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Embedded Non-Contact Displacement Sensors for Industrial Control Applications
by Bryan Manning and Robert L. Foster, Capacitec

A large and healthy segment of the sensor market that is showing persistent growth is Industrial Control. As manufacturers continue their quest to improve the quality of their products, while simultaneously reducing costs through automation and diminished scrap, they are seeking out improved measurement solutions supplied by an array of industry specialists.

Growth of Specialized Suppliers and Industrial Control OEMs

A typical industrial control solution for a manufacturing company is comprised of the following five components:

Sensors
Signal electronics
Computer hardware
Software
Mechanical interfaces.

An important strategic decision that manufacturing companies are faced with is whether they prefer to develop and maintain expertise in each of these diverse disciplines or subcontract portions of, or even the whole industrial control solution, to outside specialists.

The global trend in this area is away from vertical integration and towards manufacturers choosing to subcontract out to specialists in the various chosen fields. A driving force behind the decision for many manufacturers lies in the one of the more important principles of new product development - namely Reducing Product Time to Market. The product life cycle of high technology products continues its downward direction. Since almost all manufactured products today involve some sort of high technology, keeping their products up to date with the latest technology, is crucial to maintaining global competitiveness. In addition and, as importantly, the faster a manufacturer can get their product to market, the higher their return on capital employed.

This global outsourcing trend is therefor driving the demand for Industrial Control Solutions suppliers at all levels of integration. Some companies make the choice of using a single outside vendor to supply the complete solution. These total solution suppliers are referred to by a variety of names ranging from Systems Integrators to Industrial Control OEMs.

In the process of designing a custom solution for the manufacturer these OEMs then in turn subcontract a portion of the solution, maybe the sensor or the software, to the next level of specialized suppliers. Some Industrial Control OEM's are particularly knowledgeable in mechanical interfaces and turn to others for the remaining components. Others are strong in Computer Hardware and Software but lack the sensor expertise. The most successful Industrial Control OEMs tend to specialize in certain Industries or niche applications such as Silicon Wafer Processing or Automotive Disc Brake Testing.

Some manufacturers choose to only outsource a portion of the Industrial Control solution such as the sensor or mechanical interface and handle the rest in-house due to existing expertise or for proprietary reasons.

Whether specified by the manufacturer or the Industrial Control OEM, the sensor and sensor electronics still remain a crucial component of any Industrial Control Solution. The sensor is the "front end" of any Industrial Control solution and many times, without the sensor, the system would not exist. The specifying Engineer must be careful in choosing the optimum sensor technology for their individual application. They must then choose a sensor supplier that offers sensor product features and benefits that will maximize the performance of the sensor and electronics in the their particular Industrial Control System application.

Non Contact Displacement Sensors

Unlike Process Control Markets where Temperature, Pressure and Flow sensors are of particular interest, Industrial Control Markets are generally more interested in sensors that measure dimension, position and displacement. As new Quality Control methods such as In-process and 100% parts inspection become more commonplace, displacement sensors are being required in a greater number of locations throughout the manufacturing process. Non contact displacement sensors, in particular, are more often the optimum choice in displacement sensors due to the many advantages outlined later in this article. Although there are many determinants driving the increased demand for these sensors, the overriding benefits fall into the two major categories of improving quality and reducing cost.

Improved Quality
Brought on by global competition and more demanding end user customers, improved quality continues to be a major theme for manufacturers around the world. Manufacturers have learned that one of the best long-term values that they can offer their customers is consistent high quality products. As technology progresses so too do the quality methods employed. Older quality methods, such as AQL and other sampling methods are now consistently being replaced with preferred methods such as 100% inspection and zero defects. This change in philosophy requires displacement sensor technology capable of measuring a new array of difficult targets, locations and configurations.

