Using Photoelectric Sensors in High Ambient Temperatures

Photoelectric sensors with laser and red-light are widely used in all areas of industrial automation. A clean, dust-free and dry environment is usually essential for the proper operation of photoeyes, however, they can be the best choice in many dirty and harsh applications. Examples of this are raw steel production in steel mills and further metallurgical processes down to casting and hot-rolling.

Cutting of billets at casting – Photo: M.Münzl
Cutting of billets at casting – Photo: M.Münzl

Photoelectric sensors are especially useful in these environments thanks to their long sensing distance and their ability to detect objects independent of their material.

Most photoelectric sensors are approved to work in ambient temperatures of 55 to 60 °C. The maximum temperature range of these sensors is most often limited by the specifications of the optical components of the sensor, like the laser-diodes, but by taking certain precautions photoelectric sensors can provide optimal use in much hotter applications.

Maximize the distance
In steel production many parts of the process are accompanied by high ambient temperatures. Liquid steel and iron have temperatures from 1400 to 1536 °C. Material temperature during continuous casting and hot-rolling are lower but still between 650 and 1250°C.

The impact of heat emission on the sensors can be reduced significantly by placing the sensor as far from the target object as possible, something you can’t do with inductive sensors which have a short range. Very often the remote mounting will allow the sensor to operate at room temperature.

If you intend to detect quite small objects with high precision, the maximum distance for the installation might be limited. For this purpose chemical resistant glass fibers are suitable and can handle temperatures up to 250 °C. These pre-fabricated fiber optic assemblies can be easily attached to the sensor. The sensor itself can be mounted on a cooler and protected place.

Detect Glowing Metals
If you want to reliably detect red-hot or white glowing steel parts with temperatures beyond 700 °C, you won’t be able to use standard laser or red-light sensors. Red-hot steel emits light at the same wavelength that it is used by photoelectric sensors. This can interfere with the function of the sensor. In such applications you need to use sensors which operate based on infrared light.

Add Protection

Sensor enclosure and protective cable sleeve
Sensor enclosure and protective cable sleeve

At many locations in the steel production process, the extensive heat is only temporary. In a hot rolling mill, a slab runs through a rougher mill multiple times before it continues to a multi-stage finishing mill stand to be rolled to the final thickness. After that the metal strip runs into the coiler to be winded up.
This process runs in sequence, and the glowing material is only present at each stage of production for a short time. Until a new slab runs out of the reheating furnace, temperatures normalize.

Standard sensors can work in these conditions, but you do run the risk of even temporary temperature hikes causing sensor failure and then dreaded downtime. To protect photoelectric sensors against temporary overheating, you can use a protective enclosure. These can provide mechanical and thermal protection to the sensors which often have plastic bodies. Additional protection can be achieved when a heat resistant sleeve is used around the cable.

Photoelectric sensors do have their limits and are not suitable for all applications, even when precautions are taken. Ask yourself these questions when deciding if they can be the right solution for your high temperature applications.

  • Which distance between the hot object and sensor can be realized?
  • What is the maximum temperature at this location?
  • How long will the sensor be exposed to the highest heat levels during normal operation and at breakdown?

Industrial sensors with diagnostic functionality

Self-Awareness
For monitoring functionality in industrial processes two aspects are relevant: Environmental awareness and self-awareness. Environmental awareness analyzes impacts which are provided by the environment (e.g. ambient temperature). Self-awareness collects information about the internal statuses of (sub)systems. The diagnostic monitoring of industrial processes, which are typically dynamic, is  not as critical as the monitoring of static situations. If you have many signal changes of sensors due to the activity of actuators, with each plausible sensor signal change you can be confident that the sensor is still alive and acts properly. A good example of this is rotation speed measurement of a wheel with an inductive sensor having many signal changes per second. If the actuator drove the wheel to turn but the sensor would not provide signal changes at its output, something would be wrong. The machine control would recognize this and would trigger a stop of the machine and inspection of the situation.

