Method of and apparatus for detecting the rotation rate of an air moving fan

A method and sensor for sensing the rate of rotation of a fan in order to monitor and detect a variation in the rate of air flowing through the fan, such as to indicate a restriction in the air flow path or failure of the fan. A Hall-effect transistor (HET) is placed in the fixed field of a magnet adjacent the path of a preferably ferrous moving element capable of changing the magnetic field coupled to the HET. As the ferrous element passes the magnet and HET, the strength or alignment of the field changes sufficiently to produce an output pulse from the HET. The pulse rate decreases with slowing of the fan such as may occur with a restriction or blockage in the air flow path.

FIELD OF THE INVENTION 
The present invention relates to a method of and apparatus for sensing the 
rate of rotation of an air-moving fan, especially a decrease in the 
rotation rate. More particularly, the present invention is directed to 
apparatus and methods for directly monitoring and detecting a variation in 
the rotation rate of an air-moving fan in order to indirectly monitor and 
measure the rate of air flowing past a point so as to indicate a 
restriction in an air flow path affecting the fan speed. 
BACKGROUND OF THE INVENTION 
It is often desirable to detect when an airflow path becomes detrimentally 
obstructed, such as when a filter becomes clogged or a heat exchanger 
becomes clogged or freezes over. 
Modem automotive and other heat exchangers and air conditioners use 
electric fans to accelerate the airflow. A fan moving air through a 
clogged airflow path may suffer a short life and lead to engine damage 
and/or increased vehicle maintenance costs. Because such fans are often 
located in hard to reach locations, a failed fan for an automobile air 
conditioner or cooling system can significantly increase repair costs. It 
is therefore useful to detect and correct airflow obstructions when such 
problems occur and before repairs are required. The problem of determining 
when an airflow path becomes detrimentally obstructed has recently been 
addressed by placing a sensor in the air flow path and detecting when the 
airflow rate decreases. Sensors in previous apparatus or this purpose 
include electrical current sensors (a heavily loaded fan draws additional 
current), vane switches, pressure switches, and the like. 
It is known from U.S. Pat. No. 4,479,115 to Holzhauer to provide a 
mechanism for automatically determining the speed of a fan, such as a fan 
used to cool electronic equipment, and/or to determine when the fan has 
failed, i.e., slowed down or stopped. Holzhauer discloses that one 
possible approach is to monitor the fan speed by sensing the rate at which 
an optical path is interrupted by the fan blades. Holzhauer also discloses 
that it is possible to monitor the fan speed by using a Hall effect device 
(such as a Hall Effect Transistor or "HET") to sense the rate at which the 
magnetic field created by the fan motor is rotating. 
However, Holzhauer points to a significant disadvantage of the Hall effect 
device sensors, namely, that the HET must be installed when the fan is 
manufactured. The relatively heavy magnet required to trigger the HET 
unbalances the fan blade when added near an external HET sensor. Thus, 
such an arrangement is not readily retrofit or incorporated into existing 
installations. That is, it must be an integral part of the initial fan 
design and cannot, therefore, be conveniently added to existing fans or 
readily incorporated into existing fan designs. Moreover, stray magnetic 
fields are well-known to negatively affect the operation of HET sensors. 
The electric fan motor, of course, generates strong interfering magnetic 
fields in normal installations unless it is shielded. 
In the typical HET, a Hall element, including a plurality of semiconductor 
materials of different conductivity types, is subjected to a changing 
magnetic field so as to deflect charge carriers in the Hall element 
produced by passing a current therethrough. The HET device measures the 
extent of charge carrier deflection and thus the flux density in terms of 
a variable voltage appearing between terminals at opposite ends of the 
Hall element. Such voltage is generally proportional to the flux density 
through the Hall element, and may therefore be used to measure or 
otherwise represent a number of different parameters which can be 
translated into a magnetic field. U.S. Pat. No. 3,835,373 to Matula 
teaches that a principal difficulty in the use of HET's to measure 
parameters arises because the magnetic flux density varies hyperbolically 
in a proportional air gap producing a non-linear relation between position 
and Hall element output voltage. This nonlinear relationship is highly 
undesirable in many measurement applications. 
