W07 Sensors and Measurement (1/2) Yrd. Doç. Dr. Aytaç Gören

Transcription

1 W07 Sensors and Measurement (1/2) Yrd. Doç. Dr. Aytaç Gören

2 ELK Contents W01 Basic Concepts in Electronics W02 AC to DC Conversion W03 Analysis of DC Circuits (self and condenser) W04 Transistors and Applications (H-Bridge) W05 Op Amps and Applications W06 Midterm W07 Sensors and Measurement (1/2) W08 Sensors and Measurement (2/2) W09 Basic Concepts in Digital Electronics (Boolean Algebra, Decimal to binary, gates) W10 Digital Logic Circuits (Gates and Flip Flops) W11 PLC s W12 Microprocessors W13 Data Acquisition, D/A and A/D Converters. 2 Yrd. Doç. Dr. Aytaç Gören

3 ELK 2018 W01 Contents 1. Measurement 2. Position Sensors 3. Velocity Sensors 4. Range Sensors 5. Acceleration Sensors 6. Straingauges, Load Cells and Force Transducers 7. Motion Sensor 3 Yrd. Doç. Dr. Aytaç Gören

4 Reminder N i 1 x i x 1 x 2 x 3 x N Summation function N xi x x x x i Product function N The mean is a measure of the centrality of a set of data. x 1 N N x i i 1 x N x x x x N x g N N i 1 i Mean (arithmetical) Mean (geometric) The geometric mean is primarily used to average ratios or rates of change. Yrd. Doç. Dr. Aytaç Gören

5 Reminder Mean (harmonic) Root mean square (RMS) x rms N x x x x N 1 N N i 1 x 2 i

6 x w N i 1 N x i 1 i w w i i Reminder The Weighted Mean The mode is the value that occurs most often. The midrange is the mean of the highest and lowest values. The median is the value for which half of the remaining values are above and half are below it. I.e., in an ordered array of 15 values, the 8th value is the median. If the array has 16 values, the median is the mean of the 8th and 9th values. The variance is the mean of the squared differences between individual data points and the mean of the array. V 1 N N i 1 ( x i x) 2 The Variance

7 Reminder The standard deviation is the square root of the variance. Standard deviation is not the mean difference between individual data points and the mean of the array. N 1 2 V ( xi x) The Standard Deviation N i 1 CV 100 x The Coefficient of Variation x x N Standard Deviation (or Error) of the Mean

8 Probability Reminder P( x;, ) ( e x ) / 2 The probability of x in a Gaussian distribution with mean and standard deviation z

9 Precision, Range and Accuracy In virtually every engineering application there is the need to measure some physical quantities, such as displacements, speeds, forces, pressures, temperatures, stresses, flows, and so on. These measurements are performed using physical devices called sensors, which are capable of converting a physical quantity to a more readily manipulated electrical quantity (ref: ).

10 Precision, Range and Accuracy The key issues in the selection of sensors are: (a) the field of view and range; (b) accuracy; (c) repeatability and resolution; (d) responsiveness in the target-domain; (e) power consumption; (f) hardware reliability; (g) size; and (h) interpretation reliability. Often the active element of a sensor is referred to as a transducer. Most sensors, therefore, convert the change of a physical quantity (e.g. pressure, temperature) to a corresponding and usually proportional change in an electrical quantity (e.g. voltage or current). Often the direct output from a sensor needs additional manipulation before the electrical output is available to the user.

11 Precision, Range and Accuracy The accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual (true) value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which repeated measurements under unchanged conditions show the same results. Range is the maximum and the minimum values that can be measured.

