The Design of Control System of Slurry Mixer

 
Abstract: In order to improve the mixing quality and automation, a dense slurry mixing system based on programmable logical controller is designed. The principle of the process control flow and hardware interface circuit of control system for slurry mixer are introduced in detail. The system is composed of PLC, transducer, ultrasonic concentration meter, electromagnetic valve, and it uses PID  to complete data processing. It realizes the automatic control of the pulp concentration and liquid level ,through the detection of pulp concentration and liquid level. The system which is improved on the basis of the original device has the advantages of simple structure, convenient application, etc. The system can be used as the design basis of slurry evenly mixer.
 
Key words: PLC control; mixing efficiency; concentration control; variable-frequency
 
 
 
Introduction
 
    Slurry mixer is an important component part in a whole set of slurry mixer devices, and it is widely used in mining production, medicine, chemical, synthetic materials productive lines[1]. In order to assure the good slurry mixing effect ,the existing devices mostly are equipped with high power motors on mixers, but the special features of slurry such as: a high concentration , a high viscosity, a high density though, reduce the occurrence of sinking, inevitably increase the power consumption and reduce the production efficiency[2]. In addition, when the level h increases, the natural pressure of the spindle is reduced, accordingly, but correspondingly increasing the electricity consumed by slurry and reducing the effective volume of mixer, which is unfavorable to the operation of slurry mixer. These conditions improve the enterprise’s the consumption of energy and reduce production efficiency. And Slurry mixer the uses excessive motor for a long time , under overload, will reduce the working life of equipment , which causes a very bad influence on the economic benefits of enterprises. Therefore , considering the controlling respects, the development of high -efficiency mixers that is capable of adjusting mixing intensity according to the requirement of actual conditions, to meet the production need , is the pressing desire of many mining enterprises.
 
  For solving the above problems, we developed a slurry mixing control system based on PLC , by detecting stirring density ,to adjust the motor’s output revolving speed, achieving the actual time adjustment of stirring density and the purpose of ensuring layout concentration.
 
 
1.The Design of Slurry Mixing Devices
 
 
  1.1 The Principle of Design
 
   The ultimate purpose of slurry mixing devices is to improve the stirring effect . Large metal slurry mixing devices aim to make solid and liquid system heterogeneous spread out, so that they can be in suspense state[3]. The kind of mixing effect of this devices can be represented by the concentration. When the additive quantity of materials is same, the axial concentration of particles rise with the increase of rotation speed of mixing devices[4-5]. This is the motor output high speed provides high stirring power for equipment. Under the action of high stirring power, the mixed materials can be stirred more evenly, lowering the amount of material accumulation at the bottom of the tank. The low density of slurry suggests the need to improve mixing intensity. So this device is designed according to this principle. So this device is designed according to this principle. By detecting the change of the slurry density, correspondingly to adjust the motor rotation speed within the range to guarantee the quality of the slurry suspension. to insure the effect of mixing , meanwhile to avoid unhealthy phenomenon such as the motor power is too big, causing a phenomenon of large power consumption and low production efficiency .
 
In the specific applications, according to the working principle of DF - 6420 ultrasonic concentration meter , when the ultrasonic is spread in slurry suspensions, based on the solid content in slurry and the relationship between the particle size and its amplitude change to get the corresponding density by the measurement of ultrasonic attenuation. According to the principle of acoustics, when plane ultrasonic is spread in slurry, as the propagation distance is a shape like L, the change of the voltage E can be showed by following type :
 
 
 
                                          (1)
 
     
Type: : the launch voltage
:  attenuation coefficient
The attenuation rate caused by suspended particles can be obtained by type:
 
 
 
                (2)
 
 The acoustic amplitude accepted by immersion sensor will attenuate, along with the increase of slurry density . The converted voltage value, corresponding acoustic amplitude, also attenuate along with the increase of slurry density . After the calibration of the concentration - voltage decay curve ,we can get the density value from measuring the voltage level. Therefore, concentration of the input signal , controlled by PID algorithm, correspondingly, according to a linear relationship between concentration and motor rotation speed ,through giving output analog signal by adjust of the instruction to frequency converter, so as to control the motor.
 
