Design of pressure control system for deep sea environment simulation experimental device

There are a large number of mineral resources and microbial communities on the deep seabed. Research on biological mineralization and the origin of life in this environment will help to clarify the metabolic mechanism of deep-sea microorganisms regulated by pressure and obtain valuable extreme environmental genetic resources, which is of great significance to a series of disciplines such as geology, geochemistry and life sciences. However, due to the particularity of being in the extreme environment of the deep sea, it is difficult to observe and study it in person. It is necessary to establish a simulation system of the extreme environment of the seabed in the laboratory to conduct experiments to cooperate with relevant scientific research.
In view of the particularity of the deep-sea environment, a set of high-temperature and high-pressure simulation test equipment has been developed. This paper mainly discusses the relevant issues in the design of the pressure precise and stable control circuit and the development of control software for the device.

1 Design requirements and system composition
1.1 Design requirements
The research work and the technical indicators to be achieved are as follows:
1) The pressure control range is 0-20 MPa, the accuracy is +2% FS, and the minimum pressure gradient is 1%/min;
2) Accurate and stable control of pressure, which can be continuously adjusted within the full working range;
3) Real-time curves and digital display of pressure and flow, with curve recording and playback functions.
1.2 System composition
The simulation experiment device mainly includes a deep-sea environment simulation system composed of a water pump, a deep-sea environment simulation chamber, a water tank and pipelines, and its monitoring and control system. It can simulate the extreme marine environment of the seafloor hydrothermal vent, as well as the general marine environment. It can also complete the cultivation, addition and sampling of samples. The flow rate of the pump can be adjusted steplessly within the output flow rate range as needed, and the function switching can be achieved by opening or closing the stop valve. The monitoring and control system mainly completes the monitoring and control of parameters such as temperature, pressure and flow. The entire system is shown in Figure 1.

This article mainly introduces the design of accurate and stable pressure control system, and other parts are introduced in another article. The motor in the system uses a three-phase brushless DC motor with good speed regulation performance, small size and high efficiency. Because the single-chip microcomputer has low price, rich on-chip resources, and can be flexibly programmed, a control system with a single-chip microcomputer as the core is adopted. When working, the sensor feeds back the detected pipeline pressure and load speed to the single-chip microcomputer, and further triggers the speed regulation system composed of PI to regulate the motor speed in PWM mode. The motor drives the oil pump to work and provide a continuously adjustable pressure source.

2 Control system hardware circuit design
The overall block diagram of the control system is shown in Figure 2. This control system is mainly composed of control circuit, drive circuit, display circuit, and RS485 interface circuit. This system is a speed closed-loop system. After the position signal of the Hall position sensor is processed and sent to a dedicated driver chip, a speed pulse signal is generated. After being processed by the single-chip microcomputer, it is converted into a speed. Then, the incremental Pl algorithm is used to obtain a PWM control signal. The photoelectric coupling circuit drives the dedicated integrated driver chip to close the loop to control the motor speed. At the same time, the single-chip microcomputer also monitors the operating status of the control system. When the system has a short circuit, overcurrent, overvoltage and other faults, the single-chip microcomputer will block the PWM output signal, stop the motor, and display the fault through the LED circuit. Considering that different application occasions have different requirements for the control system, this paper considers the independence of each functional component when designing it and retains the corresponding interfaces to form a complete system.

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2.1 Control circuit design
The control circuit is mainly composed of Atmega8L microcontroller, PWM signal generation and processing circuit, current detection circuit, speed detection circuit, isolation circuit and interface circuit.
2.1.1 PWM signal generation and processing circuit design
In this control system, the internal timer of ATmega8L is mainly used to generate a fixed-frequency and width-modulated PWM wave signal to control the speed of the brushless DC motor. Here, ATmega8L's Timer2 is used to work in fast PWM mode to generate a high-frequency PWM waveform. After the waveform is generated, it needs to be processed to obtain the desired output signal. The processing circuit is shown in Figure 3.

