The following four tasks were studied: (1) modification of the calibration routines to calibrate the Inertial Measurement Unit gyroscope drifts with fixed platform heading; (2) modification of the calibration routines to calibrate the Inertial Measurement Unit accelerometers; (3) checking overall software again for errors; and (4) providing documentation on the above work describing changes to the present software, results of these changes and future operating procedures

Keck, Thomas S.

Telford, James W.

English

NASA Technical Reports Server

FINAL REPORT
Prepared for
GEORGE C. MARSHALL SPACE FLIGHT CENTER
MARSHALL SPACE _IGHT CENTER
ALABAMA 35812
CONTRACTOR: Desert Research Institute
_tmospheric Sciences Center
University of Nevada system
F.O. Box 60220
Feno, NV 89506
TITLE OF CONTRACT: #n Inertial Velocity Reference for
the NASA Airborne Doppler Lidar
DATE OF THIS REPORT: July 26, 1988
CONTRACT NO.: 5_AS8-34685
AUTHORS: _homas S. Keck and James W. Telford
(_AS_-C_-179378) AN ]NERIIAI V_Lt;CI_Y
LILA_ _inal _e;crt ([_se£t B(aEarck Inst.)
11 F CSCL 20E
N89-I 1203
Unclas
G3/36 01610E5
https://ntrs.nasa.gov/search.jsp?R=19890001832 2020-03-20T05:48:01+00:00Z
The last phase of this contract involved the following four tasks.
Task I. Modify the calibration routine_ to calibrate the Inertial Measure-
ment Unit gyroscope drifts with fixed platform heading.
Task 2. Modify the calibration routine_ to calibrate the Inertial Measure-
ment Unit accelerometers.
Task 3. Check overall software again fo_ errors.
Task 4. Provide documentation on the 8hove work describing changes to the
present software, results of these changes and future operating
procedures.
These tasks were funded in two stages, Ta;ks i and 4, and then Tasks 2 and 3.
These tasks involved designing, writing, debugging and checking in the
laboratory of calibration software to improve the calibration of the inertial
platform measuring unit gyroscopes and ac:elerometers.
The navigation program is implement_d as a wander axis system where the
heading axis is not torqued. This frees _;hesystem of azimuth torquer errors.
Azimuth gyro drift must be measured and used to calculate platform heading.
It also means that the platform alignment for navigational, use is quicker
because the cluster does not have to be physically rotated under torque drive
to line it up to true north. True north is determined by calculations and
used this way. In the original implementation of the software the gyroscopic
biases were measured by recording the torquer pulses needed to keep the
platform level during a leveled exercise at a fixed location. This enables
the torques needed to keep the platfor_J level as the earth rotates, to be
measuredand hence to derive a torque rat_ offset needed to overcome gyroscope
drifts. By measuring each gyroscope in reversed positions average offsets for
each of the two horizontal gyroscopes are determined.
This process takes several hours, _,ring which a nontorqued azimuth axis
allows direction of the gyroscope being calibrated to drift away from the axis
of the earth. While mathematically this transformation should not cause any
problems, in practice the integration of _he discrete pulses which measure the
level attitude, and the discrete nature c_f the torquing pulses may accumulate
small errors. In addition, and more important, there are forces between the
instrument and the case (mostly magnetic) which change as the heading of the
platform changes relative to the case. [[%is force cannot be properly removed
from the free azimuth calculation as its exact nature is not known.
Task i
This task was to provide the azimuth torque needed to keep the instrument
cluster aligned so true north and the gs,ro axis remain fixed during the gyro
bias measurement. Thus the effects of the case are not continually changing
and the leveling loops are more stable. Appendix A describes this process in
detail and provides the written documentation required under Task 4.
The gyrocompass software has beer modified to compensate for small
effects due to the position of the inertial instrument cluster relative to the
external case. This software is used to align the platform to the vertical
before navigation starts and is thus critical to the accuracy of the system.
It also is used to derive the gyro biases, the slow drift of an undisturbed
platform relative to the fixed stars. Inaccuracies here also affect the
platform's performance. In the past it was noted that the platform showed
less error when it was aligned at the sane orientation as when the gyro biases
were measured. When the platform is flown in an airplane the inertial
instrument cluster will be at an unpredictable angle relative to the case when
the system is powered up. In the previous software the platform was then
aligned without concern for this angle. The new version rotates the platform
to the same orientation every time.
Then as alignment proceeds, the platform is torqued about all three axes
at a rate to compensate for the earth's r_tation. Thus only drifts due to the
gyro biases remain. The gyrocompass software measures these with a series of
least square fits. Between fits the platform is adjusted so that it stays