 Motor rotation speed and mixing device concentration have numerical relationship. The test results of double -shaft mixer is in literature 7, as is shown in table 1[6]. This table is showing the relationship between the corresponding impeller linear velocity with the motor rotation speed and the quality percent concentration. So for different slurry mixing equipments’ electrical motor and slurry concentration, there exists corresponding quantity relationship.
 
                   Tab. 1 Experimental Result of Different Linear Speed
 
 
                    

Experiment number	Parameters							Results
	The ratio of Length and  width	Uniaxial phase	Biaxial phase	Biaxial arrangement	Leaf blade angle	Number of blades	Linear velocity
1	0.78		stagger	Front and back		7	1.1	0.97
2							1.2	0.74
3							1.3	0.65
4							1.4	0.59
5							1.5	0.35
6							1.6	0.39
7							1.7	0.47
8							1.8	1.05

 
 
 
 
1.2 The basic structure of the device
 
 Slurry evenly mixing control system consists of  mixing machine, ultrasonic (slurry) concentration meter, PLC control system, liquid level switch, etc. In the practical application of system, the configuration can be adjusted according to the needs of users, to adapt to the different technological requirements. Figure 1 shows the slurry evenly mixing control device’s structure diagram.
 
 
 
                       
 
Fig.1 Basic structure of Control System for slurry evenly mixer
 
     In figure 1. The mixing container; 2. The liquid level switch; 3. Incoming electromagnetic valve; 4. The impeller; 5. The middle liquid level switch; 6. The upper liquid level switch; 7. Exchange motor; 8. The speed sensor; 9. The ultrasonic concentration meter; 10. Immersion sensor; 11. Discharge solenoid valve.
 
Lower liquid level switch 1 is installed under the lower part of impeller of the mixing device, and the upper liquid level switch is installed above the mouth of discharge, the middle liquid level switch is at the same height with the discharging mouth at the bottom of the pipe, ultrasonic concentration meter is installed on the end cover of stirring device, connecting the PLC and immersion sensor respectively, immersion sensor’s test end is into the mixing device, lower than middle liquid level switch, so as to detect the slurry density on discharging mouth . Immersion sensor is installed hanging through the guardrail , and it can adapt to the operation condition of the high strength mixing environment.
 
 
2.The hardware design of PLC control system
     
 
  2.1 control requirements
 
  On the mixing device’s wall ,equipped with liquid level switch, upper level switch ST1, middle liquid level switch ST2,lower liquid level switch ST3; KM2, KM3 are incoming electromagnetic valve and discharge solenoid valve, respectively.
 
    (1) In the initial state, the container is empty, add slurry to the container, now starting mixing device can be manually when pressing the switch SB1, KM coil get electricity, and transducer and motor get three alternating power supplier. to start mixing device; slurry can also be directly put into the device, when liquid level reached the lower liquid level switch, the lower liquid level switch acts, then the mixing device starts.
      
    (2) When the slurry surface is higher than the height of discharging mouth, then the upper liquid level switch will act, the incoming electromagnetic valve close at this time, in order to reduce the liquid surface. When the liquid level down to below the middle liquid level switch, the feed solenoid valve restore to the original state of access.
 
    (3) Ultrasonic concentration measures the discharge slurry density, by PID controlling, according to the quantitative relationship between the motor rotation speed and discharge concentration’s fitted curve, outputting correspondingly motor rotation speed, guarantee the discharge meet the requirement of the slurry.
 
    (4) when the transducer failures, switch acts, KM coil loses electricity , indicator light display transducer fails, the KM cascaded between transducer and three alternating power supplier breaks, transducer loses electricity, having the effect of security protection.
 
 
 
 
2.2 The design of hardware circuit
 
 According to the control requirements, take Siemens S7-300 series PLC as an example, hardware wiring diagram of controlling system composed by PLC and frequency converter is shown in figure 3. The I/O allocation table is shown in table 1. The system make PLC processor, ultrasonic density meter, speed sensors, frequency converter and electromagnetic valve an effective combination,  making all components and actuators run according to the preset program , so as to realize the automatic control of slurry density. The system is mainly composed by the following three function modules.
 
