Disclosure of Invention
In order to solve the problem that the stability of the electric automobile is unstable due to excessive steering or insufficient steering under the extreme conditions of high speed, severe road and the like, the invention provides a method for controlling the stability envelope curve of the unmanned automobile direct yaw moment, which effectively improves the response speed and robustness of the system and improves the driving stability of the automobile under the extreme conditions of high speed, severe road and the like.
The technical scheme adopted by the invention for solving the technical problems is as follows:
the method for controlling the direct yaw moment of the unmanned vehicle based on the vehicle stable envelope line comprises the following steps:
step one, establishing a vehicle stability envelope line according to a centroid side slip angle and a yaw velocity which influence the vehicle stability;
secondly, acquiring a mass center slip angle and a yaw angular velocity of the actual vehicle through designed Kalman filtering;
step three, designing direct yaw moment through sliding mode control to keep the mass center side slip angle and the yaw velocity of the vehicle in the stable envelope line of the vehicle;
designing an approach rate to reduce buffeting of sliding mode control and generate a direct yaw moment;
and step five, the yaw moment distributor calculates the driving force or braking moment of four wheels of the actual vehicle according to the direct yaw moment calculated in the step four, so that the vehicle is kept stable.
The unmanned vehicle direct yaw moment controller based on the vehicle stability envelope line comprises a vehicle stability envelope line generator, a Kalman filter, a sliding mode controller and a yaw moment distributor;
the vehicle stability envelope generator: establishing a vehicle stable envelope line according to the centroid side slip angle and the yaw angular speed;
the Kalman filter: estimating to obtain a mass center side slip angle and a yaw angular velocity of the actual vehicle according to the lateral acceleration and the yaw angular velocity obtained by the actual vehicle;
the sliding mode controller: according to the obtained centroid side drift angle and yaw angular velocity, combining with a vehicle stable envelope line, establishing sliding mode control to generate direct yaw moment, keeping the centroid side drift angle and yaw angular velocity of the vehicle in a vehicle stable line, and designing an approach law to reduce buffeting of the sliding mode control;
the yaw moment distributor: and (3) distributing the driving force or the braking force according to the direct yaw moment generated by the slip film controller, and calculating the driving force or the braking moment of four wheels of the actual vehicle to keep the vehicle stable.
Further, the vehicle stability envelope is specifically:
and in a coordinate system with the yaw velocity of the vehicle as a vertical axis and the centroid slip angle as a horizontal axis, the stable state of the vehicle is quickly determined through the envelope line of one parallelogram. If the yaw angle and the centroid slip angle of the vehicle are within the parallelogram, the vehicle state is stable; if the vehicle yaw rate and the centroid slip angle are outside the parallelogram, then the vehicle condition is not stable.
Further, the kalman filter is designed as follows:
and establishing a linear two-degree-of-freedom vehicle dynamic equation containing parameter uncertainty and interference and noise influence according to Newton's law.
Wherein beta is the vehicle mass center slip angle, r is the yaw angular velocity, m and IZRespectively vehicle mass and moment of inertia,/FAnd lRFront axle and rear axle respectivelyDistance of heart, vxFor vehicle longitudinal speed, delta front wheel angle, CFAnd CRRespectively, front and rear wheel cornering stiffnesses.
And establishing a discrete system state equation and an observation equation by using the model.
x(k+1)=Ax(k)+Bδ(k)+ω(k)#(11)
y(k)=Hx(k)+Dδ(k)+v(k)#(12)
Wherein the system n-dimensional state vector
System state transition matrix
Random disturbance of system
Systematic observation vector
Observation matrix
System observation noise
State one-step prediction
And (3) state estimation calculation:
a filter gain matrix:
where R is the observed noise covariance matrix.
Estimating an error variance matrix:
P(k)=[1-K(k)H(k)]P(k)-#(15)
one-step prediction error matrix:
P(k)-=A(k)P(k)AT(k)+Q#(16)
and calculating the estimation of the state value by recursion of the observed value obtained by the real vehicle.
Further, the sliding mode controller is designed as follows:
defining a sliding surface s1Comprises the following steps:
where rho is [0,1 ]]To design the parameters, | Δ rmaxTo the maximum absolute value of the set yaw-rate error, | Δ β tintmaxThe maximum absolute value of the slip angle error.
Differential sliding surface s1Obtaining:
with a two-degree-of-freedom vehicle model (1), additional moments applied in the lateral direction can be obtained with respect to
The formula of (a):
substituting (19) into (18) to obtain
In order to reduce buffeting of sliding mode control, the invention designs an approach law with an exponential term:
wherein 0<p
1<1,p
2>0。
Wherein 0<c
0<1,c
1>0, c
2>0 and c
2E.g. N. H (S) is a positive value and does not affect the stability of the system. When the system is far from the sliding surface p
1H, (S) becomes smaller, p
2The ratio/H (S) is increased, thereby increasing the approaching speed and increasing the convergence speed to the sliding surface. When the system is close to the sliding surface p
1H, (S) and p
2The/h(s) are small, and therefore a small control gain is obtained, thereby reducing chattering.