close to the original alignment. This keeps the platform level and prevents
it from rotating so that the effects due i:o the case are effectively constant.
The navigation software is little _:hanged. The platform is still not
torqued about the vertical axis in naviga_ion mode, but is allowed to drift in
"wander azimuth" mode. The generation of the CRT display has been more evenly
distributed through time. In the previols version of the program there were
probably occasional time steps greater than the nominal 0.i second. The
program attempts to keep the average tittle step exactly at 0.I second to the
accuracy of the crystal clock. If a ti_e interval is longer than 0.I second
by, say, a millisecond, a later time ste_ is shortened by the same amount to
compensate.
One change in the navigation progran could affect its operation although
this is not likely. The navigation soft_are has been split off from the rest
of the program and compiled separately, r_is was done because the program had
grown to the point that the text editor _ras failing occasionally. The editor
sometimes fails when editing a file between 64K and 128K bytes, the maximum.
The surgery was done with some care, but only flight testing will assure that
the two halves of the software communicate correctly. The only testing that
has been possible so far are static drift runs.
The new gyro bias calibration soft_,are locks into the platform perfor-
mance more quickly than the old. The _revious version tended to do a slow
dampedoscillation before settling down to the bias values. This is espe-
cially true for the vertical axis. This should translate into better align-
ment before each flight. It is not possible to tell if the accelerometer
scale factors will improve performance wi_hout doing actual flight tests. The
tests so far have only been static drift runs. The scale factors do not
affect drift runs unless they are gros_ly wrong. Drift runs show average
velocity (i.e. errors) of about i knot over a 5 hour period. Again, it should
be stressed that enough changes have been made from the earlier version of
this software that it should be flight t_sted before it is used as a replace-
ment for the old version.
Task 2
In this task the software for calibrating the horizontal accelerometers
was designed, written, debuggedand checked out in the laboratory. The offset
and scale factor of the accelerometer drift slightly as they age and for the
best performance they need to be periocically recalibrated. To accomplish
this the platform cluster with the accelerometers must be placed on its side
and then rotated by 90° steps. The case needs to be mounted rigidly at an
angle tilted around the roll axis from the vertical, in excess of about 60° .
It is likely that the case can be stood on one end if blocks are provided to
keep the ball mount above the surface, provided the cooling air and the cables
function and the case is kept rigid. Ho_._everour laboratory fixture was used
in this work where the platform is mount_d in a proper fixture and rotated by
70° in roll and this, because of its rigidity is preferable. The "cage"
operation rolls the gymbals until the z axis is normal to the bottom of the
case. With the input pulses from the _Lccelerometers and the torque pulses
appropriately reassigned, the platform can be operated with the z axis
horizontal and either the x axis or the y axis vertical.
The accelerometer pulses of the vertLcal accelerometer (now the x axis or
the y axis) are then counted over a spe,_ified time of about 5 minutes. The
exact arrival time of the last pulse is measured in this system so that the
discreteness of the pulses counted does not limit the accuracy. This proce-
dure is repeated at 90° intervals so the accelerometer pulse rate for _ I g is
measured for both the x and the y accel,_rometers. Half of the sum of the +
and - counts for each accelerometer gi_es the scale factor while half the
difference provides the accelerometer bias. The z axis accelerometer is not
calibrated in this procedure, but since the vertical axis must be used in an
altitude loop a precise calibration is nc,t necessary.
Task 3
Check overall software for errors. In the expensive testing and develop-
ment no errors were discovered, so, unless flight tests reveal someproblem it
can be assumedthe software is error-fre_ .
Task 4
Provide documentation. The software has been delivered on the floppy
disk medium used in this system together with documentation to use the
software developed under Tasks i and 2. A copy of the operating instructions
is attached as Appendix A.
APPENDI>A
Operating Procedures for Calibrating
Inertial PlatformAccelerometers
Introduction
The inertial navigation software has been upgraded to include the
calibration of horizontal accelerometer scale factors as well as internal
changes to the alignment/calibration port,ion of the navigation software. The
operating instructions for the navigation1 program, J/GYRO, remain the same as
before. The only changes that affect the user are the inclusion of more
parameters in the file J/DATA. This is described below. There is a new
program, J/AXCAL, which calibrates the _orizontal axis accelerometers. This
