ADCS Interface Control Document
ADCS Interface Control Document
version 3.2
QB50
ADCS Interface Control Document
Document Version 3.2
Status Release
Date 19 July 2015
Author(s) Lourens Visagie, Mike-Alec Kearney
Version
0.1
1.0
1.1
2.0
2.1
2.2
3.0
3.1
3.2
Version history
Status
Date
Comments
Draft
13 March 2013
Initial version
– Draft
8 April 2013
After PDR
st
1 release - PDR
1 July 2013
Implemented PDR feedback
Draft
20 Oct 2013
Updated for CDR
Draft
11 November 2013 Implemeted CDR changes
nd
2 release – CDR 18 November 2013 Updated GPS power switch implementation
3rd release
18 June 2014
Updated drawing dimenstions (stack height)
Draft
21 May 2015
Update Handling section
4th Release
19 July 2015
Updated magnetorquer & battery bus spec
Approval for Release
Name
Date
Craig Underwood
Manager – SSC
Herman Steyn
Manager – ESL
Signature
ADCS Interface Control Document
version 3.2
QB50
Table of Contents
Reference documents ............................................................................................................................... 1
List of acronyms ....................................................................................................................................... 1
Changes from previous version.................................................................................................................. 1
1
Introduction...................................................................................................................................... 2
2
Functional description ....................................................................................................................... 2
3
2.1
Body axes definition and sensor/actuator mounting .................................................................... 3
2.2
Control modes........................................................................................................................... 4
2.3
ADCS control loop...................................................................................................................... 6
2.4
GPS receiver interface ................................................................................................................ 6
Electrical interface............................................................................................................................. 7
3.1
3.1.1
3.2
3.2.1
4
Power ....................................................................................................................................... 7
Power use .......................................................................................................................... 8
Communication ......................................................................................................................... 8
I2C pull-up resistors ............................................................................................................ 8
3.3
Logging ..................................................................................................................................... 8
3.4
Telemetry sampling Epoch ......................................................................................................... 8
Mechanical interface ......................................................................................................................... 8
4.1
PC104 Component stack............................................................................................................. 8
4.2
External magnetometer ............................................................................................................. 9
4.3
Coarse Sun Sensors.................................................................................................................. 11
4.4
Mass ....................................................................................................................................... 12
4.5
Inertia moments and Centre-of-mass........................................................................................ 12
5
Command interface ......................................................................................................................... 12
6
Specifications.................................................................................................................................. 12
7
6.1
Sensor specifications................................................................................................................ 13
6.2
Actuator specifications............................................................................................................. 14
Handling ......................................................................................................................................... 14
ADCS Interface Control Document
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QB50
Reference documents
[R01]
[R02]
[R03]
[R04]
[R05]
QB50 System Requirements and Recommendations - Issue 5 – 11 October 2013
INMS Requirements Compliancy Table - 20130614
FIPEX Requirements Compliancy Table - 20130619
MNLP Requirements Compliancy Table - 20130614
QB50 ADCS Reference Manual – v2.0 – 17 July 2015
List of acronyms
ACP
ADCS
bpp
CSS
EKF
FOV
ICD
H-wheel
OBC
OBDH
TBC
TBD
TC
TLE
TLM
Attitude Control Processor
Attitude Determination and Control Sub-system
Bits Per Pixel
Coarse Sun Sensor
Extended Kalman Filter
Field of view
Interface Control Document
Momentum wheel
On-board Computer
On-board Data Handling
To Be Confirmed
To Be Determined
Telecommand
Two-line Element set
Telemetry
Changes from previous version
Change
Section
Added description of high initial-rate detumbling mode
2.2
Added statements to explicitly state that GPS interfacing is optional and that
additional components and harness is required
2.4
Added battery bus voltage specification (between 7.5V and 8.5V)
3.1
Added information about UART unsolicited telemetry
3.3
Added mention of telemetry sampling epoch and adherence to sample period
3.4
Updated magnetorquer on-time resolution in specifications table (correct value
is 10ms resolution)
6.2
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1 Introduction
The QB50 ADCS will provide attitude sensing and control capabilities to 2U CubeSats in order to meet the
QB50 system requirements [R01] and science unit requirements [R02], [R03], [R04]. The QB50 mission will
carry a number of 2U CubeSats, each fitted with a Science Unit – a payload that gathers atmospheric data in
the lower thermosphere. CubeSats in the QB50 constellation require attitude control in order to
1. Minimize the influence of drag - The orbital life of a satellite will be prolonged if the effect of drag is
minimized. This will allow for more atmospheric data to be gathered
2. Ensure science payloads point towards the ram direction
The most stringent attitude requirements on the QB50 mission relate to the INMS Science Unit [R02]. The
CubeSats carrying this sensor shall have an attitude control with pointing accuracy of ±10° and pointing
knowledge of ±2° from its initial launch altitude down to at least 200km. In addition, CubeSats shal l be able to
recover from tip-off rates of up to 10 degrees/second within 2 days [R01].
