Basic Color Sensor Overview

PrintIn the past, color sensors emitted light using red, green and blue LEDs’. The sensors were then able to distinguish colors using the RGB components of the reflected light back to the sensor’s receiver. As technology has progressed true color sensors have been developed that not only can compare colors but measure them more accurately than the human eye.

Color sensors are based on diffuse technology and can be compared to a fixed focus or convergent sensor because of the focused light spot. Unlike color contrast sensors that only detect the difference between two colors based on brightness, color sensors can detect a wide range of colors.

cielabTrue color sensors typically use white LED’s which allow for a greater color spectrum evaluation. Combine this with the CIELAB color system which is one of the most versatile color systems and the result is a color sensor that equals or exceeds the human eye. The CIELAB color system is a three-dimensional independent infinite representation of colors. The L component for lightness and a and b components for color are predefined absolute values. Lightness varies from black (0) to the brightest white (100). Color channel a varies from green negative 100 to red positive 100. Color channel b varies from blue negative 100 to yellow positive 100 with gray values at a=0 and b=0.

Due to the technology, color sensors can check only a small spot of color but can check this spot amazingly fast – up to 1.5 kHz in case of the Balluff’s fiber optic BFS 33M which also has a range of 400mm. Unlike a color sensor camera, which will focus on the object’s surface pattern and may cause false readings the true color sensor will ignore patterns thus providing more accurate color detection. In addition the true color sensor will have more outputs than the color camera.

Smart color cameras are working with RGB but could work also with HSV color models. They could be used to check larger areas for the same color or color codes on a part, but have slower update rate of 50 Hz. Special cameras for faster applications are available in the market but at higher costs. It is important that the light source for the smart color cameras be a white light with a standardized white balance, and that this light must kept constant for all checks to avoid errors.

The sophistication on the front end of the color sensor can be much more advanced and still remain a cost effective option for industrial use due to the fact that a camera requires a much larger processing system. The more sophisticated the sensors are in the camera the more robust the processor must be in order to process or map the data into an image.

To learn more visit www.balluff.us.

You can also request a digital copy of our Photoelectric Handbook here.

Level Detection Basics – Where to begin?

Initially I started to write this blog to compare photoelectric sensors to ultrasonic sensors for level detection. This came to mind after traveling around and visiting customers that had some very interesting applications. However, as I started to shed some light on this with photoelectrics, sorry for the pun but it was intended, I thought it might be better to begin with some application questions and considerations so that we have a better understanding of the advantages and disadvantages of solutions that are available. That being said I guess we will have to wait to hear about ultrasonic sensors until later…get it, another pun. Sorry.

Level detection can present a wide variety of challenges some easier to overcome than others. Some of the questions to consider include the following with some explanation for each:

  • What is the material of the container or vessel?
    • Metallic containers will typically require the sensor to look down to see the media. This application may be able to be solved with photoelectrics, ultrasonics, and linear transducers or capacitive (mounted in a tube and lowered into the media.
    • SmartLevelNon-metallic containers may provide the ability for the sensors look down to see the media with the same technologies mentioned above or by sensing through the walls of the container. Capacitive sensors can sense through the walls of a container up to 4mm thick with standard technology or up to 10mm thick using a hybrid capacitive technology offered by Balluff when detecting water based conductive materials. If the container is clear or translucent we have photoelectric sensors that can look through the side walls to detect the media. You can get more information in our white paper, SMARTLEVEL Technology Accurate point level detection.
  • What type of sensing is required? The short answer to this is level right? However, there are basically two different types of level detection. For more information on this refer to the Balluff Basics on Level Sensing – Discrete vs. Continuous.
    • Single point level or point level sensing. This is typically accomplished with a single sensor that allows for a discrete or an on-off signal when the level actuates the sensor. The sensor is mounted at the specific level to be monitored, for instance low-low, low, half full (the optimistic view), high, or high-high. These sensors are typically lower cost and easier to implement or integrate into the level controls.
    • Example of in-tank continuous level sensor
      Example of in-tank continuous level sensor

      Continuous or dynamic level detection. These sensors provide an analog or continuous output based on the level of the media. This level detection is used primarily in applications that require precise level or precision dispensing. The output signals are usually a voltage 0-10V or current output 4-20mA.  These sensors are typically higher cost and require more work in integrating them into system controls.  That being said, they also offer several advantages such as the ability to program in unlimited point levels and in the case of the current output the ability to determine if the sensor is malfunctioning or the wire is broken.

Because of the amount of information on level detection this will be the first in a series on this topic. In my next blog I will discuss invasive vs non-invasive mounting and some other topics. For more information visit www.balluff.us.

