{"id":10049,"date":"2015-03-31T13:27:11","date_gmt":"2015-03-31T02:27:11","guid":{"rendered":"http:\/\/legoeng.local\/?p=10049"},"modified":"2019-03-26T18:28:55","modified_gmt":"2019-03-26T07:28:55","slug":"darkness-or-darker","status":"publish","type":"post","link":"http:\/\/legoeng.local\/darkness-or-darker\/","title":{"rendered":"Darkness or darker?"},"content":{"rendered":"<p><a href=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Brick-sweeper.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignright size-medium wp-image-10052\" src=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Brick-sweeper-244x300.png\" alt=\"Brick-sweeper\" width=\"244\" height=\"300\" srcset=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Brick-sweeper-244x300.png 244w, http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Brick-sweeper.png 399w\" sizes=\"(max-width: 244px) 100vw, 244px\" \/><\/a>In this article, we look at the difference between fixed and relative thresholds in LabVIEW, that is the \u201cdarkness\u201d and the \u201cdarker\u201d settings in the Wait for Light Sensor block.<\/p>\n<p>Note: This article builds on \u201c<a title=\"Exploring thresholds\" href=\"http:\/\/legoeng.local\/exploring-thresholds\/\">Exploring thresholds<\/a>\u201d by Josep Fargas. It is recommended that you read that article first.<\/p>\n<p>The examples in this article were written in <a href=\"http:\/\/legoeng.local\/program\/labview\/\">LabVIEW<\/a> with the LabVIEW Module for LEGO MINDSTORMS.<\/p>\n<p>A common exercise in robotics classes consists of having a robot drive on a white surface until it detects a black border and then stop. We might call this \u201cStop at the line\u201d.<\/p>\n<p>To do this, the robot has to first learn what light sensor readings correspond to the white surface and what readings correspond to the black border.<\/p>\n<p><a href=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Port-View.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignright size-medium wp-image-10053\" src=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Port-View-300x184.png\" alt=\"Port-View\" width=\"300\" height=\"184\" srcset=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Port-View-300x184.png 300w, http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/Port-View.png 326w\" sizes=\"(max-width: 300px) 100vw, 300px\" \/><\/a>In Port View students can verify the light values for white surface and black border respectively as follows:<\/p>\n<ul>\n<li>Set EV3 Brick to Port View<\/li>\n<li>Connect a Color Sensor to sensor port 1<\/li>\n<li>Set the ports reading to \u201c1: COL-REFLECT\u201d<\/li>\n<\/ul>\n<p>For example, let\u2019s say that we\u2019ve recorded the following values (remembering that 100% means brightest, and 0% means darkest):<\/p>\n<ul>\n<li>White surface: 60%<\/li>\n<li>Black border: 30%<\/li>\n<\/ul>\n<p>Using these, we can determine an average for the threshold value:<\/p>\n<ul>\n<li>Threshold:\u00a0 45%<\/li>\n<\/ul>\n<p>Note: The average (or midpoint) of the two readings isn\u2019t always the best choice. Students can explore the transition between the white surface and the black border with Port View on their robot and find an optimized threshold as distinction between bright (keeping going forward!) and dark (stop!).<\/p>\n<p>Using a \u201cWait for Darkness\u201d with a threshold of 45 gives us this LabVIEW program\u2026 <a href=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darkness.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-10054 size-full\" src=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darkness.png\" alt=\"WaitforLight-darkness\" width=\"437\" height=\"148\" srcset=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darkness.png 437w, http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darkness-300x102.png 300w\" sizes=\"(max-width: 437px) 100vw, 437px\" \/><\/a><\/p>\n<p>But what happens when the lighting changes in the room? E.g. If the Sun goes behind a cloud?<\/p>\n<p>There, however, is an alternative to \u201cWait for Darkness\u201d that means we don\u2019t have to set a fixed threshold at all, that is \u201cWait for Darker\u201d. Using this setting, we choose how much darker the light should be until the Wait block considers the light dark enough to react. But how much darker?<\/p>\n<p>Let\u2019s use the previous example:<\/p>\n<ul>\n<li>White surface: 60%<\/li>\n<li><strong>Threshold: 45%<\/strong><\/li>\n<li>Black border: 30%<\/li>\n<\/ul>\n<p>The difference between white surface (60%) and the threshold (45%) = 60% &#8211; 45% = 15% darker.