Design of measurement and control system for creative combination test bench of planar mechanism based on virtual instrument

0 Introduction

The plane mechanism creative combination test bench provides a platform for the innovative combination design of mechanism motion schemes. On the one hand, it takes the design of mechanism and mechanism combination system as the main line and the creative combination design of mechanism motion as the focus to cultivate students' hands-on ability, innovation ability and ability to analyze and solve problems; on the other hand, students obtain test data and curves through the "plane mechanism creative combination and parameter analysis test bench", and evaluate the system and design scheme by analyzing the reasons for the curve changes, such as installation accuracy, improvement methods, component motion law analysis, etc., which improves students' ability to analyze and solve problems using the knowledge they have learned, and is a big step forward compared to the simple verification experiment step by step. This requires the test bench to have an accurate detection system, and the detection system should ensure openness and flexibility on the basis of accurate testing. Based on virtual instrument technology, virtual instrument test schemes that can adapt to different application occasions can be developed to better build a test system with a high degree of automation and strong data processing and analysis capabilities. Based on this, this paper develops and designs a set of measurement and control software based on virtual instruments for this test bench.

1 Working principle and composition of the test bench

The plane mechanism creative combination test bench provides various components, kinematic pairs and a multi-layer, multi-faceted and multi-dimensional framework in the smallest unit of the mechanism. It realizes complex motion laws by changing the size of the connecting rod mechanism, realizes various mechanism motion schemes by innovative combination of different components and kinematic pairs, and splices different basic mechanisms into different components, including plane connecting rod mechanism, gear transmission mechanism, cam mechanism, intermittent motion mechanism (with grooved wheel mechanism, ratchet mechanism, incomplete gear mechanism) and other basic mechanisms. At the same time, the basic mechanisms can also be spliced, including gear-rod combination mechanism, cam combination mechanism, gear-rack combination mechanism, chain-gear mechanism, etc. The displacement, velocity and acceleration curves are measured by sensor devices, and the changes of the follower motion curve are displayed by computers, so as to organically combine testing, simulation, design and analysis.

This paper takes the JPCC-Ⅱ type planar mechanism innovative combination test bench as an example to explore the application of virtual instrument technology to the test bench's detection and analysis system. The test bench mainly consists of two parts, the mechanical structure part and the detection and analysis system. The mechanical structure is shown in Figure 1, which is mainly composed of a base (installation platform), a planar linkage mechanism, a cam mechanism, an intermittent mechanism, a gear transmission mechanism, a belt (chain) transmission and other mechanisms.

The general detection and analysis system is shown in Figure 2. It is equipped with various sensors such as speed, angular displacement, pressure, etc. to measure multiple parameters. At the same time, it uses a single-chip microcomputer combined with A/D conversion integration to collect, process and analyze data and communicate with the PC through the RS 232 interface to achieve the purpose of displaying the motion curve in a timely manner.

In this detection system, although some displacement and angle signals of the planar mechanism movement process can be displayed, it cannot achieve the processing capability of various situations in the test process during the mechanism innovation combination process, and there are still some deficiencies:

(1) Lack of openness, fixed experimental modes, poor flexibility, and only limited testing can be performed according to the system's internal settings. It is not possible to arbitrarily change some test methods, test parameters, and the presentation of test results according to changes in the conditions of the test object.

(2) The data processing capability is poor and the interface is monotonous. All data can only be processed in a certain form, and the test results can only be displayed in a certain form. For example, some errors will inevitably be caused by the working conditions of the laboratory bench, including some random errors in the acquisition process and operational errors in the operation process. These errors will inevitably have a certain impact on the results, and it cannot properly handle some errors caused by different working conditions.

In order to solve these defects of the experimental platform, virtual instrument technology based on LabVIEW software was introduced into the detection system, which greatly improved the openness, operability and data processing capability of the experimental platform, thereby improving its accuracy.

2 Detection and Analysis System Based on Virtual Instrument

By combining common modules with one or more functions, any instrument can be constructed. The same is true for virtual instruments, which are composed of three major functional modules, including signal acquisition and control, signal analysis and processing, and result expression and output.

