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Compendium of Biomedical Instrumentation
Compendium of Biomedical Instrumentation
Volume 1
Raghbir Singh KhandpurFormer Head, Medical Instruments DivisionCSIR‐Central Scientific Instruments OrganizationChandigarh, India
Founder Director, Centre for Electronics Design and Technology(Now Centre for Development of Advanced Computing)Mohali, India
Former Director General, Centre for Electronics Design and Technology of IndiaMinistry of Electronics & Information TechnologyGovt. of India, New Delhi, India
Compendium of Biomedical Instrumentation
Volume 2
Raghbir Singh KhandpurFormer Head, Medical Instruments DivisionCSIR‐Central Scientific Instruments OrganizationChandigarh, India
Founder Director, Centre for Electronics Design and Technology(Now Centre for Development of Advanced Computing)Mohali, India
Former Director General, Centre for Electronics Design and Technology of IndiaMinistry of Electronics & Information TechnologyGovt. of India, New Delhi, India
Compendium of Biomedical Instrumentation
Volume 3
Raghbir Singh KhandpurFormer Head, Medical Instruments DivisionCSIR‐Central Scientific Instruments OrganizationChandigarh, India
Founder Director, Centre for Electronics Design and Technology(Now Centre for Development of Advanced Computing)Mohali, India
Former Director General, Centre for Electronics Design and Technology of IndiaMinistry of Electronics & Information TechnologyGovt. of India, New Delhi, India
This edition first published 2020© 2020 John Wiley & Sons Ltd
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Library of Congress Cataloging‐in‐Publication Data Applied for
HB ISBN: 9781119288121
Cover Design: WileyCover Image: © Martin Barraud/Getty Images
Set in 10/12pt WarnockPro by SPi Global, Chennai, India
Printed and bound by CPI Group (UK) Ltd, Croydon, CR0 4YY
10 9 8 7 6 5 4 3 2 1
v
Contents
Preface xix
Volume 1
1 Accelerometer 1
2 Air Bubble Detector 8
3 Alcohol Analyser 11
4 Ambulatory Blood Pressure Monitor 17
5 Ambulatory Cardiac Monitor 20
6 Ambulatory Glucose Monitor 28
7 Ambulatory Sleep Monitor 33
8 Amino Acid Analyser 37
9 Anaesthesia Machine 42
10 Anaesthesia Depth Monitor 50
11 Anorectal Manometry 55
12 Antibiotic Susceptibility Analyser 60
13 Aortic Balloon Pump 64
14 Apnoea Monitor 68
15 Argon Plasma Coagulator 73
16 Arrhythmia Monitor 77
17 Arthroscope 85
18 Atomic Absorption Spectrometer 88
19 Atomic Emission Spectrometer, Flame 94
20 Atomic Emission Spectroscopy: Microwave Plasma 98
Contentsvi
21 Audiometer: Diagnostic 101
22 Audiometer: Evoked Response 106
23 Audiometer: Impedance (Tympanometry) 110
24 Audiometer: Pure Tone 113
25 Audiometer: Screening 116
26 Audiometer: Speech 118
27 Audiometric Calibrator 121
28 Autotransfusion Unit, Blood 126
29 Balance, Electronic 129
30 Ballistocardiograph 134
31 Bilirubinometer 138
32 Biosafety Cabinet 141
33 Bioelectrodes: ECG Electrodes 145
34 Bioelectrodes: EEG Electrodes 150
35 Bioelectrodes: EMG Electrodes 153
36 Biofeedback Instrumentation 158
37 Biotelemetry: ECG (Single Channel) 165
38 Biotelemetry: Multichannel 172
39 Bladder Volume Measuring System: Ultrasonic 177
40 Blood Cell Processor (Apheresis System) 181
41 Blood Flow Detector: Ultrasonic Doppler 184
42 Blood Gas Analyser 187
43 Blood Gas Monitor: Transcutaneous 194
44 Blood Glucose Meter 200
45 Blood Grouping Machine 204
46 Blood Pressure Measurement (Invasive Method) 208
47 Blood Pressure Measurement (Noninvasive Methods) 215
