A. Front Panel Design Lesson 4 Implementing a VI

Transcription

1 A. Front Panel Design Lesson 4 Implementing a VI Inputs and outputs lead to front panel design Retrieve the inputs by the following methods: TOPICS A. B. C. D. E. F. Front Panel Design LabVIEW Data Types Documenting Code While Loops For Loops Timing a VI G. Iterative Data Transfer H. Plotting Data I. Case Structures Acquiring from a device Reading directly from a file Manipulating controls Output data by the following methods: Displaying with indicators Logging to a file Outputting to a device A. Front Panel Design Labels/Captions A. Front Panel Design Color Tips Labels short descriptions Captions long descriptions Captions do not appear on block diagram Start with a gray scheme Select one or two shades of gray Add highlight colors sparingly for important settings on plots, abort buttons, and the slider thumbs A. Front Panel Design Text & Fonts A. Front Panel Design Spacing Bad Example 1

2 A. Front Panel Design Tab Controls A. Front Panel Design Decorations Use tab controls to overlap front panel controls and indicators in a smaller area Use decorations to visually group or separate objects on a front panel with boxes, lines, or arrows These objects are for decoration only B. LabVIEW Data Types Terminals B. LabVIEW Data Types Numerics Terminals visually communicate information about the data type represented The numeric data type represents numbers of various types To change the representation of a numeric, right-click the control, indicator, or constant, and select Representation from the shortcut menu B. LabVIEW Data Types Boolean Mechanical Action of Booleans Behavior of Boolean controls is specified by the mechanical action In LabVIEW, the Boolean data type is represented with the color green Use the Mechanical Action of Booleans VI located in the NI Example Finder to learn about the different switch and latch actions. DEMONSTRATION 2

3 B. Data Types String B. Data Types Enum A sequence of displayable or non-displayable ASCII characters On the front panel, strings appear as tables, text entry boxes, and labels Change the display type from the short-cut menu: Normal, \ Codes, Password and Hex Edit and manipulate strings with the String functions on the block diagram In LabVIEW, the string data type is represented with the color pink An enum represents a pair of values, a string and a numeric, where the enum can be one of a defined list of values B. Data Types Enum B. Data Types Dynamic Enum: enumerated control, constant, or indicator Enums are useful because it is easier to manipulate numbers than strings on the block diagram Stores the information generated or acquired by an Express VI Non-Express VIs do not accept the dynamic data type To use a built-in VI or function to analyze or process the dynamic data type, you must convert the data type Numeric, waveform, or Boolean data indicators or inputs automatically convert the dynamic data type when wired In LabVIEW, the dynamic data type is represented with the color dark blue C. Documenting Code Front Panels C. Documenting Code Naming Tip Strips Descriptions VI Properties Good Design Giving controls and indicators logical and descriptive names adds usability to front panels 3

4 C. Documenting Code Block Diagram Configuring Your LabVIEW Environment Use block diagram comments to: Options Dialog Box Describe algorithms Explain the data contents of wires Controls/Functions Palettes page Select Load palettes during launch to make Search Palettes immediately usable after launch Set Palette to Category (Icons and Text) Use the Labeling tool or place a free label from the Functions palette Block Diagram page Uncheck Place front panel terminals as icons to place control and indicator terminals in a compact format Configure Block Diagram Cleanup to customize your block diagram Configuring Your LabVIEW Environment Exercise 4-1 Determine Warnings VI Functions Palette Tack the Functions palette and select View»Change Visible Categories then click Select All Controls Palette Tack the Controls palette and select View»Change Visible Categories then click Select All Create and document a VI. Exercise 4-1 Determine Warnings VI GOAL D. While Loops Repeat (code); Until Condition met; End; What happens if the Max. Temp value is lower than the Min. Temp value? DISCUSSION LabVIEW While Loop Flowchart Pseudo Code 4

5 D. While Loops D. While Loops Tunnels Iteration terminal: returns number of times loop has executed; zero indexed Conditional terminal: defines when the loop stops Tunnels transfer data into and out of structures The tunnel adopts the color of the data type wired to the tunnel Data pass out of a loop after the loop terminates When a tunnel passes data into a loop, the loop executes only after data arrive at the tunnel Iteration Terminal Conditional Terminal D. While Loops - Error Checking and Error Handling Exercise 4-2 Auto Match VI Use an error cluster in a While Loop to stop the While Loop if an error occurs Use a While Loop and an iteration terminal and pass data through a tunnel. Exercise 4-2 Auto Match VI GOAL E. For Loops N=100; i=0; Until i=n: Repeat (code;i=i+1); End; How many times is the # of iterations indicator updated? Why? DISCUSSION LabVIEW For Loop Flowchart Pseudo Code 5

6 E. For Loops E. For Loops Conditional Terminal Create a For Loop the same way you create a While Loop If you need to replace an existing While Loop with a For Loop, right-click the border of the While Loop, and select Replace with For Loop from the shortcut menu The value in the count terminal (an input terminal) indicates how many times to repeat the subdiagram You can add a conditional terminal to configure a For Loop to stop when a Boolean condition or an error occurs E. For Loops Conditional Terminal E. For Loop/While Loop Comparison For Loops configured for a conditional exit have: Red glyph next to the count terminal Conditional terminal in the lower right corner For Loop While Loop Executes a set number of times unless a conditional terminal is added Can execute zero times Tunnels automatically output an array of data Stops executing only if the value at the conditional terminal meets the condition Must execute at least once Tunnels automatically output the last value E. For Loops Numeric Conversion E. For Loops Numeric Conversion The number of iterations a For Loop executes must be specified in nonnegative integers If you wire a double-precision, floating-point numeric value to the count terminal, LabVIEW converts the larger numeric value to a 32-bit signed integer Normally, when you wire different representation types to the inputs of a function, the function returns an output in the larger or wider format Coercion Dot LabVIEW chooses the representation that uses more bits However, the For Loop count terminal always coerces to a 32-bit signed integer 6