The Matula patent discloses a complex rotational positional sensor which 
utilizes a HET and a means to maintain the Hall voltage constant. The 
rotational position sensor includes a HET which is mounted in an air gap 
of variable size within a magnetic circuit. The HET is subjected to 
changes in magnetic flux density in direct relation to the rotational 
position of a cylinder member forming a part of the magnetic circuit. The 
magnetic circuit includes a C-shaped permanent magnet having opposite pole 
pieces forming air gaps with the cylindrical member. The cylindrical 
member comprises a half cylinder in the region of the air gap containing 
the HET so as to vary the effective area of the gap and thus the flux 
density as the cylindrical member turns. Such a system is very complex and 
also cannot be readily incorporated or retrofit into pre-existing fans. 
U.S. Pat. No. 4,524,932 to Bodziak describes a railroad car wheel detector 
which utilizes a Hall effect element. The HET is incorporated into a 
complex integrated circuit package including temperature compensation, 
voltage regulation, and amplification functions. It is mounted on top of a 
permanent magnet which is made of ceramic material with the critical Hall 
axis aligned with the magnet pole axis. The combined permanent magnet and 
HET assembly is mounted on the rail at a predetermined distance below the 
top of the rails so that the flange of each passing wheel occupies the air 
gap between the magnet and the rail through which the major portion of the 
magnetic flux flows. Reduction of the air gap increases the level of the 
magnet flux and thus the level of voltage output of the Hall element. 
U.S. Pat. No. 4,719,419 to Dawley discloses that one known apparatus for 
sensing precise shaft position utilizes an annular ring magnet having a 
plurality of circumferentially oriented poles. The ring magnet is 
coaxially attached to a rotary shaft and a Hall effect device is attached 
to a stationary member adjacent the ring magnet. In particular, the ring 
magnet includes a plurality of magnets connected in series to form a ring 
with the north and south poles of the magnets alternately arranged. As the 
shaft and the attached ring magnet rotate, the Hall effect device 
generates a sinusoidal electrical signal indicative of the magnetic 
induction or the magnetic flux density produced by the magnets at the Hall 
effect device. The polarity of the magnetic flux density and the generated 
electrical signal changes as each pole passes the Hall effect device. The 
number of signal cycles per revolution of the ring is a function of the 
number of poles that make up the ring. A counter counts the number of 
waveform cycles of the signal generated by the Hall effect device. The 
count is then used to determine the rotary position of the shaft. The 
accuracy of such a system is dependent upon the number of poles that make 
up the ring. That is, increasing the angular position accuracy is 
accomplished by increasing the number of magnets. Such a system is very 
complex and also cannot be readily incorporated or retrofit into 
pre-existing fan designs and fans. 
SUMMARY OF THE INVENTION 
In view of the foregoing limitations and shortcomings of the prior art 
devices, as well as other disadvantages not specifically mentioned above, 
it should be apparent that there still exists a need in the art for an 
effective fan rotation rate detector for detecting the presence of airflow 
restrictions. It is, therefore, a primary object of this invention to 
fulfill that need by providing an improved lightweight, inexpensive fan 
rotation rate sensor. 
Another object of the present invention is to sense rotation rate, or a 
variation in the rotation rate, of motor-driven fan blades in an airflow 
path. 
Another object of the present invention is to implement a HET-based sensor 
to detect the continuing rotation rate of a fan. It is intended that the 
detection occur without the necessity that the relatively heavy mass of a 
magnet be mounted on the rotating part of the fan. 
Still another object of the present invention is that the sensor not 
require significant modification or redesign of the fan elements for 
incorporation into or retrofitting to an existing fan design or 
pre-existing fans. 
An advantage arising from the novel configuration of the closely coupled 
magnet and HET combination in which the HET is located is that a small and 
lightweight interrupter element is capable of triggering the HET. Another 
advantage is that the presence of the magnet in close coupling with the 
HET isolates the HET from noise and stray magnetic field variations such 
as are inherently present in an automotive engine compartment. 