Inductive Sensors with self-awareness

DESINA
For level sensing applications in cooling liquid tanks of metalworking applications inductive sensors with self-diagnostics are often used. The inductive sensors detect a metal flag which is mounted to a float with rod fixation.

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Additionally, to the switching output these sensors have a monitor output which is a “high” signal when the sensor status is OK. In situations where the sensor is not OK, for example when there has been a short circuit or sensor coil damage, the monitor output will be a “low” signal.  This type of so called DESINA sensors is standardized according to ISO 23570-1 (Industrial automation systems and integration – Distributed installation in industrial applications – part 1: Sensors and actuators).

Dynamic Sensor control
Another approach is the Dynamic Sensor Control (DSC). Rather than using an additional monitoring output, this type of sensors provides impulses while it is “alive.”

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The sensor output provides information about the position of the target with reference to the sensor as well as status diagnostic of the sensor itself.

IO-Link
With IO-Link communication even teaching of defined switching distance can be realized. The IO-Link concept allows you to distinguish between real-time process data (like target in/out of sensing range) and service data which may be transferred with a lower update rate (in the background of the real process).

For more information, visit www.balluff.com.

This blog post was originally published on the innovating-automation.blog.

Press Shops Boost Productivity with Non-Contact Connections

In press shops or stamping plants, downtime can easily cost thousands of dollars in productivity. This is especially true in the progressive stamping process where the cost of downtime is a lot higher as the entire automated stamping line is brought to a halt.

BIC presse detail 231013

Many strides have been made in modern stamping plants over the years to improve productivity and reduce the downtime. This has been led by implementing lean philosophies and adding error proofing systems to the processes. In-die-sensing is a great example, where a few inductive or photo-eye sensors are added to the die or mold to ensure parts are seated well and that the right die is in the right place and in the right press. In-die sensing almost eliminated common mistakes that caused die or mold damages or press damages by stamping on multiple parts or wrong parts.

In almost all of these cases, when the die or mold is replaced, the operator must connect the on-board sensors, typically with a multi-pin Harting connector or something similar to have the quick-connect ability. Unfortunately, often when the die or mold is pulled out of the press, operators forget to disconnect the connector. The shear force excreted by the movement of removing the die rips off the connector housing. This leads to an unplanned downtime and could take roughly 3-5 hours to get back to running the system.

 

image

Another challenge with the multi-conductor connectors is that over-time, due to repeated changeouts, the pins in the connectors may break causing intermittent false trips or wrong die identification. This can lead to serious damages to the system.

Both challenges can be solved easily with the use of a non-contact coupling solution. The non-contact coupling, also known as an inductive coupling solution, is where one side of the connectors called “Base” and the other side called “Remote” exchange power and signals across an air-gap. The technology has been around for a long time and has been applied in the industrial automation space for more than a decade primarily in tool changing applications or indexing tables as a replacement for slip-rings. For more information on inductive coupling here are a few blogs (1) Inductive Coupling – Simple Concept for Complex Automation Part 1,  (2) Inductive Coupling – Simple Concept for Complex Automation Part 2

For press automation, the “Base” side can be affixed to the press and the “Remote” side can be mounted on a die or mold, in such a way that when the die is placed properly, the two sides of the coupler can be in the close proximity to each other (within 2-5mm). This solution can power the sensors in the die and can help transfer up to 12 signals. Or, with IO-Link based inductive coupling, more flexibility and smarts can be added to the die. We will discuss IO-Link based inductive coupling for press automation in an upcoming blog.

Some advantages of inductive coupling over the connectorized solution:

  • Since there are no pins or mechanical parts, inductive coupling is a practically maintenance-free solution
  • Additional LEDs on the couplers to indicate in-zone and power status help with quick troubleshooting, compared to figuring out which pins are bad or what is wrong with the sensors.
  • Inductive couplers are typically IP67 rated, so water ingress, dust, oil, or any other environmental factor does not affect the function of the couplers
  • Alignment of the couplers does not have to be perfect if the base and remote are in close proximity. If the press area experiences drastic changes in humidity or temperature, that would not affect the couplers.
  • There are multiple form factors to fit the need of the application.

