Online Gauging As We Know It! by Udo Skarke Erhardt-Leimer Inc.

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1 Online Gauging As We Know It! by Udo Skarke Erhardt-Leimer Inc. For many decades online measurement has made continuous processes better, faster, more accurate and more reliable. Since their introduction, thousands of gauging systems have been put in service all over the world. When it comes to online weight measurement, established technologies such as beta gauges, gamma gauges and infrared technology have delivered great results in making processes better. The right choice of measurement principle depends on the measurement range, type of material and the characteristics of a product. Every one of these measurement principles has its weaknesses and drawbacks that present challenges to the average user. Now there is a new technology available. Ultrasonic weight measurement offers clear advantages over the older measurement technologies. Beta Technology When it comes to online weight and density measurement, beta technology is by far the most common technology in service. Using the principle of absorption of beta rays, the density of any type of material is correlated to the weight or thickness of the material. Proper calibration and linearization of the measurement system is required to get a reliable measurement. Density changes, temperature, electrostatic and other environmental influences all impact the final measurement result. There are many technical features and compensation methods. Some of them work better than others, but none can totally eliminate such variable factors. A further challenge is pass line height variation that is detrimental to measurement accuracy with beta/gamma ray technology as well. Safety and regulatory issues associated with the use of radioactive sources also add to the cost of running these systems. Infrared Technology Infrared technology has been around for many years and offers many online measurement opportunities. The preferred application for IR technology is still in moisture detection, but sometimes it is also used to detect thickness on films or determination of layer thickness in composite material. When implementing IR there are other factors that have to be considered. Since it works close to the visual spectrum; significant color differences, surface structure and different materials have to be calibrated to get reliable measurement results. Erhardt + Leimer Group Page 1

2 The more products a customer has to run, the bigger the challenge to calibrate all the samples correctly. Ultrasound More than 10 years ago a new measurement principle was introduced to overcome some of the issues and difficulties conventional technologies were facing with online measurement of weight and thickness. When the technology was introduced to the market, it was the goal to eliminate most of the clear disadvantages of existing measurement technologies. All the issues and weaknesses of the older technologies had been considered when looking for an alternative. With the ultrasound measurement principle, the developers were able to find such a solution to these changes. Measurement Principle The physics is based on local oscillation. The transmission and reflection of the oscillation varies with the weight or thickness change of the material that is being measured. The oscillation is being measured with a special sensing system and further evaluated by a high speed microcontroller. For ease of explanation it can be assumed that the oscillation amplitude is inversely proportional to the surface density. As an additional performance characteristic, the relative measurement accuracy remains constant with decreasing thickness or weight, resulting in a higher absolute accuracy. Key Differentiators of Ultrasound The ultrasound measurement has overcome some of the most critical issues faced with current in line technologies. The following research details outline some of evaluations and results that were accumulated and summarized for easy comparison. Some of these significant differentiators that where researched are calibration simplicity, pass line issues, color sensitivity and material composition impact on measurement results. Erhardt + Leimer Group Page 2

3 Pass line variation Pass line variation, the height location of the web, is not critical with ultrasonic measurement. The measurement gap between transmitter and receiver ranges between 30 to 45 mm. Because the transmitted signal is in a tunnel format, the position of the measured specimen is not critical and has no impact on the measurement result. The following table shows the effect of the pass line variation on the measurement result with two different samples ranging from a low weight sample of 26 gsm up to 484 gsm. Pass line variation Sensor distance at 45mm Sample gsm a 28mm a 24mm a 19mm a 13mm The results clearly identify that variations of +/- 5mm from the center line between the transmitter and receiver have a minimum impact in the measurement result (less than 1%). Density Linearity Another study was done to determine the linearity of the measurement system and what impact the measurement range has on the accuracy and repeatability. The results are listed in the follow table: Linearity Test with Mylar Sample Tests are performed with a sensor distance of 45mm Sample gsm Mean Value Repr. Repr. % (2σ) Accuracy Acc. % With just one sample to calibrate the measurement sensor, it was possible to achieve accuracy over the full measurement spectrum better than 0.4%. Erhardt + Leimer Group Page 3

4 In order to guarantee measurement stability in an industrial environment, the system uses periodical standardization to measure the density changes of the air within the measurement gap. In addition, temperature and air pressure detection between sensors is done continuously to compensate for any significant temperature and air pressure drifts. Instead of having an array of samples needed to create a linearization to assure accurate measurement over a larger measurement range, the ultrasonic sensor needs only one sample for calibrating the instrument. Resolution The high measurement rate and low integration time needed to achieve good repeatable measurement results allows for an excellent cross profile resolution. Even with such a low integration time of 80ms, the accuracy of the measurement result was better than 0.5%. In addition to the low integration time, the sensor uses 90% of its detection signal within a diameter range of 3mm. This is a great benefit for getting a high streak resolution making the system detect any type of streaks extremely visible within the first scan. Composition Linearity The ultrasound technology also overcomes the extreme effect of the chemical composition sensibility that beta rays have. The ultrasound sensor system is not impacted by such variations, which is another reason for the simple calibration procedure you have with this ultrasound device. Color Sensitivity Figures 1, 2 and 3 compare the difference in measurement between soft x-rays, beta rays and an ultrasonic sensor on PE stretched films with different coloration. As we can see, the ultrasonic sensor signal is absolutely not impacted by any color variations. Erhardt + Leimer Group Page 4

5 Overall, the ultrasound sensor has demonstrated superior measurement abilities that surpass many of the obstacles that are being faced with conventional technology. It is a real alternative to current beta, gamma, x-ray and IR systems that are currently used for online weight and thickness measurement. Erhardt + Leimer Group Page 5