For RFID Applications, Think Throughput

A common request from many engineers I talk to is the need for a “faster” RFID read/write system.  Usually, this is due to the fact they are increasing their overall line speed and decreasing the amount of time that a work in process item dwells in one station.  This is a good thing.  We all want to make more widgets faster. However, in addition to increasing the number of widgets that come off the production line and the rate at which they come off, the demand for quality has increased significantly. This is also a good thing. This certainly leads to a win-win between the manufacturer and the consumer. As the demand for quality increases so does the amount of data. Statistical process control, lineage data, build data, etc. are represented by large amounts of data. So the tag has to have enough memory and the reader has to have enough speed to keep up with the process. The amount of data transferred over a period of time is called throughput.

In RFID readers/writers, throughput is usually represented as bytes or kilobytes of data per second or milliseconds. The read/write speeds of all RFID systems are related to the amount of data being read or written to the tag. So, if high throughput is a requirement, a feature to look for in the reader/writer is the buffer size. I don’t want to get too deep into the technical weeds of data transfer, bit rates, baud rates, etc. so I will explain it from a marketing guy’s perspective. Think of an RFID system as a data delivery system. In this delivery system an imaginary tractor-trailer is what delivers the data from the reader to the tag and the tag to the reader. The trailer represents the aforementioned buffer.  The trailer or buffer can hold a specified amount of data, 32Bytes, 64Bytes and so on. This is determined by the manufacturer of the system. Semi trucks_blogTherefore, there may be two systems that operate at the same speed, but have a totally different throughput.  Back to the tractor-trailer example, there can be two semi’s going down the road at the same speed but one has a trailer that is half the size of the other and can only carry half as much product(data in this case). So in order to transfer the same amount of data, the half-size trailer has to make two trips (cycles) whereas the larger trailer makes only one. In a case where the amount of data that needs to be transferred is multiple thousands of bytes or kilobytes, that buffer size becomes more important because the more cycles or trips that have to be completed the slower the transfer.

Ultimately, speed is a relative term in the world of moving data from one point to another. In order to future-proof your production line, look a little deeper into the features of the system to make sure you’re investing in technology that is not only fast, but fast and moves a lot of data.

For more information, visit www.balluff.com.

 

Posted in All posts, Industrial Identification, Industrial RFID Systems | Tagged , , , | Leave a comment

Position and Level Measurement for Clean Energy

A few years ago, a new gas station opened in our neighborhood situated at the edge of town.  Some new customers started showing up with camouflage painted cars, especially early in the morning or late at night. They always seemed to be a little shy, never leaving the cars alone and sometimes, while grabbing a cup of coffee, they would even cover their cars with a tarp. It became obvious that this gas station was also used as a base for the test fleet of some major car manufacturer, and even after years, it is still exciting to get a glimpse of a new prototype. Working for the international Balluff energy team, it was even more exciting to see that a new hydrogen fuel dispenser had been installed at this gas station a few months ago.

Hydrogen seems to be one of the promising possibilities to store surplus energy from wind or solar power and to realize climate friendly mobility. But the processing and transportation of hydrogen poses new challenges for the equipment with regard to pressure, temperature and material properties.

One of the  possibilities for position and level measurement in such a harsh environment is magnetostrictive measurement systems.

This type of transducer comprises current position information transferred via magnetic fields contactless through the housing wall to the internal sensor element. The operation principle enables the installation in hermetically sealed rod/housing against the process medium.

Image1

Another point to note is the presence of a possibly hazardous atmosphere that can be ignited by electric sparks or hot surfaces. Therefore the measurement systems installed need to be designed in accordance with applying explosion protection regulations.

Image2

Considering the above mentioned framework conditions, specific part testing and verification is necessary within the development process as well as in series production. In addition the necessary approvals and certificates have to be provided on an international scale.

Image3

For more information about the future market of Clean Energy, the measurement systems in use and the cooperation during the R&D process, visit the full report from Linde AG Austria (AUTlook 4/2017).

Posted in All posts | Tagged , , | Leave a comment

External Position Feedback for Hydraulic Cylinders

The classic linear position feedback solution for hydraulic cylinders is the rod-style magnetostrictive sensor installed from the back end of the cylinder. The cylinder rod is gun-drilled to accept the length of the sensor probe, and a target magnet is installed on the face of the piston. A hydraulic port on the end cap provides installation access to thread-in the pressure-rated sensor tube. This type of installation carries several advantages but also some potential disadvantages depending on the application.

