Labview Physics 3 - It Skills
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LabView Physics 3 - IT skills Miles Padgett [email protected]
1
Objectives
To acquire familiarity with the LabView Programming language To be able to write LabView programmes incorporating pre written and new VI’s To use LabView in the study of 2-D diffraction patterns You get exercise marks for completion of each milestone.
2
LabView - the basics
All LabView programmes comprise two screen types The
“front panel” (grey) which acts as the user interface The “diagram” (white) which contains the “programme”
3
LabView - getting going
Opening LabView (windows)
Opening LabView (Linux)
“Double-click” on LabView Icon LabView start-up appears From within the Linux “Shell” type “labview &” LabView start-up appears
Programs themselves are called “vi” (virtual instruments) To start a new “vi”, select “new vi” from the LabView start up screen
The LabView icon
The LabView start up screen 4
LabView - front Panel
Controls (data input) and indicators (data output) can be selected from the “controls” window and placed on the front panel. To find controls
select “show panel” from “window” menu select “show controls palette” from “window” menu
The controls palette 5
LabView - diagram
Functions (program operations) can be selected from the “functions” window and placed on the diagram. To find functions
select “show diagram” from “window” menu select “show functions palette” from “window” menu
The functions palette 6
LabView - tools
Tools (cursor/mouse functions) can be selected from the “tool” window To find tools
select “show tools palette” from “window” menu
Some issues of LabView give option for auto-tool select TURN IT OFF!
Auto tools is turn off and on by clicking on the “green” button (Dark green is off)
The tools palette
Click to turn off auto tools
7
Using LabView to add two numbers
Create new program
Select “new VI” from the file menu
Select a digital control from the controls palette and place it on the front panel (grey) Do the same again Select a digital indicator from the controls palette and place it on the front panel
Selecting a digital control
Selecting a digital indicator 8
Adding two numbers - front panel
Controls and indicator placed on front panel N.B. software automatically numbers controls of the same type in sequence The front panel 9
Adding two numbers - diagram
Data terminals automatically appear on diagram
The diagram 10
Adding two numbers - placing the function
Select and drag the “addition function onto the diagram To find addition
Selecting a numeric function
Show function panel Click on numeric Select and drag addition onto diagram
Dragging onto the diagram 11
Adding two numbers - wiring the function
Select “wiring tool” from tools palette
Cursor now changes function “click” on wire start point “drag” to end point then “release”
Use wiring tool to connect
Digital controls to function inputs Function output to digital indicator 12
Useful tips - controls and indicators and terminals
Do not confuse controls (data inputs) with indicators (data outputs)
Wiring an indicator to the input of a function will generate an error Wiring a control to the output of a function will generate an error
Double (left) clicking on any terminal on the diagram (white) will highlight the associated control or indicator on the front panel (grey) Double (left) clicking on any control or indicator on the front panel (grey) will highlight the associated terminal on the diagram (white) 13
Adding two numbers - running the program
Select “arrow” from tools palette
Running the program
Run continuously
Cursor now changes function Use arrow tool to “click” single arrow on front panel to run once Or “click” looped arrow on front panel to run continuously
Stopping the program
Stop the program Run once
Use arrow tool to “click” red stop button 14
Adding two numbers operating the program
Select “finger hand” or “text tool” from tools palette
Changing the input
Cursor now changes function Use “finger” on up/down arrows of numeric controls Or use “text tool” to highlight and edit number field (white) within numeric control
Text field
If running continuously then program is interactive
Up/down arrows
15
Adding two numbers - saving the program
Select “save” from within the file menu When prompted enter filename Note in windows LabView files have a ***.vi file extension
vi - stands for virtual instrument
DO IT NOW AND KEEP DOING IT!
16
Useful tips - changing the program
When a LabView program is running it IS possible to change the setting of the controls, i.e. change the data inputs When a LabView program is running it IS NOT possible to change to wiring of the program To make changes to the program you need to stop it first!
