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附 录中文5650字附 录Research & Development of Virtual OscillographBased on LabVIEWAbstract: This paper introduces the design process of a virtual oscillograph based on LabVIEW. Mainly analyzes the amplitude value and time base adjusting methods during the real-time display. At the same time, it simply introduces the basic thought when measuring the period and frequency of the waveform gate voltage method and its application. At the end of this paper, combining the parameter measurement and waveform display of the virtual oscillograph with the modern motor close-loop lock-phase speed control, it analyzes the parameter measurements effect in PID control.Keywords: Virtual Oscillograph; Time-Base; Motion Control. INTRODUCTIONIn the rapidly developing industry control field, measuring technologies and apparatus become more and more important. But because of the disadvantages of the traditional instruments such as high price, single function, bad expansibility, etc., they can hardly meet the industry requirement. With the development of computer technologies and virtual instruments (VI for short), the scope designed by users becomes widely. There are many different functions with the same hardware which can make two or more machines work synchronously with the advanced bus technologies such as PXI bus technology 1.Virtual instruments become more and more popular for its upstanding characteristics like low cost, multi-function, facility and so on. All measuring instruments consist of three parts: data acquisition, data analysis and results output 2. In these three parts, data acquisition can be done by the system hardware like A/D module or digital I/O modules. Data analysis and results output can be completed by software system based on computer. So, if given some necessary data acquisition hardware, a measuring instrument based on computer can be constituted. The software technology is the essential one in the virtual instrument. 3 Visual C+, LabVIEW, LabWindows/CVI, VEE etc. are all development software environment. LabVIEW is a graphic programming language called G language, which can be used in GPIB, VXI, PXI, PCI Bus and data acquisition cards based hardware system, has powerful analyzing ability. Its graphic programming method can be used to finish the total program by dataflow clearly and simply. Using its embedded board card driver interface, we can conveniently operate a board card. 4, 5 Multi-channel digital oscillograph, which mainly used in real-time data acquisition, is one of the most widely used general measuring instruments. It also can display the changes of some electric signals and compare the differences among different signals. So the research and development of virtual oscillograph is the hotspot in the area. Making use of the Graph platte in LabVIEW, you can conveniently acquire the dynamic waveforms and make them displayed. But most virtual oscillographs based on LabVIEW use the Graph platte to operate and analyze the waveform data. Although this is very facile to use, it also has many disadvantages: (1) When the waveform changes all the time, the screen will keep refreshing and the graph platte could not properly work. (2) The operation method cant satisfy most of operators because it is different from the tradition oscillograph in operation. To solve these problems, this paper brings forward some ways. In the real- time waveform display, we redevelop the waveform operation and introduce some related software arithmetic. It introduces the exploiting thinking of adjusting dynamic time base and put forward two concepts: the FIFO process and E-M process. In addition, it introduces a successful gate-voltage measure way in the measurement of the period and frequency, and based on which, the paper puts forward the application in the close-loop lock-phase System.II. RESEARCH ON THE MULTICENTER DIGITALOSCILLOGRAPHS SOFTWARE ARITHMETICThe virtual oscillograph introduced by this paper is mainly used in laboratory for the measurement and storage of various analog signals. The main functions are: data acquisition, waveform display, parameter measurement, waveform storage and replay etc. It has 64 analog signal input channels and can take 8 signal observations at the same time with the choice of switch matrix. According to the requirement, we use the NI-6133 Daq card for the data acquisition. The block diagram of virtual oscillograph is shown in Fig. 1: 6Now we will introduce the soft arithmetic to the basic functions of virtual oscillograph.A. Characteristics of Graph ControlIn LabVIEW, there are three controls for waveform displaying: Graph, Chat and x-y graph. Every control has its own advantages. This paper takes Graph control for example to discuss the soft arithmetic to the basic function of virtual oscillograph. Graph oscillograph displays all waveform data input in the screen at a time. Every time when the waveform data are input, the screen will be freshed. 7 Using its own operation tools, you can move, zoom the waveform or use the cursors to measure the parameters. But it could not work well on dynamic waveform. So its necessary to develop a more convenient operation tool to real-time display of dynamic waveform. In the virtual oscillograph introduced in this paper, there are some basic functions such as amplitude and position value adjustment, time base change, trigger mode selection etc. The oscillographs front panel is shown in fig.2.B. Amplitude Value AdjustmentMulti-center oscillograph can display more than one waveform at the same time, so it is very convenient to compare every signal change. Every waveform displayed should be operated separately through the selecting box on the front panel. At first, we distribute the screen into 1010 grids and set every channel a Y-axis. The value of every one of the 10 grids in the Y-axis is equal to the value of related amplitude knob control. With the property node of the waveform graph, you can set the minimum value of each Y-axis as -5 times much as the knob value whereas the maximum value set 5 times. So when you change the amplitude knob value, the minimum and the maximum value of the relevant Y-axis should be changed at the same time, the waveform display can be zoomed as required, and the zero point position is kept in the original location.Use an array to save the amplitude values every Y-axis changed. When a channel is selected, put its old amplitude value to the knob first; and after adjusting, replace the related array element