When we think about inductive sensors we automatically refer to discrete output offerings that detect the presence of ferrous materials. This can be a production part or an integrated part of the machine to simply determine position. Inductive sensors have been around for a long time, and there will always be a need for them in automated assembly lines, weld cells and stamping presses.
We often come across applications where we need an analog output at short range that needs to detect ferrous materials. This is an ideal application for an analog inductive proximity sensor that can offer an analog voltage or analog current output. This can reliably measure or error proof different product features such as varying shapes and sizes. Analog inductive sensors are pure analog devices that maintain a very good resolution with a high repeat accuracy. Similar to standard inductive sensors, they deal very well with vibration, commonly found in robust applications. Analog inductive proximity sensors are also offered in many form factors from M12-M30 tubular housings, rectangular block style and flat housings. They can also be selected to have flush or non-flush mounting features to accommodate specific operating distances needed in various applications.
For more specific information on analog inductive sensors visit www.balluff.com.
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.
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.
If a workpiece or similar metallic objects should 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.
If you are trying to detect non-metal objects, for example, the height of paper stacks, then 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.
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.
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.
I recently visited a customer that has a large amount of assembly lines where they have several machine builders manufacturing assembly process lines to their specification. This assembly plant has three different business units and unfortunately, they do not communicate very well with each other. Digging deeper into their error proofing solutions, we found an enormous amount of sensors and cables that could perform the same function, however they mandated different part numbers. This situation was making it very difficult for maintenance employees and machine operators to select the best sensor for the application at hand due to redundancy with their sensor inventory.
The customer had four different types of M08 Inductive Proximity sensors that all had the same operating specifications with different mechanical specifications. For example, one sensor had a 2mm shorter housing than one of the others in inventory. These 2mm would hardly have an effect when installed into an application 99% of the time. The customer also had other business units using NPN output polarity VS PNP polarity making it even more difficult to select the correct sensor and in some situations adding even more downtime when the employee tried to replace an NPN sensor where a PNP offering was needed. As we all know, the NPN sensor looks identical to the PNP offering just by looking at it. One would have to really understand the part number breakdown when selecting the sensor, and when a machine is down this sometimes can be overlooked. This is why it is so important to standardize on sensor selection when possible. This will result in more organized inventory by reducing part numbers, reducing efforts from purchasing and more importantly offering less confusion for the maintenance personel that keep production running.
Below are five examples of M08 Inductive sensors that all have the same operating specifications. You will notice the difference in housing lengths and connection types. You can see that there can be some confusion when selecting the best one for a broad range of application areas. For example, the housing lengths are just a few millimeters different. You can clearly see that one or two of these offerings could be installed into 99% of the application areas where M08 sensors are needed for machine or part position or simply error proofing a process.
For more information on standardizing your sensor selection visit www.balluff.com
Every time I enter tier 1 and tier 2 suppliers, there seems to be a common theme of extreme sensor and cable abuse. It is not uncommon to see a box or bin of damaged sensors along with connection cables that have extreme burn-through due to extreme heat usually generated by weld spatter. This abuse is going to happen and is unavoidable in most cases. The only option to combat these hostile environments is to select the correct components, such as bunker blocks, protective mounts, and high temperature cable materials that can withstand hot welding applications.
In many cases I have seen standard sensors and cables installed in a weld cell with essentially zero protection of the sensor. This results in a very non-productive application that simply cannot meet production demands due to excessive downtime. At the root of this downtime you will typically find sensor and cable failure. These problems can only go on for so long before a culture change must happen throughout a manufacturing or production plant as there is too much overtime resulting in added cost and less efficiency. I call this the “pay me now or pay me later” analogy.
Below are some simple yet effective ways to improve sensor and cable life:
Walk into any die shop in the US and nine out of ten times, we discover diffuse reflective sensors being used to detect a large part or a small part exiting a die. Many people have success using this methodology, but lubrication-covered tumbling parts can create challenges for diffuse-reflective photoelectric sensing devices for many reasons:
Tumbling parts with many “openings” on the part itself can cause a miss-detected component.
Overly-reflective parts can false triggering of the output.
Dark segments of the exiting part can cause light absorption. Remember, a diffuse sensors sensing distance is based on reflectivity. Black or dark targets tend to absorb light and not reflect light back to the receiver.
Die lube/misting can often fog over a photoelectric lens requiring maintenance or machine down time.
