ENGR 1000, Introduction to Engineering Design

Transcription

1 ENGR 1000, Introduction to Engineering Design Unit 2: Data Acquisition and Control Technology Lesson 2.1: Programming Line Outputs for the NI USB-6008 in LabVIEW Hardware: 12 VDC power supply Several lengths of wire NI-USB 6008 Device with USB cable Digital I/O Interface Board for the NI-USB 6008 Device Traffic light Objectives: Work effectively in a cooperative learning environment. Explain the characteristics/function of the digital I/O on the NI USB-6008 device. Connect the NI USB-6008 device to the USB port of the computer, the Digital I/O Interface, an external DC power supply, and digital output devices. Write a simple program in LabVIEW to control outputs using the following commands: While Loop with Stop Button DAQ Assistant (digital port output) Stacked Sequence Structure Flat Sequence Structure Boolean Constant Array Constant with Boolean Constant Wait (ms) 1

2 Data Acquisition and Control Hardware National Instruments (NI) USB-6008 Device The NI USB-6008 data acquisition and control device provides both analog and digital connections through two detachable screw terminal blocks, one of the left side of the device and the other on the right side. The left side of the USB-6008 device provides connection to eight analog input (AI) channels and two analog output (AO) channels. The right side of the device provides connection to 12 digital input/output (DIO) channels, and a digital trigger or event counter input. For this lesson we will focus on the 12 DIO channels. The 12 DIO channels are structured into two ports, port 0 and port 1. Port 0 contains eight channels identified as P0.0 to P0.7. Port 1 contains 4 channels identified as P1.0 to P1.3. Each channel can be configured individually as an input or output, or each port can be configured as a set of inputs or a set of outputs. When the NI USB-6008 is initially powered up, each channel defaults as an input and is high. When a program is run, the DIO channels configure as programmed. NI USB-6008/9 Digital I/O Interface The USB-6008 device s DIO channels are limited to 5 VDC with a maximum of 8.5 ma. Additional circuitry is necessary to increase the voltage and amperage to a useful level. The digital I/O interface board provides this function. This board provides four output channels and 4 input channels. Additional digital I/O interface boards can be daisy chained together to provide for additional inputs or outputs. Output channels for the digital I/O interface board are designed for 9V to 24VDC and require an external power supply. Trace sizes restrict outputs to a maximum of 2 amps per channel and 4 amps per board. Each output channel on the digital I/O interface has an on/off switch in its circuit for safety purposes and to conserve limited power provided by the NI USB-6008 device. Since the NI USB-6008 device, when initially powered up, defaults all DIO channels to inputs with a high status, the I/O digital interface outputs need to be turned off until a program is run. Shortly after the program is run, the outputs required should be switched on. 2

3 Connecting the NI USB-6008 Device to Control Digital Outputs To control digital outputs, the NI USB-6008 device needs to be connected to a computer and the NI USB-6008/9 digital I/O interface. Connecting the NI USB-6008 device to a computer is done using the cable provided by National Instruments. One end attaches to the top of the device and the other attaches to the USB port on the computer. Several connections are required to connect the device to the I/O interface. The ground and +5V terminals, located at the bottom of the right terminal block on the NI USB-6008 device, need to be wired to their respective terminals located at the bottom of the digital I/O interface (see photo showing attachments). Extra terminals are provided for daisy chaining additional I/O interface boards and powering analog devices. Digital input/output channels (P0.0-P0.7 and P1.0-P1.3) designated as outputs need to be wired to the USB/DO terminals on the left side of the I/O interface. There are two terminals for each digital output channel on the I/O interface that are connected by a trace on the circuit board. As a result, you can connect the wire from the USB device to either terminal for that output channel. The photo below shows channels P0.0, P0.1, and P0.2 connected to the top three output channels on the I/O interface. The digital I/O interface needs to also be connected to an external power supply and the external devices it is controlling. A 9V to 24VDC power supply is connected to the terminals at the top of the I/O interface. Again, extra terminals have been provided for daisy chaining additional I/O interface boards. The external devices being controlled by the I/O interface board are connected to the terminals on the right side of the board. Polarity (+,-) is indicated for devices where this is important. The photo shows three lamps connected. 3

