sensors-interfacing techniques

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This class notes is intended for Electronics students of SKU and other ECE students of all universities

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  • Dr.Y.NARASIMHA MURTHY Ph.D yayavaram@yahoo.com Sensors and Interfacing techniquesIntroductionSensors are sophisticated devices which will detect and measure any non-electrical physicalquantity .A Sensor converts the physical parameter (for example: temperature, blood pressure,humidity, speed, etc.) into a signal which can be measured electrically.Sensor is sometimes called a primary measuring element, which can be found simply as amercury thermometer to measure the temperature. It may be embedded in the transducer toperform its function. That means the transducer consists of a primary element (sensor) plus asecondary element (signal conditioning circuit) that transforms the passive change or smallvoltage signal into active signal range that can be easily used in other chains of the control loop.So, we can write that Transducer = Sensor + Signal conditioning circuitThe important characteristics of sensors are Accuracy :High accuracy is needed Environmental condition The performance of the sensor should not depend on environmental conditions like temperature or humidity etc.. Wide Range .Measurement limit of sensor . The range should be high. Calibration - Essential for most of the measuring devices as the readings changes with time Resolution Sensor must be able to detect even small changes in the input signal. Cost- Sensor should not be very expensive. Repeatability The sensor must be able to reproduce the same output at similar conditions. Linearity : The output of the sensor must be linear .There are various types of sensors available , which measures various physical parameters. Sensor type Physical Parameter Temperature Temperature Light Light / dark Pressure Pressure or barometric pressure or blood pressure Moisture amount of moisture present in air1
  • Dr.Y.NARASIMHA MURTHY Ph.D yayavaram@yahoo.com Water-level How full / empty a container is Movement Movement nearby Proximity How close or far something isTypes of Sensors : Based on the principle of working the sensors are divided into Resistor type Capacitor Type & Inductor type etc..Resistor type Sensors: A sensor whose resistance changes with the input signal .i.e based on thevatiation of the resistance of the sensor element , the physical parameter is detected.For example , Themistor , Platinum resistance thermometer or Thermocouple etc.. are used tomeasure the temperature with respect to the variation in the resistance of the material. In the caseof a thermistor the temperature is given by Rt = R0.e (1/T - 1/T0 ) Here , is the constant of the thermistor. Rt is theresistance of the thermistor at a temperature t. The variation of resistance of a thermistor withtemperature is given by the following graph.A strain gauge is a thin metal foil that changes resistance with applied strain. Strain gauges arethe preferred choice in stress analysis due to their small size and relatively low cost. Straingauges can measure strain levels from a few micro-strain (e) to over 100,000 micro-strain.2
  • Dr.Y.NARASIMHA MURTHY Ph.D yayavaram@yahoo.comThe changes of resistance associated with strain gauges are small and present measurementsituation. A Wheatstone bridge arrangement is commonly used to measure the small changesin resistance associated with strain gauges.Capacitive type Sensors: These sensors work on the principle of variation of capacitance withthe external input signal.The basic operation of capacitive type sensors can be seen from the familiar equation for aparallel-plate capacitor. C = K0 A /dHere K is the dielectric constant of the material, 0 is the permittivity of the free space and d isthe distance between the parallel plates and A is the area of cross section.The capacitive type sensors are used to detect the physical parameters like Humidity or pressureor proximity etc..Inductive type Sensors: These sensors are based on the principle that the inductance of a coilvaries with the change input signal.For example, if a permeable core is inserted into an inductor the net inductance is increases.The best example is the LVDT (Linear Variable Differential Transducer) which is used tomeasure displacements accurately. LVDTs can measure displacements from a few microns toseveral feet in a wide variety of environments.Inductive proximity sensors use an electromagnetic field to detect the presence of metal objects.Inductive proximity sensors are available in a variety of sizes and configurations to meet varyingapplications. The sensor incorporates an electromagnetic coil which is used to detect thepresence of a conductive metal object. The sensor will ignore the presence of an object if it is notmetal.Limitations of Sensors : The sensors have certain limitations based on their working andconstruction. Some sensors require complex signal conditioning circuitry. The output of the certain ssensors is not linear and hence additional circuitry is needed. The response time of sensors is some times very high3
  • Dr.Y.NARASIMHA MURTHY Ph.D yayavaram@yahoo.com The size of some sensors is large and occupies more space. Many times a sensor may respond to more than to one parameter. For example, gas sensors respond to more than one gas at a time. Reliability of a sensor is also a serious problem in some cases.Need and Advantages of IC sensors: With the developments in semiconductor technology andwith the advent of high speed processors ,the need of IC sensors is rapidly increasing. TheIntegrated sensors provide significant advantages in terms of overall size and the ability to usesmall signals from the transduction element.(i)They do not require complicated signal conditioning circuitry.(ii).The response time of IC sensors is low.(iii).The IC sensors are also more stable and accurate.(iv).The response of IC sensors is more linear than normal sensors.(v).The IC sensors support many latest communication protocols(I2C , SPI ,Single wire pulsewidth modulation etc..), so that they can be interfaced easily with microprocessors andmicrocontrollers.(vi).Provide better noise immunity.(vii)The smart sensors available today will also provide an alert when the sensed value exceedscertain critical value.(viii).IC Sensors are relatively simple and inexpensive.Interfacing of temperature Sensor LM 35:The LM35 series sensors are precision integrated-circuit temperature sensors, whose outputvoltage is linearly proportional to the Centigrade temperature. It gives 10mV of output voltagefor every 10C. The LM35 does not require any external calibration or trimming to provide typicalaccuracies of C at room temperature and C over a full -55 to +150C temperature range.The LM35s low output impedance, linear output, and precise inherent calibration makeinterfacing to readout or control circuitry especially easy. It can be used with single powersupplies, or with dual supplies. As it draws only 60 A from its supply, it has very low self-heating, less than 0.1C in still air. The LM35 is rated to operate over a -55 to +150Ctemperature range. The LM35 IC sensor is available at a Low cost .LM35 is a three terminal IC with ,Vcc ,Ground & Vout .4
  • Dr.Y.NARASIMHA MURTHY Ph.D yayavaram@yahoo.comThe LM 35 IC gives a 10mV analog output voltage for every degree Celsius change intemperature. The Output of the temperature sensor is analog in nature so we need an analog todigital converter for converting the analog input to its equivalent binary output.Since the output of LM35 is analog voltage, it should be converted into digital before it isapplied to a microcontroller port pin. So,with proper circuitry an ADC chip is used as shownbelow.As shown in the circuit ADC0809 is used to convert the analog voltage into its equivalent digitalvoltage.For this necessary signal conditioning circuit is developed using an operationalamplifier.The ADC0809 is an 8-input channel system. A clock signal of 100KHz is applied.The5
  • Dr.Y.NARASIMHA MURTHY Ph.D yayavaram@yahoo.comvoltage drop across the LM Zener is buffered by an op-amp LM741to produce a Vcc and Vrefof around 5V to 12Vfor the A/D converter .With this reference voltage the A/D converter willhave 256 steps of 20mV each. Since the temperature sensor signal is amplified by 2 ,each degreeCelsius of temperature change will produce an output change of 20mV.or one step on the A/Dconverter. This gives us a resolution of 10C ,which is about the typical accuracy of thesensor.The advantage of using Vref as Vcc for the device is that this voltage will not have theswitching noise that the digital Vcc lines have. The control inputs and data outputs of the ADCare connected to the Ports of the Microcontroller as shown in the circuit.A relay is used to control the temperature. When the temperature of the bath is reached a settemperature, the microcontroller sends a High bit to

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