48 Blood Recovery System 222
49 Blood Rheometer 226
50 Blood Time–Temperature Indicator 229
51 Blood Viscometer 232
Contents vii
52 Blood Warmer 237
53 Body Fat Analyser 241
54 Bone Cutting Machine 245
55 Bone Density Measurement: Dual Energy X-ray Technique 246
56 Bone Density Measurement: Ultrasound Method 249
57 Bone Healing Stimulator: External 253
58 Bone Growth Stimulator: Implantable 259
59 Bone Healing Stimulator: Ultrasound 261
60 Brachytherapy: Intravascular 264
61 Brachytherapy Machine 267
62 Breast Biopsy System 273
63 Breast Pump 277
64 Bronchoscope 279
65 Cabinet: Warming 285
66 Camera: Spot Film 287
67 Capnograph 289
68 Carbon Monoxide Analyser 295
69 Cardiac Monitor: Bedside 300
70 Cardiac Output Meter: Fick Method 307
71 Cardiac Output Monitor: Indicator Dilution Method 311
72 Cardiac Output Method: Oesophageal Doppler 316
73 Cardiac Output Monitor: Pulse Contour Method 319
74 Cardiac Output Meter: Thermodilution Technique 323
75 Cardiotocograph 327
76 Central Gas System 335
77 Centrifugal Analyser: Automated 341
78 Centrifuge: Blood Bank 343
79 Centrifuge: Cell Washing 347
80 Centrifuge: Haematocrit 350
81 Centrifuge, Laboratory 353
82 Microcentrifuge 357
Contentsviii
83 Centrifuge, Refrigerated 359
84 Centrifuge, Ultra (High Speed) 361
85 Cervical Cancer Screening System, Automated 366
86 Chloride Meter 369
87 Clinical Chemistry Analyser, Dry 373
88 Clinical Chemistry Analyser, Random Access 377
89 Clinical Chemistry Analyser, Semi‐automated 380
90 Coagulation Analyser 383
91 Coagulation Time Machine, Activated 388
92 Cobalt‐60 Machine for Radiotherapy 391
93 Cochlear Prostheses 397
94 Colonoscope 400
95 Colony Counter, Automated 405
96 Colorimeter, Photoelectric 408
97 Colposcope 416
98 Compression Machine, Intermittent Pneumatic 419
99 Computed Tomography 422
100 Computed Tomography, Single Photon 434
101 Continuous Flow Analyser: Automated 441
102 Continuous Passive Motion Machine (CPMM) 447
103 Continuous Positive Airway Pressure Machine 451
104 Crash Cart: Resuscitation 454
105 Critical Care Analyser 458
106 Cryostat 464
107 Cryosurgical Unit 469
108 Cryotherapy Machine 473
109 Cutaneous Blood Flow Monitor: Laser Doppler 475
110 CyberKnife 479
111 Cystoscope 484
112 Cytometer: Flow 488
113 Cytometer: Imaging 492
Contents ix
114 Defibrillator: External 497
115 Defibrillator: External Automated 503
116 Defibrillator: Implantable Cardioverter 507
117 Defibrillator: Pacemaker Analyser/ECG simulator 513
118 Dental Amalgamator 518
119 Dental Casting Machine 519
120 Dental Furnace 523
121 Dental Sandblaster 527
122 Differential Counter, Automated 530
123 Digital Subtraction Angiography Machine 535
124 DNA Sequencer 539
125 Dynamometer Exercise System 544
126 Echocardiograph 548
127 Electrical Safety Analyser 555
128 Electrocardiograph 563
129 Electroconvulsive Therapy Machine 571
130 Electroencephalograph 575
131 Electrogastrograph 580
132 Electrolyte Analyser 584
133 Electromyograph 589
134 Electronystagmograph 595
135 Electrooculograph 600
136 Electrophoresis Apparatus 604
137 Electrophoresis, Capillary 610
138 Electroretinograph 615
139 Electrosurgical Machine 620
140 Electrosurgical Tester/Analyser 629
141 Endoscope 635
142 Endoscopic Cyclophotocoagulator 640
143 Endoscopy Capsule (Radio Pill) 644
144 ENT Treatment Unit 647
Contentsx
145 Enteral Feeding Pump 650
146 Ergometer, Bicycle 653
147 Ethylene Oxide Analyser 656
148 Event Recorder: Cardiac 659
149 Exercise Stress Testing System 662
Volume 2
150 Flame Photometer 669
