NDTnet - March 1996, Vol.1 No.03

Automated NDT
Advantages and Disadvantages

by Godfrey Hands, Vianen, The Netherlands
About the author


Abstract:

Automation of NDT has both advantages and disadvantages. This discussion article tries to highlight these, and allows NDT users to decide for themselves if they should automate or not. The two main advantages of automation are reproducibility and man-power reduction; the disadvantages are the investment needed to implement this and the complexity of an automated system.

Table of contents


Automated NDT

Let's look at a definition of Automation, then at some of the points mentioned above, and try to analyze them. An automated NDT station can be as simple as a scanning guide combined with a monitor and audible alarm to warn of a defect to help the ultrasonic operator position his probe, ( a semi-automated system), or as complex as a multi-channel million dollar computer controlled machine that can fill a living room. Anything that mechanically or electronically assists or replaces the manual operator can be considered as some form of automation.

  • Reproducibility

    An automated NDT system, be it Ultrasonic, Eddy Current or any other type, almost always has reproducible performance. It is not significantly dependent on how much beer the operator drank last night (or at lunch time), temperature or humidity, how tired the operator is, how many hours of overtime he has worked this week, or how attentive the operator is to his work.

    Once the test system is set up, it will (or at least should) produce reproducible results, 60 seconds every minute, 60 minutes every hour, 24 hours every day. The overall sensitivity of the system may be a little less than that of a vigilant operator (but with signal processing, could provide significantly superior performance), but the reproducibility will be almost 100%. An automated system's performance is, however, so reproducible that if the operator does not adjust the machine correctly, it will test all parts incorrectly. For security, a well documented set-up procedure should be specified for the machine, to reduce the risk of this happening.

    The manual operator has very poor performance reproducibility. In many industries, detection of 80% of observable defects is considered the normal performance of a human inspector (whether the system is Ultrasonic, Eddy Current, Visual, Magnetic Particle, Penetrant, or any other.)

    Let us look at a manual ultrasonic operator testing components for defects. Under ideal conditions, such as when he is alert, just starting his shift and has just calibrated his instrument and probe, perhaps he can find defects 3mm long and larger. Assume also that an automated system can only detect defects of 5mm and longer, and that 3mm is the acceptance limit. Which is to be preferred? The more sensitive manual operator or the automated system that cannot meet the requirements of the specification ?

    defect_graph1</A defect_graph2</A
    To answer this question, we have to look at the distribution of defects in components, and at the effects of defects of different sizes on the product. Assume that the components have many very small defects (from less than 0.1mm long) and very few large defects (several centimeters long), and that a 3mm long defect could just reduce the life of the component, whilst a 3cm long defect will cause a catastrophic failure, and 5cm long may endanger life. The 0.1mm long defects occur at a rate of 1 defect in every 10 pieces, 1mm long at a rate of 1 in every 100 pieces, 10mm long at a rate of 1 in every 1 000 pieces etc. in a logarithmic progression. This means that a 5cm long defect could occur once in every 5 000 pieces.

    Because the manual operator can miss one defect in every 5 (but is less likely to miss larger ones than smaller ones), he could potentially accept one component with a life-endangering defect for perhaps every 10,000 pieces he tests. If the component that he is testing is a car road wheel (for instance), then because each car has 4 wheels in use, one car in 2 500 has a life-endangering defect. What about the automatic system ? In the example given here, it will never miss serious defects, but cannot find any of the defects that are between 3mm and 5mm long (example!). How serious is that? The manufacturer will have a few cases with reduced component life, but the reduction will probably only be noticeable in the long term, and when statistically analyzed. In many cases, the consumer will attribute the failure of the component to it having reached the end of its life. The effect on the manufacturer's sales due to dissatisfied customers will be negligible, while the manufacturer using manual operators will not be in business due to liability claims. This is an extreme case, but demonstrates the principles.

    For safety-critical parts, where it is not possible to automate the inspection process, it is common practice to "double inspect" the parts. This means that an operator will inspect the production and reject 80% of the defective parts in the batch. Another operator will then reinspect all the components that the first operator accepted. find 80% of the remaining defects (leaving about 4% of the defects instead of 20%). An additional (third) inspection will reduce this to 0.8% of the defects.

  • Manpower Reduction

    In many cases, automated systems will operate at the maximum possible throughput, limited in speed only by the laws of physics. It only takes one pulse on an ultrasonic instrument (operating with a Pulse Repetition Frequency or PRF of 2000Hz) to trigger an alarm, while it will need probably 100 pulses (or 0.05 seconds) for the alert manual operator to observe an echo exceeding his threshold. This means that the automated scan can potentially move at up to 200 times the speed of the equivalent manual scan without missing defects. The automated system can also operate with an optimal probe movement, eliminating double scanning, or missed areas when manually inspecting.

