Satellite state machine

Nine modes can be distinguished in the logic of the satellite’s payload operation. Five of them, that is the default logic, are shown in the diagram below.

KRAKsat state machine

Mode name	Mode ID
IDLE	0
DETUMBLING	1
DETUMBLING_F	2
EXPERIMENT	3
EXPERIMENT_F	4
MANUAL	5
DIAGNOSTIC	6
DIAGNOSTIC_F	7
IDLE_FOR_TIME	8
CURRENT_MODE	9
DETUMBLING_SR_MODE	10

Mode structure definition

Property	Description
mode_id	The id of the mode.
init_function	The function that is executed within initialization of the mode.
exit_function	The function that is executed immediately on the exit of the mode.
condition_exit	The function that returns state enum when at least one of stop conditions of the mode is fulfilled.
condition_entry	The function that returns state enum when all of begin conditions of the mode are fulfilled.
do_custom_function	The function that contains logic that is executed not in 1min/1s/200ms timer exception.
do_1min_cycle	The function that contains logic that is executed in 1min timer exception.
do_1000ms_cycle	The function that contains logic that is executed not in 1s timer exception.
do_200ms_cycle	The function that contains logic that is executed not in 200ms timer exception.
timer_1s	The default value that determines the maximum time of the mode duration [s].
timer_prescaler	The value that is prescaler for property timer_1s.
is_timer_active	The property that determines if timer timer_1s is active.
next_mode	The mode that will be set as a following mode if condition_entry equals SUCCESS.
failure_mode	The mode that will be set as a following mode if condition_entry equals FAIL.

IDLE

IDLE mode is a low power consumption mode that is set as a middle mode in which the state machine waits for the fulfilment of the next mode’s begin conditions.

Property	Description
mode_id	0
init_function	None
exit_function	None
condition_exit	The stop condition is equal to the begin condition of the next mode
condition_entry	None
do_custom_function	None
do_1min_cycle	IMU magnetometer sensor refresh
do_1000ms_cycle	None
do_200ms_cycle	None
timer_1s	n/a
timer_prescaler	1
is_timer_active	False
next_mode	Diagnostic
failure_mode	Diagnostic

Diagnostic

Diagnostic mode’s goal is to make measurements and control the actuator in such order in time to allow an observer to determine if all sensors and actuators are working correctly. The table bellow shows how measurement and actuator control is performed in time.

Property	Description
mode_id	6
init_function	Initialization of all actuators.
exit_function	Stop of all actuators.
condition_exit	If power voltage < 9.5 then WAIT IN IDLE; If temp F.R.W. temperature > 90 then FAIL; If timer_1s <= 0 then SUCCESS;
condition_entry	If power voltage > 10 then SUCCESS.
do_custom_function	None
do_1min_cycle	None
do_1000ms_cycle	The function is described in section "Diagnostic algorithm"
do_200ms_cycle	None
timer_1s	100
timer_prescaler	1
is_timer_active	True
next_mode	Detumbling
failure_mode	Detumbling

Diagnostic Force

Diagnostic force mode has the same function as Diagnostic mode. Only the entry and exit conditions differ.

Property	Description
mode_id	7
init_function	Initialization of all actuators.
exit_function	Stop of all actuators.
condition_exit	If power voltage < 9 then WAIT IN IDLE; If timer_1s <= 0 then SUCCESS;
condition_entry	If power voltage > 10 then SUCCESS.
do_custom_function	None
do_1min_cycle	None
do_1000ms_cycle	The function is described in section "Diagnostic algorithm"
do_200ms_cycle	None
timer_1s	100
timer_prescaler	1
is_timer_active	True
next_mode	Detumbling
failure_mode	Detumbling

F.R.W. Experiment

F.R.W. experiment mode’s goal is to control electromagnets in order to conduct acceleration of the ferrofluid. Its custom function is turned on by interrupts from timer 5.