Briefly described, the aforementioned objects are accomplished in a first 
embodiment of the invention by providing a fan rotation rate sensor 
including a lightweight magnetic field interrupter mounted to a rotating 
element of a fan (such as the hub of the fan), a magnet fixedly mounted to 
the fan relative the path of the interrupter, and a Hall effect device or 
transistor (HET), also fixedly mounted to the fan positioned near the 
magnet such that the HET is subjected to the magnetic field of the magnet. 
The magnetic field to which the HET is subjected varies as the interrupter 
closely adjoins the magnet and the HET when passing thereby during 
rotation of the fan. The magnet provides a field sufficient to fully turn 
the HET ON or OFF, until the rotating interrupter alters the magnetic 
field at the HET and the HET switches to the opposite state. 
The method objectives of the present invention are accomplished by closely 
coupling a magnet and an HET and periodically passing a magnetic 
interrupter (or coupler) element mounted on a rotatable fan element, 
through a point adjacent to the magnet which is closely coupled to the HET 
(both being mounted to a fixed fan element) such that the magnetic 
interrupter (or coupler) alters significantly the field to which the HET 
is subjected. More particularly, the magnet is field coupled to the HET 
and the interrupter/coupler is attached to the fan at a point where 
rotation of the fan causes the interrupter/coupler to periodically alter 
the magnetic field to which the HET is subjected, causing an electrical 
output from the HET which can be measured in any of several known ways, 
including interval period measurement, pulse rate measurement, or signal 
level measurement. 
Since the magnet and HET are small, and because the interrupter (or 
coupler) can be made of thin, relatively lightweight materials, the entire 
sensor can be easily retrofit into existing designs of fan motors at low 
cost and without retooling the fan elements. 
The interrupter can be a thin strip or tab of ferrous metal, a nonmagnetic 
material containing magnetic or ferrous particles therein or the 
equivalents thereof. The interrupter may simply be adhered to the hub of 
the fan blade, provided it passes sufficiently close to the HET to cause a 
significant variation in the HET current. An interrupter/coupler element 
can also be made of a strip of material which has been magnetized. Two or 
three interrupters or couplers can be used to increase the frequency of 
the output pulses provided, if desired. If a plurality of interrupters or 
couplers are used, the number should be limited, as the advantages of a 
plurality of interrupters/couplers diminishes above three. Heavier 
sensors, and sensor elements positioned at the outer periphery of the fan 
blade(s) are less preferred since they inherently contribute to imbalance 
conditions which may be difficult to overcome. 
Further, those persons of ordinary skill in the art will recognize that by 
placing the HET within the field of the magnet, the magnetic field of the 
magnet strongly affects the HET. Since the HET is thereby strongly biased 
magnetically, a significant change in the magnetic field is required to 
affect the HET sufficient to cause the HET states to switch, and thus 
adverse effects caused by stray and extraneous magnetic fields can be 
eliminated or at least significantly reduced. This arrangement 
advantageously provides significantly improved isolation of the sensor 
from extraneous signals. 
The sensor is very conveniently manufactured, either by fixing the magnet 
and HET in a simple sheet metal frame or preferably by incorporating these 
elements in a molded polymeric housing. Such frame or housing may include 
such suitable mounting features as may be needed to attach it to the fan 
or fan motor hub. A connector may be provided to facilitate electrical 
connection to the HET. 
With the foregoing and other objects, advantages, and features of the 
invention that will become hereinafter apparent, the nature of the 
invention may be more clearly understood by reference to the following 
detailed description of the invention, the appended claims, and to the 
several views illustrated in the attached drawings.

DETAILED DESCRIPTION OF THE INVENTION 
There is shown in FIG. 1 a fan 10, such a motor-driven vehicle fan, that 
may be positioned in an airflow path. A restriction in the passage of air 
through the fan will result through a slowing of the fan rotation rate due 
to an increased load on the fan motor. Failure of the fan motor or 
blockage of the fan blades, e.g., by ice, debris, etc., will result in a 
near zero or zero rotation rate. 
