Disclosure of Invention
Aiming at the problem that the traditional technical means for controlling gain by fixed way can not be converged to the expected steady-state error in the sliding mode control method of the existing power converter, the invention provides a self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection.
The invention relates to a self-adaptive continuous sliding mode control method of a power converter based on zero-crossing detection, which comprises the following steps,
establishing a mathematical model of the Buck type DC-DC converter;
according to the mathematical model, introducing integral terms of the system state of the converter on the basis of the traditional first-order sliding mode surface design aiming at two state variables to obtain a sliding mode surface s containing the integral terms; the state variable is the voltage difference x between the actual output voltage and the target output voltage of the converter 1 And the rate of change x of the actual output voltage 2 ;
Dividing the motion trail of two state variables into two stages based on a sliding mode surface s containing integral items and a switching time principle, wherein the first stage is from an initial point A to a first wave crest position point B; the second stage from the first peak position point B to the zero position point;
in the first stage, a control law u is designed 1 Enabling the motion trail of the two state variables to reach a first wave crest position point B in a limited time; control law u 1 Having a fixed control gain;
in the second phase, design control law u 2 The motion trail of the two state variables is in spiral characteristics, and gradually converges to a zero position point from a first wave crest position point B; control law u 2 Having a variable control gain; and the variable control gain adaptively adjusts the amplitude value along with the number of zero crossing points of the sliding mode surface s detected in the sampling interval.
According to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection of the invention,
the initial mathematical model of the Buck type DC-DC converter is as follows:
i in L For the current flowing through the filter inductance, t is the timeL is a filter inductance, u is a control law, E is a direct current input voltage, v c The voltage is actually output by the converter, C is a capacitor, and R is a load resistor;
definition V
ref For a target output voltage, the state variable x
1 =v
c -V
ref ,
Deforming the initial mathematical model to obtain a deformed mathematical model:
in the middle of
Is an intermediate variable +.>
According to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection of the invention,
transmission uniform-order sliding die surface s 0 The method comprises the following steps:
s 0 =c 1 x 1 +x 2 ,
in c 1 For the first design parameter c 1 >0;
Transmission uniform-order sliding mode surface s 0 Introducing integral terms of the system state of the converter to obtain a sliding mode surface s containing the integral terms:
in c 2 For the second design parameter c 2 >0。
According to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection, in the first stage, a control law u is set as the control law u 1 Control law u 1 The design process of (1) comprises:
Wherein T is a sampling interval;
the formula for the slip plane s is modified as:
mu is an intermediate variable which is used as a reference,
design control law u 1 The method comprises the following steps:
where U is the fixed control gain.
According to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection, a control law u 1 The process of enabling the motion trail of the two state variables to reach the first wave crest position point B in a limited time comprises the following steps:
solving the second derivative of the sliding mode surface s with respect to time according to the formula of the sliding mode surface s after deformation:
wherein:
y 22 (μ)=c 1 μ-μβ 1 ,
β 1 =1/RC,β 2 =ω 0 2 =1/LC;
defining four constants ζ 1 、ζ 2 、ζ 3 、ζ 4 Sum function Y 21 、Y 22 The following variable substitutions were made:
the following relationship is satisfied:
will control law u 1 The formula of (2) is substituted into the second derivative of the slip plane s with respect to time to obtain:
mu in the middle min K is a constant greater than 0, which is the minimum value of the intermediate variable μ;
further, the following is obtained:
according to
Both sides are multiplied by |s|, i.e. there is +.>
If true, then for two shapesThe motion trail of state variable is limited time t from initial point A
0 The first wave crest position point B is reached;
wherein t is 0 =t B -t A ,
Wherein t is A For the moment corresponding to the initial point A, t B The time corresponding to the first peak position point B.
