CN105784037B - All-digital waveform synthesis Coriolis flowmeter opens the device and method for the signal that shakes - Google Patents

Abstract

The invention relates to a device and method for synthesizing the start-up signal of a Coriolis flowmeter with an all-digital waveform. The device includes a microcontroller, a direct digital frequency synthesizer DDS, a multiplying digital-to-analog converter MDAC, and an audio power amplifier; the device is powered on Finally, the DDS is controlled by the microcontroller to continuously synthesize and output a 1Hz square wave signal. At the same time, the microcontroller writes the control word to the MDAC, so that the square wave signal passed through the MDAC is amplified by amplitude modulation according to the size of the control word, and then amplified by a fixed multiple after work. , so that the final amplitude of the square wave signal is adjusted to 9Vpp, and the signal is input into the flow tube to realize the rapid start-up of the flow tube.

Description

Device and method for synthesizing start-up signal of Coriolis flowmeter with full digital waveform

technical field

The invention aims at the driving system of the Coriolis mass flowmeter, and designs a 1Hz all-digital driving vibration starting device and method based on harmonic decomposition.

Background technique

Coriolis mass flowmeter (hereinafter referred to as Coriolis flowmeter) is a kind of direct mass flowmeter made by Coriolis principle. It is one of the most rapidly developing flowmeters. It can measure various conventional liquids, and can also be applied to the measurement of unconventional fluids such as slurry, liquefied gas and compressed natural gas, and is widely used in petrochemical, food and pharmaceutical industries.

The Coriolis flowmeter is mainly composed of a flow tube (also known as a vibrating tube), a sensor, a signal processing system, and a drive system (also known as an excitation system). Among them, the performance of the drive system directly affects the overall measurement accuracy of the flowmeter. The traditional drive circuit is composed of analog circuits, and the start-up process is driven by noise, which has problems such as long start-up time and poor start-up robustness. At the same time, due to the limited gain provided by noise, when the amplitude and frequency of the flow tube change, the The flow tube cannot be controlled to achieve stable vibration; at the same time, there are problems such as complex circuit structure, easy aging of components, and high environmental requirements. Digital drive schemes have become a research hotspot in recent years. Many companies and scientific research institutes have successfully developed a digital driving method based on Direct Digital Frequency Synthesis (DDS) for the driving system of Coriolis flowmeters, and achieved good performance and precision. A Chinese patent discloses an initialization algorithm for drive control in a Coriolis mass flowmeter (R.L. Majinis, Coriolis flowmeter for drive control initialization algorithm, Chinese patent, application number: 01806546.5, filing date: 2001.3.12), this method is to first apply a 1V step signal with a duration of 10ms to the flow tube, and judge whether it is necessary to increase the action time and amplitude of the step signal by checking whether the sensor feedback amplitude in the flow tube is sufficient. If the flow tube does not vibrate, increase the action time of the step to 20ms, increase the amplitude to 2V, and repeat the judgment until the vibration start requirements are met. The advantage of this method is that it does not require a priori information of the natural frequency of the flow tube, but its time accumulation is too small, and an additional amplitude is required to start the vibration; the Chinese patent also discloses a positive and negative step of the Coriolis mass flowmeter. Vibration starting method and system (Xu Kejun Coriolis mass flowmeter positive and negative step alternate excitation vibration starting method and system, Chinese patent, application number: 200910144210.7, application date: 2009.7.23), the idea of this method is in the flow If the tube vibrates at a suitable position and is excited alternately with positive and negative steps, the output of the flow tube will be continuously strengthened, and finally the flow tube will start to vibrate. However, the design of this method is extremely complicated, and the control process is cumbersome. Due to the influence of industrial site environmental noise, etc., the zero point mentioned in the method is difficult to find. Even if a hysteresis band b is set near the zero point to avoid system malfunction caused by noise , but the value of b needs to be set according to the actual noise level. This method needs to determine the prior noise information of the scene in advance in different working scenarios, and cannot adapt to the environment well. In fact, the flow tube itself is equivalent to a narrow-band filter, which has very good frequency selection characteristics. The vibration start-up signal with a frequency near the natural frequency of the flow tube will exert a positive feedback effect on the start-up of the flow tube. The start-up time and start-up effect of the flow tube have an important influence. On the basis of theoretical analysis and a large number of experiments, the present invention proposes a 1Hz digital signal driving method based on harmonic decomposition, and designs and implements a set of digital driving vibration starting device according to this method.

Contents of the invention

technical problem to be solved

In order to avoid the deficiencies of the prior art, the present invention proposes a device for synthesizing the start-up signal of a Coriolis flowmeter with an all-digital waveform.