Fan 10 includes fixed and rotating portions as is well known to those 
persons having ordinary skill in the art. The fixed portion includes an 
electric fan motor 12 centrally mounted to a fan housing 14 both of 
conventional construction. The rotating portions comprise the fan assembly 
16 including the fan hub 18 (FIGS. 2 and 5) to which are mounted the fan 
blades 20. Hub 18 and blades 20 rotate in the direction shown by the arrow 
R about the axis 13 of fan motor 12. The fan assembly 16 is also of 
conventional construction and therefore need not be further described. 
A sensor assembly 22 arranged to detect variations in the rotation rate of 
the fan comprises a rotation rate detector 24 designed to be fixedly 
mounted to the fan housing 14 and an interrupter element 26 (FIGS. 2 and 
5) attached to a rotating portion of the fan assembly 16, such as the hub 
18, as described in more detail hereinafter. 
Rotation rate detector 24 shown in more detail in FIGS. 2-5, includes a 
magnet 28 and an HET 30, the magnet and the HET being mounted in a housing 
32 affixed to a fixed portion of the fan 10. A group of electrical leads 
31 extend away from HET 30 to a connector portion of the housing 32 as 
shown in the schematic view of FIG. 2. Housing 32 is preferably molded in 
one piece of a polymeric material with the magnet 28, HET 30 and 
electrical leads 31, being molded in situ. As the fan assembly 16 rotates 
around the fan axis 13, the magnetic field interrupter 26 on hub 18 passes 
very near the detector 24 and upsets the magnetic field to which the HET 
30 is subjected by the magnet 28. 
While the present invention contemplates that an interrupter 26 is 
preferred, it should be apparent to those persons having ordinary skill in 
the art that a magnetic coupler could be substituted therefor in many 
uses. For the purposes of this description, the term "interrupter" will be 
used to describe an element capable of intentionally upsetting the 
magnetic field of HET 30, and specifically includes such a coupler. 
Upsetting of the magnetic field as contemplated herein includes an 
increase, decrease, or change in orientation of the magnetic field at the 
situs of the HET 30. The magnetic field in which HET 30 lies varies 
substantially as the interrupter 26 passes thereby. 
In order to minimize the effect of an unbalanced condition of the fan, the 
interrupter 26 is preferably a small piece of ferromagnetic material, such 
as soft steel, attached in a suitable manner to the hub 18. A small 
interrupter 26 is preferred so that a significant imbalance condition of 
the fan rotating elements is avoided. It is important that the fan 
rotating elements be maintained as well balanced as practicable. Two or 
three such interrupter elements may be used, preferably mounted on the hub 
18 at equiangular intervals about the fan axis 13 so as to maintain the 
balance of the rotating portions of the fan assembly 16. 
Interrupter 26 may comprise a clip formed of thin ferromagnetic material 
which is folded and crimped over the edge 17 fan hub 18 as shown in FIG. 
2. The interrupter 26 may be secured with an adhesive, lightweight 
mechanical fastener(s), or the equivalents thereof. The interrupter is 
selected in size and permeability such that it does not obstruct or create 
significant imbalance of the fan rotating element, and such that it alters 
the magnetic field of the magnet 28 in the vicinity of HET 30. 
In FIG. 2 magnet 28 and HET 30 are shown disposed together relative to 
interrupter 26 and fan hub 18 such that as the interrupter 26 passes 
adjacent the detector 24, it alters the field of magnet 28 in which HET 30 
lies, causing the HET output to change, thereby producing an output pulse 
which can be detected externally and used to determine either the fan 
blade speed or fan operability. The present invention contemplates a 
steady-state voltage output, such as, for example, either ON or OFF, the 
state of which changes as the field to which the HET 30 is subjected is 
altered by the passage of the interrupter 26 past the detector 24. More 
particularly, the output voltage can be either a voltage pulse displaced 
from a common or reference potential, or a steady-state voltage which 
drops to a common or reference potential at the time of interruption of 
the field. 
