Gap Sensor Technology

Innovative Technology from Capacitec improves Gap Measurement with the use of Capacitive Non-Contact Gap Sensors.

In the past, quality control, manufacturing engineering and set-up personnel have been using the traditional contact methods (plastic shims, feeler gauges, step gauges etc.) to measure gaps.

These gaps were found in a wide variety of applications from attaching wings to Airbus aircraft and measuring the fan blade tip clearance of aero engines to setting gaps in the processes of metallurgy, coating, extrusion and laminating composite materials.

Figure 1
Non Contact Capacitive Sensor and Gap Wands for Industrial Applications

Due to the increased implementation of quality control methods such as Six Sigma, SPC and ISO 9000, global companies such as General Electric, Ford and EDF are requiring the measurement, data capture and documentation of more and more measurements such as gaps in their manufacturing and assembly processes. Traditional gap measurement methods cannot meet the new quality requirements for accuracy and repeatability and are not able to automatically record and store data.

Engineers have also found limitations and major reliability problems with traditional gap measurement methods. Shims and feeler gauges have marginal accuracy when the tolerance required is better than 25 microns. Once they are removed from the gap after set-up there is no way to be sure that the gap is still within specification.

Plastic shims can vary in thickness by 7.6 microns and feeler gauges show different results from operator to operator. The accuracy of contact measurement methods is reduced over time. The constant rubbing against hard surfaces during the measurement process causes wear on the shims and feeler gauges, and can also damage the highly polished surfaces found in applications such as slot die coaters and roller to roller gaps.

Several Gauge R & Rs (Gauge Repeatability and Reproducibility Studies) have been performed by major companies to compare the capability of traditional contact methods versus those using non-contact capacitive gap sensors. A leading jet engine manufacturer, for example, showed that feeler gauges could not meet their six sigma requirements. When using the six sigma quality method, the dispersion is measured with sigma. Sigma is the measurement of standard deviation and many manufacturers are requiring six sigma. The engine manufacturer's Gage R & R showed results of 45% for feeler gauges versus 25% for Capacitec gap gauges. Since feeler gauges showed a measurement dispersion of greater than the required 30% minimum for six sigma, the engine manufacturer switched to the Capacitec. They now use a non contact capacitive gap measurement method using a Gapmanâ with remote sensing wand. (See Figure 2.)

Figure 2
Portable Gap Instrument with Contact Wand for non conductive targets

Capacitec's New Technology
Non-contact capacitive gap sensors have been in use for over 20 years in applications such as measuring the gaps between Nuclear Fuel Rods for quality control. This is done by creating a custom dual sensor wand measuring 0.9 mm thick by 20 mm wide by 350 mm long. It is used to replace contact methods as a superior method to monitor the minimum gap between the fuel rods in initial manufacture. Recently there have been some significant advances in levering this technology. Some examples:

Thin gap sensors that measure gaps down to 100 microns at 250°C.

Robust gauge block type gap sensor wands for Industrial applications that can survive temperatures up to 870°C (see Figure 3).

Non contact sabers that can measure gaps as small as 1 mm into a slot 1.5 meters in length.

Contact type sensor sabres that can measure non conductive and odd shaped targets with gap ranges to 6 mm (see Figure 4)

Alignment fixtures providing accuracy better than 25 microns and resolution down to 0.25 microns (see Figure 5)

Gapmanâ the Portable Non-Contact Electronic Thin Gap Measurement System (see Figure 6)

Sensor Electronics with improved linearity to 0.1% full scale

RS232 with system interface to computers and other data acquisition programs

Complete gap sensing solution packages for diverse applications such as Slot Die Coating measurement (see Figure 7) and Aircraft Engine Fan Blade Tip Clearance.

Figure 3
Non Contact Capacitive Sensor Gap Wands for Industrial Applications

Figure 4
Contact Type wand for non-conductive targets

Figure 5
100 micron gap wand in alignment fixture

Figure 6
Battery operated gap measurement system for roller to roller measurement

Figure 7
Slot Die Coater Gap Sensor System

Typical Gap Measurement Applications

Nuclear Fuel Rods
The gap between individual fuel rods in a nuclear fuel rod assembly is critical to the safe functioning of a nuclear plant. Fuel rod assemblies are typically made up of 16 x 16 rows of 12 mm diameter fuel rods measuring 6 meters in height. The gaps between fuel rods typically run from 1 to 3 mm. The accuracy requirement is 10 microns or better. Capacitec supplies sabres that measure 350 mm in length, 0.9 mm thick and 20 mm wide. Each saber contains dual non-contact displacement sensors. The sensors output are summarized along with the thickness of the sabre to provide total gap. This signal is sent to the sensor electronics, which is housed in an automated measurement assembly. The automated system advances and retracts the sabres between each fuel rod gap throughout the assembly and at 1 meter intervals in height. The dual sensor sabres take highly precise position compensated gap measurements between rods with an accuracy of 0.25% of full scale or better.

