Proximity Sensors and Switches 35.1 Proximity and Limit Switches Standard Basic Switches A variety of sensors are available that give ON/OFF (or yes/no) binary outputs Mechanical limit switches – often called “microswitches” – activation causes electrical contacts to either “break” (“normally closed” or NC switch) or “make” (“normally open” or NO switch) - or both NC and NO More sophisticated binary sensors are collectively known as proximity switches Standard Basic Switches Standard Basic Switches Switch Contact Configurations Mercury Switch Single pole, single throw (SPST) COM Normally Open (NO) Contacts Mercury “puddle” Single pole, single throw (SPST) COM Normally Closed (NC) NC Single pole, COM double throw (SPDT) NO NC1 COM1 NO1 Double pole, double throw (DPDT) NC2 COM2 NO2 Proximity Sensors and Switches 35.2 Photoelectric Proximity Sensor Photoelectric Sensor - Blocked VS VS VS VS Sense resistor Small current flows Current through transistor No current limiting resistor VOUT ~ 0 V VOUT ~ VS Photoelectric Sensor - Design Thru-Beam Photoelectric V VS S from Warner Current Sense resistor limiting usually large resistor 10K to usually 100K "small" ~ 1.2-1.7 V i ~10-30 mA from Eaton/Cutler-Hammer Reflective Photoelectric Cross-Beam Photoelectric from Warner from Eaton/Cutler-Hammer from Eaton/Cutler-Hammer Proximity Sensors and Switches 35.3 Automatic Door Opener Case Sorting - By Size from Warner from Warner Inductive Proximity Sensor Inductive Sensor Considerations Coil of wire forms inductor, L from Sick from Sick Proximity Sensors and Switches 35.4 Capacitive Proximity Sensors Capacitive Proximity Sensors ►Capacitive proximity sensors are similar to ► The sensing surface is inductive proximity sensors. formed by two ___________ shaped metal electrodes . capacitive proximity sensors produce an of an unwound capacitor. _________________________ field ► When an object nears the . inductive sensors produce an electromagnetic sensing surface it enters the field. electrostatic field of the ►Will sense metal as well as _____________ electrodes and changes the materials such as paper, glass, liquids, and capacitance in an _________ cloth (______________________) circuit. Capacitive Proximity Sensors Capacitive Proximity Sensors ►Capacitive sensors depend on the _______________ of the target. ►The larger the dielectric number of a material the easier it is to detect. Sr = Rated Sensing Distance Ultrasonic Proximity Sensing Ultrasonic Proximity Sensing High frequency (200 kHz) sound waves reflect from object T (Speed of sound in air) * T Distance = 2 Proximity Sensors and Switches 35.5 Proximity Switches Proximity Switches Industrial Automation by D.W. Pessen, Wiley Interscience Industrial Automation by D.W. Pessen, Wiley Interscience Sense Distance Switch Rate Sense Switch Rate (typ. max) (typ. max) Sensor Targets Distance (typ. max) Sensor Targets (typ. max) Limit switch Any 0 (physical 3 Hz Inductive Conductive Ferrous:50 300-5000 Hz contact req'd) material mm, Non- Mercury switch Any 0 (physical 3 Hz ferrous: less contact req'd) Capacitive Most solids, 30 mm 500 Hz Reed switch Magnet 20 mm 500 Hz liquids Magnetic Ferro- 50 mm 300 Hz Photo-electric Opaque 0.1 to 50 m, 100-1000 Hz inductance magnetic (depends on depends on target mass) target shape Hall effect Magnet 20 mm 100 kHz Ultrasonic Nonporous, 30 mm to 10 m 50 Hz large Wiegand Magnet 100 kHz effect Proximity Switches Proximity Switches Industrial Automation by D.W. Pessen, Wiley Interscience Industrial Automation by D.W. Pessen, Wiley Interscience Sensor Environmental Advantages Disadvantages Sensor Environmental Advantages Disadvantages Sensitivities Sensitivities Limit switch Temperature Simple, Physical Inductive Other nearby Usually fails moisture inexpensive contact, arcing sensors ON, good resolution Mercury switch Vibration, Low contact Physical mounting resistance, contact, SPST Capacitive Humidity, Complex angle sealed unit contacts only temperature circuitry, false triggering Reed switch Vibration Small size, Contact inexpensive arcing, magnet Magnetic Other nearby Good Collects debris, actuator inductance sensors resolution no static sense Photo-electric Dust, dirt, Good Hall effect Temperature Simple, Poor resolution, ambient light resolution inexpensive needs magnet Ultrasonic Noise, air Poor Wiegand No static sense, motion resolution effect magnet large target Factors in Selecting Limit & Proximity Limit & Proximity Switch Applications Switches Don't use the limit switch as a mechanical Type of output signal (high/low voltage? stop (use another component) high/low current?, DC or AC?, relay or triac?) Use cam surfaces to allow gradual Is mechanical contact with sensed object actuation OK? Don't apply side forces to the switch roller Available space or lever (will wear bearings quickly) Environmental conditions Use appropriate switch actuator for type of Nature of target: sisize,ze, shape, material, force/motion applied surface Don't switch excessive currents through the switch contacts Proximity Sensors and Switches 35.6 Factors in Selecting Limit & Proximity Switches Prox Sensor Output - NPN Proximity Sensor Sensor-to-target distance (max and min) Positional accuracy required Speed of target (will it remain in sensing area long enough?) NPN output Switching rate - how often will inputs be or presented to the sensor? Can it recover Open-Collector output quickly? or Current "sinking" output Reliability and life expectancy - can you detect a failure? Prox Sensor Output - NPN Prox Sensor Output - PNP Proximity Sensor Typically +5V, +12V or +24V Proximity Sensor V S External sense PNP output (or load) or resistor Current "sourcing" output VOUT Prox Sensor Output - PNP Both NPN and PNP outputs Proximity Sensor Typically +5V, +12V or +24V VS External sense VOUT (or load) resistor .