4 Starting and Setting Up LabVIEW Introduction LabVIEW is a powerful graphic programming language developed by National Instruments for data acquisition, signal analysis, and instrument control applications. It has been widely adopted throughout industry, government, and education in engineering and science communities. It is ideal for the mechatronics applications we will be learning and experimenting with in the following lessons. The graphical nature of the language makes it popular and fun to use. First we must start National Instruments LabVIEW 8.2 and configure our screen before programming. Starting LabVIEW From the Windows desktop select Start, select All Programs, and select National Instruments LabVIEW 8.2 from the listing of programs. An initial screen identifying the software as LabVIEW 8.2 will appear followed automatically by the Getting Started screen partially shown below. Create a Blank VI A VI is an acronym for virtual instrument. A VI can be considered a program or subprogram. To begin programming, it is necessary to create a blank VI. Select Blank VI from the Getting Started window to open the Front Panel and Block Diagram windows and the Controls Palette. The two main windows used when programming in LabVIEW will appear the Front Panel, where all virtual instruments and controls reside; and the Block Diagram where the program is written. The Controls Palette serves as a menu for interface commands associated with the Front Panel. (Note: If the Controls Palette does not appear, right click on the grid area of the Front Panel and left click on the thumb tack in the upper left corner of the Controls Palette.) 4

5 Setting Up LabVIEW To work easily between the Front Panel and the Block Diagram, the computer screen needs to be reconfigured. From the main menu that extends across the top of the programming environment of the Front Panel, select Window and scroll down and select Tile Up and Down (see graphic above). This configuration will enable you to work easily between the Front Panel and the Block Diagram. Opening Menus Three additional windows/menus need to be opened to begin your programming (one or more may already be open). The Functions Palette needs to be opened and serves as a menu for programming commands used in the Block Diagram. From the main menu that extends across the top of the programming environment of the Block Diagram, select View and scroll down and select Functions Palette. The palette (menu) shown on the right will appear. 5

6 The second window to open is the Tools Palette (menu). The Tools Palette contains the tools used to program (e.g., Position/ Size/Select tool, Edit Text tool, Connect Wire tool, etc.). To open the Tools Palette, select the View menu from the Block Diagram again and scroll down and select Tools Palette. The palette shown on the left will appear. The third and last window to open prior to programming will be the Show Context Help window. This window is very helpful. As you select various programming icons, an example graphic will appear in the help window showing wire connections and explaining the icon s function. To open this window, go to the main menu that extends across the top of the programming environment of the Block Diagram, select Help, and scroll down and select Show Context Help. Once selected, a check appears in front of the Show Context Help command and serves as a toggle switch to turn on or off the window. A shortcut to display the Context Help window is to select the yellow question mark button located in the upper right corner of either the Front Panel or Block Diagram. An example Context Help window is shown above right. Programming LabVIEW Outputs via the DAQ Assistant Introduction We can now begin programming. The first program we will write will perform the following functions in the sequence outlined below: turn on output 1 for 1 second turn on output 2 for 1 second turn on both outputs 1 and 3 for 1 second turn all outputs off for a 1/2 second repeat sequence Note: Refer to the objectives section on page one of this lesson for a listing of the commands we will use and become familiar with. Programming a While Loop LabVIEW utilizes graphic programming techniques as opposed to text based programming. LabVIEW programs are written in the Block Diagram. Our first programming task will be to insert a While Loop with a stop button pre-wired into the 6

7 Block Diagram. In LabVIEW, the While Loop is the base structure on which most programs are built. A While Loop repeats the program inside of it until the stop button on the Front Panel is pressed. An example of how a While Loop works is the phrase I will play outside while the sun still shines. This statement consists of two parts, an action and a condition. The action, playing outside, will continue to occur while the condition, the sun still shines, is true. When the condition is no longer true, the action stops. To begin programming, be sure the Block Diagram is highlighted so the Functions Palette is available. Next, close the Programming menu on the Functions Palette by left clicking on the arrow next to the Programming header. Next, open the Express menu on the same palette and select the Exec Control icon (submenu). Under the Execution Control menu, select the While Loop icon (command). Insert the While Loop by placing the curser in the upper left corner of the Block Diagram s work area and left clicking and dragging the curser to the lower right corner of the work area. This will place a While Loop with a stop button in the Block Diagram and a stop button in the upper left corner of the Front Panel. When the stop button is pressed, it will change our condition and the loop will stop. Your Block Diagram with the While Loop and the Front Panel with the stop button should look similar to diagrams below and to the left. Programming a Stacked Sequence Structure Next we will insert a Stacked Sequence Structure into our While Loop. A Sequence Structure is used to organize events chronologically. An example of a Sequence Structure might be a film strip. In a film strip, we first see the beginning frame, and then 7