According to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection, the fixed control gain U is as follows:
according to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection, in the second stage, a control law u is set as the control law u 2 Control law u 2 The design is as follows:
u in j To change the control gain, r 4 For presetting a fixed value to control gain r 4 >0。
According to the self-adaptive continuous sliding mode control method of the power converter based on zero crossing detection, the assignment method of the self-adaptive amplitude adjustment of the variable control gain comprises the following steps:
is N j For the j-th sampling interval T j The number of zero crossing points of the inner s is a reference value of the number of zero crossing points, and N is more than or equal to 2; Λ type 1 Sum lambda 2 Is two positive numbers, Λ 1 <Λ 2 ;T={T 1 ,T 2 ,...T i J=1, 2,3, … … i; i is the total number of sampling intervals; u (U) 0 Is control law u 2 Is set to the initial value of (1):
U 0 =[Y 21 (||w|| * ,|s(t 0 )|)+Y 22 u 1max +k],
u in the formula 1max For control law u 1 Is the maximum value of (2):
according to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection, a control law u 2 The convergence range of the sliding die surface s is set as follows:
|s|≤[U 0 +(1+r 4 )μ max U j ]T 2 ,
mu in the middle max Is the maximum value of the intermediate variable mu.
According to the self-adaptive continuous sliding mode control method of the power converter based on zero-crossing detection, the process for obtaining the convergence range of the sliding mode surface s comprises the following steps:
will control law u 2 Substituting the expression of s into the expression of s second derivative of the slip plane with respect to time, to obtain:
further deformation is as follows:
let i be 0 ∈[1,N j ],j 0 ∈[1,N j ]And i 0 <j 0 Wherein i is 0 Is [1, N j ]One of all zero crossing points in the inner part, j 0 Is [1, N j ]Another zero crossing of all zero crossings in the inner, then:
s(t i0 )=s(t j0 )=0;
according to N.gtoreq.2, the time point t exists
i0j0 :t
i0 <t
i0j0 <t
j0 So that
Presence;
according to the Lagrangian median theorem, at t and t i0j0 There is a time t of existence i0j0 ' the following relation is satisfied:
t is T j At any time of the sampling interval, and |t-t i0j0 |<T j ;
Similarly, at t and t i0 There is another time t i0 ' satisfy:
according to |t-t i0 |<T, the above deformation is:
and integrating the two ends at the same time to obtain the convergence range of the sliding mode surface s:
|s|≤[U 0 +(1+r 4 )μ max U j ]T 2 。
the invention has the beneficial effects that: the method is provided based on an online zero-crossing detection self-adaptive mechanism, can effectively inhibit the buffeting problem, and can ensure that the steady-state error of the system converges to a given range.
Firstly, establishing a mathematical model of a converter, improving a traditional sliding mode control algorithm from two aspects of a sliding mode surface and a control law, namely purposefully introducing an integral item of a system state into the design of the sliding mode surface, dividing a convergence track into two stages, and measuring the number of zero crossings of the sliding mode surface in real time; the expected steady-state error is incorporated into the design of the sliding mode control law, a low-pass filtering link is introduced, the continuous control law of the variable gain is deduced in stages under the constraint of a convergence track, and corresponding stability analysis is given.
Simulation and performance comparison experiments prove that the method has remarkable advantages compared with the prior art in the aspects of buffeting inhibition, response speed and control precision.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
It should be noted that, without conflict, the embodiments of the present invention and features of the embodiments may be combined with each other.
The invention is further described below with reference to the drawings and specific examples, which are not intended to be limiting.
The invention provides a self-adaptive continuous sliding mode control method of a power converter based on zero-crossing detection, which is shown in the accompanying figure 1,
establishing a mathematical model of the Buck type DC-DC converter;
according to the mathematical model, introducing integral terms of the system state of the converter on the basis of the traditional first-order sliding mode surface design aiming at two state variables to obtain a sliding mode surface s containing the integral terms; the state variable is the voltage difference x between the actual output voltage and the target output voltage of the converter 1 And the rate of change x of the actual output voltage 2 ;
Dividing the motion trail of two state variables into two stages based on a sliding mode surface s containing integral items and a switching time principle, wherein the first stage is from an initial point A to a first wave crest position point B; the second stage from the first peak position point B to the zero position point;
in the first stage, a control law u is designed 1 Enabling the motion trail of the two state variables to reach a first wave crest position point B in a limited time; control law u 1 Having a fixed control gain;
in the second phase, design control law u 2 The motion trail of the two state variables is in spiral characteristics, and gradually converges to a zero position point from a first wave crest position point B; control law u 2 Having a variable control gain; the variable control gain is along with zero crossing points of the sliding mode surface s detected in the sampling intervalThe number adaptively adjusts the amplitude.
