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PHYS 352 Position, Displacement, Pressure, Force and Motion Transducers Position, Force, Motion Transducers there exists a huge variety of transducers to make position measurements pressures and forces cause strain (e.g. bending) and can also be sensed by measuring position or displacement velocity can be measured by differentiating (measuring positions and taking differences) accelerometers work basically via F=ma, F=kx…and measure the x to find a when designing measurement systems involving position or motion transducers (or if designing the transducers themselves) be creative! 1 Position Measurements Techniques mechanical linkage optical capacitive piezoresistive (strain gauge) piezoelectric magnetic Mechanical Linkage position/motion you want to sense is mechanically attached to a potentiometer (i.e. variable resistor) joystick has two potentiometers one for each axis 2 Optical Readout of Motion optomechanical mouse ball turns wheels (one for each axis) rotary encoder counts pulses can use rotary encoder to measure linear motion also Q: how can you determine direction of rotation with rotary pulse counter? A: use two encoders out of phase! B transitions first before A if rotating one way; A transitions first if rotating the other way 3 Magnetic Rotary Encoders magnet rotates past Hall effect sensor; encoder counts the pulses Optical Mouse – Image Processing to Sense Motion …as opposed to optomechanical red LED illuminates the area under a sensor (~30x30 pixel photodiodes readout with CMOS array) imaging array records 1500 or more images a second! on-mouse digital signal processing (DSP) compares subsequent images, and computes how much the mouse has moved. 4 Other Optical Techniques for Position/Motion Sensing interferometry very precise distance measurement align on interference fringes fraction of wavelength displacement is detectable range finder laser, ultrasonic, radar time of flight determines distance (if speed is known) can use Doppler shift to determine target velocity (toward or away from detector) Capacitive Sensors either change the relative position of the two conductor or change the dielectric and the capacitance is changed four techniques considered: 1) can charge plates with DC voltage, and measure charge flow to determine changes in position e.g. condenser microphone C= εA d 5 Capacitive Sensors cont’d 2) apply AC signal (known frequency) to measure the absolute capacitance (sometimes AC Wheatstone bridge is used) e.g. non-contact position sensors guard ring 3) changing level of a dielectric changes capacitance (AC measurement of the capacitance is used also) e.g. capacitive liquid level probe Capacitive Sensors cont’d 4) proximity distorts electric field in a capacitive geometry and the changing capacitance is sensed 6 Strain Gauge (like in Lab #3) piezoresistive effect R= ρl metal or semiconductor film A resistance changes due to application of mechanical stress strain (stretching) makes wire longer and narrower metals in strain gauges respond this way some materials change resistivity under stress silicon! Strain Gauge Configurations http://www.rdpe.com/ex/hiw-sgpt.htm utilizes 4 in a full Wheatstone bridge in a bending beam configuration another load cell configuration S-type silicon pressure sensor 7 Piezoelectric some materials have asymmetric distribution of charge in lattice intrinsic polarization exists deforming these crystals (or ceramic) materials reorients charges/polarization detectable, sizable voltage change used in: acoustic sensors, to detect vibrations, in accelerometers,… no DC response requires change in stress/strain in piezo to get a signal advantages: large signals generated does not require power supply a common piezoelectric crystal: a common piezoelectric ceramic: quartz lead zirconate titanate (PZT) Magnetic motion in a magnetic field – electromagnetic induction e.g. linear variable differential transformers e.g. eddy current proximity sensors for non-contact measurement e.g. tachometer (basically a generator) 8 Accelerometers old school Pendulating Integrating Gyroscopic Accelerometer used for inertial navigation (guiding missiles, aircraft) modern accelerometers use MEMS technology (microelectromechanical systems) Summary we looked at many different ways to sense position, displacement, motion and turn it into electrical signals many different configurations and designs too many out there to look at them all! most employ one of the basic physical concepts (or variation/combination of them) that we outlined above 9