CN109655116B - System and method for controlling precision of small pulsating flow by utilizing gas micro-pressure difference based on PWM control method - Google Patents

Abstract

The invention relates to the field of differential pressure flow testing, and discloses a system based on a PWM control method for controlling the precision of pulsating small flow by utilizing gas micro-pressure difference, comprising a micro-pressure sensor, wherein the sensor is provided with a main cavity of the sensor and a first cavity of the sensor body and the second cavity of the sensor, the second cavity of the sensor and the main cavity of the sensor are both connected with the sensor, the main cavity of the sensor is connected with the precision micro differential pressure gauge, and the precision micro differential pressure gauge is also connected with the sensor first cavity A cavity is connected to the second cavity of the sensor, the second cavity of the sensor is connected to the eccentric rotor air pump, and the eccentric rotor air pump is connected to the single-chip microcomputer through the D/A converter, and the single-chip microcomputer has the PWM control function, so The single-chip microcomputer is also connected with a precision micro-differential pressure gauge and a PC. The invention can obtain that the calibrated flow controlled by the PWM is within the required precision range, and the precision can be controlled within less than 2%.

Description

A method based on PWM control method using gas micro-pressure difference to control the precision of small pulsating flow system and method

technical field

The invention relates to the field of differential pressure flow testing, and more particularly, to a system and method for controlling the precision of small pulsating flow by utilizing gas micro-pressure difference based on a PWM control method.

Background technique

The differential pressure flowmeter is measured by the working principle that there is a certain relationship between the pressure difference and the flow rate generated when the medium fluid flows through the throttling device. There are many types of differential pressure flowmeters, and the technology is also very mature. Differential pressure flowmeters include: orifice flowmeters, nozzle flowmeters, Venturi flowmeters, throttle flowmeters, etc.

Advantages of existing differential pressure flowmeters on the market: simple structure, no moving parts; high reliability; good repeatability; wide adaptability, it is suitable for single-phase fluids under various working conditions, suitable for pipelines Wide diameter range, can be equipped with general differential pressure gauge; device is standardized.

The disadvantage is that the installation requirements are strict; the front and rear of the flowmeter require a long straight pipe section, so the volume is large; the measurement range is narrow, and the general range is 3:1; the pressure loss is large; it is difficult to measure pipes with small diameters; the accuracy is not enough High (±1%～±2%); not suitable for measuring small flow and pulsating airflow.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a system and method based on the PWM control method for using the gas micro-pressure difference to control the precision of small pulsating flow. Yes, the PWM algorithm can also be used to filter out the fluctuation of the pulsating flow and control the flow accuracy to within ±2%, and the calibration flow can be set freely.

In order to achieve the above object, the technical scheme adopted in the present invention is as follows:

A system based on a PWM control method using a gas micro-pressure difference to control the accuracy of a small pulsating flow, comprising a micro-pressure sensor, wherein the micro-pressure sensor is provided with a main sensor cavity, a first sensor cavity and a second sensor cavity, so The main cavity of the sensor is connected with the precision micro differential pressure gauge, the precision micro differential pressure gauge is also connected with the first cavity of the sensor and the second cavity of the sensor, and the second cavity of the sensor is connected with the eccentric rotor air pump, so The eccentric rotor air pump is connected with a single chip microcomputer through a D/A converter, the single chip chip has a PWM control function, and the single chip chip is also connected with a precision micro differential pressure gauge and a PC.

Further, the single-chip microcomputer controls the micro-pressure differential flow through the PWM function.

The present invention also provides a method for controlling the precision of pulsating small flow rate by utilizing gas micro-pressure difference based on the PWM control method, and the method comprises the following steps:

S1, start measurement;

S2. Read the calibrated central differential pressure value A;

S3. Read the gauge pressure value B of the set upper and lower pressure difference deviation value;

S4, read the measurement value Ci of the micro-pressure sensor;

S5, calculate the sampling Ci weighted moving average C;

S6, when the C value is within the range of A±B, repeat step S2; otherwise, output the PWM control pump voltage to the D/A converter according to the control algorithm of the relationship between the pressure difference and the flow;

S7. If it is not determined to end the measurement, repeat step S2; if it is determined to end the measurement, stop the measurement.

Further, the control algorithm includes the calculation formula of standard throttling, the state equation of ideal gas and the relationship between the duty cycle and differential pressure value of the PWM control output voltage, the control algorithm is used to calibrate the flow, and the control algorithm uses To control the flow accuracy within 2%.

