Fluidic angular rate sensor employing ionized gas

A fluidic angular rate sensor employs an ionized stream of gas 104, the deflection of which is detected by an amplifier 130 connected to a pair of collection electrodes 122 and 124, motive power for the gas stream 104 being provided by momentum transfer from ions in a corona discharge.

DESCRIPTION 
TECHNICAL FIELD 
The field of the invention is that of a fluidic angular rate sensor 
improved by the use of an ionized gas as the working fluid and ion 
collectors as sensing elements. 
BACKGROUND ART 
Fluidic angular rate sensors are illustrated in U.S. Pat. Nos. 3,587,328 
and 3,626,675, in which a stream of flowing gas is deflected to one side 
by the effect of the Coriolis force, which in turn depends upon the 
angular rate of rotation of the device. In the prior art, this deflection 
was sensed by a matched pair of hot-wire anemometers. One important 
advantage of this type of angular rate sensor is that the only moving part 
is the pump used to provide the jet of gas. It is essential in these prior 
art devices that the thermal effects of the wires be extremely well 
matched and problem of manufacturing variations within the wire material 
has long plagued the art. 
DISCLOSURE OF INVENTION 
The invention relates to an improved fluidic angular rate sensor in which 
an ionized gas jet flows along an axis, the deflection from that axis 
being substantially proportional to the angular rate of rotation of the 
device, this deflection being measured by a pair of ion collecting 
elements. The invention further relates to the elimination of all moving 
parts by means of an electric wind source of pressure to move the gas jet.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 illustrates a simple embodiment of the invention in which a stream 
of flowing gas 104 enters enclosure 102 through aperture 106, powered by a 
pump not shown, proceeds along the axis and is deflected in the plane of 
the paper by rotation of the device in that same plane. This deflection is 
caused by the effect of the Coriolis force acting on the flowing molecules 
in the gas. The result of that deflection will be that a greater flow of 
gas will pass one electrode of electrodes 122 and 124, than will pass the 
other electrode of the pair. In the prior art, this difference in flow was 
detected by measuring the differential cooling of a pair of matched 
hot-wire anemometers, with problems well known in the art in matching the 
effects of the wire and in compensating for temperature problems. In this 
invention, differential flow is measured by a difference in current 
between electrodes 122 and 124. 
In operation, gas 105 is ionized by radioisotope 108 disposed about the 
left-hand portion of the inside of housing 102. This radioisotope, 
illustratively americium-241, emits radioactive particles which produce 
positive and negative ions distributed within a short range of the 
radioactive compound and extending not only in gas jet 104 but also in the 
surrounding gas 105. This wide spread of ionization is advantageous in 
that ions diffusing out of the gas jet are replaced by ions diffusing in 
from the stationary gas through which the jet passes, thus compensating 
for any ion loss as the gas jet traverses the apparatus. The combination 
of positive and negative charges distributed initially uniformly 
throughout the gas also has the advantageous feature that there is no net 
electrical repulsion driving ions away from the gas jet. 
The deflected gas jet 104 impinges on an electrode array consisting of 
center electrode 120 biased to a predetermined positive voltage and side 
ion-collector electrodes 122 and 124 which are spaced apart from electrode 
120 by a predetermined transverse distance and are connected directly to 
the input connectors of amplifier 130. The two collector electrodes will 
each attract ions on one side of center electrode 120, so the difference 
in electrode current will depend on the deflection of the gas stream and 
thus on the rate of angular rotation. Positive ions within the gas stream 
are deflected away from electrode 120 by the positive bias in voltage and 
deflected into one or the other of the collector electrodes. Electrode 120 
is shown in the drawing as having a positive bias with respect to 
electrodes, but a negative bias may also be used. The amount of the bias 
is not critical and may be conveniently set at a high enough value so that 
essentially all of the ions are collected. 
The output of amplifier 130 will, as is well known in the art, be 
proportional to the difference in magnitude of the two inputs. This output 
signal will have the same common mode problems that are inherent in the 
detection of the relatively small difference between two large signals. 
The problem of common mode rejection is well known to those skilled in the 
art and the particular amplifier design is not part of the present 
invention. High quality amplifiers capable of rejecting high common mode 
currents are well known to those skilled in the art. The amount of the 
common mode signal may be reduced by adjusting the distance between 
radioactive material 108 and the collecting electrodes, or by increasing 
the stopping power of gas 105 to reduce the number of particles that reach 
the vicinity of the electrodes, since ions generated at or near the 
electrodes will not be affected by the Coriolis force and will contribute 
only to the common mode signal. 
