bioinstrumentation & biosensors ananda vardhan.h nhce
TRANSCRIPT
BIOINSTRUMENTATION&
BIOSENSORS
Ananda Vardhan.HNHCE
Electrical quantities and units
• Essential part of any measurement.• Basic electric quantities are Electrical current & Voltage Electrical charge Resistance Capacitance Inductance Electric power
Electrical quantities & Units
Functional Elements of an Instrumentation system
An Instrument may be defined as a device or a system which is designed to maintain a
functional relationship between prescribed properties of physical variables and must
include ways and means of communication to a human observer
• Primary Sensing Element: - The Measurand is first detected by primary sensing element. The primary sensing element transfers the measurand to variable conversion element for further processing. The output signal of a primary sensing element is a physical variable such as displacement or voltage.
• Variable Conversion Element: - The output signal of a primary sensing element may require to be converted to more suitable variables while preserving its information content. This function is performed by variable conversion element and it may be considered as an intermediate transducer
• Variable Manipulation Element: - This element is an intermediate stage of a measuring system. It modifies the direct signal by amplification, filtering, etc; so that a desired output is produced the physical nature of the variable remains unchanged during this stage.
• Data Transmission Element: - when the functional elements of the measuring system are spatially separated then it becomes necessary to transmit signals from one element to another. This function is performed by data transmission element. It is an essential functional element where remote control operation is desired.
• Data Presentation Element: - usually information about the quantity being measured is to be communicated to human observer for monitoring control and analysis purpose. This is therefore, to be presented in form of human sensory capability. This function is done by data presentation element.
Characteristics of measurement systemsThe system characteristics are to be known, to choose an instrument most suited to a particular measurement application.The performance characteristics may be broadly divided into two groups, namely ‘static’ and ‘dynamic’ characteristics. • Static characteristics: the performance criteria for the measurement of quantities that remain constant, or vary only quite slowly. • Dynamic characteristics: the relationship between the system input and output when the measured quantity (measurand) is varying rapidly.
Principles of analog & digital meters
• A multimeter or a multitester, also known as a VOM (Volt-Ohm meter), is
an electronic measuring instrument that combines several measurement functions in one
unit. • A typical multimeter would include basic features
such as the ability to measure voltage, current, and resistance.
• Two types of multimeters generally used are analog and digital multimeters.
Analog & Digital meters• Analog multimeters use a microammeter whose
pointer moves over a scale calibrated for all the different measurements that can be made.
• Digital multimeters (DMM, DVOM) display the measured value in numerals, and may also display a bar of a length proportional to the quantity being measured.
Digital multimeters are now far more common than analog ones, but analog multimeters are still preferable in some cases, for example when
monitoring a rapidly varying value.
An analog meter is a device which represents the value of a measurement by a point or similar
indicator. A digital meter indicates the measure by actual displaying the number representing the value.
For example, an analog thermometer shows a
temperature of 25 degrees by pointing halfway between the 20 and 30 while a digital one displays
the digits of the number 25.
Cathode-ray oscilloscope (CRO)• Electronic-display device containing a cathode-ray
tube (CRT) that generates an electron beam that is used to produce visible patterns or graphs, on a phosphorescent screen.
• The graphs plot the relationships between two or more variables, with the horizontal axis normally being a function of time and the vertical axis usually a function of the voltage generated by the input signal to the oscilloscope.
• Because almost any physical phenomenon can be converted into a corresponding electric voltage through the use of a transducer, the oscilloscope is a versatile tool in all forms of physical investigation.
Cathode-ray oscilloscope (CRO)
Cathode-ray oscilloscope (CRO)• The central component in cathode-ray
tube, consists of an evacuated glass container with a phosphorescent coating at one end (similar to that of a television screen) and an electron gun and a system for focusing and deflecting the beam of electrons at the other.
• The electron beam emerging from the electron gun passes between pairs of metal plates mounted in such a way that they deflect the beam horizontally and vertically to control the production of a luminous pattern on the screen.
The screen image is a visual representation of the voltages applied to the deflection plates. Alternatively, the beam may be deflected magnetically by varying the currents through externally mounted deflection coils. Thus, almost any graph can be plotted on the screen by generating horizontal and vertical deflection voltages or currents proportional to the lengths, velocities, or other quantities being observed.
Cathode-ray oscilloscope (CRO)
The cathode-ray oscilloscope is one of the most widely used test instruments;
its commercial, engineering, and scientific applications include acoustic
research, television-production engineering, and electronics design.
Energy meter / Electricity meter
• Measures the amount of electric energy consumed by a residence, business or
an electrically powered device.• Electricity meters are typically calibrated in
billing units, the most common one being the kilowatt hour [kWh].
