Tài liệu Training package sensoric manual for theory

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K O L L E G Training package "SENSORIC" Manual for theory 1 Introduction 2 Inductive Sensors 2.1 Fundamental Principles 2.1.1 2.1.2 2.1.3 2.1.4 2.1.4.1 2.1.4.2 2.1.5 Basic Construction Reduction Factor Coil Size and Sensing Range Installation Problems Housing Flush Mounting Electronic Circuit 2.2 Types 2.21 2.2.1.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 Cylindrical and Rectangular proximity switches Definitions Slotted Types Ring Types Bistable Switches Sensors for use in Welding Magnetic Fields Sensors to distinguish between different materials Inductive Analogue Sensors 2.3 Interfaces for Inductive Proximity Switches 2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.3 2.3.4 Electrical Types and their Positive Effects Direct Current Switches Alternating and All voltage Switches Sensors to DIN 19234 (NAMUR) Protected and Safety Switches Reverse Polarity and Over Voltage Protection Overload Protection Safety Circuits Loads and their Characteristics Bus Connection 2.4 Manufacturing Technology 2.5 Applications 1 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 3 Capacitive Sensors 3.1 3.1.1 3.1.2 3.1.3 Fundamental Principles Sensor Construction Sensitivity Reduction Factor 3.2 3.2.1 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.3.3 Practical Model RC Oscillator Interference Suppression Interference Effects Contamination Compensation Cutting out Interference Pulses Models 3.3 Applications 4 Ultrasonic Sensors 4.1 4.1.1 4.1.2 4.1.2.1 4.1.2.2 4.1.2.3 4.1.2.4 Fundamental Principles Propagation of Sound Waves in Air Generation of Ultrasonic Waves Electrostatic Converter Bending Oscillator Membrane Oscillator L/4- Oscillator 4.2 P&F- Oscillator 4.3 Methods of Operation 4.4 Distance Measuring Ultrasonic Sensors 4.5 Ultrasonic Sensors in Through-Beam Mode 4.6 Possible Errors in distance measurements with Ultrasonic Sensors 4.7 Operating Conditions 4.8 Sensor Types 4.9 Applications 2 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 5 Photoelectric Sensors 5.1 5.1.1 5.1.1.1 5.1.1.2 5.1.2 5.1.2.1 5.1.2.2 5.1.2.3 Fundamental Principles Emitter Element Light Emitting Diodes Solid State Laser Diodes Receiver Element Photodiodes Phototransistors Position Sensitive Diode 5.2 5.2.1 5.2.2 5.2.3 Methods of Operation of Photoelectric Sensors Direct Detection Photoelectric Sensor Reflex Photoelectric Sensor Through-Beam Photoelectric Sensor 5.3 5.3.1 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.3 5.3.3.1 5.3.3.2 5.3.4 Signal Processing in Photoelectric Sensors Interference with Photoelectric Sensors Stages in the Interference Suppression Interference Suppression using Optical Modulation Interference Suppression with Band Pass Interference Suppression using Blanking Interference Suppression using Digital Filtering Function Reserve Static Function Reserve Dynamic Function Reserve Protection against Mutual Interaction 5.4 5.4.1 5.4.1.1 5.4.1.2 5.4.1.3 5.4.2 5.4.3 5.4.3.1 5.4.3.1.1 5.4.3.1.2 5.4.3.1.3 5.4.3.2 5.4.4 Types Reflex Photoelectric Sensor with Polarising Filter Polarising Filter Retro-Reflector Through-Beam Detection Direct Detection Photoelectric Sensor with Background Screening Direct Detection Photoelectric Sensor with Light Guides Light Guides Principle of Operation Glass Fibre Light Guides Plastic Light Guides Sensors with Light Guides Output Stage of Photoelectric Sensors 5.5 Triangulation Sensors 5.6 Phase Correlation Sensors 5.7 Applications 3 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 6 Magnetic Sensors 6.1 Fundamental Principles 6.2 6.2.1 6.2.2 Principal of Operation Hall Effect Sensors Magnetic Resistive Sensors 6.3 6.3.1 6.3.2 6.3.2.1 6.3.2.2 6.3.2.3 Saturated Core Probes Construction and Mode of Operation Function and Measurement Circuit Evaluation using an Oscillator Evaluation using Impulse Current Evaluation using Impedance Measurements 6.4 Applications 4 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim This handbook is a part of the training pack „SENSORIK“ (SP1). This training pack part of the complex Automation Technology includes: - Training case - Handbook of Theoretical Explanations - Collection of Experiments - Solutions and Evaluation of Results - Data Sheets - Folio Set SENSORIK - Video „New Photoelectric Sensors“ - PLC programs - CBT "Industrial Sensors 1.0" The training case is the central part of the training pack; with this set of demonstrations and exercises, experiments with different levels of difficulties, which demonstrate the function, specific characteristics, parameters and typical application for each sensor type can be performed. The theory required for the training pack is contained in the handbook covering: inductive, capacitive, photoelectric, ultrasonic and magnetic sensors. The documentation is not only to aid the further education programme, but is also suitable for self study. The theory presented covering the fundamental physical principles, method of operation, type and possible uses of the sensors has been designed for use with the training case but could be used independently to study the application of sensors in automatic control. 