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.

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.

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.

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.

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.

BMFvsVTwin

For more information visit www.balluff.us.

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.

BMF_Grippers
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.

To learn more about magnetic field sensors you can 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.