7 Exercise 4-3 Concept: While Loops versus For Loops E. For Loops Numeric Conversion Avoid coercion for better performance Choose matching data type Programmatically convert to the matching data type Understand when to use a While Loop and when to use a For Loop. GOAL F. Timing a VI F. Timing a VI Wait Functions Why do you need timing in a VI? Control the frequency at which a loop executes Provide the processor with time to complete other tasks, such as processing the user interface A wait function inside a loop allows the VI to sleep for a set amount of time Allows the processor to address other tasks during the wait time Uses the operating system millisecond clock F. Timing a VI Elapsed Time Express VI Wait Chart VI Determines how much time elapses after some point in your VI Keep track of time while the VI continues to execute Does not provide the processor with time to complete other tasks Compare and contrast using a Wait function and the Elapsed Time Express VI for software timing. DEMONSTRATION 7

8 G. Iterative Data Transfer G. Iterative Data Transfer Shift Registers When programming with loops, you often need to know the values of data from previous iterations of the loop Shift registers transfer values from one loop iteration to the next Right-click the border and select Add Shift Register from the shortcut menu Right shift register stores data on completion of an iteration Left shift register provides stored data at beginning of the next iteration G. Iterative Data Transfer Initializing Run once VI finishes Block Diagram Run again 1st run 2nd run Initialized Shift Register Output = 5 Output = 5 Not Initialized Shift Register Output = 4 Output = 8 G. Iterative Data Transfer Stacked Shift Registers Stacked shift registers remember values from multiple previous iterations and carry those values to the next iterations Right-click the left shift register and select Add Element from the shortcut menu Exercise 4-4 Average Temperature VI Exercise 4-4 Average Temperature VI Use a For Loop and shift registers to average data. You calculated the average of the last 3 temperature readings. How would you modify the VI to calculate the average of the last 5 temperature readings? GOAL DISCUSSION 8

9 H. Plotting Data Waveform Chart H. Plotting Data Chart Update Modes Special type of numeric indicator that displays one or more plots of data, typically acquired at a constant rate Displays single or multiple plots Right-click the chart and select Advanced»Update Mode from the shortcut menu Strip chart is the default update mode Scope chart and Sweep chart modes display plots significantly faster than the strip chart mode H. Plotting Data H. Plotting Data Waveform Graphs Use the Context Help window to determine how to wire multiplot data to Waveform Graphs and XY Graphs Exercise 4-5 Temperature Multiplot VI Exercise 4-5 Temperature Multiplot VI Plot multiple data sets on a single waveform chart and customize the chart view. For this exercise, is it better to use a chart or a graph? GOAL DISCUSSION 9

10 I. Case Structures I. Case Structures Have two or more subdiagrams or cases Execute and displays only one case at a time An input value determines which subdiagram to execute Similar to case statements or if...then...else statements in text-based programming languages Case Selector Label: contains the name of the current case and decrement and increment buttons on each side Selector Terminal: Wire an input value, or selector, to determine which case executes I. Case Structures Default Case I. Case Structures Input & Output Tunnels You can specify a default case for the Case structure You can create multiple input and output tunnels Inputs are available to all cases if needed You must define each output tunnel for each case If you specified cases for 1, 2, and 3, but you get an input of 4, the Case structure executes the default case Right-click the Case structure border to add, duplicate, remove, or rearrange cases and to select a default case I. Case Structures Use Default if Unwired I. Case Structures Boolean Default values are: Boolean input creates two cases: True and False Data Type Default Value Numeric 0 Boolean FALSE String Empty Avoid using the Use Default If Unwired option on Case structure tunnels Adds a level of complexity to your code Complicates debugging your code 10

11 I. Case Structures Integer I. Case Structures String Add a case for each integer as necessary Integers without a defined case use the default case Add a case for each string as necessary Strings without a defined case use the default case I. Case Structures - Error Checking and Error Handling I. Case Structures Enum Gives users a list of items from which to select The case selector displays a case for each item in the enumerated type control Use Case Structures inside VIs to execute the code if there is no error and skip the code if there is an error Exercise 4-6 Determine Warnings VI Exercise 4-6 Determine Warnings VI Modify a VI to use a Case structure to make a software decision. What happens if all the values are 10? How could you fix this? GOAL DISCUSSION 11

12 Summary Quiz Summary Quiz Answer 1. Which identifies the control or indicator on the block diagram? a) Caption b) Location c) Label d) Value 1. Which identifies the control or indicator on the block diagram? a) Caption b) Location c) Label d) Value Summary Quiz Summary Quiz Answer 2. Which structure must run at least one time? a) While Loop b) For Loop 2. Which structure must run at least one time? a) While Loop b) For Loop Summary Quiz Summary Quiz Answer 3. Which is only available on the block diagram? 3. Which is only available on the block diagram? a) b) c) d) Control Constant Indicator Connector Pane a) b) c) d) Control Constant Indicator Connector Pane 12

13 Summary Quiz Summary Quiz Answer 4. Which mechanical action causes a Boolean in the False state to change to True when you click it and stay True until you release it and LabVIEW has read the value? 4. Which mechanical action causes a Boolean in the False state to change to True when you click it and stay True until you release it and LabVIEW has read the value? a) b) c) d) Switch Until Released Switch When Released Latch Until Released Latch When Released a) b) c) d) Switch Until Released Switch When Released Latch Until Released Latch When Released 13