Analog Inductive Sensors Enable Easy Double Blank Detection in Stamping

Double sheet detection, also known as double blank detection, is an essential step in stamping quality control processes, as failure to do so can cause costly damage and downtime. Analog inductive sensors can deliver a cost-effective and easy way to add this step to stamping processes.

Most people have experienced on a smaller scale what happens when the office printer accidentally feeds two sheets of paper; the machine jams and the clog must be manually removed. Beyond the annoyance of not getting the printout right away, this typically doesn’t cause any significant issues to the equipment. In the stamping world, two sheets being fed into a machine can severely affect productivity and quality.

When two metal sheets stick together and are fed into a machine together, the additional thickness can damage the stamping dies and other equipment like the robot loaders, which can cause the production line to shut down for repairs. Even if the tool fares better and does not get damaged, the stamped product will likely be defective. In today’s highly competitive and just-in-time market, machine downtime and rejected shipments due to quality can be very costly.

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A simple solution to detect multiple sheets of metal is analog inductive sensing. This kind of sensor offers non-contact sensing with a 0…10V analog output, which can be used to determine when the thickness of the metallic material changes. As the material gets thicker, or as multiple sheets of metal stack on top of one another, the analog output from the sensor varies proportionally. These sensors can be used with ferrous or non-ferrous metals, but the operating range will be reduced for non-ferrous metals. As shown in the graph (Image 1), as the distance with the metallic target changes, the analog output increases from 0 to 10V.

 

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The pictures above, shows the technology in action. With a single sheet of aluminum, the output from the sensor is 2.946V, and for two sheets, the output is 5.67V. The user can establish these values as a reference for when there is more than one sheet of metal being fed into the machine and stop the equipment from attempting to process the material before it is damaged. These sensors can be placed perpendicular or inline with the target material and are offered in various form factors so they can be integrated into a wide range of applications.

 

 

 

Mobile Equipment Manufacturers: Is It Time to Make the Switch to Inductive Position Sensors?

Manufacturers of mobile equipment are tasked with the never-ending pursuit of making their machines more productive while adhering to the latest safety regulations, and all at less cost. To help achieve these goals, machines today use electronic control modules to process inputs and provide outputs that ultimately control the machine functions. Yet with all the changes in recent years, one component left over from that earlier era remains in regular use — the mechanical switch.  Switches offered a variety of levers, rollers, and wands for actuation, and many were sealed for an IP67 rating for outdoor use, but they came with an array of problems, including damaged levers, contact corrosion, arcing concerns, dirt or grain dust ingress, and other environmental hazards. Still, overall they were an acceptable and inexpensive way to receive position feedback for on/off functions.

Today, mechanical switches can still be found on machines used for boom presence, turret location, and other discrete functions. But are they the right product for today’s machines?

The original design parameters may have required the switch to drive the load directly, and therefore a rating of 10A@240V might be a good design choice for the relay/diode logic circuits of the past. But a newly designed machine may be switching mere milliamps through the switch into the control module. Does the legacy switch have the proper contact plating material for the load today? Switches use rare metals such as rhodium, palladium, platinum, gold, and silver in attempts to keep the contact resistance low and to protect those contacts from corrosion. Consequently, as China pursues Nonroad Stage IV standards, these metals, some also used in catalytic converters, have sharply increased in price, leading to substantial cost increases to switch manufacturers and ultimately switch users.

A better approach to position feedback for today’s mobile machines is the inductive position sensor. Inductive sensors offer a sealed, non-contact alternative to mechanical switches. Sensing ferrous and non-ferrous metals without physical contact, they eliminate many of the field problems of the past, and non-metallic substances such as water, dirt, and grain dust, do not affect the operation. These qualities make the sensor very suitable for the harsh conditions found in agricultural and construction environments.

Inductive proximity sensors come in a variety of form factors:

Threaded cylindrical – With zinc-plated brass or stainless-steel housings, the threaded barrel styles are popular for their ease of mounting and gap adjustment.  

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Low profile rectangular – These “flatpack” style sensors are great under seats for operator presence.