The attitude determination and control sub-system described in this ICD is tailored to meet the QB50 system
requirements.
2 Functional description
The QB50 ADCS will make use of a combination of magnetometer, sun and nadir sensor measurements and a
MEMS rate sensor to estimate the current attitude. It will use magnetorquers and a single reaction wheel,
operating as a momentum wheel, to stabilize and control the attitude of the satellite.
The integrated ADCS functionality is provided by three PC104 sized boards:
CubeControl
The CubeControl board interfaces to most of the sensors and the actuators. The
momentum wheel, two of the torquer rods, optional GPS receiver and the sun and
nadir sensor cameras are mounted on this board to provide a compact volume. This
board will also interface to the external magnetometer and coarse sun sensors (CSS).
CubeSense
CubeSense is a combined sun and nadir sensor. The processing unit is a PC104 sized
board and it interfaces to two CMOS cameras – one functioning as a sun sensor and
the other functioning as a nadir/horizon sensor. The cameras use wide field-of-view
optics (180°) and the sun sensor has a neutral density filter to allow only sunlight onto
the detector.
CubeComputer
CubeComputer is a generic CubeSat OBC. In this application it will serve as the attitude
control processor.
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Figure 3 CubeControl
Figure 1 CubeSense sun and nadir
sensor
Figure 2 CubeComputer
PC104 stack
External components
CubeControl
Momentum
wheel
Sun
sensor
GPS
receiver
GPS
antenna
Nadir
Sensor
Magnetometer
Y-CubeTorquer
Z-CubeTorquer
CSS
CSS
CSS
CSS
CSS
CSS
CubeSense processing unit
X-CubeTorquer
CubeComputer
Figure 5 QB50 ADCS block diagram
Figure 4 QB50 ADCS component stack
Although the components can be used individually depending on the specific functionality requirement, this
ICD will describe the integrated system and its interfaces.
2.1 Body axes definition and sensor/actuator mounting
The coordinate system definition used by the ADCS is shown below. When the ADCS is controlling the attitude
to zero roll, pitch and yaw, the ADCS coordinate system will coincide with the orbit coordinate system.
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X
Z
Y
Figure 6 ADCS coordinate system
Figure 7 Orbit coordinate system
The sun and nadir sensor mountings are such that in the nominal orientation the nadir sensor will point
towards nadir, the sun sensor will point towards zenith and the momentum wheel spin axis will align with the
orbit normal.
2.2 Control modes
The QB50 ADCS has three distinct control modes. The first high angular rate detumbling mode will serve to
recover from initial tumble conditions up to 100 °/s. Once tumble rates have been lowered to below 30 °/s,
the second Detumbling control mode will further lower the rates and place the satellite in a stable and known
tumbling motion - a so-called Y-Thomson spin1 . In this mode the satellite will end-up spinning only about the
ADCS Y-axis and the spin axis will align itself with the orbit normal. The third control mode, Y-momentum
mode, can only be activated once the satellite is in this stable tumbling state. In Y-momentum mode the
satellite will stop spinning and stabilize to the nominal orientation (zero roll, pitch and yaw angles). In Ymomentum mode the pitch angle may be controlled to a specific reference value using a telecommand.
