Basic Architecture of Electronics Instrumentation Measurement System MEASURAND TRANSDUCER SIGNAL CONDITIONING DISPLAY RECORD

Sensors and transducers a device that converts a primary form of energy into a corresponding signal take form of a sensor or an actuator SENSORS a device that detects/measures a signal ACTUATOR a device that generates a signal Example, a Heater is an actuator while a Thermometer is the sensor

Electronic sensors Generally electronic sensor consists of a primary transducer: changes “real world” parameter into electrical signal for example, heat, sound, etc for example, a microphone (input device) converts sound waves into electrical signals for the amplifier to amplify (a process), and a loudspeaker (output device) converts these electrical signals back into sound waves

Quantity being Measured Input Device (Sensor) Light Level Light Dependant Resistor (LDR) Photodiode Photo-transistor Solar Cell Temperature Thermocouple Thermistor Thermostat Resistive Temperature Detectors Force/Pressure Strain Gauge Pressure Switch Load Cells Position Potentiometer Encoders Reflective/Slotted Opto-switch LVDT Speed Tacho-generator Reflective/Slotted Opto-coupler Doppler Effect Sensors Sound Carbon Microphone Piezoelectric Crystal

Input type transducers or sensors, produce a voltage or signal output response which is proportional to the change in the quantity that they are measuring . The type or amount of the output signal depends upon the type of sensor being used. The types of sensors can be classed as two kinds, either Passive Sensors or Active Sensors.

Active Sensors Generally, active sensors require an external power supply to operate, called an excitation signal which is used by the sensor to produce the output signal. Active sensors are self-generating devices because their own properties change in response to an external effect producing for example, an output voltage of 1 to 10 V DC or an output current such as 4 to 20 mA DC.

EXAMPLE 1 – STRAIN GAUGE (ACTIVE SENSOR) It does not generate an electrical signal itself, but by passing a current through it (excitation signal), its electrical resistance can be measured by detecting variations in the current and/or voltage across it. Force

The Wheatstone bridge provides a way to convert these changes in resistance to changes in voltage, which are easy to work with. - + R2 in the diagram is set at a value equal to the strain gauge resistance when there is no force applied. R1 and R3 are set equal to each other. Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge.

Which means that before any force is applied R4 also equals to R where R4 is the resistance of the strain gauge So let say R1 = R3 = R2 = R Which means that before any force is applied R4 also equals to R Hence, any changes in R4 can be denoted as (R + R)

𝑉 𝑂 𝑉 𝑖 = 𝑅+∆𝑅 𝑅+𝑅+∆𝑅 − 𝑅 𝑅+𝑅 𝑉 𝑂 𝑉 𝑖 = 𝑅+∆𝑅 2𝑅+∆𝑅 − 𝑅 2𝑅 𝑉 𝑂 𝑉 𝑖 = 𝑅+∆𝑅 2𝑅+∆𝑅 − 1 2 𝑉 𝑂 𝑉 𝑖 = 2𝑅+2∆𝑅 4𝑅+2∆𝑅 − 2𝑅−∆𝑅 4𝑅+2∆𝑅 𝑉 𝑂 𝑉 𝑖 = ∆𝑅 4𝑅+2∆𝑅 𝑉 𝑂 𝑉 𝑖 ≅ ∆𝑅 4𝑅

What is the new resistance value of the strain gauge for Vo = 5.32 mV? No. of coins Output Voltage (mV) 1 0.769 2 1.389 3 2.108 4 2.76 5 3.38 6 3.98 7 4.64 8 5.32 9 6.35 10 REF: http://www.slideshare.net/umangIITD/transducer-and-instrumentation Consider that the excitation voltage applied is 10 V and the value of R is 120 . What is the new resistance value of the strain gauge for Vo = 5.32 mV? Calculate the percentage of increment of the resistance Answers: 120.255 , 0.2125 %

