The Challenges of Inline Thickness Measurement Parts 1 and 2

PART 1

When selecting an in-process system for measuring the thickness of film, plate or sheet materials, a number of challenges need to be considered, including sensor alignment, linearity and the effects of thermal changes, says Glenn Wedgbrow, Business Development Manager at Micro-Epsilon UK.

There are many reasons why we need to measure thickness. All materials have a tolerance in production, so any materials that are too thin or too thick could cause a problem further down the line or at the end customer’s site. Changes in thickness during production can indicate wear of components, for example, in extrusion dies or on rolling stands. Monitoring trends can point to early warning signs.

The traditional method of checking thickness is often to take a measurement sample from the start of the production run and then again at the end. But what happened in the middle? If you find the material is out of tolerance, that’s a lot of scrap to consider. So you might choose to take more readings during the process. If these checks are being done manually, they often require the production line to stop. Most process variability will come in ‘start’ and ‘stop’ phases, so keeping a line running is generally key to improving consistency. Ultimately, checking the product thickness as it’s produced ensures that the end customer receives the quality of product expected.

Target material types and the production processes used all have a bearing on how the thickness of a product can be measured. Sometimes it is based on the setting of a gap on a roller or adjustment of dies in an extrusion head. It could be liquid poured and cured such as glass, rubber or metal. It could also be part of a secondary process where the basic material is already produced, but is then combined with similar or differing materials as multiple layers, or even woven strands that are then bonded such as carbon fibre.

Let’s now consider some of the challenges of measuring thickness.

Single-sided measurements

Perhaps the simplest thickness measurement is a single-sided measurement against a reference or datum surface. First, the sensor is zero’d on the datum surface and the target to be measured is inserted. The reading of the sensor is changed or displaced by the material thickness. There are a number of uncertainties with this method. If the reference or datum moves after mastering, the reading will be incorrect. Similarly, if the target does not sit correctly on the datum surface, the air gap will also be included in the measurement. The same issue can also occur with tilted targets.

If we are only allowed to view a target from one side, we must consider if we can combine technologies to enable accurate measurements. Where we have a mix of different material types, we can utilise those different material properties to our advantage. For example, an eddy current sensor in combination with a laser triangulation sensor can be used to measure sprayed skin thickness. The eddy current sensor measures the distance to the nickel-coated spray mould and has an opening in the centre through which the laser sensor measures the distance to the sprayed part. Eddy current sensors only measure against metallic targets and therefore see straight through the non-metallic sprayed coating. The use of an air coil in eddy current sensors from Micro-Epsilon makes this combination possible, as the laser sensor can look through the eddy current sensor at the same measuring point. When both signals are subtracted, the thickness of the applied sprayed skin is measured.

Another well used sensor combination is achieved with eddy current and capacitive sensors for non-metallic materials passing over a roller. Movement of the metal roller is considered by the eddy current sensor and the capacitive sensor measures the material or coating thickness.

Single-sided measurements – special case

If the target is transparent, it is possible to make an absolute measurement of the material thickness from one side using only one sensor with Interferometer or confocal technology.

The use of light refraction creates ‘edges’ or return signals that indicate the transition between the air and the material. Knowing the refractive index of a material enables the accurate measurement of the material thickness provided it stays within the measurement or working range of the sensor.

Measuring from both sides

If single-sided measurement is not suitable or the challenges cannot be overcome, in many cases customers want to know the true material thickness by measuring the material in ‘free space’, which requires having space on both sides of the material so that a measurement can be taken from both surfaces.

When you consider this set up, there are a number of challenges that must be considered and either overcome or accepted.

Sensor alignment

The sensors must be positioned so that the measuring spots coincide ‘through’ the full measurement range of the sensor. There should be no offset, tilt or inclination of the sensors in relation to the measurement object. For example, with a sensor offset of 1 mm and inclination of 2°, the effective error equals 35 µm, and at 10 mm target thickness increases to 41 µm.

Particularly with laser triangulation sensors, the location of the beam spot to the sensor housing should be noted. It should not be assumed that two apparently identical sensors will position the spot in the same place. Standard sensor housings are not usually precise enough for precision thickness measurements unless time is taken to align them correctly. To help customers make their own thickness set ups, the ILD1900 sensor from Micro-Epsilon uses an innovative sleeve mounting arrangement to tighten the spot spacing from housing to housing.

