Method and device for measuring fluid velocities

The velocity of a fluid in a positive or negative direction is measured by se of a thermistor in a feedback circuit. The temperature of a flow thermistor is balanced with the temperature of a reference thermistor by sensing temperature through resistance by way of its voltage and adding power until the flow thermistor is at the same temperature as the reference thermistor. The ratio of reference power used as compared to flow power is indicative of the fluid's speed. Power is added by sending a signal of varying frequency to the flow thermistor; the reference thermistor is powered by a constant frequency signal. Power is measured by comparing the frequencies of the reference signal to the flow signal. Fluid direction is measured by placing a direction thermistor on either side of the flow thermistor, measuring the temperature of each direction thermistor, the cooler thermistor being upstream of the flow thermistor.

FIELD OF THE INVENTION 
This invention relates in general to devices for measuring the velocity of 
fluids and in particular to an electronic means for measuring wind speed 
for aid in the aiming of ballistic weapons. 
BACKGROUND OF THE INVENTION 
The accuracy of such weapons is very dependent on the velocity of the cross 
wind prevailing during its flight. This is especially the case as the 
travelled distance increases. Prior art wind velocity sensors are either 
excessively large, complicated and heavy, or too slow and inaccurate, or 
expensive for this intended application. 
SUMMARY AND OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a small, compact, 
accurate, light-weight, fast and inexpensive wind velocity sensor useful 
for many applications. Especially for applications such as the fire 
control system of hand carried portable ballistic weapons. 
This invention uses, in addition to other components, two thermistors. 
These thermistors have the property like many other electrical resistors 
that they generate heat in proportion to the amount of electrical power 
they receive. Some of this heat is dissipated into the surroundings and 
the rest causes the temperature of the thermistor to rise. The amount of 
temperature rise is inversely proportional to the amount of heat 
dissipated, and the amount of heat dissipated is proportional to the 
ambient temperature and the fluid velocity over the surface of the 
thermistor. One of these thermistors is placed in the flow of fluids to be 
measured, and the other is placed out of the flow but subject to the 
ambient temperature to be used as a reference. 
Another property of thermistors is that their resistance changes with 
temperature. Therefore, what has been termed a flow thermistor, while in a 
flow, will have a lower temperature and a different resistance than the 
reference thermistor. 
According to the invention a feedback circuit is employed which uses this 
difference in resistance to adjust the power in the flow thermistor such 
that the temperature of both thermistors is equal. The feedback circuit 
works by using the difference in resistance to form different voltages, 
and then this difference controls the frequency of a wave fed to the flow 
thermistor. The power received by the flow thermistor being proportional 
to the frequency of the wave. The reference thermistor is fed by a 
constant frequency wave. Therefore, since the flow of fluid across the 
flow thermistor, is proportional to its temperature compared to the 
reference thermistor, and the feedback circuit maintains the temperature 
of the flow thermistor equal to the reference thermistor by means of 
frequency, then the frequency of the flow thermistor compared to the 
reference thermistor is proportional to the flow. The ratio of the fixed 
frequency and the flow thermistor frequency can easily be measured and 
converted into the speed of the flowing fluid. This method avoids the 
complicated relationships of power, temperature and thermistor resistance 
associated with either a constant current or constant voltage drive that 
would result in non-linearity and instability of the feedback loop. 
In order to determine the wind direction smaller matched thermistors are 
placed on the sides of the flow thermistor. Depending of the direction of 
the fluid one of these smaller thermistors will be downstream and 
receiving the heat given off by the flow thermistor. This will cause the 
downstream thermistor to have a higher temperature and therefore a 
different resistance. Determining if a difference in resistance exists and 
which thermistor has the greater or lesser resistance is quite easy. This 
difference is then used to determine the direction of the flow. 
It is an object of this invention to provide fast, low cost accurate 
measurements of fluid velocities wherever needed. Examples are fire 
control systems, weather stations, low speed wind tunnels, air 
conditioning ducts, air velocities and sailboat wind speed indicators. 
This invention provides a simple and unique method of applying power to 
thermistors that is lacking in the complicated relationships of other 
methods. This inventive method makes it very easy to measure the power 
received by the thermistors and in this application to measure the fluid 
velocity. 
A further object of this invention is to create a method and device for 
measuring fluid velocities that is simple in design, rugged in 
construction and economical to manufacture. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings in particular, the invention embodied therein 
comprises two self heated thermistors R.sub.TR and R.sub.TF positioned 
inside a length of one-half inch diameter plastic tube (not shown). These 
two thermistors are used to sense wind speed and are designated as the 
reference thermistor RTR and the flow thermistor RTF. Each thermistor is 
heated above ambient temperature by an application of power. In the 
steady-state, the power and temperature rise are related by the equation: 
EQU P=.delta..DELTA.T (1) 
where .delta. is the dissipation constant of the thermistor, P is the 
thermal energy supplied and .DELTA.T is the change in temperature due to 
the power, P applied. The dissipation constant .delta. is a function of 
wind velocity flowing across the thermistor surface. The reference 
thermistor in this sense is subjected to the ambient air temperature but 
is shielded from air movement. Thus, equation (1) becomes: 
EQU P.sub.R =.delta..sub.o .DELTA.T.sub.R (2) 
where .delta..sub.o is the dissipation constant for still air. 