Relationship with Change of resistance, R = RoG Where Ro = initial resistance when there is no applied stress, G = gauge factor and  is the strain unit deformation and  = / E Where  = the mechanical stress (Pa) E = Young’s Modulus which is specific for each type of material Continue from previous example: Given G = 2.12, and the Young’s Modulus of aluminium is 72 GPa. Calculate the mechanical stress. R = RoG R = RoG [/ E] 0.255 = 120 (2.12) [/ 72x109]  = 72.17 x 106 Answers: 72.17 MPa

+ - + - Derive the output voltage equation, Vo 𝑉 𝑜 = 𝑅+∆𝑅 𝑅−∆𝑅+𝑅+∆𝑅 − 𝑅 𝑅+𝑅 𝑉 𝑖 𝑉 𝑜 = 𝑅+∆𝑅 2𝑅 − 𝑅 2𝑅 𝑉 𝑖 𝑉 𝑜 = ∆𝑅 2𝑅 𝑉 𝑖

Passive Sensors Unlike an active sensor, a passive sensor does not need any additional energy source and directly generates an electric signal in response to an external stimulus. For example, a thermocouple or photo-diode. Passive sensors are direct sensors which change their physical properties, such as resistance, capacitance or inductance etc.

V = Vout = V1 – V2 provided that temperature T3 = T4 EXAMPLE 1 – THERMOCOUPLE V = Vout = V1 – V2 provided that temperature T3 = T4

Operation: a conductor generates a voltage when subjected to a temperature gradient. a second conductor material will also generates a different voltage under the same temperature gradient. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings.

Where, SA and SB are referred to as Seebeck Coefficients (unit is V/K) and with the assumption that the coefficient remains constant through out the metal Tref Ttip + -

Seebeck coefficient relative to platinum (μV/K) Material Seebeck coefficient relative to platinum (μV/K) Selenium 900 Tellurium 500 Silicon 440 Germanium 330 Antimony 47 Nichrome 25 Chromel 22 Iron 19 Molybdenum 10 Cadmium, tungsten 7.5 Gold, silver, copper 6.5 Rhodium 6.0 Tantalum 4.5 Lead 4.0 Aluminium 3.5 Carbon 3.0 Mercury 0.6 Platinum 0 (definition) Sodium -2.0 Potassium -9.0 Nickel -15 Alumel -17. 3 Constantan -35 Bismuth -72

The thermocouple below is using Metal A as copper and Metal B as constantan. If the measured voltage in the following circuit is 3.53 mV, what is the temperature of the hot junction (in °C), if the cold junction is at 0°C? Tref Ttip + - Seebeck coefficient for copper: 6.5 V/K Seebeck coefficient for constantan: -35 V/K 𝑇 𝑡𝑖𝑝 =273.15+ 3.53 𝑚𝑉 41.5𝜇𝑉/𝐾 𝑇 𝑡𝑖𝑝 =273.15+85.1=358.25 𝐾 𝑇 𝑡𝑖𝑝 𝑖𝑛 ℃=358.25−273.15=85.1℃

A thermocouple is shown in the figure below A thermocouple is shown in the figure below. The two metals are Chromel (Seebeck coefficient is 22 V/K) and Metal B . The temperature measured by the thermocouple is 95 C and the measured differential voltage is 3.73 mV. Based on Seebeck Coefficient Table, find out what is Metal B assuming Tref as 0 C. 95=0+ 3.73 𝑚𝑉 22− 𝑆 𝐵 22𝜇− 𝑆 𝐵 = 3.73 𝑚𝑉 95 22𝜇−39.26𝜇= 𝑆 𝐵 Metal B is Alumel 𝑆 𝐵 =−17.26𝜇𝑉/𝐾

EXAMPLE 2 – PHOTODIODE A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode As for LED – Lights are emitted as a result of electrons recombine with holes. Circuit symbol for photodiode Operation: A photon (light) of sufficient energy strikes the diode, it excites electrons – creating an electron-hole pairs. If this process occurs near the depletion region, the electrons will be swept towards the negative terminal producing current flow Photodiode is connected similar to reverse-biasing of a diode

I-V Characteristic of Photodiode Dark current refers to the leakage current produced by the diode although there is no light Photovoltaic mode; no biased Photoconductive mode Diode is reverse biased

Application of Photodiode The detector is reverse biased to produce a linear response to the applied input light. The amount of photocurrent generated is based upon the incident light and wavelength and can be viewed on an oscilloscope by attaching a load resistance on the output. The function of the RC filter is to filter any high-frequency noise from the input supply that may contribute to a noisy output.

