1. BM 2203 - Sensors and Measurment G.Thiyagarajan BM 17 REC/BME

2. Block diagram of a generalized instrumentation system

3. The Bourdon Gauge

4. Block diagram of the pressure gauge based on Bourdon tube

5. A typical medical measurement system
Sensor
Data
communication
displays
Effector
Measurand
Signal
conditioning
processing
storage
Feedback
Outputs

6. Feedback with and without clinician
Instrument
Patient
Clinician
Instrument
Patient

7. A patient monitors vital signs and notify a clinician if abnormalities occur
Instrument
Patient
Clinician
Abnormal
readings

8. Detailed generalized medical measurement system

9. Alternative operational modes
Direct-indirect modes
Sampling and continuous modes
Generating and modulating sensors
Analog and digital modes
Real-time and delayed-time modes

10. Example to sampled data
Laboratory test
Typical value
Hemoglobin
13.5 to 18 g/dL
Hematocrit
40 to 54%
Erythrocyte count
4.6 to 6.2  106/ L
Leukocyte count
4500 to 11000/ L
Differential count
Neutrophil 35 to 71%
Band 0 to 6%
Lymphocyte 1 to 10%
Monocyte 1 to 10%
Eosinophil 0 to 4%
Basophil 0 to 2%
Complete blood count for a male subject.

11. Analog and digital signals
Time
Amplitude
Amplitude
Time
Analog signals can have any amplitude value
Digital signals have a limited number of amplitude values

12. Continuous and discrete-time signals
Amplitude
Amplitude
Time
Continuous signals have values at every instant of time
Discrete-time signals are sampled periodically and do not provide values between these sampling times

13. Origins of common biological signal

14. Medical measurement constraints
Range
Frequency, Hz
Method
Blood flow
1 to 300 mL/s
0 to 20
Electromagnetic or ultrasonic
Blood pressure
0 to 400 mmHg
0 to 50
Cuff or strain gage
Cardiac output
4 to 25 L/min
Fick, dye dilution
Electrocardiography
0.5 to 4 mV
0.05 to 150
Skin electrodes
Electroencephalography
5 to 300  V
0.5 to 150
Scalp electrodes
Electromyography
0.1 to 5 mV
0 to 10000
Needle electrodes
Electroretinography
0 to 900  V
Contact lens electrodes pH
3 to 13 pH units
0 to 1
pH electrode
pCO2
40 to 100 mmHg
0 to 2
pCO2 electrode
pO2
30 to 100 mmHg
pO2 electrode
Pneumotachography
0 to 600 L/min
0 to 40
Pneumotachometer
Respiratory rate
2 to 50 breaths/min
0.1 to 10
Impedance
Temperature
32 to 40 °C
0 to 0.1
Thermistor

15. Setting sensor specifications
Value
Pressure range
–30 to +300 mmHg
Overpressure without damage
–400 to mmHg
Maximum unbalance
±75 mmHg
Linearity and hysteresis
± 2% of reading or ± 1 mmHg
Risk current at 120 V
10 A
Defibrillator withstand
360 J into 50 
Sensor specifications for a blood pressure sensor are determined by a committee composed of individuals from academia, industry, hospitals, and government.

16. Specifications for ECG
Value
Input signal dynamic range
±5 mV
Dc offset voltage
±300 mV
Slew rate
320 mV/s
Frequency response
0.05 to 150 Hz
Input impedance at 10 Hz
2.5 M
Dc lead current
0.1 A
Return time after lead switch
1 s
Overload voltage without damage
5000 V
Risk current at 120 V
10 A
Specification values for an electrocardiograph are agreed upon by a committee.

17. Classification of biomedical instruments
Quantity sensed: pressure, flow, temperature etc.
Principle of transduction: resistive, inductive, capacitive, ultrasonic or electrochemical
Organ system studied: cardiovascular, pulmonary, nervous, and endocrine systems.
Clinical medical specialties: pediatrics, obstetrics, cardiology, or radiology.