Power converter system modeling:
the method of the invention is applicable to various power converters such as DC-DC, AC-AC and the like. Fig. 1 is a block diagram of a system for slip-mode control of a Buck DC-DC converter. Controllable switch tube S w MOSFET and IGBT applications are often used in many cases, and pulse width modulation is often used. In the invention, the controllable switch tube is controlled by a designed control law u.
Further, in practical system applications, the power converter is operated in continuous current mode, i.e. inductor current i L Not equal to 0, and then based on kirchhoff's circuit law, establishing an initial mathematical model of the Buck DC-DC converter shown in fig. 1 as:
i in L For the current flowing through the filter inductor, t is time, L is filter inductor, u is control law, E is DC input voltage, v c The voltage is actually output by the converter, C is a capacitor, and R is a load resistor;
definition V
ref For a target output voltage, the state variable x
1 =v
c -V
ref ,
Deforming the initial mathematical model to obtain a deformed mathematical model:
in the middle of
Is an intermediate variable +.>
Traditional first order sliding mode control: the design of the sliding mode controller comprises a sliding mode surface and a control law aiming at the mathematical model of the converter in the formula (2). Typically, a uniform slip-form surface s is imparted 0 The method comprises the following steps:
s 0 =c 1 x 1 +x 2 , (3)
in c 1 For the first design parameter c 1 >0;
x
1 And x
2 The voltage and current can be directly obtained by measuring the voltage and the current by using a Hall sensor, and the method is simple and easy to realize. Once the inverter control system converges to the slip form surface s
0 =0, the dynamic and static performance of the system depends on
I.e. output voltage deviation
Asymptotically converges to zero in an exponential fashion, and the design parameter c
1 The larger the convergence speed of the system is, the faster it is.
In terms of control law, the design of the first-order sliding mode and the second-order sliding mode control law is required to meet the sliding mode arrival condition
To ensure system stability. The buffeting suppression mechanism based on relative order from the higher order slip mode is different from the two in that: the first order sliding mode SMC directly acts the switching control item sgn (DEG) on the first derivative of the sliding mode variable +.>
On to ensure a first order sliding mode s
0 The presence of =0, followed by equation (3), then the Buck converter output voltage bias and its derivative x are implemented
1 =x
2 =0, but there is a buffeting problem. For the second order sliding mode, taking the conventional Twisting algorithm as an example, the control law is generally designed as follows:
r
1 and r
2 Are all control gains, and r
1 >r
2 >0,r
1 、r
2 Is related to the response speed and steady state error of the system. It can be seen that since the switching control term sgn ()' appears at the first derivative of the control law
The actual output u is continuously formed through the integral action, which is why the Twisting algorithm effectively solves the buffeting problem. However, it should be noted that the gain r is controlled
1 And r
2 The overall process of system convergence to the origin remains unchanged, however, the closer to the origin, the larger the control gain will destroy the steady state performance of the system.
Improved adaptive second order SMC control:
in order to improve the conventional second-order sliding mode fixed control gain problem in formulas (3) and (4), the method is improved from the two aspects of a sliding mode surface and a control law.
Sliding mode surface design:
transmission uniform-order sliding mode surface s 0 Introducing integral terms of the system state of the converter to obtain a sliding mode surface s containing the integral terms:
in c 2 For the second design parameter c 2 >0。
In actual use, the first design parameter c 1 And a second design parameter c 2 And adjusting according to the use condition.