• Periodic readings of electricity meters establishes billing cycles and energy used
during a cycle.
The most common unit of measurement on the electricity meter is the kilowatt hour [kWh], which is equal to the amount of energy used by a load of one kilowatt over a period of one hour, or 3,600,000 joules.
Mechanism of electromechanical induction meter.1 - Voltage coil - many turns of fine wire encased in plastic, connected in parallel with load.2 - Current coil - three turns of thick wire, connected in series with load.3 - Stator - concentrates and confines magnetic field.4 - Aluminum rotor disc.5 - rotor brake magnets.6 - spindle with worm gear.7 - display dials - note that the 1/10, 10 and 1000 dials rotate clockwise while the 1, 100 and 10000 dials rotate counter-clockwise.
Electronic meters• Electronic meters display the energy used on
an LCD or LED display, and some can also transmit readings to remote places.
• In addition to measuring energy used, electronic meters can also record other parameters of the load and supply such as instantaneous and maximum rate of usage demands, voltages, power factor and reactive power used etc. They can also support time-of-day billing, for example, recording the amount of energy used during on-peak and off-peak hours.
Frequency meter• A is an electronic instrument that displays
the frequency of a periodic electrical signal.• Frequency meter, device for measuring the repetitions
per unit of time (customarily, a second) of a complete electromagnetic waveform.
• Various types of frequency meters are used. Many are instruments of the deflection type, ordinarily used for measuring low frequencies but capable of being used for frequencies as high as 900 Hz. These operate by balancing two opposing forces. Changes in the frequency to be measured cause a change in this balance that can be measured by the deflection of a pointer on a scale.
Timers• A timer is a specialized type of clock for
measuring time intervals. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch; a device which counts down from a specified time interval is more usually called a timer. A simple example is an hourglass.
• Some timers sound an audible indication that the time interval has expired. Time switches, timing mechanisms which activate a switch, are sometimes called "timers".
Timers• Timers and time switches may be free-standing or
incorporated into appliances and machines. Their operating mechanism may be mechanical (typically clockwork), electromechanical, or purely electronic (counting cycles of an electronic oscillator). Timing functionality can be provided by software, typically in a computer; the program is often called a "timer".
Timers
Transducer• A transducer is a device that convert one form of
energy to other form. It converts the measurand to a usable electrical signal.
• In other word it is a device that is capable of converting the physical quantity into a proportional electrical quantity such as voltage or current.
Pressure Voltage
BLOCK DIAGRAM OF TRANSDUCERS
• Transducer contains two parts that are closely related to each other i.e. the sensing element and transduction element.
• The sensing element is called as the sensor. It is device producing measurable response to change in physical conditions.
• The transduction element convert the sensor output to suitable electrical form.
CHARACTERISTICS OF TRANSDUCERS
1. Ruggedness2. Linearity3. Repeatability4. Accuracy 5. High stability and reliability6. Speed of response7. Sensitivity8. Small size
TRANSDUCERS SELECTION FACTORS1. Operating Principle: The transducer are many times selected on the
basis of operating principle used by them. The operating principle used may be resistive, inductive, capacitive , optoelectronic, piezo electric etc.
2. Sensitivity: The transducer must be sensitive enough to produce detectable output.
3. Operating Range: The transducer should maintain the range requirement and have a good resolution over the entire range.
4. Accuracy: High accuracy is assured.5. Cross sensitivity: It has to be taken into account when measuring
mechanical quantities. There are situation where the actual quantity is being measured is in one plane and the transducer is subjected to variation in another plan.
6. Errors: The transducer should maintain the expected input-output relationship as described by the transfer function so as to avoid errors.
Contd.7. Transient and frequency response : The transducer should
meet the desired time domain specification like peak overshoot, rise time, setting time and small dynamic error.
8. Loading Effects: The transducer should have a high input impedance and low output impedance to avoid loading effects.
9. Environmental Compatibility: It should be assured that the transducer selected to work under specified environmental conditions maintains its input- output relationship and does not break down.
10. Insensitivity to unwanted signals: The transducer should be minimally sensitive to unwanted signals and highly sensitive to desired signals.
CLASSIFICATION OF TRANSDUCERS
The transducers can be classified as: I. Active and passive transducers.II. Analog and digital transducers.III. On the basis of transduction principle used.IV. Primary and secondary transducerV. Transducers and inverse transducers.
• Active transducers :
• These transducers do not need any external source of power for their operation. Therefore they are also called as self generating type transducers.
I. The active transducer are self generating devices which operate under the energy conversion principle.
II. As the output of active transducers we get an equivalent electrical output signal e.g. temperature or strain to electric potential, without any external source of energy being used.
