Ultra-Precise Control of Micro Positive and Vacuum Pressure in Sampling System - Application of Proportional Flow Control Valve

Ultra-Precise Control of Micro Positive and Vacuum Pressure in the Sampling System of Circulating Tumor Cells (CTCs) Detection Device

Abstract: In cell sorting and sampling systems such as circulating tumor cells, it is necessary to precisely control the pressure in a small positive and negative pressure range around standard atmospheric pressure to put forward higher requirements. In view of these technical problems, this article proposes a high-precision positive and negative pressure precision control solution using proportional flow control valve ( proportional needle valve), introduces the control method and the functions and technical indicators of the software and hardware in detail, so that a control accuracy of 0.5% can be reached.

1. Question

Circulating Tumor Cells (CTC) sorting has been recognized as an effective tool for cancer diagnosis and prognosis, requiring corresponding detection devices that can perform all experimental procedures automatically, quickly and sensitively without any manual intervention. For some sampling systems based on the principle of pressure-driven liquid flow, it is required to precisely control the gas pressure to ensure stable flow during the sampling process and reach automatic feedback adjustment to pump and valve action.

2. Solution

The solution proposed in this article is to reach the precise control of positive and negative pressure in the range of ±10ps (i or ±700mbar) near a standard atmospheric pressure, and the control accuracy reaches 0.5%. That is to provide a controllable air pressure source solution, using the dynamic balance method of bidirectional control mode, combined with high-precision stepper motor and micro-flow proportional flow control valve, high-precision pressure sensor and dual-channel PID controller, the air pressure source can be used for high-precision control and programmable outputs for positive, negative and one atmosphere pressure.

The basic principle of micro positive and negative pressure precision control is shown in Figure 1. The specific content is:

(1) The control principle is based on the dynamic balance method of air intake and air outlet in a closed cavity. This is a typical closed-loop control loop, the 2-channel PID controller collects the vacuum pressure sensor signal and compares it with the set value, and then adjusts the opening of the intake proportional flow control valve and exhaust proportional flow control valves, and finally makes the sensor measured value equal to the set value to reach the vacuum pressure of accurate control.

(2) The control loop is equipped with an air pump (negative pressure source) and an air source (positive pressure source) to improve the sufficient negative and positive pressure capability.

(3) In order to cover the required vacuum pressure range from negative pressure to positive pressure (such as -10psi to +10psi), configuring a high-precision absolute pressure sensor with a test range covering the required range. The absolute pressure sensor corresponds to the output of above vacuum pressure range a DC analog signal with values ​​ranging from small to large (such as 0~10VDC). This analog signal is input to the PID controller, and the PID controller adjusts the opening of intake valve and the exhaust valve to reach precise pressure control. The advantage of using an absolute pressure sensor is that it is not affected by changes in the local atmospheric pressure, and does not require pressure correction, which can ensure the accuracy and repeatability of test.

(4) When the control changes from negative pressure to positive pressure, the opening of intake control valve (intake flow) at the beginning is much smaller than the opening of exhaust control valve (extraction flow), and by automatically adjusting the intake and exhaust air. The flow reaches different equilibrium states to reach different negative pressure control, and finally the opening of intake control valve is gradually much larger than the opening of exhaust control valve, thereby reaching a series of set points or slopes in the range from negative pressure to positive pressure of continuous precision control. For the change control from positive pressure to negative pressure, the above process is reversed.

3. The specific content

The specific structure of the tiny positive and negative pressure generator involved in the solution is shown in Figure 2, which mainly includes a high-pressure gas source, a proportional flow control valve, a closed cavity, a pressure sensor, a high-precision PID controller and an air pump.

In the micro positive and negative pressure control system shown in Figure 2, the working pressure outlet on the closed cavity is connected to the detection instrument, and two FC series stepping motor proportional flow control valves are installed on the left and right of the closed cavity. The proportional flow control valve itself is a positive Negative pressure dual-purpose regulating valve, its absolute vacuum pressure range is 0.0001mbar~7bar, the maximum flow is 40mL/min, and the single step size of the stepping motor is 12.7 microns, which can fully meet the precise control of positive and negative pressure in small cavities.

In the control system shown in Figure 2, two proportional flow control valves are used to reach precise control of any set point of positive and negative pressure, and it can also be controlled linearly from positive pressure to negative pressure, or from negative pressure to positive pressure of linear change control.

For the small positive and negative pressure control in the sampling system of circulating tumor cell (CTCs) detection instruments, the requirement is to accurately control the vacuum pressure near the standard atmospheric pressure. The two-way dynamic mode of the valve, that is, through the dual-channel PID controller, one channel is used to keep the opening of the proportional flow control valve at the intake port basically unchanged, and the other channel is used to adjust the opening of the proportional flow control valve at the exhaust port according to the PID algorithm. Spend.

In addition to the above-mentioned control method of constant intake flow to adjust the exhaust flow, the control accuracy of tiny positive and negative pressure in the sampling system of circulating tumor cell (CTCs) detection instrument is mainly determined by the pressure sensor, PID controller and proportional flow control valve. The PID controller in this solution uses 24-bit AD and 16-bit DA, and the proportional flow control valve is a high-precision stepper motor, so the test accuracy of this solution mainly depends on the accuracy of pressure sensor, generally at least 0.1% of accurate pressure sensor.

For the micro pressure control in the sampling system, the closed container is often required to perform multiple reciprocating changes within the range of positive and negative pressure. Therefore, a PID controller that can store multiple editing programs is used, and the setting degree is composed of one or more polyline segments. The curve can reach the automatic program control of reciprocating change of positive and negative pressure.

In the solution described in this article, in order to reach precise control of positive and negative pressure, as shown in Figure 2, an air pump is configured for the formation of negative pressure. The suction pump is equivalent to a negative pressure source, but the use of a vacuum generator can also reach the effect of a negative pressure source. The advantage of using a vacuum generator as a negative pressure source is that the entire system only needs to be equipped with a high-pressure gas source, which reduces cost of the entire system, volume and weight, the vacuum generator can be connected to a high-pressure gas source to reach the same pumping effect.

4. Conclusion

The solution described in this article can fully reach the precise control of any set point and continuous program form of micro positive and negative pressure in the sampling system of circulating tumor cells (CTCs) detection instrument, and can reach high control accuracy and speed, and the whole process can be automated.

In addition to the automatic and precise control of small positive and negative pressure, another feature of this solution is that the system is simple, the control range of positive and negative pressure can be relatively wide, and the whole system is small and integrated, which is convenient to form a small testing instrument.

For further information on the proportional flow control valve, please click here.