Table 1 QB50 ADCS control modes
Control mode
Detumbling control mode (steady-state)
Y-momentum mode
Attitude angles
Roll = yaw = 0
Roll = yaw = 0
Pitch = θref
𝛚 = [0 𝜔𝑦,𝑟𝑒𝑓 0]
𝛚 = [0 0 0]
Pitch:
Angular rates
1
Thomson W.T., Spin Stabilisation of Attitude against Gravity Torque, Journal of Astronautical Science, No.9, pp.31 -33,
1962
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The Y-momentum mode is the nominal mode for QB50 satellites since this mode satisfies the pointing
requirements. The detumbling mode is an intermediate step towards this nominal mode.
In order to perform the required control, the ADCS needs to know the current attitude (ang les and/or rates).
This knowledge is supplied by estimation methods which make use of sensor measurements. Different
estimation methods exist with varying sensor dependencies.
The initial high-angular rate detumbling mode does not require an estimator to be active since it only uses
direct magnetometer measurements.
The normal detumbling control mode only requires an estimate of the satellite Y-body angular rate. This can
be measured directly using the MEMS rate sensor, or it can be estimated using a rate estimator that uses
successive magnetometer measurements.
The Y-momentum mode requires full attitude knowledge (angles and rates) and in order to estimate this,
current position and velocity of the satellite has to be known as well as the current time. This information will
be estimated using an SGP4 orbit propagator and requires that up-to-date TLEs and the current time is
provided (via telecommand).
The ADCS will always start up in the idle condition – that is with no estimation or control active. The orbit and
attitude estimation and control modes can be changed via telecommand. The state diagram below shows the
various states and transitions that should occur to transition from idle to stabilized state.
Idle
Send TC: change attitude estimation
mode to Magnetometer rate filter
Estimating angular rates
Send TC: change control mode to
Detumbling
Detumbling
Set current time and orbit TLEs. Send TC: Change estimation
method to full-state EKF
Steady-state Y-Thomson
tumbling – waiting for EKF
convergence
Send TC: Change control mode to Ymomentum
Y-momentum
Figure 8 QB50 ADCS state diagram
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No.
0
1
2
4
5
version 3.2
State
QB50
Attitude estimation mode
Idle
None
Estimating angular rates
Magnetometer rate filter
Detumbling
Magnetometer rate filter
Y-Thomson Tumbling – waiting for EKF to Full-state EKF
converge
Y-momentum
Full-state EKF
Control
mode
None
None
Detumbling
Detumbling
Y-momentum
The mode numbers in the following tables apply when sending telecommands to change the current mode s.
Table 2 Attitude estimation modes
Mode number
0
1
2
4
5
Mode
Notes
No attitude estimation
MEMS rate only
Magnetometer rate filter
Full-state EKF
Requires orbit position & velocity
TRIAD (magnetometer + Fine sun) and estimates from SGP4 propagator
magnetometer rate filter
Table 3 Control modes
Mode number
0
1
2
3
4
Mode
Notes
No control
High angular rate detumbling
Detumbling
Y-momentum initial
Y-momentum steady-state
Requires either the full-state EKF or TRIAD estimation
mode to be active. (attitude estimation mode >= 4)
The difference between control modes 3 & 4 is that in control mode 3, the momentum wheel speed will be
ramped up while the satellite still performs a slow Y-tumble. This will continue until the estimated pitch angle
is within 20 degrees of the commanded pitch angle. At this point the control will automatically transition to
mode 4- the steady state mode in which the pitch angle will be controlled to zero. Commands that engage Ymomentum control mode should always specify a control selection of 3 to allow the automatic transition to
occur on the ADCS.
2.3 ADCS control loop
The ADCS control loop executes on the CubeComputer at a rate of 1Hz. Sensors are thus also sampled at this
frequency.
2.4 GPS receiver interface
The CubeControl board has the capability to interface to the Novatel OEM615 GPS receiver module with
Special Space Firmware. When enabled, it is possible to read the GPS provided position, velocity and time
measurements via TLM request to the ADCS. The GPS receiver measurements are not used or required by the
ADCS.
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The use of the Novatel GPS receiver requires a harness and specific components on the CubeControl board of
the ADCS module to be populated. The harness and components are not populated on standard ADCS units.
The mechanical standoffs for the GPS receiver are also not attached by default. Users of the ADCS should
explicitly request that these components are populated.
Even when the GPS interfacing components are fitted to the ADCS, the GPS receiver itself is never supplied
with the ADCS module due to export restrictions.