Photoelectric Basics – Distance Measuring

Some photoelectric applications require not only knowing if the object is present or not but exactly where the object is while providing a continuous or dynamic value representative of the objects location.  For instance, if a robot is stacking a product is the stack at the correct height or how many additional pieces can be placed on the stack, how large is the coil or roll diameter of a product, and how high is the level or how much further can the product move before it is in position.  Distance sensors can provide this dynamic information and in some case provide a digital output as well for alarms.

RetroreflectiveThese sensors are normally based on diffuse sensing technology. However, in some cases retro-reflective technology is used for extremely long sensing distances.  As with diffuse sensors there is only one device to mount and wire.  However, due to the technology required for the higher resolutions, lensing, electronics and outputs these devices are typically much more expensive than a discrete diffuse sensor.

Similar to a diffuse sensor the distance sensor emits a pulsed light that strikes an object and a certain amount of light is reflected back to the sensor’s receiver.  The sensor then generates an analog output signal that is proportional to the distance to the target.  The technology that is utilized within the sensor to determine the distance is either Time of Flight or Triangulation.

PrintTime of Flight sensors are more immune to target color and texture than light intensity based system because of the time component.  These devices measure greater distances than the triangulation method however there is a sacrifice in resolution.

PrintTriangulation sensors emit a pulsed light towards the target object.  The light is then reflected back to the receiver.  When the light reaches the sensor it will strike the photosensing diode at some angle.  The distance between the sensor and the target determines the angle in which the light strikes the receiver.  The closer the target is the sensor the greater the angle.
Triangulation based sensors being dependent on the amount of reflected light are more susceptible to target characteristics such as color and texture.  These sensors are characterized by short to mid-range sensing distance however they provide higher resolutions than TOF sensors.

Output signals are either 0…10 volts, 1…10 volts or 4…20mA each of which has their pros and cons.  Voltage outputs, 0 – 10 or 1- 10 volts, are easier to test and there is typically a broader offering of interface devices.  However voltage outputs are more susceptible to noise from motors, solenoids or other coils and voltage drops of the wire.  In addition generally voltage output cable runs should be less than 50 feet.  Also since 0 volts is an acceptable output value broken wires, device failures, or power failures can go undetected.

Current outputs, 4 – 20 mA, provide the best noise immunity, are not affected by voltage drop and the cables lengths can exceed 50 feet.  Since the sensor will be providing 4mA at zero distance its lowest possible signal, if the sensor should fail, the cable damaged or a power failure the interface device can detect the absence of the signal and notify an operator.  Current outputs are more difficult to test and in some cases are affected by temperature variations.

For more information about photoelectric sensors, request your copy of Balluff’s Photoelectric Handbook.

The Foundation of Photoelectric Sensors

PhotoelectricsThe foundation of a photoelectric sensor is light!  Without the light you have a housing with some electronics in it that makes an interesting object to leave on your desk as a conversation starter.  Is all light the same?  Does the light source really matter?  When do you select one over the other?

Red light or red LED light sources are the most favored as they are easy to set up and confirm that the sensor is working properly since you have a bright light that you can focus on your target.  Depending on the lensing the light spot size can vary from a pin point to a spot that can be several centimeters square or round.  It is important that you aim the sensor correctly if you have the sensor installed near an operator so as the light is not shining in their eyes as it can be rather irritating.

There are several misconceptions with the laser light.  Many think that lasers are the most powerful light and can penetrate anything.  Also there is the concern that lasers will cause damage to the human eye.  Lasers in photoelectric sensors are typically available as either a Class 1 or Class 2.  Class 1 lasers are safe under normal use conditions and are considered to be incapable of damage.  Class 2 lasers are more powerful, however it is the normal response of human eye to blink which will limit the exposure time and avoid damage.  Class 2 lasers can be hazardous if looked at for extended periods of time.  In either case viewing a laser light with a magnifying optic could cause damage.

Lasers provide a consistent light with a small beam diameter (light spot) that provides a perfect solution for small part detection.  Although the light beam is small and concentrated, it can be easily interrupted by airborne particles.  If there is dust or mist in the environment the light will be scattered making the application less successful than desired.  In some cases the sensing distance will be greater with a laser light than with a red light.

Infrared LED’s will produce an invisible, to the human eye, light while being more efficient and generating the most light with the least amount of heat.  Infrared light sources are perfect for harsh and contaminated environments where there is oil or dust.  Also infrared through-beam sensors are sometimes capable of “seeing through” a package or object which is sometimes preferred to solve an application.  The ability to see though an object or dirt makes this light source perfect in very contaminated environments when the contamination builds up on the lens or reflector.

