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Magnetic Field Positioning Systems for Reliable, Accurate and Repeatable Absolute Position Feedback

Magnetic field positioning systems are increasingly popular due to their ability to provide reliable, accurate, and repeatable absolute position feedback.

These systems use magnetic field sensors to get a larger range of feedback across a pneumatic cylinder – a great alternative to traditional cylinder prox switches that may not work well in certain applications. They also allow for continuous monitoring of piston position in tight spaces, providing feedback in the form of analog voltage, current output, and IO-Link interface. And in many cases, these systems can replace the need for a linear transducer, making them a cost-effective solution for many industries.

One of the key benefits of magnetic field positioning systems is their versatility. They can be used in a wide range of industrial applications, such as:

    • Ultrasonic welding to validate set height with position feedback
    • Nut welding to verify set height with position feedback
    • Dispensing
    • Gripping for position feedback for different parts
    • Liner position indicators

While using these sensors greatly improves productivity in areas where prox sensors cannot provide the reliability needed, when selecting the magnetic field position system, it is important to consider the application requirements. The accuracy and feedback speed, for example, may vary depending on the application.

Magnetic field position systems are also available in different lengths. If the standard length does not meet requirements, you can choose a non-contact type that can be mounted on a slide with a magnetic trigger.

Overall, magnetic field positioning systems are an excellent choice for any industry that requires reliable, accurate, and repeatable absolution position feedback. With their versatility and flexibility, they are sure to improve productivity and efficiency in a wide range of applications.

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Sensing Ferrous and Non-Ferrous Metals: Enhancing Material Differentiation

Detecting metallic (ferrous) objects is a common application in many industries, including manufacturing, automotive, and aerospace. Inductive sensors are a popular choice for detecting metallic objects because they are reliable, durable, and cost-effective. Detecting a metallic object, however, is not always as simple as it seems, especially if you need to differentiate between two metallic objects. In such cases, it is crucial to understand the properties of the metals you are trying to detect, including whether they are ferrous or non-ferrous.

Ferrous vs. non-ferrous

Ferrous metals, such as mild steel, carbon steel, stainless steel, cast iron, and wrought iron, contain iron. They are typically magnetic, heavier, and more likely to corrode than non-ferrous metals, which do not contain iron. Aluminum, copper, lead, zinc, nickel, titanium, and cobalt are examples of non-ferrous metals. They are typically nonmagnetic, lightweight, and less likely to corrode.

Sensing ferrous and non-ferrous metal:

When it comes to detecting ferrous and non-ferrous metals using inductive sensors, the reduction factor plays a crucial role. The reduction factor is the ratio of the sensor’s effective sensing distance for a given metal to the sensor’s effective sensing distance for steel. In other words, it is the degree to which a metal affects the sensing range of an inductive sensor. Ferrous metals typically have less of an effect on sensing range than non-ferrous metals because inductive sensors function based on the law of induction, and magnetic metals are more likely to interact with the magnetic field created by the sensor.

The reduction factor for each type of metal varies depending on the metal’s properties. Ferrous metals typically have a higher reduction factor than non-ferrous metals, which means they can be detected from a greater distance. For example, both steel and stainless steel have a reduction factor of 0.6 to 1, which means they can be detected from the full switching distance of the sensor of 4 mm. In contrast, non-ferrous metals, such as aluminum, copper, and brass, have a lower reduction factor of 0.25 to 0.5, which means they can only be detected from a fraction of the operating switching distance, typically 1 to 2 mm.

Understanding the reduction factor for each metal allows you to answer the question of what happens when you need to differentiate between two metallic parts. If one metal is ferrous and the other is non-ferrous, then you can place the sensor at a distance that will detect the ferrous metal but not the non-ferrous metal. However, this may not be an efficient solution if the metals have similar reduction factors, or if you need to detect the non-ferrous metal over the ferrous metal.

