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Chapter 02 I/O Devices and Sensors

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  • 1. Chapter 02Supplement and reinforcement ofInput Devices and Output Actuators

2. I/O Devices I/O can be categorized into three areas or types: Binary (Discrete) (Unfortunately to add some confusion they arealso referred to as Digital) This includes all mechanical switches such as: pushbuttons,selectors, limit (micro), motor starter aux. contacts, relaycontacts, etc. It also includes all solid state sensors such as: photoelectric,inductive and capacitive proximity, etc. Digital This includes: processed video, charge coupled devices(CCD) arrays, inductive coil impulse generators, optical codewheels (encoders), etc. Analog This includes: potentiometers, linear variable differentialtransformers (LVDT), video correlation, pressure,temperature, flow, strain, etc. 3. I/O Details Mechanically operated switches are mode up of: Pole (sometimes referred to as a wiper) Contact ActuatorSPST SwitchPole orWiperSPDT RelayPole orWiperContactContactContactSPSTDB SwitchSingle Pole Single ThrowDouble BreakPole orWiperContactsSPDTDB SwitchSingle Pole Double ThrowDouble BreakPole orWiperContactsContacts 4. Mechanical SwitchesNormally Open Normally ClosedNormally OpenNormally ClosedPush ButtonsSelectorsLimit (micro) 5. Mechanical SwitchesFLOAT SWITCH PRESSURE SWITCH TEMPERATURE SWITCHNormally OpenNormally ClosedNormally OpenNormally ClosedNormally OpenNormally ClosedFLOW SWITCH FOOT SWITCHNormally OpenNormally ClosedNormally OpenNormally Closed 6. RelaysElectro-MechanicalNCNONCNOPOLEPOLENO NC 7. Switch Bounce Switch contact bounce+V V+VPLCInput 8. Switch contact bounceSwitch Bounce+V V+VPLCInputSwitchClosesSwitchcomes torestRandomBounce 9. SensorsPhotoelectric, Inductive and Capacitive 10. Sensing Theory Primer Sensors provide the equivalence of eyes, ears, nose and tongue tothe microprocessor of a PLC/PAC or computer.MicroprocessorOpticalsensorGassensorMicrophoneProbeGraphic from: Petruzella, Frank D. (2005). Programmable Logic Controllers (3rd ed.). New York, NY: McGraw-Hill 11. Review of Basic SolidState Devices Brief review of: Diodes Multiple uses within PLC/PAC control circuits DC flyback protection (Inductive surge suppressionfor DC inductive loads) Transistors Commonly used in PLC/PAC DCV output modules Silicon Controlled Rectifiers (SCR) Triac Commonly used in PLC/PAC ACV output modules 12. Diode Current flow in one direction only.Direction ofConventional CurrentAnode Cathode 13. Forward Bias A diode will conduct current when the anode is0.7V more positive than the cathode. Whencurrent is flowing the diode is Forward Biased.++Direction ofCurrent Flow 14. Reverse Bias A diode will not conduct current when the anodeis not 0.7V more positive than the cathode.When current is not flowing the diode is ReverseBiased.+ +NO CURRENT FLOW 15. Transistor Transistors are commonly used as the switching devicein PLC/PAC DCV output modules. Just like a diode, a0.7V bias is required for current flow. Transistors are available in two polarities, NPN & PNP. NPN Sink PNP SourceCollector CollectorNPN PNP+++Base Base+++Emitter Emitter 16. Field Effect Transistors Field Effect Transistors (FETs) can also beused as switches. Transistors are current operated devicesand FETs are voltage operated devices.http://www.talkingelectronics.com/projects/MOSFET/MOSFET.html 17. SCR and Triac SCRs can be used in ACV output modules but Triacs aremore commonly used as the switching device inPLC/PAC ACV output modulesGateCathodeAnodeGateMain Terminal 1Main Terminal 2 18. SCR and Triac UsageReview One of the many applications for SCRs and Triacs is forlight dimming and simple motor control. They are alsoused as AC voltage switches. A Triac in its basic form is nothing more than two SCRsin parallel, back-to-back, with their gates connectedtogether. 19. SCR Usage LightDimmer Representation of a lamp dimmer circuit usingan SCR. An SCR will only conduct on one half of the sinewave.ACVZero CrossingDetectorAdj. FiringAngle 20. SCR Usage Review Waveforms across the lamp at different firing angles. Firing at different angles changes the effective ACvoltage across the lamp (load).Fired at 30 Fired at 90Fired at 135 21. Triac Usage LightDimmer Representation of a lamp dimmer circuit using aTriac. A Triac will conduct on both halves of the sinewave.ACVZero CrossingDetectorAdj. FiringAngle 22. Triac Usage Review Waveforms across the lamp at different firing angles. Firing at different angles changes the effective ACvoltage across the lamp (load).Fired at 30 Fired at 90Fired at 135 23. AC Effective Voltage (FYI) SCRs and Triacs change the effective voltage seen by aload. Power calculations based upon a voltage midwaybetween one peak and zero are not correct because ACvoltage generally changes sinusoidal from zero to peak,rather than linearly as in DCV. The voltage value that gives the correct result is calledthe Effective Voltage because it has the same effect on apower calculation as does a DC voltage of the samevalue. Effective Voltage is equal to the square root of the meanvalue of the squares of all the instantaneous values of anAC voltage. Because of that, Effective Voltage is alsoknown as the Root Mean Square or RMS Voltage. 