17
Customising the program (text)
Select “text tool” from tools palette
Cursor now changes function
Changing the name of the input
Use “text tool” to highlight and edit the names (grey) numeric control Use “text tool” to highlight and edit the name (grey) numeric indicator
Indicator text field
Control text field
18
Customising the program (data)
Select “arrow tool” from tools palette
Cursor now changes function
Changing the allowed range and up/down increment of the controls
Selecting data range
Right click (windows) on the numeric control to generate menu then select “Data range…” Within data range can set upper limit/lower limit and increment
Now run the program again
Max/Min/Increment
19
Customising the program (display) -1
Select the front panel Show “controls panel from window menu” Use “arrow tool” to select numeric control Use “arrow tool” to select “meter” and “drag” it onto front panel Use “text tool” to optionally edit name of meter Use “text tool” to optionally highlight and edit the range of the meter
Selecting a digital meter
Name of meter
Range of meter
20
Customising the program (display) -2
Select the diagram Select “wiring tool” from tool palette Use wiring tool to connect meter terminal to existing output wire of the “addition” function or equivalently to the existing numeric indicator terminal
I.e. click on start, drag to end then release
Run the program again, note meter display matches indicator 21
Accessing LabView’s help
To get help on a function
Select the diagram Select “arrow tool” from tool palette Use arrow tool to right click (windows) on function Select help from pop up menu Function help appears in subwindow (There may be some problems with HELP under LINUX)
More general help can be obtained through the “help menu”
“Content and index” is good for specific questions LabView comes with a “learn by activities package” Also a great set of example programs 22
Useful tips - indicators, controls and terminal
Every indicator on the front panel has a terminal on the diagram Every control on the front panel has a terminal on the diagram Right clicking(windows) on any terminal, control /indicator gives the option of highlighting the corresponding control /indicator or terminal
23
Useful tips - switching between tools
Rather than picking your tool from the “tools palette” pressing the “tab” key toggles the tool between On the diagram
Arrow (allows selection and/or movement of terminals and functions around diagram Text (allow text edit of terminal and addition of extra text comments) Wiring (allows wiring connection of functions and terminals) Finger (allows selection of terminals)
On the panel
Arrow (allows movement of controls around panel Text (allows text edit of control/indicator and addition of extra text comments) Paint (allows colour change of control or indicator) Finger (allows adjustment of controls) 24
Useful tips - showing various windows
Any window or tool panel can be made active by selecting it from the window menu Alternatively “clicking” on a window will make it active
This is a quick way of switching between the diagram and the front panel
25
Useful tips - finding errors
If the current LabView programme is non executable the “run” arrow on the front panel appears broken Activating the arrow results in an error list contain the faults Selecting any fault highlight the offending part of the diagram All unconnected wires (dotted lines) can be removed by selecting “remove broken wires” from the “edit” menu
Broken “run” arrow
error list
26
Looping programs
Computers become power when you make them do something many times! LabView loops are examples of structures and placed on the diagram
Show diagram Shows functions palette Select structure
Selecting the structure sub palette from the functions palette 27
Incorporating a “while loop”
Select a “while loop” from the structure palette Place while loop on diagram to surround program
“While loop”
Click top left drag to bottom right and release
While loop will run whilst condition is true
Condition
28
Setting the condition of a “while loop”
Need Boolean (true/false) control to set state of while loop
Show front panel Show controls palette Select “boolean” Select “push button” and drag and place (release) on front panel Optionally use “text tool” to highlight and edit name (grey) of new control
29
Wiring the condition of a “while loop”
Need to wire the Boolean terminal to the condition of the while loop
Use arrow tool to move (if necessary) the terminal of the Boolean control to the inside of the while loop Use the wiring tool to connect the boolean terminal to the condition terminal
30
Running a “while loop”
Running/stopping the program within a while loop
Use finger tool to toggle Boolean control to true (dull green arrow becomes bright green) Use finger or arrow tool to “run” program (NB not continuously run) Program will now run continuously Use finger or text tool to change/edit numeric controls Use finger tool to toggle boolean control to stop program
31
To draw a sine curve in LabView
Objective To
plot a sine curve in the range 010π
Start a new program Select
“new VI” from the file menu
Save it now Select
“save” from the file menu and when prompted provide name 32
Using a “for loop”
Show the diagram Select a “for loop” from the structure palette Place the “for loop” on the diagram
“click” “drag” “release”
A “for-loop” will run N times then stop i increments from 0 to N-1
For loop
Terminal for N
Terminal for i 33
Wiring N on a “for loop”
Show the front panel
Show diagram
Select digital control from the controls palette Place control on front panel (If necessary) move terminal of digital control to outside of loop Wire digital control terminal to N terminal of for loop
NB one terminal is blue the other orange!
