Implementation of an industrial system

Despite the uncertainty over the quantied eects of the tilt of the detection transducer on the thickness measurements the measurement technique is very close to industrial viability. The advantage of only needing access to a single side of the sample, not to mention the lack of reliance on radiographic methods [10], makes the measurement technique attractive for the on-line measurement of thickness of cold rolled aluminium foils. For on-line measurements the aluminium foil would move past a stationary transducer arrangement, making the dual coil technique of measuring the thickness signicantly more suitable. Provided that the distance between the aluminium sheet and the detection coils can be maintained then the consistent nature of the measurements should allow the measurements to be performed. Measuring the S0 wave velocity

with an independent measurement may require some level of positional encoding of the sheet being measured to ensure that the velocity of both waves in the same regions are being compared. Further work may also have to be performed in optimising the transducer separation, to ensure that the signal levels are maximised, while minimising the eect of spurious reections on the thickness measurement.

However, although the measurement technique is promising, a number of problems may arise. Due to the highly sensitive nature of the phase velocity measurements these may be more sus- ceptible to electrical noise, which may be problematic in an industrial environment. Another potential concern is the length of time over which the measurements are acquired, as the work presented in this thesis is typically performed while averaging 512 times, rather than the single shot measurements likely to be required of an on-line measurement system. Additionally, it may be necessary to perform some form of correction to disentangle the wave motion from the motion of the sheet, possibly by performing the phase velocity measurement both in the direction of travel of the sheet, as well as in opposition to its motion. Furthermore, any change in the lift-o between the sheet and the detection transducer could have severe eects on the reliability of the measurement, so steps would have to be taken to minimise this.

in any industrial implementation. The ability to reduce the amount of metallic swarf collected by the magnets, or at least to periodically remove it, must be considered, as must the physical hardening of the transducers themselves, potentially using a ceramic wear surface. Additionally, the transducers must be able to be rapidly removed from proximity to the sample in the event of a buckle in the line. In order to satisfy the requirement to minimise any change in the lift-o between the transducer and the sample, the vertical position of the sensor may have to be actuated with a fast responding and highly accurate system.

Chapter 7

Measurement of lift-o using eddy

current techniques: Experimental

Details

Due to the terms of the research contract the details presented

in chapters 7-10 are to be treated as condential.

The second half of this thesis covers a second project, with research of a substantially dierent nature to the thickness measurements presented in chapters 3-6. Chapters 7-10 discuss lift- o measurements using an eddy current sensor, with the aim of performing non-contact prole measurements on electrically conducting samples. The measurement technique devised in this work was designed to meet the requirements of a research contract from a third party, who required a technique to measure the relative diameter proles of large industrial rollers, and the research covers the advancement of the technique from TRL3 to TRL7. The company in question manufactures roller grinders which are used in large steel mills to manufacture the rollers in-situ. The rollers are used in the production of either hot or cold rolled steel, and undergo a large amount of wear during normal operation. As a result of this, steel mills often have the facilities to grind new rollers on site, and these roller grinders are the items which are manufactured by the company for which this research was conducted. As part of the manufacturing process each roller is individually tested in a number of ways, including using eddy current testing to detect surface cracks and ultrasonic measurements to assess the bulk of the material. Although it is not known if techniques such as

laser heating are used to harden the surface of the roller, these are assumed to have been applied uniformly, so that any change in the eddy current signal along the length of the roller is due to changes in the prole of the roller.

The measurement of interest for the research presented here is the measurement of the diameter proles of the rollers. The rollers are typically manufactured in pairs, and in order to ensure a consistent thickness in the rolled material the proles of the roller pairs must be known to a very high degree of precision. Currently these measurements are performed using a pair of contact sensors on callipers that are scanned along the roller, measuring changes in the displacements of the callipers in order to determine the roller diameter. There is some concern about the eect that this contact method has on the surface nish of the roller, and hence the new technique for prole measurement devised in this research must be non-contact. A new non-contact method of measuring the diameter of the roller by accurately determining the lift-o of two eddy current probes is presented in these chapters.

7.1 Target specication

In order to meet the requirements of the research contract a target specication to which the measurement must perform was created:

ˆ 1 μm resolution in measurement of lift-o over a 2 mm dynamic range

ˆ Performance on curved samples with a radius of curvature (RoC) as small as 125 mm ˆ Insensitivity to transverse misalignment between the probe and the crest of the curved sample

of up to ± 1 mm, shown in gure 7.1

ˆ Insensitivity to water or oil contamination on the surface of the sample ˆ Operation with minimal calibration

ˆ Suciently rapid data acquisition to allow operation at up to 5 m per minute when scanning rollers

ˆ Insensitivity to the surface condition, such as lack of polish or small supercial scratches, and to the bulk material of the rollers

ˆ Operation over a range of temperatures from 0 to 70 °C

probe

roller

crest

± 1 mm

2mm

dynamic

range

Figure 7.1: Schematic diagram describing the transverse misalignment of the probe to the crest of the roller.

Although the required measurement is that of the diameter prole of the roller an ultrasonic thickness measurement, such as those discussed in section 2.1.2, is not particularly suitable, as the composition of the roller varies through its diameter. Additionally, the exact surface roughness of the rollers is not known, although given the client's concerns over a contact measurement damaging the surface, a very low degree of roughness is expected.

As discussed in chapter 1, a number of methods exist for the measurement of the surface prole of a metal sample. A large number of contact systems exist, and are widely used in metrology, as well as in the current version of the measurement system. However, one of the key requirements from the client was for a non-contact measurement system to be investigated. An optical system, such as an optical interferometer, could be used these methods have a number of drawbacks. Firstly, the sensitivity of these techniques is on the order of the wavelength of the light used, and thus is in the order of 500 nm, which is far too sensitive for the industrial samples, as it would require that the scanning stage be stable enough to ensure that the change in lift-o was due to the change in prole of the roller rather than vibration in the probe itself. Although techniques exist to extend the dynamic range of these measurements [160], optical measurements are inherently susceptible to variations in the surface condition. Thus, in order to achieve the requirements for dynamic range and insensitivity to the presence of water-oil contamination on the surface of the sample, an eddy current technique has been selected for this project, building on previous research using this to quantify lift-o by measuring the change in impedance of the eddy current coil [20, 92, 93, 96]. Although the dynamic ranges explored in the references given are consistent with the specication required, the accuracy of the measurement system developed here is considerably higher than in any previously reported work. An eddy current system also lends itself to the range of operating temperatures required, as well as the limitation on the weight of the probe and electronics, as the

probes themselves can be extremely lightweight (less than 50g).

In this chapter a renement to the analytical model for a coil in proximity to a conductive sample introduced in section 2.5 is discussed, and the experimental set-up which was developed in order to prove the resolution of the measurement technique is described. An initial measurement technique which drives the eddy current coils at a single frequency is described, as is the improvement to this technique, where the eddy current coils are driven at their resonant frequency. The results when using these two styles of measurement are presented in chapters 8 and 9 respectively, while the results from eld trials on industrial samples are discussed in chapter 9.