CHAPTER 11. DMM TEST STAND
83
11.1
Technical Approach
NI Vision Assistant Script
NI Vision provides the
IMAQ Read LCD
instrument reader VI that accepts an
image of a digital multimeter (DMM) display and interprets the pattern of active
segments as a numerical value; it can also identify the decimal point in the dis-
play.
IMAQ Read LCD
works with either dark segments on a light background
(LCD with passive illumination) or light segments on a dark background (backlit
LCD, LED, and fluorescent).
The instrument reader is not available within Vision Assistant, therefore the
script simply needs to convert the color webcam image to grayscale; see
for details.
The
IMAQ Read LCD
VI located in the
Vision and Motion | Machine Vi-
sion | Instrument Readers
subpalette requires a grayscale image buffer reference
and a compound ROI (array of rectangular ROIs) with a single ROI for each
digit. The companion VI
IMAQ Get LCD ROI
will automatically define the
required multi-part ROI, however, this VI requires all segments to be active.
Digital meters normally do this as part of their power-on self test, however,
capturing this brief moment as an image is somewhat difficult and not worth
effort. Fortunately you can easily create the compound ROI yourself on the
main image display by drawing the first ROI as usual (right-click on the dis-
play and choose “Clear ROI,” if necessary) and then press and hold the “Ctrl”
button as you draw the remaining ROIs. The order in which you draw the
ROIs dictates their position in the compound ROI array, therefore it is important
that you begin with the leftmost digit and work your way to the right. See
Compound ROI from Image Display
for details.
You will obtain best results when your meter display is aligned horizontally
along the pixel rows and when the ROI boundaries fall in the
center
of the
segments as shown in Figure 11.1 on page 81. You do
not
need to enclose the
decimal points within the ROIs because the instrument reader knows where to
look between your specified ROIs. You will need to experiment with the meter
position to avoid the reflected image of the webcam itself (this is why the DMM
is off center in Figure 11.1 on page 81). The instrument reader can handle a
certain amount of lighting variation but will get confused if the background is
not sufficiently uniform.
Study the video tutorial
to learn how to completely
(1:02)
(2:37)
(7:45)
Содержание NI myRIO
Страница 1: ...Ed Doering NI myRIO Vision Essentials Guide...
Страница 2: ......
Страница 11: ...Part I Machine Vision Fundamentals...
Страница 12: ......
Страница 31: ...Part II Introductory Machine Vision Projects...
Страница 32: ......
Страница 38: ...32 CHAPTER 4 CAMERA SETUP...
Страница 44: ...38 CHAPTER 5 STEREO VISION...
Страница 61: ...Part III Machine Vision Application Projects...
Страница 62: ......
Страница 72: ...66 CHAPTER 8 COIN COUNTER...
Страница 85: ...CHAPTER 10 KEYED OPTICAL LOCK 79 Figure 10 3 Typical expected results for the Geometric Matching step...
Страница 86: ...80 CHAPTER 10 KEYED OPTICAL LOCK...
Страница 92: ...86 CHAPTER 11 DMM TEST STAND...
Страница 134: ...128 CHAPTER 17 MARBLE SORTER...
Страница 135: ...Part IV Appendices...
Страница 136: ......
Страница 143: ...B MXPandMSPConnectorDiagrams Figure B 1 MXP myRIO eXpansion Port connector diagram...
Страница 144: ...138 APPENDIX B MXP AND MSP CONNECTOR DIAGRAMS Figure B 2 MSP miniSystem Port connector diagram...
Страница 148: ...142 APPENDIX D BUILD A STAND ALONE APPLICATION...
Страница 149: ...E MarbleSorterConstructionDetails To be continued...
Страница 150: ...144 APPENDIX E MARBLE SORTER CONSTRUCTION DETAILS...