Angle sensor for a steerable boring tool

A roll sensor is provided for determining the roll angle of a rotatable member. The roll sensor includes a plurality of elements arranged in a predetermined manner, each element having a first state and a second state. A controller controls the respective state of the plurality of elements. A detector detects an output of the elements which are in the first state. A liquid responsive to the rotation of the rotatable member causes the output of at least one of the elements in the first state detected by the detector to be different than the output of the remaining elements in the first state detected by the detector to determine the roll angle of the rotatable member.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The present application is related by subject matter to commonly assigned, 
copending application Ser. No. 07/539,551, entitled "Angle Sensor Using 
Thermal Conductivity For A Steerable Boring Tool" filed Jun. 18, 1990. 
TECHNICAL FIELD 
The present invention generally relates to an angle sensor for determining 
an orientation of a member in a plane and, more particularly, to a roll 
angle sensor for determining the roll angle of a steerable horizontal 
earth boring tool. 
BACKGROUND OF THE INVENTION 
The determination of the presence and location of concealed underground 
objects, such as gas and water pipes, power cables, and telephone and CATV 
cables or conduits, is a necessary prerequisite to excavation and/or the 
laying of new lines or cables. In some applications, an underground 
steerable boring tool is utilized to form an underground tunnel in which 
cables, telephone lines, etc. are subsequently positioned. When using such 
a steerable boring tool, it is important to know the location are 
orientation of the boring tool relative to underground objects to be able 
to appropriately steer the boring tool and thus position the new lines or 
cables to avoid existing lines and cables. 
One method of boring is described in commonly assigned, U.S. Pat. No. 
4,953,638 (the '638 patent) filed on Jun. 27, 1988, incorporated herein by 
reference. As generally indicated in FIGS. 1 to 3, the method includes 
positioning a boring machine on the surface of the earth adjacent a 
selected borehole entry point. The boring machine includes facilities to 
axially advance and to selectively rotate a drill string. The present 
invention, however, is not restricted to the method in the '889 
application, but has broad applicability to other methods as well, such as 
positioning the boring machine in a subsurface pit. The drill string is 
typically in the form of a plurality of lengths of pipe which are provided 
with male threads on a first end and female threads on a second end so 
that the lengths of pipe may be interconnected together in sequence to 
provide a drill string. At the end of the drill string, a drill bit as 
shown in FIGS. 2 and 3 is provided for performing the boring operation. 
The drill bit includes a blade which is inclined at an angle to the axis 
of the drill string to which the bit is attached. The angled blade of the 
illustrated drill bit generates a non-axisymmetric resultant force as it 
is thrusted through the ground, causing it to deviate off a straight line 
path if the bit is not rotated as it is advanced. 
The drill string is simultaneously rotated and advanced by means of the 
boring machine to establish a borehole in the earth. The drilling 
operation wherein the pipe is simultaneously rotated and axially advanced 
is continued until a change in direction of the borehole is desired, such 
as to avoid a known obstacle or to correct a course deviation. In order to 
change the direction of the borehole, the following sequence is employed: 
1. The rotation of the drill string is stopped. 
2. The rotational position of the drill string is oriented so that the 
drill bit blade is inclined at an angle relative to the axis of the drill 
string toward the desired new direction of the borehole. 
3. The drill string is axially advanced without rotation to axially advance 
the drill bit a short distance or as far as possible when in difficult 
drilling conditions such that the blade moves the drill bit in the earth 
toward the new desired direction. 
4. Simultaneous rotation and axial advancement of the drill string may be 
resumed for a short distance. 
5. Sequentially repeating steps 1, 2, 3 and 4 until the direction of the 
borehole is in the new direction desired. Thereafter, the drill string is 
axially advanced and simultaneously rotated until it is again desired to 
change directions. To again change the direction of the borehole, the 
above sequence is repeated. 
Referring to FIG. 1, the above-described technique will be illustrated. The 
boring machine is generally indicated by the numeral 10 and is shown 
resting on the earth's surface 12 and in position for forming a borehole 
14 underneath an obstruction on the earth such as a roadway 16. 
Alternatively, the boring machine may be placed in a subsurface pit, as is 
well-known to those skilled in this technology. As shown in FIG. 1, by 
using the machine 10 the direction of the borehole can be changed as the 
borehole passes under roadway 16. This illustrates how the machine 10 can 
be utilized to form a borehole 14 under an obstruction without first 
digging a deep ditch in which to place a horizontal boring machine, and, 
also, without having to dig a deep ditch on the opposite side of the 
obstruction where the borehole is to be received. 
A typical drill bit 58 is illustrated in FIGS. 2 and 3. The drill bit 
includes a body portion 62 which has a rearward end portion 64 and a 
forward end portion 66. The rearward end portion 64 includes an internally 
threaded recess 68 which receives the external threads 70 at the drill 
string forward end 56. A blade 72 is affixed to body portion 62. The plane 
of blade 72 is inclined at an acute angle to the axis 74 of the bit. Axis 
74 is also the axis of the drill string 44. The blade 72 is preferably 
sharpened at its outer forward end 72A. When rotated, the blade cuts a 
circular pattern. 
To form a borehole 14 in the earth, the operator attaches the drill pipe 
and drill bit to the boring machine, begins rotation of the drill pipe and 
at the same time, causes the boring machine to linearly advance in the 
travel path of the frame towards the forward end thereof. The drill bit 
58, rotating and advancing, enters the earth and forms a borehole therein. 
As long as the bit 58 is rotated as it is advanced, the borehole generally 
follows the axis of the drill pipe; that is, the borehole continues to go 
in the direction in which it is started. When the borehole is started at 
the earth's surface to go under an obstruction such as a highway, the 
borehole must first extend downwardly beneath the roadway. When the 
borehole has reached the necessary depth, the operator must then change 
the direction of drilling so as to drill horizontally. This can be 
accomplished in the following way. When it is time to change direction, 
the operator stops drilling and rotates the drill string so that the drill 
bit blade 72 is oriented in a desired direction. In the situation 
illustrated in FIG. 1, the direction of the borehole is first changed so 
that instead of being inclined downwardly, it is horizontal. To effect 
such a change in direction, the operator will rotate the drill string 
until an indicator indicates that the blade 72 is facing downwardly as in 
FIG. 3, so as to cause the drill bit to be deflected upwardly when 
advanced without rotation. 
With rotation stopped and the blade properly oriented, the operator causes 
the drill machine to move forward without rotating the drill pipe. After 
forcing the bit as far as possible, the operator may begin rotation of the 
drill bit and continue to advance the drill string for a short distance. 
This facilitates the turning process in some soils. The procedure may be 
sequentially repeated until the direction of drilling has changed to that 
which is desired. After the borehole has been oriented in the desired 
direction, such as horizontal, the drilling can continue by simultaneous 
rotation and advancement of the drill string, adding new lengths of drill 
pipe as necessary until it is again desired to change direction of 
drilling. 