ACTIVE AND PASSIVE TRANSDUCERS
CLASSIFICATION OF ACTIVE TRANSDUCERS
• Passive Transducers :
I. These transducers need external source of power for their operation. So they are not self generating type transducers.
II. A DC power supply or an audio frequency generator is used as an external power source.
III. These transducers produce the output signal in the form of variation in resistance, capacitance, inductance or some other electrical parameter in response to the quantity to be measured.
PASSIVE TRANSDUCERS
CLASSIFICATION OF PASSIVE TRANSDUCERS
PRIMARY AND SECONDARY TRANSDUCERS
• Some transducers contain the mechanical as well as electrical device. The mechanical device converts the physical quantity to be measured into a mechanical signal. Such mechanical device are called as the primary transducers, because they deal with the physical quantity to be measured.
•The electrical device then convert this mechanical signal into a corresponding electrical signal. Such electrical device are known as secondary transducers.
CONTD
•Ref fig in which the diaphragm act as primary transducer. It convert pressure (the quantity to be measured) into displacement(the mechanical signal).•The displacement is then converted into change in resistance using strain gauge. Hence strain gauge acts as the secondary transducer.
CLASSIFICATION OF TRANSDUCERSAccording to Transduction Principle
STRAIN GAUGE
• The strain gauge is a passive, resistive transducer which converts the mechanical elongation and compression into a resistance change.
• This change in resistance takes place due to variation in length and cross sectional area of the gauge wire, when an external force acts on it.
TYPES OF STRAIN GAUGE
• The type of strain gauge are as
1. Wire gauge
a) Unbonded
b) Bonded
c) Foil type
2. Semiconductor gauge
UNBONDED STRAIN GAUGE
• This gauge consist of a wire stretched between two point in an insulating medium such as air. The wires may be made of various copper, nickel, crome nickle or nickle iron alloys.
• The element is connected via a rod to diaphragm which is used for sensing the pressure.
• The wires are tensioned to avoid buckling when they experience the compressive force.
• the diaphragm act as primary transducer. It convert pressure (the quantity to be measured) into displacement(the mechanical signal).
• The displacement is then converted into change in resistance using strain gauge. Hence strain gauge acts as the secondary transducer.
• The unbounded meter wire gauges used almost exclusively in transducer application employ preloaded resistance wire connected in Wheatstone bridge as shown in fig.
• At initial preload the strain and resistance of the four arms are nominally equal with the result the output voltage of the bridge is equal to zero.
• Application of pressure produces a small displacement , the displacement increases a tension in two wire and decreases it in the other two thereby increase the resistance of two wire which are in tension and decreasing the resistance of the remaining two wire .
• This causes an unbalance of the bridge producing an output voltage which is proportional to the input displacement and hence to the applied pressure .
Resistance thermometers / Resistance temperature detectors (RTDs),
• Sensors used to measure temperature by correlating the resistance of the RTD element with temperature.
• Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it.
• The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature.
Resistance thermometers / Resistance temperature detectors (RTDs),
RTD Applications
RTD Sensor assemblies can be categorized into two groups by how they are installed or interface with
the process
immersion or
surface mounted.
RTD Applications- Immersion sensors• Process connection fitting. • They are installed into the process with sufficient
immersion length to ensure good contact with the process medium and reduce external influences.
• These styles are used to measure fluid or gas temperatures in pipes and tanks. Most sensors have the sensing element located at the tip of the stainless steel tube.
• An averaging style RTD however, can measure an average temperature of air in a large duct. This style of immersion RTD has the sensing element distributed along the entire probe length and provides an average temperature.
• Lengths range from 3 to 60 feet.
RTD Applications- Surface mounted• Used when immersion into a process fluid is not
possible due to configuration of the piping or tank, or the fluid properties may not allow an immersion style sensor.
• Configurations range from tiny cylinders to large blocks which are mounted by clamps, adhesives, or bolted into place.
• Most require the addition of insulation to isolate them from cooling or heating effects of the ambient conditions to ensure accuracy.
RTD Applications• Other applications may require special
waterproofing or pressure seals. A heavy-duty underwater temperature sensor is designed for complete submersion under rivers, cooling ponds, or sewers.
• Steam autoclaves require a sensor that is sealed from intrusion by steam during the vacuum cycle process.
Immersion sensors generally have the best measurement accuracy because they are in direct
contact with the process fluid.
Linear variable differential transformer (LVDT) The (also called just a differential transformer,
linear variable displacement transformer, or linear variable displacement transducer )is a type of
electrical transformer used for measuring linear displacement (position).