The requirement of increased precision measurement is another major factor in today's quality control environment. Component and subassembly dimensions are getting more and more precise. This is in turn driving the need for increased precision in the measurement systems controlling them. In many cases the components themselves are becoming smaller requiring smaller and often more precise displacement sensors. This trend is especially true in industries such as semiconductors, office automation and medical electronics. Today Industrial Control OEMs and in-house designers are looking for displacement measurement systems that can measure in millionths of an inch instead of tenths .

Manufacturers are also continuing their push to control quality problems earlier on in the process. Outdated quality methods relied on the Post-Process Measurement approach where finished products were only tested at the very end of the manufacturing process. In that case the products would be sorted in good and bad piles. The bad products would then be scrapped or reworked at the end of the process after most of the cost had already been built into the product. Preferred modern quality methods call for dimensional inspection prior to the completion of the product through In-Process and in some cases Pre-Process measurements. In-Process measurement (also referred to as in-line measurement) allows dimensional measurement and adjustment as the product is being manufactured. The Pre-Process approach employs non contact displacement sensors, as components of a setup tool, to check and adjust tooling, jigs and other manufacturing fixtures prior to the start of production.

In the past technical barriers hampering the use of non contact displacement sensors for In-Process inspection have been:

High temperature
Severe environments
Small target sizes
Difficult to access locations

Reduced Cost
As global competition continues to put pressure on prices, manufactures are in a constant search to find new areas of efficiency to reduce labor and overhead costs. There are several ways to reduce costs in manufacturing but few offer the rewards that come with reducing scrap and maximizing thruput. Industrial Control Systems employing the latest technology non contact displacement systems can provide just the control required. When measurements can be made early on in the process, and adjustments made quickly, significant cost savings are the result.

Capacitec differentiation factors
Capacitec has distinct advantages over other types as well as other suppliers of non contact displacement sensors.

Capacitec's sensor electronics design, for example, allows for a larger linear range in relation to the size of the sensor. Their sensors can be as small as 0.010" (0.25mm) diameter (overall probe diameter of 0.040"/1.0mm) but still offer a ±0.2% FS linear range of 0.078" (0.2mm). This small sensor size/large linear range relationship offers significant advantages in Industrial Control applications where small sized sensors are crucial. When embedding sensors in an industrial process there is usually an advantage to small size. Examples of applications where small sized sensors are critical include hard disk drives, parts inspection, IC manufacturing processes and jet engine manufacturing and rebuilding.

Another advantage of Capacitec sensors is their ability to withstand extreme environments. Their ability to withstand high levels of nuclear radiation (up to 10 to 12th power RADS accumulated dosage) allowed the measurement of critical displacement values in a nuclear fuel reprocessing application avoiding the necessity of burying high level nuclear waste. Capacitec also manufactures high temperature model displacement sensors with operating temperatures up to 1000°C. Used in applications such as glass making, turbine engine test and strength of material testing, these sensors allow real time control of specialized process at extremely elevated temperatures.

Capacitec sensors are also not effected by high magnetic fields (e.g. 2 Tesla). Particular Industrial Control systems require sensors with a high resistance to magnetic fields. Examples of these applications are Diesel Fuel Injectors, magnetic levitation motors for trains and particle physics experiments. Other applications require that the sensor itself does not influence sensitive magnetic detectors such as magnetometers.

A significant factor in the differentiation of Capacitec Sensor Systems over others is their proven reliability over very long periods of time. At Square D, Capacitec sensor systems have been measuring electrical switch contacts, on-line, five to seven days per week, during two shifts, for over 12 years without failure. At nuclear fuel bundle assemblers, where quality assurance requirements are second to none, Capacitec sensors have measured thousands of fuel rod gaps daily, for over 20 years, without failure.

Small size, high precision, excellent linearity and repeatability, combined with the ability to withstand harsh environments and outstanding long term reliability are all factors that differentiate Capacitec non contact displacement and gap sensing systems from the competition.