with the new value. And then the amplitude value change function is finished.C. Time Base AdjustmentTime base adjusting is one of the basic functions in oscillograph. The time base adjusting knobs value shows the time of every one grid of X-axis in the screen which is the nodus during the oscillograph design procedure.1) Basic ClewAccording to the characteristics of the Graph control, it displays all the data input at a time. So distribute the X- axis into 10 grids and make every grids time t. If the waveform could bestride the whole X-axis, the time spent to collect all the data is 10t. Suppose the boards sampling rate is f, in other word, the board collects f data every second. So the number N needed in the waveform is: N= fl0t =l0ft （1）Keeping the board sampling at the frequency of f, the program reads N data points from the board memory and put them to the oscillograph in every loop. Change the t value is to change the N value read from board memory every time, and thus adjust the time base in the real-time sampling.But through the experiment we can see, when the time base is too long (100ms) or too short ( 500us), the waveform displayed has time lags to different extend. That is because when the time base is too long (if the length of the needed waveform exceeds 1 second), we must wait for enough time to get all needed data collected by the board, and display them on the screen at a time. So that can cause discontinuous waveform displayed in the screen. When the time base is too short, the N value read every loop is too small, the number of data points in one second should be read for many times. There is another work to be done at the same time every loop in addition. So it will cause that the old data can not be read in time and may be covered by the new data because the board is sampling all time, the board memory will overflow. So it is needed to dispose the number read from board at long and short time base separately.2) Long time baseTo avoid the time lag discussed above, it must reduce the number of data read from sampling board every loop. Considering the display characteristics of Graph control, it can not put the data points read from boards to the graph every loop because it will make the oscillograph refreshing all the time, and the waveform displayed could not bestride the whole X-axis. An array could be set to save the waveform data points displayed. The arrays capacity is N, which is the number of data points calculated via Eq. (1). Suppose the number of data points read from board card every time is a fixed value m, which is the number of data points to be read at the proper time base.The whole procedure is made up of two parts. First, set the whole array NULL. At the beginning of the display procedure, the array is not full. So put the new m data points into the end of the array (Enqueue), and then display the whole array value in the graph control. A continuously moving waveform should be showed in the screen. Second, when the array is full, get rid of first m elements of the array; move the rest N-m elements forward the beginning of the array; and put the new m data points into the end of the array. We call this process FIFO. Then a full waveform should be showed in the screen, from one side to the other side. The Enqueue and FIFO are shown in Fig 3.Because the number of data points m read from board card every time is suitable, the time lag caused by waiting for enough data is properly solved. It ensures the synchronization between sample and display. To reduce the time spent to calculate the FIFO procedure by system, sampling frequency should be reduced at the long time base to reduce the capacity of the display array.3) Short time baseIn this part, the problem to be solved is that the time lag and memory overflow because the number N calculated via Eq. (1) is too small. The number N could not increase blindly because that will cause the disaccord between the set time base and the waveform displayed. So, a method called E-M (expand-move) is put forward. Fix the number to be read in every loop, which usually should be the one at proper time base. Suppose it as m, and the needed number at the short time base calculated via Eq.(1) as N ( N m ). Expand the t value of the waveform data by m/N times (expand). At the same time, change the maximum and minimum value of the X-axis, display the m data points ( t expanded) several times in turn, then get into next loop.Until now, the whole procedure is finished. The diagram is shown in Fig.4 and Fig.5.D. Position Adjustment and zero markChanging the Y value of the waveform data can change the waveforms position displayed on the screen. Increasing or decreasing the Y value of the waveform data can move the waveform up or down. To mark every waveforms zero position, add a button control to each waveform displayed. Drag the control into a line to mark the zero position.Initialize the control, relative to the oscillograph panel, to the middle of the right screen edge. When changing the position of the waveform, change the coordinate of the zero mark at the same time. So the zero mark will move with the waveform. In addition, set the mark button control invisible outside the top and bottom edges, that the effect can be truer. As seen in Fig.2.E. Other functionsWhat discussed above are the 3 basic functions in a virtual oscillograph. For other functions such as trig mode, couple mode, cursor, data storage, replay and print etc. are not discussed in this paper. According to actual needs, all the functions mentioned above can be achieved well through making full use of the property node in LabVIEW.III. CALCULATION OF FREQUENCY AND PERIODPeriod and frequency are key parameters to a periodic signal. Traditional measurement is to count the standard signal during the gate pulse duration in hardware so as to calculate the period of the measured signal. But there areusually no counters on hardware to use for a virtual oscillograph based on PC. It must be measured through software. However, the significance in a virtual oscillograph is that it can conveniently analyze the data collected by board cards, and then get the waveforms eigenvalue. So the basic clew to measure the waveforms period is to find the time slot between the Kth period and the (K+1)th period from the collected waveform data. Base on this thought, we introduce one method called gate voltage measuring to measure the signal period. 