===================================================****** All The World's An Input- Part 8===================================================

Let's take a look at one of the most common types of sensor in use in factories today... the photoelectric sensor. All photoelectric sensors operate on the basic principle of light presence or light absence.

There are many different types of photoelectric sensors and we'll examine most all of the them. But in the end, they all operate on just one fundamental principle... either they see light or they don't.

Many of us have photoelectric sensors in our houses. For example, the automatic garage door often has one. Generally it is located on the sides of the door opening. On one side there is a light beam that is shooting to the other side where a light receiver is located. When the receiver sees the light from the sender it knows there is nothing in the path of the door opening. It is then ok for the door to close.

If there is no light being received it knows that there is something in the door opening. Something must be there because the light is being blocked. In this case it would not be wise to allow the garage door to close.

This type of sensor is called a thru-beam sensor. Other sensor types are diffuse reflective, retro-reflective and position sensitive. Each photoelectric sensor type has its own purpose and specialty. Using the proper one for the application will be necessary. We'll examine each type later and see their features and strong points as well as when to use them for your application.

So, we've seen that photoelectric sensors are quite basic in their functionality. Applying them requires a basic understanding of their operation though. Let's examine some of the principles that are universal to each regardless of the type.

The light source of the sensor is pulsed on and off repeatedly. This is referred to as modulating the light and is important for two reasons. It allows us to get a brighter light beam and it allows us to filter out other sources of light that could cause the sensor to 'false trip'.

If we pulse the light source with a lot of energy for a very short time (I mean millionths of a second...) we get a ton of light out of the sensor and don't cause the light source to heat up and burn out. The receiver is taught to look for pulsed light. If it doesn't see pulsed light but rather sees continuous light it knows it is from another source (the sun shining through a window, for example) and won't react.

It also looks for the light to be pulsed at a specific speed so that even if it sees pulsed light but at a different speed it won't react. This is useful when two sensors are next to each other but detecting different things. Then one sensor won't mistakenly trip the other.

Something else common to all sensor types is referred to as excess gain. This is simply light quantity above the minimum amount of light that allows the sensor to 'trip'. The receiver needs to see a certain amount of light to be able to say that the target is there or not. Any light received above this minimum threshold is excess. The excess gain is the percentage of light quantity received over that threshold.

As the sensor gets old the light gets less bright. Dust, oil mist, etc also block some of the light. So as time goes by the receiver will start to receive less and less light. Eventually it hits a point of being unable to detect the target. It will either need to be replaced or some maintenance will need to be performed. More excess gain allows longer maintenance time frames and a greater usable life.

That's a lot of knowledge to absorb. Take some time to reread it if necessary. Next time we'll start looking into each of the sensor types and configurations.

===================================================****** All The World's An Input- Part 9===================================================

Now that we understand the basic function of a simple photoelectric sensor, let's take a look at the different types of sensing available to us. Each has its advantages as well as disadvantages. This time we'll examine the thrubeam method. Although they are most often called thrubeam sensors, they are also referred to by other names. It's important to know these names in case you come across them in the field. Some other 'common' names are:

  opposed mode sensors    break beam sensors    thru scan sensors    transmitted beam sensors

As you may recall, a thrubeam sensor consists of two separate parts. One part is the light source (transmitter) and the other end is the part that sees the light sent (receiver). We align the transmitter and receiver so that they face each other. In this manner the light coming out of the transmitter is received by the receiver. If anything blocks the light from entering the receiver the sensor will know something is present. In this case it senses the target... hence the name 'sensor'(ingenious naming).

The setup of these separate 'eyes' is very important to the outcome of whether or not the target is sensed or not. The key factor is in the alignment of the two separate parts. In general, when you align them you look for the most light possible to be received by the receiver. In this case it will shoot light through dust, etc and still detect the target. Of course, you'll need to verify it won't shoot the light through the target itself if it's thin or translucent, for example. Once you get the alignment considerations fixed you can expect quite reliable performance in most applications. The reason is because the excess gain is typically very high.

As you may recall, excess gain is the extra light available above and beyond the amount of light needed to make a detection. Since we are focusing the light from the transmitter directly on the receiver the excess gain is often huge.