Compared with the prior art, the advantages of the present invention are: this scheme does not need to consider the structure and size of the flowmeter, as long as the structure of obtaining a sufficient differential pressure value can be considered, and the PWM algorithm can also be used to filter out the fluctuation control of the pulsating flow. The flow accuracy is within ±2%, and the calibration flow can be freely set.

Description of drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative efforts.

Fig. 1 is the micro-pressure difference and small flow precision control schematic diagram based on PWM control method of the present invention;

Fig. 2 is the present invention utilizes the single chip PWM function to control the micro-pressure difference flow control flow chart;

Fig. 3 is the schematic diagram of the built-in pump flow of the present invention;

FIG. 4 is a flow rate and precision curve obtained by the present invention using formula 3. FIG.

Detailed ways

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, and the protection scope of the present invention can be more clearly defined.

Please refer to FIG. 1-FIG. 3. The present invention provides a system for controlling the accuracy of pulsating small flow rate by using gas micro-pressure difference based on PWM control method, including a micro-pressure sensor, and the micro-pressure sensor is provided with a sensor main cavity, The first cavity of the sensor and the second cavity of the sensor, the main cavity of the sensor is connected with the precision micro differential pressure gauge, the precision micro differential pressure gauge is also connected with the first cavity of the sensor and the second cavity of the sensor, the The second cavity of the sensor is connected to the eccentric rotor air pump, and the eccentric rotor air pump is connected to the single-chip microcomputer through a D/A converter, the single-chip microcomputer has a PWM control function, and the single-chip microcomputer is also connected to a precision micro differential pressure gauge and a PC. .

The single-chip microcomputer controls the micro-pressure difference flow through the PWM function; the method includes the following steps: S1, start measurement; S2, read the calibrated central pressure difference value A; S3, read the set upper and lower pressure difference deviation value gauge pressure value B; S4, read the measured value Ci of the micro pressure sensor; S5, calculate the weighted moving average C of the sampling Ci; S6, when the C value is within the range of A±B, repeat step S2; otherwise, control the algorithm according to the relationship between the pressure difference and the flow rate Output the PWM control pump voltage to the D/A converter; S7, if it is not determined to end the measurement, repeat step S2; if it is determined to end the measurement, stop the measurement.

The control algorithm includes the calculation formula of standard throttling, the state equation of ideal gas and the relationship between duty cycle and differential pressure value of PWM control output voltage, the control algorithm is used to calibrate the flow, and the control algorithm is used to control the flow Accuracy is within 2%.

Formula 1 is a calculation formula approved by the International Metrology Organization for standard throttling devices.

In formula 1: q _v - is the volume flow (m ³ /s); C - outflow coefficient (dimensionless); β - ratio of orifice aperture to tube inner diameter (dimensionless); ε - compressibility coefficient (dimensionless); d- orifice diameter (m); △p-pressure difference (Pa); ρ-gas density (kg/m3)

Formula 2 is the state equation of ideal gas (Gay Lussac's law), in formula 2: T ₀ -absolutely stable 273.15; ρo-1.29(kg/m3) (air density at 0℃1atm); △T-actual Celsius.

At room temperature of 25°C, formula (2) is substituted into formula (1), and then other parameters of formula (1) can be simplified to the form of formula (3) according to the actual structure. From formula (3), it can be known that the actual flow rate is proportional to the square root of the pressure difference.

First of all, the calibrated flow rate of a product is 0.1CFM=2.83 (L/min). In order to meet the pressure difference under this flow rate is not too high, the volume of the flow measurement parts should not be too large, and the designed orifice diameter is less than 2mm. The length is less than 20mm, so that the flow velocity through the aperture is less than 50m/s.

In formula 1, C is the outflow coefficient, and the outflow coefficient of the short hole (Re>2000) is generally set to 0.8; ε is the compressibility coefficient: when the flow velocity of the orifice is less than 50m/s or Ma (Mach number) is less than 0.2, the air It can be regarded as an incompressible gas (the pressure density error is usually less than 1% and can be ignored), so ε can be set to 1; the ratio of β orifice aperture to tube inner diameter: much less than 1 can be ignored.

In this way, formula (3) is obtained; using formula (3), the relationship between the differential pressure value and the flow rate can be simulated as shown in Figure 4. It can be seen from Figure 4 that to obtain the calibrated flow rate of 0.1CFM (2.83L/min), the central differential pressure value is about 510Pa.

As shown in Figure 1, in the actual flow measurement, the power source for extracting air is the eccentric rotor air pump; the pump speed is 5000-45000 (rpm), resulting in the pulsation of the air flow between 80Hz-750Hz; as shown in Figure 1 The differential pressure value actually measured by the precision micro differential pressure gauge is also fluctuating; the flow rate measured without filtering and measuring also fluctuates high and low, and the control flow accuracy is below 10%.