EDITED Image_HF

Position Sensor Mounted Internally in a Hydraulic Cylinder
(Image credit: Cowan Dynamics)

Advantages of in-cylinder sensor mounting include:

  • Simplicity. The cylinder manufacturer “preps” the cylinder for the sensor and may install it as an extra-cost option.
  • Ruggedness. The sensor element is protected inside the cylinder. Only the electronics head is exposed to the rigors of the industrial environment.
  • Compactness. The sensor is contained inside the cylinder, so it does not add to the cross-sectional area occupied by the cylinder.
  • Direct Position Measurement. Because the target magnet is mounted on the piston, the sensor is directly monitoring the motion of the cylinder without any interposing linkages that might introduce some position error, especially in highly dynamic, high-acceleration / deceleration applications.

Potential disadvantages of in-cylinder sensor mounting may include:

  • Sensor Cost. Cylinder-mounted position sensors require a rugged, fully-sealed stainless-steel sensor probe to withstand the dynamic pressures inside a cylinder. This adds some manufacturing cost.
  • Cylinder Cost. The procedure of gun-drilling a cylinder rod consumes machine time and depletes tooling, adding manufacturing cost over a standard cylinder. Refer to additional comments under Small Cylinder Bores / Rods below.
  • Cylinder Delivery Time. Prepping a new cylinder for a sensor adds manufacturing time due to additional processing steps, some of which may be outsourced by the cylinder manufacturer, increasing overall shipping and handling time.
  • Overall Installed Length. Because the sensor electronics and cabling protrude from the back end of the cylinder, this adds to the overall length of the installed cylinder. Refer to additional comments under Small Cylinder Bores / Rods below.
  • Service Access. In case sensor repair is required, there must be sufficient clearance or access behind the cylinder to pull out the full length of the sensor probe.
  • Small Cylinder Bores / Rods. Some cylinder bores and rod diameters are too small to allow for gun-drilling a hole large enough to install the ~10.2 mm diameter sensor tube and allow for proper fluid flow around it. In tie rod cylinders, the distance between the rod nuts may be too small to allow the flange of the position sensor to fully seat against the O-ring. In these cases, a mounting boss must be provided to move the mounting position back past the tie rods. This adds cost as well as increases overall installed length.

Edited Image 2

In cases where the advantages of in-cylinder mounting are outweighed or rendered impractical by some of the disadvantages, an externally-mounted position sensor can be considered. The list of advantages and disadvantages looks similar, but reversed.

FINAL Profile transducer - hydraulic frame piercing press_HF

Position Sensor Mounted Externally on Hydraulically-Actuated Equipment

Advantages of external sensor mounting include:

  • Sensor Cost. Externally-mounted magnetostrictive position sensors are typically made from an aluminum extrusion and die-cast end caps with gaskets, saving cost compared to all-stainless-steel welded and pressure-rated construction.
  • Cylinder Cost. The cylinder can be a standard type with no special machining work needed to accommodate installation of the sensor.
  • Cylinder Delivery Time. Since no additional machine work is needed, the cylinder manufacturer can deliver within their standard lead time for standard cylinders.
  • Overall Installed Length. Typically, the external sensor is mounted in parallel to the cylinder, so overall length is not increased.
  • Service Access. The externally-mounted sensor is easily accessible for service by simply unbolting its mounting brackets and pulling it off the equipment.

Disadvantages of external sensor mounting may include:

  • Complexity. The machine designer or end user must provide the means to mount the sensor brackets and the means to position a floating magnet target over the sensor housing. Alternatively, a captive sliding magnet target may be used with a length of operating rod and swivel attachment hardware.
  • Exposure to Damage. Unless guarded or installed in a protected area, an externally mounted position sensor is subject to being mechanically damaged.
  • Space Requirements. There must be enough empty space around the cylinder or on the machine to accommodate the sensor housing and operating envelope of the moving magnetic target.
  • Indirect Position Measurement. Any time a floating target magnet is mounted to a bracket, there is the potential for position error due to the bracket getting bent, flexing under acceleration / deceleration, mounting bolts loosening, etc. In the case of operating rods for captive sliding magnets, there will be some mechanical take-up in the swivel joints upon change of direction, adding to position hysteresis. There is also the potential for rod flexing under heavy acceleration / deceleration – particularly when the rod is acting under compression vs. tension. Take note of the amount of sliding friction of the captive magnet on the sensor rails; some sensor magnet designs offer high friction and stiff resistance to movement that can increase operating rod deflection and resultant position error.