Technical solutions

A device for synthesizing the start-up signal of a Coriolis flowmeter with an all-digital waveform is characterized in that it includes a microcontroller, a direct digital frequency synthesizer DDS, a multiplying digital-to-analog converter MDAC and an audio power amplifier; two data outputs of the microcontroller The pins are respectively connected to the input pins of DDS and MDAC to realize the control function of waveform synthesis and amplitude modulation. The output pin of DDS is connected to the input pin of MDAC, and the output pin of MDAC is connected to the input pin of audio power amplifier. The power amplifier outputs the start-up signal to the Coriolis flowmeter.

A method for using said device to synthesize a Coriolis flowmeter start-up signal, characterized in that the steps are as follows:

Step 1: After the system is powered on, the microcontroller clears all interrupts and interrupt flags, defines the pins connected to DDS and MDAC, and configures DDS to work in the working state of square wave output; clears MDAC output, and configures MDAC from the microcontroller The control command data of the controller is locked into the shift register at the falling edge of the clock;

Step 2: The microcontroller writes the frequency control word to the DDS, so that the DDS continuously sends out a 1Hz square wave signal;

Step 3: The microcontroller writes the amplitude control word to the MDAC, so that the MDAC adjusts the 1Hz square wave signal from the DDS according to the ratio corresponding to the amplitude control word;

Step 4: After the power amplifier amplifies the signal output by the MDAC, it outputs a 9-10Vpp start-up signal.

Beneficial effect

The invention proposes a device for synthesizing the start-up signal of a Coriolis flowmeter with an all-digital waveform. The device and method overcome the limitations of analog start-up and other digital start-up signals, and have the following beneficial effects:

1. The overall device and control method are simple. Compared with other complex hardware circuits and control methods, the control complexity can be greatly reduced. At the same time, the vibration starting device also reuses the subsequent control flow tube stabilization vibration device, without adding additional Hardware overhead, only need to increase the software control part to start the vibration;

2. Compared with the analog drive system, which mainly relies on noise to initiate vibration, the vibration initiation speed is significantly improved, and the vibration initiation effect is stable and reliable. Compared with other digital vibration starting signals, not only can the algorithm complexity and control steps be greatly reduced, but the vibration starting speed is not affected at the same time. Actual experiments show that the vibration signal method can start vibration within 3.0s for flow tubes with high natural frequency (203Hz), and can start vibration for flow tubes with low natural frequency (91Hz) within 4.0s. ;

3. The vibration start signal has strong versatility. For flow tubes with a natural frequency of 40-400Hz, this device and method can be used to achieve rapid vibration start. The actual test shows that the start time of this method does not exceed 6.2s at most;

4. Good robustness, basically not affected by noise such as environmental noise, and can realize rapid vibration start.

Description of drawings

Fig. 1 is the hardware connection relation of the present invention

Fig. 2 is working steps of the present invention

Detailed ways

Now in conjunction with embodiment, accompanying drawing, the present invention will be further described:

Aiming at the all-digital drive system of the Coriolis flowmeter, the present invention proposes a method for synthesizing the start-up signal of the Coriolis flowmeter with a 1Hz all-digital waveform based on harmonic decomposition, and designs and implements a set of all-digital drive devices according to the method. The device includes: a microcontroller, a direct digital frequency synthesizer (hereinafter referred to as DDS), a multiplying digital-to-analog converter (hereinafter referred to as MDAC), and an audio power amplifier (hereinafter referred to as power amplifier). The hardware connection relationship of the device is shown in Figure 1. The data output pin of the microcontroller is connected to the input pin of DDS and MDAC, the output pin of DDS is connected to the input pin of MDAC, the output pin of MDAC is connected to the input of the power amplifier, and the output pin of the power amplifier is the output of the device.

After the device is powered on, the DDS is controlled by the microcontroller to continuously synthesize and output a 1Hz square wave signal. At the same time, the microcontroller writes the control word to the MDAC, so that the square wave signal passing through the MDAC is amplitude modulated and amplified according to the size of the control word. Amplify with a fixed multiple, so that the final amplitude of the square wave signal is adjusted to 9Vpp, and the signal is input into the flow tube to realize the rapid start-up of the flow tube. The reason for using MDAC amplitude modulation and power amplifier amplification here is that, on the one hand, the present invention only involves the vibration start process, and does not involve the subsequent amplitude stabilization control process. Requirements, so it is necessary to use a power amplifier to amplify. MDAC mainly realizes that the magnification is adjusted according to the feedback signal of the flow tube through the program, and the power amplifier mainly realizes that the signal can be amplified according to the requirements of the vibration energy of the flow tube. At the same time, it is hoped that the device can be reused in the flow tube start-up vibration and flow tube steady-amplitude working conditions, so as to reduce additional hardware overhead.

The hardware connection relationship of the vibration starting device proposed for this starting signal is shown in Figure 1. The microcontroller is connected to the DDS and MDAC respectively to realize the control functions of waveform synthesis and amplitude modulation; the output of the DDS is connected to the analog input of the MDAC. The input of the power amplifier is connected with the output of the MDAC, and the output of the power amplifier is the output of the drive system.

