Albaha University Faculty of Engineering Mechanical Engineering Department Lecture 5: Displacement measurement Ossama Abouelatta o_abouelatta@yahoo.com Mechanical Engineering Department Faculty of Engineering Albaha University 2012 Aims This lecture aims: This lecture first reviews the state of sensor technology for manufacturing process monitoring in general. Methods of measuring pressure Electrical pressure measuring equipment Strain gauge and pressure transducers General information Principle of piezo-electric sensors Absolute and differential pressure sensors Experiment - Absolute pressure sensor.. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (2)
Outline Introduction Absolute sensors Principle of inductive sensors Design of inductive sensors Functioning of a differential inductor Inductive proximity sensors Inductive proximity switch 'Inductive displacement sensor' card SO4203-5U General Principle of inductive sensors Experiments - Inductive displacement Zero-point calibration of the measuring bridge Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (3) Introduction In the sphere of electrical instrumentation, a distinction is made between analog and digital measuring techniques. An analog technique is when a display method is used that shows the value continuously. A digital technique is one where the measured variable is detected in a quantized form with a discrete minimum resolution. Frequent characteristics of digital methods are a numerical procedure or a coded enumeration of the measured values. Familiar examples of these two differing techniques are the speed sensors (speedometers) or distance measuring devices (odometers) in cars. To measure variables, appropriate sensors are required to provide an analog or digital electrical signal. The nature of the signal can be changed from one kind to another by means of analog-to-digital or digital-to-analog converters. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (4)
Introduction Displacement and angle sensors are suitable for measuring distance, length, position and angle of rotation. Analog versions are mainly used as passive resistance sensors with potentiometers (e.g. with face-plate or helical structures) or in combination with strain gauges. They may also be manufactured as inductive sensors. Inductive immersion armature measuring system for oil-based hydraulics Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (5) Introduction For industrial measurement of displacement or angle analog sensors are very often used in spite of the modern trend towards digitalization. This is always the case when continuous resolution is essential or when transitions in the signal need to be continuous rather than occurring in steps, for example. Usually such sensors operate without contact using optical, inductive or magnetic measurement techniques. The inductive and magnetic systems are usually enormously superior to optical systems in terms of robustness and resistance to dirt. In industry the trends are leading increasingly towards digital processing of measurements. This means that either a measuring system that intrinsically provides a digital signal must be used or an analog-to-digital converter is needed to simply convert analog signals into digital form. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (6)
Introduction The group of analog sensors, includingsine and cosine potentiometers, continues to play a key role alongside purely digital methods since it is easy to digitalize the signals from them. Digital displacement and angle converters include incremental sensors that, although they may be built on the basis of resistive or inductive sensors, may also involve electrodynamics and electro-optical devices. Quantizing the sensor can be a means of generating a discontinuous or discrete signal from the initial analog signal. Each of the elements, i.e. quantized displacement unit, produces a pulse that can be input to a counter. The counter status represents the measured displacement. These two steps, quantization and counter display characterize the incremental method as a digital sensor technique. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (7) Introduction a) Fine-wire rotary voltage divider b) Curved disc d) Toothed disc with passive inductive (left) or active electrodynamics measuring technique (right). e) Perforated or slotted disc with photo-electric measuring technique α) Angle of rotation a) b) Speed and angle transducer c) d) e) Analog systems with characteristic f) Digital systems with logic analyzer trace Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (8)
Introduction The disadvantage of the simple technique is obvious. Since each individual value is dependent on the previous one, any faulty reading means that all subsequent results are erroneous. In the event of a power failure the current measured value is lost entirely. A relative technique of this kind does, however, have the advantage that its zero point can easily be set to be at any point. For example, in order to detect the direction of motion using positional methods, incremental methods require two sensors separated by half of one quantization unit, the signals from which can then be evaluated by a logical network before being fed to an up/down counter. It is not sufficient to detect the static signal from each track of the pattern and to link them together, but the edges where the signal changes (e.g. light-dark transitions of elements) must also be detected. This requires quite a complicated circuit. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (9) Introduction Further refinements such as reference marks on a third track of the quantized pattern, enable an immediate and well-defined usage of the counter after a power failure. With special opto-electronic systems errors can even be detected by means of external disturbance pulses or by the loss of certain pattern markings. (Moiré strip technique). Electro-optical methods have become widespread due their lack of wearing components being entirely contactless and due to their relative lack of susceptibility to electrical interference. There are versions where the light passes right through the components (transmission) or others where light is reflected (reflection). This applies to both incremental and absolute sensors. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (10)
Absolute sensors For sensors detecting absolute displacement,the entiremeasuringrangeneeds to be divided into discrete, numbered, quantized regions - as for all digital methods. In contrast to incremental methods, these quantized regions are not simply counted by are all provided with unique markings, e.g. by means of distinct quantized codes of a ruler. These markings and thus each position correspond to a distinct and fixed numeric value. The practical display of these numbers is achieved by means of suitable coding. Absolute displacement and angle sensors are thus called decoded displacement sensors (encoders). Typical applications include digital displacement and angle sensors used for numerically controlled tools (NC or CNC). Incremental systems currently cover about 80% of this market. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (11) Absolute sensors Position sensing for machine control a) Direct measurement of position b) Indirect measurement of position Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (12)