Other boring techniques are also commonly utilized. For example, in a 
percussive mole such as shown in U.S. Pat. No. 4,907,658 to Stangl et al. 
which is incorporated herein by reference, the forward or boring end 
generally includes an anvil which is hit by an internal striker powered by 
compressed air. Generally, the rearward end of the mole is connected to a 
whip hose which in turn is connected to a flexible air hose connected to a 
source of compressed air on the surface. The percussive mole can also be 
adapted to both push or pull pipes through the ground. 
As discussed above, the orientation of the angled blade of the drill bit 
determines the direction in which the boring tool will advance when it is 
thrusted through the ground without rotation. Thus, in order to 
appropriately steer boring tools such as those described above in a 
particular direction, the orientation of the angled blade must be known 
accurately. Addititonally, this angular orientation information must be 
effectively presented to the operating crew in order to permit efficient 
underground boring to be carried out. 
The prior art contains a number of techniques of determining this angular 
orientation information. U.S. Pat. No. 4,714,118 to Baker et al., for 
example, discloses a method and apparatus for monitoring the roll angle of 
a boring device. The arrangement includes a cylindrical support housing 
and an electrical resistor element mounted concentrically about an inner 
surface of the housing. The resistor element forms part of an overall 
potentiometer which also includes a brush or contact member extending 
radially from and mounted to a support arm. As the boring device rotates, 
the resistor element rotates relative to the brush, thereby increasing or 
decreasing the resistance of the potentiometer. This permits a 
determination of roll angle in accordance with the resistance. 
Another prior art roll sensor is disclosed in U.S. Pat. No. 4,672,753 to 
Kent. This type of sensor provides 360.degree. of roll angle indication 
but does not lend itself to miniaturization and is difficult to 
manufacture. 
Another means of indicating roll angle utilizes one or more mercury 
switches. Such a roll sensor typically can provide only one or two 
position indications within 360.degree. of rotation. Therefore, actual 
tool face positioning for a desired steering direction must be relative to 
one of these positions. This is accomplished by marking the drill string 
and rotating the desired amount therefrom. Since the actual tool face 
angle cannot be measured (unless the desired tool face angle happens to 
coincide with a position where the mercury switch is in the ON position), 
errors can occur due to the incorrect marking of the reference position, 
incorrect amount of rotation therefrom, or from windup in the drill 
string. Additionally, creating a narrow ON position with a mercury 
switched device creates manufacturing difficulties. Further, these 
switches exhibit inconsistent operation when subjected to vibration or 
when inclined more than 10-20 degrees from the horizontal as might occur 
when boring down a steep hill. 
Mechanical systems for determining the orientation, or roll angle, of the 
blade have physically marked the position of the blade on a first length 
of drill pipe. As each successive length of drill pipe is added, a 
corresponding mark is placed on the drill pipe. This process is 
cumbersome, time-consuming, and inaccurate. 
In order to appropriately steer a boring tool in a particular direction, an 
operator must be alerted that the drill is not on the proper course due to 
obstacles. While electromagnetic locating techniques may be used to 
effectively map out underground obstacles such as pipes and cables, other 
obstacles such as rocks and tree roots are often not discernible from a 
visual inspection of the surface features and cannot be located using 
electromagnetic techniques. An operator must nonetheless be made aware of 
such obstacles and counteract their effect on the drill bit in order that 
the borehole does not deviate off the intended path and the boring 
equipment is not damaged. Thus, it would be desirable to provide a device 
which alerts an operator that the drill is being deflected off course by 
metallic obstacles as well as rocks and/or roots. Such a tracking or 
location system would allow the operating crew to determine the location 
and depth of the drill bit relative to a desired path for the bore and to 
orient the bit to maintain this path whenever deviations occur. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an angle 
sensor which provides an accurate determination of the orientation of a 
member in a plane despite being subject to shock, vibration, severe 
environmental conditions, and steep pitch angles. 
It is a further object of the present invention to provide an angle sensor 
which is simple to manufacture, easy to operate, and accurate. 
In accordance with the present invention, an angle sensor for determining 
an orientation of a member in a plane is provided. The angle sensor 
includes an array of electrical elements which have a predetermined 
relationship to the orientation of the member in the plane. A fluid 
responsive to movement of the rotatable member varies an electrical 
characteristic of at least one of the electrical elements relative to the 
remaining electrical elements. A detecting circuit detects the electrical 
characteristics of the electrical elements. A determining means coupled to 
the detecting circuit determines the orientation of the member in the 
plane in accordance with the detected electrical characteristics of the 
electrical elements and the predetermined relationship of the electrical 
elements to the orientation of the member in the plane. 
Also in accordance with the present invention, a method of determining an 
orientation of a member in a plane is provided. First, electrical elements 
are arranged so as to have a predetermined relationship to the orientation 
of the member in the plane. An electrical characteristic of at least one 
of the electrical elements is varied relative to the remaining electrical 
elements in response to movement of the member. The electrical 
characteristics of the electrical elements are detected and the 
orientation of the member in the plane is determined in accordance with 
the detected electrical characteristics and the predetermined relationship 
of the electrical elements to the orientation of the member in the plane. 
Also in accordance with the present invention, a sensing device for 
determining an orientation of a member is provided. The sensing device 
comprises at least two angle sensors, each angle sensor comprising an 
array of electrical elements which have a predetermined relationship to 
the orientation of the member in a respective plane. A fluid responsive to 
movement of the rotatable member varies an electrical characteristic of at 
least one of the electrical elements relative to the remaining electrical 
elements in each array. A detecting circuit detects the electrical 
characteristics of the electrical elements. A determining means coupled to 
the detecting circuit determines the orientation of the member in 
accordance with the detected electrical characteristics of the electrical 
elements and the predetermined relationship of the electrical elements to 
the orientation of the member in the respective planes. 
Also in accordance with the present invention, a method of determining an 
orientation of a member is provided. First, at least two arrays of 
electrical elements are arranged so as to have a predetermined 
relationship to the orientation of the member in respective planes. An 
electrical characteristic of at least one of the electrical elements in 
each array is varied relative to the remaining electrical elements in each 
respective array in response to movement of the member. The electrical 
characteristics of the electrical elements in each array are detected and 
the orientation of the member is determined in accordance with the 
electrical characteristics and the predetermined relationship of the 
plurality of elements to the orientation of the member in the respective 
planes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although the present invention is discussed below in terms of an angle 
sensor for a steerable boring tool, the invention is more broadly 
concerned with sensors which determine the direction of gravitational 
acceleration. Such sensors may be referred to as gravitational encoders. 