2.1.2 Current detection circuit design
This paper uses Allegro's ACS712 integrated chip to expand the peripheral circuit for measurement. ACS712 consists of Hall element, Hall current drive element, deviation adjustment circuit, signal recovery circuit, and signal amplification circuit. It has the characteristics of low price, high precision, and good insulation performance. The current detection circuit is shown in Figure 4. When the current of ACS712 is zero, the 7-pin Vo outputs 2.5 V, so a precision resistor RP1 is designed to divide the voltage to generate a 2.5 V voltage, so that the output voltage Uo of the amplifier circuit changes linearly from 0 V; in order to improve the load capacity of the resistor divider, even if the 2.5 V voltage is not affected by the subsequent circuit, a first-level voltage follower is used here to make the output Ui2 2.5 V. According to the superposition principle, Uo in Figure 4 can be calculated as:

From formula (3), we can see that the output voltage is proportional to the current between pins 1, 2 and 3, 4. This voltage is sent to the A/D converter of ATmega8L to get the working current of the motor. This current can be used to perform torque closed-loop control and over-current protection on the brushless DC motor. [page]

2.1.3 Speed ​​detection circuit design
Speed ​​detection is very important for the control system. Since the control system is mainly a closed-loop control system composed of speed, obtaining the motor speed is the key to the control system. The drive circuit outputs the position signal through the Hall IC. The brushless DC motor outputs 12 pulse FG signals per revolution, but these pulse signals have large interference and cannot be directly processed by ATmega8L, so the pulse FG signal must be filtered and extracted. As shown in Figure 5, since the drive circuit outputs an open circuit, the circuit adds a pull-up resistor R11. The FG signal has many harmonics, and C4 is set to play a filtering role. Its value is difficult to determine and needs to be adjusted through experiments. The FG signal generates a relatively stable, harmonic-free FG' signal at pin 3 through P521, where G5 cannot be selected too large, otherwise the FG' signal will be distorted, so that ATmega8L cannot recognize it. The internal Timerl of ATmega8L has a 16-bit input capture unit, which captures external events through the external pin ICP1. When reading ICR1, the capture register first reads the low byte ICR1L, and then reads the capture register high byte ICR1H. The motor speed can be calculated based on the difference between the two capture registers.

The isolation circuit is to prevent the drive circuit and its interface circuit from being affected by strong voltage. Photoelectric coupling isolation is added to the external circuit interface line to ensure the normal operation of the circuit.
2.2 Design of drive circuit
The drive circuit is the bridge between the main control circuit and the brushless DC motor. This control system uses Hitachi's dedicated integrated brushless DC control chip ECN30206. The ECN30206 dedicated integrated brushless DC control chip is suitable for three-phase brushless DC motors with position sensors with a DC voltage of 500 V, less than 1 A, and a power of 20 to 300 W. The ECN30206 driver chip consists of 6 internal full-bridge IGBT switch tubes (upper bridge arm and lower bridge arm) and each IGBT has a freewheeling diode with protection, a charge pump circuit for boosting the voltage of the three upper bridge arms, a brushless DC motor direction control circuit, a surface acoustic wave generation circuit for generating clocks for the ECN30206 driver chip, a PWM generation circuit, a three-phase non-distributor, an overcurrent and undervoltage protection circuit, and a rotor position detection circuit. The three-phase distributor has a commutation control table inside, and the on-off state of each bridge arm is reasonably allocated according to the values ​​in the table. According to the internal principle analysis of the ECN30206 integrated driver chip and the working principle of the Hall switch chip, the DC motor drive circuit can be designed as shown in Figure 6.

The system uses a brushless DC motor with four pairs of magnetic poles, so a mechanical angle is 90° and an electrical conduction angle is 30°. Therefore, three Hall switch integrated chips EW632 need to be placed every 30° to detect the position of the rotor of the brushless DC motor. The corresponding relationship between the input signals of the three EW632 chips and the conduction state of the switch tube is shown in Table 1.

1) Determination of internal PWM parameters
The frequency of the internal PWM wave is determined by the surface acoustic oscillator (SAW), the capacitor CTR connected to pin 11 and the resistor RTR connected to pin 12, as shown in the formula:
fpwm=0.494/(CTRxRTR) (4)
The CTR selected in this system is 1 800 PF and the RTR is 22 kΩ. According to formula (4), the PWM frequency is 12.5 kHz.
The PWM duty cycle is determined by the analog voltage VSP input to pin 13. When the value of VSP is less than the minimum value Vsawl of the SAO amplitude, the PWM duty cycle is 0% and all IGBT tubes will be turned off; when the value of VSP is greater than the maximum value VsawH of the SAO, the PWM duty cycle is 100%; when VsawL≤VSP≤VsawH, the duty cycle P is linear with the size of VSP:
P=(VSP-Vsawl)/(VsawH-VsawL) (5)
Therefore, only by changing the size of VSP can the motor be linearly and steplessly regulated.
2) Determination of the number of FG pulses per motor revolution
The brushless DC motor used in the pump station has 4 pairs of magnetic poles, and the number of FG pulses per motor revolution is 12 pulses.
3) Determination of the external circuit parameters of the charge pump
In order to open the bridge arm IGBT power drive switch, the gate voltage must be increased before it can be opened. The ECN30206 has a charge pump circuit inside, and the user only needs to set the parameters of each device in the external circuit. The size of the capacitor determines the charging time, that is, the time it takes for the IGWT to drive the switch to open. Therefore, it cannot be too large, otherwise the opening time will be too long and cause an accident. Here, the capacitor is selected to be 1μF.

