Proyectos Inmobiliarios en la Comuna de Concepción

Chemical sensors are important as, within a nuclear cave, it is necessary to determine areas that may have increased toxicity or contain dangerous substances that need to be identified for disposal. This requires a mobile robot to have a chemical sensor on board or to be able to take

samples and transport them outside the cave. Transporting chemicals outside the cave would allow a more in-depth investigation into the structure of the compound, but in situ measurements allow more rapid mapping of dangerous areas and reduced complexity.

In general, systems that are miniaturised, for example for use on a small mobile robot, have issues with sensitivity, stability and reproducibility. When looking at chemical sensors it is important to realise there are two important characteristics: selectivity and sensitivity. Selectivity describes the degree to which a sensor will respond to only the target species. Sensitivity measures the minimal concentration, or change in concentration, that may be repeatably and reliably sensed.

A further factor to consider is that many chemical sensors can only be used for a few measure- ments. This is because the sensor can be easily damaged by the target chemical and this renders the sensor inert.

This section will assess: chemiresistors, chemFET sensors and electrical and electrochemical transducers.

3.7.1 Chemiresistors

Chemiresistors are polymer films that are capable of adsorbing chemical species onto their surface causing them to swell. This swelling increases their resistance as a physical response to the presence of a chemical species. This means that it is possible to design a polymer film to adsorb a particular chemical, giving it good selectivity [84].

It is possible to have a chemiresistor that is capable of response times as low as one second, however a more average response time is around 10 seconds. This is a reasonable response time for a mobile robot in a cave environment, as it would allow measurements to be taken often.

Chemiresistors have been used to create electronic noses that are capable of artificial olfaction [24] [232]. An electronic nose has been implemented on a mobile robot in order to identify objects based on their olfactory properties [147]. This could be utilised by a heterogeneous swarm in a nuclear cave to locate chemical spills and identify the contents.

3.7.2 ChemFET

ChemFET is a chemical field effect transistor, which utilises a gas-selective coating or series of coatings between its transistor gate and the analyte. The device is given a control input which modifies conduction in relation to select chemical species. This detector can detect multiple chemicals. When used for the detection of hydrogen or organic material the sensor will not oxidise and over time will produce more stable results [4].

It is also possible to build an array of chemFET/ resistive sensors that can measure various properties of gas and liquid, using the same sensing material for both; as the FET sensor is embedded within the resistive sensors [54]. This would be useful in a nuclear cave where it is not

certain either what properties of a chemical are required to be known, or which chemicals may need detecting.

3.7.3 Electrical and Electrochemical Transducers

There are several different kinds of electrical and electrochemical transducer. However, they all rely on a direct measurement of the electrical properties of the target analyte or the effect of the analyte on the electrical properties of another material. Due to the simple design of most electrical and electrochemical transducers, it is possible to use them in a harsh environment, such as a nuclear cave. The various types of transducers are as follows:

• Metal-oxide Semiconductor Devices (MOS) - detects a change in concentration of a reactive species which translates as a change in resistance. They are usually constructed of a semiconducting sensitive layer, an electrical connection to measure resistance of that layer and a heater to control the temperature of the device.

• Electrochemical Sensors - one electrode is left in the presence of the reaction, while another is isolated. The separation of these electrodes allows either a current to flow, a potential difference to form, or a change in resistance to take place between the two electrodes.

• Potentiometric Sensors - utilise the effect of the concentration of a target species on the equilibrium of redox reactions occurring at the electrode-electrolyte interface, in an electrochemical cell. Such reactions may encourage the development of a potential difference. This potential may then be measured in order to determine the concentration of the target species.

• Conductometric Sensors - measures the change in conductivity of electrolyte in an electrochemical cell, brought about by a change in concentration of a target species. • Optical Transducer - measures the interactions between various forms of light or

electromagnetic radiation and a target chemical species by detecting the modulation of some property of the radiation. These modulations include intensity, polarization and velocity in a medium.

Of all the chemical sensors, tranducers are the least expensive and most commercially available. Most electrical and electrochemical transducers are better suited to lab measurements than in situ measurements in a nuclear cave, as they are difficult to transport. This would mean that a mobile robot within a cave would need some form of sample extraction in order to use such a sensor; as is discussed for application in the oil industry by Heyer [101]. If a robot were capable of taking samples, this method would give the most accurate results for chemical sensing.