12 Linearity, Sensitivityand Accuracy Linearity maximum deviation from a straight-line response normally expressed as a percentage of the full-scale value. Sensitivity a measure of the change produced at the output for a given change in the quantity being measured. Physical phenomenon Measurement Output

13 Yrd. Doç. Dr. Aytaç Gören Position Sensors potentiometer The most commonly used of all the "Position Sensors", is the potentiometer because it is an inexpensive and easy to use position sensor. It has a wiper contact linked to a mechanical shaft that can be either angular (rotational) or linear (slider type) in its movement, and which causes the resistance value between the wiper/slider and the two end connections to change giving an electrical signal output that has a proportional relationship between the actual wiper position on the resistive track and its resistance value. In other words, resistance is proportional to position. The output signal (V out ) from the potentiometer is taken from the centre wiper connection as it moves along the resistive track, and is proportional to the angular position of the shaft.

14 Yrd. Doç. Dr. Aytaç Gören Position Sensors potentiometer When used as a positional sensor the moveable object is connected directly to the shaft or slider of the potentiometer and a DC reference voltage is applied across the two outer fixed connections forming the resistive element while the output signal is taken from the wiper terminal of the sliding contact as shown below thus producing a potential or voltage divider type circuit output.

15 Yrd. Doç. Dr. Aytaç Gören Position Sensors One type of positional sensor that does not suffer from mechanical wear problems is the "Linear Variable Differential Transformer" or LVDT for short. This is an inductive type position sensor which works on the same principle as the AC transformer that is used to measure movement. It is a very accurate device for measuring linear displacement and whose output is proportional to the position of its moveable core. It basically consists of three coils wound on a hollow tube former, one forming the primary coil and the other two coils forming identical secondaries connected electrically together in series but 180 o out of phase either side of the primary coil. A moveable soft iron ferromagnetic core (sometimes called an "armature") which is connected to the object being measured, slides or moves up and down inside the tube. LVDT

16 Yrd. Doç. Dr. Aytaç Gören Position Sensors LVDT A small AC reference voltage called the "excitation signal" (2-20V rms, 2-20kHz) is applied to the primary winding which inturn induces an EMF signal into the two adjacent secondary windings (transformer principles).

17 Yrd. Doç. Dr. Aytaç Gören Position Sensors LVDT A typical application of this type of sensor would be a pressure transducers, were the pressure being measured pushes against a diaphragm to produce a force. Advantages of the linear variable differential transformer, or LVDT compared to a resistive potentiometer are that its linearity, that is its voltage output to displacement is excellent, very good accuracy, good resolution, high sensitivity as well as frictionless operation and is sealed against hostile environments.

18 Yrd. Doç. Dr. Aytaç Gören Position Sensors LVDT

19 Position Sensors Proximity Sensors Yrd. Doç. Dr. Aytaç Gören Another type of inductive sensor in common use is the Inductive Proximity Sensor also called an Eddy current sensor. While they do not actually measure displacement or angular rotation they are mainly used to detect the presence of an object in front of them or within a close proximity, hence the name proximity sensors. Proximity sensors, are non-contact devices that use a magnetic field for detection with the simplest magnetic sensor being the reed switch. In an inductive sensor, a coil is wound around an iron core within an electromagnetic field to form an inductive loop. When a ferromagnetic material is placed within the eddy current field generated around the sensor, such as a ferromagnetic metal plate or metal screw, the inductance of the coil changes significantly. The proximity sensors detection circuit detects this change producing an output voltage. Therefore, inductive proximity sensors operate under the electrical principle of Faraday's Law of inductance.

20 Yrd. Doç. Dr. Aytaç Gören Position Sensors An inductive proximity sensor has four main components; The oscillator which produces the electromagnetic field, the coil which generates the magnetic field, the detection circuit which detects any change in the field when an object enters it and the output circuit which produces the output signal, either with normally closed (NC) or normally open (NO) contacts. Inductive proximity sensors allow for the detection of metallic objects in front of the sensor head without any physical contact of the object itself being detected. This makes them ideal for use in dirty or wet environments. The "sensing" range of proximity sensors is very small, typically 0.1mm to 12mm. Proximity Sensors

