|
|
|
 |
| Figure 1. Capacitance-based displacement sensors have been incorporated into photocopiers, where they detect path, optics, and roller alignment. |
Manufacturers are also continuing their push toward moving QC practices forward in the process. The preferred strategy now is to use in-process, and in some cases pre-process measurements to inspect a product. In-process (also called in-line) measurement checks a product's dimensions and adjusts them as the product is being made. The pre-process technique uses noncontact displacement sensors during setup to check and adjust tooling, jigs, and fixtures before production begins.
Manufacturers are always interested in reducing their costs, and two of the best ways to do that are to minimize scrap and maximize throughput. Taking displacement measurements early in the process and making the necessary adjustments right then and there is a much more cost-effective tactic than trying to correct a flaw in a finished product or throwing it into the reject bin.
Severe environments and small targets that are hard to get at kept displacement sensors out of the industrial control arena until fairly recently. Capacitec's line of noncontact displacement sensors was designed to meet these challenges. Its sensor electronics design, for example, features a favorable ratio of linear range to sensor size. Sensors measuring 0.010 in. (0.25 mm) in diameter (overall probe diameter of 0.040 in., or 1.0 mm), for instance, offer a 0.2% F.S. linear range of 0.0078 in. (>=0.2 mm). This is of significant advantage in industrial control applications such as hard disk drive assembly, parts inspection, IC fabrication, and jet engine manufacture.
The sensors are also built to withstand extreme environments. For example, they can handle nuclear radiation up to 1012 RADS accumulated dosage. Some, with operat-ing temperatures of up to 1000ºC, are used in glass making, turbine engine testing, and strength-of-material evaluation. Ability to work in the presence of high magnetic fields (e.g., 2 T) makes them suitable for use on diesel fuel injectors, magnetic levitation motors for trains, and particle physics experiments. Let's look at the details of some typical--and demanding--applications.
Photocopier and Printer Manufacture
Makers of photocopiers and laser printers need precise measurements and tight tolerances for:
 |
| 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. |
Noncontact displacement sensors help set the paper path in photocopiers during manufacture. In Figure 1, two Model HPD-4 sensors, one on either side of a 24 in. (61 cm) stainless steel wand, measure the gaps between rollers hidden deep inside a photocopier. These gaps range from 0.008 in. to 0.040 in. (0.20–1.0 mm).
The accuracy of the gap in a photocopier's or laser printer's print engine has a direct relationship to the quality of the image produced. Capacitec codesigned a special configuration of two sensor wands that are placed between two rollers to set a gap with a typical adjustment window of 0.0004 in. (0.01 mm). The sensors are also used for precise alignment of the inkjet cartridge holder into each printer during production. For one manufacturer, Capacitec embedded an HPB button-series sensor in a mockup inkjet cartridge that then became the alignment standard for cartridge holder positioning.
Glassmaking
Noncontact displacement technology is used in the manufacture of automotive windshields and commercial plate glass. Capacitec's HPC-500 high-temperature sensors control the tooling geometry that ensures consistent windshield shape. This glass-molding operation exposes the sensors to temperatures as high as 1292ºF (700ºC) and daily thermal shock cycles from 77ºF to 1292ºF (25ºC–700ºC).
 |
| Figure 3. In semiconductor wafer manufacture, capacitive displacement sensors can be incorporated in the bottom deposition plate to measure the parallelism of the deposition plate to the wafer holder. |
In the plate glass application, sheets of glass measuring 13 ft by 16 ft (4 m by 5 m) are produced in a float glass process in which the molten glass is forced through a narrow opening (similar to a plastic extruder) to form large thin sheets that solidify while floating on water. A crucial step is to precisely adjust the extrusion opening along its full length. Too much variation produces glass of uneven thickness. After a sheet of glass solidifies, two to three different coatings are typically applied to it for strength, insulation, or protection against UV rays. These coatings must be applied in a consistent manner across the full length of the glass and held to tolerances of 0.039 in. (0.1 mm). Capacitec sensors can measure the extruder gaps, which range from 0.157 in. to 0.4 in. (1.0–4.0 mm), with a linearity of 0.2% F.S.
Disc Brake Wear Analysis
The increased demand for tighter tolerances and rapid prototype designs has forced automotive brake system and testing engineers to look for new ways to verify engineering predictions and 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, brake manufacturers can measure the following characteristics at temperatures to 1100ºF (600ºC):
This new sensor system (see Figure 2) is capable of taking dynamic brake system measurements on-vehicle at test track facilities, as well as in testing laboratories using dynamometers.
Semiconductor Wafer Parallelism Measurement
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 conveyor.
During semiconductor wafer production, multiple layers of various materials are deposited in sequence on the wafer surface to achieve the electrical characteristics specified by the customer. The deposition process takes place in a vacuum chamber, often at temperatures up to 842ºF (450ºC). A wafer holder device places the wafer onto a deposition plate, where a layer of material is deposited onto the wafer. To ensure uniformity of each layer's thickness, both the wafer holder and the deposition plate must be kept parallel to within 10 microns. An array of three noncontact displacement sensors incorporated into the bottom deposition plate (see Figure 3) measures the parallelism of the deposition plate to the wafer holder before the wafer is placed in the holder. Alternatively, the probes could be mounted in a removable tool that could be positioned between the wafer holder and the deposition plate. These probes have a case o.d. of 1.0 in. (25.4 mm) and a linear range of up to 0.325 in. (8.25 mm). When matched with Capacitec's 4100-SL instrumentation amplifiers, the sensors can provide an accuracy and repeatability of ±0.2% and ±0.01%, respectively, over the actual measurement range.
100% Inspection of Manufactured Parts
Groupe Schneider makes, among other electrical and electronic components, large electric switches called contactors. The company produces millions of these 0.157 in. by 0.236 in. (4.0 mm by 6.0 mm) units per year. To ensure that the contacts are perfectly matched, Schneider requires 100% in-process inspection for flatness (see Figure 4). Five Model HPC-40 probes (three are shown) with 0.187 in. (4.7 mm) diameter sensors are positioned above the contacts as they move by on a small conveyor at 6.0 ips (150 mm/s). The sensors measure the distance to three points on the contacts, allowing their flatness to be evaluated against spec.
Summary
Overcoming many obstacles to the use of noncontact displacement sensors, Capacitec units combine small size, high precision, and excellent linearity and repeatability with the ability to withstand harsh environments. They have demonstrated themselves in many manufacturing operations, from static preprocess adjustments to high-speed, 100% online part inspection.
Bryan Manning is a marketing consultant for B&D International LLC, 1050 Brook Rd., Milton, MA 02186; 617-698-1123.
Robert L. Foster is Director of Engineering, Capacitec, PO Box 819, 87 Fitchburg Rd., Ayer, MA 01432; 978-772-6033, fax 978-772-6036.
|
|
