Car air conditioners operate in all seasons to keep the cabin temperature cool in summer and warm in winter. In summer, for air conditioning systems without humidity sensors, the system cannot maintain a comfortable humidity level because the dehumidification function stops when the compressor stops. However, a system with humidity sensors can maintain a comfortable level because the system operates with on/off switching control of the compressors based on the information of humidity sensors, even when the engine is stopped. In winter, to reduce the heating load, the air conditioning system, in conjunction with the engine, controls defogging of the windshield using humidity sensors in addition to reducing ventilation loss by increasing the recirculation rate. By adding humidity information, the control of humidity in summer and defogging in winter can minimize the operation of compressors and the engine, and lead to improved fuel consumption without loss of comfort. In particular, in order to prevent energy loss, it is necessary to sense the humidity around windshields to prevent windshields from fogging up in winter.
Automobile humidity sensors are mainly resistive. Resistive sensors monitor the impedance change of the humidity sensing membrane, absorbing water molecules responding to ambient humidity changes. Impedance changes occur as a change in the quantity of ions, such as the alkaline metals, anchored in the sensor membrane. The ions become capable of moving in the membrane due to the absorption of water molecules.
Resistive sensors that use polymer membranes containing ammonium salt encounter three main types of issues. The first issue is that the sensing range of the temperature and humidity is small. Resistive humidity sensors cannot function at around 100% RH and below 0 °C. The second issue is the dissolution of the humidity sensing membrane. When water beads are formed on the humidity sensing membrane from dew condensation and the beads drop off, movable ions in the membrane dissolve into the beads and also drop off. The third issue is the slow responsiveness to changes in the surrounding humidity. It takes time for water molecules to diffuse into the membrane because the membrane thickness of resistive sensors is over 10 um.
In order to solve these issues, capacitive humidity sensors have been developed that monitor changes in the relative permittivity of membranes in response to humidity changes in the surrounding environment. Polyimide or cellulose is used for humidity sensing membranes of capacitive sensors. The relative permittivity of the polyimide in a humidity membrane is about 3, and that of water molecules is about 80. If membranes absorb water, changes in the apparent relative permittivity occur due to water molecules. As a result, the humidity can be sensed using this technology.
Capacitive sensors can function at ranges of around 100% RH and below 0 °C. The membrane will not dissolve even if water beads are formed on the membrane due to dew condensation etc. because water-resistive materials like polyimide are used for the humidity sensing membrane. Thus, it has become possible to install the capacitive sensors at the optimum position for sensing humidity on windshields to prevent fogging. In addition, the responsiveness of the sensors becomes extremely quick to humidity changes because the membranes are very thin.
It is necessary to improve the polyimide materials to reduce changes in characteristics. Hence, countermeasures were studied to suppress hydrolysis and swelling. In hydrolysis, since polar groups in the humidity sensing membranes increase after endurance testing, it is necessary to suppress the increase of polar groups. Thus, two possible countermeasures were studied. The first countermeasure is to introduce a structure that helps reduce the water molecule absorption, which causes hydrolysis in the membrane itself, by adding fluorine atoms as hydrophobic functional groups. The second countermeasure is to reduce the amount of functional groups to be hydrolyzed. This results in a reduction of the amount of imide groups per volume by extending the length of straight-chain monomer molecules.
As a countermeasure against swelling, the enhancement of bonding strength of the molecular interchain is implemented by the cross-linking of molecular chain ends because the molecular interchain is considered to be bonded electrically, the bonding strength is low, and the absorption space of the interchain space is expanded by the entry of clustered water molecules in a high-temperature and high-humidity environment. Thus, in order to suppress swelling, acetylene is added to form a honeycomb structure, which allows for stronger bonding of the molecular interchain, as a result of the formation of benzene rings by heat treatment.
Through these improvements, capacitive humidity sensors can be achieved for automobile use even in severe-application environments. The widespread use of this humidity sensor in the automobile markets will contribute to improved fuel efficiency.
This work was done by Naohisa Niimi and Takahiko Yoshida of DENSO Corporation, and Toshiki Isogai of Nippon Soken, Inc. The full technical paper on this technology is available for purchase through SAE International at http://papers.sae.org/2013-01-1337.