The measuring and control system for improved model based diastat filling quality

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Abstract

The article describes the development and testing of a measurement and control system in industrial environment. This system enables fast and accurate membrane expansion measurements. The membrane is part of the sensor system called diastat, which is filled with a special oil. The diastat is part of mechanical capillary thermostat. To demonstrate the right selection of the measurement equipment and data processing methods, several tests and analysis were performed: the dynamic response of the diastat membrane during filling, measuring accuracy, nonlinearity and temperature stability of the measurement system with integrated distance sensor and the most important verification measurements with reference control procedures in manufacturing process. It was demonstrated that a number of novel approaches need to be introduced enabling installation of the measurement and control system in the production of the thermostat diastats.

Highlights

► The system is used for control of the thermostat membranes filling with special oil. ► Several measurement approaches of diastat expansion was tested. ► For better measurement performance a non-linearity compensation was implemented. ► The measurement system enables accurate measurements in dirty industrial environment. ► The quality of developed system was verified by reference quality control.

Introduction

The article describes the design, development, tests and evaluation results of the measurement and control system needed for control of filling procedure of a product called the diastat. The funder (project partner) for this development project was ETA Cerkno company from Slovenia. The diastat is a part of the mechanical capillary thermostat (Fig. 1) filled with a special oils. The implementation of the presented system was necessary to overcome problems with proper diastat filling and resulting quality of their products.

The main reason is to improve the quality of the manufactured products, to meet the requirements of international standards, such as the ISO standard, and to decrease the quantity of false products. These are present due to inadequate input materials or deficient manufacturing (Vacharanukul & Mekid, 2005). The following categories are often incorporated in the manufacturing production processes: computer aided manufacturing (Golnabi, 2003), various measuring systems (Rejc, Činkelj, & Munih, 2009) and automated visual inspection (AVI) systems (Chin and Harlow, 1982, Rejc et al., 2011). When these systems are introduced into the production, they need to work flawlessly in a longer time period (Nurminen, Karonen, & Hätönen, 2003).

The approach presented in our system belong to the first and second category. Several measurements are influencing the control system. The entire system consists of oil temperature control, oil pressure regulation and very accurate membrane expansion measurements.

In the field of industrial measurements, several parameters are often observed, such as pressure, temperature, distance, viscosity, mass or velocity. The most frequent in industry is distance measurement. The literature (Thiel, Pfeifer, & Hartmann, 1995) states that the most frequent distance measurement range is from 0.1 m to 40 m where contact micrometers (Zeitouny et al., 2011), calipers and incremental probes are mostly in use. In the class of contactless sensors, ultrasound, inductive (Sydenham, Taing, Mounsey, & Wen-Xin, 1995) and capacitive sensors (Zhu, Spronck, & Heerens, 1991) are dominant. Very accurate (Xing et al., 1987) distance sensors based on laser light are also being installed. The working principle can be triangulation (Ji & Leu, 1989), conoscopic holography (Spagnolo, 2006) or interferometry (Bapna, Verma, & Joshi, 1992).

Beside the distance measurement also temperature and pressure measurements are of upmost importance in industrial environments. Temperature can be measured through direct contact with the measured object or with observation of heated or cooled material (Childs, 2003). We can find several temperature measurement methods that differ in their speed and accuracy. In industry most frequently used temperature sensors are thermocouples, resistance temperature detectors and integrated temperature sensors (Campbell, 1970, Liu et al., 2011). In the field of pressure measurements the capacitive pressure sensors (Kumar, Kumar, Jain, & Kashyap, 1999) are most frequently used, but also other measurement principles can be found (Harada, Ikeda, Kuwayama, & Murayama, 1999), for instance the piezoresistive measurement approach.

The most important part of our investigation was to enable very accurate distance measurements. Therefore, the developed measuring system with the integrated distance sensor was verified in detail. Comparative measurements were performed by using a certified contact micrometer and real objects. The entire measurement and control system performance was verified with reference control procedures in manufacturing process. The measurement approaches and test results are presented and discussed.

Section snippets

Diastat

The diastat is one of the components in the capillary thermostat, used in everyday life for temperature regulation in household appliances (European standard, 2003, Peffer et al., 2011). Capillary thermostats differ in dimensions and performances. The project partner produces 6 different types of capillary thermostats, covering the temperature ranges from −5 °C to +550 °C.