LVDTs are robust, absolute linear position/displacement transducers; inherently
frictionless, they have a virtually infinite cycle life when properly used.
• Precision of LVDT– Movements as small as a few millionths of an inch– Usually measurements are taken on the order of ±12 inches – Some LVDT’s have capabilities to measure up to ±20 inches
Linear variable differential transformer (LVDT)
LVDTs do not contain any electronics, they can be designed to operate at cryogenic temperatures or
up to 1200 °F (650 °C), in harsh environments, under high vibration and shock levels.
LVDTs have been widely used in applications such as power turbines, hydraulics, automation, aircraft,
satellites, nuclear reactors, and many others. These transducers have excellent repeatability.
Linear variable differential transformer (LVDT)
The LVDT converts a position or linear displacement from a mechanical reference (zero, or null position)
into a proportional electrical signal containing phase (for direction) and amplitude (for distance)
information. The LVDT operation does not require an electrical contact between the moving part (probe or core
assembly) and the coil assembly, but instead relies on electromagnetic coupling.
LVDT Components
Signal conditioning circuitry
Primary coil
Secondary coil
Secondary coil
Bore shaft
Ferrous core
Source: http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/lvdt_primer.pdf
Cross section of a DC-LVDT
Epoxy encapsulation
Stainless steel end caps
High density glass filled coil forms
Magnetic shielding
LVDT applications
Automation MachineryCivil/Structural EngineeringPower GenerationManufacturingMetal Stamping/FormingPulp and PaperIndustrial ValvesR & D and TestsAutomotive Racing
Uses (cont.) Civil/Structural Engineering Examples
Displacement measurement of imbedded concrete anchors tested for tensile, compression, bending strength and crack growth in concrete
Deformation and creep of concrete wall used for retaining wall in large gas pipe installation
Dynamic measurement of fatigue in large structural components used in suspension bridges
Down-hole application: measuring displacement (creep) of bedrock
PIEZO ELECTRIC TRANSDUCER According to Transduction Principle
:
•In piezoelectric induction the measurand is converted into a change in electrostatic charge q or voltage V generated by crystals when mechanically it is stressed as shown in fig.
•In electromagnetic transduction, the measurand is converted to voltage induced in conductor by change in the magnetic flux, in absence of excitation.•The electromagnetic transducer are self generating active transducers•The motion between a piece of magnet and an electromagnet is responsible for the change in flux
ELECTROMAGNETIC TRANSDUCTION
Current induced in a coil.
OPTICAL SENSORS• NEW REVOLUTION OF OPTICAL FIBER SENSORS• IT IS A “SPIN-OFF” FROM OTHER OPTICAL
TECHNOLOGIES• SEEING THE POTENTIAL IN SENSING
APPLICATIONS – DEVELOPED AS ITS OWN FIELD
WHY OPTICAL SENSORS• ELECTROMAGNETIC IMMUNITY • ELECTRICAL ISOLATION • COMPACT AND LIGHT • BOTH POINT AND DISTRIBUTED CONFIGURATION • WIDE DYNAMIC RANGE • AMENABLE TO MULTIPLEXING• REACHING IN-ACCESSIBLE REGIONS OF BODY
Working principle
•LIGHT BEAM CHANGES BY THE PHENOMENA THAT IS BEING MEASURED•LIGHT MAY CHANGE IN ITS FIVE OPTICAL PROPERTIES i.e INTENSITY, PHASE, POLARIZATION,WAVELENGTH AND SPECTRAL DISTRIBUTION
Types - PHOTOMETRIC TRANSDUCERS
Used to monitor changes in amplitude or frequency of reflected light …… Localized observations…. Blood or tissue …. Ex. oximeter used for calculating % saturation of oxygen in blood.
Types - Physical transducers
• Measure temperature and pressure• Ex. Temperature sensors based on optic fibres
Nortech's fiber-optic temperature sensor probe consists of a gallium arsenide crystal and a dielectric mirror on one end of an optical fiber and a stainless steel connector at the other end.
Types
CHEMICAL SENSORS• REMOTE SPECTROSCOPY• GROUNDWATER AND SOIL CONTAMINATION• MAJOR PLAYERS IN CHEMICAL SENSORS
1) PHARMACIA BIOTECH (SWEDEN) 2) FIBERCHEM 3) THE QUANTUM GROUP
Transducers – applications
• Strain gauges – resistive transducers : muscle force and stress
• Thermistors – temperature, respiration• Inductive transducers – flow rate, muscle
movement (LVDT)• Photoelectric transducers – pulse , BP and
Oxygen analysis• Chemical transducers – pH monitoring• Piezoelectric transducers – pulse and
ultrasonic blood flow monitoring
