Improved Quantum-Inspired Evolutionary Algorithm for Engineering Design Optimization

1 Department of Computer Science, National Pingtung University of Education, 4-18 Min-Sheng Road, Pingtung 900, Taiwan 2 Institute of System Information and Control, National Kaohsiung First University of Science and Technology, 1 University Road, Yenchao, Kaohsiung 824, Taiwan 3 Department of Electrical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kung Road, Kaohsiung 807, Taiwan 4 Department of Healthcare Administration and Medical Informatics, Kaohsiung Medical University, 100 Shi-Chuan 1st Road, Kaohsiung 807, Taiwan


Introduction
Solving engineering design optimization problems usually requires consideration of many different types of design variables and many constraints.Practical problems in engineering design often involve a mix of integers, discrete variables, and continuous variables.These constraints are often problematic during the engineering design optimization process.
Since the 1960s, researchers have attempted to solve this problem, which is known as the mixed discrete nonlinear programming MDNLP problem.One of the most effective solutions reported so far is a nonlinear branch and bound method BBM for solving nonlinear and discrete programming in mechanical design optimization 1, 2 .In BBM, however, subproblems result from portioning the feasible domain to obtain solutions by ignoring discrete conditions, and the number of times the problem needs to be resolved increases exponentially with the number of variables 3 .The better method, such as the sequential linear programming SLP , was developed by Bremicker et al. 4 and by Loh and Papalambros 5 to solve general MDNLP problems.The linearized discrete problem is solved by the simplex method to obtain information at each node of the tree.Their SLP approach is compared with the pure BBM, where the sequential quadratic programming is used to solve the nonlinear problem to obtain information at each node.The study shows the SLP method to be superior to the pure BBM.Other approaches to solving MDNLP problems include the penalty function approach 6-8 and the Lagrangian relaxation approach 9 .The penalty function approach to treat the requirement of discreteness is to define additional constraints and construct a penalty function for them.This term imposes penalty for deviations from the discrete values.The difficulties with a penalty approach are the introduction of additional local minima and repeated minimizations by adjusting the penalty parameters 3 .The Lagrangian relaxation method is similar to the penalty function method.The main difference is that the additional terms due to discrete variables are added to a Lagrangian function instead of a penalty function.The Lagrangian relaxation approach does not guarantee finding a global solution, even if it is a convex problem before discrete variables are introduced.It is observed that some of the methods for discrete variable optimization use the structure of the problem to speed up the search for the discrete solution.These methods are not suitable for implementation into general purpose applications.The BBM is the most general methods; however, it is time consuming.In recent years, the focus has shifted to applications of soft-computing optimization techniques that naturally use mixed-discrete and continuous variables for solving practical engineering problems.These approaches include genetic algorithms GAs 10-18 , simulated annealing 19 , differential evolution 20, 21 , and evolutionary programming approach 22 .The major challenge when solving MDNLP problems is that numerous local optima can result in the methods becoming trapped in the local optima of the objective functions 12 .Therefore, an efficient and robust algorithm is needed to solve mixed discrete-continuous nonlinear design optimization problems in the engineering design field.
In the past decade, the emerging field of quantum-inspired computing has motivated intensive studies of algorithms such as the Shor factorizing algorithm 23 and the Grover quantum search algorithm 24, 25 .By applying quantum mechanical principles such as quantum-bit representation and superposition of states, quantum-inspired computing can simultaneously process huge numbers of quantum states simultaneously and in parallel.To introduce a strong parallelism in the evolutionary algorithm, Han et al. 26 and Han and Kim 27-29 proposed the quantum-inspired genetic algorithm QGA .For solving combinatorial optimization problems, QGA has proven superior to conventional GAs.Malossini et al. 30 showed that, by taking advantage of quantum phenomena, QGA improves the speed and efficiency of genetic procedures.Quantum-inspired evolutionary algorithms have also been used to solve optimization problems such as partition function calculation 31 , nonlinear blind source separation 32 , filter design 33 , numerical optimization 34-36 , hyperspectral anomaly detection 37 , multiple sequence alignment 38 , thermal process identification 39 , and multiobjective optimization 40 .However, the performance of the simple QGA is often unsatisfactory, and it is easily trapped in the local optima, which results in premature convergence.That is, the quantum-inspired bit Q-bit search with quantum mechanism must be well coordinated with the genetic search with evolution mechanism, and the exploration and exploitation behaviors must also be well balanced 35 .Therefore, a big challenge is to improve QGA capability of exploration and exploitation and develop an efficient and robust algorithm.
The efficient and robust Latin square quantum-inspired evolutionary algorithm LSQEA proposed in this study solves global numerical optimization problems with continuous variables and mixed discrete-continuous nonlinear design optimization problems.The LSQEA approach integrates Latin squares 41-43 and QGA i.e., quantum-inspired individual and mechanism with GAs .The concept of the use of QGA came from the works of Han et al. 26 and Han and Kim 27-29 , while the development steps were implemented by authors and shown in Section 3 .The role of the Latin square is to generate better individuals by implementing the Latin square-based recombination since the systematic reasoning ability of Latin square of the Taguchi method is, in due course, incorporated into the recombination operation to select better Q-bits.This role is important for improving the efficiency of the crossover operation in generating representative individuals and better-fit trial individuals.The Latin square is applied to recombine the better Q-bits so that potential individuals in microspace can be exploited.The QGA is used to explore the optimal feasible region in macrospace.Therefore, the LSQEA approach is highly robust and achieves quick convergence.
The paper is organized as follows.Section 2 gives the problem statements.The LSQEA for solving the mixed discrete-continuous nonlinear design optimization problems is described in Section 3. In Section 4, the proposed LSQEA approach is compared with QGA and real-coded GA RGA 44-47 in terms of performance in solving global numerical optimization problems with continuous variables.The LSQEA approach is then used to solve mixed discrete-continuous nonlinear design problems encountered in the engineering design field, and the results obtained by LSQEA are compared with those obtained by existing methods reported in the literature.Finally, Section 5 concludes the study.