The diastat consists of a membrane, capillary tube and a sensing element (Fig. 2), filled with a special oil as a

The diastat filling procedure

Each diastat is filled with oil. The filling procedure of approximately 90% of manufactured diastats is performed on rotary filling machines (Fig. 4). The quantity of ceramic button membranes diastats that are filled on rotary machines is three times larger than those with the metallic button. In production hall six rotary machines are situated, each with 48 filling heads. The filling procedure is as follows. The workers manually position the sensor element of the diastat into the filling head,

Influence of diastat filling on quality of the thermostat temperature control

Industrial environment requires very accurate distance measurements. The expansion of the membrane during filling can range from 0.07 mm to 0.5 mm, depending on the diastat type. The expansion of a kitchen oven thermostat diastat membrane for 0.01 mm represents a difference in temperature of 3 °C. These two facts require the measuring accuracy of the implemented measuring system within the range of 5 μm, corresponding to ±1–2 °C error for calibrated thermostat.

The European standard EN 14597 (European

Diastat filling quality control

The initial oil filling pressure on all filling machines is calculated by legacy mathematical equations. In the past, this calculation was used to fill a few hundreds of diastats and a dozen of them was taken for the quality control, where back and forward displacement of the diastat membrane is checked. The filling pressure was adjusted in accordance with the information and the same procedure was repeated. The drawback of this procedure is the delay of 15 to 30 min and many inadequately filled

Equipment of measuring and control system

First task of the team working on the project was to develop the approach for controlling the filling procedure on existing filling rotary machines. The working principle is shown in Fig. 7.

The presented system must be supplied with the following momentarily data to control properly the filling pressure: oil temperature, filling pressure and membrane expansion. These data and the company’s mathematical model of membrane expansion are used to calculate and adjust the new filling pressure.

The Keyence PT-165 distance measuring sensor

The distance triangulation distance sensor head Keyence PT-165 with controller PT-A160 was tested in out study. The receiver of the LED light is a PSD (Position Sensitive Device) sensor. The measuring range of this sensor is 4 mm or ±2 mm at 22 mm distance from the head. The repeatability is specified as 3 μm on a white sheet of paper. The beam diameter is approximately 1.5 mm. The specified temperature working range for the head and the controller is between 0 °C and 50 °C. This information is very

Microprocessor system

The Atmel microprocessor runs the dedicated software, which represents a bridge between the diastat expansion measurements and the transmission of measured data to the central computer.

For the operation of the entire system to run smoothly, the program in the microprocessor requires roughly three parts, which are displayed in Fig. 9. These parts are:

  • 1.

    sampling and mean value calculation of diastat expansion,

  • 2.

    nonlinearity compensation,

  • 3.

    user interface.

The first part of the program contains sampling

The testing system with reference distance contact sensor

For the initial tests of the distance measurement equipment a special mechanical system was designed (Fig. 11). It consists of the Keyence PT contactless measuring system and a certified contact micrometer Mitutoyo MHD-164-161 with a measurement uncertainty of ±2 μm and resolution 1 μm. Between the two measurement systems a special mechanical system can be seen. This system allows the translation of a mechanical slider between both measurement systems. This mechanical system is intentionally very

Nonlinearity determination of the measurement system

Fig. 12 shows the measurement characteristics of the entire measurement system taken from the reference micrometer values. The horizontal axis represents the reference Mitutoyo sensor values in μm and the vertical axis shows the error of the measurement system related to the reference values. In four displayed measurements can be noticed very high repeatability with maximum error 17 μm. In addition to the four characteristics, also a thicker black line is drawn, representing polynomial

Discussion

Developed was a measuring and control system for a highly demanding industrial environment. It enables very accurate measurements of diastat ceramic button membrane expansion. By using the measurements the system can control the filling pressure of existing rotary filling machines. At the beginning of the project the thermostat manufacturer imposed a very narrow permissible error range for expansion measurements. This range was set to ±5 μm. The current non-contact distance sensors enable better

Conclusions

Developed, tested and installed was the measurement and control system for diastat membrane expansion supervision. The diastat is the most important part of the mechanical capillary thermostat.

Greatest challenge was to develop a contactless measuring system for measuring the expansion of the membranes. The main part of this system is a contactless distance measuring sensor Keyence PT-165 with an analog controller PT-A160 and microprocessor system for sampling, processing and transmission of the

Acknowledgment

This work was financially supported by ETA Cerkno company in Slovenia.

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