34
Understanding data types
Computers store number in different forms, e.g. Integers,
8 bit, 16bit, 32 bit - BLUE in
LabView Floats single precision, double precision Orange in LabView
35
Converting Data type
Show either diagram or front panel Right click (window) on control or terminal and select “representation” “click” on data type of choice to convert numeric Do this to change numeric control to I32 integer Edit numeric value to 100
36
Drawing a sine curve - defining the range (1)
To define the 0-10π range
Show diagram Select and place constant outside loop edit to 10 Select and place π outside loop Select and place multiply function outside loop Wire “10” and “π” into “x” function
Selecting a constant
Selecting π
37
Drawing a sine curve - defining the range (2)
Select and place divide function inside loop Wire “i “into numerator of divide Wire numeric input through wall into denominator of divide Select and place multiply function Wire output of divide into multiply Wire “10 x π” multiply through loop wall into multiply 38
Drawing a sine curve calculating the value
To calculate the sine value
Select and place sine function inside loop Wire output of multiply to input of sine Wire output of sine to loop wall
Selecting sine
39
Drawing a sine curve displaying the curve
Show front panel
Select and place “waveform graph” on front panel
Show diagram
(if necessary) move waveform graph terminal outside loop Wire wall of loop to waveform graph terminal
Selecting waveform graph
40
Drawing a sine curve displaying the curve (2)
Show diagram
(if necessary) move waveform graph terminal outside loop Wire wall of loop to waveform graph terminal
NB orange wire on outside of loop is thicker than inside
Indicates wire carries an array of numbers
41
Drawing a sine curve - running the program
Use arrow tool to “run continuously the program Adjust numeric control to change number of points calculated Use arrow tool to “stop” program when finished Use text edit tool to rename x-axis of graph (angle), name of numeric control (number of data points) and name of waveform graph
42
Drawing a sine curve modifying the program
To make 10π range variable
Select and place digital control on front panel Show diagram Delete 10π product structure and wire from outside loop to leave unwired loop entry Move new numeric terminal to similar position Wire in terminal to loop entry
Run continuously and experiment with changing the range
43
Drawing a sine curve extending the program FFT (1)
To obtain an FFT of the sine wave
Select and place FFT function on diagram near existing waveform graph terminal - the route to FFT is • • • •
Functions Analyse Signal processing Frequency domain
Wire waveform graph (or neighbouring wire to Input of FFT function
Selecting FFT function
44
Drawing a sine curve extending the program FFT (2)
Wire waveform graph (or neighbouring wire to Input of FFT function On front panel place additional waveform graph On diagram wire output of FFT function to terminal of waveform graph Use text tool to edit name of waveform graph and x axis of graph on front panel
45
Drawing a sine curve - seeing the FFT
Run program continuously Update range control Examine FFT of sine wave Note the FFT has two peaks Note FFT peaks have +ve and -ve values
46
Drawing a sine curve - seeing the power spectrum (1)
More usual to consider the power spectrum Need to take modulus squared of each FFT component
Delete wire between FFT and waveform graph terminal (if necessary) move terminal of waveform graph away from FFT Select and place modulus function after FFT Select and place multiply function after modulus Wire FFT to modulus Wire modulus to both inputs of multiply Wire multiply to waveform graph terminal
47
Drawing a sine curve - seeing the power spectrum (2)
Power spectrum is +ve Still twin peaked Run program
Note that a higher “frequencies” of sine wave power spectrum peaks move towards centre Double peaks can be through to represent +ve and -ve frequency
48
Using LabView to draw a 2D function
Objective To
plot an “egg box” type pattern
Start a new program Select
“new VI” from the file menu
Save it now Select
“save” from the file menu and when prompted provide name
49
Double nested “for loop” A single for loop will create a vector of N elements To create an array use a for loop within a for loop
50
Useful tips - copying and moving items on the diagram
To move a selection of functions and wires on a diagram
Use arrow tool to define rectangle of interest, i.e. click upper left and drag to lower right Use cursor keys to move selected region
To copy a selected section of the program
Use arrow tool to define rectangle of interest, I.e. click upper left and drag to lower right Use standard copy and paste functions to replicate program
51
Calculating the “egg box”
Assume an egg box is generated by taking the product of two sine functions (one in the x-direction and one in the y) Use double nested loop and repeat logic of previous program, note
Both N terminals of loop wired to control i indices of loops wired to form x and y axis Out wired through inner loop to form output from outer loop
52
Visualising the “egg box” (1)
Show front panel
Select and place intensity graph on front panel N.B. intensity graph is different from intensity chart! Use text tool to rename x-axis, yaxis and name of graph
Selecting an intensity graph
53
Visualising the “egg box” (2)
Show diagram (if necessary) move terminal of intensity graph to outside of nested loop Wire output from outer loop to terminal of intensity graph NB note orange wire becomes “double wire” which indicates it is an array
54
Useful tips - Polymorphic
In LabView, most functions are polymorphic.
