Sensors 1 PDF

Experimental Methods MACE 6X021 2024-2025 Sensors 1 – Choice of Sensors Dr Mark Quinn – [email protected] Dr Andrew Kennaugh – [email protected] School of Mec...

Experimental Methods MACE 6X021 2024-2025 Sensors 1 – Choice of Sensors Dr Mark Quinn – [email protected] Dr Andrew Kennaugh – [email protected] School of Mechanical, Aerospace and Civil Engineering Sensors 1 and Sensors 2 The lectures on sensors have been divided into 2 areas, namely: 1. Choice of Sensors 2. Use of Sensors The first lecture will consider what you need to think about when selecting a sensor for a certain purpose The second lecture will look at how you would use a sensor and what additional equipment may be needed Sensors Why do we care? As engineers you need information to help you understand what’s happening in the physical world around you. give you data to help you design, validate, control, alert…. You will (hopefully) ask yourself the question “How do I measure……?” There isn’t a right or wrong answer. The answer may be different depending on a number of factors: accessibility, budget, availability, technology, speed, DAQ Sensors Sensor : Definition Physical “load” T, P, U, a, F A device which detects or measures a physical phenomenon and records, indicates, or otherwise generates a measurable output. A sensor, therefore, generates an output (usually electronic) in response to some physical phenomenon. Volts, mA digital value As engineers we measure that output to understand what we are studying. Aims After the end of this lecture you should: Understand that there are many different sensors, even for measuring the same physical property. Be familiar with some of the terminology associated with sensors Be aware that most sensors need additional electronics, either as power supplies and/or controllers as well as amplifiers in order to operate and generate measurable signals. Be aware that additional circuitry or components may be needed to use sensors effectively (resistors or capacitors… usually on data sheet) Sensors You have already used a sensor, the potentiometer. It, like most sensors Has a power input voltage (5V) Has a ground voltage (0V) + Has a signal voltage (0-5V) * Responds to a physical input (twisting the knob… changing an angle) Has a limited range of physical measurement (0-280 deg) Has a calibration (physical units/volts) Has a designed mounting arrangement (tabs* and through hole soldering+) It, unlike many sensors, is ratiometric (voltage output depends on the input) Volts Sensors In Engineering we are interested in many different physical properties or parameters and need to be able to measure them accurately Pressure sensors Strain gauge, Thermocouple, thermistor Hall effect sensor load cell, force sensor and IR camera and magnet Sensors - Terminology When looking at sensors you will come across 2 particular terms. Transducer : literally a device that converts an input in one form to an output in another form, e.g. mechanical or physical to an electrical signal, typically a voltage. Transmitter : a device that transmits a signal that is usually a 4-20mA current. Transducer is the more generic term for a sensor. Sensor Use Sensors are used in two main ways, as far as the user is concerned. For Monitoring – checking parameters in a process to ensure standards are maintained and/or to optimise performance or to maintain performance or for safety. Manufacturing, production, planes, trains, automobiles, ships, elevators, printers, photocopiers, health (heart rate, blood pressure, oxygen levels), security (biometrics), gas sensing (CO, NOX, O2) But……. how many times are false signals generated? Aircraft diverted due to false warnings. Cars with tyre pressure sensors giving low pressure warnings. Critical systems have redundancy; separate sensors measuring the same parameter, but not necessarily in the same place (either side of cockpit on aircraft) For Experiments – gathering data for research/testing purposes. Sensor Use In most production and processing scenarios, major factors are durability, reliability and stability. It’s expensive to close a production run to change a sensor It can be inconvenient if a sensor stops working – elevator, stairlift. If you’re designing a process you may need to consider what happens if a sensor fails. At the extreme, there are some sensors that can’t be replaced so they do have to last a lifetime under, potentially, severe or inaccessible operating conditions. Nuclear reactor core temperature sensors. Too dangerous to replace Sensors on satellites and other intergalactic devices. Too far away Sensors Sensors are used to measure many different physical properties Pressure (gas, liquid) Force, acceleration (drag of a car, weight of a truck….. Vibrations) Temperature (body… food… weather… combustion… chemical process) Velocity (derived from pressure… observed with a camera… flow rate) Position, distance, displacement (where is it, how far has it moved) Angle (rotation of an item… aircraft, turntable, manufacturing) Sound intensity or pressure (safety, environmental, detection) Light intensity (solar panels, laser intensity, manufacturing) Contamination/particulates (fires, manufacturing) Sensors : Physics Sensors use many different physical phenomena in order to generate a useful output. Pressure acting on materials in pressure sensors Force acting on materials in force and acceleration sensors Magnetic field strength in distance measurements or angle measurements EM radiation IR for thermal measurements IR laser for distance measurements IR LED for pulse generation in encoders Capacitance in distance and pressure measurements (C=εA/d) Eddy currents in distance measurements Sensors : Physics Induced voltages in LVDTs Heat transfer in flow velocity or pressure measurements Thermoelectric effects in temperature measurements Resistance changes in strain gauges, platinum RTD temperature sensors, hot wires, thermistors, position sensors Ionising radiation in smoke detectors Sensor Output The output of the sensor may vary linearly with the input or the variation may be non-linear. Some devices can offer both depending on the configuration and the internal electronics/processing. Bigger voltage change at low pressures https://www.sensirion.com/fileadmin/user_upload/customers/sensirion/Dokumente/8_Differential_Pressure/Sensirion_Differential_Pressure_Sensors_SDP8xx _Analog_Datasheet_0v4.pdf Sensor Output Some sensors generate a digital data output and this is transferred to other devices (computers) using one of a range of protocols like I2C, SPI, CANbus, RS232 or RS485. Digital data has a bit resolution; 8 bit is a value between 0 and 255, 16 bit is a value between 0 and 65535. (u8 and u16 respectively in LabView.) 8 bit data can also be in a range -128 to 127. (i8) 16 bit data can also be in a range -32768 to 32767. (i16) These data values can be converted to the physical values with suitable calibration values. Sensors A company like Digikey (digikey.co.uk) has vast listings of different types of sensor and over 195,000 items (August 2024) There are other companies (RS Components, Farnell (~63k), Mouser (104k)) Sensors Sensors There are a large number of other manufacturers of specialist sensors who do not sell their goods through 3rd party suppliers like Digikey or RS Components or Farnell. Dantec and TSI : flow measuring equipment (hotwire, LDA, PIV) Laser Optical Measurement Systems and Sensors (dantecdynamics.com) TSI - Flow Meters, Flow Sensors, and Flow Analyzers Omega : general flow/force measurement OMEGA Engineering │ Thermocouples, Pressure Transducers, Flow Meters, PID Controllers Brüel and Kjaer : Sound and vibration B&K | Sound and Vibration Measurement | Brüel & Kjær (bksv.com) As engineers it is useful to know who can supply what you need Sensors There are many other manufacturers specialising in particular areas Heat transfer/temperature measurements Hukseflux: Market leader in solar radiation & heat flux measurement (hukseflux.com) greenTEG: Heat Flux & Temperature Sensing Solutions - greenteg Rdf: RdF HFS-A // Heat Flux Sensors: Ordering Specs (rdfcorp.com) Load Cells/forces ATI-IA: ATI Industrial Automation: Robotic End Effectors and Automation Tooling (ati- ia.com) Aerodynamic test equipment Aerotech ATE: ATE - Advanced Test Equipment Structural testing Instron: Instron : Materials Testing Machines for Tensile, Fatigue, Impact, Rheology and Structural Testing | Instron Sensors When we do any experiment we need to select appropriate sensors. Understand the data we are likely to obtain, i.e. the range of values expected. Analytical or CFD information may be useful as well as any background experience or other knowledge (books, journals, other people). Get a sensor that goes beyond the range of expected data But ensure the range isn’t too high Measuring 900kg on a loadcell with range 1000kg is better than measuring 900kg on a loadcell with 10000kg. Sensor Measurement Range If you look for pressure sensors or force sensors or accelerometers, amongst other devices, you will find a number of different devices that cover different measurement ranges. Because the possible measurement range is very wide it is not possible for one sensor to cover all ranges. Masses from a feather (grammes) to a truck (tonnes) Pressures from ventilation (a few Pa) to hydraulic systems (atmospheres, MPa) Force Sensors Digikey, for example, have loadcells with maximum loads ranging from 0.1kg to 22680kg. Force Sensors But…… the measurement range of a loadcell must include any “dead weight”. If you’re