Using ferrous-only or non-ferrous-only sensors

The better solution is to use a ferrous-only or non-ferrous-only sensor. These sensors are specifically designed to detect only one type of metal and ignore the other type, resulting in a reduction factor of zero. Ferrous-only sensors detect only ferrous metals, and their reduction factors range from 0.1 to 1 for steel and stainless steel, while the reduction factors for non-ferrous metals such as aluminum, copper, and brass are zero. Non-ferrous-only sensors detect only non-ferrous metals, and their reduction factors range from 0.9 to 1.1 for aluminum, copper, and brass, while the reduction factors for ferrous metals are zero. Using ferrous-only or non-ferrous-only sensors eliminates the need to adjust the mounting distance of a standard inductive sensor to detect a ferrous metal over a non-ferrous metal.

Overall, selecting the right sensor for your application depends on the type of metals you need to differentiate and detect. If you are dealing with ferrous and non-ferrous metals, you can use a standard inductive sensor, but you need to be aware of the reduction factor for each metal type and adjust the mounting distance accordingly. If you need to detect only one type of metal, however, a ferrous-only or a non-ferrous-only sensor is the better option. These sensors are specially designed to ignore the other metal type, eliminating the need to adjust the mounting distance.

By understanding the differences between ferrous and non-ferrous metals and the capabilities of different sensors, you can optimize the metal detection system for maximum efficiency and accuracy.

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Inductive Sensors and Their Unlimited Uses in Automation

Inductive sensors (also known as proximity sensors or proxes) are the most commonly used sensors in mechanical engineering and industrial automation. When they were invented in the 1960s, they marked a milestone in the development of control systems. In a nutshell, they generate an electromagnetic field that reacts to metal targets that approach the sensor head. They even work in harsh environments and can solve versatile applications.

There are hardly any industrial machines that work without inductive sensors. So, what can be solved with one, two, three, or more of them?

What can you do with one inductive sensor?

Inductive sensors are often used to detect an end position. This could be in a machine for end-of-travel detection, but also in a hydraulic cylinder or a linear direct drive as an end-of-stroke sensor. In machine control, they detect many positions and trigger other events. Another application is speed monitoring with a tooth wheel.

What can you do with two inductive sensors?

By just adding one more sensor you can get the direction of rotational motion and take the place of a more expensive encoder. In a case where you have a start and end position, this can also be solved with a second inductive sensor.

What can you do with three inductive sensors?

In case of the tooth wheel application, the third sensor can provide a reference signal and the solution turns into a multiturn rotary encoder.

What can I do with four inductive sensors and more?

For multi-point positioning, it may make sense to switch to a measurement solution, which can also be inductive. Beyond that, an array of inductive sensors can solve identification applications: In an array of 2 by 2 sensors, there are already 16 different unique combinations of holes in a hole plate. In an array of 3 by 3, it would be 512 combinations.

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Weld Immune vs. Weld Field Immune: What’s the difference? 

In today’s automotive plants and their tier suppliers, the weld cell is known to be one of the most hostile environments for sensors. Weld slag accumulation, elevated ambient temperatures, impacts by moving parts, and strong electromagnetic fields can all degrade sensor performance and cause false triggering. It is widely accepted that sensors will have a limited life span in most plants.

Poor sensor selection does mean higher failure rates which cause welders in all industries increased downtime, unnecessary maintenance, lost profits, and delayed delivery. There are many sensor features designed specifically to withstand these harsh welding environments and the problems that come along with them to combat this.

In the search for a suitable sensor for your welding application, you are sure to come across the terms weld immune and weld field immune. What do these words mean? Are they the same thing? And will they last in my weld cell?

Weld Immune ≠ Weld Field Immune

At first glance, it is easy to understand why someone may confuse these two terms or assume they are one and the same.

Weld field immune is a specific term referring to sensors designed to withstand strong electromagnetic fields. In some welding areas, especially very close to the weld gun, welders can generate strong magnetic fields. When this magnetic field is present, it can cause a standard sensor to perform intermittently, like flickering and false outputs.