document gives operating procedures for .r/AXCAL.
The Parameter File
The file J/DATA is associated witP a particular inertial platform. It
provides the navigation and axis caliLration programs with data about the
platform. Under the old program only the first 3 lines of this file were
significant. Now a fourth and fifth l_ne have been added for the accelerom-
eter scale factors and zero offsets. ?rial values for the scale factors may
be obtained from the platform case. _ere should be a sticker labeled "Gyro
Constants". The scale factor for axis I is labeled "x" and that for axis 2 is
labeled "y". The scale factor for axi_ 3 should always be set to 1.0. The
offsets should be set to zero initially. The following table details this.
Line
4
Columns Item Trial Value
i-I0 axis I scale factor from case
11-20 axis 2 scale factor from case
21-30 1.0 (axis 3 scale factor) 1.0
I-i0 axis I zero offset 0.0
11-20 axis 2 zero offset 0.0
21-30 0.0 (8xis 3 zero offset) 0.0
After running J/AXCAL the trial values should be replaced with the computer
calibrated values. The ED editor may be used to modify the file J/DATA.
(This editor is very similar to the UNI>[ editors ed and ex.) Note: neither
J/GYRO nor J/AXCAL will function without the data on lines 4 and 5.
Axis Calibration
The calibration of the horizontal accelerometers is accomplished by
placing the platform on its side so that each horizontal axis can be set
vertically. The earth's gravity provides a constant acceleration along the
axis being tested. The platform is then slewed 90 ° to bring the next axis
into a vertical position. Four measur_ments are needed: the two axes are
oriented each upward and downward. The most important datum needed for this
procedure is the local acceleration of gravity in ft/sec 2. This should be
known to at least one part in 105 . Accuracy greater than this is not useful
since the calibration procedure is not repeatable to better than a few parts
in 105 .
During calibration mount the platlorm solidly on its side in an almost
horizontal position. There should be a minimum of vibration and the case
should not shift. (A concrete floored ¢oom in the lower stories of an office
building would be ideal.) The platfo,_m should not be totally on its side
A- _
(with the connectors uppermost, and rotated clockwise about the north direc-
tion); a tilt of 60 ° to 80 ° from its norN al orientation is all that is needed.
In detail the procedure is as follo_s.
I. Assemble the platform hardware in its normal configuration except that
the platform lies tipped on its side.
2. Bootstrap the computer.
a. Turn on the computer and CRT.
b. When "*" characters appear, type U and then I, but no return
characters.
c. Wait for the message to appear
END SUBMIT LOGIN
followed by a "-" prompt character.
3. Start the axis calibration program.
a. Turn on the platform power.
b. Type J/AXCAL. After a few moD_ents a display with a flashing screen
appears with a request to type in the date, time, and gravitational
acceleration. The typed information will appear in the next to the
bottom line of display.
c. Type in the date. If the date were July 4, 1976, type:
DATE 7 4 76 _return character at end of line)
d. Type in the time. If the time were 9:37 type:
TIME 9 37
e. Type in the local acceleratio_l due to gravity in ft/sec 2. If this
were 32.1578, type:
GRAV 32.1578
A-3
..
a.
b.
C.
When the date, time, and gravity (it any order) have been received by the
program, the screen will stop flashing and the calibration can proceed.
Let the hardware warm up. The display will announce a number of modes
such as CAGING, FAST LEVEL, SLEW, a,_d AXIS CALIBRATION. By the time the
program has been in AXIS CALIBRATI_)N mode for 15 minutes, the hardware
has warmed up sufficiently to proceed.
Take the first measurement.
Type RECAL to restart the accumulation of data.
Wait 5 minutes for sufficient ¢iata to be taken.
Type ADV to advance the platform through 90 ° for the next step. The
program will enter SLEW mode taking about i0 minutes to rotate the
next axis to vertical.
Each of the next steps is the sa1_te.
When the display returns to AKIS CALIBRATION mode, wait 5 minutes
for the orientation of the pla_:form to stabilize.
b. Type RECAL to start the collec_:ion of data.
c. Wait 5 minutes for data to acc1_ulate.
d. Type ADV to advance to the nex:: step.
The ADV command at the end of step 8 sends the platform back to the same
orientation that existed in step 5 This is reflected by the displayed
variable STAGE which cycles througil 1,2,3,4 for each of the 4 possible
orientations of the platform. Th_ actual STAGE that occurs first will
depend on how the platform was sitting inside its case when calibration
started. The final result of the calibration is available when the ADV
command is typed in step 8. The li_e labeled:
.
A-4
AXIS I CALIBRATED SCALE FACTOR
gives the scale factor and offset for axis i and the line labeled=
AXIS 2 CALIBRATED SCALE FACTOR
gives those for axis 2. Axis 3 is ,lot calibrated. These numbers should
be entered into the file J/DATA so 1:hat they can be used for the naviga-
tion program J/GYRO.
A-5


An inertial velocity reference for the NASA airborne Doppler lidar

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