Instrumentation & Control Lecture 3 PDF

Instrumentation & Control Lecture 3 : Position measurement Mechanical and proximity switches Aisha Shoaib Assistant Professor [email protected] Department of Mechatronics and Control Engineering University of Engineer...

Instrumentation & Control Lecture 3 : Position measurement Mechanical and proximity switches Aisha Shoaib Assistant Professor [email protected] Department of Mechatronics and Control Engineering University of Engineering and Technology, Lahore Ultrasonic Proximity Sensors: An ultrasonic proximity sensor is a proximity sensor that uses a pulse of sound waves to detect the presence of an object. They can be found in parking technology and anti-collision safety systems. Ultrasonic sensors are also used in robotic obstacle detection systems and manufacturing engineering. Compared to infrared (IR) sensors in proximity sensing applications, ultrasonic sensors are less susceptible from smoke, gases, and other airborne particles. Sound waves travel toward the target and are reflected toward the sensor. Frequencies range from 65 kHz to 400 kHz, depending on the type of sensor used. The elapsed time between the pulse generation and the detection of the reflection is related to the distance to the target. Photoelectric Proximity Sensors: A Photoelectric Sensor is a proximity sensor that consists primarily of an Emitter for emitting light and a Receiver for receiving light. When emitted light is interrupted or reflected by the sensing object, it changes the amount of light that arrives at the Receiver. The Receiver detects this change and converts it to an electrical output. The light source for the majority of Photoelectric Sensors is infrared or visible light (generally red, or green/blue for identifying colors). An LED is usually used as the light or infrared source. The three common modes of photoelectric detection are: 1. Diffused (proximity) Mode 2. Retro-reflective Mode 3. Throughbeam detection Mode Diffused Mode: A diffused-mode photoelectric sensor is a photoelectric sensor that directs its source against a target object and detects a reflection from the target object. The reflection is diffused and is thus not very strong. An infrared light source is much stronger than a visible light source and is thus better suited to this type of sensor. The color, finish, and size of the target object have a significant impact on whether this photoelectric mode is suitable for an application. Shiny targets reflect more light, but only at a specific angle. Therefore the sensor must be aimed directly at the target. Retro-Reflective Mode: A retro-reflective mode photoelectric sensor is a photoelectric sensor that uses a focused beam directed across the path of a target object and reflected back to the sensor. The sensor is actuated when there is no object in the path of the beam. The sensor is deactivated when the target object blocks the beam. This requires the reflector to be placed farther away than the object itself, such as across a conveyor. The longer distance that the beam has to travel normally suggests that the strongest source be used. The strongest source is an invisible infrared source, but a visible light source makes it easier to adjust the mirror. A special corner cube reflector is typically used to reflect the light beam back to the detector. The corner cube reflector has a triangular grooved surface that returns the light beam on a parallel axis. Through-Beam Mode: A through-beam mode photoelectric sensor is a photoelectric sensor that uses a beam aimed directly at a target object with a separate receiver to sense the beam. The presence of an object interrupts the beam and actuates the circuit. The through-beam mode of sensing provides the greatest range. Because of the tightly focused source, the throughbeam mode is less susceptible to atmospheric contamination. An advantage of a separate emitter and receiver is the ability to use convergent sensing, or fixed-focus sensing. Since the beam source is tightly focused, it can be directed to reflect off the object. The receiver needs to be located to sense the reflected beam. This arrangement allows a through-beam photoelectric sensor to monitor an exact point, such as a bottle cap or a liquid level. Advantages of Photoelectric sensor: o It senses all kinds of materials. o It has a longer life. o It has a long sensing range and is very reliable. o It has a very fast response time. o It is less costly. o Diffuse photoelectric sensor detects small objects including color mark and label detection. o Retro-reflective type can detect transparent objects. o Thru beam type can detect long range and it is tolerant of dirty environment. Disadvantages of Photoelectric sensor o Over coarse of time lens get contaminated. o It’s sensing range is affected due to color and reflectivity of the target. o Thru beam type requires transmitter (Tx) and receiver (Rx) at two separate locations. Retroreflective type requires reflector in addition to Tx/Rx. This makes system installation complex. Digital Sensors for Motion Measurement Digital transducers are ideal devices for motion measurement. They produce a digital output which can be interfaced to the computer. They have become increasingly attractive because of the following properties. Signal conditioning simplicity Minor susceptibility to electro-magnetic interference While they are used to measure linear or angular displacement, digital transducers also are used to measure force, pressure, and liquid level with the appropriate mechanical or electromechanical translators. Digital Encoders: Encoders are widely used for applications involving measurement of linear or angular position, velocity, and direction of movement. Encoders are used in tensile-test instruments to precisely measure the ball screw position used to apply tension or compression to the test specimen. They are used in automated test stands used when angular positions of windshield wiper drives and switch positions are tested. Construction: An encoder is a circular device in the form of a disk on which a digital pattern is engraved. The inscribed pattern is sensed by means of a sensing head. The rotary disk is normally coupled to a shaft. As the shaft rotates, a different pattern is generated for each resolvable position. The sensing mechanism can be a photoelectric device with slots acting as transparent optical windows.  An optical encoder generally is used to precisely measure rotational movement. Its main advantages are simplicity, accuracy, and suitability for sensitive applications. Optical encoders are considered one of the most reliable and least expensive motion-feedback devices available and are used widely in a broad range of modern applications. Shaft Encoders can be classified into two categories depending on the nature and method of interpretation of the output: Incremental Encoders An incremental encoder provides a simple pulse each time the object to be measured has moved a given distance. Absolute Encoders An absolute encoder provides a unique binary word coded to represent a given position of the object. Incremental encoders: Incremental encoders for angular measurement consist of a sensing shaft attached to a disk which is divided into an equal number of sectors on the circumference. Incremental rotary encoders are very useful for measuring shaft rotation and primarily consist of three components: a light source, a coded wheel, and a photoelectric sensor. Figure shows an encoder measuring system which uses transmission gratings. As the movable grating translates with respect to a fixed grating, the pulses are counted to provide position information. Absolute encoders: Absolute encoders use a unique "word" for each position, meaning that an absolute encoder provides both the indication that the position has changed and an indication of the absolute position of the encoder. Provides information in the form of unique output for every movement of the shaft rotation (in Binary, BCD or Gray Code). Advantage over incremental encoder => Position is maintained after a power-down. The absolute position is recovered upon power-up without requiring a home cycle or any shaft rotation. Potentiometers: o A potentiometer is a variable electrical resistance. A length of resistance material has a voltage applied over its ends. o A slider moves along it (either linear or rotary) and picks off the voltage at its position or angle. o The tracks may be made from carbon , resistance wire or piezoresistive material.  Potentiometers: o Potentiometric Principle A displacement transducer using variable resistance transduction principle can be manufactured with a rotary or linear potentiometer. A potentiometer is a transducer in which a rotation or displacement is converted into a potential difference. o The displacement of the wiper of a potentiometer causes the output potential difference obtained between one end of the resistance and the slider. o This device converts linear or angular motion into changing resistance, which may be converted directly to a voltage or current signal. o The position of the slider along the resistance element determines the magnitude of the electrical potential. o The voltage across the wiper of linear potentiometer is measured in terms of the displacement, d, and given by the relationship Rotary Potentiometer If the movement of the slider is in a circular path along a resistance element, rotational information is converted into information in the form of a potential difference. The output of the rotary transducer is proportional to the angular movement. Applications Used for position monitoring of products on assembly lines and checking dimensions of the product in quality control systems. Rotary potentiometers are used in applications involving rotational measurement for application ranging from machine tools to aircraft.