PHYS 5061 Lab 1: Introduction to LabVIEW

Transcription

1 PHYS 5061 Lab 1: Introduction to LabVIEW In this lab, you will work through chapter 1 and 2 of Essick s book to become familiar with using LabVIEW to build simple programs, called VI s in LabVIEW-speak, short for Virtual Instruments. Eventually we will use LabVIEW to control experiments, including control of instruments and collection, display, and analysis of data. Keep your work in a sensibly named folder on the Windows desktop so the instructor can find your work easily. (1) Work quickly through the chapter 1, which provides some basic orientation to navigating LabVIEW. You will find that some features in the LabVIEW version we are using don t match the description in the 3 rd edition of Essick s book. Don t worry about documenting anything at this point. (2) Move on to work through chapter 2 and complete the do-it-yourself exercise at the end of chapter 2. Make sure each person spends time working the controls. (3) Complete problems 5 and 8 from the end of chapter 2. For the metronome VI, add in some audio with the beep.vi built into LabVIEW to produce a 262 Hz tone for 100 ms on each tick of the metronome along with the blinking. You may work collaboratively in developing these final VI s if you wish, but in each pair, each person should take lead responsibility in producing at least one of the finished VI s. After you have finished a VI and think it s perfect, make a final test of it by closing it, loading it afresh into LabVIEW and see if it performs as desired. You may discover LabVIEW has decided on assigning some default conditions or values to parameters that make for unexpected or undesired behavior and you should figure out how to change this behavior. Add some comment labels to your block/wiring diagrams to make your code intelligible to others. Print out the VI s (Front panel, Block diagram) and tape them into your lab notebook. (Do this for the DIY and two end-of chapter 2 problems.) Save the VI s in a folder on the Windows desktop so the instructor can test them out.

2 PHYS 5061 Lab 2: More getting up to speed with LabVIEW (1) Work through chapter 3 of Essick, mastering the FOR loop and the waveform graph. (2) Complete and save the VI for the final do-it-yourself coin flipper VI at the end of chapter 3. (3) Modify it (the DIY VI) to do 50 coins in a toss and save as a new VI. (Nevermind creating 50 heads/tails indicators for this situation! That d be cruel. Simulating a flip of 50 coins can be done with a loop within a loop doing each one of the 50 coins at a time, rather than flipping all 50 in parallel. ) To total up the number of heads, there is a useful hint in problem 1 of chapter 3 on summing elements in an array, or you can look ahead to chapter 7, where shift registers are introduced as a means of retaining and updating a value from one loop iteration to the next. You only need to look at the first two pages of chapter 7 to get this. There s also an advanced histogram VI that you may want to explore as a somewhat nicer way to get the histogram. Comment this VI by adding text to the block diagram explaining to someone what/how this VI does its thing. And add a brief instruction to the front panel on how to use it for someone who picks it up to try six weeks from now. (4) (a) Complete problem #3 from chapter 3 and save the VI. Same requirement on commenting/documenting your VI. (b) Create a VI that extends the series sum in (a) to contain a user-specified number of terms and updates the plot after each term is added. You probably want to put a delay in somewhere so you can see successive plots at a human-friendly pace. And you may really, really want to look at that shift register mumbo jumbo mentioned above now. Print out the VI documentation for your various VI s and put them in your lab notebook with any additional comments you may have. Remember: After you have finished a VI and think it s perfect, make a final test of it by closing it, loading it afresh into LabVIEW and see if it performs as desired. You may discover LabVIEW has decided on assigning some default conditions that make for unexpected behavior.

