Transcript Slide # 2
Displacement sensor LVDT sensor LVDT stands for Linear Variable Differential Transformer • For operation, the LVDT's primary winding is energized by alternating current of appropriate • amplitude and frequency, known as the primary excitation. • The LVDT's electrical output signal is the differential AC voltage between the two secondary windings, which varies with the axial position of the core within the LVDT coil. • Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use. Slide # 1 Displacement sensor LVDT sensor Advantages: • Friction-Free Operation One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems. • Infinite Resolution Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display's resolution. • Unlimited Mechanical Life Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. • Low output impedance and noise/interference Slide # 2 Hall Effect sensor: Basics Lorentz force and Hall effect F qv B Jx q B nq nq e E x q B nq VH qE y q w VH LVH VH e wE x B wRx IB BI sheet VH I sheet e B nsheet sheet 1 q q sheet Slide # 3 Hall Effect sensor: Chip and Spec ANALOG DEVICES - AD22151YRZ IC, HALL EFFECT SENSOR, LINEAR, 8-SOIC Slide # 4 Hall Effect sensor: Analog and Digital o/p Analog Output circuit Digital Output circuit Slide # 5 The Schmitt trigger waveforms A small positive vi would make the o/p voltage positive (saturated to supply voltage usually), or a small negative voltage would saturate the o/p voltage to negative supply voltage. Suppose Vo = +5 V. Then as long as we have Vin > -5 V, we have the vi > 0, and the o/p voltage remains at +5 V. However, if Vin < -5 V, then vi < 0, and the o/p will switch to -5V. Then vo will remain at -5 V, unless vin exceeds +5 V. 6 Slide # Hall Effect sensor: Applications II • To sense displacement or rotation, Hall Effect sensors will have to use a magnet mounted to a moving part • The magnet can be brought closer or further, or interrupted to produce a square wave signal • In some applications, 4 Hall Sensors are arranged in a quad cell (like a wheatstone bridge) to sense rotary signal Slide # 7 Optical sensors: Fiber Optic sensors Vibration and displacement sensor • Fiber optic sensors are great because they sense by non-contact means, which at the very basic level is simply based on the changes in light intensity • A common application is to couple light from one fiber to another by means of a movable mirror, which based on changes such as pressure, would couple different intensity of light to the receiving fiber. • Another applications is in sensing the level of liquid as shown in the adjacent figure. When the liquid is in touch with the bent section of the tube, more light scatter into water, and less light is detected at the other end. Liquid level sensor Vibration and displacement sensor utilizing FO cable with microbends FO strain sensors Slide # 8 Linear Optical sensors I • Used for precision position sensing over the short and long range • An IR LED is typically used to send out the signal, which is then sensed from a object using a position sensitive detector • The detector consists of p and n+ doped Si layers positioned on opposite side of the bar. Wherever the light beam falls, it creates a low resistance path connecting the top and the bottom layers • The movement of the object can be calculated from the movement of the spot on the detector Mathematical Analysis: If the beam hits at a distance x from the electrode A, and the corresponding resistance is Rx, then the overall photoelectric current I0 will be split into IA and IB as I A I0 RD Rx RD I B I0 Rx RD RD is the resistance between the terminals A and B Slide # 9 Linear Optical sensors II Assuming linear variation of resistance with distance, IA and IB can be written as: Dx I A I0 D I B I0 x D Then the ratio of the current P is given as: P IA D 1 IB x or x D P 1 Applying similar triangle principles from Fig. 7.38 we have: Thus we have: L0 f LB x L L0 f B L0 f x LB P 1 D Slide # 10