CN113297679B - Propellant mass flow observation method of variable thrust rocket engine - Google Patents
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Abstract
The invention discloses a propellant mass flow observation method of a variable thrust rocket engine, which comprises the following steps: s1, obtaining a state space expression of the electric pump system according to the kinetic equation of the motor and the pump; s2, discretizing a state space expression of the electric pump system; s3, designing a Kalman filter through a program according to the discretization relational expression of the electric pump system; and S4, bringing the Kalman filter into the whole system pipeline, observing the mass flow of the liquid oxygen path and the methane path, judging whether the set filtering effect is achieved, if so, completing the design, otherwise, repeating the step S3 and adjusting the noise covariance and the initial prediction error covariance matrix in the Kalman filter design process until the set filtering effect is achieved. The method can effectively weaken the error of the environmental noise on the mass flow measurement, and can obtain a real and reliable mass flow value of the propellant.
Description
Technical Field
The invention relates to the technical field of aerospace technology and control, in particular to a propellant mass flow observation method of a variable thrust rocket engine.
Background
With the development of the aerospace field, many countries are beginning to focus on the research of advanced technologies such as aircraft repeatable recovery or planet surface soft landing. In order to realize soft landing, the depth variable thrust liquid rocket engine is an indispensable power device, and particularly for the surface soft landing of an atmospherical-free planet, the variable thrust liquid rocket engine is the only power device for realizing the soft landing.
For a single liquid rocket engine, thrust can be achieved by changing the type of propellant, the flow rate of the propellant, the outlet area of the nozzle and the throat of the nozzle. But due to the limitations of physical structure and heat flow, it is difficult to change the propellant type, nozzle throat and outlet area. Adjusting mass flow is the simplest way to adjust engine thrust. With the development of motor and battery technologies, the electric pump liquid rocket engine has the characteristics of low cost, high reliability, simple adjustment, easy realization of depth-to-thrust and the like, and is more and more valued.
For variable thrust liquid rocket engines, the accuracy of propellant mass flow observation determines the accuracy of thrust adjustment. The mass flow of the propellant is effectively observed and controlled, and the method has great significance for realizing the task of soft landing on the surface of the planet. However, in the case of a complex mechanical structure such as a rocket engine, strong noises such as the booming of the engine and various mechanical vibrations are accompanied during operation. Therefore, in order to accurately observe the mass flow of the propellant in the working process of the variable thrust rocket engine, a corresponding method for observing the mass flow of the propellant of the variable thrust rocket engine needs to be designed.
Disclosure of Invention
The invention aims to provide a propellant mass flow observation method of a variable thrust rocket engine, which overcomes the defects in the prior art.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a propellant mass flow observation method of a variable thrust rocket engine comprises the following steps:
s1, obtaining a state space expression of the electric pump system according to the kinetic equation of the motor and the pump;
s2, discretizing a state space expression of the electric pump system;
s3, designing a Kalman filter through a program according to the discretization relational expression of the electric pump system;
and S4, bringing the Kalman filter into the whole system pipeline, observing the mass flow of the liquid oxygen path and the methane path, judging whether the set filtering effect is achieved, if so, completing the design, otherwise, repeating the step S3 and adjusting the noise covariance and the initial prediction error covariance matrix in the Kalman filter design process until the set filtering effect is achieved.