An angle sensor in accordance with a preferred embodiment of the present 
invention is utilized to determine the direction of gravitational 
acceleration relative to an array of sensing elements in a transmitter 
mounted in or behind a steerable horizontal earth boring tool. In certain 
applications, sensing is only required relative to the roll axis of the 
boring tool and thus the angle sensor is typically referred to as a roll 
angle sensor. The roll axis is defined as either the rotational centerline 
of the boring tool or the centerline of the drill string. In typical 
applications, these centerlines coincide although some offset in order to 
accommodate the sensors and their associated circuitry is acceptable. The 
roll angle sensor is installed in the steerable boring tool in such a way 
that the sensing elements thereof are in a known, fixed relationship with 
the steering feature of the boring tool, typically the drill bit or tool 
face. Thus, the sensor can be used to determine tool face angle relative 
to the direction of gravity. 
As discussed above, it is important that an operator have the capability of 
quickly and accurately determining the orientation of the drill bit blade 
in order to appropriately steer the boring tool. The ability to quickly 
determine the orientation reduces the time needed to form the borehole. 
The ability to accurately determine the orientation also enables the 
operator to better control the path followed by the boring tool. The 
present invention uses a roll angle sensor 5, as shown in FIGS. 4 and 5 
for example, mounted in the drill bit and having a fixed and known 
relation to the blade. Roll angle sensor 5 provides a direct and accurate 
indication of the orientation of the blade to the operator through the use 
of associated transmitter and receiver elements described below. 
A roll angle sensor in accordance with a first embodiment of the present 
invention will be described with reference to FIGS. 4 and 5. The roll 
angle sensor of the first embodiment utilizes stored charge to determine a 
roll angle of a boring tool. Angle sensor 5 includes sixteen capacitor 
electrode plates 15 which are arranged in a circular array on a substrate 
10. Preferably, substrate 10 is a circular alumina ceramic disc which is 
approximately one inch in diameter, although it will be readily 
appreciated that the physical characteristics of the sensor will vary in 
accordance with the apparatus in which the invention is implemented. 
Capacitor electrode plates 15 preferably comprise a metal such as a 
silver/platinum alloy. The silver platinum alloy is screened on and then 
fused into the ceramic disc. The silver facilitates the soldering of 
electrical connections to the capacitor electrodes. Openings are formed 
through ceramic disc 10 such as by a laser in order to couple the 
electrodes to external circuitry. 
Cup-shaped member 20 is placed over the capacitor electrode plates and 
attached to ceramic disc 10 by crimping the outer peripheral edges of 
cup-shaped member 20 over the outer edge of ceramic disc 10 as indicated 
at 16 and sealing the resultant structure with a silicone greased 
elastomeric O-ring 22. Cup-shaped member 20 is designed such that when it 
is crimped over the outer edge of ceramic disc 10, it makes electrical 
contact with a conductive ring (not shown) positioned on the outer planar 
surface of ceramic disc 10. This electrical contact electrically grounds 
cup-shaped member 20. Cup-shaped member 20 is, in effect, a grounded 
electrode common to all the capacitive electrodes 15. Cup-shaped member 20 
is preferably formed of an inert material such as stainless steel or 
anodized aluminum. The separation 21 between the inner planar surface of 
cup-shaped member 20 and the inner planar surface of ceramic disc 10 is 
approximately 50 to 60 thousandths of an inch (0.050 to 0.060"). 
Cup-shaped member 20 and ceramic disc 10 define an interior space 24. As 
illustrated in FIG. 4 and discussed in greater detail below, sensor 5 is 
coupled via male connectors JP3 and JP4 to female connectors JP3' and JP4' 
of transmitter 600. Transmitter 600 includes sensor control and data 
acquisition circuitry as further detailed in FIG. 7. 
Capacitor plates 15 are covered with a one to two mil thick coating of a 
high dielectric material, preferably having a dielectric constant of 
approximately 50, although other values may be used. The dielectric 
coating should not have any holes or openings therethrough. A preferred 
technique for forming the dielectric coating comprises screening a first 
high dielectric constant layer of barium titanate onto ceramic disc 10 and 
fusing the layer at high temperature. A second layer of barium titanate is 
then screened on and fused. Statistically, any holes or openings formed in 
the first and second layers will not coincide. Next, a relatively low 
melting point glass having fewer holes, but a relatively low dielectric 
constant is placed over the second fused layer. It will be apparent to 
those skilled in the art that a dielectric coating having no holes may be 
provided by utilizing various thin dielectric coatings such as polymer 
coatings with a relatively high dielectric constant. 
A conductive fluid 27 is placed within the interior space or cavity 24 such 
that the fluid covers approximately one-half of the plurality of 
capacitive electrodes. The actual number of electrodes covered is not 
critical to the present invention. Any number of capacitive electrodes 
from one through one less than the total number of electrodes may be 
covered. An amount of fluid covering approximately one-half of the 
capacitive electrodes is a manufacturing convenience. An opening 26 is 
provided as a fill hole and is sealed by screw 25 and elastomeric O-ring 
28. The conductive fluid 27 is preferably a liquid having moderate to low 
resistivity such as salt water, ethyl glycol, mercury, or cupric nitrate. 
Methyl alcohol has also been found to work well and is, in fact, the 
preferred liquid. Assuming the liquid has a moderate to low resistivity, 
it effectively forms an electrode common to the individual capacitor 
electrodes. Since the bulk resistance of the liquid is preferably low, the 
liquid provides a relatively low resistance path to cup-shaped member 20. 
When angle sensor 5 is mounted in the drill bit and set on edge, ceramic 
disc 10 is generally vertical and thus conductive fluid 27 runs to the 
bottom of cavity 24 under the influence of gravity and lies between a 
plurality of the capacitor electrode plates 15 and the inner planar 
surface of cup-shaped member 20. 
Fluid dynamics, the chemical reaction between the fluid and the various 
sensor components and thermal effects must be considered when implementing 
the invention. The viscosity of the fluid determines the damping effect or 
responsiveness of the angle sensor. For example, methyl alcohol has a very 
low wetting coefficient and very low viscosity. Thus, it provides for a 
very fast responding angle sensor. Alternatively, for example, ethyl 
glycol could be used for a slower responding, damped angle sensor since 
the viscosity of ethyl glycol is higher than that of methyl alcohol. The 
conductive fluid must be chemically compatible with the materials 
comprising cup-shaped member 20, elastomeric O-ring 28, and the dielectric 
coating. Since the angle sensor may be subjected to adverse environmental 
conditions, as in a boring tool, a conductive liquid must not freeze or 
boil over the intended operating and storage temperature range. Methyl 
alcohol and ethyl glycol have excellent freezing temperature 
characteristics and the above-described sensor assembly is preferably 
constructed to withstand a high internal vapor pressure, thus raising the 
effective boiling points of liquids contained therein. 
FIG. 6 is a schematic diagram of the stored charge angle sensor 5. As 
illustrated, capacitor plates 15 and cup-shaped member 20 constitute 
sixteen individual capacitors. Signal lines S0-S15 are respectively 
coupled to each of the capacitor electrodes 15 through connectors JP3 and 
JP4. Similarly, cup-shaped member 20 is coupled to the common (ground) of 
the main circuit board through connector JP4. The signals generated at the 
capacitor plates 15 are transmitted via signal lines S0-S15 to a beacon 
transmitter as described below. 