This method adopts a 1Hz signal driving method based on harmonic decomposition, and the reason why it is successfully driven with a 1Hz signal is introduced below.

The flow tube vibration system itself is equivalent to a narrow-band filter, which has very good frequency selection characteristics. When the frequency of the driving signal is equal to or close to the natural frequency of the flow tube, the energy required for the flow tube to start vibration is the smallest, and the vibration will start quickly at this time , and attenuate the amplitude of other frequency signals, so as to reach the resonance state. According to the harmonic analysis method, any periodic function about time can be expanded into a Fourier series, that is, the weighted sum of infinitely many sine wave components and cosine components containing the fundamental frequency and a series of harmonics that are integer multiples of the fundamental wave, Let f(t) be a periodic function, then we have

in, is the fundamental angular frequency, T is the period, a _n and b _n are the coefficients of the Fourier series, the first term of the polynomial Represents the DC component, and n represents the nth order harmonic.

In this design, a square wave of f ₀ =1Hz is used, then the fundamental angular frequency is ω ₀ =2πf ₀ , and its harmonic decomposition can be expressed as:

Since its fundamental frequency is 1Hz, the high-order harmonics are odd multiples of the fundamental frequency, and its harmonic components are 1Hz, 3Hz, 5Hz, ..., which increase in odd multiples in turn, and there is only a difference between the harmonic frequencies of the signals 2Hz, that is, if DDS is used to synthesize the waveform, the 1Hz square wave is used as the starting signal, and its harmonic frequency components are the most. In the early stage of the flow tube vibration, when the natural frequency of the flow tube is not detected, the square wave with rich harmonic components is easier to cause the flow tube to vibrate than other waveforms, so the 1Hz square wave vibration signal must have high-order harmonics. The wave is near the natural frequency of the flow tube; in addition, since the 1Hz square wave can maintain this amplitude for as long as 1 second at a higher amplitude after being amplified by the power amplifier, it can also meet the energy requirements for the flow tube to start vibration. Based on the above two reasons, the device and method can finally realize the rapid start-up of the flow tube.

The working steps of this method are:

1. Initialization. After the system is powered on, the microcontroller clears all interrupts and interrupt flags through the program, defines the pins connected to DDS and MDAC, and configures DDS to work in the square wave output working state; initializes the MDAC function, and the microcontroller clears MDAC output, and configure the control command data from the microcontroller to be locked into the shift register at the falling edge of the clock;

2. DDS enable. The microcontroller writes the frequency to the DDS, so that the DDS continuously outputs a 1Hz square wave signal, which uses the default amplitude of the DDS and is set to V1;

3. MDAC enable. The microcontroller writes the amplitude control word to the MDAC, so that the MDAC adjusts the signal from the DDS according to the proportion corresponding to the control word. The adjustment coefficient here is E1; for the DDS default output TTL voltage amplitude of 3.3Vpp, the amplification factor here is for (1800/4096);

4. Enable the power amplifier. The power amplifier amplifies and outputs the signal from the MDAC by 6-7 times (the amplification factor is E2), so that the amplitude of the final output signal (ie V1·E1·E2) is between 9-10Vpp.

Claims (1)

1. A method for fully synthesizing a Coriolis flowmeter start-up signal adopts a device for fully digital waveform synthesis of the Coriolis flowmeter start-up signal, and the device for fully digital waveform synthesis of the Coriolis flowmeter start-up signal comprises a microcontroller, a direct digital frequency synthesizer (DDS), a multiplication digital-to-analog converter (MDAC) and an audio power amplifier; two data output pins of the microcontroller are respectively connected with input pins of the DDS and the MDAC to realize the control functions of waveform synthesis and amplitude modulation, the output pin of the DDS is connected with the input pin of the MDAC, the output pin of the MDAC is connected with the input pin of the audio power amplifier, and the audio power amplifier outputs a vibration starting signal to the Coriolis flowmeter; the method is characterized by comprising the following steps:

step 1: after the system is powered on, the microcontroller clears all interrupts and interrupt marks, defines pins connected with the DDS and the MDAC, and configures the DDS to work in a square wave output working state; clearing the MDAC output, and configuring the MDAC to lock control command data from the microcontroller into the shift register at the falling edge of the clock;

step 2: the microcontroller writes frequency control words into the DDS to enable the DDS to continuously send 1Hz square wave signals;

and step 3: the microcontroller writes an amplitude control word into the MDAC, so that the MDAC adjusts the 1Hz square wave signal from the DDS according to the corresponding proportion of the amplitude control word;

and 4, step 4: and after amplifying the signal output by the MDAC, the power amplifier outputs a 9-10Vpp vibration starting signal.