Experimental methods Sensors of displacement and its derivation 1
Position sensors with discrete signal Limit Switches (contact) Source: www.honeywell.com 2
Position sensors with discrete signal Inductive proximity sensors (non-contact) Main technical parameters: Operating voltage Operating current Assured operating distance Max. switching freq. hystereis Protection Switching output (PNP x NPN) Balluf induction sensor 3
Hall effect sensors (non-contact) A hall effects sensor is a transducer that varies its output voltage in response to a magntic field. Ip * B 3 - UH RH V; m C 1,A,T,m d where R H is Hall materiál constant d is the semi-conductor thickness in the direction of magnetic flux density B Hall effects sensors are used for proximity switching, positioning, speed detection, and current sensing applications. 4
Hall effect sensors (non-contact) k UH * B B U f x k * B k H * nonlinear f x and Differencial layout of the hall effect sensor linearization of the voltage/distance characteristic 5
Examples of usage of Hall effect sensors Hall effect sensor TLE4905L: 3 24V, 40 +150 C, P-SSO-3-2, 3x4x1,5mm Speed hall effect sensor, XS 06A905161B, Skoda Octavia 6
Other sensors type of position with discontinous signal Optical sensors Ulstrasonic sensors Laser sensors Capacity sensors 7
Displacement sensors with continous signal Potentiometer Magnetostrictive sensor Inductive sensors Inductosyn (Linear encoder) IRC (Incremental Rotary Encoder) 8
Potentiometer Source: www.electronics-tutorials.ws 9
Potentiometer K z R R z U z 1 x 1 x K z U x V 10
Resistive sensor of displacement Advantages: Absolute values in the whole measuring range Simple construction inexpensive Disadvantages: Output voltage singal Low reliability (lifespan) Temperature dependancy Output signal noise 11
Magnetostrictive sensor of displacement (continous output signal) Advantages: Absolute values in the whole measuring range Simple construction inexpensive Disadvantages: Output voltage singal Low reliability (lifespan) Temperature dependancy Output signal noise 12
Magnetostrictive sensor of displacement A - electrical signal is initialised in the magnetostrictive wire, B current pulse and magnetic field excitates a mechanical impulse - strain (Viedemann effect) in magnetostrictive material, the excited impulse is propagating along the wire and detected in the induction pickup coil. Position of magnet (measured quantity) is evaluated from the time between the initial signal and reflectected pulse. (elastic wave in ferromagnetic material v=3000 m/s) Source: www.electronics-tutorials.ws 13
Magnetostrictive sensor of displacement Technical parameters: Measuring lenght segments up to 7 600 mm (standard 50 up to 1 500 mm) System resolution min 10 μm Repeatibility min 20 μm Sampling rate 1 up to 10 khz Output signal - absolute http://www.balluff.com/balluff/mde/en/products/overview-micropulse-transducers.jsp 14
Inductance displacement sensor 15
Inductive displacement sensor 16
Inductive displacement sensor Example of inductve displacement sensor WA 200 (Hottinger Baldwin Messtechnik) Stroke: 200 mm Input signal: 80 mv / V Supply voltage and frequency: 2,5 V ef / 4,8 khz Linearity tolerance: 0,2 % Maximal acceleration: 2 500 m/s 2 Weight: 130 g 17
Inductive encoders: Inductosyn (linear displacement) Resolver (angular displacement) Measured element that is connected to a rider with 2 coils (mutualy shifted of 0,25 loop) is sliding above a static scale. Riders coils are supplied with AC voltage with 90 o phase shift. u t kusin t and u12 t kucos t 11 Due to the magnetic flux induction voltage in the static slace is: u 2 t u11 t u12 t ku sin t sin cos t cos kucos t 18
Inductosyn (measuring of linear displacement) 19
Resolver measuring of angular displacement u t U sin t 20
Resolver measuring of angular displacement u u u sin cos t t U sin t U cos t t kusin t 21
Resolver measuring of angular displacement Advantages: High precision High Measuring Stroke (in the lenght of CNC machine) Independancy on distance change of scale and rider Reliable for dusty environment Disadvantages: Cyclical absolute measruement (referent positioning required) Electronic for the step counting required 22
Optical rotary encoders: Incremental encoders (relative angular displacement) Absolute position encoders (absolute angular displacement) 23
IRC sensor measuring of relative angular displacement Measurable values Position (turning angle) Relative Rotation speed Principle of function Optical (contactless switches) Output: logic signals Using CNC machines Industry automatization Laboratory experiments 24
IRC sensor measuring of angular displacement Two pairs of LED diodes + phototransistors Light beam chopped by rotary disk 90 phase shift between output signals A, B 25
IRC sensor measuring of angular displacement IRC sensors are mainly used for the angular velocity measurement (given by pulse frequency) Relative angular position (given by pulse count requires indexing of the output pulse-signal) Direction given by phase difference CW A leads B CCW B leads A 26
IRC sensor measuring of angular displacement 27
Absolute Postion Encoder measuring of absolute angular displacement Measurable values Position (turning angle) Absolute Rotation speed Principle of function Optical (contactless switches) Output: logic signals Using CNC machines Industry automatization Laboratory experiments 28
Velocity sensors linear: angular: inductive - electromagnetic (with movable magnet) inductive - electrodynamic (with movable coil, appropriate for measuring of vibration NOT FREQUENCY) laser ultrasonic incremental (inductosyn in the speed measuring mode measure of the output pulse frequency) DC tachogenerators (AC tachogenerators) stroboscope incremental ( IRC in the speed measuring mode measure of the output pulse frequency) 29
Laser Doppler Vibrometer The laser vibrometer serves for contactless and very fast measuring of vibrations. The output of the laser vibrometer is a signal of vibration velocities. It operates at a distance up to 3 meters from the monitored object. 30
Laser Doppler Vibrometer Laser vibrometer Brüel & Kjær 8338 and its adjustement 31
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Acceleration sensors 3 basic types of acceleration sensors: - Piesoelectric (appropriate for measuring of vibration, unable to measure static acceleration) - Piesoresistive ( Accelarated mass pressure exerted on a piezoresistor the resistivity varies accordingly. Capable of measuring of static accleration) - Capacity (Evaluation of the capacity of two plates affected by the acceleration 33
The principal function of acceleration sensors is based on the 2. Newton s Law (applying a force F on mass m): F = m * a Mechanical dynamic system: m - mass of the coil ( seismic mass ) M - sensor cover connected with measuring object b - k - u - damping (depending on velocity (viscouse damping) spring stiffness induced voltage 34
Piesoelectric sensors of acceleration Use of piesoelectrical crystal for generation of electric charge as an results of mechanical stress. Pair of piesoelectric elements are used for higher precision. Material damping of piesoelectric material is very low, enabling to measure vibration up to 3*10 4 Hz. 35
Inductive (electrodynamic) sensors of acceleration Based on system motion, coil vibration in the magnetic field of permanent magnet induces voltage in the coil that is function of velocity The resonance frequency of electrodynamic sensors is in the range of 5-10 Hz If additonal damping (damping element placed below the coil) the frequnecy can be in the range 1 to 3000 Hz 1- measuring coil, 2- damping coil, 3- core of magnetic system, 4- permanent magnet, 5- membrane 36
Capacity sensors of acceleration S C 0 d C capacity capacitor (F) ε 0 permitivity of vaccum (= 8,859. 10-12 F/m) ε - relative permitivity of dielectricum (-) S plates area (m 2 ) d - plates distance (m) Capacity sensor MEMS (Micro-Electro-Mechanical-System) Ancors connection with vibration object Main beam Seismic mass Cell Differential capacity sensor 37