The solution: Super Long Range Inductive Sensors placed under chutes
Most metal forming personnel are very familiar with smaller versions of inductive proximity sensors in tubular sizes ranging from 3mm through 30mm in diameter and with square or “block style” inductive types (flat packs, “pancake types”, etc.) but it is surprising how many people are just now discovering “Super Long Range Inductive Proximity” types. Super Long Range Inductive Proximity Sensors have been used in metal detection applications for many years including Body-In-White Automotive applications, various segments of steel processing and manufacturing, the canning industry, and conveyance.
Benefits of Using A UHMW Chute + Super Long Range Inductive Proximity Sensor in Part Exit/Part-Out Applications:
It is stronger and quieter than parts flowing over a metal chute, readily available in standard and custom widths, lengths and thicknesses to fit the needs of large and small part stampers everywhere.
UHMW is reported to be 3X stronger than carbon steel.
UHMW is resistant to die lubes.
UHMW allows Super Long Range Inductive Proximity Sensors to be placed underneath and to be “tuned” to fit the exact zone dimension required to detect any part exiting the die (fixed ranges and tunable with a potentiometer). The sensing device is also always out of harm’s way.
Provides an option for part detection in exiting applications that eliminates potential problems experienced in certain metal forming applications where photoelectric sensing solutions aren’t performing optimally.
Not every Part Exit/Part-Out application is the same and not every die, stamping application, vintage of equipment, budget for sensing programs are the same. But it’s important to remember in the world of stamping, to try as consistently as possible to think application specificity when using sensors.That is, putting the right sensing system in the right place to get the job done and to have as many technical options available as possible to solve application needs in your own “real world” metal forming operation. We believe the UHMW + Super Long Range Inductive System is such an option.
When looking at a data sheet for an inductive proximity sensor, there are usually several different specifications listed with regard to the switching distance (or operating distance). Which of these various specifications really matter to someone trying to use a prox sensor in a real-world application? How can a specifier or user decide which sensor is going to work best in their situation?
Fortunately, there is an international standard that defines sensor switching distances and spells out test methods to assure that sensor specifications from product to product and even manufacturer to manufacturer can be directly compared “apples to apples.”
This standard is IEC 60947-5-2Low voltage switchgear and controlgear – Part 5-2: Control circuit devices and switching elements – Proximity switches.
Operating (switching) distance s
In the diagram shown here, the letter “s” refers to a given sensor specimen’s actual switching distance when tested. It is defined as the distance (between the standard target and the sensing face of the proximity switch) at which a signal change is generated. For a normally open sensor, the target approaches the sensor axially, that is, the sensor approaches the active surface from the front (not the side). There are several subscripts used to describe different aspects of a sensor’s switching behavior.
Rated operating distance sn
… is the nominal switching distance of the sensor. It is simply used as a standard reference value. The rated operating distance is the best figure to use when comparing different sensor models to get an idea of their essential sensing distance capabilities.
Effective operating distance sr
…is the range of actual switching distances that any given proximity sensor will fall into when measured under specified conditions of mounting, temperature, and supply voltage. For well-designed and manufactured sensors, the sensor will be triggered between 90% and 110% of the rated operating distance. For example, various samples of a proximity sensor model with a rated operating distance (sn) of 8mm may deliver switch-on points anywhere between 7.2mm and 8.8mm.
Usable operating distance su
…takes into account the effects of the sensor’s full ambient temperature range (low to high) and variation of the supply voltage from 85% to 110% of the nominal voltage rating. The IEC standard requires the usable operating distance (su) to be between 90% and 110% of the effective operating distance (sr). For our example of a sensor with a rated operating distance (sn) of 8mm, the usable operating distance would fall between 6.5mm and 8.8mm. Pop quiz: why is the max of usable operating distance not 9.7mm (sr of 8.8mm * 110%)? Answer: the usable operating distance can always be less than but can never be greater than the maximum effective operating distance.
Assured operating distance sa
This is the distance of the target to the sensor where the sensor can be guaranteed to have turned on. If a target approaches within the assured operating distance, you can be confident that the sensor will detect it. It is 90% of sr which is in turn 90% of sn, which is in effect 81% of sn. Going back to our example of a sensor with a rated operating distance (sn) of 8mm, sa would be 81% * 8mm = 6.5mm. So in essence, sa = su(min).
Differential travel H
Now when the target recedes, at what distance will the sensor switch off? All good-quality sensors have a built-in property called hysteresis, which means that the sensor will turn off when the target is further away from the sensor than the point where it turns on. This is necessary to prevent chattering and instability when the target approaches the sensor. We want the sensor to turn on and stay on, even if the target might be vibrating as it crosses the threshold of detection. For most sensors, it is defined as ≤ 20% of the effective operating distance sr. The differential travel is added to the value of sr to define the switch-off point.