A core technology of virtual instrument system is software technology. The “software is the instrument” put forward by NI Company of the United States shows the importance of software to virtual instrument. In virtual instrument system, flexible and powerful computer software is used to replace some hardware of traditional instruments, especially the system uses computer to directly participate in the generation of test signals and the analysis of measurement characteristics, so that some hardware in the instrument "disappears" from the system, and their functions are completed by the software and hardware resources of the computer.

The software of the virtual instrument test system mainly includes instrument drivers, instrument panel control software and general I/O interface software. The virtual instrument driver is a set of software modules at the application level, which is a kind of software that handles the control and communication with a specific instrument; the instrument panel control software is the test management layer, which is the link between the user and the instrument to exchange information; in the virtual instrument system, the I/O interface software is a link between the upper and lower layers of the virtual instrument system structure, and its modularization and standardization are becoming more and more important.

The overall design of this system adopts the PC-DAQ solution. Through multi-sensor acquisition and data fusion, coupled with a PC platform and virtual instrument software, it constitutes a data acquisition control instrument and system for various input and output parameters of the planar mechanism test bench.

The data to be detected by the test bench mainly include the rotation angle signal when the main shaft rotates, the swing angle signal when the pendulum swings, and the linear displacement signal when the slider moves. In the designed detection system, the sensor collects the signal, and then the electrical signal collected by the sensor is sent to the computer through a special shielded cable through the virtual instrument dedicated DAQ board. The obtained data is processed by software. The structural block diagram is shown in Figure 3.

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2.1 System Hardware Design

This system uses a PXI-1024 modular chassis, which provides a solid modular packaging structure for the system. It has 8 PXI/Compact PCI slots. The first slot is the system controller, and the remaining 7 slots are peripheral expansion slots that can be used to install data acquisition cards, oscilloscopes, etc. The chassis also has a high-performance PXI backplane that can independently provide PXI10 MHz (PXI-CLKl0) system clock signals to each peripheral expansion slot for multi-module synchronous measurement or system control.

At the same time, a PXI-6251 multi-function data acquisition card was selected and inserted into the third slot on the left end of the chassis.

The acquisition of physical signals is achieved through sensors, among which the encoder collects the rotation angle signal, the angular displacement sensor collects the swing angle signal, and the linear displacement sensor collects the linear displacement signal.

For data acquisition systems, noise signals will be generated due to the influence from power lines or machines. In this case, a low-pass filter is used to achieve maximum suppression. In addition, due to the low sampling rate, signal aliasing occurs. The Nyquist theorem points out that if an analog signal is sampled, all signals with frequencies exceeding 1/2 of the sampling rate will appear as low-frequency signals. This distortion can only be avoided by eliminating all signals with frequencies exceeding 1/2 of the sampling rate before sampling. Based on the above two reasons for the possible occurrence of noise signals, this system uses a combination of analog low-pass filters and digital filters. Analog filters are usually placed in front of the A/D converter to eliminate high-frequency noise and interference in the signal channel between A/D conversions. Digital filters are placed after the A/D converter to reduce noise on frequencies within the passband by using averaging technology.

This system uses a dual second-order loop filter circuit, using an operational amplifier circuit composed of more than two adders, integrators, etc., and introduces appropriate feedback according to the required transfer function to form a filter circuit. Its outstanding features are low circuit sensitivity, very stable characteristics, and can achieve a variety of filtering functions. Here, low-pass filtering is used to suppress some noise signals. The specific circuit is shown in Figure 4.

When forming a low-frequency filter, the circuit's natural frequency (unit: Hz) and passband gain are as follows:

2.2 Software Implementation

Software is the soul of the detection system. This test bench measurement and control system uses the virtual instrument graphical software LabVIEW, which has the characteristics of fast calculation speed, friendly interface, convenient human-computer exchange, and intuitive and convenient operation. The main program interface of the test system is shown in Figure 5: it realizes the effective selection of different items to be tested, and on the basis of selecting the input items, select the items to be tested from the main program interface, click, and the system enters the functional module instrument software panel of the performance test system to be carried out.

Among them, each functional module calls its sub-modules respectively, which mainly include data acquisition module, sensor calibration module, error processing module, data analysis and processing module, data waveform output module, output data storage module and data recovery module.

