ELECTRONIC INSTRUMENTATION EKT314/4 3. Transducers

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Transducers …definition Device that converts energy from one physical form into another. Physical variable into signal variable Sensor – input transducer Actuator – output transducer Two types of transducers Mechanical Electrical

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Electrical Transducers Sensing device that transform the physical, optical or mechanical quantity measurement directly to the electrical voltage or current relative to the input measurand.

Electrical Transducers: Parameters Sensitivity Electrical output per unit change in the input measurand. Linearity Linear relationship between physical parameter and electrical signal. Dynamic Range Can be use under wide range of measurement conditions.

Electrical Transducers: Parameters …continued Repeatability Input output relationship should be constant over a period of time. Physical Size Physically minimal in weight and volume.

Electrical Transducers: Advantages Electrical amplification and attenuation can be easily done. Output can be recorded and indicated remotely. Output can be modified to meet the requirements of indicating or controlling units. Signals can be conditioned and mixed to obtain any combinations.

Electrical Transducers: Advantages …continued Miniaturisation due to size and shape of electrical transducers. Contour design and dimensions can be chose not to disturb the measurand phenomenon. Low power required to control the system. Effects of friction are minimised. Mass-inertia effects are minimised.

Electrical Transducers: Disadvantages Low reliability due to ageing and drift of the active components. Can be expensive when associated with the signal conditioner. Less accuracy and resolution (in some cases).

Transduction elements Conversion of non-electrical quantity into electrical signal by the transducers may need it to be in two parts; Sensing element – part that responds to the changes in the physical quantity Transduction element – this part transforms the output from sensing element into electrical signals Transduction elements sometime can be refer as secondary transducer. It occupy a different electrical phenomenon.

Transduction elements… continued Resistive Inductive Capacitive Thermoelectric Photo emissive Piezoelectric Electromagnetic Photo resistive Potentiometric Frequency generating

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Classification of Transducers Two categories: Active Self generating devices Generates electrical signal directly in response to the physical parameter. Does not required external power source. Passive Operate under energy controlling principles. Requires external electrical source.

Electrical Parameters Principle of Operation Active Transducers Type of Transducers Electrical Parameters Principle of Operation Thermocouple and Thermopile Voltage and Current EMF is generated across the junction of two different metals or semiconductors when that junction is heated. Photovoltaic Voltage is generated across semiconductors junction device when radiant energy stimulated the cell. Piezoelectric Pickup EMF is generated when external force is applied to certain crystalline materials, such as quartz. Moving Coil Generator Voltage is generated from the moving of coil in magnetic field.

Electrical Parameters Principle of Operation Passive Transducers Type of Transducers Electrical Parameters Principle of Operation Photomultiplier Tube Voltage and Current Secondary electron emission due to incident radiation on photosensitive cathode. Photoemissive Cell Electron emission due to the incident radiation upon photo emissive surface. Hall effect Pickup Potential difference generated across a semiconductor plate when magnetic flux interacts with the applied current. Ionisation Chamber Electron flow induced by ionisation of gas due to the radio-active radiation.

Passive Transducers …continued Type of Transducers Electrical Parameters Principle of Operation Potentiometer Resistance Variation of resistance in a potentiometer or bridge circuit due to the positioning of the slider by an external force. Thermistor Resistance of certain metal oxide with negative temperature coefficient of resistance varies with temperature. Photoconductive Cell Variation of resistance of a cell as a circuit element with incident of light. Resistance Hygrometer Variation of resistance of a conductive strip with moisture content.

Passive Transducers …continued Type of Transducers Electrical Parameters Principle of Operation Dielectric Gauge Capacitance Variation of capacitance due to the changes of dielectric. Capacitor Microphone Sound pressure varies the capacitances between a fixed plate and a moveable diaphragm. Magnetic Circuit Breaker Inductance Variation of self or mutual inductance of an AC excited coil by changes in the magnetic circuit. Reluctance Pickup Reluctance of the magnetic circuits is varied by changing the position of the iron core of the coil.

Typical Applications of Transducers Pressure Displacement Force Torque Temperature Sound Power Current Magnetic Flux Vibration Velocity Light Position Humidity …

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Selection of Transducers Electrical Output Characteristics Compatibility of output impedance, frequency response and the response time of the transducer output signal with the recording devices or measurement system. Physical Environments Transducer selected should be able to endure the environmental conditions. Accuracy Able to reproduce exact output signal when same measurand is applied.

Selection of Transducers …continued Operating Range Range of transducer must be large enough to cover all expected magnitudes of the measurand. Sensitivity Provide sufficient output signal per unit of measured input. Errors Should be as minimise as possible.