151 Flow Injection Based Analyser 674
152 Fluorometer 677
153 Foetal Heart Detector, Ultrasonic 682
154 Foetal Vacuum Extractor 687
155 Freezer, Blood Plasma 690
156 Freezer, Ultra‐Low Temperature 694
157 Functional Electrical Stimulator 697
158 Fundus Camera 702
159 Gait Analyser 706
160 Gamma Camera 710
161 Gamma Counter 717
162 Gamma Knife 720
163 Gas Chromatograph 725
164 Haematology Analyser 732
165 Haematology Analyser, Handheld 739
166 Haemodialysis Machine 742
167 Haemoglobin Meter 749
168 Headlight, Operating 752
169 Hearing Aid 755
170 Hearing Aid Analyser 759
171 Hearing Screening Device, Neonatal 763
172 Heart–Lung Machine 766
173 Heart Rate Monitor 772
174 Heart Valve, Prosthetic 775
Contents xi
175 Heat and Cold Therapy Device 783
176 Haemodynamic Monitor 787
177 High Performance Liquid Chromatograph 792
178 Hollow Fibre Dialyser 797
179 Hospital Beds 800
180 Humidifier, Home 806
181 Humidifier, Respiratory Gas 809
182 Hyperbaric Oxygenation Chamber 812
183 Hyperthermia System 817
184 Hyperthermia, Systemic 823
185 Hyperthermia, Ultrasonic 826
186 Immunoassay Analyser 831
187 Impedance Cardiograph 836
188 Impedance Spectroscopy 840
189 Incinerator, Hospital 843
190 Incubator, Anaerobic 849
191 Incubator, BOD 852
192 Incubator, CO2 854
193 Incubator, Infant 859
194 Incubator, Microbiological 862
195 Incubator, Neonatal 866
196 Inductively Coupled Plasma Optical Emission Spectrometer (ICP‐OES) 869
197 Infusion Pump Analyser 874
198 Infusion Pump, Patient Controlled Analgesia 879
199 Infusion Pump, Syringe 882
200 Infusion Pump, Volumetric 885
201 Injector, Power 888
202 Insufflator 891
203 Insulin Pump 894
204 Intracranial Pressure Monitor 899
205 Ion-Selective Analyser 903
Contentsxii
206 Keratometer 909
207 Lactate Analyser 913
208 Laparoscope 916
209 Laryngoscope 921
210 Laser, Argon Photocoagulator 924
211 Laser, Carbon Dioxide 928
212 Laser, Diode 931
213 Laser, Excimer (Ophthalmic) 935
214 Laser, Holmium:YAG Lithotriptor 939
215 Laser, Navigating, Photocoagulator 942
216 Laser, Nd:YAG 946
217 Laser, PASCAL (Pattern Scanning Laser) 948
218 Laser, Thulium:YAG 952
219 Left Ventricular Assist Device 955
220 Lensometer 960
221 Light, Surgical 964
222 Line Isolation Monitor 968
223 Linear Accelerator Machine 971
224 Lithotripter, Extracorporeal 976
225 Lithotripter, Intracorporeal 981
226 Lyophilizer 985
227 Magnetic Resonance Imaging System 991
228 Mammography 999
229 Manikin 1006
230 Mass Spectrometer, Inductively Coupled Plasma 1009
231 Mercury Analyser 1015
232 Microbial Detection Systems 1019
233 Microbioreactor 1021
234 Microelectrodes 1025
235 Microplate Strip Washer 1031
236 Microscope 1035
Contents xiii
237 Microscope, Atomic Force 1039
238 Microscope, Bright Field 1043
239 Microscope, Confocal 1047
240 Microscope, Dark Field 1052
241 Microscope, Dissecting 1056
242 Microscope, Fluorescence 1060
243 Microscope, Inverted 1065
244 Microscope, Near‐Field Scanning Optical 1068
245 Microscope, Operating/Surgical 1072
246 Microscope, Phase Contrast 1076
247 Microscope, Polarizing 1080
248 Microscope, Scanning Electron 1084
249 Microscope, Scanning Tunnelling 1091
250 Microscope, Transmission Electron 1096
251 Microtome 1102
252 Microtome, Cryostat 1107
253 Microtome, Laser 1110
254 Microtome, Ultra 1113
255 Microwave Diathermy Machine 1115
256 Nebulizer, Pneumatic/Jet 1119
257 Nebulizer, Ultrasonic 1122
258 Neonatal Monitoring System 1124
259 Nephelometer 1129
260 Neurological Monitor 1132
261 Neutron Activation Analyser 1136