Problem Statements
This section states the considered problems, which include a global numerical optimization problem with continuous variables and a mixed discrete-continuous nonlinear programming problem.
The following global numerical optimization problem with continuous variables is considered: where X x 1 , x 2 , . . ., x i , . . ., x n is a continuous variable vector, f X is an objective function, X L x L 1 , . . ., x L i , . . ., x L n , and X U x U 1 , . . ., x U i , . . ., x U n define the feasible solution vector spaces.The domain of x i is denoted by x L i , x U i , and the feasible solution space is defined by X L , X U .For this problem, g j X and j 1, 2, . . ., t are the constraint functions.Although many design problems can be cast as the above optimization problem, efficiently obtaining optimal solution is difficult because the problem involves designs that are high-dimensional, nondifferentiable, and multimodal 35 .
The mixed discrete-continuous nonlinear programming problem is expressed as follows 12 : minimize f X subject to g j X ≤ 0 j 1, 2, . . ., t , i is the lower bound of x i ; N i is the natural number corresponding to x i ; Δx i is the discrete increment of the ith discrete variable; nq is the number of discrete variables with equal spacing.Let x i e M i ,1 for the discrete variables with unequal spacing, where nq < i ≤ nd, M i denotes the natural number corresponding to x i , M i is generated from 1, w i , e M i ,1 denotes the M i th element of the w i × 1 vector E i , E i represents the w i × 1 vector of values of discrete variables with unequal spacing, and w i is the maximum permissible number of discrete values for the ith discrete variable with unequal spacing.

The LSQEA Approach for Solving Mixed Discrete-Continuous Design Problems
This section describes the details of the LSQEA approach for solving mixed discretecontinuous nonlinear programming problems.

Q-Bit Representation
In quantum-inspired computing, the smallest unit of information stored in a two-state quantum computer is called a quantum bit.It may be in a "0" state, a "1" state, or any superposition of the two.The state of a quantum bit can be represented as where α and β are complex numbers that describe the probability amplitudes of the corresponding states.The |α| 2 and |β| 2 are probabilities of the quantum bit being in the "0" state and "1" state, respectively, such that |α| 2 |β| 2 1.The use of a Q-bit to represent an individual in this study was inspired by quantum computing concepts.The advantage of the representation is the capability to use linear superposition method to generate any possible solution.A Q-bit individual can be represented by a string of n Q-bits such as where Since Q-bits represent a linear superposition of states, a Q-bit representation provides better population diversity compared to other representations used in evolutionary computing.For example, for following three Q-bits system with three pairs of amplitudes the states of the system can be represented as The above result means that the probabilities to represent the states |000 , |001 , |010 , |011 , |100 , |101 , |110 , and |111 are 1/16, 3/16, 1/16,3/16, 1/16, 3/16, 1/16, and 3/16, respectively.By consequence, the above three Q-bits system contains the information of eight states.

