# Research on the Autonomous Control Technology Used in the Slurry Mixing System of Cementing Units
## 2 Working Principle and Model Establishment of the Slurry Mixing System  
### Working Principle of the Slurry Mixing System  
Figure 1 shows the structure of the automatic slurry mixing system of cementing units. The slurry mixing system includes a water pump, a water flowmeter, a mixing tank, a mechanical agitator, a recirculating pump, a densitometer, an averaging tank, and a boost pump. The automatic mixing system ensures the quality and efficiency of the cement mixing process.  
The water pump is used to pump water to the slurry mixing tank. The water flowmeter is used to measure the water flow rate and send the flow rate signal to the controller. The shape of the slurry mixing tank is subject to the specific configuration of the cementing unit, and it is used to store the cement slurry temporarily and provide proper space for mixing water and dry cement together so as to mix cement slurry of uniform density. The agitator is mainly used to stir the slurry in the slurry mixing tank to ensure that the dry cement and water are fully mixed to produce high-quality slurry. The recirculating pump removes the slurry from the slurry mixing tank to recirculate the slurry for secondary and multiple mixing of the slurry. At the same time, the slurry re-enters the mixing tank after passing through the densitometer in the branch pipe. The densitometer is used to monitor the slurry density in real time and send the density signal to the controller. The averaging tank is able to make the slurry more uniform, improving the quality of the slurry further. The boost pump is used to pressurize the slurry from the slurry mixing tank and then pump it to the suction manifold of the triplex plunger pump.  
The speed, stability, and anti-interference ability of cement slurry density control in the cementing process reflect the control performance of the control system, and the set density value of cement slurry can be adjusted at any time according to the specific working conditions of downhole cementing. When the set density changes during operation, it is equivalent to the step response, which requires that the slurry mixing control algorithm can immediately respond to the adjustment, and the controller can act quickly to output the cement slurry that meets the requirements. The specific control process is as follows:  
Figure 1: Control schematic of the automatic slurry mixing system.  
the water flow sensor detects the water flow and feeds the data back to the computer, and the computer calculates the valve position of the lower gray valve through the water flow; as long as the water flow changes, the lower grey valve will respond accordingly; the controller reads the density measured by the densitometer, and the control algorithm combines the set density with the actual density to calculate the new value of the valve position of the lower gray valve and adjusts the valve opening degree. At this point, if the density of the lower gray valve changes, the control algorithm will respond immediately to adjust the valve position degree; at this point, if there is a certain degree of deviation in the lower ash valve position, the feedback control loop begins to act so that the deviation reaches its minimum.  
The automatic control system in the specific operation process may encounter hysteresis problems; the mixing tank mixing cement slurry density and triplex plunger pump discharge cement slurry density cannot be synchronously changed; there will be a specified lag time difference. In order to avoid the above problems, there is a need for changes to occur in the system. When the density of the change in the computer is based on the actual situation of rapid output, the lower gray valve position quickly changes to reduce the mixing slurry obtained from the root tank cement slurry density reaction time difference. Figure 2 shows the principle of the automatic control system.