Wellbore Navigation, Inc.

Directional Surveying Equipment

15032 Redhill Ave., Suite D and
Tustin, CA 92780 Engineering Division
Ph: (714) 259-7760 Fax: (714) 259-9257
E-mail: info@welnavinc.com

WelNav Primer on Inertia Gyroscope Inner Gimbal

Corrective Drift Balancing Procedures

Phase I

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 Wellbore Navigation, Inc.

WELLBORE NAVIGATION, INC.

15032 RED HILL AVE., STE. D
TUSTIN, CA. 92780
(714) 259-7760 PH, (714) 259-9257 FAX
www.welnavinc.com
info@welnavinc.com

Gyro Drift and Corrective Action of a

Wellbore Navigation and Humphrey/Goodrich

1.50” Diameter Inertia Gyroscope

Phase I – Welnav 101 primer on Inertia Gyroscope Inner Gimbal corrective drift
balancing procedures, page 6.

What is the cause of gyro drift?

► Friction

► Inner Gimbal out of balance condition

► Outer Gimbal out of balance condition

► Intercardinal condition

► Low Spin Motor RPM

► Bad Motor Bearings

► Bad Gimbal Bearings

► Bad Electrolytic Sensor

Page 13 for component causes.

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How does a WelNav , Humphrey / Goodrich - gyroscope work?

The question is often asked, “How does our gyro work”? And our answer is fine, but sometimes this answer is
not enough, so the purpose of this paper is to give you some gyro operating principals and explain some
common gyro terms and point out why our gyros are best.

Let’s talk first about gyro configurations it all starts with a spinning wheel called “the rotor” or “spin motor”,
which is mounted in a frame called “Gimbals”.

The frame has ball bearings between the base surface and the Outer Gimbal and between the Outer Gimbal and
the Inner Gimbal. Also the Spin Motor is held in the Inner Gimbal by Rotor bearings.
These ball bearings are naturally called Gimbal Bearings.

Now the principal advantage of our spinning wheel is that it’s “lazy”, it wants to stay where it is in space!
This means that if the base surface turns around the Outer Gimbal axis or around the Inner Gimbal axis, the
gyro spin axis stays put! In other words, the gimballing system isolates the spin motor from the base rotation.

The spin axis then is stabilized in space. If some kind of angle measuring gadget is used, such as a 360° sine /
co-sine Synchro is used to measure the rotation of the base surface of the Outer Gimbal axis we can then
determine how far the base surface has moved from a stabilized reference line of the gyro spin axis.

What if the stabilized reference line has been given some particular orientation? These angle measuring gadgets
(pickoffs) will tell us how far the spin axis has deviated from some predetermined position. When the stabilized
reference line is horizontal, the unit is called a “Directional Gyro”.

The unit we have talked about so far is a “two degree of freedom” gyroscope, that is, the base surface can rotate
around two quadrature axes (axes at right angle to each other) such as the Inner Gimbal axis and the Outer
Gimbal axis without disturbing the stabilized reference line.

Let’s start with “vectors”. A vector is a physical quantity that has magnitude (how much) and direction (which
way). For example, “velocity” has magnitude (miles/hour) and direction (north).Two vectors are helpful in
explaining the action of a gyro. First, there’s the spin vector. The spin vector describes the way in which the
gyro rotor rotates. The magnitude of the spin vector is measured in revolutions per minute.

The direction of the spin vector is found by wrapping the fingers of the right hand in the direction of spin
rotation. Note: rotor spin axis for a Directional Gyro is in the horizontal plane. Now, if the thumb assumes the
usual hitch hiking position, the thumb points in the direction of the spin vector (horizontal plane). This method
of determining vector direction is known as “the right hand rule”.

Next, there is the Torque Vector. Torque is something (force) which tends to produce “rotation or twisting”.
The magnitude of torque is measured in the shortest distance between the force and what would be the axis of
rotation. Finding the “direction” of the torque vector is just a bit tricky, but with the aid of the “right hand rule”,
a torque vector direction can be established.

If the fingers of the “right hand” are wrapped in the same direction of the applied force of the “rotor rotation”
the thumb will point in the direction of the torque vector. Note: When facing the open window of the WelNav
gyro in a caged condition, which puts the Inner Gimbal caging cam towards the right side, the spin motor rotor
is turning in a counterclockwise rotation. Imagine gripping a pencil with your right hand in a horizontal position
with the eraser in back of the little finger and your thumb resting towards the pencil end.
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Now that we know a little about “Spin Vectors” and “Torque Vectors” we come to “The Law of Gyroscopic
Precession”. A gyroscope is a fairly surprising device, if you push on it in one direction it moves off in another
direction. The way it moves though, is quite predictable.