In conclusion, be sure to consider all aspects of an application requiring cylinder position feedback and choose the approach that maximizes the most important advantages and eliminates or minimizes any potential disadvantages. It may be that an externally-mounted position sensor will solve some of the challenges being faced with implementing a traditional in-cylinder application.

For more information about internally- and externally-mounted cylinder position sensors, visit www.balluff.com.

Posted in All posts, Linear Position and Distance Measurement | Tagged , , , , , | Leave a comment

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.

Posted in All posts, Magnetic Field Sensors, Object Detection Sensors | Tagged , , , , , , , , , , , , , , , | Leave a comment

Sensing Types of Capacitive Sensors

Similar to inductive sensors, capacitive sensors are available in two basic versions.  The first type is the flush or shielded or embeddable version however with capacitive sensors they are sometimes referred to as object detection sensors.  The second type is the non-flush or non-shielded or non-embeddable version however again with capacitive sensors they are sometimes referred to as level detection sensors.

CapacitiveTypes1

The flush or object detection capacitive sensors are shielded and employ a straight line electrostatic field.  This focused field is emitted only from the front face of the sensor allowing the sensor to be mounted in material so that only the face of the sensor is visible.

The highly focused electrostatic field is perfect for detecting small amounts of material or material with low dielectric constant.  The typical range of a flush 18mm capacitive sensor is approximately 2 to 8mm depending on the objects dielectric constant.  As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes2

If the sensors are mounted adjacent to each other the minimum gap should be equal to the diameter or the adjusted sensing distance whichever is less.  These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

CapacitiveTypes3Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick.  If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

CapacitiveTypes4The non-flush or level detection capacitive sensors are not shielded and employ a spherical electrostatic field.  This field is emitted from the front face of the sensor and wraps around to the sides of the sensor head.  Unlike the flush sensor this version cannot be mounted in material where only the face of the sensor is visible.  Non-flush sensors have better characteristics and better performance in applications with adhering media.

CapacitiveTypes5The spherical electrostatic field provides a larger active surface and is perfect for detecting bulk material and liquid either directly or indirectly.  The typical range of a flush 18mm capacitive sensor is approximately 2 to 15mm depending on the objects dielectric constant.  As with any capacitive sensor the sensor should be adjusted after installation.

CapacitiveTypes6If the sensors are mounted adjacent to each other the minimum gap should be equal to three times the diameter or the adjusted sensing distance whichever is less.  These sensors can also be mounted opposing each other however the distance should four times the diameter of the adjusted sensing distance whichever is less.

Shielded or flush capacitive sensors are perfect for detecting solids or liquids through non-metallic container walls up to 4mm thick.  If you are detecting liquid levels through a sight glass with the sight glass mounting bracket then the flush mounted sensor is the preferred choice.

Capacitive sensors are perfect for short range detection of virtually any object regardless of color, texture, and material.

To learn more about capacitive sensors visit www.balluff.com.

Posted in All posts, Capacitive Sensors, Object Detection Sensors | Tagged , , , , , , , , , , , | 1 Comment

Absolutely Incremental – Innovations in Magnetic Linear Encoder Technology

Linear encoders – absolute or incremental?  Incremental encoders are simple, inexpensive, and easy to implement, but they require that the machine be homed or moved to a reference position.  Absolute encoders don’t require homing, but they’re usually more expensive, and implementation is a bit more involved.  What if you could get an incremental encoder that also gave you absolute position?  Would that be great, or what?  Read on.

IncrementalEncodersIncremental encoders are pretty simple and straightforward.  They provide digital pulses, typically in A/B quadrature format, that represent relative position movement.  The number of pulses the encoder sends out correspond to the amount of position movement.  Count the pulses, do some simple math, you know how much movement has occurred from point A to point B.  But, here’s the thing, you don’t actually know where you are exactly.  You only know how far you’ve moved from where you started.  You’ve counted an increment of movement.  If you truly want to know where you are, you have to travel to a defined home or reference position and count continuously from that position.