262 Nitrogen/Protein Analyser 1140
263 Nitrous Oxide Analyser 1143
264 Oesophagoscope/Gastroscope 1145
265 Oesophagus Manometry 1150
266 Ophthalmoscope, Direct 1156
267 Ophthalmoscope, Indirect 1158
Contentsxiv
268 Optical Tweezers 1161
269 Osmometer 1165
270 Otoacoustic Emission Testing System 1168
271 Otoscope 1172
272 Oxygen Analyser 1175
Volume 3
273 Pacemaker, Cardiac External 1185
274 Pacemakers, Implantable 1190
275 Pacemakers, Rate Responsive 1194
276 Pacemaker Function Analyser 1198
277 Paraffin Dispenser 1201
278 Particle Counter 1204
279 Patient Monitoring System, Central 1210
280 Patient Warmer 1214
281 Peak Flowmeter 1217
282 Pedometer 1220
283 Peritoneal Dialysis Machine 1225
284 Personal Cascade Impactor 1228
285 pH Meter 1232
286 Phacoemulsification Machine 1240
287 Phonocardiograph 1244
288 Phototherapy Unit 1249
289 Picture Archiving and Communication Systems 1252
290 Plasma Thawing Equipment 1258
291 Platelet Aggregation Analyser 1260
292 Platelet Agitator 1264
293 Platelet Counter 1266
294 Plethysmograph 1269
295 Pneumotachometers 1275
296 Point-of-Care Analyser 1278
297 Positron Emission Tomography 1283
Contents xv
298 Proton Beam Radiotherapy Machine 1288
299 Pulmonary Function Analyser 1294
300 Pulse Oximeter 1299
301 Radiant Warmer, Neonatal 1303
302 Radiation Dosimeter 1307
303 Radiation Dosimeter, Electronic Personal 1310
304 Radiation Dosimeter, Geiger–Muller Counter 1314
305 Radiation Dosimeter, Ionization Chamber 1318
306 Radiation Dosimeter, Optically Stimulated Luminescence 1321
307 Radiographic Dosimeter, Photographic Film 1323
308 Radiation Dosimeter, Radiochromic Film 1325
309 Radiation Dosimeter, Scintillation Counter 1327
310 Radiation Dosimeter, Thermoluminescent 1331
311 Radiation Therapy Simulator 1335
312 Radiation Therapy CT Simulator 1339
313 Radiofrequency Ablation Machine 1343
314 Radiography Machine, Analog 1348
315 Radiography Machine, Digital 1355
316 Radiography/Fluoroscopy 1360
317 Radiography, Mobile Machine 1364
318 Radiographic Unit, Dental 1370
319 Radioimmunoassay Analyser 1374
320 Radiology Information System 1377
321 Radiotherapy, Intraoperative Therapy Machine 1383
322 Radiotherapy Treatment Planning System 1388
323 Refractor, Auto 1392
324 Refrigerator, Blood Bank 1396
325 Refrigerator, Blood Bank, Ice‐Lined 1401
326 Refrigerator, Blood Bank, Solar‐Powered 1403
327 Renal Transplant Perfusion Machine 1405
328 Respiration Rate Monitor 1408
Contentsxvi
329 Robotic Surgery System 1415
330 Scale, Infant 1421
331 Scale, Patient 1423
332 Scintillation Counter 1426
333 Scintillation Counter, Liquid 1429
334 Short‐Wave Diathermy Machine 1433
335 Simulator, ECG 1438
336 Simulator, Multiparameter 1441
337 Skin Temperature‐Measuring Devices 1445
338 Slit Lamp 1450
339 Spectrofluorometer 1454
340 Spectrometer, Gamma 1459
341 Spectrometer, NMR 1462
342 Spectrophotometer, Infrared 1468
343 Spectrophotometer (UV–Visible) 1478
344 Sphygmomanometer 1487
345 Spirometer 1491
346 Stem Cell Separator, Automated 1498
347 Sterilizer, Dry heat 1503
348 Sterilizer, Gas 1506
349 Sterilizer, Plasma 1510
350 Sterilizer, Radiation 1514
351 Sterilizer, Steam 1518
352 Stethoscope 1522
353 Stethoscope, Electronic 1525
354 Stimulator, Bladder 1529
355 Stimulator, Deep Brain 1534
356 Stimulator, Peripheral Nerve 1538
357 Stimulator, Peripheral Nerve (Regional Anaesthesia) 1541
358 Stimulator, Phrenic Nerve 1545
359 Stimulator, Spinal Cord 1547
Contents xvii
360 Stimulator, Vagus Nerve 1552
361 Suction Apparatus 1556