Referring now to the embodiment of the invention shown in FIGS. 3-5, the 
molded housing 32 has a generally "L" shape comprising a first end portion 
34 and a second end portion 36. Second end portion 36 is also a generally 
"L" shaped member as best seen in FIGS. 3 and 5. At the location where the 
end portions 34, 36 are joined, there is provided a pair of mounting ears 
38, 40 for attaching the detector housing 32 to a fixed portion of the 
fan, in the described embodiment, the fan housing 14. Preferably, the ears 
38, 40 are provided with holes 42 for suitable fasteners, such as screws 
or rivets, which are used to fasten the detector 24 to corresponding ears 
(not shown) on the fan housing 14. The detector may also be affixed to the 
fan housing by other suitable means, such as adhesives, clamps or the 
like. 
The constructional details of the detector 24 are shown more specifically 
in the cross-sectional view of FIG. 4 in which second end portion 36 of 
housing 32 has molded integrally therein magnet 28 and HET 30 so as to 
position HET 30 in close proximity to curved surface 44, the radius of 
which is located on a common center with the fan axis 13. The HET 30 is 
also located in proximity to the magnet 28 so as to establish a 
steady-state voltage output of HET 30 as explained above. Electrical leads 
46 are also molded integrally into second end portion 36 and are connected 
the electrical leads or terminals 31 of the HET 30 and extend into the 
first end portion 34 which is adapted to receive a conventional electrical 
connector (not shown) in cavity 48. 
In lieu of molding magnet 28, HET 30, and electrical terminals 46 in situ 
in the housing 32, the housing 32 may be molded with appropriate recesses 
into which the components 28, 30 and 46 may be secured, e.g., by 
press-fitting, adhesives or the like. 
FIG. 5 illustrates the manner in which the detector 24 and interrupter 26 
are mounted to the 'fixed and rotating portions of the fan 10. The fixed 
portion of the fan includes the fan housing 14 which comprises a 
conventional annular shroud 48 to which fan motor 12 is securely fastened. 
The L-shaped second end portion 36 of the detector housing 32 is 
positioned over the shroud 48 in such manner that curved surface 44 faces 
the outer periphery 50 of fan motor 12. The rotatable portions of the fan 
comprising the fan hub 18 and fan blades 20 are connected to the fan motor 
shaft (not shown) along fan axis 13 (FIG. 1). Ears 38, 40 shown in FIGS. 3 
and 4 are fastened to the fan housing 14 by appropriate fasteners so that 
the connector cavity 48 and electrical terminals 46 therein extend 
radially outwardly from the fan axis for ease in making the electrical 
connection with the detector 24. The interrupter 26 comprises a thin strip 
of ferromagnetic material, such as shim stock, and is fastened by adhesive 
bonding or other means to the inner circumferential surface 52 of the hub 
18. Interrupter 26 extends beyond the edge 17 of hub 18 into close 
proximity with the curved surface 44 of the detector 24. The interrupter 
26 is sufficiently small in size so as not to detrimentally affect the 
overall balance of the rotating portions of fan 10. If desired, an 
identical interrupter or interrupters (not shown) may be positioned on the 
hub 18 at equiangular spacing(s) from the location of interrupter 26 shown 
in FIG. 5 (e.g., 180.degree., 120.degree., etc.) to insure the fan remains 
balanced. Such additional interrupter(s) are only necessary when the mass 
of the interrupter required to operate the detector 24 is large enough to 
affect the balance of the fan. 
The circuit for determining the fan speed from the HET 30 output pulses is 
not disclosed as it does not form a part of the present invention. It is 
noted, however, that those persons having ordinary skill in the electronic 
measurement and instrumentation art will be familiar with many circuits 
that could be used to accomplish this objective. The fan speed can be 
determined either by counting the number of pulses per unit of time, or by 
measuring the period between pulses. In the event, the fan does not rotate 
at all because of a failed fan motor or obstructed fan blades, e.g., by 
ice or debris, the input to the measuring circuit will correspond to the 
steady-state output of the HET 30 as described above so that those 
defective conditions may also be detected. 
Although certain presently preferred embodiments of the invention have been 
described herein, it will be apparent to those skilled in the art to which 
the invention pertains that variations and modifications of the described 
embodiment may be made without departing from the spirit and scope of the 
invention. Accordingly, it is intended that the invention be limited only 
to the extent required by the appended claims and the applicable rules of 
law.