Nuclear Generators
Jeumont uses Capacitec to measure gaps between 0.22 to 4.0 mm in the manufacture of Nuclear Generators. The sensors are manufactured with special materials that allow them to be subjected to high levels of radiation without problems. Capacitec sensors can handle radiation up to 10 (to the 8th) Rads accumulated dosage.

Aircraft Assembly
Airbus and Boeing use the Capacitec Gapmanâ to measure and control gaps on commercial aircraft skins and structural components. Typical gaps range from 0.22 mm to 3.2 mm. In the assembly of tail sections, a Gapmanâ is used to measure gaps 20 cm inside of the subassembly. It is also used to measure gaps between metallic and composite materials. In a shim automation program, gap readings from the Gapmanâ are sent to a CNC machine, which manufactures custom shims that fit perfectly in the void between two structural components of the aircraft.

Metallurgy - Continuous flow sheet metal
Over the past several years one of the major breakthroughs in steelmaking has been the development of an improved method to make cold rolled laminated steel. This new method uses a continuous process where the hot molten steel flows directly into sheets instead of going through the intermediary steps of cold rolling. The advantage of this method is a major savings in time and reheating energy that results in a cost reduction of a minimum of 10%, while maintaining the same level of quality. The principle of the process is simple. Instead of forming an ingot which is then rolled while hot, the molten steel is passed directly between two cylinders of 1500 mm (1.5 meters) in diameter - rotating in opposite directions. The gap between the rollers represents the thickness (3 to 4 mm) of the steel sheets produced and is thus a critical dimension in the process.

The diagram in Figure 8 shows an overall view of the process.

Figure 8
Overview of the Continuous Flow Process

To set the width of the steel sheets, one of the rollers is maintained in a fixed position as a reference, while the other roller is set in position to determine the gap size. A 3 mm gap would thus produce 3 mm thickness steel sheets; a 5 mm gap produces 5 mm sheets.

Capacitec gap sensors are used to measure the position of each of the two rollers relative to each other (this sets the gaps size or thickness of the sheet metal); the position of the side face of the roller during production and the position of the mobile roller relative to other parts of the equipment.

Metallurgy - Aluminum sheet fabrication
Capacitec gap sensors are used by Pechiney to control the fabrication of aluminum sheets. A high temperature version capacitive sensor wand (similar to those found in Figure 3) is placed between the rollers at each end of the rolling mechanism. This is to control and maintain the gap changes between the cylinders to distance of 3.5 mm maximum.

Xerox has used Capacitec thin gap technology extensively in the production of photocopiers since 1982. In next generation color copiers and presses Capacitec non-contact gap measurement sensors are used to control various settings. The very critical distances between photoactive components to paper media must be maintained to a minimum of 0.2 to 0.3 mm. Paper paths must be preset to gaps from 0.3 mm to 0. 6mm. Alignment of printer heads in ink jet and laser printers are also typically held in a nominal window of between 0.7mm to 0.8 mm using this technology.

Ceramic Extrusion
DSM Holland is a well known manufacturer of multi-layer ceramic capacitors. These capacitors are made through a process of extruding ceramic layers. To improve product quality they worked with Capacitec to develop a custom slot die coating gap measurement system using the Gapmanâ. This system controls the extrusion process to ensure the quality of the ceramic coatings on their substrates.
Slot Die Coating
Troeller, a worldwide leader in the manufacture of high quality Slot Die coaters uses the Capacitec Slot Die Coater Gap Sensor System to control gaps in their systems. A slot die gap uniformity of 0.25 microns is maintained along the 1 to 2 meter length of the dies.

Photographic Film Manufacturing
In the photographic film and adhesive label industries, large one to two meter parallel stainless steel plates are used in the coating process. These plates have a machined air gap of between 0.5 mm and 3 mm which must be maintained to better than 0.5 micron or better repeatability across the full length to achieve coating uniformity. The Capacitec Bargrafxä is used to test alignment prior to production and to maintain alignment after various weekly maintenance procedures.

Other Applications
Additional Capacitec applications include; battery component gaps, checking rotor to stator air gaps in electric motors, Polyester film roller gaps, PC board lamination presses, die gaps on large automotive metal forming presses, plate and automotive glass manufacturing etc.

more info >> Automotive applications
more info >> Industrial Control applications

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