e.g. the same addition function will add two numbers or two vectors or two arrays (vectors or arrays must have the same dimensions The exception to “dimension matching” is that one can add a number to every element in vector or array
Adding numbers
Adding vectors Adding arrays Adding a number to each element of an array 55
Running the “egg box”
Show front panel Use finger or text tool to set numeric controls to ≈100 loop iterations and a plot range of ≈30 Run or run continuously the program Need to set z-axis of graph to autoscale • “right click” (window) on graph, select zscale and select autoscale z
Auto-scaling the z-axis of an intensity graph 56
Useful tips - changing the number format
Previously we have seen how to change the range over which a numeric control can be varied To change the way it is displayed
Show front panel “rick click” (windows) on control Select “format and precision” Edit format and precision window as desired NB this only affects the display NOT the precision of the calculations 57
The challenge - diffraction patterns
To calculate and display the far field diffraction pattern of a circular aperture
The far field diffraction pattern is the same form as the Fourier-transform (but in 2D) In the first instance “forget” about the wavelength (which sets the scaling between the aperture and the diffraction pattern) - just concentrate on the “shape”! i.e. just take the 2D FFT of an array whose numeric values are the transmission of a circular aperture (1’s and 0’s)
Read the next few pages for some hints! 58
Useful functions for challenge (1)
“Comparison” functions allow logic decisions
When defining the aperture, you may need
NB wire carrying logic, i.e. Boolean (0..1) date are green
“select” “less” or “greater”
Use LabView’s help to understand these functions 59
Useful functions for challenge (2)
“array” functions allow manipulations of array and vectors
When doing the FFT, you may need
Wire containing array data appear a two parallel wires. These may be blue (for integer)or orange (for floats)
“transpose” “rotate 1D array”
Use LabView’s help to understand these functions
60
Useful functions for challenge (3)
If the N terminal is left unwired, “for loops” will selfindex
vector to element
array to vector
When a vector is wired into a “for loop” the loop will split the array and run it on each element When an array is wired into a “for loop” the loop will split the array and run it on each row
To switch off “self-indexing” right click (windows) on wire entry to loop and select “disable indexing” (not needed in challenge) 61
Useful functions for challenge (4)
Standard LabView does not have a function for 2D FFTs Given a 2D array, a 2D FFT can be completed by doing 1D FFTs on each row and each column You may need this when calculating the diffraction pattern - note use of “self-indexing”
Part of a “diagram” for completing a 2D FFT
62
Diffraction pattern
Run your programme
Do you get a diffraction pattern centred in the four corners? This is a issue with most FFT algorithms - where should the zero “frequency” be located? At the centre or the edges? We need to centre our zero in the middle of the image
63
2D FFT with zero at centre
Can use “1D rotate function to move zero to centre (NB need to rotate by N/2) Note also modulus squared added to output of FT to give power spectrum (i.e. light intensity)
64
Calculating a diffraction pattern
One program that works! Autoscale-z
is
switched off and z-scale is manually set to reveal structure
65
Another useful function (but not needed for challenge)
Often one uses each loop iteration to modify the value of a number
The new number can be used in the next iteration
In LabView this is called a shift register To add a shift register to a loop
Right click (windows) on the left edge of the loop and select “add shift register” The shift register looks like a down and up arrow on the left and right sides of the loop
67
Shift-registers at work
The shift register needs a starting value
The shift register gives an output when the loop has finished
Wired to the down arrow on the left side of the loop
“shift register”
Wired from the up arrow on the right side of the loop
For example calculating the factorial of a number
68
1
Objectives
To acquire familiarity with the LabView Programming language To be able to write LabView programmes incorporating pre written and new VI’s To use LabView in the study of 2-D diffraction patterns You get exercise marks for completion of each milestone.