Combining equations (22) and (21), the control rate is obtained as follows:
further, the yaw moment distributor is specifically designed as follows:
the longitudinal force distribution of the vehicle tire is distributed according to the axle load proportion, and the front and rear axle load values are as follows:
in the formula axFor longitudinal acceleration, FZF,FZRThe vertical load of the front and rear shafts is adopted, and h is the height of the mass center of the whole vehicle.
The longitudinal force of each wheel should satisfy the following conditions while satisfying the yaw moment and the total longitudinal force requirement:
the four tire forces are obtained according to the yaw moment and the vertical load, and the longitudinal force of each wheel distributed according to the load ratio is as follows:
in the formula Fx1、Fx2、Fx3、Fx4The longitudinal forces of the left front wheel, the right front wheel, the left rear wheel and the right rear wheel respectively, FxThe total longitudinal driving force is determined by the pedal opening, B is the wheel base, and L is the wheel base.
The invention has the beneficial effects that:
1. the invention establishes a vehicle stability envelope curve based on a two-degree-of-freedom vehicle model and a Pacejka tire model to obtain a centroid side deviation angle and yaw velocity envelope curve which can enable the vehicle to maintain stable, and the vehicle can maintain stable as long as the centroid side deviation angle and the yaw velocity of the actual vehicle are in the envelope curve. The problem of obtaining ideal slip angle and yaw rate under the limit working condition is solved.
2. The method comprises the steps of obtaining the mass center slip angle and the yaw angular velocity of an actual vehicle through designed Kalman filtering; the problem that the centroid slip angle is difficult to measure is solved.
3. According to the invention, the direct yaw moment is designed through sliding mode control to keep the mass center side slip angle and the yaw velocity of the vehicle in the stable envelope curve of the vehicle, so that the control failure caused by inaccurate modeling and external environment condition change of the system is solved, and the rapidity and the robustness of the system are improved.
4. According to the method, the buffeting of the sliding mode control is reduced by designing the approach rate, a direct yaw moment is generated, and the buffeting problem in the sliding mode control is solved.
5. The method is easy to realize and is suitable for wide popularization and application.
Detailed Description
The invention will be further explained with reference to the drawings.
As shown in fig. 1, the direct yaw moment controller of the electric vehicle based on the vehicle stability envelope of the present invention is implemented by a vehicle stability envelope generator, a sliding mode controller, a yaw moment divider, and a kalman filter. And a vehicle stability envelope generator that establishes a vehicle stability envelope based on the centroid slip angle and the yaw rate. The Kalman filter is used for obtaining lateral acceleration and yaw angular velocity through the actual vehicle and inputting the lateral acceleration and the yaw angular velocity into the Kalman filter to obtain a mass center lateral deviation angle and the yaw angular velocity of the actual vehicle; and the sliding mode controller is used for establishing sliding mode control to generate direct yaw moment according to the obtained mass center side offset angle and yaw velocity in combination with the vehicle stable envelope line, so that the mass center side offset angle and the yaw velocity of the vehicle are maintained in the vehicle stable line, and a closing law is designed to reduce buffeting of the sliding mode control. And a yaw moment distributor for distributing the driving force or the braking force by passing the obtained direct yaw moment through the yaw moment distributor. The invention realizes the driving stability of the automobile under the conditions of high-speed turning and obstacle avoidance.
As shown in fig. 2, a vehicle stability envelope is constructed for the centroid slip angle and yaw rate.
The invention relates to an electric vehicle direct yaw moment control method based on a vehicle stable envelope line, which comprises the following concrete implementation steps:
1) generating a vehicle stability envelope
The slip angle β and the yaw rate r of the vehicle state have a crucial effect on the stability of the vehicle. The vehicle stability envelope curve is shown in fig. 1, and as long as the centroid side offset angle beta and the yaw rate r of the vehicle running are within the range of the envelope curve, the running stability of the vehicle can be ensured. The beta-r borderline formula is as follows:
a1=tanαr,peak#(3)
CD and AB are the left boundary and the right boundary of the vehicle stable envelope line mass center side deflection angle respectively, and BC and AD are the upper boundary and the lower boundary of the vehicle stable envelope line yaw angular speed respectively. lRIs the distance of the center of mass to the rear wheel, vxIs the longitudinal velocity, αr,peakMu is the road surface friction coefficient for the maximum slip angle of the rear wheel.