3 Electrical interface
The QB50 ADCS will make use of the standard PC104 stacking header for electrical interfacing. The following
pins will be used by the ADCS:
H2
H1
2
4
6
8
1
3
5
7
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52
9
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
PC104 Interface pins
H1
41
SYS I2C_SDA
System I2C Data
H1
43
SYS_I2C_SCL
System I2C Clock
H1
47
ADCS +5V
+5V ADCS supply
H1
48
ADCS +3.3V
+3.3V ADCS supply
H1
50
GPS +3.3V
GPS 3.3V supply
H2
27/28
+3V3
connect to 3V3 (eith er switched on with H1-48 or always on)
H2
29
GND
Ground connectio n
H2
30
GND
Ground connectio n
H2
32
GND
Ground connection
H2
45
V Bat
Battery bus
H2
46
V Bat
Battery bus
H1
21
ADCS I2C_SCL
ADCS I2C Clock (used internally)
H1
23
ADCS I2C_SDA
ADCS I2C Data (used internally)
H2
20
CubeSen se Enable
Enable lin e to contro l CubeCon trol power switch (used internally)
PC104 Reserved pins
Figure 9 ADCS pin allocation on the PC104 header
3.1 Power
The ADCS should be supplied with 3.3V on pin H1-48, and 5V on pin H1-47. Both supplies must be switched on
simultaneously. The ADCS has internal power switches for the different components (Cu beSense and
CubeControl). The internal power switches are controlled by the CubeComputer depending on the current
mode selection. The switch states can be read via telemetry rest and can also be forced on using a
telecommand. The ADCS also requires a 3.3V connection on pin H2-27 and H2-28. This supply is only used to
power I2C buffers and may be connected to a non-switched (always on) supply. The power draw from this line
is minimal.
Current and voltages are measured at various points in the ADCS and can be requested using TLM requests.
The ADCS will also use this to switch off components in case of latch-up. The battery bus (pins H2-45 and H246) supplies power to the momentum wheel, the magnetometer boom deployment and the GPS receiver
patch antenna LNA.
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The GPS receiver itself is powered from pin H1-50 (3.3V supply. Only applies if the GPS receiver is fitted). The
GPS does not have to be powered if the ADCS is switched on. The GPS can only be switched on when needed.
The ADCS will sense the GPS enabled status and react accordingly by setting up logs and storing telemetry.
The battery bus voltage should be between 7.5V and 8.5V.
3.1.1 Power use
The power use of the ADCS in various operating conditions is listed in the following table.
Condition
Attitude sensing only (rate estimator)
Attitude sensing only (full-state
estimator)
Detumbling control
Y-momentum control (eclipse)
Y-momentum control (daylight)
From 3.3V
supply
102
102
Average current (mA)
From 5V supply From battery
bus
0
0
23
0
102
102
102
36
10
48
0
9
9
Total
power
(mW)
336
450
516
456
644
The GPS will consume an additional 1W of power from the 3.3V supply when switched on.
3.2 Communication
The QB50 ADCS will communicate using the system I2C interface on the standard PC104 header (pin H1-41
and H1-43 on the standard header). The CubeComputer will act as a slave on the system I2C bus, responding
to commands and telemetry requests as detailed in the Reference Manual [R05].
3.2.1 I2C pull-up resistors
The ADCS will have no pull-up resistors on the system I2C bus.
3.3 Logging
The ADCS has the ability to automatically log data and save it to SD memory card. It is also possible to set-up
unsolicited telemetry logging over UART – this is the preferred way of obtaining telemetry using the UART.
For detail about the logging, please consult the Reference Manual [R05].
3.4 Telemetry sampling Epoch
If the on-board telemetry logging functionality is not being used, and telemetry logging is to be performed by
an external master, the telemetry requests shall by synchronised to idle part of the ADCS processing loop. For
details please consult the Reference Manual [R05].
4 Mechanical interface
4.1 PC104 Component stack
The stacked components make use of the standard “self -stacking” CubeSat mechanical interface. The
dimensions of the stacked components and mounting locations are shown below.
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Figure 10 QB50 ADCS PC104 stack dimensions
The ADCS stack will have spacers to support the inter-PCB spacing.