2
LabView - the basics
All LabView programmes comprise two screen types The
“front panel” (grey) which acts as the user interface The “diagram” (white) which contains the “programme”
3
LabView - getting going
Opening LabView (windows)
Opening LabView (Linux)
“Double-click” on LabView Icon LabView start-up appears From within the Linux “Shell” type “labview &” LabView start-up appears
Programs themselves are called “vi” (virtual instruments) To start a new “vi”, select “new vi” from the LabView start up screen
The LabView icon
The LabView start up screen 4
LabView - front Panel
Controls (data input) and indicators (data output) can be selected from the “controls” window and placed on the front panel. To find controls
select “show panel” from “window” menu select “show controls palette” from “window” menu
The controls palette 5
LabView - diagram
Functions (program operations) can be selected from the “functions” window and placed on the diagram. To find functions
select “show diagram” from “window” menu select “show functions palette” from “window” menu
The functions palette 6
LabView - tools
Tools (cursor/mouse functions) can be selected from the “tool” window To find tools
select “show tools palette” from “window” menu
Some issues of LabView give option for auto-tool select TURN IT OFF!
Auto tools is turn off and on by clicking on the “green” button (Dark green is off)
The tools palette
Click to turn off auto tools
7
Using LabView to add two numbers
Create new program
Select “new VI” from the file menu
Select a digital control from the controls palette and place it on the front panel (grey) Do the same again Select a digital indicator from the controls palette and place it on the front panel
Selecting a digital control
Selecting a digital indicator 8
Adding two numbers - front panel
Controls and indicator placed on front panel N.B. software automatically numbers controls of the same type in sequence The front panel 9
Adding two numbers - diagram
Data terminals automatically appear on diagram
The diagram 10
Adding two numbers - placing the function
Select and drag the “addition function onto the diagram To find addition
Selecting a numeric function
Show function panel Click on numeric Select and drag addition onto diagram
Dragging onto the diagram 11
Adding two numbers - wiring the function
Select “wiring tool” from tools palette
Cursor now changes function “click” on wire start point “drag” to end point then “release”
Use wiring tool to connect
Digital controls to function inputs Function output to digital indicator 12
Useful tips - controls and indicators and terminals
Do not confuse controls (data inputs) with indicators (data outputs)
Wiring an indicator to the input of a function will generate an error Wiring a control to the output of a function will generate an error
Double (left) clicking on any terminal on the diagram (white) will highlight the associated control or indicator on the front panel (grey) Double (left) clicking on any control or indicator on the front panel (grey) will highlight the associated terminal on the diagram (white) 13
Adding two numbers - running the program
Select “arrow” from tools palette
Running the program
Run continuously
Cursor now changes function Use arrow tool to “click” single arrow on front panel to run once Or “click” looped arrow on front panel to run continuously
Stopping the program
Stop the program Run once
Use arrow tool to “click” red stop button 14
Adding two numbers operating the program
Select “finger hand” or “text tool” from tools palette
Changing the input
Cursor now changes function Use “finger” on up/down arrows of numeric controls Or use “text tool” to highlight and edit number field (white) within numeric control
Text field
If running continuously then program is interactive
Up/down arrows
15
Adding two numbers - saving the program
Select “save” from within the file menu When prompted enter filename Note in windows LabView files have a ***.vi file extension
vi - stands for virtual instrument
DO IT NOW AND KEEP DOING IT!
16
Useful tips - changing the program
When a LabView program is running it IS possible to change the setting of the controls, i.e. change the data inputs When a LabView program is running it IS NOT possible to change to wiring of the program To make changes to the program you need to stop it first!
