7.4 Design Methodology
245
(3) Diagnosis and handling methods of the hovering and below: If the speed of the
lunar lander body axis Y B or Z B was large, the horizontal position control was
used to reach the safe landing point.
(4) Propellant shortage: If the 7500 N throttling thrust engine had a lower actual
thrust than the nominal value, it would cause the main deceleration to take
longer and consume more propellant, causing inadequate propellant for followup phases, in this case, the strategy of cancel the approaching was used to reduce
propellant consumption.
(5) Safe mode guidance law design: The principles of safe mode guidance law
were: prefer to ensure vertical safety and propellant consumption as less as
possible. The vertical optimization guidance law was adopted to meet the
vertical height and speed constraints, and guide the lunar lander to reach the
target altitude at a predefined velocity, and ensure vertical safety. To simplify
calculations and satisfy vertical constraints, only vertical channel used polynomial guidance laws. The horizontal guidance law, in order to reduce the
consumption of propellant as much as possible, controlled the velocity and
acceleration only in the horizontal direction without controlling the position,
and also eliminated the horizontal velocity as quickly as possible to reduce
unsafe landing factors.
7.5 Typical Technology
7.5.1 Procedure of GNC Powered Descent Phase [8]
GNC subsystem hardware during the powered descent phase included sensors, actuators, and controllers. The close loop of GNC subsystem of the Chang’E-3 lunar
lander during the powered descent phase is shown in Fig. 7.9. The controller was
divided into a control computer and an image processing unit. The navigation, guidance and attitude control algorithms were run on the control computer; the lunar
surface hazard identification and safe landing zone selection algorithms were run
on the image processing unit. The control computer and the image processing unit
could exchange information and data.
The general work flow of guidance, navigation and control of powered descent
phase was: the navigation sensor measured and sent the measurement data to the
control computer; the navigation algorithm processed the navigation measurement
data and sent the navigation results to the guidance and attitude control algorithm;
the guidance algorithm was based on navigation results and guidance parameters to
form the main engine command and attitude command, and the attitude command
was sent to the attitude control algorithm; the attitude control algorithm formed the
thrusters command according to the navigation results and the attitude command; the
control computer sent the main engine thrust command and the thrusters command
to the main engine and thrusters, then the main engine and thrusters executed the
commands.
245
(3) Diagnosis and handling methods of the hovering and below: If the speed of the
lunar lander body axis Y B or Z B was large, the horizontal position control was
used to reach the safe landing point.
(4) Propellant shortage: If the 7500 N throttling thrust engine had a lower actual
thrust than the nominal value, it would cause the main deceleration to take
longer and consume more propellant, causing inadequate propellant for followup phases, in this case, the strategy of cancel the approaching was used to reduce
propellant consumption.
(5) Safe mode guidance law design: The principles of safe mode guidance law
were: prefer to ensure vertical safety and propellant consumption as less as
possible. The vertical optimization guidance law was adopted to meet the
vertical height and speed constraints, and guide the lunar lander to reach the
target altitude at a predefined velocity, and ensure vertical safety. To simplify
calculations and satisfy vertical constraints, only vertical channel used polynomial guidance laws. The horizontal guidance law, in order to reduce the
consumption of propellant as much as possible, controlled the velocity and
acceleration only in the horizontal direction without controlling the position,
and also eliminated the horizontal velocity as quickly as possible to reduce
unsafe landing factors.
7.5 Typical Technology
7.5.1 Procedure of GNC Powered Descent Phase [8]
GNC subsystem hardware during the powered descent phase included sensors, actuators, and controllers. The close loop of GNC subsystem of the Chang’E-3 lunar
lander during the powered descent phase is shown in Fig. 7.9. The controller was
divided into a control computer and an image processing unit. The navigation, guidance and attitude control algorithms were run on the control computer; the lunar
surface hazard identification and safe landing zone selection algorithms were run
on the image processing unit. The control computer and the image processing unit
could exchange information and data.
The general work flow of guidance, navigation and control of powered descent
phase was: the navigation sensor measured and sent the measurement data to the
control computer; the navigation algorithm processed the navigation measurement
data and sent the navigation results to the guidance and attitude control algorithm;
the guidance algorithm was based on navigation results and guidance parameters to
form the main engine command and attitude command, and the attitude command
was sent to the attitude control algorithm; the attitude control algorithm formed the
thrusters command according to the navigation results and the attitude command; the
control computer sent the main engine thrust command and the thrusters command
to the main engine and thrusters, then the main engine and thrusters executed the
commands.