The capacitance of a parallel plate capacitor is given by the equation 
##EQU1## 
where .kappa. is the dielectric constant of a dielectric disposed between 
the plates, .epsilon..sub.0 is the permittivity of free space, A is the 
area of the capacitor plates and d is the separation of the plates. 
Without the conductive fluid disposed between the plates thereof, each 
capacitor and the wiring associated therewith has a capacitance of about 
10 pF. When the conductive liquid is opposite a given capacitive 
electrode, the capacitance is typically in a range of about 60 pF to about 
100 pF. The capacitance values given here are illustrative of a preferred 
embodiment but the invention is not limited in this respect. It is 
important that the difference in capacitance give rise to a signal which 
accurately distinguishes capacitors having the conductive liquid disposed 
between the plates thereof and capacitors not having the conductive liquid 
disposed between the plates thereof. 
FIG. 7 is a schematic diagram of a beacon transmitter 600 which may be 
utilized with the present invention. Beacon transmitter 600 is merely 
illustrative of a transmitter for use with the present invention. Details 
of transmitter 600 are described in commonly assigned copending U.S. 
application Ser. No. 07/539,851 entitled "An Improved System For Locating 
Concealed Underground Objects", expressly incorporated herein by reference 
thereto, and such details will only be outlined below. The beacon 
transmitter of FIG. 7 performs two functions. First, it broadcasts a 
29,430 Hz electromagnetic field signal to provide accurate tool location. 
Second, it determines the roll angle of the boring tool relative to 
gravity and broadcasts the angle via a digital communication system to a 
compatible receiver. The operation of the beacon transmitter is under the 
control of a micro-controller such as a Motorola MC68HC705. The 
micro-controller further controls the operation of the angle sensor and 
generates the digital signals for transmitting the angle information to 
the receiver. It will be apparent to those skilled in the art that 
different transmitters may be designed in order to perform these 
functions. The beacon transmitter described below is the preferred 
embodiment for transmitting the angle information to a receiver, but the 
invention is not limited to a particular transmitter for performing this 
function. 
Referring to FIG. 7, oscillation circuitry 615 including a crystal is 
coupled to inputs OSC1 and OSC2 of micro-controller U1 for timing and 
carrier generation purposes. The electromagnetic field signal and the 
angle information is output via PC2 and PC3 to an antenna through output 
section 620. Angle sensor 5 is interfaced to beacon transmitter 600 at 
JP3' and JP4'. Signal lines S0-S7 are respectively coupled to 
micro-controller input/output pins - via connector JP3'. Signal 
lines S8 and S9 are respectively coupled to micro-controller input/output 
pins PB0 and PB1 via connector JP3'. Signals lines S10-S15 are 
respectively coupled to micro-controller input/output bits PB2-PB7 
respectively. 
Power supply 640 supplies the power and the necessary operating voltages 
for the operation of the beacon transmitter 600. Input section 650 
comprises a plurality of DIP switches which may be used by the operator to 
set selected inputs of micro-controller U1. The input DIP switches may be 
used, for example, to configure the transmitter to only generate the 
electromagnetic field signal, or to generate the electromagnetic field 
signal and transmit angle information. The DIP switches may also be used 
to configure a duty cycle such that the transmitter transmits for a given 
period and then "sleeps" or is turned off for a given period in order to 
conserve battery power. For example, the unit may be programmed to 
transmit for nine hours and then sleep for fifteen hours. Micro-controller 
U1 may also test the operation of angle sensor 5 in accordance with a 
particular setting of the DIP switches. Such procedures may for example 
include polling the sensor elements to determine whether they are active. 
The microcontroller clock is 2 MHz and the carrier frequency is 29.43 kHz. 
The digital communication system bit rate [BAUD] is 75 Hz. As shown in the 
flowchart of FIG. 8A, micro-controller U1 samples the angle sensor at one 
second intervals. That is to say, after initialization procedures (551) 
including resetting of the one second interval timer (552), one or more 
transmitter carrier cycles are generated (553) until a one second interval 
elapses (554). Thereafter, the "Read Angle Sensor" subroutine (555) 
further detailed in FIGS. 8B, 8C, and 8D is run and subsequently a 
determination is made as to whether the angle has changed (556) since the 
last transmission. If it has, micro-controller U1 transmits a new angle 
(558). If the angle has not changed since the last transmission, the 
transmission is skipped. However, if the angle has not changed for ten 
samples (557), the angle is nonetheless retransmitted (558). The carrier 
signal may be interrupted for internal processing purposes. 
A first implementation of angle sensing as illustrated in FIG. 8B is 
governed by the physics of discharging a charged capacitor. The voltage of 
a discharging capacitor is described by the equation V=V.sub.0 e.sup.-t/RC 
where V.sub.0 is the initial voltage, R is the resistance through which 
the capacitance C is discharged, and t is time. The quantity RC is known 
as the time constant of the circuit. In this first implementation, 
microprocessor U1 discharges each of the sixteen capacitors (560). A 
selected capacitor (559) is charged (561) by setting the corresponding 
output bit high in Port A (-) or Port B (PB0-PB7) and programming 
all the bits of the ports as outputs. The selected capacitor is charged to 
approximately five volts (5 V) while the other capacitors are maintained 
in a discharged condition. The selected bit in the port is then programmed 
as an input (562), thereby connecting the capacitor as a high impedance 
input to the sensor circuitry. Bit PC0 is held low so the common 
connection of resistor pack RN1 is maintained close to ground potential. 
After a delay (563) as shown in FIG. 8B (about ten microseconds), 
micro-controller U1 reads (564) the selected bit. If the selected 
capacitor does not have the conductive liquid disposed between the plates 
thereof, the RC time constant is determined principally by the stray 
capacitance of the circuitry and the resistance associated with the 
particular electrode in question which is 270K. This yields an RC time 
constant of (270K.times.10 pF) or 2.7 microseconds. The input impedance of 
the Port A and Port B lines connected to the capacitor plates is extremely 
high since it is a CMOS device. If the fluid is present, the capacitive 
electrodes will have a capacitance of about 60 to about 100 pF yielding a 
time constant of at least (270K.times.60 pF) or 16.2 microseconds. 
Micro-computer U1, in this implementation, samples the input associated 
with a capacitor electrode at approximately 10 microseconds to determine 
the charge on the electrode. In the preferred embodiment, the transition 
between high and low, as sensed by microprocessor U1, is approximately 2.5 
volts. If the selected capacitor has the conductive liquid disposed 
between the plates thereof, the time constant is at least 16.2 
microseconds and the selected bit will still be high (greater than 2.5 V) 
after 10 microseconds. If however, the fluid is not present, the selected 
bit will be low (less than 2.5 V) after 10 microseconds and an appropriate 
flag bit is cleared (567). Microprocessor U1 upon examining the capacitor 
plate under test can thus determine whether a given capacitor has 
conductive fluid 27 disposed between the plates thereof and set an 
appropriate flag bit high (565) and increment the count of active 
capacitors when the liquid is present. By sequentially examining all 
sixteen capacitors (568), microprocessor U1 can determine the location of 
conductive fluid 27 by examining each flag bit for a high or a low and 
thus determine the direction of gravitational acceleration relative to the 
array of capacitors. "Sequentially" as used herein refers to any 
predetermined order. Since the array of capacitors is in a fixed 
relationship relative to the drill blade, the orientation of the drill 
blade may be determined (569). 