Initial Population
The initialization procedure produces p s Q-bit individuals where p s denotes the population size.

Crossover Operation
The crossover operators are the one cut-point operator, which randomly determines one cutpoint and exchanges the cut-point right parts of the Q-bits of two parents to generate new offspring.If the ith position is selected as one cut-point, the one cut-point crossover operator is used for Q-bits as shown in 3.5

3.5
For example, for following four Q-bits system, the 2nd position is selected as one cut-point, the crossover operation is shown below 3.6

Mutation Operation
Mutation of Q-bits is performed by randomly determining one position e.g., position i and then exchanging the corresponding α i and For example, for following four Q-bits system, the 2nd position is selected for mutation, the mutation operation is shown below. 3.8

Q-Bit Rotation Operation
The purpose of a rotation gate U θ is to update a Q-bit individual by rotating the Q-bit toward the direction of the corresponding Q-bit to obtain a better value.The α i , β i of the ith Q-bit is updated as follows: where θ i is a rotation angle by 0, 0.05π .
The probability of |0 becomes larger, and the probability of |1 becomes smaller.

Penalty Function
When using evolutionary method to solve a constrained optimization problem, a penalty function is used to relax the constraints by penalizing the unfeasible individuals in the population.This method improves the probability of approaching a feasible region of the search space by navigating through unfeasible regions and by reducing the penalty when a feasible region is approached.To clarify this point, it is important to distinguish between feasible and unfeasible individuals.The unfeasible individuals violate constraints included in the range 1, R where R is the number of design constraints.The higher this index of violation is, the larger the penalty should be.Given these considerations, the penalty value P is defined as follows: where y j is a value computed from the constraint function when the values of design variables are determined; U j and L j are the upper and lower bounds, respectively, of the constraint function; w p is a value distinguishing the feasible from the unfeasible individuals; and w L and w U denote the weights.Additionally, if y j < L j , then w L 1 and w U 0; if y j > U j , then w L 0 and w U 1; if L j ≤ y j ≤ U j , then w L 0 and w U 0. Equation 3.10 generally requires that each value calculated for y j of the constraint function should be limited to its upper and lower bounds.If the value is located within the feasible region, the value is not punished.Otherwise, the value is punished by being multiplied with a large number w p .The penalty value equals 0 when the optimization process is complete since the values of the design variables no longer violate the design constraints.

Latin Square
The Latin square experimental design method screens for the important factors that impact product performance.Therefore, it can be used to study a large number of decision variables with a small number of experiments.The design variables parameters are called factors, and parameter settings are called levels.The name Latin square originates from Leonhard Euler, who used Latin characters as symbols.The details regarding the experimental design method can be found in texts by Phadke  The better combinations of decision variables are also determined by integrating the orthogonal array of the Latin square and the signal-to-noise ratio of the Taguchi method.The concept of Taguchi method is to maximize signal-to-noise ratios used as performance measures by using the orthogonal array to run a partial set of experiments.The signal-to-noise ratio η refers to the mean-square deviation in the objective function.For cases of the larger-the-better characteristic and the smaller-the-better characteristic, Taguchi defined η, which is expressed in decibels, as η −10 log 1/n If only the degree of η in the orthogonal array experiments is being described, the previous equations can be modified as η i y i if the objective function is to be maximized larger-the-better and as 1/y i if the objective function is to be minimized smaller-thebetter .Let y i denote the evaluation value of the objective function of experiment i, where i 1, 2, . . ., m, and m is the number of orthogonal array experiments.The effects of the various factors variables or individuals can be defined as follows: where i is the number of experiments, f is the factor name or number, and l is the level number.
The main objective of the matrix experiments is to choose a new Q-bit individual from the two Q-bit individuals at each locus factor .At each locus factor , a Q-bit is chosen if the E fl has the highest value in the experimental region.That is, the objective is to determine the best level for each factor.The best level for a factor is the level that maximizes the value of E fl in the experimental region.For the two-level problem, if E f1 > E f2 , the better level is level 1 for factor f ∈ 1, Z .Otherwise, level 2 is better.After the best level is determined for each factor, the best levels can be combined to obtain the new individual.Therefore, systematic reasoning ability of the orthogonal array of the Latin square combined with the signal-tonoise ratio of the Taguchi method ensures that the new Q-bit individual has the best or closeto-best evaluation value of the objective function among the 2 Z combinations of factor levels, where 2 Z is the total number of experiments needed for all combinations of factor levels.
For the matrix experiments of an orthogonal array of Latin squares, generation of better individuals requires random selection of two Q-bit individuals at a time from the Qbit population pool generated by the one cut-point crossover operation.A new individual generated by each matrix experiment is superior to its parents by using the systematic reasoning ability of an orthogonal array of Latin squares and by following the below algorithm 46 .The two individuals recombine the better Q-bits to be a better-fit individual, so that potential individuals in microspace can be exploited.The detailed steps for each matrix experiment are described as follows.