The sensing accuracy of the thru beam is also quite good. This is because of the 'effective beam' of the sensor. The effective beam is the light received by the receiver. It is dependant upon the size and shape of the lens on the transmitter and receiver. We can think of the effective beam as being a rod of light extending out the transmitter lens and going to the receiver lens. Anything in this light rod will be seen by the sensor.

How much of that light rod that needs to be blocked is an adjustment available on the sensor. This is done in the calibration part of setting up the sensor to work in your application.

Some advantages of the thrubeam detection type of sensor are:  1- It has the greatest light/dark contrast ratio. Therefore, it doesn't make any difference what color the target is.  2- It's not sensitive to the target distance from the transmitter or receiver.  3- It can tolerate a very long separation distance between the transmitter and receiver.  4- There is no inadvertent detection of objects beyond the sensing 'rod'.

Some disadvantages of the thrubeam detection type of sensor are:  1- Two parts (transmitter and receiver) need to be wired across the detection zone.  2- Extra labor is involved as you need to align the heads.  3- They are often more expensive than the one piece models since there are two pieces to purchase.

Hopefully the above has got you thinking about starting out with a thru beam sensor for your photoelectric application. In general, use athru beam unless you can't. Only then should you go with a one piece model.

===================================================****** All The World's An Input- Part 10===================================================This time we'll take a look at one of the most common types of photoelectric sensor detection, diffuse sensing. You may recall from last time that thrubeam sensing is a more indirect method of sensing a target because it requires the target to break a beam of light. Diffuse sensing, however, is more of a direct method of sensing a target because it requires the target to simply be there or not. Sounds confusing, doesn't it? Let me explain more.

Diffuse sensing requires us to detect the same light that the transmitter sends out. It relies on the target to reflect the light that the transmitter has sent back into the receiver.

Both the transmitter and receiver are mounted in the same 'box'. They are setup up side-by-side. The transmitter is generally perpendicular to the target and the receiver is usually mounted on a slight angle. Unlike a thrubeam sensor, the transmitter and receiver are therefore in the same case.

So, the transmitter sends out a beam of light. If there is no target in front of it the light will keep shooting out into space. The receiver will never see the light. If there is a target in front of the sensor, however, the light will reflect off the target and be seen by the receiver. The sensor will then know that the target is in front of the sensor and turn on its output accordingly.

The important thing to understand about this sensing method is what happens to the light when a target is in front of the sensor. The light that hits the target 'diffuses'. This simply means that the light gets reflected back in all directions. Only some of the light that is sent out by the transmitter is received. Most of it never makes it back to the receiver. Let's think about why the above is true. Well, consider all the different materials available in the world for us to detect. There are obviously countless types of materials and the reflectivity of each varies tremendously.

A shiny piece of stainless steel reflects light back right where it came from without diffusing it almost at all. Black rubber on a tire absorbs most all of the light and hardly reflects any back. So, we can see that a diffuse sensor will detect different target materials from different distances because the amount of light a target reflects varies based on the material itself.

The standard target in the sensor industry is white matte finished paper. The detecting distance stated by the manufacturer on their specification sheets is most commonly stated while looking at a target made of white matte paper. If you're not going to be detecting white matte paper in your application you need to make sure the sensor willwork for you to detect your target at the distance you need to see it at.

An easy way to estimate what distance that will be for your real target is to consider the following reflectivity adjustments:

multiply the stated distance by X for a target made of Y    X- 1    Y- white matte paper  X- 0.6  Y- newspaper  X- 0.15  Y- black rubber  X- 0.7  Y- white plastic  X- 0.75  Y- wood  X- 1.5  Y- anodized aluminum  So, if our manufacturer stated detecting distance is 1 inch and the target we're detecting is white matte paper:X * Y = distance1 * 1 = 1 inchIf the target we're detecting is wood:1 * 0.75 = 0.75 inches  The above chart is just a small 'taste' of targets but should give you a general idea. The less light gets absorbed by

the target the further away you can be to detect it.

How do you know if a diffuse sensor is good for your application?It's always best to ask the advice of the sensor manufacturer, but consider the below advantages/disadvantages too.

