System and method for visually aligning terahertz light beam

A system for measuring a coating thickness on a target surface includes a terahertz spectroscopy device and a reference image projector. The terahertz spectroscopy device includes a radiation head that is operable to project a terahertz radiation beam onto the target surface and receive a reflected beam. The reference image projector includes a visible light device and is operable to project a reference image using the visible light device onto the target surface. A visual characteristic of the reference image indicates at least one of distance, rotational alignment, and angular alignment of the radiation head relative to the target surface.

FIELD

The present invention relates to a system and method for aligning a terahertz radiation beam for measuring thickness of multiple layers on a surface.

BACKGROUND

The exterior of a vehicle generally includes multiple layers of paint and/or other coatings, such as electrocoat, primer, basecoat, and clear coat. Each layer has a minimum film build designed to inhibit the degradation and potential delamination of the exterior due to, for example, UV and visible light, and to provide the appropriate appearance/color to the vehicle, and to protect the substrate from environmental damage.

While various paint thickness measurement techniques are available for measuring a single layer of paint, there are a limited number of non-destructive measurement techniques for measuring multiple layers. One such measurement technique utilizes ultrasound technology in which an ultrasonic transducer is placed on the exterior surface, and sends an ultrasonic signal through the exterior surface. A liquid couplant, usually water, is used to transmit the signal into the coating material. The ultrasonic signal generates an echo at the layer interfaces, and the thickness is determined based on the time difference between the successive echoes. Sound velocity values vary among the different coatings, so calibration is performed on all layers in addition to the various basecoat colors.

While the ultrasound technique is effective, there are some issues with this technique. For example, the transducer size and the tool used for the transducer may not allow measurement of certain vehicle surfaces, such as a windshield flange, and thus, a separate procedure is usually employed to obtain data of those areas. Another issue is that the transducer requires a large (e.g., 10 mm diameter) flat area in order to generate adequate waveforms. This requires selecting points on a vehicle based on their flatness rather than being able to select locations on the vehicle that are of interest but may not be flat. Furthermore, the transducer physically contacts the vehicle. Although damage from the transducer may not occur, the water left on the body affects other quality control measures, such as a dirt detection quality check.

Another technique for measuring a multi-layer surface includes the use of a radiation beam having a terahertz (THz) frequency. Using a THz light source to generate a THz radiation beam, a THz head is positioned at a designated offset and is normal to a target surface of the vehicle before the measurement is performed. For example, the radiation head can be attached to a robot or some other piece of automation to allow it to track the contour surfaces and the complex geometries. The THz radiation beam is emitted from the Thz radiation head and reflects off the vehicle due to a change in refractive index. The time difference between the emission and reflection is used to calculate the thickness. The THz signal reflects off of the coating interfaces, due to a change in refractive index, and the time difference of the reflection is used to calculate the thickness.

Since the THz head is fairly compact and does not contact the surface of the vehicle, it can be used to measure places not measurable by an ultrasonic transducer, such as the windshield flange. The THz radiation beam is typically 1 mm in diameter which enables measurement of multiple regions that have a flat section of that size.

However, for an optimal measurement, the THz radiation head should be aligned normal to a target surface of the vehicle so that the emitter of the radiation head aligns with the detector of the radiation head. When the emitter and detector are aligned, the amplitude of the reflected radiation signal is usually at the maximum value. If the radiation head is not normal to the surface, the reflected radiation signal may not align with the detectors, which results in a lower peak amplitude. This loss in signal may affect the results of the thickness measurement. Misalignment during the calibration procedure would also result in an incorrect calibration file and bad data. These and other issues are addressed by the teachings of the present disclosure.

SUMMARY

In one form, the present disclosure is directed towards a system for measuring the thickness of a coating on a target surface. The system includes a terahertz spectroscopy device and a reference image projector. The terahertz spectroscopy device includes a radiation head that is operable to project a terahertz radiation beam onto a target surface and receive a reflected beam. The reference image projector includes a visible light device and operable to project a reference image using a visible light source onto the target surface. A visual characteristic of the reference image indicates at least one of distance, rotational alignment, and angular alignment of the radiation head relative to the target surface.

