Curtis Assemble & Test installs confocal sensor to perform precision alignments to 2 micron accuracy
UK-based engineering and test company Curtis Assemble & Test Ltd has installed a non-contact confocal chromatic sensor from Micro-Epsilon on an automated assembly and test machine that checks the alignment of a precision disc pin assembly, part of an automotive fuel injector valve. These high precision confocal displacement sensors are able to measure the alignment of the valve disc pin assembly to within a clearance of 2 microns.
Based in Colchester, Curtis Assemble & Test works with a wide range of customers, from mainstream automotive production to Formula 1 teams. Testing and measuring customers' products requires an extraordinarily high level of cleanliness and so the company has developed a variety of in-process and clean room high-pressure wash units, specialising in hydraulic, pneumatic and leak testing, selective high precision assembly techniques and vision systems.
As Steve House, Works Director at Curtis Assemble & Test Ltd states: "The high level of precision required by our customers means that in terms of measurement technologies, our machines and test rigs must be equipped with the very best sensors and measurement devices."
"Recently, one of our customers required us to check the mechanical alignment of an automotive fuel injector valve pin assembly to within a few microns. Basically, the assembly machine that we've developed presses the two parts together - a 15mm diameter valve disc onto a 2mm diameter injector pin - and checks that these parts are mechanically aligned to a set clearance of +/- 2 microns, which is critical in order to ensure that the valve moves freely. We therefore required a displacement sensor that was even more accurate than this," he continues.
Prior to this, says House, the customer had been checking the alignment using a high end, clean room-mounted laser measuring device, which took approximately ten minutes per component to complete the necessary alignment check - which was too long. "We were therefore asked by the customer to develop a special purpose test and assembly machine that would reduce the time taken for alignment checks. Now, we transfer the valve, disc and pin assembly into the special purpose automatic assembly machine using a robot handling system. In order to avoid any measurement interference due to vibration from the moving robot, we mounted Micro-Epsilon's confocal displacement sensor on a separate frame and stand, although still in close proximity to the parts to be measured. The machine is now fully tested and proven, enabling us to check the alignment within 25 seconds."
Curtis Assemble & Test did consider alternative displacement sensors, including laser-based versions, but as Steve House explains, "these were prone to interference from the thin film of oil present on the metal components that were being measured, as well as the wet environment the sensors needed to operate in. With the confocal sensor, we don't have any of these issues. In addition, the confocal sensor works very well on highly reflective, polished metal parts."
"We were familiar with Micro-Epsilon sensors as we had trialled some of the company's confocal sensors on another previous assembly machine. So we knew the sensors could do the job and meet the very high levels of precision required for this particular project," he continues.
After discussing the application, Micro-Epsilon recommended its confocalDT IFS 2405-3 non-contact confocal displacement sensor, which is able to measure on shiny, highly reflective metal surfaces and which could also provide the extremely high measurement precision required for the machine.
As Steve House concludes: "We've learned a great deal about Micro-Epsilon's confocal displacement sensors on this project. The experience and knowledge that we've gained from this will stand us in very good stead for future customer projects, particularly where high precision measurements on shiny, highly reflective metal components are required."
The confocalDT IFS 2405 series of confocal sensors are designed for measurement tasks that require maximum precision - typically research and development tasks, laboratory and medical, semiconductor manufacturing, glass production and plastics processing. As well as distance measurements on highly reflective, shiny metal components, the sensors can also be used to measure on dark, diffuse materials, as well as for one-sided thickness measurement of clear film, boards or layers. The sensors also benefit from large stand-off distances (up to 100mm), providing users with greater flexibility in terms of the variety of applications in which the sensor can be used. In addition, the tilt angle of the sensor has been increased significantly (up to 34 degrees), which provides better performance when measuring across changing surface features, for example, from shiny metal to dark, diffuse objects.
The confocalDT IFS 2405 series comprises five sensors with measuring ranges from 0.3mm up to 30mm. Spot diameter is from 6µm up to 50µm. Maximum resolution is 0.01µm and maximum linearity is 0.3µm.
Confocal measuring principle
The confocal chromatic measuring principle works by focusing polychromatic white light onto the target surface using a multi-lens optical system. The lenses are arranged in such a way that the white light is dispersed into a monochromatic light by controlled chromatic deviation (aberration). A certain deviation (specific distance) is assigned to each wavelength by a factory calibration. Only the wavelength that is exactly focussed on the target surface or material is used for the measurement. This light reflected from the target surface is passed through a confocal aperture onto a spectrometer, which detects and processes the spectral changes. Both diffuse and specular surfaces can be measured using the confocal chromatic principle.
Confocal measurement offers nanometre resolutions and operates almost independently of the target material. A very small, constant spot size through the measurement range of the sensor is achieved. Miniature radial and axial confocal versions are available for measuring the internal surfaces of drilled or bored holes, as well as the measurement of narrow apertures, small gaps and cavities.
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