Week 5 - PLC Pneumatic Interfacing.pdf

Week # 5 Sensors Part 1 ( Digital) A sensor (also called detectors) is a device that measures a measurable attribute and converts it into a signal which can be read by an observer or by an instrument. 2 Energy System Engineering 1 Contact sensors must have physical contact with the object being sens...

Week # 5 Sensors Part 1 ( Digital) A sensor (also called detectors) is a device that measures a measurable attribute and converts it into a signal which can be read by an observer or by an instrument. 2 Energy System Engineering 1 Contact sensors must have physical contact with the object being sensed – Limit switch Non-contact sensors detect without touching the component – Electronic (often transistor outputs) More reliable than mechanical devices Faster than mechanical devices 3 Industrial sensors can be divided into two broad classifications: – Digital Generally transistor outputs Either ON or OFF Easiest to use – Analog Continuous range of voltage or current 4 Energy System Engineering 2 5 6 Energy System Engineering 3 7 8 Energy System Engineering 4 9 10 Energy System Engineering 5 11 There are many types and styles of digital sensors The media used for sensing are: – Light – Sound – Electric Field 12 Energy System Engineering 6 Using light to sense objects: – A light source (emitter) usually an LED – A light detector (receiver) usually a phototransistor Most common to use infrared light Emitter will usually be pulsed (turned on and off repeatedly) to increase life 13 Optical sensors can be light sensing or dark sensing. Many can be switched from one mode to the other Light-on: Sensor is on when receiver detects beam from emitter Dark-on: Sensor is on when receiver cannot detect beam from emitter 14 Energy System Engineering 7 Types of Optical Sensors Also Called Diffuse Reflective The emitter sends out light , which bounce off the product to be sensed. The reflected light returns to the receiver where it is sensed Less sensing distance 15 Types of Optical Sensors The emitter sends out light across a space and the receiver senses the light. If the product passes between the emitter and receiver, it stops the light from hitting the receiver. Not able to sense transparent objects. 16 Energy System Engineering 8 Types of Optical Sensors A special photo sensor senses shiny objects using a special reflector The reflector consists of small prisms that polarize the light form the sensor. 17 Types of Optical Sensors Also called Focal Length Sensor. It emits light to a specific focal point. The light must be reflected form the focal point to be sensed by the sensor’s receiver. 18 Energy System Engineering 9 Types of Optical Sensors The actual emitter and receiver are the same but with a fiber-optic cable attached. Small size and flexible. 19 Types of Optical Sensors Optical Sensor that can differentiate between colors. The object’s background colour is and important consideration. Some of them can not sense black color. 20 Energy System Engineering 10 Encoders Encoders can be used for position and/or velocity information A transparent disk will have a mask of opaque lines on it LED/Phototransistor pairs are used Can be ABSOLUTE or INCREMENTAL 21 Absolute Encoders A unique pattern (of up to 8 bits) is used represent each position of the disk A separate emitter-receiver is needed for each bit Common patterns are Gray code, natural binary and BCD 22 Energy System Engineering 11 Incremental Encoders Available in various resolutions More resolution than absolute Easier to interface because of fewer outputs Two types: – Tachometer (single track) – Quadrature (multitrack) 23 Ultrasonic Sensors Using a beam of ultrasonic sound as a sonar system Can be used to measure distance by measuring the time for the sound wave to return Distance can also interferometer sensor be measured by using an 24 Energy System Engineering 12 Electric Field Sensors Usually called Proximity sensors Two types: – Inductive Used to detect ferrous objects – Capacitive Used to detect any type of object 25 Inductive Proximity Sensors Š Š Š An oscillator and coil in the sensor produce a (weak) magnetic field Small eddy currents will be induced in a metallic object entering the field Because of this interference, the amplitude of the oscillator drops, reducing voltage, which is sensed by the detector circuitry 26 Energy System Engineering 13 Capacitive Proximity Sensors Š Š Š An oscillator and electrode produce an electrostatic field An object acts as the second plate of a capacitor As the field grows, the amplitude of the oscillator increases, increasing voltage, which is sensed by the detector circuitry 27 Range of Proximity Sensors ¾ Inductive ¾ Capacitive ƒRelated to size of ƒCannot be flush inductor coil mounted ƒShielding reduces range ƒRange effected by mass but allows flush mounting and material of target ƒSmall temperature effect ƒSensitive to temp. & 5% humidity changes (1520%) ƒSometimes sensitivity can be adjustable 28 Energy System Engineering 14 Sensor Wiring Two wiring schemes for sensors: – Load powered Two wire sensors – Line powered Three (or four) wire sensors Always important to be sure current limit is not exceeded 29 Load Powered Š Š Š One wire of the sensor is connected to power The second wire is connected to the load (PLC input) Then the load is connected to ground to complete the circuit 30 Energy System Engineering 15 Line Powered Š Š The three wire variety has two power leads, and an output lead Can be Sourcing or Sinking Sourcing type (PNP sensor) Current flows from sensor to load Sinking type (NPN sensor) Current flows from load to sensor 31 Energy System Engineering 16