If you exert a torque on the Inner Gimbal (in an uncage condition) by pushing with your finger of a wooden rod,
you would think that the Inner Gimbal would rotate about the Inner Gimbal axis in the direction of push, but no,
instead the Outer Gimbal will begin to rotate about the vertical axis of the gyro.

This rotation of a gyroscope in response to an applied torque is called “Precession”. When a torque is applied to
a gyroscope, the spin vector tries to move into the torque vector, which means an axis in quadrature to both the
spin vector and the torque vector. If a torque is applied to the Inner Gimbal axis, the gyro Outer Gimbal axis
will rotate ( precess ) about the Outer Gimbal seated frame, and conversely, if a torque is applied to the Outer
Gimbal, a rotation about the Inner Gimbal axis will commence.
Your “right hand” can be most helpful in visualizing these relationships. Arrange your thumb, index and middle
fingers into three quadrature axes. Assume that your “thumb” is pointing in the direction of the “spin vector”
(horizontally), and your “index finger” in the direction of the “torque vector”. If your fingers are kept rigidly in
this configuration and you try to rotate your “thumb” ( spin vector ) into your “index finger” ( torque vector ),
your hand ( the gyro ) moves around your “middle finger” ( precession axis ) just the way a gyro does.

Also the “middle finger” points in the direction of the precession vector. Note: Precession is a vector, just like
the spin of the motor. Try putting your thumb in the direction of the spin vector ( in a horizontal plane ), your
index finger at 90 degrees from the spin vector, and your middle finger pointing down for the Outer Gimbal
axis. Apply a torque downward on your thumb and your middle finger will move. Remember, when a torque is
applied to a gyro, the spin vector tries to move into the torque vector giving rise to precession about an axis in
quadrature to both vectors.

What force creates precessional movement? Rotor Inertia, which the dictionary defines as: “ the property of
matter by which it will remain at rest unless acted upon by some external force”. The product of Inertia X Spin
velocity is called “angular momentum”. Angular momentum is essentially that property of a spinning wheel and
mass, which determines its degree of stability.

The precession we’ve been talking about so far is characteristic of a two degree of freedom gyro, this precession
in response to an applied torque can be a “good feature” or a “bad feature”. For example, to use a gyro as a
“directional gyro”, one would want it to remain fixed in space. The gyro isn’t perfect, however, and as a result
of stray torques from “bearing friction, pick offs (slip rings, contact points)”, etc., the gyro precesses or wanders
in a random fashion. This wander is called “random drift”.

This “random drift” for a Directional Gyro is about 15 degrees per hour ( .25° /min. ). Stray torques aren’t very
much torque, but do significantly degrade the performance of the gyro. Hence, this is a bad feature of
precession in response to a torque.

On the “good side”, one can adjust for random drift. If a gyro has been properly assembled and all contact
points, bearing pre-loads, motor RPM and gimbal balancing have been accomplished, then final corrective drift
characteristics can be done with Inner Gimbal “balancing screws”.

Now that you know all about a two degree of freedom Directional Gyroscopes, we now go on to what makes a
WelNav / Humphrey / Goodrich gyroscope tick, and we’ll start with the Inner Gimbal Assembly.
Inner Gimbal balancing screws are part of the make up of the Inner Gimbal assembly during assembling
procedures, which include the spin motor rotor assembly, motor bearings, shields, caging cam, torque motor
wiper arm, and an Electrolytic Sensor. On final assembly, the Inner Gimbal assembly is precision statically
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balanced, until it is perfectly balanced within a 360 degree circle. This means that it can be stopped at any point
during static balancing or as much as possible as the Electrolytic Sensor will allow, due to fluid flow. The Inner
Gimbal provides the Angular Momentum required through 37,000 RPM – 40,000 RPM to offset friction and
stabilized stability during a survey.

The Electrolytic Sensor is a fluid filled sensor which is electrically charged and controls minute voltage changes
in the leveling horizontal plane when it becomes un-leveled with fluid proportional change or equivalent
voltage change. A common bar and two electrode tips, one on each end of the tubed sensor, sense voltage
change. One tip and the common bar control CW torque, while the other controls CCW torque.It sits atop of the
Inner Gimbal and maintains the spin axis in a horizontal plane by applying minute torques, clockwise or
counterclockwise, whenever it sees minute un-leveling to a Torque Motor Stator situated on the seated frame of
the gyro via a harden laminated torque rotor mounted concentrically on the Outer Gimbal bearing journal. All
this will be pointed out to you during the course of training.