17
Customising the program (text)
Select “text tool” from tools palette
Cursor now changes function
Changing the name of the input
Use “text tool” to highlight and edit the names (grey) numeric control Use “text tool” to highlight and edit the name (grey) numeric indicator
Indicator text field
Control text field
18
Customising the program (data)
Select “arrow tool” from tools palette
Cursor now changes function
Changing the allowed range and up/down increment of the controls
Selecting data range
Right click (windows) on the numeric control to generate menu then select “Data range…” Within data range can set upper limit/lower limit and increment
Now run the program again
Max/Min/Increment
19
Customising the program (display) -1
Select the front panel Show “controls panel from window menu” Use “arrow tool” to select numeric control Use “arrow tool” to select “meter” and “drag” it onto front panel Use “text tool” to optionally edit name of meter Use “text tool” to optionally highlight and edit the range of the meter
Selecting a digital meter
Name of meter
Range of meter
20
Customising the program (display) -2
Select the diagram Select “wiring tool” from tool palette Use wiring tool to connect meter terminal to existing output wire of the “addition” function or equivalently to the existing numeric indicator terminal
I.e. click on start, drag to end then release
Run the program again, note meter display matches indicator 21
Accessing LabView’s help
To get help on a function
Select the diagram Select “arrow tool” from tool palette Use arrow tool to right click (windows) on function Select help from pop up menu Function help appears in subwindow (There may be some problems with HELP under LINUX)
More general help can be obtained through the “help menu”
“Content and index” is good for specific questions LabView comes with a “learn by activities package” Also a great set of example programs 22
Useful tips - indicators, controls and terminal
Every indicator on the front panel has a terminal on the diagram Every control on the front panel has a terminal on the diagram Right clicking(windows) on any terminal, control /indicator gives the option of highlighting the corresponding control /indicator or terminal
23
Useful tips - switching between tools
Rather than picking your tool from the “tools palette” pressing the “tab” key toggles the tool between On the diagram
Arrow (allows selection and/or movement of terminals and functions around diagram Text (allow text edit of terminal and addition of extra text comments) Wiring (allows wiring connection of functions and terminals) Finger (allows selection of terminals)
On the panel
Arrow (allows movement of controls around panel Text (allows text edit of control/indicator and addition of extra text comments) Paint (allows colour change of control or indicator) Finger (allows adjustment of controls) 24
Useful tips - showing various windows
Any window or tool panel can be made active by selecting it from the window menu Alternatively “clicking” on a window will make it active
This is a quick way of switching between the diagram and the front panel
25
Useful tips - finding errors
If the current LabView programme is non executable the “run” arrow on the front panel appears broken Activating the arrow results in an error list contain the faults Selecting any fault highlight the offending part of the diagram All unconnected wires (dotted lines) can be removed by selecting “remove broken wires” from the “edit” menu
Broken “run” arrow
error list
26
Looping programs
Computers become power when you make them do something many times! LabView loops are examples of structures and placed on the diagram
Show diagram Shows functions palette Select structure
Selecting the structure sub palette from the functions palette 27
Incorporating a “while loop”
Select a “while loop” from the structure palette Place while loop on diagram to surround program
“While loop”
Click top left drag to bottom right and release
While loop will run whilst condition is true
Condition
28
Setting the condition of a “while loop”
Need Boolean (true/false) control to set state of while loop
Show front panel Show controls palette Select “boolean” Select “push button” and drag and place (release) on front panel Optionally use “text tool” to highlight and edit name (grey) of new control
29
Wiring the condition of a “while loop”
Need to wire the Boolean terminal to the condition of the while loop
Use arrow tool to move (if necessary) the terminal of the Boolean control to the inside of the while loop Use the wiring tool to connect the boolean terminal to the condition terminal
30
Running a “while loop”
Running/stopping the program within a while loop
Use finger tool to toggle Boolean control to true (dull green arrow becomes bright green) Use finger or arrow tool to “run” program (NB not continuously run) Program will now run continuously Use finger or text tool to change/edit numeric controls Use finger tool to toggle boolean control to stop program
31
To draw a sine curve in LabView
Objective To
plot a sine curve in the range 010π
Start a new program Select
“new VI” from the file menu
Save it now Select
“save” from the file menu and when prompted provide name 32
Using a “for loop”
Show the diagram Select a “for loop” from the structure palette Place the “for loop” on the diagram
“click” “drag” “release”
A “for-loop” will run N times then stop i increments from 0 to N-1
For loop
Terminal for N
Terminal for i 33
Wiring N on a “for loop”
Show the front panel
Show diagram
Select digital control from the controls palette Place control on front panel (If necessary) move terminal of digital control to outside of loop Wire digital control terminal to N terminal of for loop
NB one terminal is blue the other orange!