For example, in the event that the liquid level is only sufficient to be 
disposed between the plates of one capacitor and assuming that capacitor 
plate 15a of FIG. 5 is keyed such that when conductive liquid 27 is 
disposed between plate 15a and grounded cup-shaped member 20, the blade of 
the drill is angled upwardly or at 0.degree. (i.e. facing downwardly as in 
FIG. 3). In practice, if at some point in time, micro-controller U1 
ascertains that conductive liquid 27 is disposed between plate 15a and 
cup-shaped member 20, the blade may actually be oriented in a range of 
0.degree..+-.11.25.degree. and thus the present embodiment has a precision 
of 22.5.degree. or 360/16. In general, the precision may be found in 
accordance with the relation 
##EQU2## 
where n is the number of plates. Thus, the more plates utilized, the more 
precisely the roll angle may be determined. As discussed below, beacon 
transmitter 600 will transmit one of a possible sixteen values of the roll 
angle in accordance with the location of the fluid. For example, if 
micro-controller U1 determines that conductive fluid 27 is disposed 
between plate 15h and the metal cup-shaped member 20, beacon transmitter 
600 will transmit a signal indicating the blade had rotated 157.5.degree. 
in a clockwise direction from the position defined as 0.degree.. Again, 
since the sensor has a precision of 22.5.degree., the actual blade angle 
may be within a range from 146.25.degree. to 168.75.degree.. 
In a preferred embodiment where the liquid level is disposed between the 
plates of approximately half of the capacitors, vibrational forces and the 
like may cause the fluid to break up so that one or two of the "bottom" 
capacitors or segments do not indicate the presence of the liquid. 
Additionally, other factors such as a disconnected wire may cause a 
particular segment to fail to register. 
FIGS. 8C and 8D are illustrative of a run length procedure for examining 
various patterns of flag bits such that accurate readings through an 
averaging technique may be obtained notwithstanding the aforementioned 
circumstances. For example, in FIG. 8C the longest run of consecutive 
active segments with flag bits set high may be determined (570), and if 
the longest run is greater than half the total of high flags (571), such a 
determination may be used to determine the true angle code (576). Under 
some circumstances, however, (572 through 574), the flag bit patterns 
would be insufficient to determine the true angle code, and an error 
condition would be indicated (557) thus requiring a new test. Where, 
however, two longest runs exist and do not appear opposite to each other, 
such as where three consecutive high bit flags are followed by one low and 
three more high bits, the midpoint may be determined (575), and the true 
angle code determined (576) after adding in the zero offset constant or 
the aforementioned "fixed relationship relative to the drill blade." 
As illustrated in FIG. 8D, an exemplary procedure for the determination of 
the longest run may be obtained by first zeroing or resetting counters for 
accumulating indications of the maximum and current run lengths (578 and 
579). Thereafter, the initially selected segment is determined to be 
either active or inactive (580), and if active and the current run length 
is zero (581), the position of said segment is recorded (582). If the 
current run length is not equal to zero, the current run length is 
incremented (583). Thereafter, the current run length total is compared 
with the maximum permissible length (584), and if greater, an error is 
indicated (577). However, where the error test is negative, the exemplary 
process is stepped (585) so that the next segment is tested (580). Where 
the next tested segment is inactive, it is then known that the current run 
of active segments is finished or that all segments will be inactive. In 
the latter event, at step 586 the routine would branch to step 591 to 
determine whether all segments will be tested inactive or, alternatively, 
will branch to step 587. 
At step 587, a determination is made as to whether the current run length 
is greater than, equal to, or less than the maximum length. If the current 
length equals the maximum length, it is known that a new run of active 
segments may be instituted and the new position is recorded (589). If the 
current length is greater than the currently recorded maximum length, the 
maximum length is set equal to the current length (588), and the maximum 
position is also set to the current position (590). 
In the event that the current length is less than the currently recorded 
maximum length, the exemplary procedure would branch to step 591 for a 
determination of whether all of the segments have been tested. Ultimately, 
a determination is made as to the longest run of active segments as well 
as a determination of the second longest run, if it exists. 
The above noted procedures for obtaining accurate angle codes are merely 
exemplary and not exhaustive of the procedures which may be used for such 
determinations under various operating conditions. Clearly, other similar 
or equivalent procedures will occur to those skilled in the art for 
implementing the above-described embodiment. Similar observations may be 
made with regard to the implementation of the additional embodiments which 
follow. 
A second implementation of this first embodiment utilizes the same 
arrangement of components as described above. The difference resides in 
the firmware which controls micro-controller U1. The second implementation 
is governed by the physics of charging a discharged capacitor. 
Micro-controller U1 maintains all sixteen capacitors at approximately 5 
volts by programming all bits of Ports A and B (-, PB0-PB7) as 
output bits and setting all the bits high. Bit 0 of Port C is set high so 
the common connection of resistor pack RN1 is also at approximately 5 
volts. To examine an individual capacitor, micro-controller U1 sets the 
corresponding bit in Port A or Port B low, thereby discharging the 
selected capacitor. As in the first implementation, the selected pin in 
the port is then programmed as an input, causing the selected capacitor to 
be connected as a high input impedance to the sensor circuitry. 
Ten microseconds after programming the selected bit as an input, 
micro-controller U1 reads the selected bit. If the capacitor does not have 
conductive fluid 27 interposed between the plates thereof, the capacitor 
will have charged up to nearly five volts after ten microseconds and the 
selected bit will be high (i.e. greater than 2.5 V). If the capacitor has 
conductive fluid 27 disposed between the plates thereof, the bit will 
still be low (i.e. less than 2.5 V) after ten microseconds. 
This second implementation of the first embodiment may have particular 
utility when certain electrolytes are utilized as conductive liquids in 
combination with certain metals as the capacitor plates. In practice, 
thin, reliable insulating films are difficult to form on the capacitor 
plates. It is well-known that certain combinations of capacitor plates and 
electrolytes are "self-healing." That is, the insulating film on the 
plates is a metal oxide, and if it is punctured, an electrochemical 
reaction between the electrolytes and the metal of the capacitor plates 
forms new oxide and repairs the damage to the film. Since the second 
implementation maintains a potential between the capacitor plates and the 
grounded cup, it can maintain an insulating oxide film on the capacitor 
plates. 
As noted above, the angle is transmitted from the beacon using a 
communication system described in the incorporated copending application 
entitled "An Improved System For Locating Concealed Underground Objects." 
