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Story 24: Considering a Weighing Device for Production Lines

  • Production Line Weighing Sensors
June 2, 2013

Considering a Weighing Device for Production Lines
Naoto Izumo
R&D Division 5, A&D Company, Limited

Investment is continuing in production facilities in East Asian countries such as Korea, Taiwan and China, which form the world's main manufacturing center. Accordingly, uses of weighing devices on these many production lines are also increasing. In particular, while domestic growth in production facilities remains in the doldrums in Japan, opportunities are increasing for production facilities designed in Japan being introduced to these now leading manufacturing countries, or for directly exporting to the region the main pieces of equipment used on these production lines.

The main areas exhibiting this growth are parts and components production, such as lithium ion batteries, integrated circuits, liquid crystal, LED or solar energy generation parts. All of these new fields are areas that Japanese makers developed but were beaten in competition in the international arena.

But what is perhaps interesting in these markets is that while Japan may have lost its share of finished products, it remains competitive against various other Asian countries in supplying production equipment for those products. Also, Japanese products are still preferred for the key components that are essential in this production equipment such as different types of sensors, weighing devices and dispensing devices.

These facts are not unrelated to the predicaments facing major Japanese manufacturers of consumer electronic products, who have had their market share stolen by fresh new players from other Asian countries like Korea, Taiwan and China.

Taking the market for liquid crystal display TVs as an example, Japanese producers of the essential materials such as glass, films, bonding agents or resists have quiet strong export figures. This phenomenon reminds one that in the past, the first generation robots in the field of factory automation(*1), and later personal computers, came to be produced by the easy assembly of parts and components. This is because only black box makers will enjoy an advantage when devices are completed simply by the assembly of black boxes components; a black box being defined as a constituent of a device whose technology is unknown, even when the device is disassembled.

There is a suggestion that Japanese makers of light electrical appliances have started to suffer due to the Lehman Shock, the steep appreciation of the yen after that and attempts to respond to the unique and particular consumer demands of the Japanese domestic market. But if we conjecture from events of the past, we can say that Japanese makers lost their dominant market position due to a loss of planning ability for new products, with more parts becoming black box items. With this, quality standards in other Asian countries caught up with Japan, and as a result Japan has lost their competitiveness in technology and price, which has lead Japan to a prolonged period of stagnation. In other words, the extended period of manufacturing decline experienced by the United Kingdom and then the United States could also be connected with what is presently happening in Japan as well.

This development has been said to have repercussions in the automotive industry as well, with the growth of the market for electric cars without complicated internal combustion engines leading to the continued development of a market for products assembled from a series of different units, such as a motor, battery, chassis, etc.

Under these circumstances, it is considered that in the future Japan must hurry to establish specialist technology in fields where it cannot be easily replicated, advance development of black box parts or elements which cannot be quickly overtaken by competitors and increase its planning ability for new products. Also, regardless of the scale of the company, Japanese companies which depend disproportionately on a single item and thus have only limited markets to sell finished products to tend to have already lost their product planning and development capabilities in the new fields. Consequently, they often lack adaptability to markets where there is room to expand and may well encounter difficulties sustaining future business enterprises.

While that was quite a long introduction, the purpose of this development story is actually to summarize points of interest regarding the use of weighing devices for automated machinery.

In the field of automated machinery, many balances are used, from the commonly-called microbalances with a minimum display value of 1µg (one millionth of 1g) up to large scale electronic balances with a weighing capacity of several dozen kilograms. In particular, microbalances were previously only used in specialist fields such as organic microanalysis, mainly for measurement of analysis samples of just a few micrograms. But as a result of the growth of the smart phone market, application of resist ink to their small liquid crystal displays has meant an adjustment from several hundred milligrams for the previously dominant large screen television market, to just a few milligrams in recent years. Accordingly, there has also been a shift in demand for the minimum display of weighing devices from 0.1mg to a highly sensitive 0.001mg (1µg).

Particularly with 1µg measurements, it is clear that very subtle influences such as people's body heat or breath, vibrations or changes in pressure from people's movements, as well as slight ripples in temperature or the gentlest of breezes from air conditioning, can lead to a deterioration in measurement error and repeatability.

At present, there are only a handful of manufacturers producing weighing devices for production lines with 1µg sensitivity. All of these manufacturers will thoroughly check the performance of the device at their own premises before shipping it. At A&D as well, we spend close to an entire day to check each device for continual repeatability with an automated tester before delivering it. As human operation is the main cause of error in micro measurement, critical performance appraisal and confirmation of 1µg repeatability is not recommended by hand for weighing devices designed for automatic operation. In other words, there is no point in testing the device with those external disturbances mentioned above only at the time of receipt when actual use of the device will not be under human operation.

We can summarize the problems that arise when the weighing devices are utilized as part of an automated process, as well as the solutions to those problems, into the following, based on our previous experiences.

1) Vibrations
With automated machinery, the weighing device is often installed in the same space used to house some drive system, with vibrations from the drive system often being transferred to the weighing device through its mounting base. To avoid this, measuring and operating the drive system at different times, applying a "vibration adapter" between the weighing device and its mounting base, and slowing down the movement of production line equipment near the weighing device to reduce air movement (wind pressure) are all effective measures.