In another form, the terahertz spectroscopy device includes a terahertz light source that is operable to generate the terahertz radiation beam, and the radiation head that includes an emitter to emit the terahertz radiation beam and a detector for receiving the reflected beams.

In yet another form, the visible light device is positioned with the radiation head.

In one form, the system further includes a controller configured to control the position and orientation of the radiation head with respect to the target surface based on the visual characteristic of the reference image.

In another form, the reference image comprises at least two patterns that are superimposed with each other. Each pattern has a visual characteristic that is dependent on the position of the radiation head with respect to the target surface, and is independent of the other patterns.

In yet another form, one pattern of the at least two patterns is a plurality of dots arranged in a matrix form and another pattern of the at least two patterns is multiple rings having different diameters and concentrically positioned with one another.

In one form, the at least two patterns are different geometric shapes that are superimposed with each other.

In another form, the visual characteristic of the reference image includes at least one of size, deformation, and rotational position.

In one form, the present disclosure is directed toward a method for aligning a terahertz radiation head of a spectroscopy device with a target surface. The method includes: projecting, by a visible light source, a reference image onto the target surface; analyzing a visual characteristic of the reference image to determine the alignment of the terahertz radiation head with respect to the target surface; and aligning the terahertz radiation head with the target surface such that the visual characteristic of the reference image is within a calibrated visual characteristic. A terahertz radiation beam emitted from the terahertz radiation head is surrounded by the reference image, and the visual characteristic of the reference image is indicative of at least one of distance, angle, and rotational orientation of the terahertz radiation head.

In another form, the calibrated visual characteristic is representative of the reference image when an alignment of the terahertz radiation head with respect to the target surface is optimal for receiving a reflected terahertz radiation beam from the target surface.

In yet another form, the projecting the reference image further includes projecting at least two patterns that are superimposed with each other on the target surface to form the reference image.

In one form, the present disclosure is directed toward an alignment method for a terahertz radiation head of a spectroscopy device with a target surface of a vehicle. The method includes: projecting an image onto the target surface using a visible light source; and aligning the terahertz radiation head with the target surface until a visual characteristic of the image meets a calibrated characteristic. The calibrated characteristic is representative of the image when the terahertz radiation head is at a calibrated position.

In another form, the projecting the image further includes projecting two patterns that are superimposed with each other on the target surface to form the reference image. Each pattern has a visual characteristic that is dependent on the position of the terahertz radiation head with respect to the target surface, and is independent of the other pattern.

In yet another form, the visual characteristic of the image is indicative of at least one of distance, angle, and rotational orientation of the terahertz radiation head.

DETAILED DESCRIPTION

A terahertz radiation beam is not visible to the human eye, and thus may be difficult to align the radiation head, such that is its perpendicular to a target surface. While some systems include a single visible laser beam aligned with the radiation head to aid in the alignment of the radiation head, the laser beam does not provide the operator with an indication of distance from the radiation head nor does it provide an indication of rotation about the radiation head.

The present disclosure is directed toward a terahertz sensory system that includes a reference image projector for projecting a reference image onto a target surface of the vehicle that provides a visual tool for aligning a radiation head of the system with the target surface. As described further herein, a visual characteristic of the reference image is used to indicate, for example, distance and rotational alignment of the radiation head relative to the target surface.

Referring toFIG. 1, a terahertz (THz) sensory system100for measuring the thickness of one or more paint layers on a vehicle body102. The system100includes a light source106, a radiation head108coupled to the light source106, and a controller110. The light source106is operable to generate a radiation beam112within the THz frequency range. Accordingly, the radiation beam112is in a region of the electromagnetic spectrum that includes microwaves and infrared light wave. The radiation beam112can penetrate a wide variety of materials and travel in a line of sight.