measuring a 100kg variation on something with a mass of 500kg then a 1000kg load cell would be more appropriate because of the total load not just the load of interest. This is an important point when designing experiments. The presence of a model, or rig or other equipment can have an effect on the range of what you are measuring, especially if it is loads such as masses or forces. Pressure Sensors Sometimes suppliers With pressure sensors the choice is get their specifications wrong. Don’t believe more complicated. what you read. There are different units, Pa, kPa, psi, mbar, bar, inch H2O, mmHg. Pressure may be gauge or absolute The minimum pressure measurable is usually -101kPa or -15psi (vacuum) Maximum pressure (on Digikey) is 20kpsi On digikey.com there are over 8700 different pressure sensors Pressure Sensors An important factor needs to be considered when looking at pressure sensors, and some other devices. When measuring a pressure difference using a pressure sensor it may be at a high background pressure. For example an experiment may look at a low flow rate in a pressure vessel where the background pressure is 20 or 30 bar but the pressure difference is only 100 or 200Pa. In this situation you can’t use any transducer rated to 200 or 500Pa because the 20 or 30 bar pressure will destroy it. Pressure Sensors In this case, the background pressure is known as a Common Mode pressure, and when selecting a device, this pressure also needs to be considered. Common Modes also occur in electric circuits where small voltage changes may be measured against 240V, for example. Why not just use two 20 or 30 bar transducers and measure the difference between them? ? Sensor Terminology Visit a manufacturer’s website and you may be presented with a range of different possibilities. Basic, compensated, amplified…. Sensors and Transmitters Media Compatibility (dry gas, liquid, etc) You need to understand what these mean. http://www.first-sensor.com/en/products/pressure-sensors/pressure- sensors-and-transmitters/ Terminology - Pressure Sensors Type Output Corrections Cost Basic Low voltage None Cheapest 0-100mV Compensated Low voltage For temperature 0-100mV variations Amplified Medium voltage None 0-5V Compensated and Medium voltage For temperature Most expensive Amplified 0-5V variations Sensors in Use Measuring Thrust on a propeller (ATI-IA balance) and RPM (Hall effect sensor and magnet) Hall effect Balance sensor Magnet on motor casing Results Video https://youtu.be/MlAaVwgDDxw Ratiometric Devices You may see the term ratiometric used with certain sensors. What it means is that the output voltage is proportional to the input voltage. For some devices like loadcells it is implicit in the output specifications of the device that are in terms of mV/V, i.e. the output mV depends on the input V. This is important in experiments. If the power supply changes then the output of the sensor will change. You need to keep the power supply at a known and constant level and you may want to measure the power supply voltage. Amplification Some sensors generate very small voltage outputs, for example thermocouples and strain gauges – a component of load cells – and the output needs amplifying to a level that can be read by a data acquisition system. A thermocouple output is typically in the range of 10-60µV/K A strain gauge changes resistance due to deformation and in a loadcell the output is typically a few mV. Because loadcells are resistive the output is proportional to the input voltage Load Cell Amplifiers Full bridge (4-wire) load cell amplifier. http://www.datum-electronics.co.uk/products/type-131-load-cell-amplifier/ Note the mV/V specification. This is the maximum output of the load cell per volt at the rated load. 2mV/V with a 15V supply would have an output of 30mV at the maximum load of 300kg. Heat Flux Sensor Amplifier The output from a heat flux sensor is small, e.g. 22µV/W/m2 So 1kW/m2 -> 22mV Many industrial panel meters are designed to be used with small voltages and internal scaling can convert volts to physical units Sensor Excitation “Excitation”, when referred to sensors, is the voltage that is supplied to the sensor to make it work. Some sensors can use a standard DC power supply or even batteries. Other sensors need special and specific power supplies in order to operate. Capacitance or eddy current Pressure or force dynamic sensors using materials that develop a charge. Sensors and Controllers Eddy current and Capacitive sensors Sensors and Controllers Dynamic pressure, force and acceleration sensors Steady State or Dynamic Measurements? When considering the design of an experiment, you need to consider the type of data that will be obtained and how quickly a certain parameter may be changing. In certain experiments, for