21 Position Sensors Proximity Sensors (capacitive) Yrd. Doç. Dr. Aytaç Gören

22 Position Sensors Encoders Yrd. Doç. Dr. Aytaç Gören Rotary Encoders resemble potentiometers mentioned earlier but are noncontact optical devices used for converting the angular position of a rotating shaft into an analogue or digital data code. In other words, they convert mechanical movement into an electrical signal (preferably digital). All optical encoders work on the same basic principle. Light from an LED or infra-red light source is passed through a rotating high-resolution encoded disk that contains the required code patterns, either binary, grey code or BCD. Photo detectors scan the disk as it rotates and an electronic circuit processes the information into a digital form as a stream of binary output pulses that are fed to counters or controllers which determine the actual angular position of the shaft. There are two basic types of rotary optical encoders, Incremental Encoders and Absolute Position Encoders. Inductive sensor Opto-switch sensor

23 Position Sensors Encoders Yrd. Doç. Dr. Aytaç Gören Inductive sensor Opto-switch sensor Incremental Encoders, also known as quadrature encoders or relative rotary encoder, are the simplest of the two position sensors. Their output is a series of square wave pulses generated by a photocell arrangement as the coded disk, with evenly spaced transparent and dark lines called segments on its surface, moves or rotates past the light source. The encoder produces a stream of square wave pulses which, when counted, indicates the angular position of the rotating shaft. Incremental encoders have two outputs called quadrature outputs that are 90 o out of phase and the direction of rotation can be determined from output sequence. The number of transparent and dark segments or slots on the disk determines the resolution of the device and increasing the number of lines in the pattern increases the resolution per degree of rotation. Typical encoded discs have a resolution of up to 256 pulses or 8-bits per rotation.

24 Yrd. Doç. Dr. Aytaç Gören Position Sensors Encoders The simplest incremental encoder is called a tachometer. It has one single square wave output and is often used in unidirectional applications where basic position or speed information only is required. The "Quadrature" or "Sine wave" encoder is the more common and has two output square waves commonly called channel A and channel B. This device uses two photo detectors, slightly offset from each other by 90 o thereby producing two separate sine and cosine output signals.

25 Yrd. Doç. Dr. Aytaç Gören Position Sensors Encoders By using the Arc Tangent mathematical function the angle of the shaft in radians can be calculated. Generally, the optical disk used in rotary position encoders is circular, then the resolution of the output will be given as: θ = 360/n, where n equals the number of segments on coded disk. Then for example, the number of segments required to give an incremental encoder a resolution of 1 o will be: 1 o = 360/n, therefore, n = 360 windows, etc. Also the direction of rotation is determined by noting which channel produces an output first, either channel A or channel B giving two directions of rotation, A leads B or B leads A. This arrangement is shown below.

26 Position Sensors Absolute Position Encoders Yrd. Doç. Dr. Aytaç Gören Absolute Position Encoders are more complex than quadrature encoders. They provide a unique output code for every single position of rotation indicating both position and direction. Their coded disk consists of multiple concentric "tracks" of light and dark segments. Each track is independent with its own photo detector to simultaneously read a unique coded position value for each angle of movement. The number of tracks on the disk corresponds to the binary "bit"-resolution of the encoder so a 12-bit absolute encoder would have 12 tracks and the same coded value only appears once per revolution

27 Position Sensors Absolute Position Encoders Yrd. Doç. Dr. Aytaç Gören

28 Yrd. Doç. Dr. Aytaç Gören Velocity Sensors Encoders and Tachometers may be also used as velocity sensors. Other types of velocity sensors can be specifeid as, Doppler Sensors GPS sensors

29 Velocity Sensors Doppler Sensors Yrd. Doç. Dr. Aytaç Gören

30 Yrd. Doç. Dr. Aytaç Gören Range Sensors Scanning laser range finders provide a relatively new and exciting high-resolution robotics sensor. Common in high-end robotics for many years, these sensors are becoming more common on relatively inexpensive robotics applications due to the rich, high resolution, and high frequency data they generate. Laser Range Sensors USB Interface 240º Field of View 0.36º Angular Resolution 10Hz Refresh Rate 20mm to 4m

