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12.2.4 Solid State Relays

• These are fully solid state and are well suited to AC loads.

12.3 CONTACT DETECTION

• At times we want to know when a physical object is present. This may involve touch.

12.3.1 Contact Switches

• Normally open/normally closed

12.3.2 Reed Switches

• These switches are like relays, but using a moving permanent magnet.

With this device the magnet is moved towards the reed switch. As it gets closer the switch will close. This allows proximity detection without contact, but requires that a separate magnet be attached to a moving part.

12.4 PROXIMITY DETECTION

• At times we want to know when a physical object is present. This may be a non-contact detection.

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12.4.1 Optical (Photoelectric) Sensors

•Optical sensors can detect part presence using a light source and detector.

•Emitters generate light in visible and infrared light bands. These are usually LEDs or laser diodes.

•Detectors are designed to vary electrically as light intensity varies. The most common used is the phototransistor.

•Ambient light can interfere with a simple optical beam. As a result most sensors now use a modulated pulse with a frequency up to the low KHz range. This allows better detection at longer distances with lower power.

square wave

smaller signal

+V

+V

lens

lens

light

amplifier

oscillator

demodulator

detector and

switching circuits

LED

phototransistor

•The relative locations of the source and detectors, as well as surface conditions have a major impact on the selection of sensor types. These include,

-distance to target

-target characteristics (transparent, reflective, diffuse, etc.)

-target size

•The simplest form uses a detector only with ambient or radiated light.

-ambient light requires care in scene lighting

-radiated light requires some sort of photometric phenomenon such as a hot part will radiate infrared light.

•Optic sensors can often be separated for space and other constraints.

-fiberoptics allow the lens to be separated from the LED or phototransistor.

-the phototransistors and LEDs can be separated from the other circuitry to fit the sensors

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into smaller parts.

• When the emitter and detector are separated and the beam is interrupted this is known as opposed mode.

emitter

object

detector

• When the emitter and detector are in a single unit this is known as retroreflective.

reflector

emitter

object

detector

Note: the reflector is constructed with polarizing screens oriented at 90 deg. angles. If the light is reflected back directly the light does not pass through the screen in front of the detector. The reflector is designed to rotate the phase of the light by 90 deg., so it will now pass through the screen in front of the detector.

• Polarized light can be generated using filters.

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have filters for emitted light rotated by 90 deg.

emitter		reflector
	object
detector
	light reflected with
	same polarity
	light rotated by 90 deg.
emitter
		reflector
detector

• Diffuse sensors are like the retroreflective type, except that the returning light does not need to be polarized.

emitter

object

detector

Note: with diffuse reflection the light is scattered. This reduces the quantity of light returned. As a result the light needs to be amplified using lenses.

• Alignment of the emitter is necessary, and can be a problem if the sensors are separated by a large distance and the beam intensity decreases.

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	effective beam
	effective	detector
	detector
	angle
emitter
	effective				alignment
	beam angle				is required

intensity 1

----2 r

• The beam of emitted light should generally be less than the width of the detected part.

emitter		object		detector

the smaller beam width is good (but harder to align

• Separated sensors can detect reflective parts using specular reflection. This needs a reflective surface.

emit t e r

reflective surface

			ctor
		e
	t
e
d

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• By focussing emitters and detectors optics we can sense presence at a specific distance. This is known as convergent beams sensing.

focal point

emitter

detector

• Fixed field sensors use a physical setting.

distance 1

distance 2

lens

emitter

lens

detector 2

detector 1

• Opposed beams can also be for a large range light curtains.

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• Typical reflectivity values are given below [Banner Handbook of Photoelectric Sensing]

			Reflectivity
nonshiny materials		Kodak white test card	90%
		white paper	80%
		kraft paper, cardboard	70%
		lumber (pine, dry, clean)	75%
		rough wood pallet	20%
		beer foam	70%
		opaque black nylon	14%
		black neoprene	4%
		black rubber tire wall	1.5%
shiny/transparent materials		clear plastic bottle	40%
		translucent brown plastic bottle	60%
		opaque white plastic	87%
		unfinished aluminum	140%
		straightened aluminum	105%
		unfinished black anodized aluminum	115%
		stainless steel microfinished	400%
		stainless steel brushed	120%

Note: For shiny and transparent materials the reflectivity can be higher than 100% because of the return of ambient light.

• Many sensors have sensitivity adjustments that will need to be adjusted to the materials.