In one form, the radiation head108is coupled to the light source106by way of a fiber optic cable, and is arranged and attached to a moveable member114, such as a robotic arm. The moveable member114is operable to adjust the orientation and the position of the radiation head108. Referring toFIG. 2, the radiation head108includes an emitter202and a detector204. The emitter202emits or radiates the radiation beam112generated by the light source106toward a target surface208along the vehicle body102. The detector204receives one or more reflected radiation beams210reflected from the vehicle body102. The radiation head108is communicably coupled to the controller110by way of, for example, wires, and transmits data indicative of the reflected radiation beams210to the controller110.

The controller110is a computer that includes, for example, a processor, a computer readable medium, and other electronic components. The controller110is further connected to one or more user interface115, such as a keyboard and a monitor (e.g., liquid crystal display) for allowing an operator to view one or more graphical user interface configured for operating the system100. The controller110is configured to control the light source106and the radiation head108for emitting the THz radiation beam112. The controller110further analyzes the signals received from the radiation head108to determine the thickness of one or more paint layers of the target surface208. An example of such analysis is provided in Applicant's co-pending application, U.S. Ser. No. 14/829,888, filed Aug. 19, 2015 and titled “ROBOTIC VEHICLE PAINTING INSTRUMENT INCLUDING A TERAHERTZ RADIATION DEVICE” which is commonly owned with the present application and the contents of which are incorporated herein by reference in its entirety.

To align the radiation head108with the target surface208, the system100further includes a reference image projector116(FIG. 1) that projects a reference image onto the target surface208using one or more visible light devices. Referring toFIGS. 3A to 3D, in one form, the reference image projector116includes two visible light devices302A and302B (i.e., collectively known as light devices302) for generating the reference image. The light devices302are disposed with the radiation head108and can be any suitable visible light device, such as a visible laser device.

The light devices302are configured to project different images to form the reference image. In one form, the light device302A forms a linear pattern304that includes a plurality of dots arranged along two orthogonal axes, and the light device302B form a circular pattern306of a plurality of rings having varying diameters and concentrically disposed with one another. In another form, the circular pattern306may be offset to a shallow angle such that a slight change in angle results in a large distortion of the projected pattern for a visual assessment described further below. The light devices302are configured to project the respective images such that the images are superimposed to form a reference image308. The light devices302can be configured to form other suitable geometric shapes and/or patterns, and thus the patterns and reference image are not limited to the dots and rings illustrated herein.

With the reference image projector116arranged with the radiation head108, the position and rotation of the radiation head108influences one or more visual characteristics of the patterns projected by the light devices302. For example,FIG. 4Aillustrates a rotational relationship of the radiation head108and the linear pattern304about a z-axis andFIG. 4Billustrates a rotational relationship of the radiation head108and the circular pattern306about a X-Y axes. As illustrated, visual characteristics of the patterns304and306are dependent on the position of the radiation head108, and thus, may be correlated to one or more positional characteristics of the radiation head108relative to the target surface208.

The reference image projector116is arranged with the radiation head108such that the radiation beam112is surrounded by the reference image308and the visual characteristics of the reference image308is within one or more calibrated visual characteristics when the radiation head108is at a measurement position relative to the target surface208(e.g., the radiation head108is normal to and at a designated offset from the target surface208). The visual characteristics include but are not limited to at least one of size, deformation, and rotational position of the reference image308. For example,FIG. 3Dillustrates the reference image308having calibrated visual characteristics when the radiation head108is at the measurement position. At the measurement position, the circular pattern306is positioned within the boundaries formed by the linear pattern304, the linear pattern304forms a square grid made of the plurality of dots, and the circles of the circular patterns306are concentrically arranged with each other.