Absolute sensors Incremental measurement of position Absolute measurement of position Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (13) Principle of inductive sensors Design of inductive sensors Inductive sensors usually consist of one or more coils with variable inductance. The figure to the above-right shows an example of a cylindrical coil and the formula for determining its inductance L, where l is the length of the coil, A is the surface area of the coil, N is the number of windings μ 0 is the field constant and μ r is the relative permeability of the medium inside the oil. The principle of the sensor involves the influence that the quantity to be measured (distance or displacement) has over the inductance of the coil(s). The change in inductance can be detected using a suitable measuring bridge. Industrial displacement sensor for the range 0... 2 mm Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (14)
Principle of inductive sensors Functioning of a differential inductor The following animation shows how a so-called differential inductor for displacement measurement works. Two identical coils L 1 und L 2 are located within a cylindrical housing. The core of the coil is a armature that moves along the axis and detects the displacement. When the armature moves, the permeability of its material causes the inductance of one coil to increase while the inductance of the other decreases by the same amount. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (15) Principle of inductive sensors Functioning of a differential inductor The following graphic shows the measuring circuit used. The circuit is supplied with an AC voltage U ~. The resistors R 1 and R 2 form a full bridge along with the two inductors. The measured voltage U M is then directly proportional to the displacement of the armature. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (16)
Principle of inductive sensors Inductive proximity sensors Inductive proximity sensors detect the distance between the sensor and an approaching object and can be used for distance measurement or for metal detectors, for example (see right). The principle of this type of sensor involves the degree of damping by the detected object of a magnetic field produced by an oscillator. The object must therefore be made of conducting material. The operating range is usually no more than a few millimeters. The animation alongside illustrates the principle of an inductive proximity sensor. The magnetic field produced by the oscillator spreads into the surrounding space due to the open shell core. If an object made of conducting material comes within the field, eddy currents are induced within it that result in damping of the field due to the law of induction and therefore to a reduction in the amplitude of the voltage. The voltage amplitude is therefore a measure of the distance between the object and the sensor. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (17) Principle of inductive sensors Inductive proximity switch The inductive proximity sensor described above can easily be used as a proximity switch (proximity initiator), where the voltage amplitude is processed via a Schmitt trigger. This is configured so that the switch goes "on" or "off" when the distance between the sensor and the object is below a certain threshold, which can be calibrated. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (18)
'Inductive displacement sensor' card SO4203-5U Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (19) General The UNI-TRAIN Inductive displacement sensor card contains an inductive displacement sensor in the form of a differential inductor and the appropriate circuit. Displacementisdetectedbythemotionofanironcore inside the inductor coils. The motion causes the two inductances L 1 and L 2 to change, which can be detected by means of an AC powered measuring bridge. The circuit for an inductive displacement sensor. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (20)
Principle of inductive sensors The sensor consists of a differential inductor with movable iron core. A voltage u D is generated via a measuring bridge (1) powered by relatively high frequency AC voltage. The amplitude and phase of the voltage (either 0 odder 180 ) depends on the position of the iron core. The null point of the bridge can be calibrated by means of potentiometer R 2. The bridge voltage is converted by a differential amplifier (2) into ground referenced voltage. The output of the differential amplifier is tapped via a 2-mm socket. The AC voltage measured is converted into DC by means of a sample-and-hold circuit (4). The pulse is supplied to the sampling circuit via a pulse generator (3) that shapes a short square pulse from the supply voltage. The phase ϕ of the pulse can be adjusted by a potentiometer to between 0 and 90 with respect to the input voltage. For the best results, the phase should be set to about 90. The output of the sample-and-hold circuit is then a DC voltage u out proportional to the motion of the iron core. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (21) Experiments - Inductive displacement Zero-point calibration of the measuring bridge The following experiment starts by setting the zero point of the measuring bridge to be at the center point of the coil core. The input voltage to the bridge is set using the Function generator virtual instrument and the output is measured on the Oscilloscope virtual instrument. The following diagram shows the appropriate measurement circuit. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (22)
Experiments - Inductive displacement Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (23) Experiments - Inductive displacement Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (24)
Experiments - Inductive displacement First open the function generator and set the following parameters: Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (25) Experiments - Inductive displacement Now open the oscilloscope and set the following parameters: Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (26)
Experiments - Inductive displacement Now move the core of the coil to its centre position (i.e. to scale division 20). Set the potentiometer for setting the zero point so that the amplitude of the bridge voltage reaches its minimum value. Copy the resulting oscilloscope trace into the placeholder below. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (27) Experiments - Inductive displacement What are the amplitude and frequency of the bridge voltage after the calibration has been successfully completed? Enter your answer into the text box below. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (28)
Experiments - Inductive displacement Parameters for the differential amplifier The following experiment investigates how the differential amplifier functions. Expand the set-up for the previous experiment so that channel B of the oscilloscope shows the output from the differential amplifier (see following graphic). Change the settings for the oscilloscope as follows: Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (29) Experiments - Inductive displacement Now move the core of the coil to its lower limit (i.e. to scale division 0) and perform the experiment again. Copy the resulting oscilloscope trace into the placeholder below. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (30)
Experiments - Inductive displacement Now move the core to its upper limit (i.e. to scale division 40) and repeat the experiment again. Copy the resulting oscilloscope trace into the placeholder below. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (31) Experiments - Inductive displacement What is the phase difference between the bridge voltage and the output of the differential amplifier? What is the gain of the differential amplifier? Enter your answers into the text box below. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (32)
Thank You Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha University Albaha, KSA email: o_bouelatta@yahoo.com Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (33)