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Digital Control Amplifier of Electro-hydraulic Specific Gravity Directional Throttle Valve

Introduction The
  electro-hydraulic proportional directional throttle valve is composed of a valve body (mechanical structure) and a proportional control amplifier. There is no proportional control amplifier. The function of this type of valve is similar to a general electromagnetic directional valve. Proportional amplifiers are mostly dedicated analog proportional control amplifiers. The digital control amplifier introduced in this article not only realizes the digitization of the control amplifier, improves the performance of the valve, but also takes into account the needs of the valve control system, and improves the reliability, economy and flexibility.

1 Hardware structure
  The hardware structure of this control amplifier is shown in Figure 1, and its main functions are as follows: It has an RS-232 serial port, which can be connected to a standard RS-232 interface, and can form a remote control system with an upper computer or perform multi-computer coordinated control , The sending and receiving of data are all in the form of interruption. Four-channel high-power photoelectrically isolated PWM pulse width modulation output, each output current reaches more than 800 milliamperes, and has a freewheeling function to prevent high-voltage damage to the proportional solenoid coil. Each PWM output is equipped with LED display. The 8098 chip provides 10-bit A/D input, which can be expanded to 16 channels after being expanded by multiple switches. With keyboard and display, realize the man-machine interface. 8K of external RAM and 16K of external EPROM. In addition to the aforementioned A/D and PWM outputs, it also provides two programmable parallel I/O expansion interfaces for the valve control system. The main interface circuit to achieve the above functions is as follows: 1.1 Interface with the proportional solenoid The effect of the control amplifier on the valve body is to apply a PWM signal with a certain duty cycle to the proportional solenoid, so that the proportional solenoid obtains a corresponding amount of current, and then An electromagnetic force is generated to push the spool. Since the dynamic response requirements of the proportional control system are not very high, the drive circuit shown in Figure 2 is adopted. In the figure, the drive tube uses photoelectric isolation to isolate the drive part from the digital part of the controller, which improves the anti-interference ability of the controller.

  The PWM signal output by the controller is amplified by the drive circuit. Due to the inertia of the electromagnet, a DC current with an average value proportional to the duty cycle and an AC ripple component can be obtained on the coil. Therefore, the output current has a certain dithering effect; but because its frequency is the same as the frequency of the PWM signal, the amplitude is a function of the duty cycle of the modulation signal, the modulation frequency and the load time, and the frequency and amplitude restrain each other. It cannot be adjusted independently, which limits the chattering effect of this AC ripple. The frequency of the PWM modulation signal of this controller is 500 Hz, which is 2 to 10 times that of the traditional low-frequency modulation method, which reduces the influence of the coil current amplitude and the delay time of the switching characteristics on the coil current; on the other hand, the electrical feedback of the valve The closed loop encloses the entire mechanical part of the valve, and the hysteresis of the valve is better overcome. 1.2 The interface with the spool displacement sensor Because of the analog signal output by the spool displacement sensor, its voltage is between 3.8V and 9.0V, and it cannot be directly sampled. We use the signal processing circuit in Figure 3 to filter the input signal. , Zero adjustment and amplification processing, so that the output voltage is in the range of 0 ~ 5 volts, and the current provided is not less than 5 mA, so as to provide the A/D sampling of the 8098 chip; in order to avoid the influence of power fluctuations, the power supply is adjusted Filtering processing. Since the power supply here is introduced into the signal input terminal to pull down the output level, fluctuations in the power supply will directly cause the processed output fluctuations. Therefore, despite the hardware filtering of the power supply, in order to ensure the accuracy requirements, the corresponding filtering algorithm is adopted in the software, and the filtering is performed again.

1.3 The output of other interface actuators for the valve control system is detected by the grating, because the output of the grating digital meter used is 20-bit BCD code, which can be read in through the 8255 extended PA, PB and PC ports, and processed by software Converted to binary number. The output of the pressure sensor, because its voltage and current meet the A/D input requirements of the 8098 single-chip microcomputer, it can be directly used for conversion and sampling.
2 Software structure and design
2.1 Overall structure The whole software includes two parts, namely monitoring software and control software. The monitoring software mainly completes the man-machine interface function; the core of the whole software is the control software, which completes the control of the valve and the system. Figure 4 is a simplified block diagram of the entire program, in which, due to the real-time requirements of control, the return from the control program to the monitoring program is reset by the software. The storage space used by the monitoring software and the control software can be overlapped by most Without causing confusion.

