The smallest of causes can lead to large effects in the functional reliability of machines. This can be very similar to searching for a needle in a haystack. Even during development, it's important to detect faults and error sources in detail. The high-speed video analysis tool from Mikrotron is the key to success.

A little less slip, chatter, oscillations and vibrations, or other deviations from the actual values to the target can have significant effects on quality, productivity, and maintenance of machines. The mechanical challenges for a sewing machine are very complex, but at the same time the need for higher precision and functional reliability must be met. The sewing machines of the Swiss company Bernina International AG - in Steckborn on Lake Constance - are well-known for their high quality and long service life. They guarantee fast, low-vibration, quiet, and wear-resistant operation. They must stop with high precision and carry out movements with perfect timing and repeatability.

Exact movement analysis down to the micro range

The precise mechanical and electronic interplay of all components of sewing machines is influenced by many factors. For example, structural imbalance can lead to oscillations. The movements to be controlled must still be carried out very precisely. Even the smallest of deviations in the micro range can have serious influences on the functional precision of a sewing machine at high vibration frequencies. At very high speeds, the upper and lower threads must move exactly as specified. Material-dependent differences, however, can lead to movements that cannot be calculated in very close spaces.

To analyze the resulting problems unambiguously, the Design and Development Department at Bernina relies on very precise, efficient, and reliable measurement devices.

Experience has already been gathered with the goal-oriented use of high-speed cameras. In 2009, a decision was made to invest in a significantly more powerful system. The  Motionblitz Eosens mini2 from Mikrotron was the latest product at the time, and Bernina management found it to be the most powerful system in its price category.

The high-speed camera was purchased in the same year and used in the Design and Development Department. Mr. Durville, head of department, and Mr. Schwyn, test engineer, were most impressed by its flexibility, features, ease of use, precision due to high image resolution, as well as its compact design.

"The camera quickly established itself as an effective tool for many testing purposes," remarks Mr. Schwyn. This technology permitted Bernina engineers to view and test their new technologies in real time. "We use the camera consistently nearly every day," he continues. Many changes and optimizations in the design go back to the clear results of high-speed recordings. It should also be noted that the camera has already proved its value as an analytical system in other applications as well, such as production optimization.

Reliable thread movement

One particular challenge in the development of a sewing machine is the controlled movement of a thread. The market offers threads in a wide variety of materials for different tasks. Threads have different properties in terms of surface, thickness, hardness, and bending behavior. So it's difficult to keep the path of the thread under exact control. Under tension, the thread can reach a speed of up to 2 m/sec. But even when entirely loose, it's still thrown through the air at high speed through very narrow spaces. The danger is that it could get caught on components or become disengaged from the thread guiding parts. It is incredibly important for a regular, high-quality seam that the pulling movements of the thread can be defined and controlled at all times and everywhere. Thanks to the high resolution and slow motion provided by the Motionblitz Eosens mini2, the movement of the thread can be made easily visible to the human eye, and can therefore be usefully influenced.

Motion control of components under spring tension

Many components of a sewing machine are held in place by spring tension, or are allowed by spring tension to carry out the necessary movements without play. The tension generally moves between greater and lesser values. Higher tension leads to higher friction and wear, while lower tension is associated with lower functional reliability. The high-speed camera helps to find the right arrangement of springs more quickly and in a more goal-oriented manner.

Needle gripper movement

The core of any sewing machine is the so-called "gripper". The gripper grips threads passed to it from the needle. Only a few hundredths of a millimeter make the difference whether the thread is caught by the gripper or not, thus whether a seam will be created or not. The needle is guided from the upper part of the sewing machine, with the gripper guided from below. The long mechanical lever arms, the high speed, and the large number of moving parts cause vibrations that put this vital function at risk. The high-speed camera sheds light on this process as well, providing important information for the improvement of the design. Specifically in the area of gripper movement, important optimizations can be made. In particular in international competition, the highest possible quality is an important aspect of customer loyalty.

Time management of the sequence of movements is of paramount importance for the quality of sewing machine function and sewing results.

A sewing machine can only fulfill its task when the temporal sequences are executed with the precision normally only expected of Swiss watches.

Examples:

1. The material may only move when the needle is not penetrating it.
2. The needle in turn may only execute a ZZ movement when it is not currently penetrating the material.
3. The actuation motor for the material transport axis may only move when it is not currently transporting material.
4. The gripper must be in position exactly when the needle has formed a small loop of thread.

Since the cameras were purchased, a number of needed analyses have been carried out more quickly. The economic value each improvement finally had is difficult to trace. But the Design Department at Bernina is convince that the investment was worthwhile and that amortization was fast. Besides, the camera will certainly still be providing important service for years to come.