362 Suction Pump, Surgical 1559
363 Surgical Dermatome 1562
364 Tablet Counter 1563
365 Temperature Data Logger, Blood Bank 1567
366 Thermocycler (PCR Machine) 1570
367 Thermography, Infrared Camera 1574
368 Thoracic Aspirator 1580
369 Thyroid Uptake System 1583
370 Tissue Processor 1586
371 Tonometer, Arterial 1589
372 Tonometer, Ophthalmic 1593
373 Traction Unit 1597
374 Transcranial Blood Flow Doppler Machine 1599
375 Transcutaneous Electrical Nerve Stimulator 1607
376 Transcranial Magnetic Stimulator 1611
377 Ultrasonic Cleaner 1615
378 Ultrasonic Dental Scaler 1620
379 Ultrasonic Imaging System 1624
380 Ultrasonic Surgical Machine, Harmonic 1630
381 Ultrasonic Therapy Unit 1634
382 Ultrasound Thrombolysis System 1637
383 Urine Chemistry Analyser 1641
384 Urodynamic Measurements 1645
385 Uroflowmeter 1648
386 Uterine Aspirator 1652
387 Ventilator, Anaesthesia 1654
388 Ventilator, Continuous 1659
389 Ventilator, High Frequency 1662
390 Ventilator, ICU 1667
Contentsxviii
391 Ventilator, Lung 1672
392 Ventilator, Neonatal 1677
393 Ventilator, Transport 1680
394 Ventilator Tester 1685
395 Videoconferencing System (Telemedicine) 1688
396 Vital Signs Monitor 1694
397 Vitrectomy Machine 1701
398 Water Bath 1705
399 Wavefront Measurement Device 1707
400 X‐ray Film Processor 1711
Index 1715
xix
Biomedical instruments and devices today occupy an important place in the delivery of healthcare at all levels of medical facilities from primary healthcare to tertiary‐level facilities. The range of these instruments and devices is spectacular and variety baffling. It is difficult to imagine a medical speciality where some kind of instruments are not required and used. They are employed for clinical diagnosis through measurement of physical parameters, laboratory analytical techniques, and various imaging modalities. On the other hand, therapeutic devices have altered the way the diseases are treated. Advanced research in the unknown realms of functioning of various physiological phenomena of living beings is becoming possible with MEMS and computer‐based instruments. The availability of a bewildering array of such instrumental techniques in clinical practice and a large variety of commercially available equipment have presented a great challenge before all those who are responsible for managing these technologies by way of their usage, operation, and maintenance and those engaged in advancing measurement techniques through research and development. The book is designed mainly for the active workers involved in hands‐on functions rather than peers in their respective fields.
The publication is a compilation of 400 instruments and devices arranged alphabetically. Effort has been made to cover almost the entire range of important and most popular instruments used for diagnosis, imaging, analysis, and therapy. Each instrument description covers four aspects: (i) purpose of the instrument, (ii) principle of operation covering physics, engineering and electronics, and data processing, (iii) brief specifications, and (iv) major applications. No attempt has been made to include historical developments of a particular instrument or device, as most state‐of‐the‐art instruments have been given a place in the publication. However, some instruments based on older‐generation technologies have been included for legacy purposes as they are still in use in some medical facilities. It has also been tried to include generic specifications as much as possible.