2.2 Monitoring software After the 8098 chip is reset, the program counter points to the 2080 address unit, and the system automatically starts from here and runs the program. Therefore, set a close command here, and then perform some initialization, wait for input from the keyboard, scatter according to the key value, and execute the corresponding program. The program block diagram is shown as in Fig. 5. 2.2.1 Initialization (a) Close all interrupts (b) Set the stack pointer. In this system, access to the stack is mainly generated by program calls and interrupts, and the maximum push depth does not exceed 64 bytes. (c) The internal RAM and related external RAM are cleared. (d) Block the PWM output and A/D output to prevent the system from malfunctioning and causing adverse effects. (e) 8279 initialization; keyboard/display mode set command word, sent to control 10H through CPU, namely eight-digit display, left entry, code scan, double key lock mode. For the clock command word, in order to obtain the 100 KHz required by the 8279, the ALE frequency must be divided by 6, so the command word is 26 H. Read the display RAM, the address automatically increases by 1 after reading, so its command word is 70 H; when writing display RAM, the address automatically increases by 1 after reading and writing, the command word is 90 H; clear the display RAM and write to the display RAM Enter “P”.
2.2.2 After the above initialization of the man-machine dialogue, the man-machine dialogue can be started. Its main function is that the CPU recognizes the commands and data sent by the staff from the keyboard, and then executes the corresponding program. The keyboard is equipped with 16 data keys 0~F, four function keys LAST, RESET, NEXT/PRO and EXEC keys. Among them, the system will automatically power on and reset after RESET is pressed, and no CPU recognition processing is required. When the EXEC key is pressed, the initialization program of the control program will be executed. The NEXT and LAST keys are used to display and modify the contents of the next and previous unit of the current address unit; if no unit is currently displayed, pressing LAST is invalid at this time, and NEXT turns to the “PRO” key function, that is, this key The unit that is pressed, displayed and modifiable is the first address 7FF0H that stores control commands and data. 2.3 Control software Considering the requirements of the system, the control software needs to complete valve adjustment, system adjustment, interruption, timing, and corresponding data acquisition, input and processing necessary to ensure that the program is executed in a certain series. Figure 6 is the overall block diagram of the control software; each part is described as follows: 2.3.1 Initialization When the interrupt is turned off, the following initialization tasks for the control program are realized. (a). Clear some flags, input E and output U in the calculation of PID program (valve adjustment). The Qi and Mi calculated by Kp, Ki, Kd, ​​sampling period T and the time constant Tf of the low-pass filter are assigned to its storage unit. In the PID program, the representation and calculation of numbers are processed by fixed-point numbers. Although 8098 itself has a rich instruction system, there are instructions for adding, subtracting, multiplying, and dividing signed numbers and unsigned numbers, which brings great convenience to programming; however, in the operation process of PID programs, due to The display range of the points is limited, and overflow is inevitable in the middle. For this reason, there are several judgment flags, which need to be initialized before running the PID program.
  (b). The sampling time is up to the flag to be cleared; (c). The basic value assignment of the proportional valve flow dead zone; set the period of PWM, the maximum output current (including the frequency and amplitude of the dither signal); the output current is cleared, In order to avoid the random value of the output unit causing the system to malfunction. (d). Software timing interrupt and HSO interrupt initialization; open these two interrupts, that is, INT-MASK assigns #28H, and at the same time, put the entry addresses of the two interrupt service subroutines into the corresponding interrupt vector unit. (e). Set the stack pointer, SP points to the 00FEH unit in the internal RAM. 2.3.2 Sampling and calculation program (a). Read the given displacement; the input displacement is put into 7FF0H~7FF2H in the form of decimal number. Therefore, it is necessary to call the program to convert the three-byte BCD code into a binary number once , The result is put into the three byte units of 40H~42H, and saved during the operation of the whole control program. (b). Calculate the incremental PID formula Un+1=∑QiEi+Qi, Mi in Un. 2.3.3 The main program of the control program In the main program of the control program, because there is at least one PID program (valve adjustment), and the operation parameters in the PID are related to the sampling period T, for the correct realization of the PID control algorithm, this control The program guarantees a fixed sampling period through software timing. The acquisition of the fixed sampling period is actually by waiting, which equalizes the time gap between each cycle of the program. The prerequisite for this measure is that the sampling period must be greater than the longest time required to complete a cycle operation, that is, there is a limit to the maximum value of the sampling frequency. The following are the functional modules of the control software. (a). Sampling and inputting system feedback data. The data that needs to be sampled or read are the displacement of the spool, the displacement of the actuator and the system pressure and other parameters required by the valve control system. The data sampled by A/D (parameters such as spool displacement and pressure) need to be filtered and put into the designated unit. For the displacement of the actuator, the BCD code obtained from the secondary instrument also needs to be processed by the digital-to-digital conversion program. This article only uses the collection and filtering of the spool displacement, the others are prepared for the valve control system. (b). The valve control system adjustment program is the control strategy required by the valve control system. This article is only used as a signal generator. (c). The valve adjustment program, according to the relationship between the given amount and the feedback amount, the result of processing and calculation is put into the input unit of HSO, It is used as the input of PWM modulation and amplification. The valve adjustment method is PID control which is widely used in engineering. 2.3.4 Interrupt service program In the interrupt service program, there is a software interrupt timer, which is mainly used to set the start flag whenever the sampling time is up, and the main program can run. Another interrupt program is the interrupt output of HSO, which is the output of the controller. 2.3.5 Test test program In order to test and analyze the performance of the system, some auxiliary test programs are added to the control program. These procedures mainly include: signal setting and test data recording procedures. The recorded data is processed offline to obtain a dynamic curve.
3 Test results
  test research includes static performance and dynamic performance test. In the static test, the valve control system adjustment program is set as a linear signal generator, and in the dynamic test, it is set as a step signal generator. The dynamic data is recorded by the test data recording program, and the static test data is recorded by the XY function recorder. It can be seen from Figure 7 and Figure 9 that the electro-hydraulic proportional directional throttle valve composed of digital control amplifier has satisfactory dynamic and static performance. It can also be seen from Figure 8 that the flow characteristics of the valve are very poor near the zero point, and there are serious nonlinearities such as dead zones and hysteresis, which are inherent characteristics of this type of throttle valve.
4 Conclusion
  The digital control amplifier proposed in this paper has achieved satisfactory results. Compared with the analog control amplifier, it integrates the system control function, making the valve control system more economical, reliable and flexible. In the valve control system, the severe non-linearity of the flow characteristics of the throttle valve is a problem that needs to be considered and solved. On this basis, because the proportional valve is more economical and convenient to use and maintain than the servo valve, it has Broader application prospects.

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