Selection of Transducers …example Source: RS Data Sheet – Thermistors - 1999

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Photoelectric Transducers Primary types Photoemissive Cell Photoconductive Cell Photovoltaic Cell Photojunctions

Photoemissive Cell Electron emission due to the incident radiation upon photo emissive surface. Also known as phototube. Three basic types of phototube: Vacuum Gas-filled Photomultiplier

Photoemissive Cell …continued Vacuum Phototube Consists of rod anode and curvature cathode in the vacuum glass. Cathode is coated with emissive materials that emit electrons when light radiation occur on them. Stable, consistent characteristics over time when operate at low voltage and protected against excessive light. Moderate sensitivity due to small current flow in the vacuum tube. Source: http://cache.eb.com/eb/image?id=62961&rendTypeId=4

Photoemissive Cell …continued Gas-filled Phototube Same construction as vacuum phototube except the presence of inert gas, usually argon into tube. Emitted electrons are accelerated by the electric field and cause ionisation. Anode current increases due to high collision. Provide gain over response up to 10 compared to vacuum phototube. Not stable as vacuum type, the characteristics are not linear.

Photoemissive Cell …continued Photomultiplier Phototube Consists of evacuated glass envelope containing photo cathode, anode and dynodes (additional electrodes). Each dynode is at higher voltage than the previous dynode. Electrons emitted by cathode are attracted to first dynode and emitted again (secondary emission) to the following dynodes. High sensitivity and high frequency response but large in size and expensive.

Photoemissive Cell …continued Source: [Dally1993] pg. 148

Photomultiplier tube

Photoconductive Cell Fabricated from semiconductor materials such as Cadmium Sulphate (CdS) or Cadmium Selenide (CdSe). The increase of current with light intensity while the voltage remain constant makes the resistance of semiconductors decrease. Also known as Photo Resistor, Light Dependent Resistor (LDR). Simple, high sensitivity and low cost. Variations of temperature may affect resistance for a particular light intensity.

Photoconductive Cell… continued Construction Typical Curves of Resistance vs. Illumination Source: [Kalsi2005] pg. 421

Photoconductive Cell… example The relay of (a) of figure above is to be controlled by a photoconductive cell with the characteristics shown in (b) in figure above. The potentiometer delivers 10mA at a 30V setting when the cell is illuminated with 400 lm/m2 and is required to de-energized when cell is dark. Calculate (i) the required series resistance, and the (ii) dark current level. ([Kalsi2005] pg. 422 & 423).

Photoconductive Cell… example From the characteristic in (b), cell resistance at 400 lm/m2 is 1kΩ. Therefore, So,

Photoconductive Cell… example The cell dark resistance is 100kΩ Dark current mA

Photovoltaic Cell Semiconductor junction devices for converting radiation energy into electrical energy (voltage). Also known as solar cell. Conversion efficiency depends on the spectral content and illumination intensity. Main advantages is its ability to generate voltage at fast response. Can be used as energy converter (power provider) directly.

Photovoltaic Cell …continued Source: http://www.aurorasolarcar.com/solartech/celltech.html

Photojunctions Photodiodes Silicon diode with the lens on its case to focus the incident of the light to the junction. Without bias, operates like photovoltaic devices or voltage source. When reverse bias operates like photoconductive device. Important advantage is fast response compared to photoconductive device. It is also small and inexpensive. Source: [Kalsi2005] pg. 424

Photojunctions …continued Phototransistor NPN device by addition of junction to photodiode. Provide larger output current compared to photodiode for a given amount of illumination on a very small area. More sensitive than a photodiode up to the factor of 100. Source: [Kalsi2005] pg. 424

Contents Electrical Transducers Classification Selection of Transducers Photoelectric Transducers Temperature Transducers

Temperature Transducers Transducers that can be used to measure temperature. Resistance Temperature Detectors (RTD) Thermocouples Thermistors Other temperature transducers

Resistance Temperature Detectors (RTD) Usually make use of platinum, nickel or resistance wire elements. Resistance varies with the change of temperature. Almost all metals give high resistance when temperature increases. High value of temperature coefficient is required to sense a small changes in the temperature.

Resistance Temperature Detectors (RTD)… continued The temperature ranges and coefficient of resistance of various resistance wire can be tabulated as in table below; Material Range (°C) Coefficient of Resistance (Ω/Ω/°C) Platinum -200 ~ 850 0.0039 Copper -200 ~ 260 0.0067 Nickel -80 ~ 300 0.0043

Resistance Temperature Detectors (RTD)… continued Expression below relate the resistance of the conductors and the temperature; Rt is resistance of the conductor at temperature t°C Rref is resistance of the reference temperature (typically 0°C) α is temperature coefficient of resistance Δt is the difference between operating and reference temperature

Resistance Temperature Detectors (RTD)… continued Platinum RTD is the most widely used. Advantages: Wide operating temperature range. Stability at high temperature. Linearity. Disadvantages: Low sensitivity. Expensive. Easily affected by contact resistance.