3 PHYS 5061 Lab 3: Math tricks in LabVIEW, building re-usable VI s, saving data. Open up LabVIEW 8.6 for this lab! LabVIEW s graphical programming leads to programs driven by data flow, which is convenient in situations where one collects and processes data from an experiment. But it may seem that one often spends a fair amount of time building a diagram that does some relatively simple mathematical calculation. For example, think about what the block diagram to compute an array of values of A sin( t ) for t = 0, 0.01, 0.02,, , with values set by controls for the amplitude, frequency (which a user would prefer to give in Hz) and phase. That requires putting down VI icons for the trig function, a constant (2 ), a few multiplications, and an addition, along with generating the array of input time values, along with wiring it all up and placing things so the next person who looks at it can figure it out. It s not that hard (although the figuring-out by the next user often is), but sometimes good old-fashioned text-based programming languages can achieve the same goals (functionality and ease-of-understanding) with significantly less effort. LabVIEW offers that capability through two of its programming structures. The first is the function node. This icon expands to a region where one can put in simple text-based programming instructions. It allows you to wire up input values (from other icons, constants, or front panel controls) to tunnels on the math function node frame and then do text-based calculations, i.e. calculate functions of the input values, and take the results from output tunnels to further processing or display. The second and fancier option is the MathScript node, which does much the same thing, but knows a large set of high level commands. Its use will be familiar if you have previously used Matlab. It understands the same syntax and has many of the same functions built into it as Matlab. It also provides an interactive MathScript window that is reminiscent of Matlab s interpreter interface, with different regions for commands, variables, outputs, plots, etc. (Note: The drawback to the enhanced functionality of Mathscript is that it is no longer a standard part of our LabVIEW package. LabVIEW 8.6 has it, but it is missing from 2010 on. It is still included in recent student editions of LabVIEW. Note, however, that VI s created in recent versions of LabVIEW may work in v. 8.6 only if you take advantage of the Save for previous version option when saving them. So, if navigating between versions, keep that in mind.) (1) Work through chapter 3 of Essick to become familiar with the basic functionality of these features. This chapter introduces you to how to make your VI a re-usable entity. Having written a carefully designed VI to perform a useful task, you can give it its own icon, with input and output connections, save it, and use it as just another element you simply drop into a VI to perform a more complicated task. Frequently repeated tasks can be coded in LabVIEW once, with controls to input key variables, and then used repeatedly in future work. Be sure to save your AM Wave VI from the Do it yourself

4 exercise at the end of the chapter, and figure out how to print out the front panel and block diagram for your VI. (2) Complete problem 6 from chapter 3. Add a control named Save to the front panel so that when it is true, the θ,r data is written to a text file suitable for input to other programs. Write the data (both θ and R values, one column for each) to a text file using the write-to-spreadsheet file VI described at the start of chapter 5. You can assemble the θ and R data into a single 2-D array and use the 2D data option on the file-writing VI. There is also a transpose option on that VI that is useful in getting data out in columns vs. rows. Be sure to look at your output data file to be sure it s human-readable. Save your complete VI and print out the front panel and block diagram. (3) Complete problem 8 at the end of chapter 3 and save the VI. Print out the VI front panel and block diagram. Always comment the block diagrams of your VI s to provide guidance as to how the VI works.

5 Previous lab 3 component (3) Complete problem 1 from chapter 3, to form a square wave from a Fourier series of harmonics of a fundamental sine wave, but with several twists for your finished VI described here. Most importantly, instead of a fixed number of terms in the series, extend the sum to include an arbitrary number of harmonic terms in the Fourier series, which will be specified by the user through a front panel control. (Make the default choice 5 terms.) You should be able to infer the form of the general term in this series. Use one Mathscript node to calculate the array of time values required. You can use another MathScript node inside a loop (which executes as many times as the number of terms the user has specified) to calculate the contribution to the waveform from each successive harmonic. Make use of a shift register to accumulate the sum of the series. Shift registers are described at the start of chapter 6. (Yes, a clever MathScript node probably could do everything for us all at once, but that s not desired here.) Display the waveform with an x-y graph on the front panel as each successive harmonic is added to the waveform. (You might need to put a delay in the loop to watch the waveform evolve at a reasonable pace.) Write the final waveform data (both t and y values, one column for each) to a text file named squarewave.txt. Use the write-to-spreadsheet file VI described at the start of chapter 5. You can assemble the t and y data into a single 2-D array and use 2D data option on the file-writing VI. Save the VI and print it out. Always comment the block diagrams of your VI s to provide guidance as to how the VI works.

