CSE 323b: Measurements & Testing (1)b

About the course Course webpage: http://www.staff.zu.edu.eg/amabd/ Grading Scheme 50 Semester work Lab Final 150 Total Course webpage: http://www.staff.zu.edu.eg/amabd/ http://www.amelanany.faculty.zu.edu.eg/

References: William Dunn, Introduction to Instrumentation, Sensors, and Process Control, Artech House, 2006. William Bolton, Instrumentation and Control Systems, Elsevier, 2004. Curtis Johnson, Process control instrumentation technology, Prentice-Hall, 6th edition, 2000. William Bolton, Programmable Logic Controllers, Elsevier, 4th edition, 2006.

Topics to be covered in this course: Several types of sensors: Displacement Pressure Flow Liquid level Temperature Programmable logic controllers. Serial interfaces. Motors A/D and D/A converters

Lecture 1: Displacement

Position and Motion Sensing Many industrial processes require information on and control of both linear and angular position, and rate of motion. These are required in robotics, rolling mills, machining operations, numerically controlled tool applications, and conveyors.

Displacement sensors Measure linear displacement Measure angular displacement Detect motion (and then issue an alarm or turn light on) Detect the presence of some object - using the so called proximity sensor - e.g. to detect if a work piece is available for the tool to work on

Displacement sensors Displacement sensors can be classified into: Direct contact. Non-contacting (using e.g. light, infrared beams, or magnetic effect) In these slides, we are going to learn about some position and motion measuring devices.

Potentiometer Potentiometers are a convenient cost-effective method for converting displacement to an electrical variable. The wiper or slider arm of a linear potentiometer can be mechanically connected to the moving object. Where angular displacement is involved, a single or multi-turn rotational type of potentiometer can be used. The lifetime of the potentiometer may be limited by wear.

Linear variable differential transformers (LVDT) LVDTs are devices used for measuring small distances. The device consists of a primary coil with two secondary windings, one on either side of the primary.

Linear variable differential transformers (LVDT) A movable core, when centrally placed in the primary, will give equal coupling to each of the secondary coils. When an ac voltage is applied to the primary, equal voltages will be obtained from the secondary windings, which are shown wired in series opposition. This gives zero output voltage. When the movable core travelled away from the center, an output voltage proportional to displacement is obtained. A variety of LVDTs is available, with linear ranges from ±1 mm to ±25 cm. The transfer function is normally expressed in mV/mm. These devices are not as cost-effective as potentiometers, but they are more accurate, and have better longevity than potentiometers.

Capacitive Capacitive devices can be used to measure angular or linear displacement. The capacitance between two parallel plates is given by: Where K is the dielectric of the material between the plates, A is the area of the plates in square meter, and d is the distance between the plates in meter.

Capacitive Therefore, there are three methods of changing the capacitance: changing the distance between the plates, moving one plate with respect to the other plate to reduce the overlapping area between the plates, and moving a dielectric material between fixed plates.

Capacitive Capacitance variation is a very accurate method of measuring displacement, and is used extensively in micromechanical devices where the distance is small, giving high capacitance per unit area. Capacitive devices are used in the measurement of pressure, acceleration, and level.

Strain Gauge Strain gauge is a metal foil strip used to measure strain by converting it into a change in resistance. Strain gauge can be used also to measure weight and pressure. In order to compensate for temperature, additional (dummy) gauges are used.

Ultrasonic Sensor Ultrasonic devices can be used for distance measurement. In this device, the transmitter send an ultrasonic pulse. The time taken by the pulse to travel to an object and be reflected back to a receiver is measured. Given this time and the speed of ultrasonic waves (340 m/s), the distance can be calculated. Ultrasonic waves can be used to measure distances from 1m to approximately 50m.

Optical encoders An encoder is a device that provides a digital output as a result of an angular or linear displacement. Position encoders can be grouped into two categories: absolute encoders, which give the actual position directly. incremental encoders, which detect changes in displacement from some datum position, and

Absolute Optical Encoder Absolute position encoders are made using patterned disks. An array of LEDs with a corresponding photodetector for each LED can give the position of the wheel at any time.

