discrete sensors - 4.11

4.3 PRESENCE DETECTION

There are two basic ways to detect object presence; contact and proximity. Contact implies that there is mechanical contact and a resulting force between the sensor and the object. Proximity indicates that the object is near, but contact is not required. The following sections examine different types of sensors for detecting object presence. These sensors account for a majority of the sensors used in applications.

4.3.1 Contact Switches

Contact switches are available as normally open and normally closed. Their housings are reinforced so that they can take repeated mechanical forces. These often have rollers and wear pads for the point of contact. Lightweight contact switches can be purchased for less than a dollar, but heavy duty contact switches will have much higher costs. Examples of applications include motion limit switches and part present detectors.

4.3.2 Reed Switches

Reed switches are very similar to relays, except a permanent magnet is used instead of a wire coil. When the magnet is far away the switch is open, but when the magnet is brought near the switch is closed as shown in Figure 4.10. These are very inexpensive an can be purchased for a few dollars. They are commonly used for safety screens and doors because they are harder to trick than other sensors.

Note: 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.

Figure 4.10 Reed Switch

discrete sensors - 4.12

4.3.3 Optical (Photoelectric) Sensors

Light sensors have been used for almost a century - originally photocells were used for applications such as reading audio tracks on motion pictures. But modern optical sensors are much more sophisticated.

Optical sensors require both a light source (emitter) and detector. Emitters will produce light beams in the visible and invisible spectrums using LEDs and laser diodes. Detectors are typically built with photodiodes or phototransistors. The emitter and detector are positioned so that an object will block or reflect a beam when present. A basic optical sensor is shown in Figure 4.11.

square wave

smaller signal

+V

+V

lens

lens

light

amplifier

oscillator

demodulator

detector and

switching circuits

LED

phototransistor

Figure 4.11 A Basic Optical Sensor

In the figure the light beam is generated on the left, focused through a lens. At the detector side the beam is focused on the detector with a second lens. If the beam is broken the detector will indicate an object is present. The oscillating light wave is used so that the sensor can filter out normal light in the room. The light from the emitter is turned on and off at a set frequency. When the detector receives the light it checks to make sure that it is at the same frequency. If light is being received at the right frequency then the beam is not broken. The frequency of oscillation is in the KHz range, and too fast to be noticed. A side effect of the frequency method is that the sensors can be used with lower power at longer distances.

An emitter can be set up to point directly at a detector, this is known as opposed mode. When the beam is broken the part will be detected. This sensor needs two separate

discrete sensors - 4.13

components, as shown in Figure 4.12. This arrangement works well with opaque and reflective objects with the emitter and detector separated by distances of up to hundreds of feet.

emitter

object

detector

Figure 4.12 Opposed Mode Optical Sensor

Having the emitter and detector separate increases maintenance problems, and alignment is required. A preferred solution is to house the emitter and detector in one unit. But, this requires that light be reflected back as shown in Figure 4.13. These sensors are well suited to larger objects up to a few feet away.

reflector

emitter

detector

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.

Figure 4.13 Retroreflective Optical Sensor

discrete sensors - 4.14

In the figure, the emitter sends out a beam of light. If the light is returned from the reflector most of the light beam is returned to the detector. When an object interrupts the beam between the emitter and the reflector the beam is no longer reflected back to the detector, and the sensor becomes active. A potential problem with this sensor is that reflective objects could return a good beam. This problem is overcome by polarizing the light at the emitter (with a filter), and then using a polarized filter at the detector. The reflector uses small cubic reflectors and when the light is reflected the polarity is rotated by 90 degrees. If the light is reflected off the object the light will not be rotated by 90 degrees. So the polarizing filters on the emitter and detector are rotated by 90 degrees, as shown in Figure 4.14. The reflector is very similar to reflectors used on bicycles.

	emitter		reflector
		object
have filters for	detector
		light reflected with
emitted light		same polarity
rotated by 90 deg.		light rotated by 90 deg.
	emitter		reflector
	detector

Figure 4.14 Polarized Light in Retroreflective Sensors

For retroreflectors the reflectors are quite easy to align, but this method still requires two mounted components. A diffuse sensors is a single unit that does not use a reflector, but uses focused light as shown in Figure 4.15.

discrete sensors - 4.15

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.

Figure 4.15 Diffuse Optical Sensor

Diffuse sensors use light focused over a given range, and a sensitivity adjustment is used to select a distance. These sensors are the easiest to set up, but they require well controlled conditions. For example if it is to pick up light and dark colored objects problems would result.

When using opposed mode sensors the emitter and detector must be aligned so that the emitter beam and detector window overlap, as shown in Figure 4.16. Emitter beams normally have a cone shape with a small angle of divergence (a few degrees of less). Detectors also have a cone shaped volume of detection. Therefore when aligning opposed mode sensor care is required not just to point the emitter at the detector, but also the detector at the emitter. Another factor that must be considered with this and other sensors is that the light intensity decreases over distance, so the sensors will have a limit to separation distance.

discrete sensors - 4.16

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

intensity	1
	----
	r2

Figure 4.16 Beam Divergence and Alignment

If an object is smaller than the width of the light beam it will not be able to block the beam entirely when it is in front as shown in Figure 4.17. This will create difficulties in detection, or possibly stop detection altogether. Solutions to this problem are to use narrower beams, or wider objects. Fiber optic cables may be used with an opposed mode optical sensor to solve this problem, however the maximum effective distance is reduced to a couple feet.

emitter				detector
		object

the smaller beam width is good (but harder to align

Figure 4.17 The Relationship Between Beam Width and Object Size

Separated sensors can detect reflective parts using reflection as shown in Figure 4.18. The emitter and detector are positioned so that when a reflective surface is in position the light is returned to the detector. When the surface is not present the light does not return.

discrete sensors - 4.17

emit t er

reflective surface

detector

Figure 4.18 Detecting Reflecting Parts

Other types of optical sensors can also focus on a single point using beams that converge instead of diverge. The emitter beam is focused at a distance so that the light intensity is greatest at the focal distance. The detector can look at the point from another angle so that the two centerlines of the emitter and detector intersect at the point of interest. If an object is present before or after the focal point the detector will not see the reflected light. This technique can also be used to detect multiple points and ranges, as shown in Figure 4.20 where the net angle of refraction by the lens determines which detector is used. This type of approach, with many more detectors, is used for range sensing systems.

focal point

emitter

detector

Figure 4.19 Point Detection Using Focused Optics

discrete sensors - 4.18

distance 1

distance 2

lens

emitter

lens

detector 2

detector 1

Figure 4.20 Multiple Point Detection Using Optics

Some applications do not permit full sized photooptic sensors to be used. Fiber optics can be used to separate the emitters and detectors from the application. Some vendors also sell photosensors that have the phototransistors and LEDs separated from the electronics.

Light curtains are an array of beams, set up as shown in Figure 4.21. If any of the beams are broken it indicates that somebody has entered a workcell and the machine needs to be shut down. This is an inexpensive replacement for some mechanical cages and barriers.

Figure 4.21 A Light Curtain

The optical reflectivity of objects varies from material to material as shown in Fig-