In the first stage, control law u is set as control law u 1 Control law u 1 The design process of (1) comprises:
Wherein T is a sampling interval;
in combination with formula (2), formula (5) can be transformed into:
mu is an intermediate variable which is used as a reference,
control law design:
in the design of the control law u, the method of the invention divides the motion trail into two phases according to the system convergence process based on the switching time principle, as shown in fig. 2, namely, the 1 st phase is from the initial point A to the first peak position point B, the 2 nd phase is after the point B, and is divided into sampling intervals with equal interval T, which is expressed as { T } 1 ,T 2 ,...T i }. In particular, where the desired steady state error Δ is incorporated into the improvement of the conventional twist control law of equation (4), the two-stage control law u is decomposed into u 1 And u 2 The design process is as follows.
First stage, movement of point a to point B:
in FIG. 2, it is assumed that the time of the initial point A is t
A The corresponding position is (t
A ,s
A ) The moment of the first wave crest position point B is t
B The corresponding position is (t
B ,s
B ) And has
Comparative formula (4), design control law u
1 The method comprises the following steps:
where U is the fixed control gain.
The fixed control gain U is:
wherein k is>0 is a constant; the method comprises the steps of carrying out a first treatment on the surface of the Mu (mu) max And mu min The maximum and minimum values of μ are defined by equation (7), respectively.
Control law u 1 The process of enabling the motion trail of the two state variables to reach the first wave crest position point B in a limited time comprises the following steps:
similarly to equation (6), the second derivative of the sliding mode variable s with respect to time is further calculated according to the equation of the sliding mode surface s after deformation, and the switching control term sgn () is displayed, that is, the second derivative of the sliding mode surface s with respect to time can be derived from equation (6):
wherein:
y 22 (μ)=c 1 μ-μβ 1 , (10)
β 1 =1/RC,β 2 =ω 0 2 =1/LC;
for convenience of the following description, four constants ζ are defined by formulas (9) - (10) 1 、ζ 2 、ζ 3 、ζ 4 Sum function Y 21 、Y 22 The following variable substitutions were made:
the following relationship is satisfied:
theorem 1: for the Buck converter in the formula (2), if the design of the sliding mode surface is shown as the formula (5), the control law in the first stage is shown as the formulas (14) - (15), and the limited time of the system reaches the point B.
First, the existence of the point B in FIG. 2 is demonstrated because it is the first peak position point, satisfying
For this purpose, formula (14) is substituted into formula (8), i.e., control law u
1 The formula of (2) is substituted into the second derivative of the slip plane s with respect to time and combined with formula (12), then there is
Mu in the middle min K is a constant greater than 0, which is the minimum value of the intermediate variable μ;
further, the following is obtained:
according to
Both sides are multiplied by |s|, i.e. there is +.>
If so, for the motion trail of the two state variables, a finite time t is set from any initial point A
0 The first wave crest position point B is reached;
wherein t is 0 =t B -t A ,
Wherein t is A For the moment corresponding to the initial point A, t B The time corresponding to the first peak position point B.
Further, in the second phase, the converging motion after point B:
in FIG. 2, when t>t B The system then enters a second stage of converging motion. Comparing the formula (5), setting the control law u as the control law u 2 Control law u 2 The design is as follows:
u in j To change the control gain, r 4 For presetting a fixed value to control gain r 4 >0。
The assignment method of the variable control gain self-adaptive adjustment amplitude comprises the following steps:
according to the variable control gain U j At T j The self-adaptive change is realized by detecting the zero crossing point s in the sampling interval, namely:
is N j For the j-th sampling interval T j The number of zero crossing points of the inner s is a reference value of the number of zero crossing points, and N is more than or equal to 2; Λ type 1 Sum lambda 2 Is two positive numbers, Λ 1 <Λ 2 ;T={T 1 ,T 2 ,...T i J=1, 2,3, … … i; i is the total number of sampling intervals; u (U) 0 Is control law u 2 Corresponds to the initial value of the first stage B point control law u 1 The maximum value is represented by formula (14):
U 0 =[Y 21 (||w|| * ,|s(t 0 )|)+Y 22 u 1max +k], (20)
u in the formula 1max For control law u 1 Is set at the maximum value of (c), i w i * The upper limit of w is:
theorem 2: for the Buck converter of formula (2), if the improved sliding mode surface is designed as formula (5) and the variable gain control law of the second stage is designed as formulas (18) - (19), the control law u can be ensured 2 The convergence range of the sliding die surface s is set as follows:
|s|≤[U 0 +(1+r 4 )μ max U j ]T 2 , (22)
mu in the middle max Is the maximum value of the intermediate variable mu.