In short, press automation can gain a productivity boost, by simply changing out the connectors to non-contact ones.

 

Improve Your Feeder Bowl System (and Other Standard Equipment) with IO-Link

One of the most common devices used in manufacturing is the tried and true feeder bowl system. Used for decades, feeder bowls take bulk parts, orients them correctly and then feeds them to the next operation, usually a pick-and-place robot. It can be an effective device, but far too often, the feeder bowl can be a source of cycle-time slowdowns. Alerts are commonly used to signal when a feed problem is occurring but lack the exact cause of the slow down.

feeder bowl

A feed system’s feed rate can be reduced my many factors. Some of these include:

  • Operators slow to add parts to the bowl or hopper
  • Hopper slow to feed the bowl
  • Speeds set incorrectly on hopper, bowl or feed track
  • Part tolerance drift or feeder tooling out of adjustment

With today’s Smart IO-Link sensors incorporating counting and timing functions, most of the slow-down factors can be easily seen through an IIoT connection. Sensors can now time how long critical functions take. As the times drift from ideal, this information can be collected and communicated upstream.

A common example of a feed system slow-down is a slow hopper feed to the bowl. When using Smart IO-Link sensors, operators can see specifically that the hopper feed time is too long. The sensor indicates a problem with the hopper but not the bowl or feed tracks. Without IO-Link, operators would simply be told the overall feed system is slow and not see the real problem. This example is also true for the hopper in-feed (potential operator problem), feed track speed and overall performance. All critical operations are now visible and known to all.

For examples of Balluff’s smart IO-Link sensors, check out our ADCAP sensor.

Polarized Retroreflective Sensors: A Solution for Detecting Highly Reflective Objects

The complexity of factory automation creates constant challenges which drive innovation in the industry. One of these challenges involves the ability to accurately detect the presence of shiny or highly reflective objects. This is a common challenge faced in a variety of applications, from sensing wheels in an automotive facility to detecting an aluminum can for filling purposes at a beverage plant. However, thanks to advancements in photoelectric sensing technologies, there is a reliable solution for those type of applications.

Why are highly reflective objects a challenge?

Light reflects from these types of objects in different directions, and with minimum energy loss. This can cause the receiver of a photoelectric sensor to be unable to differentiate between a signal received from the emitter or a signal received from a shiny object. In the case of a diffuse sensor, there is also the possibility that when trying to detect a shiny object, the light will reflect away from the receiver causing the sensor to ignore the target.

So how do we control the direction of the light going back to the receiver, and avoid false triggering from other light sources? The answer is in polarized retroreflective sensors.

Retroreflective sensors require a reflector which reflects the light back to the sensor allowing it to be captured by the receiver. This is achieved by incorporating sets of three mirrors oriented at right angles from each other (referred to as corner cubes). A light beam entering this system is reflected by all three surfaces and exits parallel to the incident beam. Additionally, corner cubes are said to be optically active as they rotate the plane of oscillation of the light by 90 degrees. This concept, along with polarization, allow this type of sensor to accurately detect shiny objects.

Polarization

Light emitted by a regular light source oscillates in planes on dispersal axes. If the light meets a polarizing filter (fine line grid), only the light oscillating parallel to the grid is let through (see figure 1 below).

Figure 1_AR
Figure 1

In polarized retroreflective sensors, a horizontal polarized filter is placed in front of the emitter and a vertical one in front of the receiver. By doing this, the transmitted light oscillates horizontally until it hits the reflector. The corner cubes of the reflector would then rotate the polarization direction by 90 degrees and reflect the light back to the sensor. This way, the returning light can pass through the vertical polarized filter on the receiver as shown below.

Figure 2_AR
Figure 2

With the use of polarization and corner cubed reflectors, retroreflective sensors can create a closed light circuit which ensures that light detected by the receiver was sourced exclusively by the emitter. This creates a great solution for applications where highly reflective targets are influencing the accuracy of sensors or causing them to malfunction. By ensuring proper operation of photoelectric sensors, unplanned downtime can be avoided, and overall process efficiency can be improved.