Applications

Photocopiers/Printers
Photocopier and printer manufacturers require very precise measurements and very tight tolerances in several difficult to access and hidden locations throughout their equipment. Non contact displacement sensors are used to successfully solve these application challenges.

Critical zones in a photocopier where parts tolerance and the setting of precise dimensions have a direct effect on copier performance are as follows:

Paper path alignment is critical to optimum paper thruput and reduced paper jams
Alignment of the optics to the copy original is critical to copy quality
Roller alignments of the printing mechanism are critical to print quality

Non contact displacement sensors are used by photocopier manufacturers to set the paper path gaps in photocopiers in the process of manufacturing. In Figure 1 two Capacitec model HPD-4 sensors are configured on either side of a long (e.g. 24 inches/61cm) stainless steel wand to measure the gap between rollers hidden in board in the photocopier. These gaps range from 0.008" to 0.040" (0.20mm to 1.0mm).

 

Figure 1
Capacitance-based displacement sensors have been incorporated into photocopiers, where they detect path, optics, and roll alignment.

Another crucial application is the setting of gaps in the Print Engine for photocopiers and printers. In this application the quality of the gap has a direct relationship to the image produced. The gap is between two rollers. Working with an Industrial Control OEM, Capacitec has designed a special configuration of four sensor wands that are placed between the bar and the roller to set the gap with a typical adjustment window of 0.0004" (0.01mm).

A further application where non contact displacement sensors are used is for a leading manufacturer of Ink Jet Printers. A critical application for them is the precise alignment of the Ink Jet cartridge holder into each individual printer during production. In this case a Capacitec HPB button series sensor was embedded in a mock-up ink jet cartridge. This standard jet cartridge is subsequently used as a type of metrology standard to set the alignment of the cartridge holder for each of the individual printers manufactured.

Glass Making
A major European manufacturer of Automotive windshield and commercial plate glass has been successfully using Capacitec non contact displacement technology over the past 12 years to maximize the thruput and quality of their high volume glass products.

In one application car windshields are manufactured in high volume 24 hours a day under very demanding environmental conditions. Capacitec HPC500 high temperature sensors are used to control the tooling geometry which in turn assures that the windshields are produced with a consistent shape.

The sensors are exposed to 1292°F (700°C) in this high temperature glass molding operation. The sensors are also required to survive daily thermal shock cycles from 77°F (25°C) to 1292°F (700°C). Laser technology sensors were tried in this application in the past but could not survive the high temperature environment.

Another application where the same glassmaker chose Capacitec non-contact capacitive sensors was in the production of their commercial plate glass. These large sheets of glass (about 13 by 16 feet/4 by 5 meters) are produced in what is referred to as a float glass process. The molten glass is forced through a narrow opening (similar to a plastic extruder) to form large thin sheets, which solidify while floating on water. A crucial step in this process is to set the extrusion opening to a precise measurement along the full length of the opening. If the opening has excess variation, the glass will be produced at different thicknesses along its length, causing breakage and other problems.

Commercial plate glass is also manufactured with a variety of coatings. There are typically two to three different coatings applied to each sheet. The different coatings are added for strength, insulation or protection against ultra violet rays. These coating must also be applied in a consistent manner across the full length of the glass and held to tolerances of 0.0039" (0.1mm).

Capacitec sensors were chosen for this application for several reasons. First the sensors are able to handle the elevated temperatures in the glass making process. The extruder gaps, which measure from 0.157" to 0.4" (1.0 to 4.0mm), can also be measured with a linearity of 0.2% FS and 0.4% FS at the elevated temperature of 1292°F (700°C).

Disc Brake Wear Analysis Sensors
The increased demand for closer tolerances and rapid prototype designs has forced automotive brake system and testing engineers to look for new ways to verify engineering predictions and/or explain the dynamic physical characteristics of braking system components.