When you have two sensors looking at the same target, you must consider that each sensor has its own cycle time. If your target is vibrating or moving in the gap between the sensors then it’s very easy to introduce an error. Consider a target oscillating up and down by 1mm at 20Hz (times a second). A difference in capture time of 1ms between your sensors would equate to an error of 125µm.

Positioning of the sensors/measuring range

The next challenge is the relative position of the sensors and their measurement ranges.

Depending on how the sensors are arranged, the position of the target edges must stay within the measuring field. If the sensors are set so that the measuring zones do not overlap, then situations can arise where one sensor may not see the target. Consideration should also be given to process ‘start’ and ‘stop’ conditions, for example, is the material held in tension at all times? Speed changes in the line can induce upward or downward movement. Is the setup able to capture these events if needed? Ideally, the sensor ranges should overlap and cover the full range of material movement or at least control the movement.

Linearity

The accuracy of a sensor is often referred to as its linearity. The linearity value describes the deviation from the ideal, straight characteristic curve. Each measurement sensor has its own measurement uncertainty, or non-linearity. This means that at any given point in the measuring range the actual reading from a sensor can vary by a percentage of its measuring range. So taking just two sensors without any additional processing means that both sensors’ uncertainties need to be considered with respect to the accuracy looking to be achieved. For example, without adjustment, just moving a target 200 micron up or down can result in errors of 8 micron. The position in the range also has potential to affect the true value. To solve this, the sensors must be calibrated together as a whole.

Effects of thermal changes

Even when the sensors have been aligned and synchronised, there is still a further challenge that can affect everything – thermal changes. When measuring a target thickness in free space, the gap between the sensor and the opposite sensor is critical, as this is the constant on which the differential measurement is based. Taking a mechanical frame with a pair of sensors and cycling the temperature shows that the effective change with just a 5°C swing is up to 20 micron.

System capability

The final challenge – if the previous ones have been overcome or the errors associated have been accepted – is to prove the capability. Exactly how do you check or confirm the performance of your solution? There are two factors to consider here: the system repeatability, i.e. how much variability in the measurement system is caused by the measurement device, and reproducibility – how much variability is caused by different operators.

For more information, please visit www.micro-epsilon.co.uk

PART 2

What is important when selecting a suitable thickness measurement system from a supplier is to understand the combined real-world errors that can occur when using a non-contact system and how these errors can be eliminated or compensated for. While many suppliers state on their datasheets that the measurement system meets a certain resolution and linearity, in the real world, this performance is affected by a number of environmental influences. Errors associated with real world thickness measurement are not always obvious, but can combine to create significantly large errors. It is therefore critical to select a system based on system accuracy, not just sensor accuracy.

To overcome these challenges, as a systems supplier, we must take each application on its own and assess the needs of the customer, consider the tolerances required and whether sensors alone are able to solve the task. Our systems division is focused on thickness and width gauging solutions for strip processing materials including metals, plastics and rubber. All our key competences can be found within the systems division and so we are able to use the many different measurement sensor technologies from within the Micro-Epsilon group. We also develop our own software and algorithms, including all the automation, linear guiding, traversing and programming of robots. Most important is that we understand the mechanics of the systems we build, our knowledge of the surrounding environmental influences such as temperature and how to overcome these, which is critical to the success and continued operation of our systems. We now have more than 500 system installations worldwide.

So how do our systems solve the challenges of inline thickness measurement?

Two of the big challenges, alignment and synchronisation, are taken care of when purchasing a system from Micro-Epsilon. Knowing the overall gap or distance between the two sensors allows the calculation of thickness to be made. To establish this value, we first need to calibrate the system. A certified gauge block is positioned in the measuring path. This known thickness combined with the readings from the sensor above and below the target equal the overall measurement gap. Our systems will periodically return to the calibration position to check for changes and readjust values accordingly. Once the gap is confirmed and stored, the system can then be moved onto the target to be measured. The thickness of the material will now be the known gap, the sum of the individual measurements from the upper and lower sensor.

