Experimental Methods MACE 6X021 2024-2025 Sensors 2 – Use of Sensors PDF

Experimental Methods MACE 6X021 2024-2025 Sensors 2 – Use of Sensors Dr Mark Quinn – [email protected] Dr Andrew Kennaugh – [email protected] School of Mechan...

Experimental Methods MACE 6X021 2024-2025 Sensors 2 – Use of Sensors Dr Mark Quinn – [email protected] Dr Andrew Kennaugh – [email protected] School of Mechanical, Aerospace and Civil Engineering Use of Sensors Why do we care? Using sensors correctly is part of doing an experiment well and, hopefully, getting the best results. This includes installation, power, wiring, calibration and DAQ This lecture will cover many of the things you will need to think about when you want to use sensors in experiments. Topics include the electronic circuits that may be necessary, any additional electronic components, calibration, and considerations when using a data acquisition (DAQ) system. Lecture Aims At the end of this lecture you will, hopefully, appreciate that There are many different requirements to use sensors effectively Mounting Electrical and electronic considerations Power Calibration DAQ parameters There are lots of different hardware connections Plugs and sockets Bulkhead fittings Screw terminals Cases Use of Sensors You will remember from the last lecture, Sensors 1, that there are many different types of sensor and there are many additional considerations to think about before using sensors. Many sensors come as standalone devices that need electrical power for them to work and some sort of simple circuit to get the power to the sensor and the output from the sensor. Other sensors have to have additional power supplies and controllers that are more complicated and usually bought with the sensors What makes these K-type thermocouples different? Rated to 250C Rated to 350C All K-type thermocouples Rated to 1100C are made from the same metal. Use of Sensors with Analog Data Acquisition One important factor that needs to be considered when using sensors in a Data Acquisition system is the voltage output, especially when you are using a number of different sensors. Some sensors may have a voltage output of 0.5-4.5V, others may have an output of 0-50mV. It adds complexity to coding to use both and maintain accuracy. With a DAQ card you usually have a fixed voltage range for all measurement channels. 0.5-4.5V is OK in a 0-10V range. 0-50mV only uses a small part of the 0-10V range. It is possible to allocate different measurement ranges to different channels, but each channel needs to be treated individually Use of Sensors with Data Acquisition DAQ cards have different measurements ranges, for example (NI PCI- 6255) http://www.ni.com/pdf/manuals/375215c.pdf Use of Sensors with Data Acquisition Most DAQ cards only have one ADC chip. If you are gathering data from a number of different sensors at the same time, samples may be collected simultaneously into a series of sample and hold amplifiers before processing in the ADC. Multiplexer reads each Data saved channel in sequence simultaneously into saving data into another each hold amplifier hold amplifier before data is transferred to ADC http://www.analog.com/media/en/training-seminars/tutorials/MT-090.pdf Use of Sensors with Data Acquisition Historically, this only worked well if the ADC had the same measurement range for all the samples. In some cards it is possible to apply a different measurement range to each channel, but it takes a certain time for the internal electronics to settle in the change from one range to the next. So it may not be possible to take accurate measurements at different measurement ranges, i.e. you can’t jump between ranges +/-10V and +/-100mV in successive samples and maintain accuracy, especially at high sample rates. Use of Sensors with Data Acquisition Some cards do have a separate ADC for different channels, but they have far fewer channels and are a lot more expensive per channel. Note: 8 duplicate components For the 8 channels PXIe-6345 PXI-6123 £3090 ex VAT £5840 ex VAT 40 differential channels 8 differential channels 1 ADC = £77.25/channel 8 ADC = £730/channel Calibration Before using sensors, it is always good experimental practice to calibrate them. Calibration is generating data that matches the output of a sensor to a corresponding physical parameter (pressure, force, temperature, etc) Calibration is the best guarantee you have that the measurement is accurate, i.e. that the measured voltage output (or current) is an accurate representation of the physical value. Calibration Some sensors come with individual calibrations, for example dynamic pressure sensors, microphones and multi-axis balances (usually unique or low production run or material dependent devices). Other sensors come with a specification, i.e. the expected output for the given physical input. In many cases the specification is accurate and it may not be necessary to do a calibration, but you should always calibrate to get the best results. Note that devices can drift from their calibration over time so they do not remain accurate. Individual Calibration Note that individual calibrations give the serial number of the sensor Calibration Calibration is expensive if you have to outsource it to a testing agency or the original manufacturer of equipment. Many pieces of equipment can only be tested at the manufacturers because they have the specialist equipment and facilities to do it. It is very expensive to maintain equipment in calibration and it will not be cost effective to have calibration rigs for occasional use so returning to the manufacturer is the only option. Calibration ATI-IA balance Calibration matrix to transform voltages to forces and moments The internal structure of the balance means that the calibration matrix is complicated It has a lot of what are known as cross couplings – all the components affect each other. You need accurate (aligned) equipment to calibrate it 0.13282 0.05932 0.56579 -31.66247 0.28784 32.06085 -0.42744 37.20859 0.14364 -18.30735 -0.25567 -18.63575 18.02391 -0.53175 18.37657 -0.69213 19.25115 -0.95064 0.13252 0.09324 -31.7275 1.13746 33.61048 -1.63693 35.86783 -1.00266 -18.14812 0.66608 -19.20849 0.88954 0.20263 -18.43851 0.44695 -18.2642 -0.39182 -18.59208 Calibration Laboratories like at Universities may have calibration standards that devices can be calibrated against. Devices such as thermocouples and pressure transducers can be readily calibrated “in-house” Furness FCO560 pressure calibrator 0-2000Pa, 0-20000Pa. Has internal Omega Dry block temperature calibrator pump to provide pressure. (~£2500) How Do You Measure the Drag on a Cyclist? Measuring forces with loadcells Bicycle mounted on stanchions that connect to a series Under the turntable of loadcells under the turntable Loadcell with flexures Calibrate the Balance Frame sits where the bicycle sits and known loads are applied in different locations Calibrating the Balance The balance has 6 loadcells 3 for lift/pitch – 2 at the back, 1 at the front 2 for side force/yaw 1 for drag The calibration process involves applying loads along the different axes to represent all expected aerodynamic and weight loads so that when loads are measured on individual load cells they can be converted to the aerodynamic load and weight. It involves many measurements in different combinations of lift, drag and side force load as well as where the load is applied. Sensor Mounting An important consideration when using many sensors is how and where they are going to be used. For many devices such as loadcells or strain gauges that are mechanically connected or pressure sensors that are screwed into pipes/pressure vessels, the application usually defines their use. For other devices, especially the microelectronic devices such as pressure sensors, accelerometers, and encoders, the electrical connection is important. Sensor Mounting Most sensors are bought as standalone devices and need additional circuitry (wires, power supplies) to be usable 3 or 4 pins usually indicates 2 2 terminals means that the sensor generates a for power +/- and 2 for output change that is measured elsewhere (a strain +/-. Sometimes the –ve power gauge has a resistance change and a is common with the –ve output thermocouple generates a voltage) so only 3 pins are used. Sensor Mounting There are plenty of other sensors that have more than 4 terminals but many are not used. They are only there because they are mounted in a standard 8 or 12 or 16 pin package and unused connections are for support. If you check specification sheets you will usually see a N/C next to a terminal indicating it is Not Connected or DNC for Do Not Connect. Note: only 3 pins are used Power (Vs), Output (VOUT) and Ground (GND) SMT, SMD, DIP(P), SIP(P) The above abbreviations tell you how the sensor is electrically connected in a circuit SMT – surface mount technology SMD – surface mount device DIP(P) – dual in-line pin (package) SIP(P) – single in-line pin (package) SMT, SMD, DIP(P), SIP(P) Many sensors need soldering or fitting into circuit boards. For many applications, in a University, it is quickest and easiest to get stripboard or prototyping breadboard to manufacture the appropriate circuits for the sensors if they are DIP or SIP devices. Breadboards shouldn’t be a permanent solution – bad connections. SMT, SMD, DIP(P), SIP(P) Surface mount devices (SMD) are usually much smaller than DIP or SIP devices and need specific boards purchasing or manufacturing for them. This evaluation board, for accelerometers, allows developers to test devices more easily than making their own boards. Note that the IC for this board is 3.25mm x 3mm x 1mm in size. So, it will be fiddly to solder into place SMT, SMD, DIP(P), SIP(P) There are a number of prototyping breakout boards that smaller IC devices can be soldered to. These don’t allow additional components, they are just for individual ICs. Typical standards include TSSOP, SOT and SOIC. TSSOP-28 SOT23-6 SOIC-28 Thin shrink small outline package Small outline transistor Small outline integrated circuit SMT, SMD, DIP(P), SIP(P) It is possible to design and print your own circuit boards for specific purposes or to optimise a design where stripboard is inefficient or does not allow suitable placement of components. These can be prototyped on breadboards or stripboards before manufacture so the design is correct. It takes time and money to create boards so it’s best to get things right first time. There are many companies that can print custom PCBs and doing an internet search on “PCB printing” will bring up many. Note that designing circuit boards does require a good knowledge of electronics. It is easy to make bad circuits. PCB Connections As well as soldering to a PCB or stripboard there are a large number of other types of connectors that allow you to fit devices or wires without soldering them. Making something removable can help if items need removing or replacing. DIL socket Push on PCB connector Screw terminals Crimped wire tab Electronic connections The electrical connection to the sensor is one of the most important aspects of any experiment. Any sensor needs a power connection, generates an output and may be some distance from the data acquisition system. The wiring for all of these can contribute to noise on the signal, drift or offset, reduced dynamic response and, at worst complete loss of the signal. Old connections can be worse due to corrosion. This can also be a problem with older computers with the memory or I/O cards Noise Electrical noise is a particular problem associated with voltage generating sensors. Noise can come from a number of sources within the sensor itself, From the power supply Within the wires connecting the sensor to the power supply and to the DAQ system because they are in an EM environment – mains electricity, machinery, switches. Some devices like stepper motors can be troublesome because they, or their controllers, generate electrical spikes. Dynamic Response For any rapidly changing electronic signal we need to be aware of the capacitance and inductance of wires as well as the resistance. All of these combine to give what is known as the impedence (Z). Z2=R2+(XL-XC)2 (series connection, XL~ ωL, XC~ 1/ωC) For off the shelf items, these problems will have been foreseen and specific wiring requirements will be given, for example the length of wire to be used. For custom made devices it is possible get the wrong results because the dynamic response of wiring connections has not been considered. Loss of Signal Some sensors are low power devices, i.e. they generate a small current. Electronically speaking, the current is what allows the voltage to be transferred to the DAQ system to be measured. If the current is low it may not be enough to overcome the wire resistance of the signal wires and if the wires are too long the voltage will reduce, eventually to zero. Some devices may need a 1:1 gain amplifier to boost the current, but the gain won’t be exactly 1:1 just using an amplifier IC. Electronics knowledge is important here. Wires and Cables Usually a wire is seen as a single conductor and a cable is seen as multiple conductors, but the terms are used interchangeably. Wires and Cables There are lots of different wires that can be bought Single strand – one single metal wire Multi strand – wire made up of fine strands Multi core – a lot of wires in the cable Coaxial – copper core with outer braided return wire Data – for internet and other digital communication Shielded – with outer braided earth shield Unshielded – without outer earth shield. Armoured – with heavy duty outer shield for physical protection Different Wires Multi core Single core Shielded Multi strand If you have a number of sensors it can make sense to use multi core cable to connect the output from the sensors Coaxial to a DAQ system. Wires The shielding around wires is used to earth the EM environment so that the noise generated in the wires can be reduced. The outer shielding in such cables would be connected to a suitable earth point in the circuit. It is important to have only one earth point. You don’t earth each end of the shielding – if you do it then becomes an aerial and can increase noise. Wiring In some experiments the sensors may be in a different location to the DAQ system. Choice of power supply is important. In many industrial settings, the main supply is 3 phase, i.e. three 240V supplies in parallel. To balance the load, different parts of a building may be supplied from different phases and these can have different earth potentials. The different earth levels can can cause errors in measurements, for example an offset in voltage. Wherever possible, use the same mains source (phase). Sensors DAQ DAQ signal 240VAC 240VAC Power Supplies You know that the myRIO has a number of different output voltages that can be used as power supplies (3.3V, 5V) but actual voltages may be lower and will decrease with more devices attached. For most situations with only a few sensors, these power supplies may be sufficient, though the current draw may be a limitation. Where more power or different voltages or currents may be needed there are other power supplies that can be used. Switch mode power supply Bench power supply Battery Power Supplies The commonest types of supplies are known as switched mode power supplies (SMPS) and these power most consumer devices that are available. Your phone charger or laptop charger are switched mode power supplies. A SMPS operates by switching the input on and off at a very high frequency and using other circuitry to convert the voltage to the desired output level. Because of the switching they are electrically noisy, but they are efficient and small. Their design is why they can be used at different mains voltages, i.e. 110V and 240V. Switch Mode Power Supply SMPS can have different voltage outputs and some devices can have more than one output connection, e.g. +5V, -5V, +12V, -12V, 0V Plug-in power supply Note the adjuster Power supply modules Power Supply Bench power supplies provide variable voltages and currents, but may be expensive. Some limited to 2A maximum output. Some won’t drive motors – they see the low resistance as a short circuit. Batteries are useful for remote systems (or where mains interference needs to be avoided) but may need some means of charging – wind turbine or solar panel. Aim-TTi EL302RD Digital EA Elektro-Automatik EA-PS Bench Power Supply, 2 2042-10B Digital Bench Power Output 0 → 30V 0 → 2A Supply, 1 Output 0 → 42V 0 → 120W 10A 160W Cases For safety it may be useful or necessary to put sensors and power supplies in a case. This protects the equipment from damage, dust, etc, and also stops electric shocks if there are any high voltages. And it makes things look neater. There are lots of different cases in different sizes and shapes and materials. Instrument Cases Instrument cases usually have detachable tops and removable front and rear panels. They can be metal or plastic, or both. This makes it easier to drill the holes for the various plugs, sockets and other connections for the instrumentation in the case. Instrument Case Fittings In order to connect to instrumentation you may need a range of electrical connections or airline fittings as well as connections for a power lead. IEC mains socket Output Pneumatic airline fitting DC, low voltage, power plug and socket Thermocouple socket and Input mounting plate Instrument Case Fittings Multiway signal connectors – there are many different types “D” sockets and kit with casing Multiway circular connectors “Rules” of Input and Output Connectors Outputs from a system, or cable, are always covered, or shielded, or hidden……… ESPECIALLY MAINS VOLTAGES. NEVER have output power connections that can be touched. IEC mains socket Output DC power connector. Outer contact is earth, Computer power connector inner contact is + voltage. Serial port (input/output) Video port (output) Input Summary Getting a sensor is only the start of using it. There are many other things to consider, including Power supply and safety Wiring, noise environment Calibration Issues with multiple sensors on a DAQ system Accessories – cases, connectors, fittings All of these may have an impact on how well the sensors work and the accuracy of results as well as the ease of use.