Design of control system for large centrifugal casting machine

Large centrifugal casting machines are mostly used to produce rolls. In centrifugal casting, the rotational speed of the mold (also called the tube mold) is very important, especially in the roll production, which requires secondary casting (adding core), and particularly requires strict control of the speed, time and temperature. In order to meet the casting process requirements of the rolls, we designed a control system for the large centrifugal casting machine. The system controls and dynamically tracks the real-time control and precisely controls the process of centrifugal casting rolls. In the past, the control systems of large centrifuges mostly used open-loop automatic/semi-automatic control and electromagnetic speed control systems, which had poor working stability and many artificial factors. Through the improved design of hardware and software, we use Siemens AC drive (6SE70 series) or DC speed control system (6RA70 series), sensor input temperature, speed and host vibration measurement signal, programmable controller is calculated in real time, as long as By setting corresponding working parameters on the interface, the set process curve can be automatically tracked; the whole production process realizes real closed-loop and automatic tracking control, eliminating the influence of manual intervention and other factors, improving product quality and production efficiency.

1. Main process parameters of large centrifugal casting machine

(1) Initial speed increase rate: the speed at which the centrifuge rises from standstill to the time required for casting;

(2) Maintaining the solidification speed: the rotation speed maintained by the centrifuge after pouring;

(3) Stop deceleration speed: The deceleration stop speed is the stop speed.

2. System hardware

The hardware consists of industrial computer, programmable controller PLC, analog input, output, DC speed control device or AC inverter and peripheral equipment. Figure 1 is a block diagram of the control system of a large centrifuge.

The industrial computer mainly completes the remote real-time monitoring and management report production of the centrifugal casting machine through PLC, and monitors the on-site centrifuge speed, cavity metal liquid temperature, bearing temperature and centrifugal casting machine vibration working status in real time. During the centrifugal casting process, the computer can continuously and automatically record the temperature profile of the molten metal in the mold.

Working principle: The main process parameters of the centrifugal casting machine are set by the interface or the process parameters of the original product are adjusted. When the current product is met, the current product control curve is displayed, which is confirmed according to the actual needs, and the pouring is started by the control button. Casting, secondary casting and other signals; PLC according to the process parameters of the product and through the field analog input to get the actual speed of the current centrifugal casting machine, after calculation, through the analog output a practically required speed, while passing the analog input centrifuge Vibration signal, temperature signal, real-time diagnosis of the working state of the centrifugal casting machine; the motor working current signal of the centrifugal casting machine diagnoses the load of the centrifuge in real time, displays the working curve of the product in real time, and confirms the quality of the product. Figure 2 and Figure 3 show the electrical schematics of the large centrifuge used in the Wuhan Iron and Steel Plant.

3. Software design

The purpose is to enable the user to arbitrarily set or call the process parameters and process curves of various products according to their own products, that is, to refresh the production plan in the form of a formula, and to pass the process parameters of the site and the working state of the centrifuge according to the needs of the user. The real-time acquisition of the working state of the motor displays the numbers and curves in real time in the form of animations, and displays the warnings at the same time, providing the user with a solution to solve the actual working problems in real time.

Firstly, various process parameters are saved from the interface, and various process parameters can be reset to generate a new formula, as shown in Table 1 for the roll casting process parameters.

Since the time required for the secondary casting in the roll generation is strict, it is the key to the roll production whether the solidus temperature can be confirmed in time and correctly. In the past, the solid phase temperature was confirmed by manual observation, so the stability of product quality could not be guaranteed. Now through the software, the solidus temperature can be automatically confirmed and the downtime can be automatically controlled, thus ensuring the stability of the roll quality. At the same time, the software can automatically predict the working state of the centrifuge through the automatic measurement of the vibration amplitude of the centrifugal casting machine itself and the bearing temperature of the centrifuge. If there is any problem, it can promptly prompt to ensure the normal operation of the centrifuge.

The verification of the temperature inflection point is based on the following two formulas:

Inflection point velocity Vi calculation formula: Vi = V setting - a (V setting - V actual) (where a is the empirical coefficient);
The formula for calculating the slope Ki of the inflection point temperature curve:
Ki=∑Fj1-∑Fj/Ti
Where j1 is (i-10~i-6), j is (i-5~i), that is, the first 5 values of the first 10 values of the inflection point are subtracted from the 5 values divided by the time to obtain the slope of the temperature inflection point. 4.

The pouring temperature is 1350 ° C, the solidus temperature is about 1100 ° C, the centrifugal casting machine roller speed is 1400 r / min (can be determined according to the inner diameter of the casting), braking time: large parts within 5 min, gravity coefficient 100 ~ 110g.

According to the above hardware configuration and software design principle, the process parameters are set in advance on the interface or computer before the start of production, and the system is started after the check is correct. Firstly, the control system converts the initial speed increase rate into an analog output signal through program design and calculation. The analog quantity signal drives the motor through the DC speed control device or the AC drive to accelerate the centrifuge from the stationary state to the initial increase in time 1. Speed: After the centrifuge reaches this speed, the second step is to pour the high-temperature molten iron. After the pouring, the rotation speed is maintained for cooling and solidification molding; the centrifuge performs the mold cavity temperature detection during the high-speed rotary cooling solidification forming process. When the mold temperature drops to the solidus temperature (such as 1100 °C in Figure 4), the control system must control the centrifuge to start deceleration, and the centrifuge speed must be reduced from speed 2 to speed in time 3. 3, that is, the shutdown state; then proceed to the next process, that is, the second casting.