Instrumentation Handbook for Biomedical Engineers

Mesut Sahin

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781466504844
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The book fills a void as a textbook with hands-on laboratory exercises designed for biomedical engineering undergraduates in their senior year or the first year of graduate studies specializing in electrical aspects of bioinstrumentation. Each laboratory exercise concentrates on measuring a biophysical or biomedical entity, such as force, blood pressure, temperature, heart rate, respiratory rate, etc., and guides students though all the way from sensor level to data acquisition and analysis on the computer. The book distinguishes itself from others by providing electrical circuits and other measurement setups that have been tested by the authors while teaching undergraduate classes at their home institute over many years.

Key Features:
• Hands-on laboratory exercises on measurements of biophysical and biomedical variables
• Each laboratory exercise is complete by itself and they can be covered in any sequence desired by the instructor during the semester
• Electronic equipment and supplies required are typical for biomedical engineering departments
• Data collected by undergraduate students and data analysis results are provided as samples
• Additional information and references are included for preparing a report or further reading at the end of each chapter

Students using this book are expected to have basic knowledge of electrical circuits and troubleshooting. Practical information on circuit components, basic laboratory equipment, and circuit troubleshooting is also provided in the first chapter of the book.

Studio 1

Body Thermometer Using a Wheatstone Bridge and the Projection Method

S1.1 Learning Objectives

The objectives of the studio are for the students to:

1. 1.Learn the advantages of using a Wheatstone Bridge.
2. 2.Become familiar with the projection method based on exponential function.
3. 3.Understand the operation of a thermistor.

S1.2 Background

A Wheatstone Bridge is a method of using two voltage dividers to gain increased sensitivity to small resistive value changes. The circuit for one voltage divider is shown in Figure 1.1.

FIGURE 1.1 Voltage divider.

The voltage at B with respect to C can be calculated as follows:

V B = V B T 1 R 2 / ( R 1 + R 2 )

If we set R₁ equal to R₂ then:

V B = V B T 1 / 2

If for example BT1 is 9 volts, then V_B would be 4.5 volts. If the value of R₂ changed slightly, so that the value of V_B decreased by a few millivolts, it would be hard to accurately make the measurement because 1 millivolt out of 4.5 volts is five parts in 10,000. However, if we add a second divider, as in Figure 1.2, then we would be measuring the difference between two similar voltages. In addition, the measurement would be less sensitive to changes in V_BT1 since the resultant output voltage change would affect both dividers. If we now set R₁ = R₂ = R₃ = R₄ then V_B = V_D and meter M1 would read 0 volts. This would be true for any battery voltage. Now if we replace R₂ with a thermistor, RT1 and replace R₄ with the appropriate potentiometer (or trimpot), for any given thermistor temperature, we could adjust potentiometer R₄ so that the bridge is balanced and reads 0 volts.

FIGURE 1.2 Wheatstone Bridge.

When measuring the temperature of an object, the temperature of the sensor is initially at a different value than that of the object, which in many cases would be the room temperature. Then we put the sensor in contact with the object and wait for the temperature of the sensor to change and stabilize at the new temperature before making the reading. To measure a person’s body temperature the sensor is usually placed in the person’s mouth. The time it takes to complete the measurement would be dependent on the thermal time constant of the sensor, which is the time it takes the sensor’s temperature to reach the temperature of the body. Since it is somewhat uncomfortable for the subject to keep the sensor in their mouth, we would like to complete the reading as quickly as possible. It is possible to make the measurement quicker by extrapolating the result from a number of sampled data points over time, as the sensor is stabilizing at the new temperature to calculate the final value before the sensor temperature actually stabilizes, and report that value. When designing a modern digital thermometer, a simple microcontroller would be used that is powerful enough to quickly complete this calculation without impacting the cost of the product, thus decreasing the time it takes to complete a measurement.

S1.3 Overview of the Experiment

This laboratory exercise will introduce the concepts of the Wheatstone Bridge and of extrapolation. A Wheatstone Bridge will be built incorporating a thermistor, data will be sampled and the initial readings will be used to calculate the final value by extrapolation. The projected value will be compared to the actual final value.

S1.4 Safety Notes

There is no health risk in this studio other than standard precautions that need to be observed in dealing with electronics.

S1.5 Equipment, Tools, Electronic Components and Software

Additional information about the use of the items required in this studio can be found in the introduction and the appendices of this book.

• Equipment
  1. a. 9 V battery.
  2. b. Multi-meter.
  3. c. Data Acquisition Card (DAQ) installed into a computer.
• Tools
  1. a. Breadboard (protoboard).
  2. b. Two BNC-to-micro clips cables.
• Electronic Components
  1. a. Thermistor EPCOS (TDK) B57164K0103J000.
  2. b. Two 10K 1% resistors.
  3. c. 20 kΩ trimpot (10-turn is preferred).
• Software
  1. a. MATLAB®.

S1.6 Pre-Lab Questions

Questions to be answered before starting to read and/or implement the sequence of experimental steps in the following section 1.7:

1. 1. Do the students have knowledge of using trimpots and its terminal configuration? (e.g., the terminal that is physically in the middle is also shown in the middle in the schematic and the side ones are interchangeable).
2. 2. Do the students know how to measure resistances using an ohmmeter and what the tolerance color codes of the resistors mean?
3. 3. Do the students have an understanding of thermal resistance between the thermistor and the environment and how it affects the time constant of the...

Table of contents

• Cover
• Half-Title
• Title
• Copyright
• Contents
• Foreword
• Preface
• About the Authors
• Abbreviations
• Introduction
• Studio 1 Body Thermometer Using a Wheatstone Bridge and the Projection Method
• Studio 2 Electrophysiological Amplifier: Recording Electrocardiograms Through A Breadboard
• Studio 3 Small Signal Rectifier-Averager for EMG Signals
• Studio 4 Digital Voltmeter: Usage of Analog-to-Digital Converters
• Studio 5 Force Measurements with PZT Transducers
• Studio 6 Oscillometric Method for Measurement of Blood Pressure
• Studio 7 Electronic Stethoscope: Heart Sounds
• Studio 8 Transmission Photoplethysmograph: Fingertip Optical Heart Rate Monitor
• Studio 9 Measurement of Hand Tremor Forces with Strain-Gauge Force Transducer
• Studio 10 Optical Isolation of Physiological Amplifiers
• Studio 11 Extraction of Respiratory Rate from ECG (ECG-Derived Respiration-EDR)
• Studio 12 Heart Rate Variability Analysis in Frequency Domain
• Studio 13 AC Impedance of Electrode-Body Interface
• Appendix I: Using Electronic Components and Circuit Design
• Appendix II Required Equipment and Materials
• Index