"Puzzle-Piece" Monitoring Gives More Quality Through Less Effort

By: Thomas Kopka and Anton Schwer, of Schwer + Kopka GmbH
Topics: Envelope Monitoring, Error Detection, Change-overs, Metal Forming, Puzzlemaster, IMPAX-SK

New puzzle-piece monitoring logic brings expert know-how to every operator, with the touch of a button

In-line process monitoring systems are a common sight on production machinery. They represent the state of the art in metal forming applications and have reached a high level of performance. Yet, it is often the case that these systems are not fully utilized due to the fact that advanced monitoring features increase the need for operator understanding and knowledge. New designs are thus geared towards automating operational procedures, by having the monitoring parameters self-adapting to the characteristics of the monitored processes.

An alternate revolutionary concept, pioneered by IMPAX-SK, is the introduction of "selector switch/puzzle-piece" technology which automatically activates pre-programmed routines with special expert knowledge, to provide perfect-fit monitoring limits for certain types of forming operations, and to catch certain specific errors.

IMPAX-SK process monitoring systems are the state of
the art in the modern metalworking environment

Process monitoring systems ensure product quality and protect machines and tooling

Many producers of high volume industrial products today rely on process monitoring systems to survey running production. Process monitors ensure a more productive mode of operation at higher quality levels when producing high volume parts such as fasteners, stampings, pressings, turned parts, or when assembling components.

Process monitors typically observe a manufacturing process through suitable sensors, "learn" the correct sensor signal profile for a machine cycle, and then verify that each following sensor signal profile is identical or nearly identical to the learned profile, and that it falls within acceptable control limits. Such sensor signals can be force, torque, acoustics, strain, tension, electrical motor load or other methods suitable to measure the production process. Should the system detect any unacceptable deviations from the learned pattern such as tool breakages, misfeeding, or raw material defects, the production machine is stopped immediately to protect the machine and tooling from overloading, and to prevent the production of further bad parts.

Over time, ever-refined monitoring algorithms were developed aiming at the detection of smaller and smaller defects. A few years ago, dynamic enveloping was established as the most reliable method of detecting process errors and proved to be a vast step forward as compared to the previously-used peak load monitors. Still, enveloping was further developed as more powerful micro-controllers became available, which allowed analyzing the incoming sensor signals with very high resolution even at high machine speeds. This puts previously undetectable types of errors within the reach of the monitoring system.

Technology must be operated for it to be useful

Progressive metal working companies will often invest in the latest monitoring technology, but on the other hand, the advanced monitoring procedures will only work properly if the relevant features are used at all, and set with the right parameters.

In practice, it is often found that although manufacturing is equipped with the latest in fault detection technology, there is a vast difference between the technical capabilities on the one side and their effective utilization on the other side. This "gap" is becoming larger: as monitoring strategies become more complex, operating personnel are less trained in using the monitoring equipment properly.

Advanced process monitoring features
are often unused or under-used

Monitoring quality drops further with frequent change-overs

Single purpose machines which produce the same part every day can be monitored effectively even if all monitoring parameters are set manually. Over time, the operator will achieve intimate knowledge of his process characteristics, and will fine tune the monitoring limits perfectly to the individual behavior of this operation.

But the situation is completely different on machines with frequent change-overs. All relevant monitoring parameters must be newly adjusted to the current process and the characteristics of the measured force signals. Often, monitoring quality deteriorates rapidly with increasing numbers of change-overs.

Looking at a multi-die cold forming machine makes the point clear: a 5-die machine currently produces a part with a piercing operation in the 5th station. Piercing typically is a very erratic process which necessitates that this operation must be monitored by peak load rather than the more sensitive enveloping technique. Now the machine is changed over to a different part where the piercing takes place in the 4th station. Consequently, the operator now must go to the set-up menu of the monitoring system and correctly set all relevant parameters. Depending on what was done in the 4th station before, this could be a lengthy job. As a result, machines with frequent job changes and a large variety of different parts are often set so coarsely that all jobs are run with the same monitoring parameters. The necessary fine-tuning is not carried out, and the desired monitoring performance is not reached.

Once set, parameters are often forgotten

Another example emphasizes the additional dilemma of reduced monitoring accuracy due to raw material variation within one coil of wire. If a rather inconsistent coil must be processed irrespective of poor material specification, this will have an impact on the stability of the manufacturing process and on the settings of the monitoring parameters. The monitoring limits will have to be widened to avoid unnecessary machine shut downs due to material variation as long as this coil is on the machine.

Very often, however, the limits are left at the coarse settings even if the following coil is of perfect condition, and the monitoring quality itself remains at low levels for the upcoming production runs.

Automated routines improve monitoring efficiency

In response to all of these concerns, the development of highly automated monitoring routines has helped to maintain a high quality level in the use of monitoring systems. For example, it is quite common today that the active timing window "zooms" itself in to the actual process signal and cuts any idle portions of the machine cycle out of the monitored segment. In addition, modern process monitors utilize intelligent algorithms which automatically adjust the relevant parameters such as the envelope width and constantly revalidates the settings to match the dynamic process behavior.