example the oscillations in a large structure, it may only be necessary to gather data once a second and in other experiments you may need to gather at megasamples per second when parameters are changing very rapidly and/or the experiment is of a very short duration and you need sufficient temporal resolution. Piezoresistive and Piezoelectric sensors Dynamic sensors need to react very quickly and one method that is used by manufacturers is to use a piezo material, i.e. a material that changes or generates an electrical output when compressed. A piezoresistive device contains a semiconductor type material that changes its resistance when put under load. A piezoelectric device contains a quartz type material that generates an electrical charge when compressed. This can ONLY be used with dynamic pressures or forces or accelerations because the charge gradually dissipates so the output from a steady load will decrease with time. Sensor Outputs Sensors generally fall into 3 different groupings Analogue transmitter – analogue current output Analogue transducer – analogue voltage output Digital transducer – digital output Analogue Transmitter Analogue transmitters can be considered as “industrial” devices where there is a standard output across all types of different sensor to match with the main industrial instrumentation standard of 4- 20mA. There are a number of historical reasons why 4-20mA is used. The main thing to note is the 4mA. Why not a range of 0-20mA instead of 4-20mA? Voltage Based Sensors Many sensors that are used generate a voltage output. Pressure sensors may have outputs like 0-33mV, 0-50mV, 0-100mV and others may have outputs like 0-5V, 1-6V, 0.5-4.5V. There is a lot less standardisation compared to 4-20mA devices. Voltages are relatively easy to measure, but can have problems with loss of signal if wires are too long, or noise. Voltage Based Sensors A pressure sensor with a measurement range of +/- 2000Pa and output voltage of 0.5V to 4.5V 2.5V at 0Pa. 0.5V at -2000Pa. 4.5V at +2000Pa. Voltage always > 0V Voltage Based Sensors Other sensors, like loadcells or thermocouples, use external devices like amplifiers to generate a measurable output. They may have positive or negative output voltages depending on what is measured. A thermocouple may have an output of 0V at 0C (273K), a negative output voltage at temperatures < 0C and positive voltage at temperatures >0C. A loadcell may be zeroed have an output of 0V at 0N load and negative output under compression and positive output under tension. Digital Sensors With modern microelectronics there are an increasing number of sensors that are digital rather than analog. Digital sensors convert any analog voltage to a digital signal within the device itself and then the data is transferred over a data bus to a controller computer or other acquisition system. With digital sensors there are a number of different standards, e.g. I2C, SPI, CanBUS, Wireless, Bluetooth and each operate differently. Digital Sensors The disadvantage with digital sensors is that you also need access to the appropriate data bus, through suitable hardware, to read the output from the sensors. The myRIO can communicate with I2C and SPI devices. Arduino devices can also communicate with I2C and SPI devices. There are many other microprocessor/microcontroller devices that can communicate with I2C, SPI and other digital protocols. Digital Sensors Digital sensors are addressed individually, either through their own ID code or on an individual digital address line. This is a major limitation on the number of devices you can use, particularly of the same type, because there are limited addresses for the devices or limited data lines on a DAQ system. Summary This lecture has introduced you to a number of different aspects of choosing a sensor There are many different types of sensor that are available for different types of measurements Additional equipment may be needed to operate them that may cost more than the sensors themselves. There can be different data outputs, voltage, current and digital There are a number of different options or parameters that are important when choosing sensors Datasheets are important and provide a lot of useful information There are different suppliers of sensors Summary Sensors for a particular measurement may have different packaging or different mounting arrangements or be different sizes. This can be very important when designing a test rig. If you do not understand all the information relating to sensors, you can buy the wrong device, you can find it doesn’t fit, if it’s a surface mount device (SMD) you may not be able to solder it, if it has the wrong output you may not be able to measure the output. Sensors may have lead times of days, weeks or even months.