## 3 Furrowing Depth Stability Control--Design of Profiling Adjustment Device
### Active Profiling Adjusting Device
#### 3.2.1. Hydraulically Driven Downforce Adjustment Device  
Hydraulically driven downward pressure adjustment device mainly utilizes a hydraulic device instead of a mechanical spring mounted on a parallel four-bar mechanism to adjust the furrowing depth by adjusting the planter unit. [PERSON] et al. designed an automatic control test bed for furrowing depth where a hydraulic cylinder is mounted on a parallel four-bar, ultrasonic sensors are utilized to detect surface undulation, and the hydraulic cylinder adjusts the furrowing depth [51]. [PERSON] et al. mounted a downforce sensor on the limit pin of the depth-limiting wheel for sensing the downforce and mounted a hydraulic cylinder on a parallel four-bar so that it could acquire the downforce information, thus realizing the real-time adjustment of furrowing depth. The test showed that the seeding depth pass rate of the downforce control device was 25.19 percentage points higher than that of the mechanical adjustment method [46,52]. [PERSON] et al. achieved furrowing depth regulation by adjusting the hydraulic cylinder downforce on the four-link mechanism. The field test showed that the sowing depth qualification rate of electrohydraulic active adjust  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Model Number** & **Enterprise Name** & **Machine Pictures** & **Sowing Crops** & **Main Features** \\ \hline
1745 Series [47] & John Deere & & Soybean; corn & Active pneumatic downforce profiling adjustment \\ \hline
1725C Series [48] & John Deere & & Cotton & Active pneumatic downforce or single-row hydraulic downforce profiling adjustment \\ \hline Maximetro Series [49] & Maschio & & Corn; soybean; beet & Active pneumatic downforce profiling adjustment \\ \hline
5900 Series [50] & Kinze & & Corn; sugar beets; milo; cotton; oil sunflower; soybean & Single-row hydraulic downforce adjustment; true depth downforce control system \\ \hline \hline \end{tabular}
\end{table}
Table 3: Common active profiling planners.  
ment was 8.66% higher than that of mechanical adjustment. The coefficient of variation of sowing depth was 7.75% lower in electrohydraulic active regulation than in mechanical regulation [53]. [PERSON] et al. designed a furrowing depth control device, mainly composed of a ground surface height detection mechanism and a four-link imitation mechanism. Displacement sensors were used to sense the undulation of the profiling wheel and transmit it to the PLC, which controlled the expansion and contraction of the hydraulic cylinder according to the undulation of the ground surface and realized the real-time adjustment and closed-loop control of the furrowing depth [54]. Some experts and scholars have also proposed to connect the hydraulic device directly with the furrow opener to realize the real-time adjustment of the furrow depth. [PERSON] et al. installed a flexible tubular element above the furrow opener, which has a cross-section that can be deformed under the control of a hydraulic system, thus actively controlling the depth of the furrow opener [55]. [PERSON] et al. designed a garlic sowing depth adjustment device based on a hydraulic drive, which directly connects the furrow opener to the hydraulic cylinder and controls the furrow depth of the opener in real time. The test results showed that the average error of seeding depth adjustment was 4.7%, and the coefficient of variation was 5.3%, which could meet the test requirements [56].  
Hydraulically driven downforce adjustment device mainly adopts the tractor as the hydraulic input source. Hydraulically driven downforce regulators can provide large force and torque, sizeable adjustable range, and high stability and can realize more accurate adjustment in different terrain and soil conditions. Hydraulically driven downforce adjustment device has fast response and is suitable for high-speed operation of seeding equipment, and is primarily installed in high-speed and large-sized operational seeding machines.