5 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 1 INTRODUCTION 1.1 Technical - economic importance of sensors Automatic control has been introduced in production, process engineering, warehousing, materials handling or administration. The following are the main aims in doing this: * * * * * Improvement in product quality Savings in energy and raw materials Increase in productivity Reduction in damage to the environment Humanization of the work place Usually the required control engineering is achieved using a computer or an PLC as the central element. In the end the system can only fulfill the required tasks if it is supplied with reliable process information. This is achieved with the use of sensors, which operate according to the most widely different physical principles. These sensors convert non-electrical process measurements such as distance , angle, position, level, temperature or pressure into electrical signals in order that the controller or regulator can operate. At the present time over 100 physical, chemical and biological effects are known for which „technical feelers“ are on the market or under development. The sensors, because of their different operating principles, are only suitable for specific range of applications. This must be taken into account during the planning stage of an installation. 6 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 1.2 Definitions As stated above, sensors are signal converters, which change a non-electrical magnitude into an electrical magnitude ( only in a few applications will pneumatic output signals be produced). In automatic control sensors replace the function of the human sensory organs. non-electrical signals mechanical p l 2 1 E R Q chemical ω v pH thermal % magnetic γ B,H T 3 ∆t C optical 4 E ∆t U E U R U R U 5 W electrical signal p l v ω = = = = pH % = = T B H γ = = = = Pressure Distance, Gap Speed Angular velocity, Speed of rotation Ion concentration Volume % Gas concentration Temperature Flux density Field strength photon U R Q ∆t C E W 1 2 3 4 5 = = = = = = = Voltage Resistance Quality factor of a resonant circuit Time interval Capacitance Electric field Electrical energy Ultrasonic sensor Inductive sensor Capacitive sensor Magnetic sensor Photoelectric sensor Diagram 1.1: Survey of signal conversion with sensors 7 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim Often in automatic control a binary signal is required to signal that an object is in a particular position or not in that position. Proximity switches, which are a special category of sensors, are used for such tasks. In these sensors the output is obtained as follows; the signal converter output is connected to a threshold circuit (e.g. Schmitt trigger), which operates when the converter signal is greater than or less than a preset value, or an adjustable value, allowing the output circuit to operate. Sensors, which operate without physical contact, have a number of advantages over mechanical contacts: no power required, no feedback and no contact bounce Greater number of switching operations and high switching frequency No contact wear Maintenance free Resistant to harsh environments Subsequently explanation of some terms: Sensor: other names are primary element, detector, measuring transformer, measuring transducer, pick-up Initiator: Referred to as proximity switches Sensor element: the part of the sensor, which detects the quantity to be measured, but cannot operate alone as the signal processing element and the connectors are also required. Example: Coil of the saturated core of a magnetic sensor,or the transducer of an ultrasonic sensor. Multi-Sensor System:A sensor system in which a number of the same type of sensors or a number of different types of sensors are used together to complete the required task. Due to the concentration the analysis of individual elements is achieved electronically, by the use of logic or mathematics. Example: The combination of a number of initiators to distinguish between production parts of different shapes and materials or a combination of gas analyses sensors; where the operating ranges of the sensors overlap and the total of their measurements by intelligent analysis gives more information than that obtained from individual sensors. 