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Block designs – The compact, cubed package is ideal for larger sensing ranges.

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Large cylindrical – These large “pancake” style sensors are great for detecting suspension movements and other applications requiring extreme ranges.

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Inductive position sensors are more than just a discrete product used for detecting linkage, operator presence, or turret stops; They can also perform the duties of a speed sensor by counting teeth (or holes) to determine the RPM of a rotating shaft. Other models offer analog outputs to provide a continuous feedback signal based on the linear location of a metal linkage or lever. Safety rated outputs, high temperatures, and hazardous area options are some of the many product variants available with this electromagnetic technology.
So, perhaps it’s time to review that legacy switch and consider an inductive sensor?
To learn how an inductive position sensor performs its magic, please take a look at an earlier blog:

Basic Operating Principle of an Inductive Proximity Sensor

Why Sensor & Cable Standardization is a Must for End-Users

Product standardization makes sense for companies that have many locations and utilize multiple suppliers of production equipment. Without setting standards for the components used on new capital equipment, companies incur higher purchasing, manufacturing, maintenance, and training costs.

Sensors and cables, in particular, need to be considered due to the following:

  • The large number of manufacturers of both sensors and cables
  • Product variations from each manufacturer

For example, inductive proximity sensors all perform the same basic function, but some are more appropriate to certain applications based on their specific features. Cables provide a similar scenario. Let’s look at some of the product features you need to consider.

Inductive Proximity Sensors Cables
 

·         Style – tubular or block style

·         Size and length

·         Electrical characteristics

·         Shielded or unshielded

·         Sensing Range

·         Housing material

·         Sensing Surface

 

·         Connector size

·         Length

·         Number of pins & conductors

·         Wire gage

·         Jacket material

·         Single or double ended

 

Without standards each equipment supplier may use their own preferred supplier, many times without considering the impact to the end customer. This can result in redundancy of sensor and cable spare parts inventory and potentially using items that are not best suited for the manufacturing environment. Over time this impacts operating efficiency and results in high inventory carrying costs.

Once the selection and purchasing of sensors and cables is standardized, the cost of inventory will coincide.  Overhead costs, such as purchasing, stocking, picking and invoicing, will go down as well. There is less overhead in procuring standard parts and materials that are more readily available, and inventory will be reduced. And, more standardization with the right material selection means lower manufacturing down-time.

In addition, companies can then look at their current inventory of cable and sensor spare parts and reduce that footprint by eliminating redundancy while upgrading the performance of their equipment. Done the right way, standardization simplifies supply chain management, can extend the mean time to failure, and reduce the mean time to repair.

Press Shops Boost Productivity with Non-Contact Connections

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

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

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

 

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

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

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

Some advantages of inductive coupling over the connectorized solution:

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

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

 

Operational Excellence – How Can We Apply Best Practices Within the Weld Shop?

Reducing manufacturing costs is absolutely a priority within the automotive manufacturing industry. To help reduce costs there has been and continues to be pressure to lower MRO costs on high volume consumables such as inductive proximity sensors.

Traditionally within the MRO community, the strategy has been to drive down the unit cost of components from their suppliers year over year to ensure reduce costs as much as possible. Of course, cost optimization is important and should continue to be, but factors other than unit cost should be considered. Let’s explore some of these as it would apply to inductive proximity sensors in the weld shop.

Due to the aggressive manufacturing environment within weld cell, devices such as inductive proximity sensors are subjected to a variety of hostile factors such as high temperature, impact damage, high EMF (electromagnetic fields) and weld spatter. All of these factors drastically reduce the life of these devices.

There are  manufacturing costs associated with a failed device well beyond that of the unit cost of the device itself. These real costs can be and are reflected in incremental premium costs such as increased downtime (both planned and unplanned),  poor asset allocation, indirect inventory, expedited freight, outsourcing costs, overtime, increased manpower, higher scrap levels, and sorting & rework costs. All of these factors negatively affect a facility’s Overall Equipment Effectiveness (OEE).