34
Understanding data types
Computers store number in different forms, e.g. Integers,
8 bit, 16bit, 32 bit - BLUE in
LabView Floats single precision, double precision Orange in LabView
35
Converting Data type
Show either diagram or front panel Right click (window) on control or terminal and select “representation” “click” on data type of choice to convert numeric Do this to change numeric control to I32 integer Edit numeric value to 100
36
Drawing a sine curve - defining the range (1)
To define the 0-10π range
Show diagram Select and place constant outside loop edit to 10 Select and place π outside loop Select and place multiply function outside loop Wire “10” and “π” into “x” function
Selecting a constant
Selecting π
37
Drawing a sine curve - defining the range (2)
Select and place divide function inside loop Wire “i “into numerator of divide Wire numeric input through wall into denominator of divide Select and place multiply function Wire output of divide into multiply Wire “10 x π” multiply through loop wall into multiply 38
Drawing a sine curve calculating the value
To calculate the sine value
Select and place sine function inside loop Wire output of multiply to input of sine Wire output of sine to loop wall
Selecting sine
39
Drawing a sine curve displaying the curve
Show front panel
Select and place “waveform graph” on front panel
Show diagram
(if necessary) move waveform graph terminal outside loop Wire wall of loop to waveform graph terminal
Selecting waveform graph
40
Drawing a sine curve displaying the curve (2)
Show diagram
(if necessary) move waveform graph terminal outside loop Wire wall of loop to waveform graph terminal
NB orange wire on outside of loop is thicker than inside
Indicates wire carries an array of numbers
41
Drawing a sine curve - running the program
Use arrow tool to “run continuously the program Adjust numeric control to change number of points calculated Use arrow tool to “stop” program when finished Use text edit tool to rename x-axis of graph (angle), name of numeric control (number of data points) and name of waveform graph
42
Drawing a sine curve modifying the program
To make 10π range variable
Select and place digital control on front panel Show diagram Delete 10π product structure and wire from outside loop to leave unwired loop entry Move new numeric terminal to similar position Wire in terminal to loop entry
Run continuously and experiment with changing the range
43
Drawing a sine curve extending the program FFT (1)
To obtain an FFT of the sine wave
Select and place FFT function on diagram near existing waveform graph terminal - the route to FFT is • • • •
Functions Analyse Signal processing Frequency domain
Wire waveform graph (or neighbouring wire to Input of FFT function
Selecting FFT function
44
Drawing a sine curve extending the program FFT (2)
Wire waveform graph (or neighbouring wire to Input of FFT function On front panel place additional waveform graph On diagram wire output of FFT function to terminal of waveform graph Use text tool to edit name of waveform graph and x axis of graph on front panel
45
Drawing a sine curve - seeing the FFT
Run program continuously Update range control Examine FFT of sine wave Note the FFT has two peaks Note FFT peaks have +ve and -ve values
46
Drawing a sine curve - seeing the power spectrum (1)
More usual to consider the power spectrum Need to take modulus squared of each FFT component
Delete wire between FFT and waveform graph terminal (if necessary) move terminal of waveform graph away from FFT Select and place modulus function after FFT Select and place multiply function after modulus Wire FFT to modulus Wire modulus to both inputs of multiply Wire multiply to waveform graph terminal
47
Drawing a sine curve - seeing the power spectrum (2)
Power spectrum is +ve Still twin peaked Run program
Note that a higher “frequencies” of sine wave power spectrum peaks move towards centre Double peaks can be through to represent +ve and -ve frequency
48
Using LabView to draw a 2D function
Objective To
plot an “egg box” type pattern
Start a new program Select
“new VI” from the file menu
Save it now Select
“save” from the file menu and when prompted provide name
49
Double nested “for loop” A single for loop will create a vector of N elements To create an array use a for loop within a for loop
50
Useful tips - copying and moving items on the diagram
To move a selection of functions and wires on a diagram
Use arrow tool to define rectangle of interest, i.e. click upper left and drag to lower right Use cursor keys to move selected region
To copy a selected section of the program
Use arrow tool to define rectangle of interest, I.e. click upper left and drag to lower right Use standard copy and paste functions to replicate program
51
Calculating the “egg box”
Assume an egg box is generated by taking the product of two sine functions (one in the x-direction and one in the y) Use double nested loop and repeat logic of previous program, note
Both N terminals of loop wired to control i indices of loops wired to form x and y axis Out wired through inner loop to form output from outer loop
52
Visualising the “egg box” (1)
Show front panel
Select and place intensity graph on front panel N.B. intensity graph is different from intensity chart! Use text tool to rename x-axis, yaxis and name of graph
Selecting an intensity graph
53
Visualising the “egg box” (2)
Show diagram (if necessary) move terminal of intensity graph to outside of nested loop Wire output from outer loop to terminal of intensity graph NB note orange wire becomes “double wire” which indicates it is an array
54
Useful tips - Polymorphic
In LabView, most functions are polymorphic.