    Automation can therefore bring a possible throughput increase into the manufacturing (or inspecting) process of up to maybe 50 times, but anyway should not reduce it. If the manufacturer has large enough production amounts to warrant it, he will be able to inspect his parts with less personnel. If he only manufactures small quantities, and set-up and calibration times of an automatic system are high, then he is unlikely to have any significant benefit in this area.

    Let us assume that the manufacturer can reduce his manpower for inspection by only 50% through automation. If he only has two operators, he cannot continue with one tenth of an operator unless the operator is also employed in other areas of the factory. Thus, the cost of one man is (an assumed) $30 000 per year - the limit of the manufacturer's direct financial advantage. If he can reduce his inspection manpower by 5 or even 10 men for the investment of one machine, his savings become very significant.

  • Investment

    An automated NDT station will not be cheap. Depending on the complexity of the system, the price could range from $10,000 to $10,000,000, but let us assume that a machine costs about $100,000. In pure financial terms, if it can replace three operators, it may come close to paying for itself in just over a year. One has to consider, however, the interest on capital borrowed to purchase the machine (or at least loss of interest by not having the capital to invest), running costs etc., as well as possibly extra training for the operators in the new technology. Consider also the situation that you could be in if the machine breaks down, so you have to have a back-up of some type, or a significant investment in spare parts and a good TPM (Total Preventative Maintenance) scheme. For smaller companies, more complex test systems may prove to be too high an investment to make economic sense.

  • Complexity
  • Now let's look at the complexity of an automated system. It is not practical to take a 10 meter long ultrasonic immersion system into the field and test pipelines in production, but less complex systems could be considered, using contact transducers. A modern high-tech ultrasonic test system that is dedicated to one high volume production component can be as complex as the supplier likes, but it has to be simple enough for an operator to use without overwhelming him with technology. A more highly qualified engineer could possibly operate the system without problems, but will probably not remain in such a monotonous occupation for more than a few months.

    The ideal system is one that is user friendly and not difficult to adjust for component changes. It can ideally be computer controlled, with the computer controlling all necessary adjustments, based purely on an operator's simple input to the system. This can be through a keyboard if he is computer literate, but if the level of personnel to be employed on such systems have no potential for computer literacy, then alternative arrangements have to be made.

    It may not take a scientist to operate the system, but it could possibly need one to adjust the system in the first place for different component types. If the system is user friendly enough, it may be possible to remotely (or in advance) program the machine so that the operator only has to press a button with his name on it to change from the production types of the last shift to those that he has to test on his shift.

  • Logistics
  • If (for example) you needed 20 Level 2 NDT operators to manually test your production on a three shift system, you may have the potential to replace these with an automated system, one level 2 (or Level 3) operator plus perhaps 4 or 5 Level 1 operators, so the savings are not only in the numbers of personnel that you need, but also in the class of personnel.

    Lets look at the pipeline inspection situation mentioned above. Supposing you are presently manually testing 15 welds per man per day, and you employ 3 Level 2 operators to make the tests (45 welds per day). An automated inspection system may take only 5 minutes to test each weld, but 15 minutes to accurately set up. You will still have the potential to test 25 welds per day per operator, but you may be able to use 2 Level 1 operators to perform the testing with two systems. One Level 2 or Level 3 will also be needed to interpret the results generated by the automated systems, so you return to about the same manpower costs, and the same throughput. Where is the advantage here? It lies in the reproducibility, plus you have the potential to store the test results digitally for future reference, which is not possible when the operator is relied upon to interpret the screen and translate this into "accept" or "reject" in real time.

  • Conclusions
  • Based purely on quality requirements, automation scores very highly. For a mass production environment, automation can have a significant economic potential. When the lot size is very small, automation becomes less attractive, unless it can be compensated for by a more complex and versatile machine that is capable of simple adjustment from one production lot to the next. Issues of investment or complexity may, however, make automation infeasible. Each individual situation should be considered on its own merits. These conclusions are also valid for any automatable NDT systems.

    Discussion Point

    Taking mass production to its extreme, we need to consider if NDT should be employed in the production process at all. If your process is producing defective parts, then the process is potentially costing you money in the form of reduced throughput (due to parts rejected), as well as inspection costs. If the cause of the defects can be determined, and the process so controlled that these defects do not occur, then the need for NDT becomes questionable.

    References

    1. Quality Control Handbook Third Edition 1979 Page 12-53 by J.M.Juran
    2. "Some individual psychological factors leading to error in ultrasonic testing" Journal of the British Institute of NDT Vol. 31 No 12 Dec 1989 by Calum Webster

    G.Hands</A The author
    Godfrey Hands started his career in NDT in 1968. His specialties are Ultrasonics and Resonant Inspection, and his NDT experience has encompassed many different industrial sectors from offshore industry and training to research. His employers have ranged from construction and the NDT service industry, through sales to his present position within a multi-national mass-production organisation.
    E-Mail:
    Godfrey@hands-ndt.co.uk
    Homepage: www.hands-ndt.co.uk

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    Rolf Diederichs 1.March.1996, info@ndt.net
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