The process of obtaining the convergence range of the slide face s includes:
will control law u 2 Substituting the expressions (18) - (19) into the second derivative expression (8) of the slip plane s with respect to time, yields:
from fig. 2, point B is the first peak position of the first stage and is also the point where the oscillation amplitude of the second stage is the largest. Thus, by combining formulas (8), (12), (13) and formulas (20), (21), then (23) can be further modified to:
by T j The sample interval is given as an example, and the analysis condition of the system zero crossing point in the single sample interval T is given. As in fig. 3, assume i 0 ∈[1,N j ],j 0 ∈[1,N j ]And i 0 <j 0 Wherein i is 0 Is [1, N j ]One of all zero crossing points in the inner part, j 0 Is [1, N j ]Another zero crossing among all the zero crossings in the inner is known from fig. 3:
s(t i0 )=s(t j0 )=0;
because of the zero crossing point set pointN is equal to or greater than 2, meaning that at least two zero crossings occur within a single sampling interval T, a certain moment T must exist according to the Roel theorem
i0j0 :t
i0 <t
i0j0 <t
j0 So that
Presence;
according to the Lagrangian median theorem, at t and t i0j0 There is a time t of existence i0j0 ' the following relational expression is satisfied by the formula (24):
wherein T is T j At any time of the sampling interval, and |t-t i0j0 |<T j ;
Similarly, at t and t i0 There is another time t i0 ' satisfy:
according to |t-t i0 |<T, formula (26) is deformed into:
and integrating the two ends at the same time to obtain the convergence range of the sliding mode surface s:
|s|≤[U 0 +(1+r 4 )μ max U j ]T 2 。
it should be particularly noted that during the second phase of motion of fig. 2, the amplitude of s becomes smaller as the system approaches the equilibrium point, and the number of zero crossings in the same sampling interval increases. From the formula (18), N j The magnitude relation to N affects the next sampling interval T j+1 Control gain U of (2) j+1 The choice of a given value N is therefore of vital importance. In practical systems, N may be determined experimentallyBy way of measurement, one can take the form of n=max {2Tf j +1}, where f j =N j and/T is the frequency of the experimentally measured s zero crossing point.
Specific examples:
in order to verify the superiority of the continuous sliding mode control method based on the self-adaptive mechanism of the online zero-crossing detection provided by the method in the aspects of buffeting inhibition, response speed and control precision, performance comparison is carried out on the continuous sliding mode control method with a second-order sliding mode method represented by a first-order sliding mode and a traditional twist algorithm, and the circuit parameters of the converter are shown in a table 1. For convenience of explanation, a first-order sliding mode method is represented by "1-SMC", a second-order sliding mode method represented by a traditional Twisting algorithm is represented by "2T-SMC", and a method of the invention is represented by "2 AT-SMC".
Table 1 circuit parameters of the converter
For the Buck converter in the formula (2), the sliding mode surfaces of the 1-SMC and the 2T-SMC adopt the form of the formula (3), and the design parameter c 1 Selected as 100,1-SMC control law designed as u=0.5 [ sgn(s) -1]In an actual system, hysteresis modulation is mostly adopted to relieve buffeting, wherein the hysteresis width is 0.01; control gain r in equation (4) of 2T-SMC 1 Take 240, r 2 Taking 120; for the 2AT-SMC method provided by the invention, the sliding mode surface parameter c in the formula (5) 1 Still take 100, c 2 Taking 0.001, the control gain U of the first stage of equation (14) is taken to be 75, and the design parameters of the second stage of equations (18) - (19) are selected to be r 4 =0.541,Λ 1 =2,Λ 2 =4,N*=8,T=25μs。
The control performance of the Buck converter under the action of three methods is compared by taking two conditions of rated working conditions and input voltage disturbance as examples.