The electronics circuit board assembly controlling the Electrolytic Sensor is called the Deadband Torquer. It
dispenses correct voltages and sends sensor signals to the Torque Motor Stator for CW or CCW torque via the
Torque Rotor. Through the “right hand rule” and gyro precession, the Inner Gimbal is maintained in a
horizontal position at all times. The Deadband stigma comes into play only when the sensor is perfectly leveled
with equal voltages, so no signals are transferred to the Torque Motor Stator. However, vibration and motion
keep the sensor operating at all times during the course of a survey.

The Outer Gimbal Assembly frame houses the Inner Gimbal Assembly with two ball bearings and is equipped
with a three ring slip ring on the lower bearing journal to provide ac and dc power and a circular slip ring to
transfer sensor transitional signals. It is also equipped with a 360 degree cam and Inner Gimbal caging bearing
rod assembly. As mentioned, the upper bearing journal houses Torque Motor Rotor and the Syncro Resolver
Rotor Stator, which will be covered shortly.

The Outer Gimbal Assembly, as was with the Inner Gimbal Assembly is extensively static balanced in a
horizontal position within a gimbal retaining fixture. It is balanced within a 360 degree circle until it can be
stopped in any position within the circle, but again, within allowance of the Electrolytic fluid placement flow.
If the Outer Gimbal Assembly is unbalanced, it reacts as a torque to the Inner Gimbal Assembly, causing the
Inner Gimbal to leave its horizontal plane. The Electrolytic Sensor senses the angle movement and reacts by
sending a corrective signal to the Torque Motor to provide a torque to the Outer Gimbal realigning the Inner
Gimbal back to the horizontal plane. However in the process, and according to the “right hand rule”, something
has to move and in this case it would be the Outer Gimbal, for it is not restrained and free to rotate within
supporting ball bearings, in which case any movement would then be regarded as drift.

Although the Outer Gimbal is at fault, due to an unbalanced condition, the Inner Gimbal is restrained from
leaving its horizontal plane by the Electrolytic Sensor and the Torque Motor corrective action. Conversely if
the Inner Gimbal is unbalanced, the Outer Gimbal would be the only one to move. Get it?

Moving on, the Outer Gimbal supporting ball bearings are housed within an aluminum tubular frame, one on
each end of an aluminum base attached to each end of the tubular frame.

On the lower section, an Outer Gimbal caging bearing rod is housed and attached to an actuator arm, which
moves up and down, allowing it to engage a 360 degree cam provided on the Outer Gimbal Assembly. This
caging bearing rod travels on the cam to a low point section of the cam, which puts the Inner Gimbal in a
correct alignment to permit the caging bearing rod to engage an Inner Gimbal caging bearing rod and in turn
engaging an Inner Gimbal cam.

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Once the Outer Gimbal caging bearing rod enters the O.G. cam slot, it locks the Outer Gimbal, while it
continues to push the I.G. caging bearing rod into a slot provided on the I.G. cam locking it in place.
What actuates the movement up and down of the O.G. caging bearing rod? A D.C. motor with a threaded shaft
at one end is threaded into a floating nut attached to an actuator plate pined at one end and the floating nut in the
center, pined to the plate as well. A flat steel spring arm .062” wide also attached to the plate is inserted into the
slotted O.G. caging bearing rod, assembled with an internal compression spring, keeps the arm from exerting
excess pressure.

With a plus & minus D.C. voltage applied to the D.C. Motor for an up movement and a reverse voltage of
minus & plus voltage applied for a down movement, locks or unlocks the O.G. and I.G. gimbals simultaneously
(caging & uncaging). Two cams attached to the actuator plate, one on each side of the plate, provide shut off
voltage to the D.C. Motor as well as reversing the voltage through two Micro Switches, one for the up position
shut off and one for the down position shut off.

A little lower we find the Transformer which is excited with a D.C. voltage input. The Transformer sends D.C.
voltage to an Inverter Circuit Board Assembly, which regulates D.C. voltage to the cage / uncage D.C. Motor. It
sends an A.C. voltage to the Deadband Circuit Board Assembly to excite the Electrolytic Sensor as well as to a
sine/co-sine electrical Syncro Resolver and also converts D.C. to A.C. regulated voltage to the Spin Motor.