24. AC Voltmeters (FYI) AC voltmeters read the AC voltage in one of three ways: Average Root Mean Square (RMS) True RMS Average responding voltmeters simply use a diode to rectify the ACsignal being measured and read the equivalent DC voltage. MostVOMs use this method for ACV measurements. (Not very accurateon non-sinusoidal waveforms). RMS voltage is a function of power and an RMS meter useselectronics to simulate an AC power measurement making the ACVmeasurement more accurate. True RMS voltage is also a function of power but also takes intoconsideration the heating characteristics of the ACV. True RMSvoltmeters use a sophisticated P based calculation that will mimic abolometer by calculating the area under the curve. This is the mostaccurate of ACV measurements. This measurement will include anyspikes or distortion on the AC signal. Fairly good source to learn more about measuring AC voltage:http://www.allaboutcircuits.com/vol_2/chpt_1/3.html 25. Inductive Flyback andProtecting the PLC/PAC+Vdc +Vac 26. Photoelectric Sensing Photoelectric Sensor An electrical device that responds to a change in the intensity ofthe light falling upon it. Photocell A photocell is a device that changes resistance when it isexposed to light. This change in resistance can then be detectedto trigger a response. The earliest method of photoelectricsensing used a photocell to sense light change. Non-modulated The earliest photo sensors consisted of an incandescent lightbulb and a photocell. The gain of the non-modulated sensor is limited to the point atwhich the receiver recognizes ambient light. This type of sensor is only powerful if its receiver can be made tosee only the light from its light source (emitter). What are some advantages and disadvantages to this type ofsensor? 27. Ambient Light Receiver Ambient light receivers are non-modulated typephotoelectric sensors that are still in frequentuse. Applications for such devices could be: Detecting red-hot metal or glass that emit largeamounts of infrared light. As long as these materials emit more light than thesurrounding light level, ambient light receivers can reliablydetect these materials. A sensor mounted under an open frame conveyorthat is reading the ambient light in the room. If a box, carton or some other material passes along theconveyor and over the sensor, it blocks the ambient lightfrom the sensor. This change in light is used to detect thepresence of an object on the conveyor. 28. Light Sources (Emitters) Light Emitting Diode (LED) A solid state device electrically similar to the diodeexcept that it emits a small amount of light when it isforward biased.RED GREEN AMBERBLUE INFRARED 29. Light Sensor (Receiver) Phototransistor A solid state device similar to a transistor except thatthe base connection is made using light. Thesedevices are widely used as photoelectric receivers.Phototransistor 30. Modulated LED Sensors LEDs can be turned on-and-off at frequenciestypically in the kilohertz(KHz) range. This switchingon-and-off is referred to asmodulating the light. The receiver can be tunedto this frequency so that itonly sees the light signalsthat pulse at this frequency. This is what gives the LEDsensor its apparent power.Picture borrowed from the Banner Photoelectric HandbookPicture borrowed from the Banner Photoelectric Handbook 31. Photoelectric SensingModes Opposed Mode Retroreflective Mode Proximity Mode Diffused mode Divergent mode Convergent Beam mode Fixed-field (sometimes called backgroundsuppression mode) Adjustable-field 32. Opposed Mode SensingReceiverEmitterThe emitteris a lightsourceObjectPicture borrowed from the Banner Photoelectric Handbook Often referred to asDirect Scanning orBreak Beam mode. In this mode theemitter and receiverare positionedopposite each otherso that the light fromthe emitter is aimedat the receiver. An object is detectedwhen it interrupts theeffective beam oflight between the twosensing components. 33. Effective Beam Photoelectric sensors will sense a change inlight when the effective beam is completelyblocked.Effective BeamRadiation PatternField of ViewEmitter Receiver 34. Shaping the EffectiveBeam The effective beam can be shaped by usingdifferent sized lenses on the emitter and/orreceiver.Effective Beam is:Cone ShapedEmitter (or receiver)with large lensEmitter (or receiver)with small lens 35. Shaping the EffectiveBeam Apertures can also be placed on the lenses to shape theeffective beam for sensing small objects that would notnormally be large enough to break the effective beam.Picture borrowed from the Banner Photoelectric Handbook 36. Retroreflective Mode This mode is also calledreflex mode or simplyretro mode. The emitter and receivercircuitry of these sensorsare in the same package. The light beam