31 Range Sensors Ultrasonic Sensors Ultrasonic sensors (also known as transceivers when they both send and receive) work on a principle similar to radar or sonar which evaluate attributes of a target by interpreting the echoes from radio or sound waves respectively. Ultrasonic sensors generate high frequency sound waves and evaluate the echo which is received back by the sensor. Sensors calculate the time interval between sending the signal and receiving the echo to determine the distance to an object. Systems typically use a transducer which generates sound waves in the ultrasonic range, above 18,000 hertz, by turning electrical energy into sound, then upon receiving the echo turn the sound waves into electrical energy which can be measured and displayed. Yrd. Doç. Dr. Aytaç Gören

32 Range Sensors Ultrasonic Sensors Yrd. Doç. Dr. Aytaç Gören In reflection mode (also known as echo ranging ), an ultrasonic transmitter emits a short burst of sound in a particular direction. The pulse bounces off a target and returns to the receiver after a time interval t. The receiver records the length of this time interval, and calculates the distance travelled r based on the speed of sound c: r = c * t

33 Yrd. Doç. Dr. Aytaç Gören Acceleration Sensors Acceleration is related to motion, a vector quantity, exhibiting a direction as well as magnitude. The direction of motion is described in terms of some arbitrary Cartesian or orthogonal coordinate systems.

34 Acceleration Sensors Yrd. Doç. Dr. Aytaç Gören

35 Yrd. Doç. Dr. Aytaç Gören Acceleration Sensors Piezoelectric accelerometers are widely used for general-purpose acceleration, shock, and vibration measurements. They are basically motion transducers with large output signals and comparatively small sizes and they are self generators not requiring external power sources. They are available with very high natural frequencies and are therefore suitable for high-frequency applications and shock measurements. where q is the charge developed and dij is the piezoelectric coefficient of the material.

36 Yrd. Doç. Dr. Aytaç Gören Acceleration Sensors Piezoelectric accelerometer Nonzero lower cutoff frequency (0.1 1 Hz for 5%) Light, compact size (miniature accelerometer weighing 0.7 g is available) Measurement range up to +/- 500 g Less expensive than capacitive accelerometer Sensitivity typically from mv/g Broad frequency bandwidth (typically khz) Operating temperature: C Capacitive accelerometer Good performance over low frequency range, can measure gravity! Heavier (~ 100 g) and bigger size than piezoelectric accelerometer Measurement range up to +/- 200 g More expensive than piezoelectric accelerometer Sensitivity typically from mv/g Frequency bandwidth typically from 0 to 800 Hz Operating temperature: C

37 Acceleration Sensors Piezoresistive Accelerometers Piezoresistive accelerometers are essentially semiconductor strain gauges with large gauge factors. High gauge factors are obtained since the material resistivity is dependent primarily on the stress, not only on the dimensions. This effect can be greatly enhanced by appropriate doping of semiconductors such as silicon. Most piezoresistive accelerometers use two or four active gauges arranged in a Wheatstone bridge. Extra precision resistors are used, as part of the circuit, in series with the input to control the sensitivity, for balancing, and for offsetting temperature effects. The sensitivity of a piezoresistive sensor comes from the elastic response of its structure and resistivity of the material. Wire and thick or thin film resistors have low gauge factors, that is, the resistance change due to strain is small. Yrd. Doç. Dr. Aytaç Gören

38 Yrd. Doç. Dr. Aytaç Gören Acceleration Sensors Strain-gauge accelerometers Strain-gauge accelerometers are based on resistance properties of electrical conductors. If a conductor is stretched or compressed, its resistance alters due to (a) dimensional changes, and (b) the changes in the fundamental property of material called piezoresistance. This indicates that the resistivity ρ of the conductor depends on the mechanical strain applied onto it. Electrostatic accelerometers are based on Coulomb s law between two charged electrodes; therefore, they are capacitive types. Depending on the operation principles and external circuits they can be broadly classified as (a) electrostatic-force-feedback accelerometers, and (b) differential-capacitance accelerometers.