The upper section of the gyro houses the Torquer Motor Stator and the Electrical Syncho Resolver Stator.
Comment was made earlier to the Syncro Resolver Rotor Stator, which is mounted on the upper bearing journal
of the Outer Gimbal. Also mounted at the tip of the O.G. journal is a two ring slip ring, which sends A.C.
voltage from the Transformer to the Syncro Resolver Rotor Stator. The Syncro Resolver Rotor Stator in placed
in line with the Electrical Syncro Resolver Sator concentrically as well as the harden laminated torque rotor,
which is also mounted on the upper O.G. bearing journal. The Electrical Syncro Resolver provides a sine/cosine
output voltage that is converted to 360 degrees for an azimuth directional heading.

See reference engineering drawing C-59265 Outer Gimbal Frame Assembly for reference information
pertaining to the above description of gyro operating components.

Now that we know what makes a WelNav /Humphrey/Goodrich gyroscope tick, we’ll proceed into Phase I –
101 of the WelNav Primer on Inertia Gyroscope Inner Gimbal Corrective Drift Balancing Procedures.

Phase l
WelNav Primer on Inertia Gyroscope Inner Gimbal
Corrective Drift Balancing Procedures
WelNav Gyroscopes:

1. 1.50” Diameter SRG Gyroscope P/N 300201-1

2. 1.50” Diameter Photographic Gyroscope P/N 300645-2

3. 1.50” Diameter Humphrey / Goodrich type Gyroscope P/N DG69-0901-4

The WelNav Primer is designed to give a Survey Engineer the opportunity to maintain his / her gyroscope to a
minimize gyro drift over time, if the only problem seen is drift characteristics. The object is to increase the life
of the gyro performance before having to submit the gyro for Spin Motor overhaul by re-balancing the Inner
Gimbal to minimize drift.
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Lab Equipment requirements:

1. P/N 100569-16 Bristol Wrench – To interact with Balancing Screws on Inner Gimbal.
2.
3. P/N 100586-1 Multi-meter – To check Sine / Co-Sine Resolver Voltage Output

4. P/N 300377-1 Gyro Warm Up Box – To regulate DC voltage control and monitor current to gyroscope.

5. P/N 300490-1 Gyro Control Box – Provides electrical Caging and Uncaging functions to gyroscope via
Gyro Warm Up Box.

6. P/N 100509-1 Scorsby Table with P/N 300207 Test Fixture to retain gyroscope. Provides Roll, Pitch and
Yaw at ±15° movement, oscillating back and forth every six clockwise and counterclockwise
revolutions.

7. P/N 200549-1 Clamp, Scorsby Table

8. P/N 200547-1 Thumbscrew, Scorsby Table

9. P/N 300207-1 Test Fixture, Gyroscope

10. P/N 302742-1 Resolver Test Box – Converts gyroscope Resolver voltage from Sine / Co-Sine output to
0° - 360° Azimuth in 0.01° digital increments.

11. P/N 102865-1 Power Adapter, Resolver Test Box

12. P/N 104630-1 Gyro Tachometer – Checking Gyro Spin Motor Rotor RPP

13. 302006-1 Cable, Test, Gyro Warm Box to Gyroscope

Calibration Test 1:

To begin with, for the ease and purpose of demonstrating the Inner Gimbal Balancing Procedures, a Humphrey
type 2.50” Diameter Photographic Electrical Cage/Un-cage Inertia Gyroscope, Model DGO2-0806-1 will be use
to demonstrate these procedures. See envelope drawing DG02-0803-1 and 41042 Outer Gimbal Frame Assy.

Since this particular model has a rather large Inner Gimbal Assembly ( Spin Motor ) it is used by WelNav as the
first phase of Inner Gimbal balancing training.

1. With hands on, the gyroscope access window is opened and the internal workings of the Inner Gimbal
Assembly including location of the Inner Gimbal Balancing Screws are shown to the engineer trainee
with some explanation of the Inner and Outer Gimbals of the gyroscope. See engineering drawing 41065
Outer Gimbal Assembly and 41064 Inner Gimbal Assembly at this time.

2. Following the visual and handling of the DG02, it is placed on a Scorsby Table and secured in place via
the single indexing pin and bottom connector. Note: It is permissible to retain the DG02 with only the
Index Pin and Connector for this purpose.

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3. The gyroscope is then electrically hooked up to the Gyro Warm Up Box and Gyro Control Box. The
Resolver Test Box is not hooked up at this time, due to the fact that Model DG02-0806-1 Gyroscope is a
Photographic type Gyroscope and the equipped Azimuth Compass Card of the gyroscope will be used to
monitor CW & CCW drift of the gyroscope.

