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.

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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.

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.


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Reed Switches vs. Magnetoresistive Sensors (GMR)

In a previous post we took a look at magnetic field sensors vs inductive proximity sensors for robot grippers. In this post I am going to dive a little deeper into magnetic field sensors and compare two technologies: reed switches, and magnetoresistive sensors (GMR).

Reed Switches

PrintThe simplest magnetic field sensor is the reed switch. This device consists of two flattened ferromagnetic nickel and iron reed elements, enclosed in a hermetically sealed glass tube. As an axially aligned magnet approaches, the reed elements attract the magnetic flux lines and draw together by magnetic force, thus completing an electrical circuit.

While there are a few advantages of this technology like low cost and high noise immunity, those can be outweighed by the numerous disadvantages. These switches can be slow, are prone to failure, and are sensitive to vibration. Additionally, they react only to axially magnetized magnets and require high magnet strength.

Magnetoresistive Sensors (GMR)

PrintThe latest magnetic field sensing technology is called giant magnetoresistive (GMR). Compared to Reed Switches GMR sensors have a more robust reaction to the presence of a magnetic field due to their high sensitivity, less physical chip material is required to construct a practical GMR magnetic field sensor, so GMR sensors can be packaged in much smaller housings for applications such as short stroke cylinders.

GMR sensors have quite a few advantages over reed switches. GMR sensors react to both axially and radially magnetized magnets and also require low magnetic strength. Along with their smaller physical size, these sensors also have superior noise immunity, are vibration resistant. GMR sensors also offer protection against overload, reverse polarity, and short circuiting.

Basic Sensors for Robot Grippers

Robot gripper with inductive proximity sensors mounted
Robot gripper with inductive proximity sensors mounted

Typically when we talk about end-of-arm tooling we are discussing how to make robot grippers smarter and more efficient. We addressed this topic in a previous blog post, 5 Tips on Making End-of-Arm Tooling Smarter. In this post, though, we are going to get back to the basics and talk about two options for robot grippers: magnetic field sensors, and inductive proximity sensors.

One of the basic differences is that detection method that each solution utilizes. Magnetic field sensors use an indirect method by monitoring the mechanism that moves the jaws, not the jaws themselves. Magnetic field sensors sense magnets internally mounted on the gripper mechanism to indicate the open or closed position. On the other hand, inductive proximity sensors use a direct method that monitors the jaws by detecting targets placed directly in the jaws. Proximity sensors sense tabs on moving the gripper jaw mechanism to indicate a fully open or closed position.

Robot gripper with magnetic field sensors mounted

Additionally, each solution offers its own advantages and disadvantages. Magnetic field sensors, for example, install directly into extruded slots on the outside of the cylinder, can detect an extremely short piston stroke, and offer wear-free position detection. On the other side of the coin, the disadvantages of magnetic field sensors for this application are the necessity of a magnet to be installed in the piston which also requires that the cylinder walls not be magnetic. Inductive proximity sensors allow the cylinder to be made of any material and do not require magnets to be installed. However, proximity sensors do require more installation space, longer setup time, and have other variables to consider.

Magnetic Field Sensors – Not Just For Cylinder Applications

When thinking of magnetic field sensors the first form factors that come to mind are C or T slot style sensors designed to fit into specific cylinders. These popular types of magnetic field sensors are used to sense through the aluminum body of a cylinder and detect a magnet inside the housing (cylinder wall) of the cylinder. This is a very reliable sensor type that simply detects the extended or retracted position of a cylinder.

BMF Application2But did you know you can achieve a wide range of applications when using tubular style magnetic field sensors? These types of magnetic field sensors typically come in tubular sizes that range from 6.5mm – M12x1 and can be used with various size magnets to cover several application specifications. These offerings offer precise reliable switch points, robust housings for harsh applications and they are also short circuit protection. For example, if a target is too far away for a traditional inductive proximity sensor or maybe too reflective for a photoelectric sensor, a tubular magnetic field sensor and a mating magnet can reliably sense that magnetic field from 90 mm away! Great distance, switching frequency at 10k Hz, and with a small mounting footprint!

BMF ApplicationApplications include pallet detection, high speed impeller, gear, cog detection, many more in a wide range of industry disciplines.

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5 Tips on Making End-of-Arm Tooling Smarter

Example of a Flexible EOA Tool with 8 sensors connected with an Inductive Coupling System.

Over the years I’ve interviewed many customers regarding End-Of-Arm (EOA) tooling. Most of the improvements revolve around making the EOA tooling smarter. Smarter tools mean more reliability, faster change out and more in-tool error proofing.

#5: Go Analog…in flexible manufacturing environments, discrete information just does not provide an adequate solution. Analog sensors can change set points based on the product currently being manufactured.

#4: Lose the weight…look at the connectors and cables. M8 and M5 connectorized sensors and cables are readily available. Use field installable connectors to help keep cable runs as short as possible. We see too many long cables simply bundled up.

#3: Go Small…use miniature, precision sensors that do not require separate amplifiers. These miniature sensors not only cut down on size but also have increased precision. With these sensors, you’ll know if a part is not completely seated in the gripper.

#2: Monitor those pneumatic cylinders…monitoring air pressure in one way, but as speeds increase and size is reduced, you really need to know cylinder end of travel position. The best technology for EOA tooling is magnetoresistive such as Balluff’s BMF line. Avoid hall-effects and definitely avoid reed switches. Also, consider dual sensor styles such as Balluff’s V-Twin line.

#1: Go with Couplers…with interchangeable tooling, sensors should be connected with a solid-state, inductive coupling system such as Balluff’s Inductive Coupler (BIC). Avoid the use of pin-based connector systems for low power sensors. They create reliability problems over time.

Reliable Sensors for Reliable Process Quality

Here’s a real-world application where the reliability of the sensors is directly related to the reliability of the process in producing quality results.

Pictured below is a pneumatic actuator for a vacuum valve.  Inside the actuator, a magnetic ring is installed around the moving piston by the manufacturer of the actuator (this is an option that must be requested when placing the order for the actuator).  The magnet acts as a target to activate the sensors as it moves under them during operation.  It’s important to note that the wall material of the actuator must be non-magnetic in order for this concept to work properly; typically aluminum or non-magnetic stainless steel is used.

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