1. Field of the Invention
The present invention relates to a particle sensor for detecting the solid particles contained in a fluid.
2. Description of Related Art
When solid particles are present in a fluid (a liquid or a gas), it is necessary in some cases to detect presence of those the solid particles. Detection of such solid particles is particularly important when the solid particles present in the fluid detrimentally affect the intended action of the fluid.
Lubricating oils (e.g. engine oils) are used in internal combustion engines of automobiles or heavy machinery in order to reduce the frictional resistance and abrasion of the rotating surfaces and sliding surfaces of the engines. Operation of such internal combustion engines generates solid particles (e.g. metal fine particles) caused by abrasion, and the solid particles are taken into the lubricating oils and accelerate the abrasion of the rotating surfaces and sliding surfaces. The solid particles, etc. present in the lubricating oils are ordinarily removed by the use of a filter, such as oil filter or the like. The condition of the lubricating oils can be monitored by detecting the solid particles present in the lubricating oils.
Hydraulic oils and flushing oils are used in power transmission mechanisms (e.g. transmission), oil hydraulic pipe systems (e.g. oil hydraulic servo valve), rolling, pressing, etc. It is important to control the condition of these oils by detecting the solid particles present in the oils and/or examining the viscosities of the oils.
It is also important to detect the particles suspended in the air or examine their concentration in the air in order to monitor the level of air pollution. Such detection includes detection of the particles suspended in the exhaust gas discharged from a plant, a factory or the like.
For such detection of solid particles present in a fluid, a particles sensor comprising a sensor element using a piezoelectric film is disclosed in Japanese Patent Application Laid-Open (Kokai) No. 7-301594. With this particle sensor, solid particles present in a fluid collide with the detecting section (having a piezoelectric film) of the sensor element or with the vibrating section (mounting the detecting section thereon) of the sensor element, thereby causing the vibrating section and the detecting section to vibrate. The piezoelectric film converts the vibration into electric signals, which are outputted by electrodes holding the piezoelectric film between them.
In detecting solid particles contained in a fluid by the use of a particle sensor such as mentioned above, the vibrating section of the sensor element of the particle sensor is vibrated not only when the solid particles in a fluid collide with the vibrating section but also when the flow of the fluid becomes turbulent as a result, electric signals are outputted from the detecting section of the sensor element. Moreover, electric noise signals may enter the particle sensor from outside.
No method has heretofore been established to clearly distinguish the above-mentioned three kinds of electric signals, i.e. (1) the electric signals caused by the collision of solid particles (the signals are hereinafter referred to as "particle signals"), (2) the electric signals caused by the turbulent flow of fluid (the signals are hereinafter referred to as "turbulent flow signals") and (3) the electric noise signals which may enter the particle sensor from outside (the signals are hereinafter referred to as "electric noise signals"). This is one reason that the detection of solid particles in fluid has been unreliable.