AVR-based closed-loop control system for experimental loading

　　Separate hydraulic jacks are widely used in various engineering structure loading operations such as production construction, scientific experiments, etc.

　　The equipment is generally composed of an electric high-pressure oil pump + a compression (tension) jack. The specifications of the loading system discussed in this article are shown in Table 1.

　　The oil pump motor directly drives the pump shaft to rotate, the plunger to reciprocate, the oil inlet valve/oil outlet valve to work, and the hydraulic oil enters the jack through the high-pressure oil pipe. The user can manually turn the unloading valve in the pump to different positions through the handle on the pump station to achieve the system's loading/unloading, thrust/pull, loading rate and pressure stabilization operations.

　　In scientific research and experimental loading situations where high precision is required for force and displacement values ​​or stabilization time, it is obvious that this manual operation method is not competent. Adding a computer measurement and control system to the equipment has become a technological innovation that keeps pace with the times and is also the best solution to improve the cost-effectiveness of such equipment.

　　1 Lower computer

　　The lower computer control core is composed of ATmega128 (see Figure 1), which mainly performs A/D conversion on the working signals transmitted by each channel amplifier module and drives the stepper motor according to the instructions of the upper computer to achieve the working purpose of the control execution system.

　　1.1 Data Collection

　　The test of force and displacement signals is completed by a bridge sensor composed of resistance strain gauges , and the signal amplifier uses an AMP1 amplifier module. The proper matching between the two is crucial to whether the single-chip microcomputer can successfully perform data acquisition. The key technology is to regulate the balance of the bridge. After the signal enters the single-chip microcomputer, if there is a small amount of offset voltage, it can be compensated by software. If the offset is too large, this channel will not work. Therefore, it is necessary to use to adjust the analog signal with a large offset. The zero adjustment circuit is shown in Figure 2.

　　1.2 Equipment Control

　　The key to equipment control is to use electric power to replace manual power in the rotation of the oil pump handle. Fix the handle plate on the shaft, arrange several position electrode points on the plate, and use a stepper motor plus gear transmission to solve the problem of sensing and moving the handle position, as shown in Figure 3.

　　The PWM, clock and forward and reverse signals of the single-chip microcomputer Mega128 enter L297 through the optical coupler to generate a phase sequence signal, which is driven by the full bridge of L298N, so that the stepper motor drives the gears to rotate together according to the control of the single-chip microcomputer. The circuit diagram of driving the stepper motor is shown in Figure 4.

　　1.3 A/D and serial communication

　　In the serial port 0 initialization programming, ICCAVR uses the code UCSROB=(1<

　　2 Host computer programming

　　The host computer is programmed with VB, which mainly includes window layout, data communication and processing, voltage stabilization function, etc.

　　2.1 Form Layout

　　Considering the needs of practical applications, when designing the host computer, the system window interface layout is as simple, intuitive and clear as possible. In programming, the parameter calibration process and the seamless link between test data and large application software are particularly highlighted. As shown in Figure 5. [page]

　　2.2 Data Communication and Processing

　　Settings for the communication control MSComm:

　　Since InByte() is a dynamic array, InByte(0)=1 and InByte(3)=2 are defined as the force and displacement data numbers. To prevent the upper and lower computer data from being out of sync, the data sent to the receiving buffer by the lower computer must be reconfirmed:

　　These 6 bytes are two sets of data:

　　For monitoring data, if it is equal to or greater than the upper limit, the handle returns to position 3# (stop/stable pressure). To prevent the sensor data from hovering near the critical point, causing data instability, you should consider jumping out of stickiness when programming.

　　The computer system clock is used to realize the voltage stabilization and delay function.

　　3 Conclusion

　　The built-in 10-bit ADC function of the AVR microcontroller makes the design and debugging of the A/D conversion circuit of the lower computer simple and easy. The AMP1 amplifier module has a wider adjustment range after adding the bridge zeroing circuit. In actual use, due to the differences in the internal structure and manufacturing process of the hydraulic system of each manufacturer, when the positioning control of the fully automatic handle plate is not good, the solid-state relay can be directly controlled by the microcontroller I/O port , and then the oil pump motor can be controlled. It can also obtain better working results.