The visual characteristics of the reference image308indicate the positional relationship of the radiation head108relative to the target surface208. Such positional relationship includes but are not limited to distance, rotational alignment, and angular alignment of the radiation head108relative to the target surface208. For example,FIGS. 5A to 5Cillustrate the reference image when the radiation head108is at positions other than the measurement position with respect to the target surface208. InFIG. 5A, the circular pattern306is larger than that ofFIG. 3Dand correlates to a position in which the distance of the radiation head108is too far from the target surface208. Conversely, inFIG. 5B, the circular pattern306is smaller than that ofFIG. 3Dand correlates to a position in which the distance of the radiation head108is too close to the target surface208. InFIG. 5C, the linear pattern304is rotated and the circular pattern306is deformed which correlate to a position in which the radiation head108is too close to the target surface208and is not normal to the target surface208. Other correlations between the visual characteristics of the reference image and various positions of the radiation head108are possible, and within the scope of the present disclosure.

Using the calibrated visual characteristics and predetermined correlations between the visual characteristics of the reference image and the position of the radiation head108, a visual assessment of the reference image projected on the target surface208is conducted to align the radiation head108with the target surface208. In one form, the visual assessment is performed by that operator that adjusts the position of the radiation head108by operating the moveable member114until the reference image projected on the target surface208is within the calibrated visual characteristics.

In another form, the system100is configured to include a visual recognition instrument that compares visual characteristics of the reference image to the calibrated visual characteristics and uses pre-stored correlations between visual characteristics and positions of the radiation head108to align the radiation head108. For example, the visual recognition instrument has a camera that captures an image of the reference image and transmits the captured image to the controller110. The controller110is configured to determine the position of the radiation head108based on the captured image of the reference image, and align the radiation head108by controlling the moveable member114until the visual characteristics of the reference image substantially match that of the calibrated visual characteristics. In yet another form, the visual assessment is performed by a combination of an operator and the controller110. For example, the operator may make an initial adjustment of the radiation head108when the visual characteristics of the reference image are notably different from the calibrated visual characteristics, and then then operate the controller110to perform a further analysis to further tune the position of the radiation head.

In one form, the controller110may display, an animated representation of the moveable member114, the radiation head108, and the target surface208, on the monitor. In the animated the representation, the controller110displays one or more indicia for indicating the position of the radiation head108relative to the target surface208based on the visual assessment of the reference image. For example, the controller110may show a circular bubble that is displayed in different colors and/or sizes for indicating the distance between the radiation head108relative to the target surface. Other indicia may be used for indicating angle measurement. Such visual indicators assist the operator in aligning the radiation head108with the target surface.

Referring toFIG. 6a radiation head alignment routine600executed by the system100is provided. At602, the system100projects the reference image on to the target surface, and analyzes one or more visual characteristics of the reference image. For example, the controller110operates the reference image projector116to project the reference image on the target surface. The visual characteristics of the reference image may be analyzed by the controller110, the operator of the system100, or a combination thereof.

At604, a visual assessment is performed to determine whether one or more visual characteristics of the reference image are within respective calibrated visual characteristics. For example, the size, rotation, and/or deformation of the reference image being projected is compared to a calibrated size, calibrated rotation, and/or calibrated deformation.

If the one or more visual characteristics are not within respective calibrated visual characteristics, the position of the radiation head108is adjusted based on the analysis of the visual characteristics and on predetermined correlation information, at606. For example, in one form, the size of the reference image correlates to a distance of the radiation head108relative to the target surface208. Thus, if the size of the reference image is smaller than the calibrated size, then the radiation head108is too close to the target surface. From606, the visual characteristics of the reference are again compared to the calibrated visual characteristics at604. If the visual characteristics are within the calibrated visual characteristics, at608, the radiation head108is determined to be properly aligned with the target surface208for performing a paint thickness assessment and the alignment routine600ends.

Through the use of the reference image projector, the invisible terahertz radiation beam can be visualized to accurately align the radiation head relative to the target surface by an operator and/or a controller. In the example provided herein, the reference image comprises two patterns that are superimposed with each other. By superimposing both patterns on top of each other it is possible to determine rotation and distance about three axes of the terahertz radiation head.