It is obtained through experiments that the following relationship exists between the duty ratio of the voltage output that controls the rotational speed of the pump and the output value of the differential pressure gauge (the relationship between the duty ratio of the PWM control output voltage and the differential pressure value).

In the formula, a is the hysteresis coefficient (there is a hysteresis time from the output voltage to the motor speed increase and feedback to the voltage difference), Vi is the voltage of each output, ti is the time of each output voltage, Vi is set to a fixed value, formula 4 can be Expressed as the following formula.

Δp=a·f(t) (5)

In the formula, t is the duty ratio of the output of the single-chip microcomputer, and it is brought into formula 3 to obtain the following formula.

Using the above formula 6 for PWM control, the flow is controlled within ±2%.

First embodiment:

The calibration flow can be freely set in the range of 0-5L/min using the device shown in the figure below. The required flow rate of the built-in pump particle sensor is 0.1CFM=2.83L/min, and the central differential pressure value at 2.83L/min can be set at about 510Pa with the independently developed PC software. The calibration center value A and the precision control deviation value B are written into the microcontroller.

Second embodiment:

It is used for the flow control of the built-in pump series of the particle sensor. As shown in Figure 2, firstly read the calibrated and set pressure difference value and the pressure difference value up and down line in the first case; weight the value sampled by the precision micro differential pressure meter The moving average processing filters out the pulsation of the differential pressure to a certain extent, and then the connection hole structure is added in part B shown in Figure 2 to improve the pulsation and control hysteresis.

Determine whether the weighted moving average of the sampling is within A±B, if it exceeds the PWM control program calculated by formula 6 in the execution of the single-chip microcomputer, the part shown in A in Figure 2; Make circular judgments.

In this way, it can be obtained that the calibrated flow rate of PWM control is within the required accuracy range, and the accuracy can be controlled within less than 2%. This is the working principle of using a precision differential pressure gauge to control the accuracy of small flow based on the PWM control method. The content described in detail belongs to the prior art known to those skilled in the art.

Although the embodiments of the present invention are described in conjunction with the accompanying drawings, the patent owner can make various changes or modifications within the scope of the appended claims, as long as the protection scope described in the claims of the present invention is not exceeded, all should be within the protection scope of the present invention.

Claims (4)

1. A system for controlling the precision of small pulsating flow by utilizing the micro differential pressure of gas based on a PWM control method is characterized in that: including the micro-pressure sensor, be equipped with sensor main cavity, the first cavity of sensor and sensor second cavity in the micro-pressure sensor, the sensor main cavity is connected with accurate differential pressure gauge, accurate differential pressure gauge still is connected with the junction of the first cavity of sensor and sensor second cavity, the sensor second cavity is connected with eccentric rotor aspiration pump, eccentric rotor aspiration pump passes through the DA converter and is connected with the singlechip, the singlechip has PWM control function, the singlechip still is connected with accurate differential pressure gauge and PC, and the sensor main cavity, the first cavity of sensor and sensor second cavity communicate in proper order, and the sectional area of the sensor main cavity is greater than the sectional area of sensor second cavity, and the sectional area of sensor second cavity is greater than the sectional area of the first cavity of sensor.

2. The system for controlling precision of pulsating small flow with gas micro-pressure difference based on PWM control method according to claim 1, wherein: the single chip microcomputer controls micro differential pressure flow through a PWM function.

3. A method of using the system for controlling precision of pulsating small flow with micro pressure difference of gas based on PWM control method according to claim 1, characterized in that: the method comprises the following steps:

s1, starting measurement;

s2, reading a calibrated central differential pressure value A;

s3, reading the set upper and lower differential pressure deviation value gauge pressure value B;

s4, reading a measured value Ci of the micro-pressure sensor;

s5, calculating a weighted moving average C of the samples Ci;

s6, when the C value is in the range of A +/-B, repeating the step S2; otherwise, outputting the PWM control pump voltage to the D/A converter according to a pressure difference and flow relation control algorithm;

s7, if the measurement is not determined to be finished, repeating the step S2; and determining to finish the measurement and stopping the measurement.

4. The method of claim 3, wherein: the control algorithm comprises a standard throttling calculation formula, an ideal gas state equation and a duty ratio and differential pressure value relation of PWM control output voltage, the control algorithm is used for calibrating flow, and the control algorithm is used for controlling the flow accuracy within 2%.

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