In practice, for any group of sensors, the minimum value of H would be zero and the maximum value would be sr(max) + 20% of sr(max). For our example of a sensor with a rated operating distance (sn) of 8mm, 7.2mm ≤ sr ≤ 8.8mm. So, the range of switch-off points would be 7.2mm ≤ sr+H ≤ 10.6mm. It might sound like a large range, but for any given sensor specimen the switch-off point is never greater than 20% of that particular sensor’s switch-on point.
The good news is that you don’t have to conduct sensor tests yourself or go through all of these calculations manually to determine a sensor’s performance envelope. The sensor manufacturer provides all of these useful figures pre-calculated for you in the sensor data sheet.
Learn more about the basics of the most popular automation sensor here.
Applications where sensor contact is unavoidable are some of the most challenging to solve. Metal forming processes involving over travel can also damage or even destroy a sensor causing failure and expensive unplanned downtime. Manufacturers often try to remedy this with in-house manufactured spring loaded out-feed mechanisms but those are expensive to make by experienced tool and die personnel who have more important things to do . Over the years, I’ve seen this as a pervasive problem in the stamping industry. Many of these issues can be solved with the use of a simple yet effective sensor actuator system known as a plunger probe.
Plunger probe solves a few key issues in Progressive stamping:
The flexible trigger/actuation point is fully adjustable to meet sensitive or less sensitive activation points, not possible with “fixed” systems with substantial “over travel” built into the design.
It is fully self-contained (minimizing any risk of sensor damage and resulting unplanned machine down time).
The device can be disassembled and rapidly cleaned, reassembled, and placed back in service in the event that die lube or other industrial fluids enter the M18 body that can potentially congeal during shut down periods.
Integrating sensors in washdown applications can be confusing when considering the various approvals. So what do they all mean? If a sensor is an IP69K rated sensor does that mean it will survive everything? In the world of sensors there is IP54, IP67, IP68 and IP69 so if my sensor is IP69K that means it is the best right? The short answer is no. Let’s take a brief look at the differences.
IP ratings will generally have two digits with the first digit referring to the solid particle protection. The second digit indicates the level of protection against the ingress of water.
Sensors rated for IP54 indicates they are dust protected, meaning that dust can get inside the sensor, however, it cannot be enough to interfere with the operation of the equipment – this is designated by the 5. The 4 indicates that the sensor withstands splashing water on the housing from any direction with no detrimental effect. The test for the splashing of water lasts at least five minutes with a water volume of 2.64 gallons per minute with a pressure of 7.25 to 21.76 PSI.
IP67 rated sensors are the most commonly used sensors on the market. Even most electrical enclosures used in automation are IP67 rated. The 6 indicates these devices will not allow the entry of dust. The 7 indicates that the sensor can be immersed in water to a depth of 1 meter for 30 minutes.
IP68 rated sensors are dust tight sensors that can be immersed in water continuously under conditions specified by the manufacturer. Typically the depth of the immersion is 3 meters.
The IP69K rating is based on a dust tight sensor that can withstand high pressure sprays. The devices are sprayed with a pressure of 1,160 to 1,450 PSI. The water temperature can be as high as 176°F with a flow rate of 3.7 to 4.2 gallons per minute. The distance from the nozzle to the device is 4 to 6 inches. The sensor is placed on a rotary table that rotates at 5 revolutions per minute and the sensor is sprayed for 30 seconds at four angles 0°, 30°, 60°, and 90°.
The ultimate sensor would have a rating of IP67/IP68/IP69 indicating that it will survive submersion and high pressure washdown. Also, some of these sensors are 316L stainless meaning they have low carbon content and are more corrosion resistant than other stainless steel grades. Are all IP69K sensors stainless steel? No, some sensors utilize polycarbonate-ABS thermoplastic.
Usually during washdown applications in the food and beverage industry the spray is not just water but some sort of cleaning chemical or disinfectant. These aggressive cleaning and disinfecting agents can attack different housing materials. This is addressed by the ECOLAB certification.
The ECOLAB test consists of testing the housing and sensor materials to exposure to these aggressive cleaning and disinfecting agents. The devices are tested for 14 to 28 days at a room temperature of 68° F. During this time the sensor is visually inspected for swelling, embrittlement, or changes in color.
Don’t forget that even though the sensor has the correct IP rating for your application that the mating connector has to meet the same specifications. For example, if the sensor is IP69K rated and a IP67 mating cable is used then the lower IP rating has precedence.
If you are interested in what sensors and cables meet washdown requirements, please visit www.balluff.us.
The 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)
The 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.
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.
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.