The communication system is a digital system and provides an accurate and 
efficient means for the transmitter to communicate with the receiver. The 
communication medium is the electromagnetic field produced by the 
transmitter. Data and associated control bits are encoded by amplitude 
modulating the carrier frequency. The communication system uses the 
standard UART (Universal Asynchronous Receiver Transmitter) 
non-return-to-zero (NRZ) format. 
The roll angle is transmitted from the beacon transmitter of FIG. 7 at 
predetermined periodic intervals. When the above-ground receiver has been 
set-up in a proper operation mode to receive information from the beacon 
transmitter, the roll angle information is received and displayed on an 
angle display of the receiver. 
As noted above, the roll angle assumes one of sixteen values ranging from 
0.degree. to 360.degree. in increments of 22.5.degree.. It will be 
appreciated that the number of possible values transmitted from the beacon 
transmitter is dependent on the precision of the angle sensor incorporated 
therein. A more precise angle sensor will have a greater number of 
possible angle values. 
FIG. 9 illustrates an angle display of the receiver in accordance with one 
embodiment of the invention. Angle display 800 includes eight LCD segments 
804-811. Each of the LCD segments 804-811 includes a pointer 815. The 
pointers are fixed at 0.degree., 45.degree., 90.degree., etc. Each of the 
LCD segments 804-811 represents an interval of 45.degree.. Thus, display 
segment 804 indicates angles between -22.5.degree. and +22.5.degree.; 
display segment 805 between +22.5.degree. and 67.5.degree., etc. Roll 
angles which correspond to one of the eight pointer angles are indicated 
by lighting the corresponding segment. Thus, to indicate an angle of 
45.degree., segment 805 would be lit. The remaining eight intermediate 
angles are indicated by displaying the two adjacent principal angle 
segments. Thus, to indicate a roll angle of 22.5.degree., LCD segments 804 
and 805 are illuminated. 
It will be recognized that the above-described display is only one example 
of how the roll angle may be displayed and the invention is not limited in 
this respect. For example, a LCD digital readout of the roll angle may be 
provided. Additionally, a sixteen segment LCD may be utilized. 
Preferably, an audible or visual indication such as a beep or an indicator 
light is provided whenever the receiver receives an angle signal from the 
beacon transmitter. The indication enables the operator to determine that 
the receiver is in fact continuing to receive angle data from the beacon. 
The construction of a drilling assembly including the angle sensor of the 
present invention will be explained with reference to FIGS. 4 and 10-13. 
As shown in these Figures, connectors JP3 and JP4 of angle sensor 5 are 
coupled to connector JP3' and JP4' positioned at one end of PCB 601 of 
transmitter 600. PCB 601 and angle sensor 100 coupled thereto are 
positioned inside a two piece transmitter housing 611. The pieces of the 
transmitter housing are coupled to each other by the use of mating threads 
616. 
FIGS. 12 and 13 illustrate a drill bit particularly adapted for percussive 
boring. Drill bit 700 includes a body portion 705 which has a forward end 
portion 706 and a rearward end portion 707. Reference should be made to 
the above-identified U.S. Pat. No. 4,907,658 for the details of 
operatively coupling a drill bit to a mole body and an anvil to permit 
directional percussive boring. Bit 700 includes an angled cutting face 
720. Transmitter housing 611 containing the PCB 601 and angle sensor 100 
is positioned within internal opening 701 bit 700 before installation onto 
a suitably threaded percussion mole such as in U.S. Pat. No. 4,907,658. 
The transmitter housing must be positioned within internal opening 701 
such that the electrical components or sensing elements have a 
predetermined relationship to the angled cutting face 720. PCB 601 
includes two edge mouldings (not shown) about one inch long and centered 
on the edges of PCB 601. The mouldings engage slots in the transmitter 
housing 611 to insure that the angle sensor is fixed so as to have a 
predetermined relationship with outer slotted notch 713 in the housing 
611. Slotted notch 713 engages a screw 725 which is inserted via opening 
725 of steel bit 700 to rotationally fix all the tool components with 
respect to the steering feature of the boring tool. Other techniques may 
be utilized to provide this fixed relationship and the invention is not 
limited to the technique discussed above. 
As illustrated in FIG. 20, a second sensor 100a may be positioned in the 
drill bit to provide an indication of the pitch of the drill blade 
relative to the horizontal. Pitch of the drill bit can be thought of as 
rotation of the bit within a vertical plane. The rotation is an arc 
(partial revolution) of radius which must not violate the allowable bend 
radius of the drill string. Otherwise, damage to elements of the drill 
string may occur. This information is particularly useful when the drill 
is being deflected upward or downward by obstacles such as rocks or tree 
roots. Knowledge of pitch angle provides additional information on the 
orientation of the drill bit, i.e., whether it is level or inclined upward 
or downward. This knowledge, for example, gives advance warning that the 
bit has been deflected off course or that the bit is in fact, reacting to 
an up or down steering correction before the change can actually be 
detected by monitoring only the depth of the head. 
The angle sensors may be arranged such that an angle of 90.degree. is 
established between the axis of the sensors. The pitch angle of the bit or 
head can best be evaluated when the pitch sensor is approximately 
vertically oriented (such as within .+-.45.degree.) when the rotational 
centerline of the drill bit (and transmitter 600) is approximately 
horizontal. It will be apparent that other means of implementation, are 
possible. It will be appreciated that it may not be necessary to provide a 
full 360.degree. range for the pitch angle sensor. For example, a range of 
180.degree. may be implemented to define a range between up and down. In 
most cases, a range of 90.degree. (.+-.45.degree. from level) will be 
sufficient. The pitch angle may be displayed to an operator in a manner 
similar to that of the roll angle. 
As shown in FIG. 14, in practice, an operator 900 utilizes a receiver 905 
to receive signals from a transmitter 910 positioned in an underground 
boring tool 915. As illustrated, the field lines 930 produced by the coils 
of the transmitter are bipolar and axial. Receiver 905 tracks the progress 
of transmitter 910 as it moves under ground or underwater. The receiver 
has several operating modes, including an active tracking mode in which it 
is tuned to the frequency of the subsurface transmitter. In this mode, it 
can locate the boring tool, measure its depth, and display the roll angle 
or pitch angle. It may also be possible to transmit the angle information 
to a receiver remote from the location of the drill bit through the use of 
an appropriate transmitter/receiver system. A wire-based system inside the 
drill string could also be used to obtain angle information from the 
sensor. Although described in conjunction with a steerable underground 
boring device, it can be appreciated that the angle sensor embodiments 
could have utility in any number of applications. 
The above-described angle sensor is compact, simple, and low in cost. This 
makes it especially well-suited for applications such as sensing tool face 
angle within a tracking transmitter in the steerable head of a horizontal 
earth-boring machine where very little space is available. Further, the 
stored charge angle sensor is very fast, i.e. readouts may be obtained in 
ten microseconds. While the above description discusses a sequential 
reading of the capacitors, the reading may be of two or more or all of the 
capacitors in parallel. 
