Development Of Torque Target Flowmeter For Liquid Fertilizer

The agriculture in China is developing from the traditional rough style to modern precision,variable technology is an important method for precision agriculture.Variable fertilization can effectively reduce environmental pollution,improve crop quality and yield,reduce fertilizer input,reduce production costs,and has far-reaching strategic significance in the development process of moderm precision agriculture.The ultimate goal of variable fertilization is to realize precise fertilization according to the soil fertility and crop nutrition status.In liquid fertilizer variable fertilization control system,realizing precision fertilization need to adjust the fertilizer outlet flow real-time according to the current target fertilizing amount and flowmeter value in fertilization system.There are many kinds of liquid flowmeters,and each has its own scope of application.The target flowmeter has the characteristics of simple structure,strong vibration resistance,can measuring the high viscosity and fluid containing tiny solid particles and so on,is suitable for variable fertilization in liquid fertilizer.At present,the main target flowmeter are strain type and capacitance type.Strain type target flowmeter,the adhesive processing of strain requirements are very strict,easy to appear zero drift and temperature drift;Capacitive target type flowmeter in a large range ratio of measurement range,the signal output is nonlinear,prone to measurement error and even error.Aiming at the problem of appeal,this paper proposes a liquid fertilizer torque target flowmeter with a new force transfer mode(force-torque tube-strain voltage),in order to meet the measure requirements of liquid fertilizer variable fertilization system.The main research work of this paper is as follows:(1)According to the characteristics of the flow measurement of the liquid fertilizer variable fertilization system and the research status of target flowmeter,a kind of liquid fertilizer torque target flowmeter was designed.The design of flowmeter structure is completed by the new force transfer mechanism(force-torque tube-strain voltage),analyzed the measuring principle of the torque target type flowmeter,established the mathematical model of fluid flow which is square root relation with strain voltage.The error source of the torque target flowmeter in the measurement was analyzed,including the influence of the fluid flow area,target and measurement pipeline is not concentric,fluid flow state and the effects of temperature and fluid density,and proposed the corresponding solution.(2)The influence different target shapes and different diameters ratio on the pressure loss,sensitivity and resolution of the torque target was analyzed.Nine groups model of torque target flowmeter which contained different target shapes(disc shape,conical,spherical)and different diameter ratio(0.5,0.6,0.7)did numerical simulation calculation in eight groups with different flow rate based on Fluent software,then the pipe internal flow field,pressure loss and target force size were analyzed,the relationship between the pressure loss,the sensitivity,the resolution of torque target flowmeter and the different target shape and different diameter ratio are determined.Then,according to the actual demand,the optimal target shape(circular)and diameter ratio(0.6)were selected.(3)According to the working principle and measurement requirement of the torque target flowmeter,the hardware circuit of the torque target flowmeter was designed.The hardware circuit is composed of signal processing module,power supply module,human-computer interaction module,current output module and serial communication module and other modules.The characteristics of STM32 microprocessor were analyzed in detail;Designed the complex power circuit with 3.3V,5V,+ 12V,24V output;Flow signal detection circuit was designed by using the signal detection circuit design ideas on "primary amplifier,low pass filter and two stage amplifier";Human-computer interaction circuit,current output circuit and serial communication circuit were designed;In the light of the interference of hardware circuit,the corresponding measures of PCB anti interference were put forward.(4)According to the work flow and system requirements of the torque target flowmeter,the software system of the torque target flowmeter was designed.Software system is composed of system initialization module,flow detection module,flow calculation module and serial communication module.The modular design method of "top-down" was adopted in the software system.First,according to the work process to develop the overall plan,and then,the various needs of the program was divided into the sub module with separate function,respectively,achieved the function of each subroutine,at last,the flow measurement was completed by the main program scheduling routine.The software digital filter technology was adopted in flow detection subroutine to reduce the noise interference,and the data was reliable;In the flow calculation routine,against on the effect of environmental temperature change and fluid density change on flow rate,temperature and density compensation method was proposed,improved measurement accuracy of flow meter.(5)The experimental performance test of the designed torque target flowmeter was carried out.Including flowmeter calibration experiment,performance test experiment and liquid fertilizer flow test.Experimental results show that:In this paper,the design of the torque target type flowmeter measurement results are stable and reliable,the full scale error is within 2.0%,the measured minimum flow rate is 0.203 m3/h,the maximum flow rate is 3.986 m3/h,and the range ratio is 20:1,can meet the requirement of the flow measurement in the variable rate fertilization system of the liquid fertilizer.