AbsoluteEncodersAbsolute encoders, on the other hand, provide a unique output value everywhere along the linear travel, usually in the form of a serial data “word”.  Absolute encoders tell you exactly (absolutely) where they are at all times.  There’s no need to go establish a home or reference position.

So absolute is better, yes?  If that’s so, then why doesn’t everyone use them instead of incremental encoders?

It’s because incremental encoders typically cost a lot less, and are much easier to integrate.  In terms of controller hardware, all you need is a counter input to count the pulses.  That counter input could be integral to a PLC, or it could take the form of a dedicated high-speed counter module.  Either way, it’s a fairly inexpensive proposition.  And the programming to interpret the pulse count is pretty simple and straightforward as well.  An absolute encoder will usually require a dedicated motion module with a Synchronous Serial Interface (SSI, BiSS, etc.).  These interfaces are going to be both more expensive and more complex than a simple counter module.  Plus, the programming logic is going to be quite a bit more involved.

So, yes, being able to determine the absolute position of a moving axis is undoubtedly preferable.  But the barriers to entry are sometimes just too high.  An ideal solution would be one that combines the simplicity and lower cost of an incremental encoder with the ability to also provide absolute position.

Fortunately, such solutions do exist.  Magnetic linear encoders with a so-called Absolute Quadrature interface provide familiar A/B quadrature signals PLUS the ability to inform the controller of their exact, absolute position.  Absolute position can be provided either on-demand, or every time the sensor is powered up.

How is this possible?  It’s really quite ingenious. You could say that the Absolute Quadrature encoders are “absolute on the inside, and incremental on the outside”.  These encoders use absolute-coded magnetic tape, and the sensing head reads that position (with resolution as fine as 1 µmeter and at lengths up to 48-meters, by the way).  But, during normal operation, the sensor head outputs standard A/B quadrature signals.  Remember though, it actually knows exactly where it is (absolute inside…remember?), and can tell you if you ask.  When requested (or on power-up, if that’s how you have it configured), the sensor head sends out a string, or burst, of A/B pulses equal to the distance between the home position and the current position.  It’s as if you moved the axis back to home position, zeroed the counter, and then moved instantly back to current position.  But no actual machine movement is necessary.  The absolute burst happens in milliseconds.

So, to sum it up, Absolute Quadrature linear encoders provide a number of advantages:

  • Economical: Compatible with standard A/B incremental interfaces – no absolute controller needed
    • No need to upgrade hardware; can connect to existing control hardware
    • Get the advantages of absolute, but maintain the simplicity of incremental; eliminate the need for homing
  • Easy implementation: Simple setup, no (or very minimal) new programming required
  • Accurate: Resolution down to 1 µm, over lengths up to 48 meters

If you’d like to learn more about linear encoders with Absolute Quadrature, go to: http://www.balluff.com/local/us/news/product-news/bml-absolute-quadrature/

Posted in All posts, Linear Position and Distance Measurement | Tagged , , , , , , , , , | Leave a comment

Solving Analog Integration Conundrum

These days, there are several options to solve the integration problems with analog sensors such as measurement or temperature sensors. This blog explains the several options for analog integration and the “expected” benefits.

Before we describe the options, let’s get a few things cleared up.  First, most controllers out there today do not understand analog at all: whenever a controller needs to record an analog value, an analog-to-digital converter is required.  On the other end of the equation is the actual sensor measuring the physical property, such as distance, temperature, pressure, inclination, etc.  This sensor, a transducer, converts the physical property into an analog signal.  These days with the advanced technologies and with the cost of microprocessors going down, it is hard to find a pure analog device.  This is because the piezo-electronics inside the sensor measures the true analog signal, but it is converted to a digital signal so that the microprocessor can synthesize it and convert it back to an analog signal.  You can read more about this in a previous blog of mine “How Do I Make My Analog Sensor Less Complex?