4. To insure that the Inner Gimbal Assembly is unbalanced for the purpose of balancing technique
procedures, the gyroscope is electrically started up for no more them 5 seconds to allow electrical un-
caging of the Inner Gimbal. Once the Inner Gimbal is un-caged, power is shut down. To insure there is
no nutation ( oscillation ) of the gimbals, a thumb is placed within the window access to hold the Inner
Gimbal while un-caging. Instructor to demonstrate.

5. The two ( 2 ) balancing set screws located in two corners of the Inner Gimbal (see engineering drawing
41064 ) are turned with a #4-40 Bristol Splined Wrench CCW until completely removed.

6. The two balancing screws are then re-screwed in the two locations until approximately ¼ of the length
of the screw is entered.

7. On completing this procedure, the Outer Gimbal Assembly is then turned until Compass Card North is
aligned to Zero. Hold in Inner Gimbal in place with thumb, aligning the Inner Gimbal approximately in
the horizontal plane. Switch on power for approximately 5 seconds via Gyro Control Box , and
electrically cage gyroscope. Instructor to demonstrate.

8. The gyroscope is now electrically caged, and will be allowed to warm up for approximately 15 to 20
minutes. Note: Warm up is required to stabilize internal workings of the gyroscope as well as
components composed of Aluminum material to normalize expansion and contraction obtained during
original calibration procedures.

9. Following warm up, uncage gyroscope and allow too run in the static uncaged condition for
approximately 10 minutes. This is a preliminary static drift check. Note: Due to the large Rotor Size, the
angular momentum of the rotor spinning mass, little drift will be noted for the 10 minute time duration
( approximately 5° ).

10. After 10 minutes, look at the compass card to see which way gyro drift has moved away from zero. CW
or CCW ( West Drift or East Drift ).

11. To determine corrective action, use a small wooded stick ( ¼” diameter maximum X 4 to 6 in. ), and
holding one end touch and gently push very lightly the lower left corner balancing screw or Inner
Gimbal. If a wooden stick is not available, use index finger.

12. While slightly pushing gently, observe the Compass Card to determine if this action is moving the
Compass Card in a corrective manner. If not and it is increasing, stop and do the same procedure to the
upper right corner.

13. If the action on the upper right hand corner is moving North of the Compass Card of the gyroscope to
zero, it is now known that the balancing screws require to be screwed CW to put weight on the
unbalance side. West Drift.

14. Turn each balancing screws approximately five ( 5 ) full turns by holding the Inner Gimbal with thumb
and index finger, while turning.

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 15. On adjusting screws, hold Inner Gimbal and align North to Zero and position approximately in the
horizontal position. Cage gyroscope while holding the Inner Gimbal, which will move slightly while
locking the Inner and Outer Gimbals in place.

16. Look at Scorsby Table and insure that it is set for 6 cycles per minute and for ± 15° tilt angle. Switch on
power and un-cage. Put one hand on each end of square table and feel rotation of table. Look under table
to see rotating Hub. When Hub is aligned with Up and Down Motion Arm, then gently push down on
table as it comes around. While exerting pressure, the square plate will find the least resistance to
moving the plate to the 15° of angle.

17. Allow to rotate in the Roll, Pitch and Yaw condition for 10 to 15 minutes.

18. Following 10 – 15 minutes of Scorsby action, place hands on square table once again and bearing lifting
pressure, and again looking under the Scorsby Table and aligning Hub to Up and Down Arm, gently lift
plate until feel of plate begins to rise until plate is in a near horizontal position. Apply pressure until
insured that plate is locked in a horizontal condition.

19. Look at Compass Card North to see where position of North is with respect to Zero. If touching lower
left hand corner moves North to Zero, then additional weight is required on the opposite side. West
Drift.

20. Turn the two ( 2 ) balancing screws CW for five ( 5 ) additional turns and repeat steps 14. to 19 until
balancing screw weight is over balanced.

21. When overbalance is seen (East Drift ), turn balancing screws two ( 2 ) turns CCW , Cage / Un-cage and
run Scorsby test for 10 – 15 minutes.

22. Repeat until Azimuth remains stable on Zero for 10 – 15 minutes following Scorsby testing..

23. If stable at approximate Zero, cage Gyro and uncage for a static drift check for 10 – 15 minutes.

24. Again if stable once more, balancing has been completed and gyroscope is ready to be caged and power
turned off and allowed to run down for a minimum of seven ( 7 ) to eight ( 8 ) minutes before moving.
Note:
As a reminder, what has been experienced on calibration Test l as you recall is “Gyro Precession”. This
Phenomenon is caused by the spinning mass ( angular momentum ), which sustains its equilibrium via mass and
Rotor Revolutions Per Minute ( RPM ).