8 the second, and then the third, etc. The Stacked Sequence Structure looks like one frame of a film strip. At the end of the lesson, we will change it to a different kind of a Sequence Structure called a Flat Sequence Structure which is equivalent to a Stacked Sequence Structure but will look even more like a film strip. To program a Stacked Sequence Structure first, close the Express menu by selecting the arrow next to the Express header and then open the Programming menu. Select the Structures submenu and then select the Stacked Sequence Structure command. Insert the Stacked Sequence Structure into the While Loop using the same technique (click and drag) used to insert the While Loop. Your program should look similar to the graphic below. Programming a DAQ Assistant as a Digital Line Output Next, an interface command needs to be created for our application. The NI DAQ Assistant will be used to simplify this process. The NI DAQ Assistant interfaces the software with the hardware. It is the part of the program that queries the hardware in the case of an input or writes to the hardware in the case of an output. In this lesson we will only be using outputs, so you can think of the DAQ Assistant that we will configure today as the part of the software that gives commands to the hardware. At this point, you need to make sure the NI USB-6008 data acquisition and control device is connected to your USB port or a virtual device is set up on your computer. First, close the Programming menu and open the Measurement I/O menu on the Functions Palette. Select the DAQmx Data Acquisition submenu. Select the DAQ Assistant command and place inside the Stacked Sequence Structure. The DAQ Assistant command will initialize, and the Create New Express Task window will open. We will now configure our DAQ Assistant to communicate with our hardware. Select and open the Digital I/O submenu from the list of options on the right. Select Line Output from the submenu. Press and hold Ctrl while selecting Port0/Line0, Port 0/Line1 8

9 and Port0/Line2 from the Supported Physical Channels and select Finish. The DAQ Assistant window will open, and everything should be configured correctly 1 Sample (On Demand) should be selected, and Invert Line unchecked. Select OK in the lower right corner. This procedure establishes the 3 lines (0-2) we have selected as outputs. The DAQ Assistant will take a moment to configure. To help interpret your program, it will be helpful to change the name of the DAQ Assistant to identify what it specifically controls. The easiest way to change the name is to have your Tools Palette s automatic feature turned on; it is turned on when the program is first run. You can tell if the automatic feature is on or off by the green indicator at the top of the palette; green indicates it is on. The automatic feature is programmed to be toggled on or off with a left click of the mouse. With the feature on, double click on the name of the DAQ Assistant command and change its name to P0.0-2 OUT, identifying it as an output controlling port 0, lines 0 to 2. Programming an Array Constant with a Boolean Constant The DAQ Assistant interface we are using requires an array for its input data. Since we configured our DAQ Assistant (P0.0-2 OUT) as digital line outputs, each output line will be controlled by an element in our array of Boolean values. We will use an array of Boolean constants. The easiest way to install this type of constant is to right click on the data bar of the DAQ Assistant and select Create and Constant in the submenu. An array of Boolean constants will appear and be wired to the data in node of the DAQ Assistant. Each Boolean constant in this array will represent and control a different line. The left number (0) is an index representing various lines, and if changed, can cause problems for this application. As a result, we will remove it. Right click on the index (left 0) and highlight Visible Items and on its submenu, uncheck Index Display. The Boolean constant that is visible controls the 0 th line configured in our DAQ Assistant. We have three lines total that we need to control, so we will make two more constants visible by expanding the view of our array of Boolean constants. Hover over the bottom edge of the Boolean Array until the cursor turns to a double arrow. (Make sure you are hovering over the bottom edge of the array and not the bottom edge of the constant element.) Click and drag the edge of the array down until it expands down two spaces. Once it is down two spaces release the button and three Boolean Constants that we will be using should be visible. Since we want line 0 of port 0 to go high, we need to change the Boolean value of the 1 st constant to true (T). Hover over the Boolean Constant until the tool changes to the selector tool (it should look like a hand with a pointing finger) and then click on the true (T) box. When you click on the constant it will change from being dull green to a brighter green. This indicates that the constant is now activated. Activate the other two constants by clicking on them and changing their values back to false (F). Be careful if experimenting with this array. If a Boolean Constant located in this array is not activated that should be activated the program will experience problems and not run correctly. If a Boolean Constant in this array that should not be activated is activated the program will give you an error. 9