4.2 External magnetometer
The external magnetometer is housed on a small external PCB and encased inside an aluminium housing. The
magnetometer housing measures 16 x 17 x 6mm.
The magnetometer housing is supplied with a deployable boom. The boom will give a separation of 8cm from
the rest of the spacecraft to limit the effect of electromagnetic interference and result in low noise
measurements.
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The deployable boom consists of a rod with the magnetometer in a metal enclosure at one end. The other end
is connected to a mounting bracket with a spring. The bracket will be mounted on the outside of the satellite
structure. The boom will be constrained when stowed and deployment takes place upon a telecommand. The
deployable boom and mounting interface is illustrated below:
Figure 11 Deployable magnetometer boom stowed (left) and deployed (right)
Figure 12 Deployable magnetometer dimensions and mounting locations
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Both the magnetometer and the boom deployment actuation will be connected to the CubeControl PCB in the
ADCS stack using a harness. The harness between the magnetometer and the CubeControl is broken by a
circular 11-way connector with harness length as indicated below.
The connector is an Omnetics nano-circular connector with part number NCP-11-WD-18.0-C for the plug
(CubeControl side) and NCS-11-WD-18.0-C for the socket (magnetometer side).
300 mm
CSS
50 mm
CSS
CSS
CSS
CSS
CubeControl
PCB
CSS
TBD mm
50 mm
Figure 13 CSS and magnetometer harness lengths
4.3 Coarse Sun Sensors
Each Coarse Sun Sensor (CSS) consists of a photodiode mounted on a small PCB. There are 6 of these CSS in
total. The dimensions of each CSS are 4mm x 11mm x 2mm. The CSS do not have mounting holes – they should
be staked down to the satellite structure using epoxy. The sensors should be placed on the 6 different facets
of the spacecraft.
The CSS are not critical for the operation of the ADCS, and it is expected that it will not be possible to place a
CSS on the +X (RAM direction) facet due to the presence of the Scie nce Unit. The ADCS will still be able to
estimate sun vectors from the CSS if only 5 are used.
The 6 CSSs will be connected to the CubeControl PCB by wire harness. The wire harness is broken by a single
12-way rectangular connector as in Figure 13. The connector is a latched Harwin Gecko 12-way connector with
part number G125-204-12-96-L0 for the socket (CubeControl side) and G125-304-12-96-L4 for the plug (CSS
side).
The CSS connector pin assignment is specified in the table below:
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Table 4 CSS connector pin assignment
pin
signal
1
CSS1 - cathode
5
CSS3 - cathode
9
CSS5 - cathode
pin
signal
2
CSS1 - anode
6
CSS3 - anode
10 CSS5 - anode
pin
signal
3
CSS2 - cathode
7
CSS4 - cathode
11 CSS6 - cathode
pin
signal
4
CSS2 - anode
8
CSS4 - anode
12 CSS6 - anode
4.4 Mass
The mass of the QB50 ADCS stack and optional and external components are summarized in the table.
Table 5 mass of ADCS components
Component
PC104 component stack (excluding GPS receiver)
External components
External magnetometer assembly (including boom)
6x CSS (including harness)
Mass
335g
15g
5g
4.5 Inertia moments and Centre-of-mass
The moment of inertia about any satellite body axis for the host satellite on which the QB50 ADCS will be used
shall not exceed 0.02 kgm2.
The host satellite shall have a dominant moment of inertia about the ADCS Y-axis for configurations in which
the detumbling mode will be used. The Y moment of inertia (Iyy) shall be at least 5% larger than the X and Z
inertia moments (Ixx and Izz ).
The host satellite centre-of-mass and deployable panels should be arranged such that a positive (restoring)
aerodynamic torque is generated for all possible angles of attack.
5 Command interface
The ADCS attitude control processor (CubeComputer) acts as an I2C slave node on the system bus with 7-bit
addressing. The default 8-bit read and write addresses of the node are:
Table 6 I2C node address
I2C write address
0x24
I2C read address
0x25
For more detail on the communications interface including protocols for telecommands, telemetry and file
downloads, please consult the QB50 ADCS Reference Manual [R05].