The motivation of the work arose from interactions with various biomedical instrumentation engineers particularly those who join in healthcare facilities after receiving their undergraduate degrees and get involved in managing medical technology, including rendering of advice on procurement of new instruments and gadgets. It falls on them to first understand the working principle of the instrument proposed to be procured and also to have an appreciation of the salient specifications. The service engineers are expected to broadly know as to what the instrument contains inside and the clinical engineer can help the clinician to adopt better measurement techniques.
Preface
Prefacexx
I would like to acknowledge with thanks the contribution of various persons and agencies who have helped me to complete the work. First of all , I wish to thank McGraw‐Hill Education (India) Private Limited, New Delhi, for their permission to use a few lines here and there from my two books published earlier by them, namely, Handbook of Biomedical Instrumentation and Handbook of Analytical Instruments. I have tried to include commercial instruments from almost all important players in the field. All of them have shown tremendous cooperation in supplying high resolution images of their instruments along with associated information. Most of them have authenticated the accuracy of the text material and made useful suggestions for the improvement of the text. While it is not possible to individually name and acknowledge their contribution here, the assistance of each one of them has been acknowledged along with the respective instrument image under different chapters.
I am thankful to my wife Ramesh Khandpur who has always been a source of encour-agement and strength in supporting my writing endeavours. I am sure my children Vimal, Gurdial, and Popila and grandchildren Ravleen, Harsheen, Manmeet, Ashna, and Gurtej will feel elated when they see this publication. Thanks are due to Gurdial and Sumit Khandpur for their timely help in preparing the response to the initial copyedit-ing work. The interest shown by Balwinder and Jaswinder in the project is gratefully acknowledged.
I would also like to place on record my deep appreciation of M/s John Wiley & Sons, UK, particularly Mr Steven Fassioms, Mr Hari Sridharan, Ms Hannah Lee, and Ms Anita Yadav of the Delhi office for their constant support during the preparation and production of this book.
Dr R S Khandpur
Compendium of Biomedical Instrumentation, Volume 1, First Edition. Raghbir Singh Khandpur.© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.
1
1
Purpose
Accelerometers are widely used as sensors for wearable medical devices to measure and assess physical activity (PA) in clinical/laboratory settings or free‐living environments. They are, however, mostly employed for ambulatory monitoring for continuously meas-uring long‐term activities of subjects in a free‐living environment. From the recorded activity data, it is possible to identify daily movements that are associated with an indi-vidual’s functional status and also to detect adverse activity, such as falls, through signal analysis and appropriate algorithm. In addition, energy expenditure is the most com-monly found application of the accelerometers.
Principle
Any bodily movement produced by skeletal muscles that results in an energy expendi-ture can be regarded as PA. Various techniques and devices have been used to measure physical activities that employ wearable or body‐fixed motion sensors. These include gadgets ranging from simple switches, pedometers, goniometers, actometers, acceler-ometers, and gyroscopes. The measurement of physical activity employing accelero-meters is the most preferred technique because the acceleration is proportional to external force and, as a consequence, can reflect intensity and frequency of human movement. Also, it is easy to obtain velocity and displacement information by integrat-ing accelerometer data with respect to time. The accelerometers are also designed to respond to gravity, thereby providing tilt sensing with respect to reference planes when accelerometers rotate with objects on which they are mounted. The resulting inclination data from accelerometers help to classify body postures or orientations, thus providing sufficient information for measuring physical activity and a range of human routine activities.
Accelerometers are sensors that are designed to measure the acceleration of an object in motion along reference axes. These sensors are basically force sensors that sense linear acceleration along one or several directions, or angular motion about one or sev-eral axes. The former is called an accelerometer, while the latter is referred to as gyro-scope. The principle on which an accelerometer operates is based on a mechanical sensing element that consists of a proof mass or seismic mass attached to a mechanical
Accelerometer
Compendium of Biomedical Instrumentation2
suspension system with respect to a reference frame. Inertial force due to acceleration or gravity causes deflection of the proof mass according to Newton’s second law. The acceleration is measured electrically based on the physical changes in displacement of the proof mass with respect to the reference frame. The most common type of sensors used to measure acceleration are based on the principle of piezoresistivity, piezoelec-tricity, or differential capacitive measurement.