18. Interfering and modifying inputs
An interfering input may shift the baseline
Original waveform
A modifying input may change the gain

19. Simplified Electrocardiographic recording system

20. Compensation Techniques
Inherent insensitivity
Negative feedback
Signal filtering
Opposing inputs

21. Negative feedback y + Gd xd - Hf

22. Signals without noise are uncorrupted
Signal filtering
Signals without noise are uncorrupted
Interference superimposed on signals causes error. Frequency filters can be used to reduce noise and interference

23. Opposing inputs Differential amplifier: v0 = Gd(vA- vB)
DC cancellation (bucking)
An input signal with dc offset
An input signal without dc offset

24. Generalized Static Characteristics
Accuracy
Precision and reproducibility
Resolution
Statistical control
Static sensitivity
Zero drift
Sensitivity drift
Linearity
Input ranges
Input impedance

25. Accuracy Data points with
Accuracy: closeness with which an instrument reading approaches the true or accepted value of the variable (quantity) being measured. It is considered to be an indicator of the total error in the measurement without looking into the sources of errors.
low accuracy
Accuracy is often expressed in percentage
high accuracy

26. Precision Data points with
A measure of the reproducibility of the measurements; i.e., given a fixed value of a variable, precision is a measure of the degree to which successive measurements differ from one another.
low precision
Number of distinguishable alternatives V is more precise than 2.43 V.
high precision

27. Resolution
The smallest change in measured value to which the instrument will respond.
Statistical control: random variations in measured quantities are tolerable,
Coulter counter example

28. Tolerance Maximum deviation allowed from the conventional true value.
It is not possible to built a perfect system or make an exact measurement. All devices deviate from their ideal (design) characteristics and all measurements include uncertainties (doubts).
Hence, all devices include tolerances in their specifications. If the instrument is used for high-precision applications, the design tolerances must be small.
However, if a low degree of accuracy is acceptable, it is not economical to use expensive sensors and precise sensing components

29. Static sensitivity A low-sensitivity sensor has low gain
signal
Measurand
Sensor
signal
Measurand
A low-sensitivity sensor has low gain
A high sensitivity sensor has high gain

30. Static sensitivity constant over a limited range

31. Zero and sensitivity drifts

32. Linearity A nonlinear system does not fit a straight line
Output
Input
Output
Input
A nonlinear system does not fit a straight line
A linear system fits the equation y = mx + b.

33. Calibration for linearity
Output
Input
Output
Input
The one-point calibration may miss nonlinearity
The two-point calibration may also miss nonlinearity
Measuring instruments should be calibrated against a standard that has an accuracy 3 to 10 times better than the desired calibration accuracy

34. Hysteresis
A hysteresis loop. The output curve obtained when increasing the measurand is different from the output obtained when decreasing the measurand.

35. Independent nonlinearity

36. Objectives At the end of this chapter, the students should be able to:
describe the principle of operation of various sensors and transducers; namely..
Resistive Position Transducers.
Capacitive Transducers
Inductive Transducers

37. Introduction Sensors and transducers are classified according to;
the physical property that they use (piezoelectric, photovoltaic, etc.)
the function that they perform (measurement of length, temperature, etc.).
Since energy conversion is an essential characteristic of the sensing process, the various forms of energy should be considered.

38. Introduction
There are 3 basic types of transducers namely self-generating, modulating, and modifying transducers.
The self-generating type (thermocouples, piezoelectric, photovoltaic) does not require the application of external energy.

39. Introduction
Modulating transducers (photoconductive cells, thermistors, resistive displacement devices) do require a source of energy.
For example, a thermocouple is self-generating, producing a change in resistance in response to a temperature difference, whereas a photoconductive cell is modulating because it requires energy.
The modifying transducer (elastic beams, diaphragms) is characterized by the same form of energy at the input and output. The energy form on both sides of a modifier is electrical.

40. Definition
The words 'sensor' and 'transducer' are both widely used in the description of measurement systems.
The former is popular in the USA whereas the latter has been used in Europe for many years. The word 'sensor' is derived from entire meaning 'to perceive' and 'transducer' is from transducer meaning 'to lead across'.

41. Definition
A dictionary definition of 'sensor' is `a device that detects a change in a physical stimulus and turns it into a signal which can be measured or recorded;
The corresponding definition of 'transducer' is 'a device that transfers energy from one system to another in the same or in the different form'.

42. Features of Sensors The desirable features of sensors are:
accuracy - closeness to "true" value of variable; accuracy = actual value - sensed value;
precision - little or no random variability in measured variable
3. operating range - wide operating range; accurate and precise over entire sensing range
4. calibration - easy to calibrate; no "drift" - tendency for sensor to lose accuracy over time.
5. reliability - no failures
6. cost and ease of operation - purchase price, cost of installation and operation

43. Sensors Types
A list of physical properties, and sensors to measure them is given below:

44. Sensors Types

45. Common Sensors
Listed below are some examples of common transducers and sensors that we may encounter:
Ammeter - meter to indicate electrical current.
Potentiometer - instrument used to measure voltage.
Strain Gage - used to indicate torque, force, pressure, and other variables. Output is change in resistance due to strain, which can be converted into voltage.
Thermistor - Also called a resistance thermometer; an instrument used to measure temperature. The operation is based on change in resistance as a function of temperature.