Absolute Optical Encoder Disk encoders are typically designed with the binary code or Gray code pattern. What is the advantage of Gray code over binary code? 3-bit binary Rotary encoder. 3-bit binary-reflected Gray code (BRGC).

Incremental Optical Shaft Encoder

Incremental Optical Shaft Encoder Light from a light emitting diode (LED) shines through windows in the disk onto a photodiode. Every time a window is between the LED and the photodiode a pulse is generated. Hence, as the shaft turns, a train of pulses is obtained. The position of the wheel with respect to its previous location can be obtained by counting the number of pulses multiplying them by 360°, divided by the number of slots in the disk. The resolution of the sensor is 360°divided by the number of slots in the disk. Reflective strips can replace the slots, in which case the light from the LED is reflected back to a photodiode. Only one slot in the disk would be required to measure revolutions per minute. Note that optical encoders can also be used to measure speed by counting the number of pulse obtained per 1 seconds.

Incremental Optical Encoder with 2 tracks A & B The problem with the previous incremental encoder with one track is the inability to detect the direction of rotation. This can be solved using two tracks which are out of phase by 90°.

Incremental Optical Encoder with 2 tracks As with the single track encoder, a counter can be used to count the number of pulses and hence detect the angle rotated. To sense the direction of rotation, the phase differences can be used in a circuit such as the one shown below.

The output from the J-K flip-flop is used to indicate direction (other circuits being used to indicate speed). In this circuit, the OR gate forms a clock to the J-K flip-flop and the A and B inputs from the encoder are the J and K inputs of the flip-flop.

Other Uses for Optical Devices Optical devices have many uses in industry, other than for the measurement of the position and speed of rotating equipment. Optical devices are used for counting objects on conveyor belts on a production line, reading bar codes, measuring and controlling thickness, and detecting breaks in filaments.

Tachogenerator for Speed Measurement Tachogenerator is simply a generator which gives a voltage proportional to the speed of rotation. Typically such a sensor can be used up to 10,000 revs per minute.

Infrared Proximity Sensor This proximity sensor is based on reflection. A LED emits infrared radiation which is reflected by the object. The reflected radiation is then detected by a phototransistor. In the case of absence of the object there is no detected reflected radiation.

Capacitive proximity sensor Capacitive proximity switches are on-off devices. They can be used to sense metals as well as nonmetallic materials such as paper, glass, liquids, and cloth. The rated sensing distance is ranged from 5 mm to 20 mm depending on the sensor type.

Capacitive proximity sensor The sensing surface of a capacitive sensor is formed by two concentrically shaped metal electrodes of a capacitor. When an object approaches the sensing surface it enters the electrostatic field of the electrodes and changes the capacitance in an oscillator circuit causing it to oscillate. When the oscillator’s amplitude reaches a specific level the trigger circuit changes the output state of the sensor. As the target moves away from the sensor the oscillator’s amplitude decreases, switching the sensor output back to its original state. Inductive proximity sensors are also available and are similar to capacitive proximity sensors.

Pyroelectric Sensor (for motion detection) There are materials such as Lithium tantalate which gives voltage when infrared radiation falling on them changes. Such sensors can be used with burglar alarms or for the automatic switching on of a light when someone walks up the drive to a house. A special lens is put in front of the detector to focus the infrared radiation emitted from the object.

Limit Switch Is used to detect the presence of some object. For example, a work piece closes the switch by pushing against it when it reaches the correct position on a work table. Another example is a light being required to come on when a door is opened, as in a refrigerator. The action of opening the door can be made to close the contacts in a switch and trigger an electrical circuit to switch on the lamp.

Reed switch Non contact switch sensor. An example of a reed switch's application is to detect the opening of a door, when used as a proximity switch for a burglar alarm.