Now let’s review the options available:

  1. The classical approach: an analog to digital converter card is installed inside the control cabinet next to the controller or a PLC. This card offers 2, 4, or even 8 channels of conversion from analog to digital so that the controller can process this information. The analog data can be a current measurement such as 0-20mA or 4-20mA, voltage measurement such as 0-10V, +5- -5V etc., or a temperature measurement such as PT100, PT1000, Type J, Type K and so on.  Prior to networks or IO-Link, this was the only option available, so people did not realize the down-side of this implementation.  The three major downsides are as follows:
  • Long sensor cable runs are required from the sensor all the way to the cabinet, and this required careful termination to ensure proper grounding and shielding.
  • There are no diagnostics available with this approach: it is always a brute-force method to determine whether the cable or the actual transducer/sensor has the issue. This causes longer down-times to troubleshoot problems and leads to a higher cost to maintain the architecture.
  • Every time a sensor needs to be replaced, the right tools have to be found (programming tools or a teaching sequence manual) to calibrate the new sensor before replacement. Again, this just added to the cost of downtime.
  1. The network approach — As networks or fieldbuses gained popularity, the network-based analog modules emerged. The long cable runs became short double-ended pre-wired connectors, significantly reducing the wiring cost. But this solution added the cost of network node and an additional power drop.  This approach did not solve the diagnostic problems (b) or the replacement problem above (c ). The cost of the network analog module was comparable to the analog card, so there was effectively no savings for end users in that area.  As the number of power drops increase, in most cases, the power supply becomes bigger or more power supplies are required for the application.
  2. The IO-Link sensor approach is a great approach to completely eliminate the analog hassle altogether. As I mentioned earlier, since the sensor already has a microprocessor that converts the signal to digital form for synthesis and signal stabilization, why not use that same digital data over a smarter communication to completely get rid of analog? In a nutshell, the data coming out of the sensor is no longer an analog value; instead it is a digital value of the actual result. So, now the controller can directly get the data in engineering units such as psi, bar, Celsius, Fahrenheit, meters, millimeters, and so on. NO MORE SCALING of data in the controller is necessary, there are no more worries of resolution, and best of all enhanced diagnostics are available with the sensor now. So, the sensor can alert the controller through IO-Link event data if it requires maintenance or if it is going out of commission soon.  With this approach, the analog conversion card is replaced by the IO-Link gateway module which comes in 4-channels or 8 channels.

Just to recap about the IO-Link sensor:

  1. IO-Link eliminated the analog cable hassle
  2. IO-Link eliminated the resolution and scaling issue
  3. IO-Link added enhanced diagnostics so that the end users can perform predictive maintenance instead of preventative maintenance.
  4. The IO-Link gateway modules offers configuration and parameter server functionality that allows storing the sensor configuration data either at the IO-Link master port or in the controller so that when it is a time to replace the sensor, all that is required is finding the sensor with the same part number and plugging it in the same port — and the job is done! No more calibration required. Of course, don’t forget to turn on this functionality on the IO-Link master port.

Well, this raises two questions:

  1. Where do I find IO-Link capable sensors? The answer is simple: the IO-Link consortium (www.IO-Link.com) has over 120 member companies that develop IO-Link devices. It is very likely that you will find the sensor in the IO-Link version. Want to use your existing sensor?  Balluff offers some innovative solutions that will allow you bring your analog sensor over to IO-Link.
  2. What is a cost adder for this approach? Well, IO-Link does a lot more than just eliminate your analog hassle. To find out more please visit my earlier blog “Is IO-Link only for Simplifying Sensor Integration?

Balluff offers a broad portfolio of IO-Link that includes sensors, RFID, SmartLights, Valve connectors, I/O hubs, and the gateway modules for all the popular fieldbuses and networks. Learn more at www.balluff.com

Posted in All posts, Industrial Networking, IO-Link | Tagged , , , , , | Leave a comment

Kicking Off Summer With a Big Announcement

Since 2010 we have been sharing best practices, technology and industry trends. Occasionally we have even gotten back to basics with how the products we use to automate actually work. As you know, we are dedicated to the successful implementation of sensor technology by the automation community. With the inevitable evolution of technology and the birth of the 4th Industrial Revolution or the Industrial Internet of Things (IIoT) we are excited to announce a few changes to the SensorTech blog. As summer kicks off we have decided to update our blog with a new look, and a new name.