Capacitive Sensors: Versatile enough for most (but not all) detection applications

capacitive 1

Capacitive sensors are versatile for use in numerous applications. They can be used to detect objects such as glass, wood, paper, plastic, ceramic, and more. Capacitive sensors used to detect objects are easily identified by the flush mounting or shielded face of the sensor. This shielding causes the electrostatic field to be short and conical shaped, much like the shielded version of an inductive proximity sensor.

capacitive 2Just as there are non-flush or unshielded inductive sensors, there are non-flush capacitive sensors, and the mounting and housing look the same. The non-flush capacitive sensors have a large spherical field which allows them to be used in level detection, including detection of liquids and granular solids. Levels can be detected either directly with the sensor making contact with the medium, or indirectly with the sensor sensing the medium through a non-metallic container wall.

Capacitive sensors are discrete devices so once you adjust the sensitivity to detect the target while ignoring the container, the sensor is either on or off. Also remember that the sensor is looking for the dielectric constant in the case of a standard capacitive sensor or the conductivity of a water based liquid in the case of the hybrid technology.

Recent technology advances with remote amplifiers have allowed capacitive sensors to provide an analog output or a digital value over IO-Link. As previously mentioned, these sensors are based off of a dielectric constant so the analog value being created is dependent on the media being sensed.

While capacitive sensors are versatile to work in many applications, they are not the right choice for all applications.

Recently a customer inquired if a capacitive sensor could detect the density of an substance and unfortunately the short answer is no, though in some applications the analog sensors can detect different levels of media if it can be separated in a centrifuge. Also, capacitive sensors may not detect small amounts of media as the dielectric constant of the media must be higher than the container that holds the media.

There are three important steps in applying a capacitive sensor — test it, test it and test it one more time. During your testing procedures be sure to test it under the best and worse conditions. Also like any other electronic device temperature can have an affect although it may be negligible there will be some affect.

For more information on capacitive sensors visit www.balluff.com.

How to keep prox sensors from latching on

For inductive proximity sensors to operate in a stable manner, without constant “chatter” or switching on/off rapidly close to the switching point, they require some degree of hysteresis.

Hysteresis, basically, is the distance between the switch-on point and the switch-off point when the target is moving away from the active surface. Typical values are stated in sensor data sheets; common values would be ≤ 15%, ≤ 10%, ≤ 5% and so on. The value is taken as a percentage of the actual switch-on distance of the individual sensor specimen. Generally, the higher the percentage of hysteresis, the more stable the sensor is and the farther away the target must move to turn off the sensor.

basic_oper_inductive_sensorBut occasionally, a sensor will remain triggered after the target has been removed. This condition is called “latching on” and it typically occurs when the sensor remains damped enough to hold the sensor in the “on” condition.

Some factors that could cause “latching on” behavior and ways to correct it are:

Having too much metal near the sensor
Using a quasi-flush, non-flush, or extended-range sensor that is too close to metal surrounding its sides will partially dampen the sensor. While it is not enough to turn the sensor on, it is enough to hold it in the on state due to hysteresis. If there is a lot of metal close to the sides of the sensor, a flush-type sensor may eliminate the latching-on problem, although it will have shorter range.

Having the mounting nuts too close to the sensor face
of a quasi-flush, non-flush, or extended-range sensor. Even though there are threads in that area, the mounting nuts can pre-damp the sensor.

Using a sensor that is not stable at higher temperatures
Some sensors are more susceptible to latching-on than others as temperature is increased. This is caused by temperature drift, which can increase the sensor’s sensitivity to metals. In these cases, the sensor may work fine at start-up or at room temperature, but as the machinery gets hot it will start latching on. The solution is to make sure that the sensor is rated for the ambient temperature in the application. Another option: look for sensors designed properly by a reputable manufacturer or choose sensors specifically designed to work at higher temperatures.

