2. Determination coefficient(\(\:{R}^{2}\))
2.4 BP Neural Network Model
The BP (back propagation) neural network is a typical multilayer network of hierarchical type with input, hidden and output layers, and the layers are mostly fully connected to each other and network trained according to the error back propagation algorithm [28, 29], the network was trained and predicted with the Levenberg-Marquardt algorithm, Levenberg-Marquardt algorithm is one of improved algorithms and it combines advantages of the method of grads descent and Newton. It absorbs not only the target function’s first derivative information, but also the second, which make the learning time shorter and speed up the convergence process. It is an efficient method for nonlinear forecasting. The advantages of the BP neural network model lie in the few parameters required and the high simulation accuracy.
The different environmental factors had a nonlinear coupling relationship with Ti and Hui. In order to reduce redundant information about the input feature and the computational complexity of the model, PCC was employed to analyze the relationship among the Ti, Hui, and others. The computation results of PCC are demonstrated in Fig. 3. Ii, Hui, and Ti are very strongly correlated, To, Huo, V, and Ti are strongly correlated, Ts and Ti are weakly correlated, whereas Hs and Ti are very weakly correlated. Thus, the primary factors affecting greenhouse temperature are Ii, Hui, To, Huo, V and Ts. Similarly, Ti, Ii, and Hui are highly correlated, To, V, and Hui are strongly correlated, and Huo and Hui are weakly correlated, while Ts, Hs, and Hui are very weakly correlated. Therefore, the main environmental factors for establishing a humidity prediction model are Ti, Ii,To, V, and Huo.
On the basis of the above analysis and thinking, the outside temperature, air humidity, solar radiation, wind speed, soil temperature, and previous inside temperatures and humidity form the input vector of the multiple-input/multiple-output( MIMO ) forecasting model.In order to further optimize the number of input features, this study divides the features derived through PCC into several groups for experimentation. The details are shown in Table 1. Among them, G1 ~ G3 are the most relevant features in PCC analysis. According to the correlation coefficient, 3,4 and 5 input features are selected respectively. G4 and G5 increase the historical data of indoor temperature and relative humidity in the previous 10 min and 20 min respectively on the basis of G1.
Table 1
Grouping of input features
Input Group
|
G1
|
G2
|
G3
|
G4
|
G5
|
Input Features
|
To, Huo, Ii
|
To, Huo, Ii,V
|
To, Huo, Ii,V,Ts
|
To, Huo, Ii,
Ti(t-t0),Hui(t-t0)
|
To, Huo, Ii,Ti(t-t0),Hui(t-t0),
Ti(t-2t0),Hui(t-2t0)
|
Ti(t-t0),Hui(t-t0) is temperature and humidity data from 10 minutes ago,while Ti(t-2t0),Hui(t-2t0)is temperature and humidity data from 20 minutes ago.