Root Cause

In selection of inductive proximity sensors for the weld manufacturing environment there are root cause misconceptions and poor responses to the problem. Responses include: leave the sensor, mounting and cable selection up to the machine builder; bypass the failed sensor and keep running production until the failed device can be replaced; install multiple vending machines in the plant to provide easier access to spare parts (replace sensors often to reduce unplanned downtime);  and the sensors are going to fail anyway so just buy the cheapest device possible.

None of these address the root cause of the failure. They mask the root cause and exacerbate the scheduled and unscheduled downtime or can cause serious part contamination issues down stream, resulting in enormous penalties from their customer.

So, how can we implement a countermeasure to help us drive out these expensive operating costs?

  • Sensor Mounting – Utilize a fixed mounting system that will allow a proximity sensor to slide into perfect mounting position with a positive stop to prevent the device from being over extended and being struck by the work piece. This mounting system should have a weld spatter protective coating to reduce the adherence of weld spatter. This will also provide extra impact protection and a thermal barrier to further assist in protecting the sensing device asset.
  • The Sensor – Utilize a robust fully weld protective coated stainless steel body and face proximity sensor. For applications with the sensor in an “on state” during the weld cycle and/or to detect non-ferrous utilize a proper weld protective coated Factor 1 (F1) device.
  • Cabling – A standard cable will not withstand a weld environment such as MIG welding. Even a cable with protective tubing can have open areas vulnerable for weld berries to land and cause burn through on the cables resulting in a dead short. A proper weld sensor cord set with protective coating on the lock nut, high temp rated and weld resistant overmold to a weld resistant jacketed cable should be used.

By implementing a weld best practice total solution as described above, you will realize significant increases in your facilities OEE contributing to the profitability and sustainability of your organization.

Ask these 3 simple questions:

1) What is the frequency of failure

2) What is the Mean Time To Repair (MTTR)

3) What is the cost per minute of downtime.

Once you have that information you will know with your own metrics  what the problem is costing your facility by day/month/year. You may be surprised to see how much of a financial burden these issues are costing you. Investing in the correct best practice assets will allow you to realize immediate results to boost your company OEE.

How to keep prox sensors from latching on

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

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

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

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

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

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

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

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

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

 

Back to the Basics: Measuring

In the last post about the Basics of Automation, we discussed how objects can be detected, collected and positioned with the help of sensors. Now, let’s take a closer look at how non-contact measurement—both linear and rotary—works to measure distance, travel, angle, and pressure.

Measuring travel, distance, position, angle and pressure are common tasks in automation. The measuring principles used are as varied as the different tasks.

Sensor Technologies

  • Magnetostrictive enables simultaneous measurement of multiple positions and can be used in challenging environments.
  • Magnet coded enables the highest accuracy and real-time measurement.
  • Inductive is used for integration in extremely tight spaces and is suitable for short distances.
  • Photoelectric features flexible range and is unaffected by the color or surface properties of the target object.

Different Sensors for Different Applications

Distance measurement

Janni1Disc brakes are used at various locations
in wind power plants. With their durability and precise measurement, inductive distance sensors monitor these brake discs continuously and provide a timely warning if the brake linings need to be changed.

In winding and unwinding equipment, a photoelectric sensor continuously measures the increasing or decreasing roll diameter. This means the rolls can be changed with minimal stoppages.

Linear position measurement

Janni4Workpieces are precisely positioned on the slide of a linear axis. This allows minimal loss of production time while ensuring quality. Magnetic encoders installed along the linear axis report the actual slide position to the controller (PLC) continuously and in real time — even when the slide is moving at a speed of up to 10 m/s.

In a machine tool the clamping state of a spindle must be continuously monitored during machining. This improves results on the workpiece and increases the reliability of the overall system. Inductive positioning systems provide continuous feedback to the controller: whether the spindle is unclamped, clamped with a tool or clamped without a tool.

Rotational position measurement

Janni5Workpieces such as a metal plate are printed, engraved or cut on a cut/print machine. This demands special accuracy in positioning it on the machine. Magnetic encoders on both rotating axes of the machine measure the position of the workpiece and ensure an even feed rate.