Having strong magnetic fields
This happens because the magnetic field oversaturates the coil, so that the sensor is unable to detect that the target has been removed. If this is the case, replace them with weld-field-immune or weld-field-tolerant sensors.

inductive-proximity-sensor-cutaway-with-annotation

For a more detailed description of how inductive proximity sensors detect metallic objects without contact, please take a look at this related blog post.

 

Measuring Distance: Should I Use Light or Sound?

Clear or transparent sensing targets can be a challenge but not an insurmountable one. Applications can detect or measure the amount of clear or transparent film on a roll or the level of a clear or transparent media, either liquid or solid.  The question for these applications becomes, do I use light or sound as a solution?

photoelectric.png
An application that measures the diameter of a roll of clear labels.

In an application that requires the measurement of the diameter of a roll of clear labels, there are a number of factors that need to be considered.  Are the labels and the backing clear?  Will the label transparency and the background transparency change?  Will the labels have printing on them?  All of these possibilities will affect which sensor should be used. Users should also ask how accurate or how much resolution is required.

Faced with this application, using ultrasonic sensors may be a better choice because of their ability to see targets regardless of color, possible printing on the label, transparency and surface texture or sheen.  Some or all of these variables could affect the performance of a photoelectric sensor.

Ultrasonic sensors emit a burst of short high frequency sound waves that propagate in a cone shape towards the target.  When the sound waves strike the target, they are bounced back to the sensor. The sensor then calculates the distance based on the time span from when the sound was emitted until the sound was received.

In some instances, and depending on the resolution required, a time of flight sensor may solve the above application. Time of Flight (TOF) sensors emit a pulsed light toward the target object. The light is then reflected back to the receiver. The elapsed time it takes for the light to return to the receiver is measured, thus determining the distance to the target. In this case, the surface finish and transparency may not be an issue.

Imagine trying to detect a clear piece of plastic going over a roll.  The photoelectric sensor could detect it either in a diffuse mode or with a retroreflective sensor designed for clear glass detection.  But what if the plastic characteristics can change frequently or if the surface flutters.  Again, the ultrasonic sensor may be a better choice and also may not require set up any time the material changes.

So what’s the best solution?  In the end, test the application with the worst case scenario.  A wide variety of sensors are available to solve these difficult applications, including photoelectric or ultrasonic. Both sensors have continuous analog and discrete outs.  For more information visit www.balluff.com.

 

Non-Contact Transmission of Power & Data on Transfer Rails & Grippers

For press shops utilizing transfer rail systems, fixed sensor connections regularly cause frustration. Cables and contacts are often subject to heavy strain. Cables can wear out and break, damaged pins or mechanical collisions can cause hours of machine downtime, and the replacement of large multi-pin connectors comes at a high cost.

Inductive couplers offer an ideal solution: By using these non-contact, wear-free products you can eliminate pin connections and simplify job changeovers on the press. Inductive couplers transfer signals and power contact-free over an air gap. The quick-disconnect units are easy to use and require no maintenance, enabling you to meet new demands quickly. Mechanical wear is a thing of the past. This increases system availability, reduces cycle time and enhances the flexibility of workflow processes.

Inductive coupling example

Replace pin connections for transfer rails

Typically, two pin-based connectors connect the transfer rail to the transfer system on the press. The connections are on both the feed and exit sides of the rail to the control. If there is any misalignment of the connections, damage regularly occurs. By replacing the connectors with pin-free inductive couplers, the connections are simplified and repair work is minimized. Additionally you don’t have open pins exposed to the environment (dust, water, oil) that can also cause nuisances in the connection process.

Replace pin connections for grippers

To connect the transfer rail on each gripper, normally a pin-based connector is used. As the grippers are changed on each tooling change, the connectors become worn and damaged with regularity. By replacing the pin connector with non-contact inductive couplers, the two sensor signals are maintained but the maintenance of these connections is reduced dramatically. An additional “in-zone signal” verifies that the gripper is installed and connected. This provides assurance during operation.