Thermal compensation

A typical system solution is a traversing C-frame measuring strip metal thickness for crown, wedge and edge drop on a decoiling line prior to slitting. Calibration checks against the certified gauge block are performed at regular intervals to avoid inaccuracies that may be caused by changes in the mechanical frame, for example, due to temperature.

Thermal compensation in an O-frame system works slightly differently. The gap between the sensors is key to accurate thickness measurement. Even in an O-frame configuration the mounting frame can expand and contract with temperature changes. Typically, the upper frame will move further than the lower frame, heat rises and so the reference for the gap between the sensors can vary, the consequence being that the system reports a thickness change even though the real thickness has not changed. Micro-Epsilon O-frame systems use a patented compensation frame made from an invar material and are designed so that thermal expansion of the frame is transferred in the horizontal direction rather than vertically. However, this is only part of the solution.

Thermally stable system

The second element is to measure the gap change between the main frame, which holds the sensors as this invar frame. This is achieved by using two Micro-Epsilon capacitive sensors, themselves extremely accurate and thermally stable. When the main frame expands or contracts, we can measure this change on the gap between both the top and bottom frame and compensate the effect into the thickness calculation. Even when the sensors are traversed across the full opening, the changing gap is continuously monitored and adjusted. As with the C-frame system, we also have a certified gauge block included that can be positioned into the measurement zone.

The effect of temperature change with no compensation employed can be significant.

A 25°C change in temperature is perceived as almost 400 micron change in thickness.

Optical sensors

Three optical sensor technologies are available for integration into the measurement systems. Single point laser triangulation sensors offer a relatively low cost entry into full thickness measurement capability. Confocal sensors provide the highest precision available, especially on shiny surfaces, whilst the laser line technology is the most commonly used due to its excellent combination of high precision and large measuring ranges. One of the main reasons for the use of laser line profile sensors is that we are able to make a best fit line of the multiple points taken from the surface. Even if some points are missing due to surface oils or textures, the average reading is still good and valid.

Another consideration in the use of the laser line sensors is that the laser line can compensate for tilts in the material. If single point measurement is used, there can be significant errors introduced by the fact that the measurement is no longer taken perpendicular to the surface. The line sensor allows for detection of the angle of the tilt and thus compensates to ensure the correct thickness value is measured. In addition, the laser line power/exposure time can also be adjusted to cope with oil films, emulsion and scale on the surface. It also offers the ability to measure profiled surfaces for both minimum and maximum thicknesses, which can be helpful in understanding the performance and wear of, for example, an embossing roller. A single point sensor cannot do this.

Advantages over X-ray and isotopic systems

The more traditional method of thickness measurement in the metals industry has been the use of Isotopic or X-ray gauges but these can have limitations when it comes to handling, ongoing support and operation. As we use optical measurement techniques, we are generally not restricted on the material type we can measure. Our measurements are made against a certified gauge block and we do not have to recalibrate for different material types. Isotopic or X-ray systems need to know the material type being measured due to material property changes that influence the systems’ measurement behaviour.

As well as the fact that we have no radioactive substance to handle or control, there is also no need for a safety area around the system, other than standard guarding.

Total system accuracy

Each individual sensor has its own measurement accuracy and therefore in order to obtain the best performance for the ‘system’ we need to understand the measurement behaviour of the individual sensors and combine them. This is achieved in the systems department through use of a known target and highly precise reference gauge, which is moved through the measurement range of the sensor. The values of the sensors are recorded and the combined readings stored into a new linearisation algorithm, which is used by the systems software to calculate the true thickness reading. This is the extra step needed to achieve the total system accuracy, which is often better than the quoted individual sensor performance.

The fact that we are measuring against a known target for calibration allows for the customer to perform their own capability checks at any time and to verify the system performance without influence from the supplier.

Each measurement system from Micro-Epsilon comes with fully capable software for operation, data capture and analysis of the required measurements. The multi-touch functionality provides a familiar user experience similar to many smartphones and touchscreens in everyday use. The software screens are customisable and can be adjusted for a customer’s own specific needs. The software also includes the means to check your system’s capability at the touch of a button, performing both reproducibility and repeatability checks whenever required and without supervision from Micro-Epsilon.

For more information, please visit www.micro-epsilon.co.uk