Dynamic enveloping adapts perfectly
to any inherent process variation

Many users of modern systems today view these automatic procedures as a major step forward because for the first time, the monitoring performance becomes independent of the individual knowledge, experience and training of the different machine operators. The automatic routines ensure that the best possible monitoring quality is uniformly achieved on all machines, for every product produced and with every operator.

Selector switch programs enhance error detection

Still, it must be stated that certain small types of errors will only have a minimal and often locally restricted impact on the measured force or acoustic emission signals. In addition, each type of error causes a unique change to the measured signals making it difficult to detect all types of errors with the same, universally-designed automated monitoring routine.

This is where the IMPAX-SK's new approach of "program selector switches" comes in. Similar to most modern appliances of day-to-day life such as washing machines or microwave ovens, the user does not need to learn exactly how to set all parameters and functions of the device to perform the desired job, but merely selects a suitable program which will do all necessary settings according to preprogrammed rules.

This feature, called Puzzlemaster, is now being used in the operation of process monitoring systems for manufacturing machines. The name of this feature indicates that the user picks from a catalogue of programs or "puzzle pieces" those items which perfectly match the monitoring requirements of the current part. He then combines them into a specific monitoring puzzle. Each sensor or monitoring type can have its own puzzle piece assigned.

Use the know-how of the expert

Each puzzle piece contains the "know-how" from many applications, and provides this expert knowledge to every user. Many measurements on different machines and parts have been collected by IMPAX-SK to clearly define how the measured force or acoustic signals change when certain types of errors occur. The monitoring system has been pre-programmed with this information such that the suitable monitoring strategies and required accuracies are applied to the proper segment of the signal, to be able to detect the errors reliably.

A set of puzzle elements for
a 5-station cold forming machine

Today, suitable puzzle pieces are available for many typical cold forming errors. For example, the randomly occurring error of rotated heads on hexagon parts can be monitored by selecting the puzzle piece "rotated heads," which automatically activates a special zoom-channel with tight envelope limits. Similarly, parts with internal drives such as Philips or Torx types are best monitored by selecting the puzzle pieces "tip breakage" or "slot breakage," which activates detection of small tool breakages or chipping. Parts with a high degree of deformation tend to produce cracked or split heads, and for those a special puzzle piece is provided, which is geared towards detecting the relatively small changes in forming force when cracks occur.

Thread rolling operations use the puzzle pieces "rolling" and the "SKRollmaster die-match indicator" to assure a proper set of monitoring parameters for the rolling operation, and to assist the operator during die set-up to find the correct die match. In sheet metal forming, a special puzzle has been developed to detect rising slugs and scrap build-up inside the stamping tool.

For multi-channel applications, each sensor can be fitted with an individual puzzle piece selected from the available list, in order to activate the best possible monitoring combination for the current part produced. Rather than lengthy programming of all relevant monitoring parameters, the required puzzle pieces are quickly selected from the menu list to produce the perfect-fit monitoring puzzle for the current part. Once the puzzle is proven, it is memorized under the part number and is available immediately when this part is being produced again later.

Improved detection of rising slugs on stamping machines
is possible by activating the appropriate puzzle element

Creating your own puzzle pieces

Individual puzzle pieces can also be created to meet the specific appearance of the measured force signals for a certain type of defect, a certain type of machine, or a certain type of part. This allows for error detection which goes beyond the capabilities of the standard puzzle pieces.

To do this, it is only necessary to know what the measured signal looks like for good part production, and how it changes when the particular error occurs. Normally, most errors do not occur when an engineer is present at the machine, but IMPAX-SK has added a feature which automatically registers and documents the characteristics of the running process, as well as the errors. This new Memory-Master software can be installed on most process monitoring units already installed on a machine and connected to force or acoustic sensors. The data accumulated by the continuous logging process is valuable information for quickly designing the new puzzle piece able to detect the specific type of error in question.

The ongoing registration of running process data also provides detailed information about how modifications in the tool design or the manufacturing process affects the sensor readings, and which benefits they may generate.

Conclusion: more quality, through less effort

The ever increasing demand for better quality has also made process monitoring systems more complex in their functionality and more difficult to use properly. Over time, a growing gap has developed between the theoretical technical possibilities and the actual usage of such features at the machines on the shop floor.

The new puzzle technology from IMPAX-SK aims at reducing the previous operational complexity down to merely pressing a few buttons which select a suitable monitoring program for a given job. Every puzzle piece incorporates the expert know-how and the experience from many different applications. The operator only needs to decide which type of error he is expecting, or if his forming operation requires special treatment. The final monitoring puzzle is easily composed by picking the suitable puzzle piece for each forming station from the available catalog of pieces. It takes just a few button presses to obtain a perfectly matched monitoring package with a high degree of error detection. Once found to be a good working puzzle, this is saved into memory under the respective part number, and is ready to be reused the next time the part runs. Thus the gap closes, and the full monitoring power of the system is reached.

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