Algorithm
Step 1. Set j 1. Generate two sets U 1 and U 2 , each of which has Z design factors variables .From the first Z columns of the orthogonal array L m 2 m−1 , allocate Z design factors, where m ≥ Z 1.
Step 2. Designate sets U 1 and U 2 as level 1 and level 2, respectively, by using a uniformly distributed random method to choose two Q-bit individuals from the Q-bit population pool generated by the crossover operation.
Step 3. Assign the level 1 values obtained from U 1 and the level 2 values obtained from U 2 to level cells of the j experiment in the orthogonal array.
Step 4. Calculate the fitness value and the signal-to-noise ratio for the new individual.
Step 6. Calculate the effects of the various factors E f1 and E f2 , where f 1, 2, . . ., Z. Step Otherwise, it is obtained from U 2 , where f 1, 2, . . ., Z. Implementing the process for each Q-bit at each locus then obtains the new Q-bit individual.

Steps of LSQEA
The LSQEA approach is a method of integrating Latin squares and QGA.The Latin square method is performed between the one-cut-point crossover operation and the mutation operation.The penalty function is considered for a constrained problem, as the fitness value is calculated.The steps of the LSQEA approach are described as follows.
Step 1. Set parameters, including population size p s , crossover rate p c , mutation rate p m , and number of generations.
Step 2. Generate an initial Q-bit population, and calculate the fitness values for the population.
Step 3. Perform selection operation by roulette wheel approach 44 .
Step 4. Perform the one-cut-point crossover operation for each Q-bit.Select Q-bit individuals for crossover according to crossover rate p c .
Step 5. Perform matrix experiments for Latin squares method, and use signal-to-noise ratios to generate the better offspring.
Step 6. Repeat Step 5 until the loop number 1/4 × p s × p c has been met.

Mathematical Problems in Engineering
Step 7. Generate the Q-bit population via Latin squares method.
Step 8. Perform the mutation operation in the Q-bit population.Select Q-bits for mutation according to mutation rate p m .
Step 9. Except for the best individual, select p s Q-bit individuals for the Q-bit rotation operation.
Step 10.Generate the new Q-bit population.
Step 11.Has the stopping criterion been met?If so, then go to Step 12. Otherwise, repeat Step 3 to Step 11.
Step 12. Display the best individual and fitness value.

Design Examples and Comparisons
This section first describes the performance evaluation results for the proposed LSQEA approach.The performance of the LSQEA is then compared with those of the QGA and RGA methods in solving nonlinear programming optimization problems with continuous variables.Finally, the LSQEA approach is used to solve mixed discrete-continuous nonlinear design problems in the engineering design field, and its solutions are compared with those of other methods reported in the literature.