The Inner Gimbal Assembly is suspended within the Outer Gimbal by two ball bearings in the horizontal plane (
one on each side - see engineering drawing 41065 ), while the Outer Gimbal Assembly is supported by two ball
bearings in the vertical plane and can rotate 360° in the azimuth plane.

By touching and slightly applying pressure via wooden stick or finger to any corner of the Outer Gimbal, this
will move the Inner Gimbal up and down depending on where touched.
The same happens to the Outer Gimbal when touching with slight pressure on the Inner Gimbal as experienced.

To keep the Inner Gimbal in a horizontal position at all times, a Sensor Leveling Switch mounted on the Inner
Gimbal( see engineering drawing P/N 41064 ) maintains the Inner Gimbal level by sending an electrical signal
to a Torque Motor (see engineering drawing P/N 41042 ).

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Whenever the Inner Ginbal strays off the level position, the Torque Motor provides a CW or CCW torque to the
Outer Gimbal rotor precessing the Inner Gimbal back to a level position continuously. This acts the same as
touching the Outer Gimbal with an object.

This ends Calibration Test l and we are now ready for Calibration Test ll.

WelNav Primer on Inertia Gyroscope Inner Gimbal

Corrective Drift Balancing Procedures
Calibration Test ll:

With experience in hand following Calibration Test l, you are now ready to calibrate a 1.50’ diameter Surface
Recording Gyro under Calibration Test ll, which has less angular momentum then the 2.50” diameter and is
more susceptible to drift characteristics. SRG Gyroscope P/N 300201-1 or a DG69-0901-4 will be used for this
exercise and thereafter identified as just Gyro.

For the purpose of this exercise, engineering drawings P/N 52966 Outer Gimbal Assembly and P/N 52940 Inner
Gimbal Assembly will be used to identify balancing screws that are to be adjusted accordingly to counteract
drift. Note: Engineering drawings are for reference only and do not reflect the actual gyroscope currently in
service.

1. Gyro has a window an access opening slightly different then the 2.50” diameter gyroscope access
window. In place of one complete case with window, Gyro has a two piece constructed case. The upper
case includes an access window that is easily rotated to access the Inner Gimbal.

2. Following handling of the Gyro and showing the approximate location of the Inner Gimbal balancing
screws ( see engineering drawing P/N 52940 ), the unit can now be placed within a Scorsby Test Fixture
and secured in place via the two indexing pins and bottom connector.

3. The gyroscope is then electrically hooked up to the Gyro Warm Up Box , Gyro Control Box and
Resolver Test Box , which will be used to monitor CW & CCW (West Drift & East Drift respectively )
drift of the gyroscope in degrees ( 360° ).

4. To insure that the Inner Gimbal Assembly is unbalanced for the purpose of balancing technique
procedures, the gyroscope is electrically started up for no more them 5 seconds to allow electrical un-
caging of the Inner Gimbal. Once the Inner Gimbal is un-caged, power is shut down. To insure there is
no nutation ( oscillation ) of the gimbals, a thumb is placed within the window access to hold the Inner
Gimbal while un-caging.

5. The four ( 4 ) balancing set screws located in two corners of the Inner Gimbal (see engineering drawing
P/N 52940 ) are turned with a # 2-59 Bristol Splined Wrench CCW until completely removed.

6. The four ( 4 ) balancing screws are then re-screwed in the four locations until approximately 1/3 of the
length of the screw is entered.

14. On completing this procedure, the Outer Gimbal Assembly is then turned until the Resolver Test Box
indicates approximately 360° Azimuth. Hold Inner Gimbal in place

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15. With thumb, align the Inner Gimbal approximately in the horizontal plane. Switch on power for
approximately 5 seconds via Gyro Control Box , and electrically cage Gyro.

9. The gyroscope is now electrically caged, and will be allowed to warm up for approximately 15 to 20
minutes. At this time check Gyro current and DC voltage to insure current and voltage meet
specifications within the first minute before continuing.
Note: Warm up is required to stabilize internal workings of the gyroscope as well as components
composed of Aluminum material to normalize expansion and contraction obtained during original
calibration procedures.

10. Following warm up, un-cage gyroscope and allow to run in the static uncaged condition for
approximately 10 minutes. This is a preliminary static drift check.

11. After 10 minutes, look at Azimuth reading to see which way gyro drift has moved away from zero. CW
or CCW ( West Drift or East Drift ).

12. To determine corrective action, use a small wooded stick ( ¼” diameter maximum X 4 to 6 in. ), and
holding one end touch and gently push very lightly the lower left corner balancing screw of Inner
Gimbal. If wooden stick not available, use index finger.