Introducing, Automation Insights. We are here to act as a resource for the industry as it navigates through the impending IIoT technology revolution, sharing information about emerging trends and standards, current best practices, success stories, technology fundamentals, and advanced implementation concepts. And we are here to offer expertise, ideas, guidance, and support for industrial identification and industrial sensor specifiers, installers, and users to help them get the most out of their automation technology investments. One thing that will not change, though, is our dedication to the successful implementation of industrial control technology by the automation community at large.

We look forward to sharing many new posts with you and are excited for you to get to know a few new contributors from around the globe.

Posted in All posts | Tagged , , , | Leave a comment

What is a Capacitive Sensor?

Capacitive proximity sensors are non-contact devices that can detect the presence or absence of virtually any object regardless of material.  They utilize the electrical property of capacitance and the change of capacitance based on a change in the electrical field around the active face of the sensor.

Capacitive sensing technology is often used in other sensing technologies such as:

  • flow
  • pressure
  • liquid level
  • spacing
  • thickness
  • ice detection
  • shaft angle or linear position
  • dimmer switches
  • key switches
  • x-y tablet
  • accelerometers

Principle of operation

A capacitive sensor acts like a simple capacitor.  A metal plate in the sensing face of the sensor is electrically connected to an internal oscillator circuit and the target to be sensed acts as the second plate of the capacitor.  Unlike an inductive sensor that produces an electromagnetic field a capacitive sensor produces an electrostatic field.

The external capacitance between the target and the internal sensor plate forms a part of the feedback capacitance in the oscillator circuit.  As the target approaches the sensors face the oscillations increase until they reach a threshold level and activate the output.

Capacitive sensors have the ability to adjust the sensitivity or the threshold level of the oscillator.  The sensitivity adjustment can be made by adjusting a potentiometer, using an integral teach pushbutton or remotely by using a teach wire.  If the sensor does not have an adjustment method then the sensor must physically be moved for sensing the target correctly.  Increasing the sensitivity causes a greater operating distance to the target.  Large increases in sensitivity can cause the sensor to be influenced by temperature, humidity, and dirt.

There are two categories of targets that capacitive sensors can detect the first being conductive and the second is non-conductive.  Conductive targets include metal, water, blood, acids, bases, and salt water.  These targets have a greater capacitance and a targets dielectric strength is immaterial.  Unlike an inductive proximity sensor, reduction factors for various metals are not a factor in the sensors sensing distance.

The non-conductive target category acts like an insulator to the sensors electrode.  A targets dielectric constant also sometimes referred to as dielectric constant is the measure of the insulation properties used to determine the reduction factor of the sensing distance.  Solids and liquids have a dielectric constant that is greater than vacuum (1.00000) or air (1.00059).  Materials with a high dielectric constant will have a longer sensing distance.  Therefore materials with high water content, for example wood, grain, dirt and paper will affect the sensing distance.

When dealing with non-conductive targets there are three factors that determine the sensing distance.

  • The size of the active surface of the sensor – the larger the sensing face the longer the sensing distance
  • The capacitive material properties of the target object, also referred to as the dielectric constant – the higher the constant the longer the sensing distance
  • The surface area of the target object to be sensed – the larger the surface area the longer the sensing distance

Other factors that have minimal effect on the sensing distance

  • Temperature
  • Speed of the target object

Sensing range

A capacitive sensor’s maximum published sensing distance is based on a standard target that is a grounded square metal plate (Fe 360) that is 1mm thick.  The standard target must have a side length that is the diameter of the registered circle of the sensing surface or three times the rated sensing distance if the sensing distance is greater than the diameter.  Objects being detected that are not metal will have a reduction factor based on the dielectric constant of that object material.  This reduction factor must be measured to determine the actual sensing distance however there are some tables that will provide an approximation of the reduction factor.

Rated or nominal sensing distance Sn is a theoretical value that does not take into account manufacturing tolerances, operating temperatures and supply voltages.  This is typically the sensing distance listed in various manufactures catalogs and marketing material.

Effective sensing distance Sr is the switching distance of the sensor measured under specified conditions such as flush mounting, rated operating voltage Ue, temperature Ta = 23°C +/- 5°C.  The effective sensing range of capacitive sensors can be adjusted by the potentiometer, teach pushbutton or remote teach wire.