Using a pair of Capacitec model HPC-150 sensors, one on either side of the disc, automotive brake manufacturers measure the following characteristics at temperatures up to 1100°F (600°C)

Rotor run-out (TIR)
Rotor coning
Wobble
Rotor thickness variation (RTV)
Plate-to-plate orientation (V-ing, barreling)
Thermal expansion

This new sensor system (see Figure 2) is capable of taking dynamic brake system measurements both on-vehicle at test track facilities as well as in Testing Laboratories using dynamometers.

Figure 2
Displacement sensors can be placed in pairs on either side of an automotive brake disc to dynamically monitor an assortment of performance parameters.

Semiconductor Wafer Parallelism Measurement
The semiconductor industry is a classic example of where control methods can make the difference between success and failure in this highly yield sensitive manufacturing process.

During semiconductor wafer production, multiple layers of various materials are deposited in sequence on the wafer surfaces in order to achieve the electrical characteristics specified by the semiconductor device customer. The deposition process takes place in a vacuum chamber and often at temperatures up to 842°F (450°C). A wafer holder device places the wafer onto a deposition plate where the layer of material is then deposited onto the wafer.
A critical element of the wafer deposition process is the uniform thickness of each of the various layers being deposited onto the wafer. In order to achieve this uniform thickness, both the wafer holder and the deposition plate must be kept parallel to within 10 microns to insure that the wafer will be positioned exactly parallel at the time that the deposition occurs.

A US semiconductor process equipment manufacturer, supplying wafer deposition equipment for 8" (200mm) and the new 12 inch (300 mm) wafers, uses an array of three Capacitec non contact displacement sensors to measure the parallelism of the deposition plate to the wafer holder prior to the wafer being installed in the holder. In this application, the sensors are mounted into the bottom deposition plate (see Figure 3). The probes could be optionally mounted into a removable tool that could be positioned between the wafer holder and the depostion plate in order to affect the measurement. The Capacitec probes have a case ouside diameter of 1.0" (25.4mm) and a linear range of up to 0.325" (8.25 mm). When matched with Capacitec's 4100-SL instrumentation amplifiers, the sensors can provide an accuracy and repeatabiltiy of 0.2% and 0.01% over the actual measurement range. The high temperature option enables the probes to operate at temperatures up to 1600°F (871°C).

100% Inspection of manufactured component parts
In response to the demand for ever higher quality and lower costs, many industries have shifted from sample inspection (e.g. AQL) of manufactured component parts to 100% in-line inspection. A Case in Point is Groupe Schneider, manufacturers of electrical and electronic components.

One of Schneider's well known product lines are large electrical switches known as contactors. These products are produced in high volume (millions per year) with many component parts manufactured using automated assembly. One of the key criteria for success with this process is high quality component parts. Capacitec non contact displacement sensors are used to assure the quality of a critical component of this assembly - namely the contacts. To assure that the contacts are perfectly matched, Schneider requires 100% in-process inspection of the flatness of these approximately 0.157" x 0.236" (4.0mm x 6.0mm) rectangular contacts (See Figure 4). Five Capacitec Model HPC-40 probes with 0.187" (4.7mm) diameter sensors are positioned above the contacts as they move by on a small conveyor at 6.0"/second (150mm/second). The Capacitec sensors measure distance at three different points on the contacts, which allows the manufacturer to evaluate their flatness against spec. The parts are inspected 100% and only those contacts that meet spec are installed in the switches.

Figure 4
The contacts in large electrical switches can be inspected for flatness with displacement sensors positioned above the contacts as they move along a conveyer.

Kelsey Hayes has a similar application where Capacitec probes are used to 100 % inspect the high volume manufacture of disc brake thickness, parallelism and TIR. This system has been functioning successfully in a continuous, 3-shift operation since 1978.

more info >> Gap Sensor Technology



For more information, please contact:-

Capacitec
UK contact: Tel: +44(0)1527 854103
Email:
sales@techni-measure.co.uk
Website:
www.techni-measure.co.uk

January 2009

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