13. While slightly pushing gently, observe the Azimuth reading to determine if this action is moving the
Inner Gimbal in a corrective manner. If not and it is increasing, stop and do the same procedure to the
upper right corner.

14. If the action on the upper right hand corner is moving the Azimuth reading of the gyroscope to zero, it is
now known that the balancing screws require to be screwed CW ( East Drift ) to put weight on the
unbalance side.

15. Turn each balancing screws approximately five ( 5 ) full turns by holding the Inner Gimbal with thumb
and index finger, while turning.

16. On adjusting screws, hold Inner Gimbal and align Azimuth reading to approximately 360° and position
approximately in the horizontal position. Cage gyroscope while holding, which will move slightly while
locking the Inner and Outer Gimbals in place.

17. Look at Scorsby Table and insure that it is set for 6 cycles per minute and for ± 15° tilt angle. Switch on
power and un-cage. Put one hand on each end of square table and feel rotation of table. Look under the
Scorsby Table at rotating Hub then gently push down on table with both hands as it comes into line with
Up and Down Motion Arm. While exerting pressure, the square plate will find the least resistance to
moving the plate to the 15° of angle.

18. Allow to rotate in the Roll, Pitch and Yaw condition for 10 to 15 minutes.

19. Following 10 – 15 minutes of Scorsby action, place hands on square table once again and bearing lifting
pressure, gently lift plate while looking under the Scorsby Table and aligning rotating Hub with Up and
Down Motion Arm, until plate is in a near horizontal position. Apply pressure until insured that plate is
locked in a horizontal condition.

20. Look at Azimuth reading to see where Azimuth position of the Gyro is. If touching lower left hand
corner moves to Zero, then additional weight is required on the opposite side ( West Drift ).

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 21. Turn the two ( 2 ) balancing screws CW ( West Drift ) for five ( 5 ) additional turns and repeat steps 14.
to 19 until balancing screw weight is over balanced.

22. When overbalance is seen, turn balancing screws two ( 2 ) turns CCW ( East Drift ), Cage / Uncage and
run Scorsby test for 10 – 15 minutes.

23. Repeat until Azimuth remains on Zero for 10 – 15 minutes following Scorsby testing..

24. If stable at approximate Zero, cage Gyro and un-cage for a static drift check for 10 – 15 minutes.

25. Again if stable once more, balancing has been completed and Gyro is ready to be caged and power
turned off and allowed to run down for a minimum of seven ( 7 ) to eight ( 8 ) minutes before moving.

26. Perform step 4. and while in the un-cage condition and power off, apply Red Glyptal via a tooth pick to
balancing screws between entry of screw into wall of Inner Gimbal and wall itself. Red Glyptal acts as
an adhesive, but does not prevent balancing screw from re-adjustment when necessary.

27. Perform step 8. and shut window when completed.

Note: You will note that Calibration Test ll is similar to Calibration Test l. Testing and calibrating of two ( 2 )
degree of freedom Inertia Gyroscopes is essentially identical for these two types of gyroscopes.

The exception between the two gyroscopes is the “Spin Axis” with respect to the indexing pin. The 2.50”
Diameter gyro’s spin axis for high angle bore holes is in line with the high angle. With its high Angular
momentum, it can achieve the higher angle of 45 degrees plus without going into gimbal lock.

For high angle bore holes on the 1.50” Diameter gyro, the spin axis must be position perpendicular to bore hole
azimuth direction. This is primarily due to ± 35 degree Inner Gimbal stops that prevent the I.G. to rotate further
then noted.

The following will be used for all practical purposes for gyroscopic Inner Gimbal drift corrections:

Drift adjustment for the 1 ½” Diameter WelNav SRG Gyroscope

As indicated previously on the WelNav SRG Gyroscope, the Inner Gimbal viewing window when open
looks directly at the four Inner Gimbal balancing screws and also will show CCW rotation of the “Rotor”,
while in the caged condition.

You will note two (2) adjustment screws at the top and two (2) at the bottom of the Inner Gimbal Assembly.

Adjustment procedure and rule of thumb:

1. Turn the balancing screws one – half turn for each 5 degree of drift.

2. Start by adjusting the two (2) top screws first, turning each screw in or out the same amount.

3. If the adjustment of the two(2) top screws does not yield a satisfactory result, then continue adjusting
using the two (2) bottom screws.