Solving Nonlinear Programming Optimization Problems with Continuous Variables
For performance evaluation, the proposed LSQEA approach was used to solve the nonlinear programming optimization problems shown in Table 1 48-50 .The test functions included quadratic, linear, polynomial, and nonlinear forms.The constraints of these functions f 1 , f 2 , f 3 , and f 4 were linear and nonlinear inequalities, and their dimensions were 13, 8, 7, and 10, respectively.The penalty function of 3.10 was used to handle constrains of linear and nonlinear inequalities for optimization.Therefore, the test functions had sufficient local minima to provide a challenging problem for the purpose of performance evaluation.
To identify any performance improvements obtained by application of Latin square and quantum computing-inspired concepts, the QGA and RGA approaches were used to solve the test functions.
Optimizing the main parameters in evolutionary environments continues to be a area of active research in this field.Studies have shown how the performance of a GA can be improved by modifying its main parameters 51, 52 .For example, Chou et al. 53 applied an experimental design method to improve the performance of a GA by optimizing its evolutionary parameters.Therefore, this study adjusted evolutionary parameters by using the same experimental design method applied in Chou et al. 53 .The evolutionary environments used for experimental computation by LSQEA, QGA, and RGA approaches were as follows.For f 1 , f 2 , f 3 , and f 4 , the population size p s was 300, the crossover rate p c was 0.9, and the mutation rate p m was 0.1.For f 1 , f 2 , and f 4 , the stopping criterion for all methods and test functions was 540000 function calls.For f 3 , however, the stopping criterion was set to only 300000 function calls because it approached the optimal value fastest.Additionally, x 6 30 ≥ 0, 3x 1 − 6x 2 − 12 x 9 − 82 7x 10 ≥ 0, −10 ≤ x i ≤ 10, i 1, . . ., 10. each test function was performed in 30 independent runs, and data collection included 1 the best value, 2 the mean function value, and 3 the standard deviation of the function values.
Table 1 shows that the test functions involved 13, 8, 7, or 10 variables factors , which required 13, 8, 7, or 10 columns, respectively, to allocate them in the Latin square used in the LSQEA approach.The Latin square L 8 2 7 was used for 7 variables because it had 7 columns.The Latin square L 16 2 15 was used for 13, 8, or 10 variables because it had 15 columns.In this case, the first 13, 8, or 10 columns were used, whereas the remaining 2, 7, or 5 columns, respectively, were ignored.The computational procedures and evolutionary environments used to solve the test functions by QGA and RGA approaches were the same as those used in the LSQEA approach.However, Latin square was not used in QGA and RGA, and quantuminspired computing was not used in the RGA.
In Table 2, the comparison of results obtained by LSQEA, QGA, and RGA approaches reveals the following.3 For all test functions, the deviations in function values are smaller in the proposed LSQEA than in the QGA and RGA.That is, the proposed LSQEA has a relatively more stable solution quality.Since the RGA is largely based on stochastic search techniques, the standard deviations in all evaluations of test functions are higher in the RGA than in the LSQEA and QGA.
Figure 1 shows convergence results on test functions f 1 , f 2 , f 3 , and f 4 by using the LSQEA, QGA, and RGA.The LSQEA requires fewer function calls to reach the best value and has the sharper decline than the QGA and GA.That is, the LSQEA has faster convergence speed than the QGA and GA.
In the computational experiment by using the systematic reasoning ability of Latin squares, it was confirmed that a new individual generated by each matrix experiment is superior to its parents, two Q-bit individuals.That is, potential individuals in microspace can be exploited.In micro Q-bit space, the systematic reasoning mechanism of the Latin square with signal-to-noise ratio enhanced the performance of the LSQEA by accelerating convergence to the global solution.In macro Q-bit space, quantum-inspired computing with the GA enhanced the performance of the LSQEA.Table 2 shows that the QGA outperformed the RGA, which indicates that quantum-inspired computing with GA improves the performance of the QGA.Therefore, the LSQEA outperforms the QGA and RGA methods in both exploration and exploitation.
García et al. 54, 55 confirmed the use of the most powerful nonparametric statistical tests to carry out multiple comparisons.Therefore, this study used the nonparametric Wilcoxon matched-pairs signed-rank test 56 to tackle a multiple-problem analysis to compare two algorithms over a set of problems simultaneously.Let d i be the difference between the performance scores of the two algorithms on ith out of n different runs.The differences are ranked according to their absolute values, and average ranks are assigned in case of ties.Let T be the sum of ranks for the different runs on which the second algorithm outperformed the first, and let T − be the sum of ranks for the opposite.Ranks of d i 0 are split evenly among the sums, and if there is an odd number of them, one is ignored Let T be the smaller of the two values, T min T , T − .If T is less than or equal to the value of the distribution of Wilcoxon for n degrees of freedom, the null hypothesis of equality of means is rejected 54, 55 .Also, to calculate the significance of the test statistic T , the mean T and standard error SE T were defined as follows 57 : where n is the sample size.Therefore, Z T − T /SE T .If Z is bigger than 1.96 ignoring the minus sign then it is significant at P < 0.05.
The sample data obtained by Table 2 include the best values, mean function values, and standard deviation of mean function values.The Z values are all 2.934 P 0.0033 in LSQEA versus QGA, LSQEA versus RGA, and QGA versus RGA.So, the tests mean there is a significant difference between LSQEA and the two algorithms, QGA and RGA.That is, the performance of LSQEA really outperform those of QGA and RGA, since the Wilcoxon test Z > 1.96 and P < 0.05.There is a significant difference between QGA and RGA.The performance of QGA is superior to that of RGA.For evaluating the LSQEA in a problem which has a relatively larger dimensionality, two test examples which are 100 dimensions were used and minimized.They are h 1 − n i 1 sin x i sin 20 i × x 2 i /π , 0 ≤ x i ≤ π, and h 2 n−1 j 1 100 x 2 j − x j 1 2 x j − 1 2 , −5 ≤ x j ≤ 10, where n 100.To ensure a fair comparison of the performance of the LSQEA with that of recently proposed algorithms which are particle swarm optimization PSO 58 and artificial immune algorithm AIA 59 , the study use the same population size that is 200 for 50 independent runs.The results of LSQEA on h 1 x in terms of mean function value standard deviation and mean function call standard deviation are −92.8300 and 178347 14362 , respectively, and on h 2 x are 0.7 0 and 60377 3368 , respectively.The results of PSO on h 1 x in terms of mean function value standard deviation and mean function call standard deviation are −92.8250.03 and 330772 29516 , respectively, and on h 2 x are 0.752 0.02 and 168736 19325 , respectively, while the results of AIA on h 1 x are −90.540.93 and 346972 29842 , respectively, and on h 2 x are 2.95 1.29 and 178048 75619 , respectively.Figure 2 shows convergence results on test functions h 1 and h 4 by using the LSQEA, PSO, and AIA.The LSQEA requires fewer function calls to reach the best value and has the sharper decline than the PSO and AIA.That is, the LSQEA has faster convergence speed than the PSO and AIA.In general, the performance of the LSQEA is superior to those of the PSO and AIA in the two examples.Additionally, the nonparametric Wilcoxon matchedpairs signed-rank test was used to evaluate the performance in two algorithms.The Wilcoxon test Z values are all 2.521 P 0.0117 in LSQEA versus PSO and LSQEA versus AIA.So, the tests mean there is a significant difference between LSQEA and the two algorithms, PSO and AIA.That is, the performance of LSQEA really outperforms those of PSO and AIA, since the Wilcoxon test Z > 1.96 and P < 0.05.There is also a significant difference between PSO and AIA, since Z value is 2.521, and P value is 0.0117.The performance of PSO is superior to that of AIA in the two examples.
Hence, the performance improvement in the proposed LSQEA is achieved by using quantum-inspired computing and the systematic reasoning mechanism of Latin square with signal-to-noise ratio.Therefore, we conclude that the proposed LSQEA approach effectively solves the six nonlinear programming optimization problems with continuous variables.After confirming this capability of the LSQEA approach, the LSQEA approach was then evaluated for use in solving mixed discrete-continuous nonlinear design problems.

