Fluidic rotational speed sensor

A device for measuring the rotational speed of a rotating object in which a tream of fluid from a back pressure sensor is directed at a chopper wheel attached to the rotating object. The output of the back pressure sensor varies as the chopper wheel rotates. The number of variations in pressure per unit time is directly proportional to the rotational speed of the rotating object.

BACKGROUND OF THE INVENTION 
Presently existing fluidic rotational speed sensors utilize a toothed 
chopper wheel attached to a rotating object, such as a rotating shaft. The 
chopper wheel is placed between a fluid supply jet nozzle and a fluid 
receiver. As the shaft rotates, the teeth of the chopper wheel 
periodically interrupt the jet of fluid. This causes the output pressure 
at the fluid receiver to appear as a pulse train. The number of pulses per 
unit time is directly proportional to the rotational speed of the shaft. 
However, this sensor inherently has a very low signal to noise ratio 
because of the flow noise in the chopper wheel. Its upper speed is also 
limited because the fluid supply jet can not reconstitute fast enough at 
high shaft speeds. As a result, it becomes very difficult, if not 
impossible, to distinguish the signal from the noise. 
SUMMARY OF THE INVENTION 
The present invention is a device for measuring the rotational speed of a 
rotating object. A chopper wheel is attached to the rotating object. The 
chopper wheel has a radius of r=r.sub.1, and at least one tooth having a 
radius of r=r.sub.2 where r.sub.2 &gt;r.sub.1. A back pressure sensor means 
directs a stream of fluid at the teeth of the chopper wheel. The back 
pressure sensor means is placed such that the stream of fluid is coplaner 
with and directed directly at the edge of the chopper wheel. 
The gap distance between the chopper wheel and the back pressure sensor 
means varies as the chopper wheel rotates from a distance of d=x.sub.1 to 
d=x.sub.2, and the output pressure, P.sub.A, of the back pressure sensor 
means varies from P.sub.A =P.sub.A1 to P.sub.A =P.sub.A2. The output 
pressure from the back pressure sensor is measured by means such as a 
pressure transducer. 
To filter noise in the output pressure from the back pressure sensor means, 
a tank capacitor in fluid connection with the output of the back pressure 
sensor means is used. The output pressure, P.sub.B, of the tank capacitor, 
varies from P.sub.B =P.sub.B1 to P.sub.B =P.sub.B2. 
OBJECTS OF THE INVENTION 
It is an object of the invention to present a fluidic rotational speed 
sensor in which the number of pressure pulses is directly proportional to 
the speed of the rotating object. 
It is another object to present a fluidic rotational speed sensor which has 
a high signal to noise ratio.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention represents an improvement to the prior art fluidic 
rotational speed sensor. The prior art speed sensor, as illustrated in 
FIG. 3, comprises a toothed chopper wheel 50 attached to a rotating object 
which in this figure is a rotating shaft 52. Chopper wheel 50 is placed 
between a fluid supply jet nozzle 54 and a fluid receiver 56. As shaft 52 
rotates, the teeth of chopper wheel 50 periodically interrupts the jet of 
fluid from fluid supply jet nozzle 54. This causes the output pressure at 
fluid receiver 56 to appear as a pulse train, as shown in FIG. 5. The 
number of pulses per unit time is directly proportional to the rotational 
speed of the shaft. However, this sensor inherently has a very low signal 
to noise ratio because of the flow noise in the chopper wheel. Its upper 
speed is also limited, because the fluid supply jet can not reconstitute 
fast enough at high shaft speeds. As a result, it becomes very difficult, 
if not impossible, to distinguish the signal from the noise. 
In order to overcome these shortcomings, the sensor of the present 
invention is presented. It comprises a toothed chopper wheel 10 attached 
to a rotating object, such as shaft 12. Set screw 11 is used to keep 
chopper wheel 10 firmly attached to shaft 12. The chopper wheel 10 has a 
radius of r=r.sub.1, and at least one tooth having a radius of r=r.sub.2, 
where r.sub.2 &gt;r.sub.1. A back pressure sensor 14 directs a stream of 
fluid at the teeth of the chopper wheel. A fluid, such as air, is supplied 
to the back pressure sensor 14 at pressure P.sub.s. The fluid travels 
through tube 16 to nozzle 18, which directs it to chopper wheel 10. The 
back pressure sensor 14 is placed such that the stream of fluid is 
coplaner with and directed directly at the edge of chopper wheel 10. A 
portion of the fluid exits the back pressure sensor 14 through tube 20. 
The pressure, P.sub.A, of the fluid flowing from tube 20 is the output 
pressure of the back pressure sensor. The output pressure, P.sub.A, is 
measured by means not illustrated, such as a pressure transducer. 
The gap distance between the chopper wheel 10 and the back pressure sensor 
14 varies as the chopper wheel rotates, from a distance of d=x.sub.1 to 
d=x.sub.2. For a given supply pressure, P.sub.s, to the back pressure 
sensor, the output pressures, P.sub.A =P.sub.A1, and P.sub.A =P.sub.A2, 
are dependent on the gap distance, d, between the chopper wheel and the 
back pressure sensor, as shown in FIG. 6. As shaft 12 rotates, the gap 
distance varies between x.sub.1 and x.sub.2, and the output pressure 
changes accordingly. For the chopper wheel shown in FIG. 1, which has only 
two teeth, there will be two output pulses for each revolution. Therefore, 
the number of pulses per unit time is directly proportional to the 
rotational speed of the shaft. FIG. 7 shows a typical plot of the output 
pressure as a function of time. 
As may be seen from FIG. 7, the output pressure from the back pressure 
sensor 14 contains a significant amount of noise. To filter the noise from 
the output pressure, a tank capacitor 22 is placed in fluid connection 
with the output of back pressure sensor 14. The output pressure, P.sub.B, 
from the tank capacitor, varies from P.sub.B =P.sub.B1 to P.sub.B 
=P.sub.B2. The output pressure, P.sub.B, is measured by means not 
illustrated, such a pressure transducer. FIG. 8 clearly shows that the 
tank capacitor effectively filters the noise in the output pressure, and 
as a result, the output wave form has a very high signal to noise ratio. 
While the invention has been described to make reference to the 
accompanying drawings, I do not wish to be limited to the details shown 
therein as obvious modifications may be made by one of ordinary skill in 
the art.