Nowadays automatic batching system has been widely used in metallurgy,building materials,chemical industry and so on accompany with technologies updated greatly,and the performance of auto control system definitely determines the accuracy of batching system. But the accuracy is actually hard to be guaranteed in practical batching process. It’s well known that batching accuracy is a key factor affecting product quality,and batching system’s efficiency also plays a crucial role during production process. Therefore in recent years many companies focus more on improving efficiency of batching system as well as batching accuracy.
Composition of auto batching system
Auto batching system consists of three parts:mechanical system,detection system and electrical control. The role of mechanical system is to store ingredients,complete transportation processes,etc.,it is mainly composed of feeder,conveyor belt,hopper,switch of material door…; Detection part includes scale body,load cells,etc.,it’s task is to weigh varieties of ingredients during batching process. For electrical control system including motor,frequency converter,weighing indicator and computer,etc.,it can control the conveying,feeding and discharging processes of materials.
How does auto batching system work?
Set each material’s value upon batching proportion first. Weighing controller or computer controls feeding process of various ingredients in sequence,detects feeding amount of ingredients,and controls feeding motor’s speed according to ingredients’ amount set by system as well,then closes the feeding valve when ingredients’ amount reaches the set point. Finally batching process is completed when all ingredients’ amounts reach the set values.
According to the structure and application features of auto batching system,some controlling methods can be referenced to improve batching accuracy in different production processes.
Working modes of batching system
1. Manual mode
Operator uses manual button to implement batching process,and computer is only for display purpose as a terminal. This way is inefficient,generally applied in system debugging and some special situations.
2. Semi-automatic mode
Computer implements batching process and achieves the right ratio of ingredients according to operator’s instructions.
3. Automatic mode
Batching system automatically completes batching tasks based on selected batching procedure,no further human intervention required during whole process. In batching process,the accuracy will be affected by many factors,including feeding speed,uniformity,hopper vibration,the remainder in dropping stroke and overshoot,etc. Hopper vibration causes that weight signal obtained by load cells can’t accurately reflect practical weight of the ingredients in the hopper. Ingredients flow into hopper from feeder or valve,a portion of the ingredient during flowing process has not yet reached the ingredients surface in the hopper,then the remainder occurs. However the overshoot is due to the delay from the time a stop signal sent by electronic device till the time the valve is actually closed or the feeder stops. The random variation of overshoot caused by the differences in specific ingredient density,flow rate,dropping distance brings great difficulties to the control of batching accuracy.
How to improve the batching accuracy in auto-batching system? Here are three control modes.
1. Computer control
Automatically weigh various ingredients based on the preset dosing plan,controlling all sections of batching to complete the whole process. The key of accurate batching in auto batching system is to control the speed,start and stop of feeding motor. During batching process,motor speed should be adjusted according to feeding amount,therefore high-speed mode supported by low-speed mode can be applied to promote both batching efficient and dosing accuracy. If ingredient amount in the hopper is far from the set value,high-speed feeding mode is adopted to achieve high efficient of batching. And low-speed feeding mode should be started when ingredient amount in the hopper is close to the set value. The motor stops rotating when the spill value is reached due to the remainder in the air. Actually batching system will set the spill value for motor stopping according to the operating conditions,because each material density is different,so the weight of each air remainder is also different after motor stops rotating. The end of feeding time is determined by drop compensation coefficient of each ingredient. Reading the final weight and comparing it with the set value after the weighing hopper is stable,that can be the basis for the next selection of drop compensation coefficient. That means the current batching will use the average value of last batching drop value and drop compensation value to compensate for drop process,this way can increase batching accuracy accompany with the increase of batching number.
2. PLC control
Combining computer with traditional relay control system can enable industrial control process more flexible and reliable. In batching system PLC is in charge of receiving external signals to judge the operating status of current system,and output a variety of signals which can control switch,relay,electromagnetic valve and other components to perform corresponding actions. In actual batching process,motor starts to run;feeding system adopts high-speed mode first;when the ingredients weight approaches the target value,dot motion control mode should be adopted to ensure accuracy. Moreover PLC is also responsible for receiving the control commands from computer to complete batching control.
3. Weighing controller
Weighing controllers are typically designed for packaging,sorting,auto batching and other applications. They can collect weight signals,deal with them and output corresponding control signals. It is one of the most common control modes in batching system as weighing controller is easy to install and debug. Batching weighing controller can detect and control ingredients with high reliability,improve batching accuracy greatly,achieve weight proportion of ingredients directly. Generally batching controller has two feeding speed:high-speed and low speed. High-speed feeding is adopted at the begging of batching,the spill value of high-speed feeding will determine the feeding amount for low-speed process. After the set value is reached,high-speed changes to low-speed,and spill value of low-speed is to compensate for the impact of ingredients remaining in the air,so actual weight can approach to the set value as close as possible. Therefore setting weighing controller properly according to pratical production process,as well as setting proper spill value of fast feeding and slow feeding,then batching accuracy can be controlled and insured.
Batching weighing controller is suitable for processes with a uniform feeding speed. That is to say,batching accuracy will be affected if feeding speed fluctuates greatly which will cause a large dosing deviation.
At present predictive control algorithm is introduced into batching system,which contributes to some uncertainty factors of batching process can be well controlled,batching error is reduced,and weighing speed is expedited as well,this mode is suitable for batching systems requiring high accuracy.