Some advantages of diffuse sensing include:  1- alignment is generally simple.  2- different color targets can be detected  3- wiring is easy since there is only one 'box'  4- low cost since only one box is needed for the sensor  Some disadvantages that might need to be considered though are:  1- the sensing distance is often very short  2- if the targets surface color changes the detecting distance will be effected  3- surface texture variances can change detecting distance  In the end, a diffuse sensor is often a good choice for an application with limited installation space or requires simplicity in setup. That's enough learning for now.

===================================================****** All The World's An Input- Part 11===================================================The retroreflective sensor is one of the most commonly used for medium to long distance detection. You may also hear it called a reflex or reflective sensor. They all refer to the same sensor type.

As in the diffuse type, the light transmitter and receiver are both in the same casing. It is a one piece sensor head. A reflector is also needed for all applications.

The light is sent out the transmitter, hits the reflector and is reflected back to the receiver. If the receiver 'sees the light' the sensor knows that no target is in between the sensor head and the reflector. If the receiver doesn't see any light, however, the sensor will know that there is a target present in between the sensor head and the reflector.

The actual detecting distance will vary depending upon the actual reflector used. As you can imagine, different types and sizes of reflectors are available in the market. There is even reflective tape (like you might find worn by a jogger who runs at night) available for when the reflector mounting surface has limited space.

Most reflectors use a system of many small spheres mounted next to each other to reflect the light back to its source. Other reflectors include more of a corner cube type reflecting system.

The corner cube reflectors will be described in more detail later because they allow us to do something 'interesting' with the light they reflect. You often see corner cube reflectors on bicycles and highway road signs. They make the signs more visible by returning the light they receive back to exactly where it came from as opposed to scattering it back in all directions like a diffuse target would.

The reflector size also plays a large factor in determining what the minimum detectable target size should be. In general, a good rule is to make sure the target is at least equal in size to the size of the reflector used.

Some advantages of retroreflective sensors include:    - long detecting distance    - unaffected by target color    - high dark to light contrast exists    - only need to wire one item

Some disadvantages would be:

    - two items need to be installed (the head and reflector)    - cost is a bit higher than other types as 2 items are needed    - shiny targets can be a problem

That last disadvantage is quite an interesting fact. Why would shiny targets be a problem? Is there any way to get around that problem? (hint... there is and we'll learn it next time)

===================================================****** All The World's An Input- Part 12===================================================The hardest items to detect with retroreflective sensors are what?That's something that experience will teach you. Unfortunately, it will not be a good experience because you will have not detected some targets. Worse yet, you may miss double detect and have to waste time troubleshooting the system.

Let me save you some time. The hardest items to detect with a retroreflective sensor are transparent and metallic targets. Transparent targets are not good targets for retroreflective sensors.

Metallic objects (like a can before the label is applied)can be a problem because they are shiny targets. They can reflect so much light back to the sensor that it is the same as the reflector so the sensor doesn't see it as a target.

More commonly though, it sees the target multiple times. Consider a shiny can passing between the sensor and reflector: The side of the can blocks the beam and reflects light away from the sensor. It detects it. Then the mid part of the can reflects back a lot of light so the sensor thinks a target has passed and is no longer there. Now, the other side of the can reflects light away from the sensor and the sensor thinks another target has passed in front of it. So, it has detected 2 targets when only one has passed in front of it.

There is a solution however... polarization. That's a word that needs some explanation in how it applies to optics.

Normally, light shines in all directions. Up, down, right, left. Or more simply stated, it shines vertically and horizontally at the same time.

Polarization filters that light into only one direction. Either it blocks horizontal light or it blocks vertical light. We do that with what else but a polarized filter. A vertical polarizing filter will let vertical light pass through it while it blocks horizontal light. Turn it 90 degrees and it becomes a horizontal polarizing filter whereby it allows horizontal light to pass while blocking only vertical light.

This type of filter will obviously block about half of the light from passing through it. So, the intensity of the light passed is greatly reduced. You probably have seen this technology applied to things around you. For example, some sunglasses are polarized. They are good for driving as they cut down on glare. They are also good for water sports for the same reason.