In a parabolic trough system,
sunlight is concentrated on parabolic troughs using parabolic mirrors allowing the heat energy to be stored. To achieve the optimal energy efficiency, the position of the parabolic mirror must be guided to match the sun’s path. Inclination sensors report the actual position of the parabolic mirror to the controller, which then adjusts as needed.

Pressure and Level Measurement

Janni7Consistently high surface quality of the machined workpiece must be ensured in a machine tool. This requires continuous monitoring of the coolant feed system pressure. Pressure sensors can reliably monitor the pressure and shut down the machine within a few milliseconds when the defined pressure range is violated.

Janni8In many tanks and vats, the fill height of the liquid must be continually measured. This is accomplished using ultrasonic sensors, which note levels regardless of color, transparency or surface composition of the medium. These sensors detect objects made of virtually any material (even sound-absorbing) including liquids, granulates and powders.

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.

Where Discrete Position Sensing Belongs in the Manufacturing Process

Unlike continuous position sensors which provide near real-time position feedback throughout the stroke of the cylinder, discrete position sensors are equipped with a switching functionality at one or more designated positions along the cylinder’s stroke. Typically, these positions are set to detect fully retracted and extended positions but one can also be used to detect mid-stroke position.

To determine which is right for you requires a review of your application and a determination of how precisely the movement of the cylinder needs to be controlled. Some hydraulic cylinder applications require no position sensing at all. These applications simply use the cylinder to move a load, and position control is either done manually or by some other external switch or stop. Moving up a step, many applications require only that the beginning and end of the cylinder stroke be detected so that the cylinder can be commanded to reverse direction. These applications are ideal for discrete position sensing.

Several types of sensors are used for discrete position detection, but one of the most common is high-pressure inductive proximity sensors, which are installed into the end caps of the cylinder. The sensors detect the piston as it reaches the end of the cylinder stroke in either direction.

These sensors are designed to withstand the full pressure of the hydraulic system. Inductive sensors are extremely reliable because they operate without any form of mechanical contact and are completely unaffected by changes in oil temperature or viscosity.

High-pressure
High-pressure inductive sensors installed in hydraulic cylinder

Discrete position sensors are used in applications such as hydraulic clamps, detection of open/closed position in welding operations, and in hydraulic compactors and balers for compacting materials until end of cylinder stroke is reached, at which point the cylinder retracts.

Additionally, it is quite common for pneumatically-actuated clamps and grippers to use discrete sensors to indicate fully extended and fully retracted positions, and in many cases, in-between positions as well. There are even applications where multiple discrete sensors are used in grippers for gauging and sizing work pieces.

By far, the most common method of providing discrete position in an air cylinder is to use externally-mounted switches that react to a magnet installed around the circumference of the piston. These magnetically-actuated switches can sense the field of a magnet embedded in the cylinder’s piston through the aluminum body of the cylinder.

magnetically actuated
Magnetically actuated sensor installed into cylinder C-slot

There are several different operating principles used in these magnetically-actuated switches, ranging from simple, low-cost reed switches and Hall-effect switches to significantly more reliable sensors that use magnetoresistive technology. One of the big advantages of magnetoresistive sensors is that they will reliably detect both radial and axial magnetic fields, making them ideal replacements for reed or Hall-effect switches.

Check out our previous blog to learn more about continuous position sensors.

Distance Measurement with Inductive Sensors

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.

Application Examples:

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For more specific information on analog inductive sensors visit www.balluff.com.

Reliable Part Exit/Part-Out Detection

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:

  1. Tumbling parts with many “openings” on the part itself can cause a miss-detected component.
  2. Overly-reflective parts can false triggering of the output.
  3. 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.
  4. 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:

  1. 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.
  2. UHMW is reported to be 3X stronger than carbon steel.
  3. UHMW is resistant to die lubes.
  4. 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.
  5. 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.
A Two-Out Die with Metallic Chute
A Two-Out Die with Metallic Chute

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.

You can learn more in the video below or by visiting www.balluff.us.