Resistance Temperature Detectors (RTD)… continued Two Lead Wire RTD Uses Wheatstone’s Bridge* to measure the resistance. Low cost but in order to achieve high accuracy, the circuit must be stable and insensitive to the variations of ambient temperature. *Refer to Kalsi2005 pg. 305 ~ 312 Es is supply voltage, Eo is output voltage, R1, R2 and R3 are fixed value resistors, RL1 and RL2 are the resistance of the two leads and RT is resistance of RTD Source: [Kalsi2005] pg. 428

Resistance Temperature Detectors (RTD)… continued Three Lead Wire RTD Practical and accurate method for most industrial applications. The bridge circuit automatically balance resistance change due to ambient temperature change. Third lead is has no effect on the bridge ratios and balance. Source: [Kalsi2005] pg. 428

Thermocouples Consists of two different materials that are in thermal and electrical contact. Thermoelectric effect - Seebeck effect. Direct conversion of temperature differences to electrical voltage.

Thermocouples… continued The junctions at different temperature causes a circulation of current. Open circuit will generate voltage that is relative to the Seebeck current. Electromagnetic force (Thomson and Peltier) come from the conductors where the density of free charge carriers increases with temperature. Materials used for wire and sensing junction temperature effect the magnitude of the voltage generated. Reference temperature junction of thermocouple also known as cold junction.

Thermocouples… continued Common thermocouple materials combination: Platinum/Platinum-Rhodium Chromel/Alumel Chromel/constantan Copper/constantan Iron/constantan

Thermocouples… continued Thermocouple output voltage with respect to temperature for various thermocouple material Source: [Kalsi2005] pg. 434

Thermocouples… continued If in thermocouple, the voltage generated by the reference junction is the same with the one generated by the sensing junction, this give null output provided that both junctions is at the same temperature. This can be solve by a process known as cold junctions compensation. The reference junction is now at 0°C. Source: [Kalsi2005] pg. 436

Thermocouples… continued The isothermal block is made of material that is good conductor of heat but poor in electricity. Industries often use an isothermal block that contains a thermistor and two reference junctions. This setup known as electronic ice point reference. Source: [Kalsi2005] pg. 436

Isothermal block

Thermocouples… continued Advantages: High speed. Cheap. Rugged. Disadvantages: Low accuracy. Placed remote from measuring devices. Reference junctions compensation.

Thermistors Also called thermal resistors as the resistance varies as a function of temperature. Manufactured in the form of beads, discs and rods. Most thermistors have a negative coefficient (NTC) of temperature resistance. Three important characteristics: Resistance-Temperature Voltage-Current Current-Time

Thermistors… continued The resistance of thermistor, RT at a temperature T can be formulated as follows; From the equation above, a and b are constants that can be determined by the structure and material of thermistor

Thermistors… continued Resistance versus temperature of a NTC Thermistor Source: [Kalsi2005] pg. 396

Thermistors… continued Major applications of thermistors are measurement and control of temperature. Other applications of thermistors: Measurement of power at high frequencies. Measurement of thermal conductivity. Measurement of level, flow and pressure of liquids. Measurement of composition of gases. Vacuum measurement.

Thermistors… continued Advantages High sensitivity especially in NTC region. Fast response over narrow temperature range. Cold junction compensation is not required. No problems on contact and lead resistance. Low cost and small size. Limitations Characteristics of resistance and temperature are non-linear. Not recommended for wide temperature range applications. Need a shielded power lines or filters and low excitation current.

Thermistors… example The circuit of figure below is used for temperature measurement. The thermistor is 4kΩ type. The meter is 50mA meter with a resistance of 3Ω, Rc is set to 17Ω, and supply voltage Vt is 15V. What will be the meter reading at 77°F (25°C) and at 150°F. ([Kalsi2005] pg. 398).

Thermistors… example From the plot of temperature versus resistance, the resistance at 25°C is 4kΩ. So, the current at 25°C is mA

Thermistors… example At 150°F, the resistance is approximately 950Ω. The meter reading will be; mA

Other temperature transducers Bimetallic Strip Two strips of different metals welded together, in the form of straight cantilever beam with one end fixed. Due different thermal coefficient, one of the metals expands more than the other when heated. Commonly used as thermostat. Simple, cheap and robust.

Other temperature transducers …continued Integrated Circuit Temperature sensing element and signal conditioning electronics in single monolithic integrated circuit package. Advantages; linearity, cheap and high output. Disadvantages; require power supply, small range, self-heating, slow and limited configuration.

Difference in performance

Other temperature transducers …continued IC Type Sensor Source: [Kalsi2005] pg. 441

Other temperature transducers …continued Radiation Pyrometers Operation based on Stefan-Boltzmann law – sense the temperature from the energy radiated from heated blackbody optically. Construct such that a heat is focused onto the hot junction of thermocouple. Used for measuring a very high temperature. Two types of pyrometer that are fixed focus and variable focus.