UNIT - 3

SENSORS & MACHINE VISIONRequirements of a sensor, Principles and Applications of the following types of sensors Position of sensors (Piezo Electric Sensor, LVDT, Resolvers, Optical Encoders, Pneumatic Position Sensors), Range Sensors (Triangulation Principle, Structured, Lighting Approach, Time of Flight Range Finders, Laser Range Meters), Proximity Sensors (Inductive, Hall Effect, Capacitive, Ultrasonic and Optical Proximity Sensors), Touch Sensors, (Binary Sensors, Analog Sensors), Wrist Sensors, Compliance Sensors, Slip Sensors. MACHINE VISIONCamera, Frame Grabber, Sensing and Digitizing Image Data Signal Conversion, Image Storage, Lighting Techniques. Image Processing and Analysis Data Reduction: Edge detection, Segmentation Feature Extraction and Object Recognition - Algorithms. Applications Inspection, Identification, Visual Serving and Navigation.

INTRODUCTIONA transducer is a device that converts one type of physical variable (E.g Force, Pressure, Temperature, Velocity, Flow rate ) into other form.Why Conversion To make it use in digital Computers.A sensor is a transducer that is used to make a measurement of physical variable. INTRODUCTIONFew examples :

Strain Gauges ThermocouplesSpeedometersPitot tubesAll sensors are transducers, but not all transducers are sensorsINTRODUCTIONCalibration is a procedure by which the relationship between the measured variable of the converted output signal is established.

SensorsExternal SensorContactNon Contact Internal Sensor

INTRODUCTIONInternal State Sensors deals with the detection of variables such as arm joint position.External State Sensors deals with detection of variables such as range, proximity, guidance.Based on the converted signal Analog TransducerDigital TransducerRequirements of SensorsAccuracyPrecisionOperating RangeSpeed of ResponseCalibrationReliabilityCost & ease of operationPIEZOELECTRIC SENSORSPiezo electric materials when stretched or compressed generate electric charges, with one face of the material becoming positively charged and the opposite face negatively charged. As a result voltage is generated.Measurement of pressure,force,accelerationPiezo material : Quartz, Barium titanate, Zirconate titanate.

PIEZOELECTRIC SENSORSThe net charge q = kx =SFx charge displacementF Force applied S Charge SensitivityCharge Sensitivity depends on 1.Material concerned(Depends on S Value)2. Orientation of the crystalLVDT

CONSTRUCTION & PRINCIPLEThe LVDT consists of a single primary winding and two secondary windings wound on a hollow former.The secondary windings have an equal number of turns but are connected in series opposition so that emfs induced in the coils oppose each other.The primary winding is connected to an a.c source, whose frequency range from 50Hz to 20kHz.CONSTRUCTION & PRINCIPLEThe position of the movable core determines the flux linkage between the a.c excited primary winding and each of the two secondary windings.

The core is made up of nickel iron alloy is slotted longitudinally to reduce eddy current loss.

CONSTRUCTION & PRINCIPLEThe displacement to be measured is applied to an arm attached to the core. With the core in the center, or reference, position, the induced EMF in the secondary winding are equal, and since they oppose each other, the output voltage will be zero

CONSTRUCTION & PRINCIPLEWhen an externally applied force moves the core to the left-hand coil. The e.m.f induced in the left hand coil > right hand coil.The magnitude of the output voltage and it is in phase with the voltage of the left hand coil.Similarly, when a core moves to a right

ADVANTAGES Infinite resolution is present in LVDT High output LVDT gives High sensitivity Very good linearity Ruggedness LVDT Provides Less friction Low hysteresis LVDT gives Low power consumption.DISADVANTAGES Very high displacement is required for generating high voltages.

Shielding is required since it is sensitive to magnetic field.

The performance of the transducer gets affected by vibrations

Its is greatly affected by temperature changes.

APPLICATIONSLVDTs provide position feedback in hydraulic applications by monitoring the performance accuracy of actuators and cylinders to improve operational efficienciesAnother example is robotic arm picking up a piece of glass. If the control system does not know when to stop the arm, based on position feedback from an LVDT, the hydraulic cylinder could drive the arm right through the piece of glass.