6 Specifications
Table 7 QB50 ADCS specifications
Specification
Physical
Mass
Value
Notes
400g
Complete system including GPS receiver
and deployable magnetometer boom
Dimensions
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PC104 stack
CSS
External magnetometer housing
Performance
Attitude update rate
Attitude measurement accuracy (>200 km)
Pitch
Roll and yaw
Pointing accuracy (Y-momentum mode)
> 300km altitude
> 200km altitude
Time to reach steady-state Y-Thomson
motion from 10°/s initial tip-off rate (at
350km altitude)
Outgassing
Total Mass Loss (TML)
Collected Volatile Condensable Material
(CVCM)
Vibrations
Sine sweep
Random vibrations
Thermal
Operating temperature range
Non-operational temperature range
QB50
90 x 96 x 60
mm
4 x 11 x 2 mm
16 x 17 x 6 mm
1 Hz
< 0.5°
< 2.0°
< 0.5° 1
< 2.5° 1
< 0.5 days 1
1σ
1σ
1σ
1σ
Maximum time from Monte-carlo
simulation of 1000 test cases.
< 1.0 %
< 0.1 %
as per [R01]
as per [R01]
-10°C to +60°C
-10°C to +60°C
1
Applies to 2U satellite with no appendages, centre-of-mass location [0.01, 0.0, 0.0] T m and moment of inertia
matrix diag (0.0027, 0.0107, 0.0101) kg.m2
The specifications for the sensors and actuators used in the QB50 ADCS are listed in the following tab les.
6.1 Sensor specifications
The sensor specifications below apply when requesting the measurements via the system I2C interface.
Individual components may have different update rates when interfacing to them directly.
Table 8 Sensor specifications
Specification
Sun and nadir sensor
Sun sensor measurement accuracy (1σ)
Nadir sensor measurement accuracy (1σ)
Field of view (both cameras)
Update rate
Image size
Image format
Coarse sun sensor
Measurement accuracy
Update rate
Magnetometer
Value
Notes
< 0.5°
< 2°
0.18°
180°
1 Hz
640 x 480 pix
8 bpp grayscale
< 40° from bore-sight
entire FOV
Entire Earth visible in FOV
< 10°
1 Hz
1σ
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ADCS Interface Control Document
Measurement accuracy
Update rate
Rate sensor
Measurement accuracy
Update rate
Angular Random Walk
Bias instability
version 3.2
QB50
< 50 nT
1 Hz
1σ (per X/Y/Z channel)
< 0.015 °/s
1 Hz
0.28°/√ℎ𝑟
24 °/hr
1σ
Averaged output
6.2 Actuator specifications
Table 9 Actuator specifications
Specification
Magnetorquers
Maximum magnetic dipole
On-time command resolution
Momentum wheel
Maximum momentum storage
Maximum wheel speed
Maximum torque
Wheel inertia
Value
0.2 Am2
10 ms
Notes
For a 1Hz control period
1.7 mNms
± 8000 rpm
0.15 mNm
2.0 kg.mm2
7 Handling
Anti-static
The bundle contains a variety of static sensitive devices. The appropriate electrostatic protection measures
must thus be implemented. The unit must never be handled without proper grounding.
Cleanliness
It is recommended that the ADCS unit be handled in a clean environment. A clean room of ISO class 8 or higher
or an appropriate laminar flow workbench is recommended.
Moisture
The unit should be kept free of moisture or liquids. Liquids and moisture could have corrosive effects on the
electronics and electronic joints which may lead to degradation and loss of reliability of the circuits.
Shock
The unit must be handled with care and dropping or bumping the unit should be completely avoided.
Camera lens cleanliness
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The camera lenses should be kept clean and free of any dirt that may obstruct the images captured by the
camera. Dust should be removed with a cloth. If required, the lens may be cleaned using ethanol and
appropriate lens cleaning equipment, but unnecessary cleaning of the lens should be avoided.
Camera lens structural integrity
The camera units are aligned to be parallel with PCB. This is important since misalignment of the cameras
influence the ADCS performance of the system. External forces on the camera module should thus be avoided
completely.
Camera lens covers
The sun and nadir optics are fitted with dust caps which should be removed before flight.
Momentum Wheel
The Aluminium shell in which the momentum wheel is enclosed should NOT be tampered with. Tampering
with this casing may damage the wheel. No attempt should be made to loosen or remove the fasteners that
secure the casing.
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