(1) Rated operating mode:
the control performance pairs of the three methods under rated conditions are shown in fig. 4 to 7 and table 2, wherein fig. 4 is the output voltage v c And FIG. 5 shows the inductor current i L As can be seen from the simulation results of (2), the three methods all realize two methodsConvergence control of the output voltage v c Converging to a given value V ref =5v, where the steady state error of 1-SMC is 13.01mV maximum, next 2T-SMC is 6.04mV,2at-SMC steady state performance is optimal, steady state error is only 1.03mV. Comparing equations (3) and (5), the method of the present invention achieves good results due to the 2AT-SMC method introducing integral terms of system state into the design of the slip-form surface. AT the same time, the system convergence speed under the control of 2AT-SMC is the fastest, which is only 0.042s, and the effect obtained by the invention is attributed to the variable gain control function of the 2AT-SMC method in combination with the comparison of the control law u of FIG. 6. Further, as shown in fig. 2, equation (14) and theorem 1, it can be seen that the initial motion trajectory of the system of the present invention oscillates maximally in the first stage, which also explains the reason that the amplitude of the control law u of 2AT-SMC is maximized in this stage; and then in the second stage, the amplitude of the second-order SMC is the smallest in the three methods along with the trend convergence of the system, particularly, the control law u of 1-SMC has obvious buffeting phenomenon, and even if hysteresis modulation is adopted for relieving, the second-order SMC buffeting inhibition performance such as 2T-SMC and 2AT-SMC is not good. Further, in fig. 7, three different zero crossing set points N are selected, 2,4,8, corresponding to output voltage steady state errors of 12.15mV,6.43mV, and 1.03mV, respectively. According to n=max {2Tf j From the formula +1, N can be found * The larger the zero crossing point is, the faster the frequency of detection is, the better the variable gain control performance of the 2AT-SMC method is, and further the influence of an online zero crossing point self-adaptive mechanism on the system performance is proved.
TABLE 2 comparison of Voltage and Current Performance under rated conditions
(2) Disturbance condition
Taking the disturbance of the input voltage E as an example, let us assume a jump from 10V to 12V at t=1s, and then back to 10V at t=2s, simulation pairs such as fig. 8 to 10 and table 3.
Comparing the rated condition of fig. 4 to 7 with the disturbance condition of fig. 8 to 10, three methods can be seen to output voltage v to the Buck converter c Inductor current i L And control law uThe effect is consistent due to the superiority of the sliding mode robust control. Specifically, the voltage v is output at t=1s c By taking disturbance as an example for analysis, it can be seen that the response speed of the 2AT-SMC of the method of the present invention is fastest, the response speed is recovered to the equilibrium state about 1.015s, the convergence time of the 2T-SMC and the 1-SMC method are respectively 1.038s and 1.042s, and the former start phase oscillates, which is due to the fact that the conventional second order sliding mode method selects the fixed gain to be natural, the comparison of the control law u in fig. 10 can also explain the reason of oscillation, namely, when the disturbance occurs AT t=1s, the control law u of the two second order SMCs of the 2T-SMC and the 2AT-SMC is close in size, but the control gain of the 2AT-SMC method provided by the present invention can be adaptively reduced along with the convergence process, while the 2T-SMC of the conventional fixed gain is always maintained AT a larger value, and the output voltage v is further enabled c Oscillations are generated during the rapid convergence process.
TABLE 3 comparison of Voltage and Current Performance under disturbance conditions
Based on the performance comparison of the Buck converter under the rated working condition and the disturbance working condition, the superiority of the 2AT-SMC based on the online zero-crossing detection self-adaptive mechanism in buffeting inhibition, response speed and control precision is demonstrated, and the output voltage quality of the converter is improved.
Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the different dependent claims and the features described herein may be combined in ways other than as described in the original claims. It is also to be understood that features described in connection with separate embodiments may be used in other described embodiments.