Solving Mixed Discrete-Continuous Nonlinear Design Problems
To evaluate the use of the LSQEA approach for solving mixed discrete-continuous nonlinear design problems in the engineering design field, this study applied the experimental design method reported in Chou et al. 53 for parameter adjustment in different evolutionary environments.
Example 4.1 compression coil spring design .Figure 3 shows the first example, which is the design for a compression coil spring under a constant load for minimum volume of material.The relationships among the three design variables can be expressed as X N, d, D T x 1 , x 2 , x 3 T , where N is an integer representing the number of coil springs; d is a discrete value representing the wire diameter according to ASTM code; D is a continuous variable representing winding diameter.As described by Sandgren 2 , the objective and constraint equations can be mathematically derived as follows: Mathematical Problems in Engineering subject to where x 2 ∈ {0.207, 0.225, 0.244, 0.263, 0.283, 0.307, 0.331, 0.362, 0.394, 0.4375, 0.500}, 1.0 ≤ x 3 ≤ 3.0.
The evolutionary environmental parameters applied in the computational experiments performed using the proposed LSQEA approach are p s population size 100, p c crossover rate 0.9, p m mutation rate 0.3, and generation number 100.The design function is performed in 30 independent runs.Table 3 shows that the computational results obtained by the proposed LSQEA approach are superior to those obtained by the methods developed by Sandgren 2 and by Rao and Xiong 12 .Another observed advantage is that, unlike the approach presented in Sandgren 2 , the LSQEA can use arbitrary starting points, which enhances its versatility and effectiveness.Table 4 further shows that the robustness analysis of where T s and T h represent discrete values, integer multiples of 0.0625 inches, while R and L are continuous variables.a Upper bound of maximum shear stress τ X on the weld:

4.10
b Upper bound of maximum normal stress σ X on the beam: c Lower bound of bulking load P c x on the beam:

4.16
Additionally, the side constraints, which are expressed as x 3 ≤ x 2 and x 3 ≥ 0.125, are considered along with the following numerical data 12 :

4.17
The variables t and b are considered discrete variables integer multiples of 0.5 ; h and l are considered integers.The parameters used in the computational experiments performed to evaluate the proposed LSQEA approach are population size p s of 10, crossover rate p c of 0.9, mutation rate p m of 0.3, and generation number of 20.The design function was performed in 30 independent runs.The computational results obtained by the proposed LSQEA approach are comparable to those obtained by Rao and Xiong 12 .The minimum cost is 5.67334, and t, b, h, l T 4.5, 1.0, 1, 2 T .Table 7 also shows the results of robustness analysis of the LSQEA.The standard deviation of 0 obtained in 30 independent runs indicates that the LSQEA finds the optimal solution each run.That is, the LSQEA is a very robust and stable method for designing the welded beam.Additionally, the solution space in this welded beam design problem is small since only 4 design variables are used, and all are discrete variables integer multiples and integers .Therefore, the LSQEA easily finds the optimal solution.Example 4.4 twenty-five bar truss design .Figure 6 shows Example 4.4, a twenty-five bar truss which is a classic test case often used to test optimization algorithms for both continuous and discrete structural optimization problems.Table 8 gives the two load conditions for this truss, which is designed under constraints on both member stress and Euler buckling stress.Since the truss is symmetrical, the member areas can be divided into eight groups: where l i denotes the length of the member i; ρ 0.1 lb/in 3 is the weight density; δ ix , δ iy , and δ iz are the x, y, and z components, respectively, of deflections in node i i 1, 2 ; ω 1 is the fundamental natural frequency of vibration.The constraints can be stated as where σ ij denotes the tension or compression stress in member i under load condition j; allowable stress S is set to 40000 psi; x l i and x u i are the lower and upper bounds of x i , which are set to be 0.1 in 2 and 5.0 in 2 , respectively; the Euler bulking stress B i (X) in member i is 25 , 4.20 in which E is the Young modulus of 1.0 × 10 7 psi.
If each x i is considered a discrete variable, the following expression is used: x i 0.1 0.1k, where k 0, 1, 2, . . ., 49.The length of each bar is calculated according to the coordinates of the nodes of the truss.The deflections δ ix , δ iy , and δ iz of node i, the fundamental natural vibration frequency ω 1 , and the stress σ ij in member i under the load condition j are obtained by finite-element analysis of the truss with ANSYS CAE Toolbox 62 .Table 9 shows the best continuous and discrete results obtained by the proposed LSQEA approach with ANSYS CAE Toolbox.In contrast, Table 10 shows the best results obtained by the Rao and Xiong 12 approach using the ANSYS CAE Toolbox.The comparison of results in Tables 9 and 10 confirm that the proposed LSQEA approach provides better results in each main objective function compared to the method developed by Rao and Xiong 12 .Meanwhile, for each specified design objective function, the other two objectives are also better than those reported in Rao and Xiong 12 .An interesting question is why the discrete results given in Rao and Xiong 12 achieve a higher optimal natural frequency compared to the continuous results Table 10 .Intuitively, the answers found in a continuous space should be best.That is, in the case in which the natural frequency is maximized, the results presented by Rao and Xiong 12 are inapplicable.Another issue is the three individual objective functions, which are apparently dependent.Reducing the weight of the truss requires an increase in deflection and a decrease in the fundamental natural frequency, and vice versa.Although reaching the global optimum is possible in singleobjective optimization problems, it is reached at the expense of performance in achieving the other two objectives.Therefore, the reasonable design specification for a practical engineering application is an essential consideration.If the defined objective is maximizing the fundamental natural frequency of vibration in the truss subject to the limited deflections in nodes 1 and 2, reasonable results are obtained.In this example, the results in the third case maximizing the fundamental natural frequency of vibration in the truss show the design characteristics.
In summary, the above results confirm that the LSQEA approach obtains robust and stable results.Therefore, we conclude that the LSQEA is highly feasible for solving the four examples that are mixed-discrete-continuous nonlinear design optimization problems.