39 Yrd. Doç. Dr. Aytaç Gören Acceleration Sensors Micro- and Nanoaccelerometers Multiple accelerometers can be mounted on a single chip, sensing accelerations in x, y, and z directions. The primary signal conditioning is also provided in the same chip. The output from the chip is usually read in the digital form.

40 Yrd. Doç. Dr. Aytaç Gören Gyroscope A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation. Although this orientation does not remain fixed, it changes in response to an external torque much less and in a different direction than it would without the large angular momentum associated with the disk's high rate of spin and moment of inertia. Since external torque is minimized by mounting the device in gimbals, its orientation remains nearly fixed, regardless of any motion of the platform on which it is mounted. Gyroscopes based on other operating principles also exist, such as the electronic, microchip-packaged MEMS gyroscope* devices found in consumer electronic devices, solid-state ring lasers, fibre optic gyroscopes, and the extremely sensitive quantum gyroscope. * Acronym for Microelectromechanical Systems

41 Gyroscope Yrd. Doç. Dr. Aytaç Gören

42 Yrd. Doç. Dr. Aytaç Gören Gyroscope Gyroscope measures Rate Of Turn. Integrate for angle Digital-output X-, Y-, and Z-Axis angular rate sensors (gyros) on one integrated circuit Digitally-programmable low-pass filter Low 6.5mA operating current consumption for long battery life Wide VDD supply voltage range of 2.1V to 3.6V Standby current: 5μA Digital-output temperature sensor Fast Mode I 2 C (400kHz) serial interface Optional external clock inputs of kHz or 19.2MHz to synchronize with system clock Pins broken out to a breadboard friendly 7-pin 0.1" pitch header

43 Yrd. Doç. Dr. Aytaç Gören Force Transducers The use of strain gages is based on the fact that the resistance of a conductor changes when the conductor is subjected to strain. The resistance of an electrically conductive material changes with dimensional changes which take place when the conductor is deformed elastically. When such a material is stretched, the conductors become longer and narrower, which causes an increase in resistance. Straingauges, Load Cells, Force Transducers

44 Yrd. Doç. Dr. Aytaç Gören Force Transducers Straingauges, Load Cells, Force Transducers Static balanced Wheatstone Bridge.

45 Force Transducers Straingauges, Load Cells, Force Transducers

46 Yrd. Doç. Dr. Aytaç Gören Force Transducers Straingauges, Load Cells, Force Transducers Both half-bridge and full-bridge configurations grant greater sensitivity over the quarterbridge circuit, but often it is not possible to bond complementary pairs of strain gauges to the test specimen. Thus, the quarter-bridge circuit is frequently used in strain measurement systems. When possible, the full-bridge configuration is the best to use. This is true not only because it is more sensitive than the others, but because it is linear while the others are not. Quarter-bridge and half-bridge circuits provide an output (imbalance) signal that is only approximately proportional to applied strain gauge force. Linearity, or proportionality, of these bridge circuits is best when the amount of resistance change due to applied force is very small compared to the nominal resistance of the gauge(s). With a full-bridge, however, the output voltage is directly proportional to applied force, with no approximation.

47 Yrd. Doç. Dr. Aytaç Gören Force Transducers Straingauges, Load Cells, Force Transducers Tension Compression Load Cell

48 Yrd. Doç. Dr. Aytaç Gören Motion Sensor Pyroelectricity is the ability of certain materials to generate a temporary voltage when they are heated or cooled. PIR sensors allow you to sense motion, almost always used to detect whether a human has moved in or out of the sensors range. The PIR sensor itself has two slots in it, each slot is made of a special material that is sensitive to Infra Red.

49 PIR Sensor Yrd. Doç. Dr. Aytaç Gören

50 Yrd. Doç. Dr. Aytaç51Gören