小麦播种量电容法检测系统设计与试验

Design and test of capacitive detection system for wheat seeding quantity

为实现小麦播种机播种量的精准检测,该文基于电容法设计了一套用于小麦播种量检测的系统,由检测分辨率和排种轮转速与采样频率约束关系确定传感器结构尺寸,建立了种子数量与电容变化量之间的线性关系。在采样周期为15 ms、排种轮转数20 r/min条件下,基于时间窗口建立了小麦播种量实时检测最小二乘回归模型。为了使检测系统适用于不同的排种轮转速,提出了一种通过改变采样周期的检测方法,即排种轮速度每增加5 r/min时,采样周期相应减少0.4ms,则上述建立的最小二乘回归模型仍适用,对不同的排种轮转速均具有较高的检测精度,相对误差介于-2.26%~2.17%之间。本文所设计的检测系统为实现小麦播种量的精准检测提供了一种有效途径,具有较好的实用性和经济性。

At present,the methods of seeding detection are photoelectric-based method,image-based method and capacitance-based method.For the photoelectric-based method,the detection accuracy of photoelectric sensor is affected by the vibration,light,temperature and other factors on the farmland.When multiple seeds fall simultaneously,the refraction phenomenon of photoelectric sensor also affects the accurate detection.For the image-based method,its high precision detection provides a new way to improve the performance of wheat-planter seeding.However,image processing technology requires special equipment with high cost,and cameras are easy to be interfered by external light.Consequently,it is difficult to be widely applied in the complex environment on the farmland.Compared with photoelectric-based and image-based methods,the capacitance-based method is less affected by light and dust and thus has a strong environmental adaptability.However,when multiple seeds fall simultaneously,the detection accuracy of the capacitance-based method still needs to be improved.In this paper,a precise detection system for wheat-planter seeding quantity was designed using the capacitance-based method.The detection resolution and the constraint relation between the seeding speed and the sampling frequency determine the structure size of the capacitance sensor.To guarantee the detection accuracy,every seed should be detected only once as far as possible when it passes through the parallel plate of capacitance sensor.Then the initial sampling period can be determined according to the above sampling method.Meanwhile,high detection accuracy is difficult to be achieved due to the small capacitance change in detection system and the influence of parasitic capacitance and environment in conditioning circuit.Therefore,the capacitance analog-to-digital conversion chip of AD7746 is utilized to effectively reduce the error caused by the above factors.The signal acquisition and processing circuit for precise detection of wheat-planter was designed based on the AD7746 and the single chip microprocessor of STM8.Ideally,the seed passing through the capacitor plate will be sampled only once by the capacitive sensor.However,the times of the seed passing through the parallel plate will be changed with different wheat-planter seeding speeds,which will affect the distribution of sampling times when the seed passes through the capacitor plate.The least squares regression model of the real-time wheat-planter seeding quantity detection was built under the condition with the sampling period of 15 ms and the wheat-planter speed of 20 r/min.The results showed that when the sampling period is 15 ms,the relative error between the number of seeds calculated by the least squares regression model and the actual number of seeds increases with the speed of the wheat planter.Consequently,to make the detection system suitable for the different seeding speeds,a detecting method was proposed by changing the sampling period,in which the sampling period was reduced by 0.4 ms when the wheat-planter speed was increased by 5 r/min.The least square regression model established above is still applicable for this case.The results showed that high detection accuracy can be obtained for different seeding speeds,and the relative error was between-2.26%-2.17%.