Inductive couplers offer IO-Link functionality

Inductive coupling with IO-Link technology adds more benefits besides replacing the pin coupling. It allows users to transfer up to 32 bytes of data in addition to power for actuation or sensors. If you connect IO-Link enabled I/O hubs or valve connectors to the remote side, you can also store identification data on the IO-Link hub or valve. When the connection is established, the controller can request the identification data from the tool to ensure that the system is utilizing the correct tool for the upcoming process.

With pin based coupling you needed up to 4-5 seconds to first engage the tool and to mate the two ends of the pin couplers and then request the identification. With inductive couplers, the base only needs to be brought closer to the remote so that you quickly couple and identify the tool before engaging the tool — this takes less than a second. Additionally the base and remote do not need to be well aligned to couple. Misalignment up to 15-20 degrees of angular offset or 2-4 mm of axial offset still provides functionality.

The benefits at a glance

  • Power and signals transfer with pin-less connectivity
  • Reduced downtime due to rail or gripper repair
  • Know that the gripper is present and powered with in-zone signal
  • Inform the controller that the rail has power and connectivity to the sensors

To decide the right coupler for your next application visit www.balluff.com.

 

Technological Alternative to Fiber Optics

Photoelectric applications with space restrictions, small part detection, high temperatures, or aggressive harsh environments may be solved using fiber optic sensors. These sensors allow the electronics to be mounted out of harm’s way while at the same time focusing the light beam on a small target. The sensing tips can be manufactured in a wide variety of housings for unique mounting requirements.

Fiber optic sensors require two components: a remote mounted amplifier, and the fiber optic cable(s). The amplifiers can be basic, with few features, or advanced with many configurable options and digital displays. The fiber optic cables are made of either plastic or glass fibers, each with advantages and application specific solutions.

Many applications, primarily those in the medical Technological Alternative to Fiber Optics 1sciences and semiconductor industries, cannot be solved with fiber optic or miniature photoelectric sensors because they are physically too large to fit in the instruments. Additionally, the cables are typically not flexible enough to be routed through the instruments.  Today, highly flexible and miniature sensors are are being incorporated in other industries due to today’s demands of smaller machines and tools.

MICROmote® sensors are miniaturized photoelectric Technological Alternative to Fiber Optics 2sensors with separate amplifiers that are also available with a variety of functionalities. Their highly flexible, electric sensor cables make them a genuine technical alternative to conventional fiber optics. The photoelectric sensor heads have extraordinarily small dimensions, excellent technical characteristics, and outstanding flexibility for application-specific solutions.

Similar to fiber optic sensors, these micro-optic photoelectric sensors function as either a through-beam or diffuse type sensor with comparable sensing ranges. Unlike fibers, the wired sensing heads are inherently bifurcated type cables so that there is only one connection to the amplifier.

Unlike conventional fiber optic cables,Technological Alternative to Fiber Optics 3 there are no significant coupling losses, minimum bending radius and cyclic bending stresses.  The patented precision elements produce extremely small beam angles with sharply defined light spots unlike standard fiber optics where the beam angle is a function of the fiber geometry.  Additional lenses must be used if the light beam of a fiber optic cable must be focused which adds to the costs.

MICROmote® photoelectric sensors for water detection use a specific wavelength at which water absorbs more light. This significantly simplifies the detection of liquids with high water content using optical sensors. The combination of an ultra-compact design and powerful micro-optics allows for reliable use in capillary tubes where other sensing devices are stretched to their limits.

These sensors can also be used as precision tube Technological Alternative to Fiber Optics 4sensors for detecting bubbles through use of either light refraction or attenuation through the air, or liquid column within the tube. They provide excellent detection for even the smallest air-to-liquid transitions and are reliable for all liquid types, even clear liquids.

In addition, these sensorsTechnological Alternative to Fiber Optics 5 are designed to detect free-floating microbubbles in transparent liquids. Microbubbles refer to little gas bubbles with dimensions smaller than the inside diameter of the tube. Uniform lighting is achieved in the liquid column by using a concentrated arrangement of multiple light beams with very uniform intensity distribution. Gas bubbles that move through this field induce a signal jump in the built-in photoelectric receiver elements

For more information on this technological alternative to fiber optics visit www.balluff.com.