e.g. the same addition function will add two numbers or two vectors or two arrays (vectors or arrays must have the same dimensions The exception to “dimension matching” is that one can add a number to every element in vector or array
Adding numbers
Adding vectors Adding arrays Adding a number to each element of an array 55
Running the “egg box”
Show front panel Use finger or text tool to set numeric controls to ≈100 loop iterations and a plot range of ≈30 Run or run continuously the program Need to set z-axis of graph to autoscale • “right click” (window) on graph, select zscale and select autoscale z
Auto-scaling the z-axis of an intensity graph 56
Useful tips - changing the number format
Previously we have seen how to change the range over which a numeric control can be varied To change the way it is displayed
Show front panel “rick click” (windows) on control Select “format and precision” Edit format and precision window as desired NB this only affects the display NOT the precision of the calculations 57
The challenge - diffraction patterns
To calculate and display the far field diffraction pattern of a circular aperture
The far field diffraction pattern is the same form as the Fourier-transform (but in 2D) In the first instance “forget” about the wavelength (which sets the scaling between the aperture and the diffraction pattern) - just concentrate on the “shape”! i.e. just take the 2D FFT of an array whose numeric values are the transmission of a circular aperture (1’s and 0’s)
Read the next few pages for some hints! 58
Useful functions for challenge (1)
“Comparison” functions allow logic decisions
When defining the aperture, you may need
NB wire carrying logic, i.e. Boolean (0..1) date are green
“select” “less” or “greater”
Use LabView’s help to understand these functions 59
Useful functions for challenge (2)
“array” functions allow manipulations of array and vectors
When doing the FFT, you may need
Wire containing array data appear a two parallel wires. These may be blue (for integer)or orange (for floats)
“transpose” “rotate 1D array”
Use LabView’s help to understand these functions
60
Useful functions for challenge (3)
If the N terminal is left unwired, “for loops” will selfindex
vector to element
array to vector
When a vector is wired into a “for loop” the loop will split the array and run it on each element When an array is wired into a “for loop” the loop will split the array and run it on each row
To switch off “self-indexing” right click (windows) on wire entry to loop and select “disable indexing” (not needed in challenge) 61
Useful functions for challenge (4)
Standard LabView does not have a function for 2D FFTs Given a 2D array, a 2D FFT can be completed by doing 1D FFTs on each row and each column You may need this when calculating the diffraction pattern - note use of “self-indexing”
Part of a “diagram” for completing a 2D FFT
62
Diffraction pattern
Run your programme
Do you get a diffraction pattern centred in the four corners? This is a issue with most FFT algorithms - where should the zero “frequency” be located? At the centre or the edges? We need to centre our zero in the middle of the image
63
2D FFT with zero at centre
Can use “1D rotate function to move zero to centre (NB need to rotate by N/2) Note also modulus squared added to output of FT to give power spectrum (i.e. light intensity)
64
Calculating a diffraction pattern
One program that works! Autoscale-z
is
switched off and z-scale is manually set to reveal structure
65
Another useful function (but not needed for challenge)
Often one uses each loop iteration to modify the value of a number
The new number can be used in the next iteration
In LabView this is called a shift register To add a shift register to a loop
Right click (windows) on the left edge of the loop and select “add shift register” The shift register looks like a down and up arrow on the left and right sides of the loop
67
Shift-registers at work
The shift register needs a starting value
The shift register gives an output when the loop has finished
Wired to the down arrow on the left side of the loop
“shift register”
Wired from the up arrow on the right side of the loop
For example calculating the factorial of a number
68
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