In all cases LED’s are modulated or turned on and off very rapidly.  This modulation determines the amount of light a photoelectric sensor can create and prolongs the life of the LED.  In addition, the sensor receiver is designed to look for the modulated light at the same frequency to help eliminate ambient light causing the sensors output to false trigger.

We have determined that all light sources are not the same each with their benefits and drawbacks.  Selection of the light source really depends on the application as often red lights have been installed in very contaminated applications that required the power of the infrared.

If you are interested in learning more about the basics of photoelectrics request the Photoelectric Handbook or visit www.balluff.us/photoelectric.

Photoelectric Output Operate Modes and Output Types

Photoelectric sensors are used in a wide variety of applications that you encounter every day. They are offered in numerous housing styles that provide long distance non-contact detection of many different types of objects or targets. Being used in such a variety of applications, there are several outputs offered to make integration to control systems easy and depending on the sensing mode when the output is activated in the presence of the target.

DiffuseDiffuse sensors depend on the amount of light reflected back to the receiver to actuate the output. Therefore, Light-on (normally open) operate refers to the switching of the output when the amount of light striking the receiver is sufficient, object is present. Likewise, Dark-on (normally closed) operate would refer to the target being absent or no light being reflected back to the receiver.

RetroreflectiveRetroreflective and through-beam sensors are similar in the fact they depend on the target interrupting the light beam being reflected back to the receiver. When an object interrupts the light beam, preventing the light from reaching the receiver, the output will energize which is referred to as Dark-on (normally open) operate switching mode or normally open. Light-on (normally closed) operate switching mode or normally closed output in a reflex sensor is true when the object is not blocking the light beam.

signalsOutputs from photoelectric sensors are typically either digital or analog. Digital outputs are on or off and are usually three wire PNP (sourcing output) or NPN (sinking outputs). The exception to this is a relay output that provides a dry or isolated contact requiring voltage being applied to one pole.

Analog outputs provide a dynamic or continuous output that varies either a voltage (0-10 volt) or current (4-20mA) throughout the sensing range. Voltage outputs are easier to integrate into control systems and typically have more interface options. The downside to a voltage output is it should not be ran more than 50 feet. Current outputs can be ran very long lengths without worry of electrical noise. As additional advantage of the analog output is that it has built in diagnostics, at its minimum there will always be some current at the input unless the device completely fails or the wire is damaged.

Some specialty photoelectric sensors will provide a serial or network communication output for communications to higher level devices. Depending on the network, IO Link, for instance, additional diagnostics can be provided or even parameterization of the sensors. io-link
Interested in learning more about photoelectrics basics? You can also request a copy of the new Photoelectric Handbook.

Detecting Small Bubbles? Consider These Factors First

BubbleDetectionBubble or air-in-line detection is a common lab automation application. In these types of applications it’s important to know whether or not liquid is flowing through a line to ensure safe and proper function in liquid-handling processes.  As these processes utilize smaller and smaller volumes of liquid — which provides cost and time saving benefits — it becomes more and more difficult to detect the potential air pockets forming inside the line. The most common approach in detecting these minute air pockets is a through-beam, photoelectric bubble sensor.

Photoelectric bubble sensors provide non-invasive detection of fluids and air pockets residing inside a tube. They have fixed opening dimensions for standard tube sizes allowing the selected tube to sit in perfect position between the sensor’s optical components. When the sensor’s light beam is blocked by fluid (or an air pocket) inside the tube, the received signal varies and external electronics determine if the signal variation is above or below the set threshold. Once the threshold is met the sensor’s output is switched.

Detecting bubbles sounds quite straightforward and simple, but in reality the application can be somewhat complicated. Several factors should be considered for reliable detection. Listed below are a few factors to consider:

  1. Tube diameters (inner and outer)
  2. Tube transparency
  3. Liquid type(s)
  4. Liquid transparency

Tube Diameters

Tube Sensor DrawingBecause a tube acts as a lens for light to travel it’s important to factor in the tube diameters. If there is a large difference between the outer and inner diameters of a particular tube, the outcome is a relatively large tube wall. A large tube wall will allow light rays to travel from the emitter through the wall straight to the detector without passing through the inner diameter of the tube, where the liquid or bubble is present. This causes unreliable detection. By accounting for both the inner and outer tube diameters a proper determination can be made in selecting what type of sensor to use to ensure that light only passes through the inner diameter of the tube and not through the wall.