Basic Transimpedance Amplifier Also known as current to voltage converter – used when current has a more linear response compared to the voltage The feedback capacitor, Cf is used to improve the stability of the circuit. Vout = - I x RF (for low frequency signal i.e DC

When the UV LED is turned on, reverse current flow through the photodiode, D1 from cathode to anode to the base of Q1 (BJT). The current is amplified and use to light up a LED. Calculate the current Id if the voltage drop across the output LED is 0.65 V and current gain  = 190 and Vout = 6 V 0.47 IC + 0.6 + 6 – 12 = 0 IC = 11.5 mA Id = 11.5 /  = 60.5 A

EXAMPLE 3 – PIEZOELECTRIC Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress.

Piezoelectric materials respond to mechanical stimuli by producing an instantaneous current or voltage due to polarization changes caused by the change of shape Example of Materials Silicon Oxide (Quartz) Zinc Oxide, ZnO Aluminium Nitride PZT - lead zirconate titanate

In the flat region (the yellow region), the sensor can be modeled as a voltage source in series with the sensor's capacitance or a charge source in parallel with the capacitance

v A = electrode area d = thickness of the piezomaterial F = force q = charge V = voltage dij and ii are piezo constants Figure above illustrates the piezoelectric effect with the help of a compression disk. A compression disk looks like a capacitor with the piezo material sandwiched between two electrodes. A force applied perpendicular to the disk causes a charge production and a voltage at the electrodes. q = dij F 𝑉= 𝑡𝑞 𝐴𝜀𝑖𝑖 = 𝑡𝑑 𝑖𝑗 𝐹 𝐴𝜀𝑖𝑖 where i and j corresponds to direction of the electrode and the force respectively

d31 mode d33 mode 1 3 2

REF: http://www. scielo. br/scielo. php

d31 = 22 pm/V 33 = 0.1062 nF 𝑉= 𝑡𝑑 𝑖𝑗 𝐹 𝐴𝜀𝑖𝑖 electrode 52 m 10 cm F A piezoelectric sensor uses PVDF strip measuring 10 cm by 10 cm, and 52 m thick. Calculate the voltage output when a 40 kg compression is applied along direction 1 𝑉= 𝑡𝑑 𝑖𝑗 𝐹 𝐴𝜀𝑖𝑖 d31 = 22 pm/V 33 = 0.1062 nF

𝑉= 52× 10 −6 22× 10 −12 40(9.8) (52× 10 −6 )(0.1)(0.1062× 10 −9 ) 𝑉=812 𝑉 electrode 10 cm 52 m F Answer: 812 V

Analogue and Digital Sensors Analogue Sensors produce a continuous output signal or voltage which is generally proportional to the quantity being measured. Examples: Temperature, Speed, Pressure, Displacement, Strain Characteristic: Continuous in nature For example, the temperature of a liquid can be measured using a thermometer or thermocouple which continuously responds to temperature changes as the liquid is heated up or cooled down. Very small signal (µV to mV) range Amplification Analogue to Digital conversions

Digital Sensors produce a discrete digital output signals or voltages that are a digital representation of the quantity being measured. Digital sensors produce a Binary output signal in the form of a logic “1” or a logic “0”, (“ON” or “OFF”). For example, digital temperature sensor, DS1620 provides temperature of device with 9-bit temperature readings. thermostat with its three thermal alarm outputs. temperature of device is greater than or equal to user defined temperature, TH then THIGH is driven high. temperature of the device is less than or equal to user defined temperature, TL then the TLOW is driven high. If the temperature of the device exceeds TH and remains high until it falls below that of TL, then the TCOM is driven high.