maximum amount it could carry. A pulse oximeter is a device which measures this saturated oxygen content using the technique of photoplethysmography. Generally, the device consists of two LEDs (Red and IR) as light sources, and a photo diode as the detector. Depending on the intensity of light transmitted or reflected to the detector, oxygen saturation is computed. Commercially available pulse oximeters require calibration, since the device is dependent on sensor and patient-dependent parameters like skin color and thickness of the digit. This is normally done by acquiring data from volunteers, after which the necessary coefficients are extracted, for computing the saturated oxygen content. Such a calibration technique is dependent on the sample population, and hence is undesirable. The work presented here, makes use of a calibration-free algorithm published earlier, incorporating it into a micro-controller, leading to a compact, power-efficient, pulse oximeter. Moreover, present day pulse oximeters do not give access to the raw PPG data, which could be used for extracting important cardiovascular parameters. The preliminary design presented here provides a serial port interface in order to acquire and communicate the PPG signal to a PC, for further offline analysis.

Simulation study of a contactless, capacitive ECG system

The electrocardiogram (ECG) is one of the primary sources of information for assessing cardiovascular function. Conventional ECG systems are not suitable for continuous, long-term monitoring. Contactless measurement of ECG using capacitive sensors is emerging as an attractive alternative. One of the disadvantages of a contactless system is that the signal quality is poor as far as the clinical acceptance of the system is concerned. The contactless ECG signal typically contains a relatively large amount of dc, due to electrode offset or artifacts, which would drive the front end of the high-gain amplifier into saturation. Hence, what is required is a circuit that would effectively block dc while coupling the differential ac signal. A circuit developed for conventional ECG systems has been made use of, for this purpose. This paper describes the simulation and analysis of a contactless ECG monitoring system, using capacitive electrodes, with an ac-coupled front end for effective dc decoupling. The performance of the circuit is studied for different material and thicknesses of clothing.