Tube Transparency

Since photoelectric tube sensors operate on the principle of light detection, light must make it through one end of the tube and out the other end. Therefore, the transparency of the tube is critical. If the tube is opaque a photoelectric sensor solution is unlikely; however, in some cases it’s possible for a photoelectric tube sensor to detect through an opaque tube.

Liquid Type(s) and Transparency

The liquid type(s) and transparency are critical when determining which photoelectric tube sensor to use. If the liquid type is non-aqueous, without factoring in its transparency, it’s best to use the principle of light refraction through the liquid. If the liquid type is aqueous and is completely transparent or semitransparent, it’s best to use the principle of light absorption through the liquid. The following table will help determine what type of sensor to use with respect to the liquid type present inside the tube.

BubbleSensingChart

Since the type of applications that require precise bubble detection range in specifications from the use of hundreds of different liquids to specialized tube dimensions, this post only touches the surface of the photoelectric sensors for bubble detection.  For more information on tube sensors, please visit the Balluff website.

Let’s Get Small: The Drive Toward Miniaturization

minisensorGoing about our hectic daily lives, we tend to just take the modern cycle of innovation for granted. But when we stop to think about it, the changes we have seen in the products we buy are astonishing. This is especially true with regard to electronics. Not only are today’s products more feature-laden, more reliable, and more functional…they are also unbelievably small.

I remember our family’s first “cell phone” back in the ’90s. It was bolted to the floor of the car, required a rooftop antenna, and was connected to the car’s electrical system for power. All it did was place and receive phone calls. Today we are all carrying around miniature pocket computers we call “smartphones,” where the telephone functionality is – in reality – just another “app”.

Again going back two decades, we had a 32″ CRT analog television that displayed standard definition and weighed over 200 pounds; it took two strong people to move it around the house. Today it’s common to find 55″ LCD high-definition digital televisions that weigh only 50 pounds and can be moved around by one person with relative ease.

LabPhotoThese are just a couple of examples from the consumer world. Similar changes are taking place in the industrial and commercial world. Motors, controllers, actuators, and drives are shrinking. Today’s industrial actuators and motion systems offer either the same speed and power with less size and weight, or are simply more compact and efficient than ever before possible.

The advent of all this product miniaturization is driving a need for equally miniaturized manufacturing and assembly processes. And that means rising demand for miniaturized industrial sensors such as inductive proximity sensors, photoelectric presence sensors, and capacitive proximity sensors.

Another thing about assembling small things: the manufacturing tolerances also get small. The demand for sensor precision increases in direct proportion to manufacturing size reduction. Fortunately, miniature sensors are also inherently precision sensors. As sensors shrink in size, their sensing behavior typically becomes more precise. In absolute terms, things like repeatability, temperature drift, and hysteresis all improve markedly as sensor size diminishes. Miniature sensors can deliver the precise, repeatable, and consistent sensing performance demanded by the field of micro-manufacturing.

For your next compact assembly project, be sure to think about the challenges of your precision sensing applications, and how you plan to deploy miniature sensors to achieve consistent and reliable operation from your process.

For more information on precision sensing visit balluff.us/minis.

There’s more than just one miniature sensor technology

As I discussed in my last blog post, there is a need for miniature, precision sensors. However, finding the right solution for a particular application can be a difficult process. Since every sensor technology has its own strengths and weaknesses, it is vital to have a variety of different sensor options to choose from.

The good news is that there are several different technologies to consider in the miniature, precision sensor world. Here we will briefly look at three technologies: photoelectric, capacitive, and inductive. Together these three technologies have the ability to cover a wide range of applications.

Photoelectric Sensors

MiniPhotoelectricPhotoelectric sensors use a light emitter and receiver to detect the presence or absence of an object. This type of sensor comes in different styles for flexibility in sensing. A through-beam photoelectric is ideal for long range detection and small part detection. Whereas a diffuse photoelectric is ideal for applications where space is limited or in applications where sensing is only possible from one side.

Miniature photoelectric sensors come with either the electronics fully integrated into the sensor or as a sensor with separate electronics in a remote amplifier.

Capacitive Sensors

MiniCapacitiveCapacitive sensors use the electrical property of capacitance and work by measuring changes in this electrical property as an object enters its sensing field. Capacitive sensors detect the presence or absence of virtually any object with any material, from metals to powders to liquids. It also has the ability to sense through a plastic or glass container wall to detect proper fill level of the material inside the container.

Miniature capacitive sensors come with either the electronics fully integrated into the sensor or as a sensor with separate electronics in a remote amplifier.

Inductive Sensors

MiniInductiveInductive sensors use a coil and oscillator to create a magnetic field to detect the presence or absence of any metal object. The presence of a metal object in the sensing field dampens the oscillation amplitude. This type of sensor is, of course, ideal for detecting metal objects.