Weld field immune sensors have special filtering and robust circuitry that withstand the influence of strong magnetic fields and avoid false triggers. This is also called magnetic field immune since they also perform well in any area with high magnetic noise.

On the other hand, weld immune is a broad term used to describe a sensor designed with any features that increase its performance in a welding application. It could refer to one or multiple sensor features, including:

    • Weld spatter resistant coatings
    • High-temperature resistance
    • Different housing or sensor face materials
    • Magnetic field immunity

A weld field immune sensor might be listed with the numerous weld immune sensors with special coatings and features, but that does not necessarily mean any of those other sensors are immune to weld fields. This is why it is always important to check the individual sensor specifications to ensure it is suitable for your application.

In an application where a sensor is failing due to impact damage or weld slag spatter, a steel face sensor with a weld resistant coating could be a great solution. If this sensor isn’t close to the weld gun and isn’t exposed to any strong magnetic fields, there is really no need for it to be weld field immune. The important features are the steel face and coating that can protect it against impact and weld slag sticking to it. This sensor would be classified as weld immune.

In another application where a sensor near the weld gun side of the welding procedure where MIG welding is performed, this location is subject to arc blow that can create a strong magnetic field at the weld wire tip location. In this situation, having a weld field immune sensor would be important to avoid false triggers that the magnetic field may cause. Additionally, being close to a MIG weld gun, it would also be wise to consider a sensor with other weld immune properties, like a weld slag resistant coating and a thermal barrier, to protect against high heat and weld slag.

Weld field immunity is just one of many features you can select when picking the best sensor for your application. Whether the issue is weld slag accumulation, elevated ambient temperatures, part impact, or strong electromagnetic fields, there are many weld immune solutions to consider. Check the placement and conditions of the sensors you’re using to decide which weld-immune features are needed for each sensor.

Click here for more on choosing the right sensor for your welding application.

 

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Back to the Basics: Object Detection

In the last post about the Basics of Automation, we discussed how humans act as a paradigm for automation. Now, let’s take a closer look at how objects can be detected, collected and positioned with the help of sensors.

Sensors can detect various materials such as metals, non-metals, solids and liquids, all completely without contact. You can use magnetic fields, light and sound to do this. The type of material you are trying to detect will determine the type of sensor technology that you will use.

Object Detection 1

Types of Sensors

  • Inductive sensors for detecting any metallic object at close range
  • Capacitive sensors for detecting the presence of level of almost any material and liquid at close range
  • Photoelectric sensors such as diffuse, retro-reflective or through-beam detect virtually any object over greater distances
  • Ultrasonic sensors for detecting virtually any object over greater distances

Different Sensors for Different Applications

The different types of sensors used will depend on the type of application. For example, you will use different sensors for metal detection, non-metal detection, magnet detection, and level detection.

Detecting Metals

If a workpiece or similar metallic objects Object Detection 2should be detected, then an inductive sensor is the best solution. Inductive sensors easily detect workpiece carriers at close range. If a workpiece is missing it will be reliably detected. Photoelectric sensors detect small objects, for example, steel springs as they are brought in for processing. Thus ensures a correct installation and assists in process continuity. These sensors also stand out with their long ranges.

Detecting Non-Metals

If you are trying to detect non-metal objects, for example, the height of paper stacks, Object Detection 3then capacitive sensors are the right choice. They will ensure that the printing process runs smoothly and they prevent transport backups. If you are checking the presence of photovoltaic cells or similar objects as they are brought in for processing, then photoelectic sensors would be the correct choice for the application.

Detecting Magnets

Object Detection 4

To make sure that blister packs are exactly positioned in boxes or that improperly packaged matches are sorted out, a magnetic field sensor is needed which is integrated into the slot. It detects the opening condition of a gripper, or the position of a pneumatic ejector.

 

Level Detection

What if you need to detect the level of granulate in containers? Then the solution is to use capacitive sensors. To accomplish this, two sensors are attached in the containers, offset from each other. A signal is generated when the minimum or maximum level is exceeded. This prevents over-filling or the level falling below a set amount. However, if you would like to detect the precise fill height of a tank without contact, then the solution would be to use an ultrasonic sensor.