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 Screw adjustment:

1. For East Drift – Turn screw out ( counterclockwise ).

Note: East drift ( clockwise ), the Azimuth Tool Face reading will decrease , I.E. “get smaller”.

a. Example: Start = 114.00° End = 113.00° This = 1° East Drift

b. Example: Start = N11.50W End = N12.00W This = ½° East Drift

2. For West Drift – Turn screw in ( Clockwise ).

Note: West Drift ( Counterclockwise ), the Azimuth Tool Face reading will increase, I.E. “get larger”.

a. Example: Start = 114.00° End = 115.00° This = 1° West Drift

b. Example: Start = S15.00E End = S14.00E This = 1° West Drift

Inner Gimbal Adjustment windows are provided on all WelNav 1.50” Diameter gyroscopes, so drift
adjustments may be made by a surveyor in the Northern or Southern hemispheres. Each zone will react a little
differently with respect to gyro drift. What the surveyor is counteracting is the Earth’s drift rate in various
zones.

Note: For Humphrey / Goodrich DG69-0901-4 Gyroscopes.

Originally static and dynamic Inner Gimbal balancing were done only during testing and final calibration at the
Humphrey / Goodrich zone locations.

A sealed outer stainless steel case prevented anyone form re-adjusting the gyroscope in a different zone and
relied on the original Inner Gimbal factory zone adjustments, but only for San Diego, California and possibly
Minneapolis, Minnesota

What is the cause of gyro drift?

► Friction:
1. Slip Ring brushe tension to tight on to Slip Ring
2. Inner Gimbal bearing pre-load to tight
3. Outer Gimbal bearing pre-load to tight
4. Any member of the Outer Gimbal Assembly rubbing or touching the I.D. of main gimbal frame

► Inner Gimbal out of balance condition

1. Shifting of the Rotor mass
2. Balancing screws shifting
3. Component became loose
4. Reacts towards the Outer Gimbal and precesses the Outer Gimbal

► Outer Gimbal out of balance condition

1. Static balancing weights become loose – screws, nuts, washers and weights
2. Reacts towards the Inner Gimbal and in turn precesses the Outer Gimbal

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► Intercardinal condition
1. During the course of a survey within high angulations bore holes of 35° or higher, and the bearing
angle sets off too 45°, 135°, 225° or 315° from the initial heading from when the gyro was un-caged, a
precessional torque will apply to the Outer Gimbal due to the Inner Gimbal spin axis ( rotor mass )
becoming more in line with the Outer Gimbal axis of rotation. This is what is known as Gimballing
Error. Depending on how the slant angle is, 1° or 2° may be lost on returning to a vertical position.
2. Emphasis is placed on positioning the Spin Axis perpendicular to bore hole direction on when seeing
slant angles of 30° to 35° of slant angle.

► Low Spin Motor RPM

1. Spin motor RPM is rated at 37,000 to 40,000 RPM to sustain angular momentum stability for the
1.50” Diameter gyroscope. Whenever the RPM drops, the sustaining angular momentum drops
as well, leaving the ability for stabilization to decrease and allowing the Rotor Mass to shift into
the area of least resistance, which would be the Outer Gimbal axis.

► Bad Motor Bearings

1. Motor bearings that have been subjected to excessive heat, shock, unusual vibration and bernalling
(harden bearing balls leave indentations on the inner and outer races) create a vibration noise level,
leaving the Inner Gimbal to become unstable, which leads to an un-balance condition.
2. Loss of bearing pre-load due to shock or rotor mass shift.

► Bad Gimbal Bearings

1. Contaminated bearings that cause stickiness between the balls, retainer and races.

► Bad Electrolytic Sensor

1. Minute crack on an Electrolytic Sensor, which in turn loses fluid and sensitivity and slow to react
2. Broken lead on one end causing the torque motor to torque in one direction only
3. Internal voltage signal loss to the Inner Gimbal via brush Wiper Arm from the Inner Gimbal to the
Outer Gimbal circular slip ring, causing the Inner Gimbal to become a free gyroscope and eventually
dumping.

In conclusion:

Although a lot of the text is repetitious pertaining to the 2.50” Diameter Gyroscope and the 1.50” Diameter
Gyroscope, WelNav wished to emphasize the difference of Angular Momentum between the two gyroscopes.

This was in order to give the trainee the feel of two different gyroscopes yet basically the same. Rotor size and
RPM represent the difference similar to an ocean going ships gyro compass, which is very low RPM but has a
Rotor Wheel approximately 12” in diameter and weights as much as 1 ¼ lbs. to 55 lbs.

For additional information on gyroscopic systems, request Wellbore Navigation, Inc.:

“How Gyro Systems Work”

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