TEST & MEASUREMENT FEATURE


Thickness profile measurement of rubber strip using Micro-Epsilon's thicknessCONTROL O- frame system


INLINE THICKNESS measurement systems


When selecting an in-process system for measuring the thickness of film, plate or


sheet materials, a number of important factors need to be considered, including the effect of combined real world errors, says Chris Jones of Micro-Epsilon


I


n metals, plastics and rubber production, thickness (and width)


measurement of strip, plate, film and sheet is a key quality control parameter that needs to be carried out accurately and as the material is being processed. All manufacturers will have a process specification that they will need to meet in order to satisfy their customers. But in reality how can a manufacturer be sure that they are meeting these specifications at all times? A number of different measurement systems can be used to measure the thickness of a material. Some of these are used offline, i.e. random samples of the material are removed from production and measured to verify that they meet the specification. A more effective approach is to install an in- process or fixed, inline non-contact measurement system that continuously measures the thickness of the material as it is processed. If measurements from these systems move towards the outer limits of the specification, machine and process control parameters can be altered to bring the thickness back into acceptable limits.


BUILD IN-HOUSE OR PURCHASE A TURNKEY SYSTEM? The first factor to consider is whether to design and build the measurement system in-house rather than purchase a


turnkey system from a supplier. While there are many suppliers of sensors that can be mounted inline with simple framework and signals output to a local display or HMI for thickness calculation, verifying the actual measurement accuracy of this type of solution can be difficult. It is considered best practice to use a measurement system that is ten times more accurate than the production tolerance. With many production tolerances reducing to below, for example, <100 microns, it is therefore necessary to produce a measurement system with a capability of less than ten microns, which is very difficult to achieve, particularly if the user has never done this before. Therefore going to a supplier of a turnkey system is often a more efficient solution. What is important when selecting a suitable system from a supplier is to understand the combined real-world errors that can occur when using a non- contact thickness measurement system and how these errors can be eliminated


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“Some suppliers of inline thickness


measurement systems provide automatic calibration features built into their system as standard, which can continuously compensate for thermal expansion errors"


or compensated for. While many suppliers state on their datasheet 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 so 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.


VERIFICATION OF


SYSTEM CAPABILITY Look for a measurement system that provides fully automatic process capability


(CPc, CPk, etc.) features with in- built statistical functions that are


available to the operator at the touch of a button. This should also include a store and print out feature that allows the operator to verify the material thickness at any time, with no supplier involvement required.


MECHANICALLY AND THERMALLY STABLE It is extremely important to design a mechanically and thermally stable sensor mounting frame. The mounting mechanism should be isolated from


INSTRUMENTATION | NOVEMBER 2017


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