8The period of a periodic signal can be defined as the time slot that the signal across a specified gate voltage from the same direction (positive or negative edge) two times. As seen in Fig. 6Suppose a signal sample Xi, (i =v and Xi-1v, the waveform is across the gate voltage from the bottom up, called positive edge; when Xiv, the waveform is across the gate voltage from the top down, called negative edge. At the same time, to eliminate the infection brought by interference, get ride of the positive and negative edges whose interval is less than 10 sample points. So the position of positive and negative edge is acquired. Because the sampling rate is unchangeable, so the time slot between two sampling data is fixed. Thus the signal frequency and period can be calculated. 9 Suppose the sampling rate is f0 , the period is T0, T0 = 1/ f0, sampling rates error coefficient is a. The measured signals frequency is f, period is T, T= 1/f; the frequency measured actually is fC, TC, TC= 1/fC. Suppose at every sample time the sample contains k full periods of the measured signal, and the exact time is kT . The time of the first edge cross the gate point phase is t. Because of the disperse sample, at the kth period, the time when signal passes across the gate point phase t+kT may have an error T0. Considering the sampling rate error, at the worst state, at the kth period, the time when signal passes across the gate point phase is t+kT+(kTT0 )，and the time the k signal periods pass is t+kT+(kTT0) .So there it is:Suppose the sample length is L, there is LTokT. Put it on the Eq. (2) above, there is:When a is far smaller than 1/L，increasing the L value can increase the measure precision consumedly. When a is as much as 1/L, there is no significance to increase the L value. a is fixed on systems hardware. With a, it can find the proper L value that make the sample and calculation process under the best precision.Suppose the square signal, its pulse duration is T, the one measured is TC, so there is:What discussed above is the period and frequency measurement in the sampling procedure, putting forward the measurement precision theory. But to the amplitude, rise time or spectrum analyze etc. are not discussed in this paper. Using the data group collected, the user can develop other better measurements. In LabVIEW, there is plenty of measuring VIs, which can measure the parameter exactly. 10 Combining the control procedure with virtual oscillograph can achieve better effect. Next, take DC motors PWM speed control for instance, it will introduce the function that using virtual oscillograph in PID and closed loop feedback controlling.IV. APPLICATION IN MOTOR SPEED CONTROLLock-phase technology plays an important role on motor speed control. With the technology, it can improve the precision of motor speed; and also, it does stepless speed variation control only by changing the specified frequency, that will be conveniently used on controlling more than one motor work synchronously. 11 The basic theory diagram that indicates the speed control system based on PPL closed-loop lock-phase is shown in Fig.7.Suppose the specified pulse met the motor speed is fR, the pulse from photo sensor is fF. Compare their frequency and phase in the phase comparator, and bring the signal voltage proportion to frequency and phase difference. This voltage controls the motor speed through the low-pass filter to synchronize the motor speed and the specified control signal. In case the load is fluctuating which changed the motor speed, the pulse output from photo sensor is changing at the same time. There is difference between it and the specified signal. So the output of the phase comparator through the low-pass filter and driver circuit is changing, and make the motor faster or slower until the two frequencies of feedback and specified become equal. At that time, the motor is steady again. The feedback frequency is locked to the specify frequency, so the system control precision is very high.Combining the phase comparator and computer, taking the advantage of measurement and control of virtual instruments, we can get the digital lock-phase closed loop circuit. The theory diagram is shown in Fig 8.In the diagram, motor speed is converted into square signal in proportion it refers to through the photo-sensor. If the speed value measured by virtual oscillograph is lower than the necessary one, the output frequency should be increased; oppositely, the output frequency should be decreased. In the actual controlling, the anticipant speed can be achieved quickly by comparing the frequency measured and specified, adjusting through PID.Following the PID theory, putting the secular equation and Jury criterion together, there are:Therefore, to keep the system working steadily, we should set KP = 1, KI = 1/2It is quickly to adjust the output frequency to the anticipated one by PID revision. We can get the proper square signal through the wave form generator with the specific frequency, and the low-pass filter can convert it into relevant control voltage. The whole procedure is quick and steady. Using the measure function of virtual oscillograph can complete the same work as lock phase speed control system. Its advantage to traditional lock- phase speed control system is that it can use the control theory in the procedure, optimizing the control method. At the same time, the whole changing signal curve can be seen in the virtual oscillograph, the operator can clearly see the control and feedback state so as to solve the problems met with better methods. Using the multicenter and bus technology, it can control more than one motor at the same time, make them work synchronously, that is propitious to product line management and remote control.V. CONCLUSIONThe virtual oscillograph in this paper not only has the functions that common oscillographs have such as data acquire and display, parameter measurement, but also can be used in industry control, as an important part in motor speed control system. In the actual application, its flexibility is popular with more and more people. With different hardware, more complex and agile measuring system will be produced. With the development of computer and measurement technology, virtual instrument technology will play a more important role in many fields.VI. ACKNOWLEDGMENTThis work is supported by natural science foundation of China under the research project 50375008 and 60575052.VII. REFERENCES1 W.Jang, and F.Yuan,Design, of multicenter virtua oscillograph, China measurement technology, Vol.30 No.4, July, 2004.
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