A second embodiment of the present invention utilizes a high dielectric 
constant fluid having a high resistance instead of the dielectric coating 
on the inner planar surface of the ceramic disc. The construction and 
associated transmitter of the angle sensor of the second embodiment is the 
same as that of the first embodiment. However, the separation between the 
inner planar surface of the ceramic disc and the inner planar surface of 
the cup-shaped member is preferably ten thousandths of an inch (0.010"). 
In the second embodiment, a permanent dielectric coating is not formed on 
the capacitive electrode plates, and a high dielectric constant, high 
resistance fluid is used in place of the conductive fluid. An example of 
such a fluid is a 3M.RTM. experimental product L-10065. In this case, the 
fluid would form the dielectric material located between the capacitive 
electrode plates and the cup-shaped member used as the common electrode. 
The operation of this embodiment is the same as for the first embodiment 
and will not be repeated here. 
FIGS. 15 and 16 illustrate a third embodiment of the present invention. The 
sensor 200 includes sixteen identical electrodes 215 which are arranged in 
a circular array on PCB 210. Preferably PCB 210 is a circular disc which 
is approximately one inch in diameter, although it will be readily 
appreciated that the physical characteristics of the sensor will vary in 
accordance with the apparatus in which the invention is implemented. A 
disc 220 is spaced from PCB 210 and secured thereto by crimping a crimp 
seal 223 around the outer peripheral edges of disc 220 and the outer 
peripheral edges of PCB 210 as shown in FIG. 16. The resultant structure 
is sealed with a silicone greased elastomeric O-ring 170. The separation 
165 between the inner planar surface of disc 220 and the inner planar 
surface of PCB 210 is approximately ten thousandths of an inch (0.010"). 
Disc 220 and PCB 210 define an interior space 240. Disc 220 includes an 
output electrode 221 positioned on an inner planar surface thereof as 
shown in FIG. 15(B). A non-conductive fluid 227 having a dielectric 
constant higher than air (such as K=10) is placed within internal space 
240 such that the fluid covers approximately one-half of the number of 
electrodes. Again, as in the first embodiment, the number of electrodes 
covered is not critical. When the angle sensor is mounted in the drill bit 
and set on edge, non-conductive liquid 227 flows to the bottom of cavity 
240 under the influence of gravity and lies between electrodes 215 and 
output electrode 221. 
FIG. 17 illustrates a beacon transmitter 940 for use with angle sensor 200. 
Oscillation circuitry 942 including a crystal is coupled to inputs OSC1 
and OCS2 of micro-controller U5 for timing and carrier generation 
purposes. The electromagnetic field signal and the angle information is 
output via PC2 and PC3 to an antenna through output section 945. Angle 
sensor 200 is interfaced to beacon transmitter 942 at JP13' and JP14'. 
Signal lines S0-S7 are respectively coupled to micro-controller 
input/output bits - via connector JP3'. Signal lines S8 and S9 are 
respectively coupled to micro-controller input/output bits PB0 and PB1 via 
connector JP3'. Signal lines S10-S15 are respectively coupled to 
micro-controller input/output bits PB2-PB7 respectively. Power supply 950 
supplies the power and the necessary operating voltages for the operation 
of the beacon transmitter 940. Micro-controller U5 controls the ON/OFF 
switching of transistor Q1 via output PC1 so as to control the application 
of power from power supply 950 to angle sensor 200. Input section 952 
comprises a plurality of DIP switches which may be used to set selected 
inputs of micro-controller U15. The input DIP switches may be used, for 
example, to configure the transmitter to only generate the magnetic field 
signal, or to generate the magnetic field signal and transmit angle 
information or to perform the same functions as discussed with respect to 
transmitter 600. 
FIG. 18 is a schematic diagram of angle sensor 200. As illustrated, 
electrodes 215 and output electrode 221 constitute sixteen capacitors. 
Micro-processor U5 of FIG. 18 sequentially pulses each of the sixteen 
capacitors one at a time by sequentially setting one bit high in port A 
(-) and port B (PB0-PB7). Output electrode 221 is coupled to the 
inverting input U1-2 of operational amplifier (op-amp) U1-A through a 
resistor R1 which functions to suppress high frequency transients. The 
inverting input U1-2 of amplifier U1-A is at a virtual ground. Amplifier 
output U1-1 thus goes negative to maintain U1-2 at a virtual ground 
potential. 
If, for example, non-conductive fluid 227 in internal space 240 has a 
dielectric constant of ten, the amount of charge transferred when 
non-conductive fluid 227 is disposed between the capacitor plates will be 
ten times as great as when there is only air between the plates. The 
output voltage pulse at pin U1-1 will therefore also be ten times as 
great. Op-amp U1-B is coupled as a Schmitt trigger, and the remaining 
circuit components are selected so that output U1-7 goes negative only 
when the liquid is disposed between the capacitor plates. This output 
signal goes to transmitter 940 via connector JP14 where it is inverted by 
transistor Q2 and is supplied to the inputs TCAP and PD7 of 
micro-controller U5. Micro-controller U5 may be programmed so that a 
transition of input TCAP generates an interrupt signal, and input PD7 can 
be examined by micro-controller U5 to determine the high/low state of the 
sensor output at any time. As will be appreciated by the artisan, since 
the positions of drive plates 220 have a predetermined relationship to the 
angular orientation of the boring tool, the determination of the 
electrodes immersed in non-conductive fluid 227 by a high input signal to 
PD7 permits the roll angle to be determined in a manner similar to that 
discussed above with reference to the first embodiment. 
The capacitances of each of the sixteen individual capacitors of capacitive 
angle sensor 200 are very small because of the small physical dimensions 
of the angle sensor. Thus, the stray circuit capacitance to ground may be 
larger. Furthermore, the combined capacitances of the fifteen "OFF" 
capacitors, which are being held near ground, exceed the capacitance of 
the single "ON" capacitor. Placing the input node of amplifier U1-A at 
virtual ground prevents these additional capacitances from shunting the 
signal and enables construction of a practical circuit. 
A fourth embodiment of the present invention is an angle sensor utilizing a 
resistive liquid to determine a roll angle of a boring tool. The structure 
and circuitry associated with the resistive liquid angle sensor is the 
same as that for the angle sensor 200 of the third embodiment. The 
resistive angle sensor includes a circular PCB with a diameter of 
approximately one inch. Sixteen electrodes are arranged in a circular 
array on the PCB. The positions of the electrodes have a predetermined 
relationship to the angular orientation of the boring tool. A disc having 
an annular output plate or electrode positioned on an inner planar surface 
thereof is secured to the PCB using a crimp seal. The separation between 
the inner planar surface of the PCB and the inner planar surface of the 
disc is approximately ten thousandths of an inch. A resistive liquid 
having a resistivity substantially lower than the resistivity of air is 
placed within the internal space defined by the PCB and the disc. When the 
sensor is mounted in the drill bit and set on edge, the resistive fluid 
flows to the bottom of the cavity under the influence of gravity and lies 
between the electrodes and the output electrode. 