Property	Description
mode_id	3
init_function	Initialization of PWM timers for F.R.W. magnetorquers.
exit_function	Stop of PWM timers for F.R.W. magnetorquers.
condition_exit	If power voltage < 9.5 then WAIT IN IDLE; if temp F.R.W. temperature > 90 then FAIL; if timer_1s <= 0 then SUCCESS;
condition_entry	If power voltage > 10 and temp F.R.W. temperature > 0 and temp F.R.W. temperature < 50 then SUCCESS;
do_custom_function	The function logic is described in section "F.R.W. control algorithm".
do_1min_cycle	None
do_1000ms_cycle	None
do_200ms_cycle	None
timer_1s	180
timer_prescaler	1
is_timer_active	True
next_mode	IDLE time
failure_mode	IDLE time

F.R.W. Experiment Force

F.R.W. experiment force mode has the same function as F.R.W. experiment mode. Only the entry and exit conditions differ.

Property	Description
mode_id	3
init_function	Initialization of PWM timers for F.R.W. magnetorquers.
exit_function	Stop of PWM timers for F.R.W. magnetorquers.
condition_exit	If power voltage < 9 then WAIT IN IDLE; if timer_1s <= 0 then SUCCESS.
condition_entry	If power voltage > 10 then SUCCESS.
do_custom_function	The function logic is described in section "F.R.W. control algorithm".
do_1min_cycle	None
do_1000ms_cycle	None
do_200ms_cycle	None
timer_1s	180
timer_prescaler	1
is_timer_active	True
next_mode	IDLE time
failure_mode	IDLE time

Detumbling

Detumbling mode’s goal is to reduce the angular velocity of the satellite. The implemented algorithm calculates and adjusts a control on 3 magnetorquers based on IMU sensor data. If detumbling doesn't succeed, the payload will go to Detumbling SR mode and trigger the algorithm embedded in the ADCS module.

Property	Description
mode_id	1
init_function	Initialization of PWM timer for ADCS magnetorquers.
exit_function	Stop of PWM timer for ADCS magnetorquers.
condition_exit	If power voltage < 9.5 or temp F.R.W. temperature > 80 then WAIT IN IDLE; If timer_1s <= 0 then FAIL; If average angular speed in all axes < 5°/s then SUCCESS;
condition_entry	If power voltage > 10 and temp F.R.W. temperature < 60 then SUCCESS.
do_custom_function	None
do_1min_cycle	None
do_1000ms_cycle	None
do_200ms_cycle	The function logic is described in section "Detumbling algorithm".
timer_1s	10800
timer_prescaler	1
is_timer_active	True
next_mode	F.R.W. experiment
failure_mode	Detumbling SR

Detumbling force

Detumbling force mode has the same function as Detumbling mode. Only the entry and exit conditions differ.

Property	Description
mode_id	2
init_function	Initialization of PWM timer for ADCS magnetorquers.
exit_function	Stop of PWM timer for ADCS magnetorquers.
condition_exit	If power voltage < 9 then WAIT IN IDLE; If timer_1s <= 0 then FAIL; If average angular speed in all axes < 5°/s then SUCCESS.
condition_entry	If power voltage > 10 then SUCCESS.
do_custom_function	None
do_1min_cycle	None
do_1000ms_cycle	None
do_200ms_cycle	The function logic is described in section "Detumbling algorithm".
timer_1s	10800
timer_prescaler	1
is_timer_active	True
next_mode	F.R.W. experiment
failure_mode	F.R.W. experiment

Detumbling SR

Detumbling SR mode triggers detumbling algorithm embedded in the OBC board. No action is performed by the microcontroller in the Payload module.

Property	Description
mode_id	10
init_function	Initialization of detumbling algorithm embedded in the OBC board.
exit_function	Stop of detumbling algorithm embedded in the OBC board.
condition_exit	If OBC transmit SUCCESS then SUCCESS; If timer_1s <= 0 then FAIL.
condition_entry	None
do_custom_function	None
do_1min_cycle	Check register table if OBC transmit SUCCESS.
do_1000ms_cycle	None
do_200ms_cycle	None
timer_1s	2700
timer_prescaler	1
is_timer_active	True
next_mode	F.R.W. experiment
failure_mode	F.R.W. experiment

Manual

Manual mode is a mode that controls ADCS magnetorquers based on commands from the ground station.

