GB2195439A

GB2195439A - Measuring coating thicknesses

Info

Publication number: GB2195439A
Application number: GB08709468A
Authority: GB
Inventors: Ian Charles Miller
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1986-09-20
Filing date: 1987-04-22
Publication date: 1988-04-07
Anticipated expiration: 2007-04-22
Also published as: GB8622702D0; GB8709468D0; GB2195439B

Abstract

A coating thickness on a non-flat surface e.g. paint on car body (10) is measured by bringing up to the surface a point thickness gauge (e.g. on infra-red absorption gauge) (24) mounted on a support (22). The support (22) carries an array of sensors (26) mounted on it which, without contacting the surface, tell the equipment when the support (22) is at the correct distance from, and at the correct orientation relative to the surface. A thickness reading can then be taken. The sensors (26) may be inductive or optical e.g. with inductive sensors mounted at the end of each limb of a cruciform shape. <IMAGE>

Description

SPECIFICATION Measuring coating thicknesses This invention relates to a method and apparatus for measuring the thickness of coatings, such as paint, applied to a substrate. The invention is particularly applicable to the measurement of such coatings on a non-flat substrate.

The current method of finishing metal objects, including car bodies, consists of a variety of chemical and paint application processes which provide the corrosion resistance and appearance specified for each particular part or product. The majority'of these treatments are in the form of paint materials that are applied as distinct layers of a required thickness. The coating weight or thickness of each layer is critical to the performance and/or appearance of the material. Therefore careful monitoring of the application processes and of the final result is of the utmost importance in order to produce maximum quality with minimum material usage.

There are a number of methods for monitoring paint coatings currently in use. In principle, most of these rely on contacting the surface with a probe whereby a reading is made, displayed and in some cases electronically recorded. The physical contact made by these gauges has generally meant that these readings have to be made manually. Taking the readings necessary to achieve a comprehensive scan of an entire car body shell is there; fore a very labour intensive function.

It is also known to measure paint thicknesses by a non-contact method using a gauge with an infra-red beam directed at the paint surface. The degree of absorption of the infra-red radiation is proportional to the coating thickness. This gauge requires that the beam be directed at the surface at a constant angle and constant distance from the surface.

For this reason, it has only been practicable to use such a gauge for measuring coating thicknesses on flat substrates.

According to the invention there is provided apparatus for measuring the thickness of a coating on a non-flat substrate, the apparatus comprising a non-contact gauge for measuring the coating thickness without touching the coating surface, and means for locating the gauge in space at a predetermined position and orientation relative to the surface, without touching the surface.

The locating means preferably comprises an array of proximity detectors mounted on a support, and the support may include means for mounting the measuring gauge.

The support may have a cruciform shape, with a detector mounted at the end of each arm and a detector mounted at the centre.

The locating means will then take up a position where the piane of the support is parallel to the substrate.

The detectors may be induction sensors (in this case it is necessary for the substrate and/or the coating to be electrically conductive), or optical sensors. preferably, however, the sensors are infra-red sensors.

The locating means is preferably mounted at the end of a multi-axis arm, and the controls for moving the arm respond to signals from the proximity detectors so as to correctly position the locating means relative to the substrate to enable coating thickness readings to be taken.

The non-contact gauge preferably emits infra-red radiation which is selectively absorbed by the coating. The infrared source may be a tungsten filament lamp or a laser. An optical arrangement is provided to focus the radiation and to collect the reflected radiation. The radiation may be supplied to this optical arrangement through optical fibres.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of apparatus in accordance with the invention for measuring paint coat thicknesses on a car body; Figure 2 is a schematic view of the measuring and locating head of the apparatus; and Figure 3 is a schematic view of a non-contact gauge.

Figure 1 shows part of a car body 10 supported on a conveyer 12 which carries it past a paint thickness measuring station 14. At the station 14, an arm 16 is mounted on a motorized pedestal 18 and has a large number of axes about which it is free to move, as is well known for arms of this type.

At the outer end of this arm is a track 20 along which a support 22 can be moved. The track 20 allows the support to move over the whole height of the body 10.

The support 22 mounts a measuring gauge 24 and a locating head 26. The position of the measuring gauge relative to the locating head is fixed, so that when the locating head is correctly positioned, the measuring gauge is also correctly positioned.

The locating head 26 (see Figure 2) has a cruciform shape, with induction sensors 28 at the end of each limb and at the -centre. The dimension across the limbs of the head can for example be 150mm. The sensors are set up so that they produce a signal which brings the locating head to a halt when the sensor is for example 50mm from the body 10. The signal can for example be an ON/OFF signal which changes state at the correct distance from the surface. When this condition is reached for all the sensors, the locating head will lie generally parallel to the body surface.