Further, the state space expression of the motor-driven pump system can be expressed as:
Further, the kinetic equation of the motor and pump is:
wherein, UmIs the motor voltage, RmIs the motor coil resistance, imIs the motor current, LmIs the motor inductance, t is the time, emIs the motor back electromotive force, ω is the motor rotational angular velocity, CeIs the motor back electromotive force and torque coefficient, Ce0Is the motor back electromotive force and the torque coefficient constant, ωRIs a reference rotational angular velocityThe degree of the magnetic field is measured,is the mass flow rate of the fuel or oxidant,is a reference value for the mass flow of fuel or oxidant, ImIs the moment of inertia of the motor, fmIs the friction coefficient of the motor, T is the load moment driving the pump, IF is the fluid inertia of the electric pump, TRElectric pump reference torque, theta is electric pump characteristic angle, N is electric pump specific speed, WH and WT are theta and Nsρ is the average density of the fluid entering and exiting the pump, g is the acceleration of gravity, a1And a2Is a constant coefficient;
three state variables x1,x2And x3Are respectively imω andthe input quantity U and the output quantity y of the system are U respectivelymAnd
wherein:
further, in the step S2, a four-step lattice stoke method is adopted to discretize the system state space expression, so as to obtain:
yk+1=h(Xk+1,uk+1)=xk+1,3;
where Δ t is the discretized time step, f1、f2、f3And f4Can be represented as:
f1=f(Xk,uk);
f4=f(Xk+Δtf3,uk)。
further, the step S3 specifically includes the following steps,
s31, giving the mean value of the initial state at time step kAnd an initial prediction error covariance matrix Pk|k;
S32, calculating the sigma point of the time step k:
wherein n is a state variable XkThe dimension(s) of (a) is,is a matrix (n + lambda) Pk|kCholesky decomposes the ith column vector, and λ may be expressed as λ ═ α2(n+k)-n;
S33, calculation through a nonlinear modelObtaining a new predicted mean value of the state variablesAnd the prediction covariance matrix Pk+1|k;
By means of the new sigma pointThe mean value of the predicted values of the measurement results is as follows:
measuring the covariance matrix Pyy,k+1|kAnd a cross-covariance matrix P of state variables and measurementsxy,k+1|kExpressed as:
s35, calculating Kalman gain K of the kth time stepk+1Mean value of state variablesSum covariance matrix Pk+1|k+1;
Kk+1=Pxy,k+1|k(Pyy,k+1|k)-1
Pk+1|k+1=Pk+1|k-Kk+1Pyy,k+1|kKk+1 T。
Further, the kalman filter is an unscented kalman filter.
Compared with the prior art, the invention has the advantages that: the invention can effectively weaken the error of the environmental noise on the mass flow measurement, can obtain the real and reliable propellant mass flow value, can provide more accurate state or output feedback for the design of the controller on one hand, and also provides certain technical support for realizing the fault detection and diagnosis of the variable thrust process and eliminating the sensor and the environmental interference on the other hand.
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In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a flow chart of the propellant mass flow observation method of the variable thrust rocket engine of the present invention.
Detailed Description
The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, and the scope of the present invention will be more clearly and clearly defined.
Referring to fig. 1, the embodiment discloses a propellant mass flow observation method for a variable thrust rocket engine, which adopts an approximately linear control strategy to control a nonlinear system, and mainly comprises the following steps:
and step S1, obtaining a state space expression of the electric pump system according to the kinetic equation of the motor and the pump.
In this embodiment, the motor dynamic equation mainly includes three partial voltage balance equations, an electromagnetic torque equation, and a motor torque balance equation.
The voltage balance equation of the armature coil of the direct current motor is as follows:
wherein U ismIs the motor voltage, RmIs the coil resistance imIs a current, LmIs the inductance, t is the time, emIs the back electromotive force and can be expressed as:
em=Ceω (2)
where ω is the angular speed of rotation of the motor, CeIs the motor back emf and torque coefficient, which can be expressed as:
Ce=Ce0im (3)
wherein C ise0Is the motor back emf and torque coefficient constant.
The electromagnetic torque equation of the motor is
M=imCe (4)
Where M is the output torque of the motor.
The torque balance equation of the motor is as follows:
wherein, ImIs the moment of inertia of the motor, fmIs the coefficient of friction of the motor and T is the load torque driving the pump.
The following is the pump kinetic equation, first defining two dimensionless parameters, as follows:
wherein v is the dimensionless volume flow rate, Q is the volume flow rate, QRIs a reference volumetric flow rate; alpha is the dimensionless speed, N is the pump speed, NRIs the reference rotational speed.