46. Sensors Types
There are several transducers that will be examined further in terms of their principles of operations.
Those include :
Resistive Position Transducers
Strain Gauges
Capacitive Transducers
Inductive Transducers
And a lot more…

47. Resistive Position Transducers
The principle of the resistive position transducer is that the measured quantity causes a resistance change in the sensing element.
A common requirement in industrial measurement and control work is to be able to sense the position of an object, or the distance it has moved.
One type of displacement transducer uses a resistance element with a sliding contact linked to the object being monitored.
Thus the resistance between the slider and one end of the resistance element depends on the position of the object.

48. Resistive Position Transducers
The output voltage depends on the wiper position and therefore is a function of the shaft position.
In figure below, the output voltage Eout is a fraction of ET, depending on the position of the wiper.
The element is considered perfectly linear if the resistance of the transducer is distributed uniformly along the length of travel of wiper.

49. Resistive Position Transducers
Example 1
An RPT with a shaft stroke of 5.5 inches is applied in the circuit as below. The total resistance of the potentiometer is 4.7kΩ. The applied voltage is ET= 3V. When the wiper is 0.9 in. from B, what is Eout?

50. Strain Gauges
The Strain Gauge is an example of a passive transducer that uses electrical resistance variation in wires to sense the strain produced by a force on the wire.
It is a very versatile detector and transducer for measuring weight, pressure, mechanical force or displacement.

51. Strain Gauges
The construction of a bonded strain gauge shows a fine wire looped back and forth on a mounting plate, which is usually cemented to the element that undergoing stress.

52. Strain Gauges
For many common materials, there is a constant ratio between stress and strain.
Stress is defined as the internal force per unit area.
S – Stress (kg/m2)
F – Force (kg)
A - Area (m2)
The constant of proportionality between stress and strain for the curve is known as the modulus of elasticity of the materials, E or Young’s Modulus.

53. Capacitive Transducers
The capacitance of a parallel plate is given by:
k= dielectric constant
A= area of the plate
o=8.854x10-12 F/m
d= plate spacing
Since the capacitance in inversely proportional to the spacing of the parallel plates, any variations in d will cause a variation in capacitance.

54. Capacitive Transducers
Some examples of capacitive transducers

55. Capacitive Transducers
Example 2:
An electrode-diaphragm pressure transducer has plates whose area is 5x10-3 m2 and distance between plates is 1x Calculate its capacitance if it measures air pressure with k=1.

56. Inductive Transducers
Inductive Transducers may be either the self-generating or the passive type transducers.
In the Self-Generating IT, it utilises the basic electrical generator principle that when there is relative motion between conductor and magnetic field, a voltage is induced in the conductor.
An example of this is Tachometer that directly converts speeds or velocity into an electrical signal.

57. Tachometers
Examples of a Common Tachometer

58. Linear Variable Differential Transformer (LVDT)
Passive inductive transducers require an external source of power.
The Differential transformer is a passive inductive transformer, well known as Linear Variable Differential Transformer (LVDT).
It consists basically of a primary winding and two secondly windings, wound over a hollow tube and positioned so that the primary is between two of its secondaries.

59. Linear Variable Differential Transformer (LVDT)
Some examples of LVDTs.

60. Linear Variable Differential Transformer (LVDT)
An example of LVDT electrical wiring.

61. Linear Variable Differential Transformer (LVDT)
An iron core slides within the tube and therefore affects the magnetic coupling between the primary and two secondaries.
When the core is in the centre , the voltage induced in the two secondaries is equal.
When the core is moved in one direction of centre, the voltage induced in one winding is increased and that in the other is decreased. Movement in the opposite direction reverse this effects.

62. Linear Variable Differential Transformer (LVDT)
In next figure, the winding is connected ‘series opposing’ -that is the polarities of V1 and V2 oppose each other as we trace through the circuit from terminal A to B.
Consequently, when the core is in the center so that V1=V2, there is no voltage output, Vo = 0V.

