Ej501 Unit 2a

EJ501

PROGRAMMABLE LOGIC CONTROLLER (PLC) AND AUTOMATION

SBO/ PMM/ EJ501/ Jun2011 1

UNIT 2a

LOGICAL SENSORS AND ACTUATORS


Objective

1. Logical Sensor� Understand Logical Sensor� Identify basic logical sensor types and their functions

� Contact Switches� Proximity� Photo optics� Capacitive� Inductive� Ultrasonic

� Explain logical sensor and switches wiring (sourcing and sinking).� Develop logical sensor application based on simple logical

control/automation.

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Logical Sensor

� Sensors allow a PLC to detect the state of a process.

� Logical sensors can only detect a state that is either true or false.Examples of physical phenomena that are typically detected are listedbelow :

• inductive proximity - is a metal object nearby?

• capacitive proximity - is a dielectric object nearby?

• optical presence - is an object breaking a light beamor reflecting light?

• mechanical contact - is an object touching a switch?

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Types of Logical Sensor

1. Contact Switches

� Contact switches are available as normally open andnormally closed. Their housings are reinforced so thatthey can take repeated mechanical forces. These oftenhave rollers and wear pads for the point of contact.Lightweight contact switches can be purchased for lessthan a dollar, but heavy duty contact switches will havemuch higher costs.

� Examples of applications include motion limit switchesand part present detectors.

Door Contact Switches

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2. Proximity

� Sensors allow a PLC to detect the state of a process.

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3. Photo Optics

� Optical sensors require both a light source (emitter) and detector. Emitters will producelight beams in the visible and invisible spectrums using LEDs and laser diodes.Detectorsare typically built with photodiodes or phototransistors. The emitter and detector arepositioned so that an object will block or reflect a beam when present.

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4. Capacitive

� Capacitive sensors are able to detectmost materials at distances up to a fewcentimeters. Recall the basic relationshipfor capacitance.

� In the sensor the area of the plates anddistance between them is fixed. But, thedielectric constant of the space aroundthem will vary as different materials arebrought near the sensor.

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5. Inductive� Inductive sensors use currents induced by

magnetic fields to detect nearby metal objects.The inductive sensor uses a coil (an inductor) togenerate a high frequency magnetic field. If thereis a metal object near the changing magnetic field,current will flow in the object.

� This resulting current flow sets up a newmagnetic field that opposes the original magneticfield. The net effect is that it changes theinductance of the coil in the inductive sensor. Bymeasuring the inductance the sensor candetermine when a metal have been broughtnearby.

� These sensors will detect any metals, whendetecting multiple types of metal multiple sensorsare often used.

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6. Ultrasonic

� An ultrasonic sensor emits a sound above thenormal hearing threshold of 16KHz. The time that isrequired for the sound to travel to the target andreflect back is proportional to the distance to thetarget.The two common types of sensors are;

� electrostatic - uses capacitive effects. It haslonger ranges and wider bandwidth, but ismore sensitive to factors such as humidity.

� piezoelectric - based on charge displacementduring strain in crystal lattices. These arerugged and inexpensive.

� These sensors can be very effective for applicationssuch as fluid levels in tanks and crude distancemeasurement.

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Logical Sensor and SwitchesWiring (sourcing and sinking)

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Ej501 Unit 2a

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