The operation of the resistive angle sensor is the same as for the 
capacitive angle sensor of FIG. 18. A micro-controller sequentially pulses 
each of the sixteen sets of electrical elements by sequentially setting 
one bit at a time high in port A or port B. The common output electrode is 
coupled to the inverting input U1-2 of operational amplifier U1-A through 
resistor R1 which functions to suppress high frequency transients. The 
inverting input U1-2 of amplifier U1-A is a virtual ground, so when the 
electrodes are immersed in the resistive fluid, the positive pulse causes 
current to flow out of the amplifier input, and the amplifier output U1-1 
thus goes negative to maintain U1-2 at ground potential. 
Operational amplifier U1-B is coupled as a Schmitt trigger, and the circuit 
components are selected so that output U1-7 goes negative only when the 
electrodes are immersed in the fluid. This output signal goes to the 
transmitter circuit board where it is inverted by transistor Q2 and goes 
to the inputs TCAP and PD7 of microprocessor U1. Microprocessor U1 may be 
programmed so that a transition of input TCAP generates an interrupt 
signal, and input PD7 can be examined by the microprocessor to determine 
the state of the sensor output at any time. Since the positions of 
electrodes have a predetermined relationship to the angular orientation of 
the boring tool, the determination of the electrodes immersed in the 
resistive fluid permits the roll angle to be determined as discussed above 
with reference to the first embodiment. 
A fifth embodiment of the present invention is an angle sensor utilizing a 
conductive fluid to determine a roll angle of a boring tool. The structure 
and processing associated with the conductive angle sensor is similar to 
that for the capacitive and resistive angle sensors and will not be 
illustrated here. The conductive angle sensor includes a circular ceramic 
disc with a diameter of approximately one inch. Sixteen metal electrodes 
are arranged in a circular array on the ceramic disc. The position of the 
electrodes have a predetermined relationship to the angular orientation of 
the boring tool. A coating of Teflon covers the inner surface of the 
ceramic disc, with a hole in the Teflon coating formed over each of the 
electrodes. A metal cup-shaped member having a common annular electrode 
positioned on an inner planar surface thereof is crimped around the outer 
planar surface of ceramic disc. The separation between the electrodes on 
the ceramic disc and the common plate is approximately ten thousandths of 
an inch. The inner planar surface of cup-shaped member is also covered 
with a coating of Teflon and further includes sixteen holes formed 
opposite the respective electrodes on the inner surface of the ceramic 
disc. A conductive liquid such as mercury is placed within the internal 
space defined by the ceramic disc and the cup-shaped member. When the 
sensor is mounted in the drill bit and set on edge, the conductive liquid 
flows to the bottom of the internal space under the influence of gravity 
and forms an electrical contact between the electrodes on the ceramic disc 
and the common plate. The Teflon coating prevents bridging of the 
electrodes by droplets of the liquid. 
The determination of which electrodes have the conductive liquid between it 
and the common plate is the same as for the resistive capacitive angle 
sensors and will not be repeated here. 
Other fluids may be disposed in the cavity and the invention is not limited 
to the embodiments described above. For example, a dielectric coating may 
be placed on the inner planar surfaces of the disc and PCB and a resistive 
or conductive fluid may be used. 
With reference to FIG. 19, a sixth embodiment of a roll sensor of the 
present invention includes printed circuit board (PCB) 510 having sixteen 
light emitting diode (LED) chips 512. LED chips 512 are surface mounted in 
a circular arrangement on PCB 510 as shown in FIG. 19(A). FIG. 19(B) 
illustrates a collimating lens 520 which is positioned opposite LED chips 
512 with a predetermined gap of approximately 50 to 60 thousandths of an 
inch therebetween. Collimating lens 520 is arranged to collect and focus 
light from LED chips 512. PCB 510 and collimating lens 520 are arranged as 
discussed above with respect to the other angle sensors. A drop of an 
opaque liquid which is large enough to block the light emitted by at least 
one of LED chips 512 at all times is placed within the angle sensor. 
Preferably, the drop of opaque liquid is large enough to block the light 
emitted by a single LED chip 15. The opaque liquid may be ethylene glycol 
mixed with an opaque organic dye, although the invention is not limited in 
this respect and other opaque liquids may be used. A photodetector (not 
shown) is mounted at focal point F of collimating lens 520. 
A microprocessor may sequentially pulse each of the LED chips and receive 
information output from the photodetector. Output ports of the 
microprocessor may output the position information. 
LED chips 512 are sequentially pulsed until no light is detected by the 
photodetector. As discussed above the liquid drop always blocks the light 
emitted by one of the LED chips 15. By sequentially switching on each of 
the LED chips, the chip or chips whose light is blocked by the liquid drop 
may be determined. Since the positions of LED chips 512 have a 
predetermined relationship to the angular orientation of the boring tool, 
the determination of the LED chips whose light is blocked by the liquid 
drop permits the roll angle to be determined. 
After the first determination of the direction of acceleration or roll 
angle, the micro-controller initiates subsequent measurements by first 
switching on the LED chip from which no light was detected in the previous 
measurement. If this LED is still blocked by the drop of liquid, no 
further action is needed. If, however, light is received by the 
photodetector when this LED is switched on, the micro-controller then 
respectively switches on the LEDs on each side of the previously blocked 
LED. Only if light is received by the photodetector from the LED chips on 
each side of the previously blocked LED chip is a full sequential scan 
made of all the LED chips. 
In a variation of this embodiment, PCB includes sixteen photodetectors 
respectively mounted opposite the LEDs on an inner planar surface thereof. 
Circuitry may sequentially pulse each of the LED chips and receive the 
information output by the photodetector opposite the pulsed LED. A 
microprocessor may control the sequential pulsing of the LEDs and receive 
and process the information from the photodetectors. Output ports of the 
microprocessor output position information. 
While the above-described embodiments have utilized planar arrays of 
sensing elements, the present invention is not limited in this respect. A 
non-planar array of sensing elements arranged in a predetermined manner 
for example on the inner surface of a sphere may be utilized for 
determining the orientation of a member in various planes. 
Variations of the above-described principles will be apparent to those 
skilled in the art. For example, a solid object responsive to the rotation 
of the sensor may be used to in certain implementations rather than a 
fluid or liquid. An acoustic arrangement may also be devised which detects 
the presence or absence of a fluid relative to an array of acoustic 
sensing elements. Further, the invention may broadly be utilized to 
measure the orientation of a member in a plane and is not limited merely 
to rotatable members. 
The invention has been described in detail in connection with the preferred 
embodiments. These embodiments, however, are merely for example only and 
the invention is not limited thereto. It will be apparent to those skilled 
in the art that other variations and modifications can easily be made 
within the scope of this invention as defined by the appended claims.