Of course, where the body surface is curved, it will not be possible for the head to be truly parallel to the surface, but the electronic circuitry which receives the signals from the sen sors 28 will perform an "averaging" function to obtain the best compromise position for the head. If infra-red proximity detectors are used, the head may stop at a position about 100mm from the surface.

Once the head is correctly positioned, for example at 200mm from the surface, the gauge 24 will also be correctly positioned. For the obtaining of accurate thickness readings, it is important that the beam of infra red radia tion emitted by the gauge 24 should impinge on the surface at the designed angle which will normally be 20 (+/--1"). This angle is required to avoid any undue influence on the reading resulting from ambient light, and is indicated in Figure 2, where the point at which the beam impinges on the surface is indicated by X, and lies centrally of the head 26.

The gauge can be based on the operating principles established by infra-red Engineering Ltd., of Maldon, Essex, UK, for their infra-red absorption gauge. Some modifications may be necessary to this gauge if it is required to measure thick paint coatings, as applied to car bodies.

As an alternative method for the measure ment of the paint thickness, a laser source 30 (Figure 3) could be used. The basic principle of operation is based on the use of two reference frequencies of infra-red radiation being compared to a single absorption frequency.

This multifrequency beam of infra-red can be supplied by one or more laser units. The ac tual laser unit(s) can be mounted in the base unit of the manipulator arm 16, and the beam transferred to the measuring head via an opti cal cable 32.

The measuring head 34 would be designed to direct the beam of infra-red, which is diverged by the use of a lens 36, onto the body surface 10. The reflected beam is then collected using a spherical mirror 38 and fo cused onto an infra-red detector 40. Using electronics, the various signals can be com pared and a measurement achieved. The com plete measuring head would be mounted on the manipulator arm 16, as described above, to enable the system to maintain the angle of incidence.

The use of optical fibres to transmit the infra-red radiation results in substantial reduc tion in inertia of the measuring head which enables the system to be more manoeuvrable.

Furthermore, the inherent properties of a laser, i.e. power and limited frequency spread, will enable more accurate measurement to be made.

The programme of movement of the arm 16 will be predetermined so that paint thickness readings are taken at the desired positions on the car body. Once the support 22 has been moved to the desired position, the locating head will approach the body 10 and adjust itself to the correct distance and orientation relative to the body. The locating head can also have additional sensors (not shown) which will detect the approach of an opening in the car body.

It is a major advantage of this invention that there is no physical contact between the measuring equipment and the article being measured. This effectiveiy prevents any possibility of the body being damaged by the equipment, but also makes it possible to measure thicknesses while the paint is still wet, which could have far-reaching implications for speeding up production processes.

Furthermore, the equipment will not need to be reprogrammed for model changes, or when a mixture of car body styles is being handled on the same line.

Claims

1. Apparatus for measuring the thickness of a coating on a non-flat substrate, the apparatus comprising a non-contact gauge for measuring the coating thickness without touching the coating surface, and means for locating the gauge in space at a predetermined position and orientation relative to the surface, without touching the surface.

2. Apparatus as claimed in Claim 1, wherein the locating means comprises an array of proximity detectors mounted on a support, and the support includes means for mounting the measuring gauge.

3. Apparatus as claimed in Claim 2, wherein the support has a cruciform shape, with a detector mounted at the end of each arm and a detector mounted at the centre.

4. Apparatus as claimed in Claim 2 or Claim 3, wherein the detectors are induction sensors and the substrate and/or the coating is electrically conductive.

5. Apparatus as claimed in Claim 4, wherein the detectors are optical sensors.

6. Apparatus as claimed in any preceding claim, wherein the locating means is mounted at the end of a multi-axis arm, and the controls for moving the arm respond to signals from the proximity detectors so as to correctly position the locating means relative to the substrate to enable coating thickness readings to be taken.

7. Apparatus as claimed in any preceding claim, wherein the non-contact gauge comprises a source of infra-red radiation and an optical arrangement for focusing the radiation on the substrate and for collecting the reflected radiation.

8. Apparatus as claimed in claim 7, wherein the infrared radiation is powered by a laser.

9. Apparatus as claimed in claim 7 or 8, wherein the radiation source is positioned remotely from the optical means, and is linked up with the optical arrangement by optical fibres.

10. Apparatus for measuring the thickness of a coating on a surface, substantially as herein described with reference to the accom panying drawings.