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4) Negative voltage and overcurrent protection
When ECN30206 detects that the voltage of the VCC pin is less than 12 V, all IGBT power drive switches are turned off until VCC is greater than 12 V to resume normal operation.
Current protection is achieved by connecting a resistor in series with the lower bridge arm IGBT to the ground GL voltage divider and feeding it back to the internal overcurrent voltage comparator of ECN30206. The reference voltage Vref of the internal overcurrent voltage comparator is 0.5 V. Here, the maximum current value Imax is set to 1 A, then the value of RS is:
Rs=Vref／Imax (6)
Substituting Imax=1 A and Vref=0.5 V into formula (6), RS is 0.5 Ω. Constantan wire is selected here.
5) Guarantee of circuit stability
In order to make the drive circuit stable and improve anti-interference performance, each pin must be processed, and a pull-up resistor or a pull-down resistor must be added to the pin appropriately. A high-frequency bypass capacitor is added to the pin with high input impedance to eliminate the sensitivity of the high input impedance pin to high-frequency noise.
2.3 Design of RS485 display circuit and communication interface circuit
The control system uses the MAX7219 driver chip of MAXIM (Maxim) Company in the United States as the LED digital display driver chip to drive 8 8-segment digital LED displays to display the speed, working current and faults of the permanent magnet brushless DC motor. ATmega8L has a series of communication modules integrated inside, so it can communicate by expanding an RS485 interface driver chip on the periphery. Here, the system uses MAXIN's MAX487 as the driver chip. In specific applications, the two enable terminals RE and DE of the chip are connected together to make MAX487 in a certain state, and at the same time save the system's I/O port. The display circuit and communication interface circuit are relatively common general circuits. Due to limited space, they will not be introduced in detail here.

3 Control system software program design
After the control system hardware circuit design is completed, software compilation work needs to be carried out. The system's software design specifically reflects the system's technical requirements and is the logical implementation of the control law of the entire system.
The software design of this control system adopts the front-end and back-end system, that is, the system consists of a dead loop program plus multiple interrupt service subroutines. When there are many tasks, a real-time operating system (RTOS) is used to improve the utilization rate of the single-chip CPU. The main program consists of the system initialization rotor speed calculation and speed PID closed-loop control, which completes most of the functional tasks of the system; the interrupt program mainly detects the interrupt time and notifies the main program to perform corresponding processing, completes the necessary real-time functions, thereby saving CPU time and making the various functions of the program run reliably; the interrupt program is mainly used for serial interrupts to receive the speed setting value sent by the host computer, timed interrupt detection current and display speed value and related faults.
3.1 Speed ​​digital PID closed-loop control program design
In order to realize the motor speed following the given value, the control system uses the PID algorithm to perform closed-loop control of the speed. After determining the parameters of the PID algorithm, it is relatively simple to implement it with the Atmega8L microcontroller. In the program, define a structure variable to store the user-set speed value, current speed value, previous error value, current error value, cumulative error value, proportional constant, integral constant and differential constant. The structure definition source program is as follows:

According to the PID control algorithm, compile the control program to control the PWM wave duty cycle of timer T2 to change the size of the VSP voltage value to achieve closed-loop speed control. As the number of sampling times increases, the cumulative error will also saturate and overflow, so anti-saturation processing must be performed.
3.2 Online communication program design
The speed setting of the motor can be achieved by adjusting the precision potentiometer, then converting the voltage value into analog-to-digital, changing the PWM duty cycle according to the analog-to-digital conversion value, and changing the driver input voltage VSP; it can also be achieved through the control system and the host computer through RS485 bus communication, and the speed value is directly sent by the host computer. Different current protection values ​​can also be set according to different types of motors through the host computer. At the same time, the control system can feedback the operation and fault information to the host computer for reference.

4 Conclusion
1) The main control circuit with Atmega8L microcontroller as the core, the drive circuit with ECN30206 as the core, and the display circuit with Max7219 as the core are independent of each other and can be selected separately to meet the needs of different occasions and form a closed-loop control system.
2) The designed pressure control system can ensure the accurate and stable control of the pressure of the deep-sea environment simulation system, and can achieve continuous adjustment.
3) The system can also provide a useful reference for other pressure control systems.