The fluid inertia IF, reference torque T is given nextRCharacteristic angle theta and specific rotation speed NsDefinition of (1):
where ρ isRFor reference density, LRFor pump reference flow path length, ARReference flow passage cross-sectional area, HRFor reference pump head, ηRFor reference efficiency, ρ is the average density of the fluid entering and exiting the pump, and g is the acceleration due to gravity.
The pump head and pump torque can be expressed as:
H=HRWH(a2+ν2) (11)
T=TRWT(α2+v2) (12)
wherein WH and WT are θ and NsThe function of (2) can be obtained by table lookup interpolation. For the same electric pump, NsIs a fixed value, therefore WH and WT are univariate functions of θ, which can be fitted experimentally, andreference may be made to the work in published literature.
Further, a dynamic equation of the volume flow rate of the centrifugal pump can be obtained:
from the volume flow rate, a mass flow expression can be obtained:
from the relationship between rotational speed and angular velocity and the relationship between mass and density, equation (6) can be written as follows:
whereinWhich represents the mass flow of fuel or oxidant,is a reference value for the mass flow of fuel or oxidant, w is the angular rotation velocity, and wR is the reference angular rotation velocity.
In equation (13), there is a pump boost pressure, and the pump boost pressure is related to the mass flow rate by fitting data:
wherein a is1And a2Are constant coefficients, i.e., two coefficients within the relationship between pump boost and mass flow.
Combining equations (1) - (16), the system of kinetic equations can be obtained as:
three state variables x defining a system1,x2And x3Are respectively imω andthe input quantity U and the output quantity y of the system are U respectivelymAndthe state space expression for the system can be expressed as:
wherein:
h(X,u)=x3 (20)
and step S2, discretizing a state space expression of the motor-driven pump system.
In this embodiment, a four-order longge stoke method is adopted to discretize the system state space expression, so as to obtain:
yk+1=h(Xk+1,uk+1)=xk+1,3 (23)
where Δ t is the discretized time step, f1、f2、f3And f4Can be represented as:
f1=f(Xk,uk) (24)
f4=f(Xk+Δtf3,uk) (27)
step S3, designing a kalman filter by a program according to the discretized relational expression of the electric pump system, specifically including the following steps.
Step S31, giving the mean value of the initial state at time step kAnd an initial prediction error covariance matrix Pk|k:
Step S32, calculating a sigma point at time step k, i.e. a sigma point or a standard deviation point:
wherein n is a state variable XkThe dimension(s) of (a) is,is a matrix (n + lambda) Pk|kCholesky decomposes the ith column vector, and λ may be expressed as λ ═ α2(n + k) -n, typically, 0 ≦ α ≦ 1 and κ ≧ 0, thereby ensuring that the covariance matrix is semi-positive, typically, κ ≦ 0 or n + k ≦ 3.
Step S33, time updating, and obtaining through nonlinear model calculation:
further, a new predicted mean value of the state variables can be obtainedAnd the prediction covariance matrix Pk+1|k;
in the case of gaussian noise, β is typically set to 2.
Step S34, in the step of updating measurement, the sigma point is required to be based onAnd Pk+1|kRecalculating:
by means of the new sigma point it is possible to obtain:
the mean value of the predicted values of the measurement results is as follows:
measuring the covariance matrix Pyy,k+1|kAnd a cross-covariance matrix P of state variables and measurementsxy,k+1|kExpressed as:
step S35, calculating parameters, and calculating Kalman gain K of the kth time stepk+1Mean value of state variablesSum covariance matrix Pk+1|k+1:
Kk+1=Pxy,k+1|k(Pyy,k+1|k)-1 (45)
Pk+1|k+1=Pk+1|k-Kk+1Pyy,k+1|kKk+1 T (47)
And S4, bringing the Kalman filter into the whole system pipeline, observing the mass flow of the liquid oxygen path and the methane path, judging whether the set filtering effect is achieved, if so, completing the design, otherwise, repeating the step S3 and adjusting the noise covariance and the initial prediction error covariance matrix in the Kalman filter design process until the set filtering effect is achieved.