8 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 1.3 Typical criteria for application Inductive Sensor: Magnetfeld-Sensor: - - metal objects sensing range up to 50 mm switching frequency up to 5 kHz up to 250 °C up to IP 68 high noise immunity DIN 19234 (NAMUR) - magnetic objects sensing range up to 60 mm switching frequency up to 1 kHz up to 70 °C up to IP 67 high noise immunity DIN 19234 (NAMUR) Capacitive Sensor: - metallic, non metallic objects, solids and fluids sensing range up to 50 mm switching frequency up to 100 Hz up to 70 °C up to IP 68 DIN 19234 (NAMUR) Ultrasonic Sensor: Optical Sensor: - - - objects which reflect or absorb sound sensing range up to 15 m reaction time > 50 ms up to 70 °C up to IP 67 lower noise immunity color independent not sensitive to dirt - objects which are lightreflecting or non-transparent sensing range up to 100 m switching frequency up to 1,5 kHz up to 300 °C (fibre optic) up to IP 67 detect smallest objects (fibre optic) DIN 19234 (NAMUR) fibre optic, adaptierbar 9 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim 1.3 Forecast for the Future The Sensor element (that is the measurement detector) and the signal processing are already very closely connected in sensor systems today. In this sector integration is also time and money. The technology and experience in the miniaturisation of electronic circuits can be incorporated in and are ideal for the sensor development. It is only a matter of time before an integrated circuit and miniature sensor element can be produced on a piece of silicon, gallium arsenide or another semiconductor material. An interesting development is the use of enzymes, microbes or whole cells as sensor elements; known as „Bio-Sensors“. By the end of this decade contact element, that is control elements, will also be come part of this development. Finally, mechanical parts, such as pressure jets, switches or even motors with gears are already available based on the micro-electronic technology in mini-format. Sensor materials • ceramic • amorphous metal • Fibre optic • Bio-components Technology • Surface mount, hybrid • IC design technology • Laser alignment • micro-machining NEW SENSORS Sensor Idea • Micro-structure • Smart transmitter • Intelligent sensors • Multi- sensor systems Communication • 2 conductor technology • Programmed wiring • Interfaces • Bus connections Figure 1.2: Forecast 10 © PEPPERL+FUCHS Kolleg GmbH, Königsberger Allee 87, 68301 Mannheim K O L L E 2 Inductive Sensors 2.1 Fundamental Principles 2.1.1 2.1.2 2.1.3 2.1.4 2.1.4.1 2.1.4.2 2.1.5 Basic Construction Reduction Factor Coil Size and Sensing Range Installation Problems Housing Flush Mounting Electronic Circuit 2.2 Types 2.21 2.2.1.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6 2.2.7 Cylindrical and Rectangular proximity switches Definitions Slotted Types Ring Types Bistable Switches Sensors for use in Welding Magnetic Fields Sensors to distinguish between different materials Inductive Analogue Sensors 2.3 Interfaces for Inductive Proximity Switches 2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.3 2.3.4 Electrical Types and their Positive Effects Direct Current Switches Alternating and All voltage Switches Sensors to DIN 19234 (NAMUR) Protected and Safety Switches Reverse Polarity and Over Voltage Protection Overload Protection Safety Circuits Loads and their Characteristics Bus Connection 2.4 Manufacturing Technology 2.5 Applications G 11 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K 2.1 O L L E G Fundamental Principles Inductive sensors, in particular in the form of inductive proximity switches, also known as initiators, are widely used in automation and the process industry. 2.1.1 Basic Construction The active elements of an inductive sensor are the coil and ferrite core (see diagram 2.1). an alternating current is passed through the coil producing a magnetic field, which passes through the core in such away that the field only leaves the core on one side; this the active face of the proximity switch. When an metallic or magnetic object is near to the active face the magnetic field is deformed. An exact picture of the magnetic field can be obtained from computer simulation (see diagram 2.2). The effect on the magnetic field of a conducting material can be seen, in this case a steel plate. The change in the magnetic field due to the steel plate, also produces a change in the coil so that it’s impedance changes. This change in impedance is evaluated by the integrated sensor electronic and converted to a switch signal. Eddy currents are induced in electrically conducting materials present in the alternating magnetic field. The damping plate may be considered as short circuited winding, and the arrangement of damping material and sensor coil can be considered as a transformer. Ferrite core damping plate coin Diagram 2.1: Principle of an inductive sensor 12 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K O L L E G damping plate Diagram 2.2: Diagram showing the lines of force of the magnetic field of an inductive sensor with and without damping plate made from ST37 