Miniature inductive sensors come with the electronics fully integrated into the sensor.

One sensor technology isn’t enough since there isn’t a single technology that will work across all applications. It’s good to have options when looking for an application solution.

To learn more about these technologies, visit www.balluff.us

Liquid Level Sensing: Detect or Monitor?

Pages upon pages of information could be devoted to exploring the various products and technologies used for liquid level sensing and monitoring.  But we’re not going to do that in this article.  Instead, as a starting point, we’re going to provide a brief overview of the concepts of discrete (or point) level detection and continuous position sensing.

 Discrete (or Point) Level Detection

Example of discrete sensors used to detect tank level
Example of discrete sensors used to detect tank level

In many applications, the level in a tank or vessel doesn’t need to be absolutely known.  Instead, we just need to be able to determine if the level inside the tank is here or there.  Is it nearly full, or is it nearly empty?  When it’s nearly full, STOP the pump that pumps more liquid into the tank.  When it’s nearly empty, START the pump that pumps liquid into the tank.

This is discrete, or point, level detection.  Products and technologies used for point level detection are varied and diverse, but typical technologies include, capacitive, optical, and magnetic sensors.  These sensors could live inside the tank outside the tank.  Each of these technologies has its own strengths and weaknesses, depending on the specific application requirements.  Again, that’s a topic for another day.

In practice, there may be more than just two (empty and full) detection points.  Additional point detection sensors could be used, for example, to detect ¼ full, ½ full, ¾ full, etc.  But at some point, adding more detection points stops making sense.  This is where continuous level sensing comes into play.

Continuous Level Sensing

Example of in-tank continuous level sensor
Example of in-tank continuous level sensor

If more precise information about level in the tank is needed, sensors that provide precise, continuous feedback – from empty to full, and everywhere in between – can be used.  This is continuous level sensing.

In some cases, not only does the level need to be known continuously, but it needs to be known with extremely high precision, as is the case with many dispensing applications.  In these applications, the changing level in the tank corresponds to the amount of liquid pumped out of the tank, which needs to be precisely measured.

Again, various technologies and form factors are employed for continuous level sensing applications.  Commonly-used continuous position sensing technologies include ultrasonic, sonic, and magnetostrictive.  The correct technology is the one that satisfies the application requirements, including form factor, whether it can be inside the tank, and what level of precision is needed.

At the end of the day, every application is different, but there is most likely a sensor that’s up for the task.

Back to the Basics – Photoelectric Light Source

Welcome to the first in a series of getting back to the basic blogs about photoelectric sensors.

LightTypeAll photoelectric sensors require a light source to operate. The light source is integral to the sensor and is referred to as the emitter. Some light sources can be seen and may be of different colors or wavelengths for instance red, blue, green, white light or laser or one you cannot see, infrared. Many years ago photoelectric sensors used incandescent lights which were easily damaged by vibration and shock. The sensors that used incandescence were susceptible to ambient light which limited the sensing range and how they were installed.

Today light sources use light emitting diodes (LED’s). LED’s cannot generate the light that the incandescent bulbs could. However since the LED is solid state, it will last for years, is not easily damaged, is sealed, smaller than the incandescent light and can survive a wide temperature range. LED’s are available in three basic versions visible, laser and infrared with each having their advantages.

Visible LED’s which are typically red, aid in the alignment and set up of the sensor since it will provide a visible beam or spot on the target. Visible red LED’s can be bright and should be aimed so that the light will not shine in an operator’s eyes. The other color visible LED’s are used for specific applications such as contrast, luminescence, and color sensors as well as sensor function indication.

Laser LED’s will provide a consistent light color or wavelength, small beam diameter and longer range however these are generally more costly. Lasers are often used for small part detection and precision measuring. Although the light beam is small and concentrated, it can be easily interrupted by airborne particles. If there is dust or mist in the environment the light will be scattered making the application less successful than desired. When a laser is being used for measuring make sure the light beam is larger than any holes or crevasses in the part to ensure the measurement is as accurate as possible. Also it is important to ensure that the laser is installed so that it is not aimed into an operator or passerby’s eyes.

Lastly, the infrared LED will produce an invisible, to the human eye, light while being more efficient and generating the most light with the least amount of heat. Infrared light sources are perfect for harsh and contaminated environments where there is oil or dust. However, with the good comes the bad. Since the light source is infrared and not visible setup and alignment can be challenging.

LED’s have proved to be robust and reliable in photoelectric sensors. In the next installment we will review LED modulation.

You can learn more about photoelectric sensors on our website at www.balluff.us