DEVELOPMENT OF A COMPACT NDIR SPECTROMETER BASED ON MOEMS COMPONENTS FOR FRUIT RIPENING MONITORING.

Authors: 
Jordi Fonollosa
Author Email: 
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Publication date: 
06/01/2009
In today’s store-houses the ripening of fruit is controlled by managing the ethylene concentration in the ambient atmosphere. Precise and continuous ethylene monitoring is very advantageous since low ethylene concentrations are produced by the fruit itself and are indicative of its ripeness, and on other occasions, ethylene is externally added when ripeness or degreening of the product must be promoted. In this Thesis an optical multi-channel non-dispersive mid-infrared (NDIR) spectrometer for ethylene measurement is built and characterized for measuring fruit status in apple’s store-houses. The corresponding optical components and signal processing electronics have been developed, tested and integrated in a compact measurement system. In addition to the ethylene channel, the spectrometer also features ammonia and ethanol channels to consider their cross-sensitivities, and a reference channel to improve long-term system stability. Moreover, these channels are useful for monitoring a potential malfunction of the cooling system and possible fouling of the fruit. Therefore, the complete system can be considered as a multipurpose instrument for controlled atmosphere management. In the trend towards miniaturization, a novel detector module containing multiple IR sensor channels is built and characterized. In its final form it contains thermopiles as IR detectors, narrow band filters to select the absorption bands of the target gas, and a four Fresnel lenses fabricated on the same silicon substrate in a combined multi-lens array to increase system sensitivity. In order to reduce the number of photolithographic steps, a new design based on sharing up to sixteen quantization steps by the four lenses is done. However, it has been found that integration of filters too close to the IR detector may lead to degraded performance due to thermal coupling. To avoid such detrimental effects two possibilities have been considered: set the IR detector in vacuum conditions and increase the solder joint height between the filter and the thermopile. Specific signal conditioning electronics were designed. They contain an analogue preamplification stage based on an instrumentation amplifier, and a digital lock-in amplifier implemented on a commercial microcontroller as a signal recovery system. The complete spectrometer has been successfully tested in laboratory and field conditions. Assuming that ethylene and ammonia may not appear simultaneously, it has been found an ethylene detection limit of 30ppm, which is low enough to detect when fruit is ripe and prevent it to decline to senescence. The detection limit corresponding to ammonia is 160ppm, which can be used to set an alarm if a leakage from the cooling system occurs.
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