Hysteresis

Hysteresis is the difference in distance between the switch-on as the target approaches the sensing face and switch-off point as the target moves away from the sensing face.  Hysteresis is designed into sensors to prevent chatter of the output if the target was positioned at the switching point.

Hysteresis stated in % of rated sensing distance.  For example a sensor with 20mm of rated sensing distance may have a maximum hysteresis of 15% or 3mm.  Hysteresis is an independent parameter that is not a constant and will vary sensor to sensor.  There are several factors that can influence hysteresis including:

  • Sensor temperature both ambient and heat generated by the sensor being powered
  • Atmospheric pressure
  • Relative humidity
  • Mechanical stresses to the sensor housing
  • Electronic components utilized on the printed circuit board within the sensor
  • Correlated to sensitivity – higher sensitivity relates to higher rated sensing distance and a larger hysteresis

How to determine a capacitive sensor’s sensitivity

Capacitive sensors have a potentiometer or some method to set the sensor sensitivity for the particular application.  In the case of a potentiometer, the number of turns does not provide an accurate indicator of the sensors setting for a couple of important reasons.  First, most potentiometers do not have hard stops instead they have clutches so that the pot is not damaged when adjusted to the full minimum or maximum setting.  Secondly, pots do not have consistent linearity.

To determine the sensitivity of a capacitive sensor the sensing distance is measured from a grounded metal plate with a micrometer.  The plate is grounded to the negative of the power supply and the target is moved axially to the sensors face.  Move the target out of the sensing range and then move it towards the sensor face.  Stop advancing the target as soon as the output is activated.  This distance is the sensing distance of the sensor.  Moving the target away and noting when the output turns off will provide the hysteresis of the sensor.

To learn more about capacitive sensor technology visit www.balluff.com.

Posted in All posts, Capacitive Sensors, Object Detection Sensors | Tagged , , , , | Leave a comment

Magnetic Encoders in Metalworking

When thinking about position sensing in machine tool applications typically glass scale systems for the CNC axis control come to your mind. These sensor principles are the most used ones in modern machine tools and are applied to requirements with resolutions to even submicrometer resolutions. Yet there are many other applications in metalworking which do not need these high end but also high priced measuring systems.

Loading and Unloading of Workpieces

In highly automated processes of loading and unloading workpieces the required repeatability of the motion axis positions is in hundreds of millimeters. This is accurate enough to achieve a reliable and accurate handling of the workpieces. Here magnetic linear encoder systems provide an optimum performance-to-cost-ratio. With significantly lower price levels compared to glass scale systems and much easier installation the total cost of ownership is much better compared to glass scale systems. These magnetic linear encoder systems are offered with both incremental and absolute output signals. Signal types for incremental outputs are quadrature or sinusoidal. Absolute outputs e.g. are used with the industrially standardized SSI and BISS interfaces. Now more and more popularity the recently also industrially standardized serial IO-Link interface has gained.

The non contact, wear free system is designed for a long lifetime and allows tolerances in alignment to a certain extent, which is especially relevant in applications of axis lengths of several meters.

Position sensing at rotating applications

The usage of CNC controls started with typically 3 axis (X-, Y, Z-). In the last years more and more 5 axis solutions have entered the market as they offer more flexibility in manufacturing. Additionaly the efficiency of these machines is higher as in many cases workpieces may be produced without the need of manually changing their orientation in the machining process.

Modular systems like rotary tables and swivel tables significantly increase the performance of machine tools. The highly compact design of magnetic rotary encoder systems supports the design of these mechatronic modular  Systems.

Another advantage of the magnetic rotary encoder principle is the generous leeway in the center of the axis which allows more room for media such as coolants as well as the power supply and signal lines.

Summary

Besides the usage of glass scale systems for the classical 3-axis control of CNC machines the automation of Metalworking processes in machine tools more and more uses magnetic encoder systems thanks to their features like compact design, cost efficiency and easy installation. Drivers for the design of new machine tool concepts will be efficiency and flexibility. Definitely magnetic encoders support these demands.

More information about magnetic encoders is available here.

Posted in All posts, Linear Position and Distance Measurement | Tagged , , , , , , , | Leave a comment