13 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K O L L E G The sensor coil forms the primary winding and the metal plate the short circuited secondary winding, see diagram 2.3. Because of the inductive coupling, represented by the mutual inductance M12 the current flowing in the secondary circuit i2 is reflected in the primary circuit. This manifests itself in the change of the coil impedance Z. This can easily derived from a comparison with the ideal transformer. Primary side: u1 = (R1+j·w·L1)·i1 + j·w·M12·i2 Secondary side: 0 = u2 = (R2+j·w·L2)·i2 + j·w·M12·i1. From the above we have: u1 ω2·M212 Z = — = R1 + j×ω×L1 + (R2 - j×ω×L2)· __________ i1 R22 + (ω×L2)2 Re (Z) = R1 + R2· ω2·M212 _________ R22 + (ω⋅L2)2 ω2·M212 Im (Z) = ω⋅L1 - ω⋅L2· _________ R22 + (ω×L2) 2 It can be seen that in the presence of a conducting material the real part of Z is increased above the resistance of the coil R1 the increase is dependent on R 2, L2, M12 and w. Experience shows that the imaginary part of Z only shows a measurable change with very small separation between the coil and the metal plate; it is only necessary to draw on the change in the real part of Z to detect an object made of conducting material. i1 Z = U1 R1 R2 M 12 L1 i2 L2 Diagram 2.3: Equivalent circuit of the transformer 14 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. U 2= O K 2.1.2 O L L E G Reduction Factor The increase in the real part of Z by the damping piece is largely dependent on the distance between the plate and the coil assembly and the material from which the plate is made, in particular from the material conductivity and permeability u. The largest change is obtained with damping pieces manufactured from mild steel (St37). The sensing range s of various materials is standardised against sn, which is the sensing range obtained with St37 and define a reduction factor, also known as correction factor: reduction factor = s/sn. Diagram 2.4 shows the dependence of reduction factor on the quotient of electrical conductivity divided by the relative permeability of the test piece; the example is for a proximity switch with 5mm sensing range ( no account is taken of the hysteresis loss of the test piece). The curve varies for each type of proximity switch, however it always has the same tendency. Diagram 2.4: Reduction factor of a proximity switch as a function of the quotient electrical conductivity / permeability of damping piece. 2.1.3 Coil size and Sensing range Diagram 2.2 shows that the magnetic field only extends over a limited distance, which in the end determines the maximum possible sensing range of an inductive proximity switch. It is evident that the extension of the field and the sensing range sn increase with increasing coil diameter. There is a moderate increase in the sensing range with increase in core diameter for proximity switches with standard sensing range (diagram 2.5). Diagram 2.5: Nominal sensing range sn for an inductive proximity switch, with standard sensing range, as a function of core diameter d. 15 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K 2.1.4 O L L E G Installation Problems The surroundings of the coil system, of inductive proximity switches, which includes conducting material outside of the active area creates a problem, in that this also has an effect on the shape of the magnetic field and therefore the impedance of the coil. 2.1.4.1 Housing Where a stainless steel housing is used for a proximity switch, the induced eddy currents. in the housing, causes an initial damping in the coil system and the oscillator, which in turn reduces the maximum sensing range. The effect can be reduced by mounting a copper ring shell core in the steel housing; the magnetic field in the housing is reduced in this way (see diagram 2.6). The eddy currents which now flow in the copper ring instead of the housing produce a lower loss, because the electrical conductivity of copper is approximately 40 times that of usual housing material V2A (see also diagram 2.4). The pre-damping is reduced to such a degree that it is possible that the sensing range is increased. coil shell core copper ring V2A-Housing Diagram 2.6: Lines of force of the magnetic field of an inductive sensor with integrated copper screening 16 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K 2.1.4.2 O L L E G Flush mounting Further undesirable losses are produced when a sensor is flush mounted in a conducting material, e.g. machine parts made from steel. The sensing range is reduced due to the additional pre-damping of the sensor magnetic field. In unfavourable cases the initiator may switch by the installation. In this situation the screening produced by the copper ring has a positive effect in that the eddy currents produced in the installation material are reduced. Sensors with increased sensing range, for flush mounting are normally provided with copper ring screening. The effect of the screening however is reduced with sensors with larger diameters, so that the flush mounting of larger sensors remains a problem. A possible solution for the future could be that the proximity switch senses it’s surroundings, this will require an increased technical effort in both the construction and the control circuit of the sensor. 