Property	Description
mode_id	5
init_function	Initialization of PWM timer for ADCS magnetorquers.
exit_function	Stop of PWM timer for ADCS magnetorquers.
condition_exit	If timer_1s <= 0 then SUCCESS; If battery_volt<9 then FAILURE.
condition_entry	If battery_volt>10 then SUCCESS.
do_custom_function	None
do_1min_cycle	None
do_1000ms_cycle	ADCS magnetorquers control and IMU magnetometer sensor refresh.
do_200ms_cycle	None
timer_1s	10800* timer_prescaler
timer_prescaler	1
is_timer_active	True
next_mode	IDLE
failure_mode	IDLE

IDLE time

IDLE TIME mode is a low power consumption mode that is set to wait for a long period of time for recharging batteries and cooling coils. IDLE TIME mode lasts for 48h or while special log buffer is not full and the battery is charged above 10V.

Property	Description
mode_id	8
init_function	None
exit_function	None
condition_exit	If timer_1s <= 0 or (battery_volt>10 and special log buffer is not full) then SUCCESS;
condition_entry	None
do_custom_function	None
do_1min_cycle	IMU magnetometer sensor refresh
do_1000ms_cycle	None
do_200ms_cycle	None
timer_1s	2880 * timer_prescaler
timer_prescaler	60
is_timer_active	True
next_mode	Diagnostic mode
failure_mode	Diagnostic mode

Diagnostic algorithm

The diagnostic algorithm is used to validate proper functioning of satellite's hardware. This procedure toggles certain actuators in predefined intervals. Simultaneously, measurements from the IMU and the temperature sensor that is attached to the payload container are taken.

Timeline	Step
15 s	Only measurements
10 s	ADCS magnetorquer X
10 s	ADCS magnetorquer Y
10 s	ADCS magnetorquer Z
15 s	Only measurements
10 s	F.R.W. electromagnets phase 0
10 s	F.R.W. electromagnets phase 1
10 s	F.R.W. electromagnets phase 2
10 s	F.R.W. electromagnets phase 3

F.R.W. control algorithms

F.R.W. control algorithms are designed to control electromagnets in order to accelerate the ferrofluid. Its custom function is turned on by interruption from timer 5. F.R.W. can work in 9 different modes which are listed in the table below. More information about control system of the Ferrofluid Reactio Wheel can be found in: Jan Życzkowski, "Ferrofluid reaction wheel for nanosatellites orientation control", Master Thesis, AGH University of Science and Technology, Cracow 2019, supervisor: Adam Piłat.

Control mode	mode ID
FRW_BAISIC_CLOCKWISE	0
FRW_BAISIC_ANTICLOCKWISE	1
FRW_ACC_CLOCKWISE	2
FRW_ACC_ANTICLOCKWISE	3
FRW_STEP_CLOCKWISE	4
FRW_STEP_ANTICLOCKWISE	5
FRW_PID_DUTY	6
FRW_PID_FREQ	7
FRW_LIST_MODE	8

FRW_ControlBasicClockwise:

FRW_ControlBasicClockwise works as a universal control. It is possible to change its parameters:

Parameter	Details
pwm_duty	duty of coils PWM
period	period with which 4 coils are working, in ms. Period of the is F.R.W. two times larger
pwm_direct	direction of the current going through coils (it is not the direction for F.R.W.)
control duty	percentage of time every phase is tuned on during cycle (e.g. 25 - one-step, 37 - one and half-step, 50 - two-step)

To control F.R.W. in opposite direction use function: FRW_ControlBasicAntiClockwiseFunction.

FRW Baisic control diagram

FRW_ControlAcceleratingClockwise:

FRW_ControlAcceleratingClockwise works like the basic mode, with changing frequency over time. It has some additional parameters:

Parameter	Details
start_period	Starting period for 4 coils
stop_period	ending period for 4 coils
acceleration_time	Time of acceleration in seconds. After this time, F.R.W.will work with period: stop_period

To control F.R.W. in opposite direction use function: FRW_ControlAcceleratingAntiClockwise.

FRW acceleration control diagram

Algorithm FRW_ControlStepClockwise:

FRW_ControlStepAntiClockwise is an experimental asymetric control with the following parameters:

Parameter	Details
period	period with which 4 coils are working, in ms
coils_on_time	Time of one phase turned on, in ms
phase_delay	delay in turning on phases, in ms
pwm_direct	direction of the current going through coils (it is not the direction for F.R.W.)
pwm_duty	duty of coils PWM

The function works when:

• period>3∗phase_delay+coils_on_time

• coils_on_time>2∗phase_delay

To control F.R.W. in the opposite direction use function: Algorithm FRW_ControlStepAntiClockwise.