Since the rear wheel is more easily saturated than the front wheel due to the large load on the rear wheel, the stability margin is determined by the rear wheel force peak, and the rear wheel maximum slip angle formula is as follows:
where μ is the road surface coefficient of friction. Bringing r into the CD and AB segments, maximum slip angle betamaxAnd minimum slip angle betaminCan be expressed as:
2) designing a Kalman filter
The centroid slip angle is particularly important for stability under extreme conditions. At present, a highly integrated sensor can measure the yaw velocity of a vehicle in the running process, but the centroid slip angle cannot be directly measured, so that the centroid slip angle is estimated through Kalman filtering.
The algorithm is as follows:
and establishing a linear two-degree-of-freedom vehicle dynamic equation containing parameter uncertainty and interference and noise influence according to Newton's law.
Wherein beta is the vehicle mass center slip angle, r is the yaw angular velocity, m and I
zRespectively vehicle mass and moment of inertia,/
FAnd l
RDistances from the front and rear axes, respectively, to the center of mass, v
xIs a longitudinal direction of the vehicleThe speed of the moving-direction is controlled,
for lateral acceleration, delta front wheel angle, C
FAnd C
RRespectively, front and rear wheel cornering stiffnesses.
And establishing a discrete system state equation and an observation equation by using the model.
x(k+1)=Ax(k)+Bδ(k)+ω(k)#(11)
y(k)=Hx(k)+Dδ(k)+v(k)#(12)
Wherein the system n-dimensional state vector
System state transition matrix
Systematic observation vector
Observation matrix
Random disturbance of system
System observation noise
ω
1(k)、ω
2(k)、v
1(k)、v
2(k) The white noise is independent and normally distributed, and the delta t is the system sampling time.
State one-step prediction
And (3) state estimation calculation:
a filter gain matrix:
K(k)=P(k)-H(k)T[H(k)P(k)-H(k)T+R]-1
where R is the observed noise covariance matrix.
Estimating an error variance matrix:
P(k)=[1-K(k)H(k)]P(k)-#(15)
one-step prediction error matrix:
P(k)-=A(k)P(k)AT(k)+Q#(16)
wherein Q is the covariance of the random perturbation of the system.
And calculating the estimation of the state value by recursion of the observed value obtained by the real vehicle.
3) Design sliding mode controller
The sliding mode controller generates a direct yaw moment, so that the mass center side offset angle and the yaw velocity of the vehicle are maintained in a vehicle stable envelope line, the purpose of keeping the vehicle stable is achieved, and buffeting of sliding mode control is reduced through a designed approach law. Defining a sliding surface s1Comprises the following steps:
where rho is [0,1 ]]To design the parameters, | Δ rmaxTo the maximum absolute value of the set yaw-rate error, | Δ β tintmaxThe maximum absolute value of the slip angle error.
Differential sliding surface s1Obtaining:
wherein (r, beta) is a point outside the stable envelope line, (r)safe,βsafe) To the nearest point on the stability envelope.
With a two-degree-of-freedom vehicle model (1), additional moments applied in the lateral direction can be obtained with respect to
The formula of (a):
wherein FFy、FRyFront and rear wheel side forces, M, respectivelyzIs the yaw moment.
Substituting (19) into (18) to obtain
In order to reduce buffeting of sliding mode control, the invention designs an approach law with an exponential term:
wherein 0<p
1<1,p
2>0。
Wherein 0<c
0<1,c
1>0, c
2>0 and c
2E.g. N. H (S) is a positive value and does not affect the stability of the system. When the system is far from the sliding surface p
1H, (S) becomes smaller, p
2The ratio/H (S) is increased, thereby increasing the approaching speed and increasing the convergence speed to the sliding surface. When the system is close to the sliding surface p
1H, (S) and p
2The/h(s) are small, and therefore a small control gain is obtained, thereby reducing chattering.
Combining equations (22) and (21), the control rate is obtained as follows:
4) distributor for designing transverse moment
The direct yaw moment output by the sliding mode controller generates driving force or braking force of wheels through the yaw moment distributor.
The vehicle tire longitudinal force distribution is generally distributed in axle load proportion. The front and rear axle load values are as follows:
in the formula axFor longitudinal acceleration, FZF,FZRThe vertical load of the front shaft and the rear shaft is adopted, and h is the height of the mass center of the whole vehicle.
The longitudinal force of each wheel should satisfy the following conditions while satisfying the yaw moment and the total longitudinal force requirement:
in combination (24) (25), the longitudinal forces of the individual wheels, distributed according to the load ratio, are obtained as follows:
in the formula Fx1、Fx2、Fx3、Fx4The longitudinal forces of the left front wheel, the right front wheel, the left rear wheel and the right rear wheel respectively, FxThe total longitudinal driving force is determined by the pedal opening, and B is the wheel track.
The above-listed series of detailed descriptions are merely specific illustrations of possible embodiments of the present invention, and they are not intended to limit the scope of the present invention, and all equivalent means or modifications that do not depart from the technical spirit of the present invention are intended to be included within the scope of the present invention.