Piezoresistive Accelerometers
Piezoresistive accelerometers, also termed as strain gauge accelerometers, operate by measuring the change in electrical resistance of a piezoresistive element when mechani-cal stress is applied to it. Figure 1.1 shows the principle of piezoresistive accelerometer. The sensing element consists of a cantilever beam and its proof mass is determined by bulk micromachining. The acceleration causes the motion of the proof mass that can be detected by piezoresistors in the cantilever beam and proof mass. The piezoresistors are placed as two arms of a Wheatstone bridge that produces a voltage proportional to the applied acceleration.
In practice, a piezoresistive accelerometer is structurally quite stable. This is because it is composed of a silicon chip formed by the semiconductor production technology. An electrical bridge is formed by piezoresistors representing a mass and a beam which are fabricated on a silicon chip. The electrical bridge so formed by such piezoresistive resistors generates electrical signals that are proportional to the applied acceleration. The piezoresistive accelerometers can measure constant acceleration with respect to gravity as they are responsive to DC voltage. The piezoresistive accelerometers are sim-ple and low cost but have lower level of the output signals and suffer from the tempera-ture‐sensitive drift. Though they have a limited high frequency response, they are preferred in high shock applications.
Piezoelectric Accelerometers
This type of sensor is based on the piezoelectric effect. When certain types of crystals are compressed by application of some force, charges of opposite polarity accumulate on opposite sides of the crystal. Figure 1.2 shows a simplified schematic of a piezoelec-tric accelerometer. The accelerometer uses an internal piezoelectric element that is coupled with a proof mass to form an accelerometer system. Here, the sensing element
MassHousing
Bondingpad
Amplifier
Base plate
Mass plate
Beam
PiezoresistorFigure 1.1 Principle of piezoresistive accelerometer.
Accelerometer 3
bends due to applied acceleration, which causes a displacement of the proof mass, that results in an output voltage proportional to the applied acceleration. The accelerometer is a charge‐sensitive device in which an instantaneous change in stress on the piezoelec-tric element produces a voltage at the accelerometer’s output terminals that is propor-tional to the applied acceleration. Piezoelectric or charge mode accelerometers require an external amplifier or built‐in charge converter to amplify the generated charge, lower the output impedance for compatibility with measurement devices, and minimize sus-ceptibility to external noise sources and crosstalk. These devices are usually integrated circuit piezoelectric (ICP) sensors.
A piezoelectric accelerometer’s sensitivity is specified in picocoulombs per gram (pC/g). Typical sensitivities are in the range of 0.5–1000 pC/g. Piezoelectric accelerom-eters only respond to AC phenomenon such as vibration or shock, rather than DC phe-nomenon such as the acceleration of gravity. The accelerometers can be applied to measure acceleration levels ranging from 4 g to greater than 100 g. The useful measure-ment range of a given system is often limited by its signal conditioning and measure-ment system.
Differential Capacitive Accelerometers
The displacement of the proof mass due to acceleration can also be identified by meas-uring changes in capacitance. Capacitive sensing accelerometers produce a voltage dependent on the distance between two planar surfaces or plates. One or both of these ‘plates’ are charged with an electrical current. When there is a change in the gap between the plates due to application of force, it changes the capacitance of the system, which can be easily measured in terms of voltage. There are several advantages of using capaci-tive accelerometers which include larger bandwidth, low power dissipation, faster response to motion, high accuracy and stability in operation. They are also less prone to noise and variation with temperature. Differential capacitive accelerometers are also responsive to static forces.
Figure 1.3 illustrates the working of the differential capacitive accelerometers. The sensing element of the accelerometer consists of two fixed plates attached to the sub-strate and a suspended plate. When the unit moves in the direction as shown in the diagram, the displacement of the suspended plate with respect to the two fixed plates changes, resulting in a change in capacitance of C1 and C2. An increase in C1 will result
Acceleration
Output
Ampli�er:need current excitation
Preloadbolt
Solid base
Piezoelectricmaterial
Seismic mass
Figure 1.2 Cross section of a piezoelectric accelerometer.