Stay tuned for future posts that will cover the essentials of automation. To learn more about the Basics of Automation in the meantime, visit www.balluff.com.

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Precision Pneumatic Cylinder Sensing

When referring to pneumatic cylinders, we are seeing a need for reduced cylinder and sensor sizes. This is becoming a requirement in many medical, semiconductor, packaging, and machine tool applications due to space constraints and where low mass is needed throughout the assembly process.

These miniature cylinder applications are typically implemented into light-to-medium duty applications with lower air pressures with the main focus being precision sensing Image 2with maximum repeatability. For example, in many semiconductor applications, the details
and tolerances are much tighter and more controlled than say, a muffler manufacturer that uses much more robust equipment with slower cycle times. In some cases, manufacturing facilities will have several smaller sub-assemblies that feed into the main assembly line. These sub-assemblies can have several miniature pneumatic cylinders as part of the process. Another key advantage miniature cylinders offer is quieter operation due to lower air pressures, making the work place much safer for the machine operators and maintenance technicians. With projected growth in medical and semiconductor markets, there will certainly be a major need for miniature assembly processes including cylinders, solenoids, and actuators used with miniature sensors.

One commonality with miniature cylinders is they require the reliable wear-free position detection available from magnetic field sensors. These sensors are miniature in size, however Image 1offer the same reliable technology as the full-size sensors commonly used in larger assemblies. Miniature magnetic field sensors play a key role as speed, precision, and weight all come into play. The sensors are integrated into these small assemblies with the same importance as the cylinder itself. Highly accurate switching points with high precision and high repeatability are mandatory requirements for such assembly processes.

To learn more about miniature magnetic field sensors visit www.balluff.com.

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Importance of Directional Sensitivity in Magnetic Field Sensor Applications

Figure 1
Figure 1: Mounting of a standard T or C-slot magnetic field sensor

When using a T-slot or C-slot (Figure 1) magnetic field sensor to determine positioning in a pneumatic cylinder, the sensing face is oriented directly toward the magnet inside of the cylinder. But on the other side of the coin, how
susceptible is the sensor to magnetic interference of some outside source that may contact the sensor from other angles?

 

Figure 2
Figure 2: The angle between the AMR Bridge current and the applied magnetization will determine the quality of the sensor output signal.

The behavior of anisotropic magnetoresistive sensing devices can vary under certain conditions. Most critically, the magnetoresistive effect can be extremely angular dependent. The angle between the AMR Bridge current and the applied magnetization on the device determines how much the resistance will change. This is depicted in Figure 2, while Figure 3 shows a demonstration of how the output can change as the angle changes.

Figure 3
Figure 3: The change in resistance of the AMR Bridge shown as a function of the angle between magnetizing current and the magnetization.

When used in a standard application with only the sensor face looking at the magnet, this is not an issue as the AMR device is angled to allow for ideal operating conditions. But in the event that the device senses a magnetic field from someplace other than directly in front of it, double switching conditions and generally unpredictable behaviors can be seen.

At this point, the question becomes “how can we minimize the risk of the sensor’s susceptibility to unintended magnetic fields?” The answer to this comes in the directional sensitivity of the AMR Bridge. AMR devices can be either unidirectional, bidirectional, or omnidirectional.

The unidirectional sensor is designed to only be activated by one of the poles, and the output turns off when the sensor is removed.

Bidirectional sensors are activated by a pole like the unidirectional is, however the output must be turned off by using the opposing magnetic pole.

Lastly, the omnidirectional sensor is capable of being activated by either pole and turns off when the magnet is removed from the sensing zone.

Since the omnidirectional device is designed to be able to detect a magnetic field coming from multiple poles and directions, it has a much more consistent response when in an application that could be prone to encountering a magnetic field that isn’t directly in front of the sensing face.

There are a handful of factors that determine directional sensitivity of an AMR chip; however, the largest comes from the handling of the resistance bridge offset.

