Household Appliance Plastics Identification by Fusing Multi-Level Feature Enhancement and Hierarchical Classification
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摘要: 针对废旧家电塑料回收中低光谱分辨率条件下识别精度不足的问题,尤其是黑色塑料在可见–近红外波段因高吸光性与光谱重叠导致的特征可分性下降,该文提出一种面向受限光谱特征空间的自动识别方法。该方法基于可见–近红外多光谱传感系统,通过多层次特征工程增强低维光谱信息,并结合分阶段层级分类策略,将多类别识别分解为粗分类与细分类的联合推理过程,以抑制复杂样本的特征混叠对分类性能的干扰。在五折交叉验证条件下,模型取得97.4%的分类准确率,在独立测试集上取得93.1%的分类准确率,相较于单阶段分类及无特征增强方法表现出更优的分类性能。结果表明,该方法在低分辨率多光谱条件下对黑色塑料及其他常见家电塑料具有稳定的识别能力,有效提升了复杂样本的分类精度,为废旧家电塑料自动化分选系统的开发提供了理论与应用支持。Abstract:
Objective Accurate identification of plastics in waste household appliance recycling remains challenging under low-resolution spectral conditions. In practical recycling environments, plastics often exhibit complex compositions, surface contamination, and aging effects, which increase classification difficulty. In particular, black plastics show strong light absorption and spectral overlap in the visible and near infrared (Vis–NIR) range, leading to reduced feature separability and degraded classification performance. Under such conditions, conventional single-stage classification models are often unable to maintain stable accuracy. To address this problem, an automated identification method for low dimensional multispectral feature spaces is proposed, aiming to improve the discriminative capability of limited spectral information and enhance classification accuracy for complex plastic categories. Methods A compact Vis–NIR multispectral acquisition system based on the AS7265x sensor is used to collect 18 channel reflectance data within the 410–940 nm range. A handheld acquisition device with a controlled optical structure is designed to reduce environmental interference and ensure measurement consistency ( Fig.3 ). A total of 576 samples from five typical appliance plastics, including ABS, high-impact polystyrene (HIPS), polypropylene (PP), acrylonitrile styrene copolymer (AS), and PC/ABS blends, are collected from waste household appliances and subjected to preliminary surface cleaning prior to spectral acquisition. To improve feature representation, a multi-level feature engineering strategy is adopted. It integrates original spectral intensity features, nonlinear polynomial expansion features, and adjacent channel ratio features to characterize both global and local spectral information. The nonlinear expansion enhances reflectance variation representation, while the ratio features capture local spectral shape changes and reduce external disturbances. These features are combined into a 53-dimensional feature vector. Linear Discriminant Analysis (LDA) is then applied to enhance inter class separability. To handle spectral overlap and class imbalance, a Hierarchical Joint Classifier (HJC) is constructed. The HJC adopts a two stage classification framework: an XGBoost based primary classifier performs coarse classification to separate easily distinguishable samples and group spectrally similar black plastics, while a TabTransformer based secondary classifier is used for fine grained classification of difficult samples (Fig. 6 ). This hierarchical design reduces classification complexity and improves discrimination for challenging categories. Model performance is evaluated using five fold cross validation and an independent test set. Evaluation metrics, including accuracy, precision, recall, and F1 score, are calculated based on confusion matrices (Fig. 7 ). Comparative experiments are conducted with traditional machine learning methods, ensemble learning models, and deep learning approaches under different feature processing strategies (Fig. 8 ,Fig. 9 ).Results and Discussions The proposed HJC achieves a classification accuracy of 97.4% under five fold cross validation and 93.1% on the independent test set ( Table 4 ). Compared with single stage classifiers and methods without feature enhancement, the proposed method shows improved performance and stability under low resolution spectral conditions. Comparative results indicate that the proposed method outperforms baseline approaches such as PCA combined with CNN, which achieves approximately 71.3% accuracy under the same dataset (Fig. 8 ). This improvement indicates that the proposed feature engineering strategy effectively enhances the discriminative capability of low dimensional spectral data. In addition, combining LDA with feature engineering further improves class separability compared with conventional PCA based methods. Confusion matrix analysis shows that misclassifications are mainly concentrated between spectrally similar black ABS and black HIPS samples, while other categories achieve high classification accuracy (Fig. 9 ). This result suggests that spectral overlap remains the main challenge under low resolution conditions. The hierarchical classification strategy mitigates this issue by focusing classification on difficult samples, thereby improving the overall generalization capability of the model. Overall, the proposed method demonstrates robustness under practical conditions, including spectral noise, limited channel resolution, and material heterogeneity, indicating its suitability for real world recycling applications.Conclusions A hierarchical classification method with multi-level spectral feature engineering is developed for plastic identification under low-resolution Vis–NIR conditions. Nonlinear and morphological features are incorporated into a two-stage framework to improve performance on spectrally similar materials. The results show consistent accuracy across different plastic types. The method is suitable for automated sorting in waste appliance recycling and can be applied to other material identification tasks with limited spectral information. -
表 1 三个子芯片对应的6通道波长说明
子芯片 波段类型 6 个通道中心波长(nm) AS72651 近红外(VIS–NIR) 610、680、730、760、810、860 AS72652 可见–近红外(VIS–NIR) 560、585、645、705、900、940 AS72653 可见光(VIS) 410、435、460、485、510、535 表 2 不同参数探测头参数以及效果说明
探测头编码 类型 内径(mm) 高度(mm) $ {\text{SD}}_{\text{mean}} $ $ {\text{Delta}}_{\text{mean}} $ 1 圆柱形 25 45 1.18949 2.979367 2 圆柱形 25 50 1.33102 3.267261 3 圆柱形 18 40 1.612278 3.736211 4 圆柱形 20 35 1.977449 5.234972 5 圆柱形 20 22 0.967459 2.218467 6 圆锥形 10/20 35 1.197082 2.911761 7 圆锥形 10/20 30 1.343622 3.401544 表 3 LDA、特征工程条件HJC模型评估指标
塑料类别 精确率 召回率 F1-Score ABS-BLACK 0.90 0.94 0.92 ABS-WHITE 1.00 1.00 1.00 AS-BLACK 0.99 1.00 0.99 AS-WHITE 1.00 1.00 1.00 HIPS-BLACK 0.92 0.87 0.89 HIPS-WHITE 1.00 1.00 1.00 PC+ABS 1.00 1.00 0.99 PP 1.00 1.00 1.00 表 4 独立测试集上不同模型分类性能及排序结果
排名 分类器 测试准确率 宏平均F1-Score 加权平均F1-Score 测试样本数 1 HJC(This Work) 0.9310 0.9202 0.9307 145 2 XGBoost 0.9103 0.8915 0.9133 145 3 ResNet 0.8966 0.8805 0.8998 145 4 TabTransformer 0.8759 0.8590 0.8752 145 5 Logistic Regression 0.8690 0.8679 0.8825 145 6 Stacking 0.8552 0.8471 0.8615 145 7 SVM 0.8552 0.8511 0.8617 145 8 Random Forest 0.8552 0.8591 0.8600 145 9 CNN 0.8207 0.8056 0.8224 145 表 5 HJC模型在独立测试集上的分类别性能指标
塑料类别 精确率 召回率 F1-Score 样本数 ABS−BLACK 0.9688 0.9143 0.9412 35 ABS−WHITE 1.0000 0.9600 0.9796 25 AS−BLACK 1.0000 0.7273 0.8421 11 AS−WHITE 1.0000 1.0000 1.0000 12 HIPS−BLACK 0.6471 0.8571 0.7273 14 HIPS−WHITE 0.8667 1.0000 0.9286 13 PC+ABS 0.9444 1.0000 0.9714 17 PP 1.0000 0.9444 0.9714 18 -
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