Detection of hydraulic system flow with time difference method

1 Introduction

The hydraulic system has the problem of high failure rate and difficult fault location. The hydraulic system vibration signal diagnostic technology, oil analysis technology has been unable to obtain the accurate diagnosis reflect the running state of the hydraulic system flow and pressure and its key parameters change; hydraulic intrusive traditional measurement methods, detection interface is limited, and difficulty, but also affects the dynamic characteristics of the system.

The flow is one of the important parameters of the hydraulic system, its size directly reflects the operation of the hydraulic system is good or bad. The real-time monitoring of the hydraulic system can be realized by measuring the flow of the system, so as to ensure the normal operation of the hydraulic system. Therefore, it is important to detect the flow of the hydraulic system.

2 time difference method of measuring hydraulic flow principle

There are many advantages of using ultrasonic to measure the velocity of fluid. Compared with the traditional mechanical flow meter, electromagnetic flow meter, it has high measuring precision, strong adaptability to the pipe diameter, non-contact fluid, easy to use, easy to digital management, etc.. In recent years, due to the development of electronic technology, the cost of electronic components greatly reduced, so that the production cost of ultrasonic flow meter is greatly reduced, ultrasonic flowmeter has begun to spread. Customers often ask questions about ultrasonic flow measurement. As a universal, we will continue to write a number of feature articles to introduce some of the relevant knowledge in order to promote and popularize the popularization and improvement of ultrasonic flow technology.

The principle of time difference measurement is that the velocity of ultrasonic wave in fluid is related to the velocity of fluid flow. The ultrasonic transducer 1 and transducer 2 are respectively arranged on the upper and lower reaches of the flow medium with a flow rate of V, as shown in figure 1.

The transducer L and transducer 2 spacing is L, the pipe diameter is D, the angle between L and V is theta. When the transducer 2 receives the ultrasonic pulse transmitted by the transducer 1, the propagation velocity of the ultrasonic wave along the L is (C-V), and the C is the ultrasonic velocity in the stationary medium. The time of ultrasonic countercurrent flow from transducer l to transducer 2:

The receiving function exchange transducer, transducer 2 transmitting ultrasonic pulse, ultrasonic transducer L received by the transducer 2 is transmitted to the downstream transducer 1 time:

Thus, the time difference between countercurrent and downstream:

Because the velocity of ultrasonic wave propagation in liquid is 1500m/s, and the fluid velocity is not very high, it can be concluded that the formula (3) is simplified as:

In this way, the average velocity of the fluid V can be determined by acoustic time difference in T, C and X under the premise of constant, linear V and delta t. Then according to the flow equation to calculate the flow Q:

In the formula, K is the correction coefficient of velocity distribution.

3 hardware system design

The hardware design of the detection system is mainly composed of ultrasonic transducer, CPLD function, drive transmitting, receiving and amplifying and zero crossing comparison. When the system works, the first single chip to send CPLD commands, internal PULSE function module of CPLD pulse driver 600ns, and the CNT function module: start timing drive pulse into the drive transmitting circuit makes the ultrasonic transducer 1 generates ultrasonic signal; the received signal is relatively weak, the composed of LF357 and LM318 three receiver amplifier circuit the amplified signal by the amplification; will be composed of MAX903 zero crossing comparison circuit, thereby providing a high level signal to stop the clock for the CNT modules in CLPD. The time data in the CNT is converted into time, and then sent by the transducer 2, the transducer 1 is received. Another group of recorded time data with the CNT, the two is obtained by subtracting the acoustic time difference t downstream and upstream of the calculated flow rate and flow system. The key of this detection system is to get the accurate driving pulse and the precise forward time. Therefore, the use of Aher CPLD series EMP240T100C5N MAX, and is equipped with 100MHz crystal oscillator, CPLD function module is the core of the hardware design of the system.

3.1CPLD function module

CPLD function module is mainly composed of 6 sub modules, as shown in figure 2. They are using the VHDL language, functions of each sub module is: DECODER MCU instructions decoded and transmitted to the CPLD within each sub module; the CNT module is responsible for timing; PULSE module to generate the drive pulse: CNT_SP sub module generates a stop timing signal CNT; SEL_2 is used to select 16 bits of data in CNT. The first 8 and last 8; TRIBUFFER can be 8 SEL_2 data transmission to the microcontroller.

Its work flow is as follows: through the P2 port CPLD. Sent by the PULSE module specific pulse signal to drive the ultrasonic transducer, CPLD and CNT in pulse emission module start time, receiving and amplifying circuit receives the signal and after zero comparison, provide high level signal to stop the clock to the CPLD PULSE_ACT port. Then CPLD will be in the 16 bit CNT data in the form of a by SEL_2, TRIBUFFER and then uploaded to the microcontroller through the P0 port. Data processing by the MCU, the last upload or direct display of data.

Functional simulation of key sub modules in 3.2CPLD

Due to the requirement of accurate driving pulse and accurate forward and backward time, the two sub modules of PULSE and CNT are the key modules of the design. The design of these 2 modules has a direct impact on the overall system performance, functional simulation and verification of the feasibility of the design.

3.2.1PULSE sub module simulation

According to the spectrum analysis, there is an optimal relationship between the pulse width and the sensor frequency. When the pulse width is satisfied, the received signal quality is the best. this