Although the embodiments of the present invention have been described with reference to the accompanying drawings, various changes or modifications may be made by the patentees within the scope of the appended claims, and within the scope of the invention, as long as they do not exceed the scope of the invention described in the claims.
Claims (6)
1. A propellant mass flow observation method of a variable thrust rocket engine is characterized by comprising the following steps:
s1, obtaining a state space expression of the electric pump system according to the kinetic equation of the motor and the pump;
s2, discretizing a state space expression of the electric pump system;
s3, designing a Kalman filter through a program according to the discretization relational expression of the electric pump system;
and S4, bringing the Kalman filter into the whole system pipeline, observing the mass flow of the liquid oxygen path and the methane path, judging whether the set filtering effect is achieved, if so, completing the design, otherwise, repeating the step S3 and adjusting the noise covariance and the initial prediction error covariance matrix in the Kalman filter design process until the set filtering effect is achieved.
3. The method of observing mass flow of propellant in a variable thrust rocket engine according to claim 2, wherein: the kinetic equation of the motor and the pump is as follows:
wherein, UmIs the motor voltage, RmIs the motor coil resistance, imIs the motor current, LmIs the motor inductance, t is the time, ω is the motor rotational angular velocity, Ce0Is the motor back electromotive force and the torque coefficient constant, ωRIs a reference to the angular speed of rotation,is the mass flow rate of the fuel or oxidant,is a reference value for the mass flow of fuel or oxidant, JmIs the moment of inertia of the motor, fmIs the friction coefficient of the motor, T is the load moment driving the pump, IF is the fluid inertia of the electric pump, TRElectric pump reference torque, theta is the electric pump characteristic angle, NsIs the specific speed of the electric pump, WH and WT are theta and Nsρ is the average density of the fluid entering and exiting the pump, g is the acceleration of gravity, a1And a2Is a constant coefficient;
three state variables x1,x2And x3Are respectively imω andthe input quantity U and the output quantity y of the system are U respectivelymAnd
wherein:
h(X,u)=x3。
4. the method of observing mass flow of propellant in a variable thrust rocket engine according to claim 1, wherein: in the step S2, a four-step lattice stoke method is adopted to discretize the system state space expression, so as to obtain:
yk+1=h(Xk+1,uk+1)=xk+1,3;
where Δ t is the discretized time step, f1、f2、f3And f4Can be represented as:
f1=f(Xk,uk);
f4=f(Xk+Δtf3,uk)。
5. the method of observing mass flow of propellant in a variable thrust rocket engine according to claim 1, wherein: the step S3 specifically includes the following steps,
s31, giving the mean value of the initial state at time step kAnd an initial prediction error covariance matrix Pk|k;
S32, calculating the sigma point of the time step k:
wherein n is a state variable XkThe dimension(s) of (a) is,is a matrix (n + lambda) Pk|kCholesky decomposes the ith column vector, and λ may be expressed as λ ═ α2(n+κ)-n;
S33, calculation through a nonlinear modelObtaining a new predicted mean value of the state variablesAnd the prediction covariance matrix Pk+1|k;
s34, in the step of updating measurement, the sigma point is required to be based onAnd Pk+1|kRecalculation
By means of the new sigma pointThe mean value of the predicted values of the measurement results is as follows:
measuring the covariance matrix Pyy,k+1|kAnd a cross-covariance matrix P of state variables and measurementsxy,k+1|kExpressed as:
s35, calculating Kalman gain K of the kth time stepk+1Mean value of state variablesSum covariance matrix Pk+1|k+1;
Kk+1=Pxy,k+1|k(Pyy,k+1|k)-1
Pk+1|k+1=Pk+1|k-Kk+1Pyy,k+1|kKk+1 T。
6. The method of observing mass flow of propellant in a variable thrust rocket engine according to claim 1, wherein: the Kalman filter is an unscented Kalman filter.
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