63. Linear Variable Differential Transformer (LVDT)
When the core is away from S1, V1 is greater than V2 and the output voltage will have the polarity of V1.
When the core is away from S2, V2 is greater than V1 and the output voltage will have the polarity of V2.
That is the output of ac voltage inverts as the core passes the center position.
The farther the core moves from the centre, the greater the difference in value between V1 and V2, and consequently the greater the value of Vo.

64. Linear Variable Differential Transformer (LVDT)
Thus, the amplitude of Vo is a function of distance the core has moved. If the core is attached to a moving object, the LVDT output voltage can be a measure of the position of the object.
The farther the core moves from the centre, the greater the difference in value between V1 and V2, and consequently the greater the value of Vo.

65. Linear Variable Differential Transformer (LVDT)
Among the advantages of LVDT are as follows:
It produces a higher output voltages for small changes in core position.
Low cost
Solid and robust -capable of working in a wide variety of environments.
No permanent damage to the LVDT if measurements exceed the designed range.

66. Linear Variable Differential Transformer (LVDT)
Example 3:
An ac LVDT has the following data; input 6.3V, output 5.2V, range ±0.50 cm. Determine:
Plot of output voltage versus core position for a core movement going from +0.45cm to -0.03cm?
The output voltage when the core is -0.35cm from the center?
The core movement from center when the output voltage is -3V?
The plot of core position versus output voltages varying from +4V to -2.5V.

67. Piezoelectric Transducers
When a mechanical pressure is applied to a crystal of a Rochelle salt, quartz, or tourmaline type, a displacement of the crystals that will produce a potential difference will occur.
This property is used in piezo- electric transducers; where a crystal is placed between a solid base and force-summing element, as shown below:

68. Piezoelectric Transducers
When externally force is applied to the plates, a stress will be produced in the upper part of the crystal.
This deformation will produce a potential difference at the surface of the crystal. This produces an electromotive force across the crystal proportional to the magnitude of the applied pressure. This effect is called piezoelectric effects.
The induced charge on the crystal is proportional to the impressed force and given by:
Q = dF; where d = piezoelectric constant.

69. Temperature Transducers
The temperature transducers can be divided into four main categories:
Resistance Temperature Detectors (RTD)
Thermocouples
Thermistors
Ultrasonic transducers

70. Resistance Temperature Detectors (RTDs)
Detectors of resistance temperatures commonly employ platinum, nickel, or resistance wire elements, whose resistance variation with temperature has a high intrinsic accuracy.
They available in many configurations and sizes and as shielded and open units for both immersion and surface applications.

71. Resistance Temperature Detectors (RTDs)
Some examples of RTDs are as follows:

72. Resistance Temperature Detectors (RTDs)
The relationship between temperature and resistance of conductors can be calculated from this equation:
where;
R= resistance of the conductor at temp t (oC)
Ro=resistance at the reference temp.
= temperature coefficient of resistance
= difference between operating and reference temp.

73. Resistance Temperature Detectors (RTDs)
Example:
A platinum resistance thermometer has a resistance of 220Ω at 20oC. Calculate the resistance at 50oC? Given that 20oC=

74. Thermocouples
A thermocouple is a sensor for measuring temperature. It consists of two dissimilar / different metals, joined together at one end, which produce a small unique voltage at a given temperature. This voltage is measured and interpreted by the thermocouple.
The magnitude of this voltage depends on the materials used for the wires and the amount of temperatures difference between the joined end and the other ends.

75. Thermocouples
Some examples of the thermocouples are as follows:

76. Thermocouples
Common commercially available thermocouples are specified by ISA (Instrument Society of America) types.
Type E, J, K, and T are base-metal thermocouples and can be used up to about 1000°C (1832°F).
Type S, R, and B are noble-metal thermocouples and can be used up to about 2000°C (3632°F).

77. Thermocouples
The following table provides a summary of basic thermocouple properties.

78. Thermocouples
Calibration curves for several commercially available thermocouples is as below:

79. Thermocouples
The magnitude of thermal emf depends on the wire materials used and on the temperature difference between the junctions.
The effective emf of the thermocouple is given as:
Where;
c and k – constant of the thermocouple materials
T1 - temperature of the ‘hot’ junction.
T2 - temperature of the ‘cold’ or ‘reference’ junction.

80. Thermocouples Example
During experiment with a copper- costantan thermocouple, it was found that c= 3.75x10-2 mV/oC and k = 4.50x10-5 mV/oC. If T1= 100oC and the cold junction T2 is kept in the ice, compute the resultant electromotive force, emf?