2.2.1 Data Collection

The data acquisition module encapsulates the driver modules of various hardware instruments in the components. When the test requirements change and new instrument hardware needs to be replaced, you only need to update the corresponding parameters and ensure that its interface to the upper layer remains unchanged. Then the new instrument hardware can run normally in the original system.

2.2.2 Calibration of sensor data

The calibration of the sensor is to establish the relationship between the sensor input and output through experiments. The sensor obtains the corresponding output by inputting the known standard value into the sensor to be calibrated. The calibration curve is obtained by plotting the output and the input standard value. The relationship between the input and output can be obtained through the calibration curve, so that the calibration coefficient can be determined. Using the calibrated data as the measurement data input improves the accuracy of the sensor.

2.2.3 Data analysis and processing

The data analysis and processing module includes logical calculation of data, graphic display of test signals, display of characteristic parameters during measurement, error analysis, signal filtering, etc. Through the work of this module, very accurate data and its corresponding curves can be output.

(1) Signal filtering

The arithmetic mean filtering method is to take n sampling values ​​continuously and average them. Its mathematical expression is:

The arithmetic mean filter is used to filter signals with random interference. The characteristic of this signal is that it fluctuates around an average value within a certain range. The degree of smoothness of the signal by the arithmetic mean filter depends entirely on N. In the case of system error, when n→∞, the noise signal approaches zero, and its average value tends to the maximum expected value. However, in fact, when N is small, the smoothness is low, but the sensitivity is high. N should be selected according to the specific situation to ensure the filtering effect and minimize the calculation time.

(2) Error processing

The rotational motion error includes transmission error and return error. To measure the transmission error, you can manually rotate the motor so that the speed does not exceed 10 r/min. When the motor rotates, the input and output angular displacements are measured by the input and output sensors respectively. The obtained test data can be displayed and recorded in the computer. Instantaneous transmission error = input angle / theoretical transmission ratio - output angle, and the corresponding curve can be fitted. When measuring the return error, remove the motor and rotate the input sensor. When at a certain angular displacement, keep the output end stationary and reverse the output end. The angle generated by the reverse direction is the return error. After testing at several angular displacements, a return error curve can be fitted.

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2.2.4 Data preservation and recovery

The data saving module can save the real-time measured data, and the data recovery module can redisplay the data curves of previous tests to facilitate comparative analysis, etc.

2.3 Experimental verification

In order to verify the feasibility of the design of the test bench measurement and control system, the swing angle of a crank rocker mechanism is tested through the test bench, mainly completing data acquisition, data input, signal processing, data analysis, result display and test result processing. The front panel of the detection system is shown in Figure 6.

The upper right corner of Figure 6 shows the system input parameters, including acquisition parameters and data processing parameters, which can be flexibly set according to different test objects and test conditions; the lower right corner shows the system output parameters, which can display the swing angle of the pendulum and the limit values ​​of angular velocity and angular acceleration according to requirements, thereby providing a certain basis for the study of vibration and noise of high-speed motion systems under certain working conditions; the left side shows the curve graph and operation buttons, which can display the curve graph of the relationship between the swing angle and time, as well as the relationship between the swing angle and the spindle angle, so that for dynamic systems with certain changes in the spindle speed, the relationship between the power and the driven parts can be further studied. The two graphs respectively represent the relationship between the swing angle and the main shaft angle and the relationship between the swing angle and time. The horizontal axis directly marks the two test methods, namely the main shaft angle and time. The meaning of the vertical axis for different curves is displayed in the upper right corner of the graph. Different colors represent different motion properties. White, red, and green represent the swing angle, angular velocity, and angular acceleration, respectively. This simple and clear description describes the different corresponding relationships and describes the different motion properties of a test object in the same graph, which is more conducive to the analysis and research of different motion characteristics of the same test object.

3 Conclusion

The creative combination test bench of planar mechanism based on virtual instrument is based on the original measurement and control test bench, and introduces virtual instrument technology to make it more versatile, more flexible experimental system components, stronger data processing capabilities, more accurate and faster measurement data, and its open environment is more conducive to the initiative and creativity of students. At the same time, LabVIEW software is a graphical language, which is more suitable for engineering and technical personnel. During the experiment, they can compile test software or modify some software modules according to their needs, so as to achieve the purpose of open experiment. Not only can basic teaching experiments be carried out, but also corresponding scientific research experimental projects can be carried out.