Figure 4
Figure 4: The transfer curve of the magnetic field vs. output voltage (resistance change across the bridge) shows an offset from the origin that must be accounted for. How this is dealt with plays a major role in determining directional sensitivity of AMR devices

The offset is simply the voltage difference when no magnetic field is present. This is a problem that arises due to the transfer characteristics (Figure 4) of the AMR sensor and is a common property on the datasheet of an AMR chip. This offset is usually handled within the AMR IC, which means that the directional sensitivity is pre-determined when you buy the chip. However, there are some AMR manufacturers that produce “adjustable offset” devices, that allow the user to determine the directional behavior.

While unidirectional and bidirectional devices have their place in certain applications, it remains clear that an omnidirectional sensor can have the most angular versatility, which is critical when there’s a possibility of magnetic fields surrounding the device. While many anisotropic magnetoresistive sensors do have built in stray field concentration, it is still a good idea to evaluate the needs for your application and make an informed decision in regards to directional sensitivity.

For more information visit www.balluff.us.

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Ensure Optimum Performance In Hostile Welding Cell Environments

The image above demonstrates the severity of weld cell hostilities.

Roughly four sensing-related processes occur in a welding cell with regards to parts that are to be joined by MIG, TIG and resistance welding by specialized robotic /automated equipment:

  1. Nesting…usually, inductive proximity sensors with special Weld Field Resistance properties and hopefully, heavy duty mechanical properties (coatings to resist weld debris accumulation, hardened faces to resist parts loading impact and well-guarded cabling) are used to validate the presence of properly seated or “nested” metal components to ensure perfectly assembled products for end customers.
  2. Poke-Yoke Sensing (Feature Validation)…tabs, holes, flanges and other essential details are generally confirmed by photoelectric, inductive proximity or electromechanical sensing devices.
  3. Pneumatic and Hydraulic cylinder clamping indication is vital for proper positioning before the welding occurs. Improper clamping before welding can lead to finished goods that are out of tolerance and ultimately leads to scrap, a costly item in an already profit-tight, volume dependent business.
  4. Several MIB’s covered in weld debris

    Connectivity…all peripheral sensing devices mentioned above are ultimately wired back to the controls architecture of the welding apparatus, by means of junction boxes, passive MIB’s (multiport interface boxes) or bus networked systems. It is important to mention that all of these components and more (valve banks, manifolds, etc.) and must be protected to ensure optimum performance against the extremely hostile rigors of the weld process.

Magnetoresistive (MR), and Giant Magnetoresistive (GMR) sensing technologies provide some very positive attributes in welding cell environments in that they provide exceptionally accurate switching points, have form factors that adapt to all popular “C” slot, “T” slot, band mount, tie rod, trapezoid and cylindrical pneumatic cylinder body shapes regardless of manufacturer. One model family combines two separate sensing elements tied to a common connector, eliminating one wire back to the host control. One or two separate cylinders can be controlled from one set if only one sensor is required for position sensing.

Cylinder and sensor under attack.

Unlike reed switches that are very inexpensive (up front purchase price; these generally come from cylinder manufacturers attached to their products) but are prone to premature failure.  Hall Effect switches are solid state, yet generally have their own set of weaknesses such as a tendency to drift over time and are generally not short circuit protected or reverse polarity protected, something to consider when a performance-oriented cylinder sensing device is desired.  VERY GOOD MR and GMR cylinder position sensors are guaranteed for lifetime performance, something of significance as well when unparalleled performance is expected in high production welding operations.

But!!!!! Yes, there is indeed a caveat in that aluminum bodied cylinders (they must be aluminum in order for its piston-attached magnet must permit magnetic gauss to pass through the non-ferrous cylinder body in order to be detected by the sensor to recognize position) are prone to weld hostility as well. And connection wires on ALL of these devices are prone to welding hostilities such as weld spatter (especially MIG or Resistance welding), heat, over flex, cable cuts made by sharp metal components and impact from direct parts impact. Some inexpensive, effective, off-the-shelf protective silicone cable cover tubing, self-fusing Weld Repel Wrap and silicone sheet material cut to fit particular protective needs go far in protecting all of these components and guarantees positive sensor performance, machine up-time and significantly reduces nuisance maintenance issues.