2.1.5 Electronic circuit The coil system of the proximity switch together with a capacitor forms a parallel resonant circuit. A simplified equivalent circuit is shown in diagram 2.7, L represents coil inductance and Rv = Re (Z) the coil resistance, which is dependent on the damping piece (object sensed). C is the parallel capacitor considered as an ideal capacitor. The resistance R v determines the Quality Factor Q of the resonant circuit. Diagram 2.7: Simplified equivalent circuit for the resonant circuit of an inductive sensor. A block diagram of an inductive proximity switch is shown in diagram 2.8. The resonant circuit is part of an oscillator and the quality factor of the resonant circuit Q = wL/Rv determines the amplitude of the resulting HF oscillations. With the approach of the damping piece the quality factor of the coil is reduced due to the increase in the loss resistance Rv and therefore a reduction in the amplitude of oscillation. When the amplitude falls below a preset value a comparator operates, which in turn operates the output circuit and the sensor switches. comparator output stage Diagram 2.8: Block diagram of an inductive proximity switch 17 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K O L L E G In diagram 2.9 the change in quality factor Q, as a function of distance s from damping piece, for a coil system of a flush mounting proximity switch with a nominal sensing range of 10 mm is presented. Diagram 2.9: The change in quality factor Q, as a function of distance s from damping piece, for a coil system of a flush mounting proximity switch with a nominal sensing range of 10 mm. Diagram 2.10 Shows the relative change in quality factor ∆Q/Q for the same case, with reference to the undamped coil. The change in quality factor, which is taken as the switch point, is in the region of 10% to 50% (in this example 10%) for sensors with standard sensing range. In the case of initiators with double the sensing range only a change in quality factor of 1% to 6% is available, which demands a higher specification of the sensing electronic particularly with regard to temperature sensitivity. Diagram 2.10: Relative change in quality factor ∆Q/Q, of the coil system of an inductive sensor with 10mm sensing range, as a function of the sensing distance s of the damping piece, with respect to the undamped system. 18 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given. K O L L E G A basic oscillator circuit is shown in diagram 2.11. The resonant circuit comprises L and C. Transistor T is connected in the common collector configuration and operates as an noninverting amplifier with a voltage amplification less than 1; because of this the transformer feedback is necessary to produce the required voltage boost. The transformer is formed by tapping on to the coil. Rb and diode D determine the DC operating point of the transistor. Continuous oscillation of the oscillator is ensured by RE, which is also used to adjust the switching point. In practice this circuit exhibits a number of disadvantages, in particular with reference to temperature stability; because of this a slightly modified version is used as shown in diagram 2.12. Diagram 2.11: Principle of the oscillator circuit Diagram 2.12: Oscillator circuit Here the diode is replaced by the base emitter of a second transistor. When both transistors are at the same temperature, which can be best obtained with a dual transistor, the temperature drift of one is compensated for by that of the other. The capacitor C in the resonant circuit is connected so that the inductance of both coil windings is used. In this way the required capacitance is reduced for a given frequency of oscillation f. This is given by : 1 f = ————. 2·p·(LC)½ Depending on the switch type this ranges from a few kHz to a few MHz and is to a large extent dependent on the size of the coil core and therefore the sensing distance sn (diagram 2.13). The current taken by the output of the oscillator is high in the undamped condition and low in the damped condition. Diagram 2.14 shows the current taken by the oscillator, for an initiator using this circuit, with 10 mm switching distance, as a function of the distance of the damping object. 19 © PEPPERL+FUCHS KOLLEG GmbH, Königsberger Allee 87, 68301 Mannheim • 6DSP1006 We reserve the right to make modifications and no guarantee of the accuracy of information contained herein is given.
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