Compendium of Biomedical Instrumentation4
in a decrease in C2 and vice versa which can be sensed as a voltage signal. A simplified circuit diagram of the signal conditioner of the differential capacitance circuit is shown in Figure 1.4. The signal from the sensor is processed to obtain a filtered and amplified linear output. Due to the small capacitances involved and in order to reduce noise and thermal drift and increase the resolution, a differential capacitance system is employed.
Microelectromechanical Sensors (MEMS)-based Accelerometers
Most of the modern accelerometers are based on microelectromechanical sensors (MEMS). All the above‐mentioned technologies for converting acceleration to an elec-trical signal, namely, piezoelectric, piezoresistive, and capacitive change, can be used in the construction of MEMS‐based sensors. However, the preferred technology is capaci-tive sensing MEMS as it offers long‐term stability with high sensitivity. For this reason, capacitance‐based MEMS are used in some of the most demanding applications. These sensors are available in one‐, two‐, or three‐axis versions. Multiple axis measurements can also be grouped into a single monitor, allowing capturing of movement in multiple planes. Figure 1.5 shows the functions of an MEMS inertial sensors to detect and meas-ure tilt, shock, rotation, vibration, or any other types of motion.
MEMS are silicon‐based micromachined sensors, which have on‐chip integration for measurements such as acceleration and vibration. The chip includes the sensor and the signal conditioning circuitry and consequently require only a few external components. Some chips also have built‐in analog‐to‐digital converter to convert the analog output of the signal conditioner to a digital format facilitating direct display on an LCD. They include adequate memory to record physical activity over 21‐day periods.
Motion
Fixed plate 1
Fixed plate 2
C1
C2 Suspendedplate
Figure 1.3 Principle of differential capacitance‐based accelerometer.
Capacitanceto voltageconverter
Gain/offset/�lter stage
0.5–4.5 VOutput signal
Capacitanceto voltageconverter
Differentialampli�er
C1
C2–
Figure 1.4 Circuit diagram for signal conditioning of differential capacitance‐based accelerometer.
Accelerometer 5
The MEMS chip comprises springs, masses, and motion‐sensing components. These sensors are fabricated using the standard IC processing technology commonly employed in wafer fabrication facilities. The sensor with a 3D structure, which allows free move-ment in all directions, is designed by using layers of oxide and polysilicon, IC photoli-thography, and selective etching techniques. The sensor is of differential capacitor type in which the plates on the wafer can be driven 180° out of phase. Any movement of the mass on application of force unbalances the capacitor and results in a square wave out-put whose amplitude is proportional to the acceleration.
Figure 1.6 shows the layout of various components of MEMS capacitance change‐based accelerometer on the chip. The digital accelerometers give output in the form of a variable frequency square wave, the method being known as pulse‐width modulation (PWM). A pulse width‐modulated accelerometer takes readings at a fixed rate, typi-cally, say, at 1000 Hz. The value of the acceleration is proportional to the pulse width or duty cycle of the PWM signal. A demodulator in each axis rectifies the signal and deter-mines the direction of acceleration. This output is given to a modulator that filters the analog signal and converts it to a duty cycle output. A microcontroller can be used to measure acceleration by timing both the duty cycle and the period of each axis. The duty cycle output would be 50% at a 0 g acceleration. A typical commercial MEMS‐based accelerometer chip is available from M/s Safron Collibrys, Switzerland, which is shown in Figure 1.7.
Figure 1.7 Typical MEMS‐based accelerometer chip.
Acceleration
VibrationMEMSsensors
Rotation
Tilt Shock
Figure 1.5 Various functions of an MEMS‐based sensor.
Torsion bar
Moving capacitor plate
Fixed capacitor plate
Support
Substrate
Figure 1.6 Layout of various components on an MEMS capacitance change‐based accelerometer.
Control logic
MultiplexerCurrent-to-
voltageconverter
Ampli�er Analog-to-digitalconverter
Display
X-axistransducer
Y-axistransducer
Z-axistransducer
Figure 1.8 Block diagram of a three‐axis accelerometer circuit including in‐built A/D converter of MMA7660FC integrated chip from M/s Freescale.