## 4 Furrowing Depth Stability Control--Furrowing Depth Detection Technology
### Ingression Detection Method  
In addition to the commonly used sensors mentioned above for detection, there are also ingression sensors for detecting furrowing depth. This type of sensor is usually installedbefore the furrowing device when the planter rises and falls. According to the different physical and chemical properties of the soil at different depths, in order to obtain different signals, the system processes the signals to obtain the actual furrowing depth of the planter. [PERSON] et al. developed an automatic control system for seeder sowing depth based on DFS. A drying front sensor senses the changes in the depth of the planter's leading edge, and the system controls the sowing depth through an electromagnetic proportional reversing valve. Its positioning error is 0.66 cm, and the optimized system response time can reach 255 ms [74]. When the seeding depth deviation detected by the drying front sensor reaches more than 2.5 cm, the system adjusts the position of the furrow opener, and its adjustment error is 0.66 mm. The characteristics of the different types of sensors are shown in Table 5. Sensors have relatively low accuracy compared with contact sensors. It is highly affected by stubble and straw residue in the field, does not measure soil firmness, and does not provide the required trenching force for the system [70]. Appropriate downforce can improve the furrow opening depth's stability and avoid compaction of seed furrows, which can improve crop emergence and yield [14; 77]. Contact sensors mainly use pressure sensors to detect the downforce of the depth-limiting wheel, which can simultaneously obtain the downforce and the depth of furrowing, providing more comprehensive information for improving the seeding effect. They have high accuracy, wide measuring range, and reliability compared with noncontact sensors, but they are susceptible to temperature and humidity. When the depth-limiting device is subjected to shock and vibration, it can cause problems such as unstable signals from noncontact sensors [26]. According to the measurement properties of the sensor, for plots with large surface undulations, the contact sensor signal is unstable and prone to damage because the depth-limiting device is susceptible to large shocks and vibrations. For this situation, noncontact sensors are considered, but it is also difficult to meet the operational requirements for precision seeding using noncontact sensors. Contact sensors are more suitable for measuring no-till surfaces because of the susceptibility of noncontact sensors to stubble and straw residue in the field. Due to the more complex soil conditions in the field, the physical and chemical properties of the soil are different in different plots and at different times in the same plot. Compared with contact and noncontact sensors, using ingression sensors to detect the depth of the seeder into the soil requires much preliminary work, and the detection error is large. In summary, it can be concluded that with the development of precision seeding technology, the requirements for measurement and control accuracy are getting higher and higher. Due to the more complex field operating conditions, it is difficult to have ideal field operating conditions suitable for a single sensor. Due to the limitations of a single sensor, it will cause a significant detection error. In order to improve the detection accuracy of furrow depth, multi-sensor fusion should be used for detection. Multi-sensor fusion can give full play to the sensor's advantages, reduce the sensor's detection error, and improve the detection accuracy. Multi-sensor detection requires the system to process the output signals of the sensors comprehensively, and the noise points generated by the sensor