10 Programming a Wait (ms) The final item that needs to be inserted into this frame is a Wait (ms) command. A Wait (ms) command is used to control the timing in a program. Computer clocks run faster than humans think. If we refrain from adding timing elements into our program, the computer will execute the commands as fast as possible. In some programming this is what is desired, but when it is a computer-human interface, it is necessary to slow the computer down so that the human can see what is happening and respond. We will use the timer to increase the length of time that the lights are on so that we can see them turn on and off. To insert a Wait (ms) command, close the Measurement I/O menu and open the Programming menu and the Timing submenu. Select the Wait (ms) command and insert it in the lower right of the Stacked Sequence Structure. To practice using the Tools Palette and Functions Palette, we will manually insert a Numeric Constant and wire it to the Wait (ms) command. Under the Programming menu, close the Timing submenu and open the Numeric submenu. Select the Numeric Constant command and insert it to the left of the Wait (ms) icon. Since the Wait (ms) command counts in milliseconds, while the Numeric Constant is highlighted enter 1000 for a delay of 1 second. With the automatic tools feature on, hover over the right side of the constant; a terminal will appear on the constant, and the curser will turn to a Connect Wire tool. Select the terminal and move the tool to the left side of the wait command and select the terminal that will appear. You should now have a wire connecting the constant to the wait command. The short cut would be to hover over the left side of the Wait (ms) command, right click and select Create and Constant in the submenu. This will insert the Numeric Constant wired to the Wait (ms) command. Finish by changing the constant s value to Your program should look like the following graphic. The structure below will perform the first sequence in the program; turn on output 1 for 1 second. 10

11 Before continuing I would like to point out a few things about the above graphics that will help in later programming. Notice the green line between the Boolean Array and the DAQ Assistant. It is thicker than the line between the stop button and the condition terminal of the While Loop. In LabVIEW the wires can indicate what type of data is being used by size; an array will have a thicker line than a single value. Also, the color is indicative of the data type. Notice that the wire between the Numeric Constant and the Wait (ms) command is blue. A green wire indicates a Boolean value while a blue wire indicates an integer value. We will also see strings, which will be pink, and doubles, which will be orange. Adding a Second Frame To allow the program to be run in stages of completion, we will write the frame for the last sequence next; turn all outputs off for a 1/2 second. This will allow us to run part of the program before the rest of it is written. The quickest way to do this is to duplicate the first frame and edit it. With the pointer, hover over the right border of the stacked sequence and right click. Select Duplicate Frame. This procedure will create and take you to the new frame. Also, note there is now a selector label at the top center of the Stacked Sequence Structure. The number to the left tells you at which frame you are currently looking. The numbers to the right represent the various frames in the stack. The arrows allow you to navigate through the stack. In addition, you will notice the DAQ Assistant has been renamed with a number 2 added. Finally, edit the Boolean constants so the Boolean data elements entering the DAQ Assistant are all false (F), turning off all outputs, and the data for the Wait (ms) command is 500, delaying the event 1/2 second. Running the Program We can now test this portion of the program. To run, select the Run arrow in the tool bar of the Front Panel. Notice Output 0 of Port 0 turns on for 1 second, then off for 1/2 second, and continues to repeat. To turn off the program, select the Stop button on the Front Panel that was created when the While Loop was placed. Avoid using the Abort Execution button (stop sign) on the tool bar as this will stop your program before completion. The Abort Execution button should be used with caution, if used at all. Adding Frames 2 & 3 After stopping the program, the other two sequences can be written using the same procedure as above by duplicating frames and editing constants. The order of duplicating 11

12 frames is important; the duplicated frame is always placed behind the frame being duplicated. When creating the second frame, be sure to duplicate it from the first frame in the stack. For the third output in the sequence, you need to turn on two output lines; as a result, you need to make two of the Boolean constants true (T); one for each output line turning on. When completed, your stacks should look similar to the graphics below. Investigating the Flat Sequence Structure The Flat Sequence Structure is similar to the Stacked Sequence Structure. To view your program written in a Flat Sequence Structure, right click on the border of the Stacked Sequence Structure and select Replace and Replace with Flat Sequence in the submenu. You will notice your program expands and all frames are visible. This type of structure may be preferred with some applications. To change the structure back, press Control and Z to undo the replacement. 12

13 Team Assignment As a team, write a program to control the outputs of the NI USB-6008 device and operate the 12 VDC traffic light. Wire the red lamp of the traffic light to Port 0 line 0, the yellow lamp to Port 0 line 1, and the green lamp to Port 0 line 2. Operate the traffic light using the following sequence: turn on the green lamp for 3 seconds turn on the yellow lamp for 1/2 second turn on the red lamp for 3 1/2 seconds turn all lamps off for 1/2 second turn all lamps on for 1 second turn all lamps off for 1/2 second repeat sequence Save your program on a storage device under the name HW2-1T# (use your team number in place of the # symbol). Have the next person representing your team turn in your assignment at the beginning of the next lab. Each person in your team should be able to perform this assignment and the lesson s objectives independently. Each will have an opportunity to demonstrate their knowledge independently on a quiz. A team is successful when all members are able to demonstrate the objectives. Have fun helping each other learn the material. 13