6 PHYS 5061 Lab 4: Data Acquisition in LabVIEW: A/D and D/A operations. Use LabVIEW 8.6 for this lab! Tools developed in the previous lab may be handy here. Chapter 4 in Essick introduces generation and measurement of signals using LabVIEW. We are making use of commercial off-the-shelf hardware and software to perform these tasks. Unlike the days of old, experimentalists can now routinely avoid worrying about the low-level aspects of communicating with the devices in many situations. Commercially available data acquisition (DAQ) hardware typically comes with software packages or drivers that may be accessed from high level programming languages, including LabVIEW, and offers great flexibility on making measurements in a variety of circumstances. The data acquisition (DAQ) hardware we will use is the NI USB-6211, which offers up to 16 channels of 16-bit resolution analog-to-digital conversion (ADC), 2 channels of 16-bit analog output (DAC), and a small number of lines for digital input and output (DIO). All this is contained in a small box that is external to the PC and communicates via USB. LabVIEW makes use of driver software (NI-DAQmx drivers) to talk to the hardware. The drivers handle the work of converting your desires into configuring the hardware and transmitting or retrieving data between your program or VI and the DAQ device. The USB-6211 features hardware timing and buffers for both input and output. For example, it is possible to create a VI that loads the USB-6211 with data to produce a single cycle of a sinewave for output and arrange for the device to automatically regenerate the sinewave to produce a continuous sinewave output without further intervention by the PC or the user. Your VI can go on and make measurements or do calculations, just as if you had a stand-alone benchtop function generator to create the sinewave. Chapter 4 makes use of some high-level VI s (the DAQ Assistant VI and Express VI s) that sacrifice some freedom for ease of configuring DAQ tasks, which will provide a gentle introduction to using the DAQ hardware. (Chapter 11 describes how to use the more basic DAQmx VI s as building blocks to have greater control over the individual choices needed in setting up a DAQ task and lays out more clearly what choices a user must make in setting up measurements.) (1) Preliminaries: connecting the DAQ device: Make sure the DAQ device is NOT connected to the PC. From the Windows Desktop or under `All Programs from the Windows menu open up the Measurement and Automation Explorer (MAX). Look in Devices and Interfaces. You may see one or more devices listed there. You may leave any GPIB devices, but if you see any USB devices listed, delete them. Then plug in your device to a USB port. It should be recognized, drivers load, and then it appears in the list of devices. Note how it is identified (Dev1 probably). Once it s installed and available in MAX, you can select it and run a self-test in MAX. Hereafter, the DAQ device should remain with that PC.

7 Swapping them between PCs may cause a second DAQ device to be created and a Dev2 entry, which can lead to confusion and frustration in the not-so-distant future. A short specifications sheet is available and an enlarged diagram with the pinouts for the device is provided. We will use the analog inputs (ai0 ) in NRSE mode (Non- Referenced Single-Ended mode), not the differential mode mentioned in Essick. (This gives 16 input channels, not that you ll need them all, but it means the wiring is a little simpler; all the channels use a common ground connection as the low side of their. In general differential mode is preferred, and at times essential.) To operate in NRSE mode, connect a 10k resistor between the analog input sense (AI SENSE) and analog in ground (AI GND). Find a small screwdriver that fits the terminal blocks and keep it handy for making connections. (2) Fire up LabVIEW (8.6). Work through Essick chapter 4. You can skip 4.11, since the hardware we have has the timing capabilities for high quality waveform generation needed beginning in sec (Sec does something similar using the more basic DAQmx VI s. ) (3) Note on DIY exercise on digital I/O (DIO) before problem: Our USB-6811 has only four general purpose digital output lines and four digital input lines. For the alternating LEDs portion, put a delay in loop to make the alternating visible to the user. (4) Do Problem 4.6. in Essick. In addition, have the VI display an estimate of the 3db point of your filter at the end of the measurement. Save the VI so it can be tested with filters that contain different components. Put some instructions for the user on the front panel for it, and include commentary so someone can make sense of you block diagram a cleanly wired diagram makes data flow and the VI s operation easier to follow.

8 Build a VI to continuously output your AM signal.

9 (1) How feedback works in an op-amp. Build a model in LabVIEW for a non-inverting amplifier. Recall that a raw op-amp does two things: takes the difference between two voltages: V + - V - and then amplifies that difference by a number that is very large compared to 1. Put such a calculation inside WHILE loop in LabVIEW. Put a control that corresponds to V+ inside the loop. For the moment, let V- be fixed at 0.