To learn more about high durability solutions visit www.balluff.com.

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Evolution of Magnetic Field Sensors

When I visit customers, often a few minutes into our conversation they indicate to me they “must decrease their manufacturing downtime.” We all know that an assembly line or weld cell that is not running is not making any money or meeting production cycle times. As we have the conversation regarding downtime, the customer always wants to know what new or improved products are available that can increase uptime or improve their current processes.

A major and common problem seen at the plant level is a high amount of magnetic field sensor failures. There are many common reasons for this, for example low-quality sensors being used such as Reed switches that rely on mechanical contact operation. Reed switches typically have a lower price point than a discrete solid state designs with AMR or GMR technologies, however these low-cost options will cost much more in the long run due to inconsistent trigger points and premature failure that results in machine downtime. Another big factor in sensor failure is the operating environment of the pneumatic cylinder. It is not uncommon to see a cylinder located in a very hostile area, resulting in sensor abuse and cable damage. In some cases, the failure is traceable to a cut cable or a cable that has been burned through from weld spatter.

Below are some key tips and questions that can be helpful when selecting a magnetic field sensor.

  • Do I need a T- or C-slot mounting type?
  • Do I need a slide-in or a drop-in style?
  • Do I need an NPN or PNP output?
  • Do I need an offering that has an upgraded cable for harsh environments, such as silicone tubing?
  • Do I need a dual-sensor combination that only has one cable to simplify cable connections?
  • Do I need digital output options like IO-Link that can provide multiple switch points and hysteresis adjustment?
  • Do I need a single teachable sensor that can read both extended and retracted cylinder position?

Magnetic field sensors have evolved over the years with improved internal technology that makes them much more reliable and user-friendly for a wide range of applications. For example, if the customer has magnetic field sensors installed in a weld cell, they would want to select a magnetic field sensor that has upgraded cable materials or perhaps a weld field immune type to avoid false signals caused by welding currents. Another example could be a pick and place application where the customer needs a sensor with multiple switch points or a hysteresis adjustment. In this case the customer could select a single head multiple setpoint teach-in sensor, offering the ability to fine tune the sensing behavior using IO-Link.

If the above tips are put into practice, you will surely have a better experience selecting the correct product for the application.

For more information on all the various types of magnetic field sensors click here.

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Simplifying wiring with sensors and Multiple Interface Blocks

When machine builders build assembly machines for their customers they want to keep the wiring as clean and clear as possible for an attractive machine but more importantly the ease of troubleshooting in the event of a failure. Simplifying connections with unnecessary cables and splitters not only makes it easier for the maintenance technicians to trouble shoot but it also saves the company money with unneeded product and components to inventory and maintain.

V-Twin sensor with one cable
V-Twin sensor with one cable

In the past it was common practice to wire sensors and cables all the way back into a terminal box located in sections of an assembly line. This could be very difficult to track down the exact sensor cable for repair and furthermore in some cases five meter cables or longer would be used to make the longer runs back to the terminal box. The terminal boxes would also get very crowded further complicating trouble shooting methods to get the assembly lines back up and running production. This is where Interface Blocks come in and can provide a much cleaner, effective way to manage sensor connections with significantly decreasing downtime.

For example: If our customer has a pneumatic cylinder that requires two sensors, one for the extended position and one for the retracted positon. The customer could run the sensor cables back to the Interface Block. This sometimes is used with a splitter to go into one port to provide the outputs for both sensors only using one port. Now we can take this a step further by using twin magnetic field sensors (V-Twin) with one connection cable. This example eliminates the splitter again eliminating an unneeded component. As you can see in the reference examples below this is a much cleaner and effective way to manage sensors and connections.

BMFvsVTwin

For more information visit www.balluff.us.