High quality current amplifiers may be constructed having a sensitivity on 
the order of 10.sup.-13 amps, which corresponds to an angular rate 
sensitivity of 0.1 degree/sec. in a fluidic angular rate sensor of 
conventional configuration. The use of AC rather than DC detection, by 
reversing the inter-electrode bias, may be used to avoid drift; and 
temperature compensation may be employed to stabilize the sensor response 
further if a particular application requires. The signal output at zero 
rotation may be nulled by a slight bending or repositioning of the 
collector electrodes. 
An alternative embodiment of the invention, suitable for a situation in 
which radioisotope 108 is extended over a large portion of the interior of 
housing 102 (in order to generate a sufficient quantity of ions) is 
illustrated in FIG. 2, in which additional compensating electrodes 132 and 
134 are connected to and biased with respect to the center input of 
amplifier 130 by voltage source 127, the bias being negative so that 
electrodes 132 and 134 attract positive ions away from collector 
electrodes 122 and 124. The purpose of these compensating electrodes is to 
collect those positive ions generated within gas 105 between the 
compensating electrodes and the collector electrodes, i.e. those ions 
which are not part of the flowing gas stream and thus are not responsive 
to the angular rotation which is being measured. By empirically 
determining the size, position and bias of the compensating electrodes, 
the ion current flowing away from the collection electrodes towards the 
compensating electrodes may be made essentially equal to the current 
flowing from the center electrode to the ion collecting electrodes. The 
net current to each collector and thus the common mode signal to the 
amplifier can thus be effectively canceled out. Initial adjustment of the 
device with the sensor stationary will result in effective nulling of the 
common mode signal and thus the signal passing through the amplifier will 
be nearly all due to the rotation. 
FIG. 3 illustrates a further refinement of the invention, in which 
radioactive source 108 is confined to a small portion of the interior 
volume of the housing and in which the source is an alpha emitter, a 
characteristic of which is that the radioactivity and ionization induced 
by the source is confined to a small area. The ions are thus generated 
only in the upstream portion of the gas jet so that a maximum fraction of 
the ions are subject to the Coriolis deflection and so that the fraction 
of ions generated in the collection region is reduced. The center 
electrode 120 is connected directly to additional electrodes 141 and 142 
which have the purpose of repelling back to the collector electrodes 122 
and 124 that portion of the ions which are not collected immediately. 
Collector electrodes 122 and 124 and compensating electrodes 132 and 134 
perform the same functions as in the previous figure, but are extended 
further upstream in order to reach into the high ionization region of the 
gas and provide better cancellation of common mode signals. 
The preceding figures all show means of collecting ions from a gas jet, the 
gas jet itself being provided by a conventional pump. The embodiment of 
the invention illustrated in FIG. 4 illustrates an alternative "pump" 
employing no moving parts. In this figure, electrode 214 is maintained at 
a high voltage on the order of several thousand volts by a voltage source 
not shown, the electrode passing through case 210 through insulator 213. 
Electrode 214 serves as a source of a corona discharge directed at the 
lower potential of case 102. Bias resistor 220 connected between case 210 
and housing 102 serves to provide the bias formerly provided by supply 
126. Separate power supplies could be used for the high voltage and the 
bias, if desired. 
Radioactive source 109 serves as a temporally stable and spatially uniform 
source of ionization, the magnitude of which is multiplied by the electric 
field surrounding electrode 214. Ions generated in a corona discharge are 
accelerated by the electric forces which generate them and thus serve as a 
small source of motive power to the gas within which the ions move. The 
pressure provided by such a discharge is extremely small but is sufficient 
for this particular application, and has the great advantage that the only 
moving part of the previous embodiments, namely the pump, is eliminated. 
Gas jet 104 passes through housing 102 and is detected by electrodes 120, 
122 and 124 as before, the gas then exiting through apertures 107 in 
housing 102 and circulating around again. Amplifier 130 functions as 
before, being connected through insulating members 211 and 212. The simple 
electrode configuration of FIG. 1 is illustrated in FIG. 4; other 
electrode configurations may be added if desired. 
The foregoing embodiments of the invention have been illustrated as having 
pin-shaped electrodes in a single plane. Electrodes having greater surface 
area may be used, resulting in more efficient ion collection and slightly 
increased turbulence in the gas. A complementary set of collection (and 
compensation) electrodes and amplifier may be added perpendicular to the 
plane of the drawing, thus providing information on rotation about two 
axes.