Conclusions
The LSQEA approach proposed in this study solves the problems of mixed discretecontinuous nonlinear design optimization.The approach combines the merit of QGA, which is its powerful global exploration capability for exploring the optimal feasible region, with that of the Latin square, which is exploitation of the better offspring.In this study, the LSQEA approach efficiently solved global numerical optimization problems with continuous variables.The computational experiments show that, compared to QGA and RGA, the proposed LSQEA is more efficient in finding optimal or near-optimal solutions for nonlinear programming optimization problems with continuous variables.Additional advantages of the LSQEA approach include its superior robustness compared to QGA and RGA and its global exploration capability.Finally, applications of the LSQEA to solve various mixed discrete-continuous nonlinear design optimization problems in computational experiments in this study confirm its superior solution quality and superior robustness compared to existing methods.Therefore, the proposed LSQEA approach can be used as a global optimization method for solving mixed discrete-continuous nonlinear design problems complicated by multiple constraints.

41 ,
Montgomery 42 , and Park 43  .For an orthogonal array, matrix experiments are conducted by randomly choosing two individuals from the Qbit population pool.Each factor of one Q-bit individual is designated level 1, and each factor of the other Q-bit individual is designated level 2. The two-level orthogonal array of Latin squares applied here is L m 2 m−1 .Additionally, each of Z number of design factors has two levels.To establish a two-level orthogonal array of Z factors, let L m 2 m−1 represent m − 1 columns and m individual experiments corresponding to the m rows, where m 2 k , k is a positive integer k > 1 and Z ≤ m − 1.If Z < m − 1, only the first Z columns are used while the other m − 1 − Z columns are ignored.For example, if each of six factors has two levels, only six columns are needed to allocate these factors.In this case, L 8 2 7 is sufficient for this purpose because it has seven columns.

4 Test function h 2 bFigure 2 :
Figure 2: Convergence results on test functions h 1 and h 2 by using the LSQEA, PSO, and AIA.

14 d
Upper bound of end deflection DEL x on the beam: DEL X ≤ δ d
Further details can be found in works byPhadke 41,Montgomery 42 , and Park 43 .

Table 2 :
Results of performance comparisons of LSQEA, QGA, and RGA.

Table 3 :
Optimal design of compression coil spring.

Table 4 :
Robustness analysis results for compression coil spring design obtained by LSQEA.Example 4.2 pressure vessel design .Figure4shows a compressed air storage tank consisting of a cylindrical pressure vessel capped by hemispherical heads at both ends 2 .The vessel design problem is formulated according to the ASME boiler and pressure vessel code.The relationships among the design variables can be expressed as X T s , T h , R, L T x 1 , x 2 , x 3 , x 4T , where T s is shell thickness, T h is spherical head thickness, R is shell radius, and L is shell length.The purpose of the objective function is to minimize the total cost, including the cost of the material and the cost of forming and welding the pressure vessel.

Table 7 :
Robustness analysis of welded beam design obtained by LSQEA.
A 10 A 11 , A 12 A 13 , A 14 A 15 A 16 A 17 , A 18 A 19 A 20 A 21 , and A 22 A 23 A 24 A 25 , thus eight independent areas are selected as continuous or discrete design variables.Three objective functions are considered in this

Table 8 :
Load conditions for twenty-five bar truss.

Table 9 :
Best results for individual objective functions obtained by LSQEA.

Table 10 :
Best results for individual objective functions 12 .