## 4 Conclusions  
Seed-fertilized drill machines with fluted rollers generally use a calibration relationship model between the mass flow rate and driving shaft rotation speed before initiation to control the application rate of seeding and fertilizing. This calibration relationship model is relatively stable for a short period, but the deviation between the calibrated relationship model and the actual model gradually increases with the operating time of the drill machine because there can be significant changes (such as deformation, damage, caking or bonding by wetted fertilizer, etc.) appearing in fluted rollers, brushes, and other working parts. Thus, the accuracy of the seed and fertilizer application rate is negatively affected. This kind of control mode is essentially open-looped. To keep a high-quality operation performance, frequent calibration of the control model is inevitable. This not only affects the work efficiency, but is also not suitable for the use of ordinary farmers, since special skill is required for calibration.  
In this paper, based on a self-developed application rate measuring system, we provided a method for the self-calibrating relationship model between the mass flow rate and driving shaft rotation speed. Because of the function of self-calibrating the control parameters, the control mode is closed-looped. Field tests showed that the calibrating system can effectively correct the control parameters based on the detected data from the application rate measuring system from a certain period. It can be used to hold a continuous high-quality control performance and improve actual productivity. This method can be applied to the existing mainstream seed-fertilized drill machines, which will benefit agricultural production.  
In contrast to other closed-looped control methods, we do not use the real-time feedback information of the application rate for controlling, and instead use the integrated information from a certain period to draw the control parameters. Compared to the real-time detection method, our method provides an easier way to obtain effective information in the complicated field operation environment.  
It is also noted that actual field operation is complex and there are still many difficulties that need to be solved. It is difficult to determine suitable duration to calibrate new control parameters. Further, the application rate measuring system can be mixed with significant vibration interference. The proposed application rate calibration strategy is not smart enough and requires further detailed study beyond the conducted extent described in this paper.  
Conceptualization, H.Y. and Y.D.; methodology, H.Y. and Y.D.; software, Z.L. and C.Y.; validation, H.Y. and X.F.; formal analysis, X.D.; data curation, H.Y.; writing--original draft preparation, H.Y.  
This work was mainly supported by the National Key Research and Development Program of China (Grant No. 2016 YFD070030403) and partially supported by the Fundamental Research Funds for the Central Universities (Grant No. KYGX201703) and by the National Engineering and Technology Center for Information Agriculture in Nanjing Agricultural University, China.  
We would like to thank Editage (www.editage.cn) for English language editing.  
The authors declare no conflict of interest.