1.  Field of the Invention
The present invention relates generally to measuring the volumetric flow of a fluid through a passage and, more particularly, to a system for detecting the passage of a follower element motivated by the fluid in the passage. 
2.  Description of the Prior Art
In the prior art fluid flowmeters or the like are known which utilize a ball or other type follower element in a manner wherein the motion of the follower element is activated by the flow of the fluid medium such that the displacement of the follower element is related to the volumetric flow of the fluid.  The path described by the follower element may be a closed circular path or the like in which the rate of rotation of the follower element is indicative of the flow of the fluid medium.  Such a system is illustrated and described in U.S. Pat.  No.  4,157,660 to Spacek issued June 12, 1979.  A follower element may also be utilized to sense other conditions in a flowing fluid wherein the location or position of the follower element in the fluid is indicative of the condition to be sensed. 
The movement of the follower element past a fixed location or the positioning of the follower element at a fixed location has been sensed optically.  An example of this which uses a light emitting diode and receiver in conjunction with an opaque follower is disclosed in U.S. Pat.  No.  4,089,220 issued to Houlburg May 6, 1978.  It is known to use a ferromagnetic follower element which can be sensed, for example, by a system using a permanent magnet in cooperation with an induction coil to produce an induced current during the passage of the follower element.  Other types of photoelectric or magnetic pick-up devices including electrodielectric devices have also been proposed. 
Whereas the prior art follower sensing devices have been somewhat successful, they all contain certain drawbacks.  Optical systems, of course, depend on the transparency of the fluid involved and cannot be used in many industrial applications where the fluid is turbid or opaque or where environmental dust or the like may interfere.  Magnetic type sensors require the use of a ferromagnetic follower.  The follower, however, may not be compatible with the fluid either in terms of specific gravity or may react chemically with the fluid of interest.