The sunglasses use a vertical polarizing filter. In this way, the horizontal glare from the asphalt and the car in front of you is cut down. The same from the water surface if you're fishing, for example. How do we apply this to a sensor? Well, we use two filters... a horizontal and a vertical one. We apply one of them to the light transmitter and the other to the light receiver. Then, for example, we will transmit vertical light but only look to receive horizontal light.

The key is in the reflector. We use a 'corner cube' reflector. This type of reflector works by flipping the light 90 degrees which in effect will change vertical light to horizontal light and vice versa.

Now imagine using a polarized retroreflective sensor with a corner cube reflector to detect our cans going by. Initially we shoot vertical light and are looking for horizontal light to be received. If we have a can going by, it may reflect strong light back but it will be the vertical light and hence our receiver won't see it. So, this technique has solved out problem and not affected other targets going by. When necessary, remember to use a polarized reflective sensor for your shiny target application!

===================================================****** All The World's An Input- Part 13===================================================The last type of photoelectric sensor we will cover is used in some special applications. The applications are not the most popular around but when you come across it you'll be glad to have the knowledge of this type of sensor. What type of photoelectric sensor am I talking about?

The specialized background suppression sensor. What does it do? Well it does exactly what it's name implies. It suppresses the background so the target can be detected.

To put that in more simple terms, consider a conveyor moving parallel to a wall behind it. Imagine boxes on that conveyor that need to be detected. There is no room for a through beam and we can't mount a reflector on the wall. What to do? Or how about the need to sense short targets laying flat on a machine base? If the distance between the target and the background is small it is highly probable that the sensor will not be able to detect the target.

The method to detect such a small difference between the background and the target is to use a distance specific or background suppression type of sensor.

These sensors typically send light our in a straight and focused line. The light hits the target and reflects back on an angle. The further away the target is from the sensor head, the smaller the received light angle will be.

The receiver is typically a position sensitive detector (i.e. PSD) The PSD is a rectangular shape and positioned lengthwise next to the light transmitter.

The light is received further (larger angle) from the light transmitter the closer the target is to the head. It hits the PSD at a location on one side of the PSD. As the target is further and further away from the head the received beam spot hits further and further away from the light transmitter side of the PSD.

In order to properly detect the target at a specified distance, the light received must be in the proper position on the PSD. If it hits too close or too far from the learned position on the PSD the reflection is ignored. In this manner, it is 'distance specific'.

This method is very useful for background suppression of the application. Can you see a need for this photoelectric sensor type?

===================================================****** Too Hot... or Not? Part 1===================================================

Temperature sensing is one of the most common applications used for specialty modules. There are quite a few different ways to measure the temperature though. We can put the methods into two different groups, contact and non-contact.Non-contact temperature sensing is generally done with infrared light. Since that is not typical used for logging or as an input to our systems, we'll focus on contact methods.

In our types of applications we're likely to come across three different contact temperature sensing methods. They are:    1- Thermocouple    2- RTD (Resistance Temperature Detector)    3- Thermistor

Of the three different methods, thermocouple probably has the largest base out there. RTD comes in second place and thermistor follows in the back.

It's important to note that there are differences between them. The thermocouple, for example, works based upon a change in voltage. The RTD and thermistor have a different operating principle. They each work based on a change in resistance. Because of this fundamental operating principle difference, they are each best used in particular sensing applications. We'll learn more about each in the future.

The most important thing for us at this stage of understanding is how to go about choosing a sensor type. Some of the most important things to consider in your application are:     1- Accuracy. You obviously need to sense temperature. How accurately do you need to sense that temperature? In other words, you need to know that the temperature is, for example, 125F +/- what?     2- Resolution. What is the smallest unit that you need to know the temperature changed by? For example, you need to know the temperature is 125.001F would be a higher resolution than say 125.01F     3- Repeatability. In simple terms, if you made the same measurement ten times (for example) the reading would be the same exact thing. In reality it will most likely deviate from reading to reading. The repeatability will determine that deviation specification.    4- Temperature Range. The reality of the various technologies is that they don't all measure in the same temperature range. Be sure the product you use can take temperature readings in the particular range that you are measuring.     5- Price. Self explanatory, don't you think?     Next time we'll start going through the operating principles of each technology so you can see why one way is better than another for your particular application.