    (1) control module, the S7-300 series of products have high-performance CPUs, its all kinds of modules with different performances can meet and adapt to automation control tasks. Simple and practical distributed structure and general ability of network make the application very flexible. When the control task increases, it can expand freely [7]. This device in the selection of S7-300 PLC CPU 314 c - 2 inhibits PTP, choose SM331 analog input module input of an analog is measured by the concentration meter, choose SM332 analog output module to the transducer output analog signal, choose SM321 digital quantity input module 5 switch to digital quantity input, select SM322 four digital output digital output module. Ultrasonic concentration meter choose DF - 6420 ultrasonic concentration meter. RS485 is ultrasonic concentration meter designated use serial communication interface.
   (2) The detection module. Using ultrasonic density meter to collects the analog signal in slurry mixing devices, and using speed sensor to detect the actual output voltage signal, and constantly adjusting the rotational speed to the required range.
     (3) Actuator, including transducer, motor, solenoid valve, etc.
       The whole section of the control system is mainly for field data collection, filtering; collected analog signal through the control loop of setting and control of PID (proportion, integral, differential) parameter , and its quantitative relation of speed to set calculation: Level control valve, etc.
 
 
                    
 
 
             Fig.2 Hardware wiring diagram of Control System for slurry evenly mixer
 
 
 
                     Tab.2 I/O Assigning address of I/O
 

Input			Output
add.	part	function	add.	part	function
I0.0	SB1/ST2	start	Q0.0	KM1	Switch on
I0.1	SB3	stop	Q0.1	3XD	breakdown
I0.2	SB4	fault protection protectio	Q0.3	KM2	Incoming electromagnetic valve electromagnetic valve
I0.3	ST1	Upper liquid level	Q0.4	KM3	Output electromagnetic valve
I0.4	ST2	Down liquid level	A01		Transducer input

 
 
 
 
 
 
 
 
 
 
 
 
3 .control algorithm and software design
 
     The device is involved in a variety of controls, the following part will focus on concentration control. In the practical application, the closed loop PID control is used for the slurry density, PID is a typical closed-loop control algorithm the and the most mature and widely way of regulating in continuous system [8]. This device is through the relationship between motor rotation speed and slurry density (W),in each circulation of the PID loop ,accordingly, to adjust motor speed, as the controller output control signal is used to complete speed regulation of slurry mixing device, namely the size of its output value and speed is one-to-one relationship, so position type PID algorithm is adopted. PLC control system design principle diagram is shown in figure 3.
 
              
             
                 Figure 3 control system working principle diagram
 
                The expression of the principle of PID controller (1).
 
       (3)
 
                          
      Type: t - time. U (t)- the output of the PID algorithm; e (t)-system settings and process values deviation; kp - amplification coefficient (gain or magnification); TI - the integral time constant; TD - differential time constant.
 
  The PID algorithm of PLC control program uses the function block of its own internal instruction to perform, the serial number of the PID loop, given value range, the proportional gain, sampling time and integration time, the PI arithmetic of concentration. According to the set of sampling period to perform the PID function block.
 
 
 
 
   
 
 
 
 
                             Fig.4 Flow chart automatic of control system
 
 
 
As it is shown in figure 4, it is working principle diagram for control system. When the parameter of wizard is set, it automatically generates No. X loop initial sub-program, interrupt routine, the symbol table and data block. Figure 5 is PID initializing instruction, to complete the initialization of the sampling time ,the proportional gain, integral and differential time. When the slurry density is below the setting value, it should raise motor rotation speed, on the other hand, lowering speed. So its proportional gain settings should be positive [8]. The system can be controlled by manual and automatic control of liquid level switch. Auto - Manual at this time is 1, the circuit is automatic simulation control (PID closed-loop control), VD200 is standardized real input values in manual mode. When performing PID arithmetic, we input analog quantity feedback of concentration to the module  through the PV – 1.After PID arithmetic , the output calculation results of AQW0, through the cycle this implementation, to realize the adjustment of the concentration by PID internal arithmetic .
           
 
                 Figure 5 PID initialized instructions
 
 
 
4. conclusion
 
  Slurry evenly mixing system conveys the concentration and rotation speed analog quality testing information and liquid level switch digital information to PLC control system for the corresponding calculation and control , achieving the adjustment of stirring intensity, according to the working condition demand, and the purpose of controlling safely liquid level, ensuring of the quality of discharge. This article mainly introduced the design principle of this device, hardware and software design. The system is based on the original equipment. The application is convenient, and the structure is simple, according with energy conservation concept, and provides proper stirring space for mixing device, realizing the automatic control of the slurry density.
 
 
 
 
 
 
 
 
 
 
 
 
 
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