FRW Step control diagram

FRW_ControlPID_duty:

FRW_ControlPID_duty aim is to stabilize satellite rotation according to Z axis to value given in INIT_FRW_MODE_DUTY (degrees/s). It uses mesurements done by gyroscope. There are 3 constants: Ki_duty, Kp_duty and Kd_duty - which can be changed from Earth using IMU paramconfig.

FRW_ControlPID_frequency:

The purpose of FRW_ControlPID_frequency is to stabilize satellite rotation according to Z axis to value given in INIT_FRW_MODE_FREQ (degrees/s). It uses mesurements done by gyroscope. There are 3 constants: Ki_freq, Kp_freq and Kd_freq - which can be changed from the Earth using IMU paramconfig.

FRW_Control_list:

FRW_Control_list enables the user to create their own control. It is done by performing all instructions from the control list. To add an instruction to the list, use command 240 which consists of:

Parameter	Details
coil	phase which should be changed
time	time till next action [ms]
dir	direction of current in phase - 0 for clockwise, 1 for anticlockwise
duty	duty for changed phase
reset	1- for cleaning all data on the list, 0 - to add instruction

Detumbling algorithm

Detumbling modes high level logic

Detumbling high level logic

Detumbling algorithm

The payload detumbling algorithm uses magnetorquers in all three satellite axes. A magnetorquer is an electromagnet which due to current flow creates a magnetic moment which interfaces with the Earth'a magnetic field. This creates a torque acting on the satellite. Thus, being able to control the current flowing through the coil (the words coil and electromagnet will be used interchangeably) enables controling the angular velocity of the satellite. The equation used to calculate the torque created by the electromagnet is shown below:

$$\vec{M}=\vec{\mu} \times \vec{B}$$

Where $\vec{M}$is the created torque, $\vec{\mu}$is the magnetic moment and $\vec{B}$ is the Earth's magnetic field's induction.

The operation of a magnetorquer

The algorithm is based on the so-called Angular Rate Feedback algorithm. The equation used to calculate the needed current in each of the coils is shown below (the current in each coil is one of the components of the current vector):

​

$$\dot{\vec{B}} = \vec{B}\times\omega$$

$$\vec{i} = -k \frac{\dot{\vec{B}}}{ \left\lVert B \right\rVert ^2}$$

The optimal value of k was chosen after conducting simulations on a model of the satellite. An appropriate PWM signal is passed on the inputs of H-bridges, which control the current of the magnetorquers.

One cycle of the algorithm consists of five steps; the time interval between those steps is 200 ms (so the whole algorithm cycle is 1s).

• Step 1: At the beginning of this step, the current in all coils is always zero. The magnetic field induction (B) is measured (because no current flows through the magnetorquers, their magnetic field has no influence on the measurement). Also the angular velocity of the satellite is acquired. The output current of the coils is calculated using the above equation and the current of the coils is changed.

• Step 2, 3, 4: In each of these steps the magnetic field induction in the satellite-oriented reference frame is estimated and the angular velocity is measured. Apart from that, the operations are the same as in step 1

• Step 5: The current of the coils is set to 0, so that the magnetic field induction can be measured in step 1. Return to step 1.

The estimation of the magnetic field induction in the satellite-oriented reference frame is conducted as follows: it is assumed that it is constant in the Earth-oriented frame and it changes because of the non-zero angular velocity of the satellite (which is measured). So, the magnetic field induction can be estimated as:

$$\vec{B}(t+\Delta t) \approx \vec{B}(t) + \dot{\vec{B}}\Delta t,$$

where:

$$\Delta t = 200 ms.$$

More information about detumbling system can be found in: Artur Hadasz, "Simulation of stabilization algorithms and building a prototype of a stabilization system for picosatellites using magnetorquers", BSc Thesis, AGH University of Science and Technology, Cracow 2018, supervisor: Paweł Zagórski and in the work: Piotr Mikołajek, "Design and simulation of a picosatellite low Earth orbit magnetic field based attitude determination system", BSc Thesis, AGH University of Science and Technology, Cracow 2018, supervisor: Paweł Zagórski.