The flow thermistor is subjected to the air flow, again at ambient 
temperature. Thus, equation (1) for the flow thermistor becomes: 
EQU P.sub.F =.delta..sub.F .DELTA.T.sub.F (3) 
where the air flow is constant and .delta..sub.F is the dissipation 
constant for that flow rate. Thus, if the ratio of equations (2) and (3) 
is taken: 
EQU (P.sub.R /P.sub.F)=(.delta..sub.F .DELTA.T.sub.F)/(.delta..sub.o 
.DELTA.T.sub.R) (4) 
and if .DELTA.T.sub.F is maintained equal to .DELTA.T.sub.R because of a 
feedback circuit, a direct measure of wind velocity is given by: 
EQU .delta..sub.F =.delta..sub.o (P.sub.E /P.sub.R) (5) 
because .delta..sub.o is given and P.sub.F and P.sub.F are measured. By 
inspection, at zero flow, .delta..sub.F =.delta..sub.o and therefore 
P.sub.F =P.sub.o. Thus, the plot of .delta..sub.F versus wind velocity 
starts at .delta..sub.F =.delta..sub.o at zero velocity. By realizing 
.delta..sub.F =.delta..sub.o .delta..sub.w, equation (5) can be rewritten 
as 
EQU .delta..sub.w =.delta..sub.o ([P.sub.F /P.sub.R)]-1) (6) 
such that the curve plots from zero at zero wind velocity. 
The assumption of .DELTA.T.sub.F =.DELTA.T.sub.R can be implemented by 
accomplishing two tasks. First, the resistance vs. temperature 
characteristics of the reference and flow thermistors should be closely 
matched. Secondly, a feedback loop may be devised to vary P.sub.F as a 
function .DELTA.T.sub.F and .DELTA.T.sub.R and drive the differential 
temperature to zero. 
FIG. 1 shows the block diagram for the disclosed wind sensor. The 
thermistors R.sub.TR being the reference thermistor and R.sub.TF being the 
flow thermistor are placed in a first bridge circuit by adding resistors 
R.sub.W1 =R.sub.W2 &gt;&gt;R.sub.T. At equilibrium and no flow, the dc 
components of V.sub.R and V.sub.F are equal because R.sub.TR and R.sub.TF 
are equal and the output of A.sub.1 is zero. Power is applied to each 
thermistor via capacitors C.sub.1, and C.sub.2 which are of equal value. 
If a square wave is applied to the capacitor input (f.sub.r and f.sub.v, 
FIG. 1), it is easily shown that the power applied to R.sub.TR and 
R.sub.TF is a linear function of frequency (P=kf) if 1/2f&gt;&gt;5R.sub.T C. 
This condition insures that all the energy added to the capacitor at each 
transition of the square wave is dissipated in the thermistor. This drive 
mode avoids the complicated relationships of power, temperature, and 
thermistor resistance associated with either constant current or constant 
voltage drives that would result in non-linearity and instability of the 
feedback loop. 
The reference frequency, f.sub.r, is maintained at a constant value which 
maintains a constant power, P.sub.R, in R.sub.TR, regardless of ambient 
temperature. Thus, from equation (2) .DELTA.T.sub.R is also a constant 
value above ambient temperature. 
Amplifier A, produces a voltage proportional to V.sub.F -V.sub.R. This 
voltage is integrated and applied to a voltage-to-frequency converter 
whose output square wave is applied through C.sub.2 to R.sub.TF (see FIG. 
1). The polarities are arranged to force V.sub.F =V.sub.R and thus, at 
equilibrium, R.sub.TR =R.sub.TF. Under no flow conditions, f.sub.r 
=f.sub.v and thus P.sub.R =P.sub.F. When the wind flows across R.sub.TF, 
.delta..sub.w increases and thus P.sub.F (and f.sub.v) is raised to 
maintain .DELTA.T.sub.F =.DELTA.T.sub.R, implying R.sub.TR =R.sub.TF. Thus 
f.sub.v is a direct function of wind velocity, which is also shown by 
rearranging equation (6) and substituting P=kf to give: 
EQU f.sub.v /f.sub.r =(.delta..sub.o +.delta..sub.w)/.delta..sub.o(7) 
FIG. 3 shows data taken using a working prototype implementation of FIG. 1. 
The frequency ratio fv/fr is plotted versus wind velocity. The curves show 
a linear function shift with changes in ambient temperature. This 
variation is easily compensated giving accurate and repeatable results. 
Wind direction is determined by placing two smaller matched thermistors 
(R.sub.T3 and R.sub.T4, FIG. 2) on either side of R.sub.TF spaced equal 
distances away. These thermistors are heated by R.sub.TF due to radiation 
and conduction through the air. A second bridge is constructed using 
R.sub.T3, R.sub.T4 and two fixed resistors, R.sub.D, of equal value. At 
equilibrium with no flow, this bridge is balanced and the output of 
A.sub.2 is zero. The bridge midpoints A.sub.2.sup.+ and A.sub.2.sup.- 
are unbalanced when air flows because the small thermistor up stream of 
R.sub.TF cools more than the downstream thermistor. A.sub.2 produces 
either a positive or negative voltage depending on whether the flow is in 
one direction of the tube (not shown) or the other, thus indicating 
direction according to voltage polarity. 
While a specific embodiment of the invention has been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.