2) Changes in air movement and temperature
As automated machinery will have a source of heat generation such as its power unit, it will often also have a fan to displace this heated air. It will be effective to handle this disturbance to measurement stability by installing a draft shield which completely covers the device or adding one that surrounds the weighing pan. Points to be careful of when using such a device are the influence that even the slightest of gaps can exert on measurements in µg units. For example, even if one side is left open, caution must be exercised to ensure the device becomes a dead end for circulating air, with all wind flow being completely cut off. If the intrusion of wind can be prevented, this will often mean that changes in temperature due to convection flows can be controlled as well.

3) Static electricity
Automated machinery is naturally accompanied by the movements of machines, etc. In particular, glass or resin containers are known to cause static build up from friction while being conveyed. Also, in the dry environment of a battery production line, the resin fixtures used to hold the batteries can easily become charged to over 10kV, and the force of this static electricity would be enough to cause measurement error at the level of dozens of micrograms. As a natural electrical discharge is not expected in a low humidity environment, a proactive neutralization strategy is necessary. In this case, in order to minimize costs of a neutralization strategy, use of a static electricity measurement device which can visualize static electricity and introduction of a DC neutralization device which, having strong neutralization effects, doesn't need to fan air have been proven to be effective. (*2 Static electricity measurement device/neutralization device)

4) Overload
Particularly in the case of measurement objects exceeding several kilograms, weighing devices can be damaged by overload. The results of tests on overload tolerance actually show the results of tests on metal fatigue failure, and with various conditions being contributing factors, the reproducibility of the test itself becomes a problem. At the weighing device's actual place of use the system is designed with the premise that the device does not break, so tolerance tests comparing different weighing devices should, due also to individual differences between devices, have little significance for actual use on location. Also, to be perfectly honest, as the devices will sooner or later become damaged, the level of maintenance at the time of damage (costs × turnaround time) is therefore important.

As a generalization, if a load placement on a weighing pan by hand is taken as 1, under identical conditions, a load placement on a weighing pan by an uncontrollable actuator such as an air cylinder will place approximately 3 times the load on the weighing device as human operation. The loads for all weighing devices are assumed to be measured under static weighing conditions, with the acceleration rate at this time equivalent to 1G (1000gal). In other words, the acceleration rate added by automated equipment is predicted to amount to several Gs, so when a weighing device is introduced to an automated production line there is a necessity to prepare a device whose weighing capacity is several times larger than the weight of the anticipated objects to be weighed. Stated another way, in order to ensure a safe rate of measurement equivalent to measurement by hand, a weighing device added to an automated process will need a weighing capacity 2 or 3 times that of a human operated one. Further, impact loading has particularly sharp peaks and extremely large values have been shown. Therefore installing an impact resistance adapter between the pan and the weighing device as a load pathway dramatically improves the measurement safety rate.

5) Calibration
Weighing devices incorporated into automated production lines are often very hard to remove again and there have been many customer requests for devices which are not only durable, but self-calibrating ones as well. However, when we think of the minimum sample weight commonly used in the pharmaceutical industry, the measurement will be reliable enough if the measured value is 3000 times the repeatability of the weighing device. The fact is that microbalances are for weighing 10mg, semi microbalances are for weighing 100mg, and even standard analytical balances of 0.1mg display are actually installed and used with the final purpose of weighing objects of several grams. On the other hand, the sensitivity drift of the weighing device is generally 2ppm/°C (2×10E-06), that is to say, with Δ10°C change in temperature, with a measurement sample of 1g: 1g×20E-06=0.00002g. In other words, even with a temperature change of 10°C, the actual difference in measurement value that arises per gram does not even exceed 0.02mg (20µg). To put it simply, as the balances will not actually be weighing masses near their capacity, but rather used for weighing fractional amounts, considering the above from a technical viewpoint, you can say there is no need for calibration in response to a change in temperature. Further, with recent balances, changes in values over time have been found to hardly occur at all (*3), and it is considered that there is actually no necessity for calibration at all, excepting circumstances where damage may potentially occur, such as dropping the object for measurement or a jarring load, etc.

To summarize the information above, more precise mass measurement on automated production lines is increasingly being seen. This is a response to customer demands to realize higher quality and productivity. Mass measurement differs from optical measurement and other methods in delivering high precision at a low cost, with the advantage of being easily able to manage the entire quantity of the sample, from the surface of solids, powders and fluids to any internal defects. On the other hand, its down sides are long measurement time and susceptibility to its installation environment. However with regard to improving the installation environment, various analytical tools have already been prepared and it is now possible to perform stable measurement even at the 1µg level. As a weighing device maker, it is our strong intention to further respond to market needs with application of these already established technologies to realize faster and more stable weighing performance.

*1 In the 1980s, Japan led the world in the global boom for robots on factory floors. Many scara robots were proposed as industrial goods, but as most manufacturers made their products using a control instrument from just one company, or combining servomotors from several companies, they invited intensive price competition and many of them were bankrupted.

*2 Static electricity measurement devices, neutralization devices: AD-1684 Non-contact Electrostatic Fieldmeter / AD-1683 DC Fanless Ionizer

*3 Summary of durability test results: Using the AD4212C-300 a durability test of 30 million times (over one year) was performed with a maximum drift in measurement values of 5mg (5 scale) confirmed for a 200g weight. For further information, please refer to A&D's product page.

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