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Analysis goes on-line to offer real-time measurements

30 January 2013

De'Longhi, a producer of coffee makers, has adopted stringent levels of quality control in the form of on-line particle size measurement to verify the performance of every grinder leaving its production plant.

De'Longhi manufactures ‘bean to cup’ coffee makers, which are able to produce a cup of coffee from a feed of beans, at the touch of a button. These machines incorporate a number of components - a grinder; a boiler; a brewing unit; and, in many cases a steamer. The machines are produced on assembly lines with each of the constituent units being individually manufactured and tested, prior to assembly of the finished system.

Late in 2006, De'Longhi began a process of re-evaluating the testing procedure carried out during production, with a target of implementing a system that would guarantee the quality of every coffee maker leaving the site. A prime requirement was for 100% product testing. Beyond this, there was a desire to quantify quality, via numerical data.

For assembly line testing of the grinder the company chose an on-line laser diffraction particle size analyzer - an Insitec system from Malvern Instruments. The Insitec laser diffraction particle sizing system is designed for use in the production environment. Capable of measuring up to four complete particle size distributions per second it can be installed on a process line or on a by-pass sample loop. With the capability to measure slurries, emulsions, suspensions, as well as dry powders it has a measurement range of 0.1 to 2500 microns.

Grinder performance testing has been an integral element of manufacture at De'Longhi for many years. Historically it was a manual procedure which saw a trained assembly line worker rubbing a sample of ground coffee between thumb and forefinger and making any appropriate adjustments to the grinder by manipulating a a gear which can be rotated to fine tune the gap between the grinding surfaces to alter the particle size distribution produced. The majority of grinders - in excess of 95% - exited this part of the production process perfectly tuned.

The drawback to this manual process was that achieving 100% quality control was highly unlikely. The manual test also provided a qualitative, not a quantitative assessment of grinder performance, hampering efforts to fine tune the particle size distribution obtained, towards optimal flavour. Finally, any production expansion would create an increasing staff training burden.

Recognising these limitations, the company began to research alternative testing strategies a laser diffraction particle size analyzer was identified as a possible option, prompting the wider evaluation of this technology.

Initially, consideration was given to the use of an off-line laser diffraction instrument. This would meet the requirement to numerically quantify the performance of the grinder and, since such systems are highly automated, would reduce training requirements, even with an expanded operation. However, the team concluded that from the point of view of speed this approach was simply impractical. The goal was to cut testing times to a minimum, to enable the attainment of 100% assurance within the constraint of increasing production rates. This would demand real-time acceptance/failure, which was not possible with an off-line system.

The company next considered on-line particle size measurement. The speed of this system, the small quantity of material required for testing and the representative nature of the data produced, suggested that the technology would be able to meet the demands of the application and De'Longhi took the decision to purchase a dedicated system for production testing.

Developing on-line testing
The Insitec was installed at a work station on one of the assembly lines, directly beneath the bench to which the grinder is delivered for testing.
 
To carry out the test the assembly line worker pours fresh coffee beans into the grinder and initiates the test sequence. The grinder starts to work and the resulting ground coffee flows out of the grinding chamber, down towards the analyzer. A venturi aspirates the arriving sample into the measurement zone of the instrument.

One of the parameters measured during laser diffraction particle size analysis is obscuration (or transmission), the amount of light penetrating the sample. Measuring at an appropriate obscuration is essential to obtain reliable particle size data and is therefore one of the issues addressed during method development. In routine operation, obscuration can be used to trigger an analysis. In this set-up the arrival of ground coffee reduces the amount of light seen by the detector, automatically triggering measurement at an appropriate concentration, without any manual intervention. The end of the analysis is marked in an identical manner.
The particle size distribution data for the sample is presented in real-time on the screen towards the back of the workstation. Data presentation has been precisely developed to meet the requirements of the application and particle size data is not the most prominently displayed information although the software does calculate a median particle size for the coffee (Dv50 – the particle size below which 50% of the population by volume lies). The focus of attention for the operator is the green areas of the screen which indicate whether the grinder is giving acceptable performance, and if not, how many turns of the gear wheel are required to correct it.

Figure 2 shows a test result where change is required, in this case a single turn in one direction. A negative sign ahead of the number would indicate a need to turn in the opposite direction. Following any corrective action a second test is carried out.

Refining the analytical solution to this level of simplicity and effectiveness required some initial effort. Key tasks were to establish suitable specifications, for each grinder type manufactured, and the development of a suitable interface.

De'Longhi worked with Malvern application specialists to exploit the full potential of the software that drives the analyzer and enables its integration with other processing equipment. The result is that data is presented in an easy to understand way that minimises the risk of error by the operator.

Testing is now completed in a few seconds and the amount of coffee used is relatively small. Because the grinder is tested with real coffee, small sample size is a distinct advantage that reduces the ongoing cost of testing. Every grinder is tested and each one leaves the testing station with numerical test data verifying performance. This data is linked to the serial number for the grinder, and ultimately the coffee maker, and form part of a matrix of QC test data for the machine, that verifies every aspect of performance.

Although the company no longer relies on the expert eye/feel of the assembly line staff but they are still trained to recognise powders in this way. Today, particle size data enables trainers to teach people to recognise the feel of specific blends and to demonstrate the sensitivity of the solution that the company has developed. Such efforts emphasise to all staff the need to control the particle size of the coffee in a statistically reliable way, and underline company commitment to manufacture of the highest quality products.

Looking further forward, particle size information is also now driving continuing development. At De’Longhi the Insitec systems are in almost constant use as development staff also use the system to gather experimental data to support research. Particle size results also enable assessment of different grinder designs, with, for example, different internals, allowing the company to refine its product offering.








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