<\/p>\n<p>Therefore when the robot senses a drop in lighting of more than 15% it will treat this as having found a black border.<\/p>\n<p><a href=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-10057 size-full\" src=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker.png\" alt=\"WaitforLight-darker\" width=\"436\" height=\"147\" srcset=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker.png 436w, http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker-300x101.png 300w\" sizes=\"(max-width: 436px) 100vw, 436px\" \/><\/a><\/p>\n<p>When I showed this to my students, it lead to a discussion about what \u201cdarker\u201d actually means\u2026<\/p>\n<p style=\"padding-left: 30px;\">Student: Darker than what?<br \/>\nTeacher: Hmm \u2026 darker than actual light conditions.<br \/>\nStudent: Well, but how does our robot realize these actual light conditions?<br \/>\nStudents\u2019 hypotheses:<\/p>\n<p style=\"padding-left: 60px;\">At start of program-run?<br \/>\nWhen light-sensor lights up?<br \/>\nWhen wait-for-light is activated?<br \/>\n\u2026 but that is too late!<br \/>\n\u2026 and it\u2019s occupied with watching out for the dark border \u2026<\/p>\n<p>So we investigated the internal coding of the \u201cWait for Darker\u201d block by double-clicking on it.<\/p>\n<p>This coding shows the guts of \u201cWait for Darker\u201d block.<\/p>\n<p><a href=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitDarker-guts.png\"><img loading=\"lazy\" decoding=\"async\" class=\" size-medium wp-image-10055 aligncenter\" src=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitDarker-guts.png\" alt=\"WaitDarker-guts\" width=\"286\" height=\"149\" \/><\/a><\/p>\n<p>Be aware! Inspecting the internal code of a block can be quite interesting, BUT do not change the inner settings because you won\u2019t recognize those changes from an outside view afterwards.<\/p>\n<p>For better understanding I integrated the internal coding above (light-sensing, subtraction minus 15 and wait\u2013for-darkness) into our LabVIEW Program \u201cwait\u2013for-darker\u201d.<\/p>\n<ul>\n<li>Get Light-Sensor from: Functions &gt; MINDSTORMS Robotics &gt; Sensor (Read EV3 color &gt; reflected)<\/li>\n<li>Get Subtraction from: Functions &gt; MINDSTORMS Robotics &gt; Programming &gt; Numeric &gt; Subtract.<\/li>\n<\/ul>\n<p><a href=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker-details.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-10056 size-full\" src=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker-details.png\" alt=\"WaitforLight-darker-details\" width=\"549\" height=\"164\" srcset=\"http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker-details.png 549w, http:\/\/legoeng.local\/wp-content\/uploads\/2015\/03\/WaitforLight-darker-details-300x90.png 300w\" sizes=\"(max-width: 549px) 100vw, 549px\" \/><\/a><\/p>\n<p style=\"padding-left: 30px;\">Student:\u00a0 That\u2019s not much more than we had already!<br \/>\nTeacher: Well, what did they really add in the \u201cWait for Darker\u201d coding?<\/p>\n<p>The students argued a bit until they understood the meaning of subtraction and then recognized that this coding simply does the same we did ourselves. (Actual light reading minus darker-value = new threshold).<br \/>\nImmediately at start of the \u201cWait for Darker\u201d function, the Light sensor takes an initial reading. Then as soon as the current light reading is 15 points lower (our setting for darker) than the initial reading, the function has finished waiting for darker and the program proceeds to the next instruction.<\/p>\n<p>This explanation engaged the students into a closer discussion of internal timing in the program and the corresponding behavior of the robot.<\/p>\n<p>They also argued about what starting position would be best to have a successful program run across the surface.<\/p>\n<p>Can you imagine that starting with the light sensor on the black border won\u2019t be a good choice and why?<\/p>\n<p>By investigating this topic the students gained a deeper understanding of<\/p>\n<ul>\n<li>how to use a light sensor.<\/li>\n<li>what we can expect from a wait-for- block.<\/li>\n<li>the internal timing in the program and the corresponding behavior of the robot. (This is important for debugging skills because quite frequently students can\u2019t imagine which part of the program is responsible for such a strange behavior of their robot.)<\/li>\n<\/ul>\n<p><em>Andi Pi<\/em><\/p>\n","protected":false},"excerpt":{"rendered":"<p>In this article, we look at the difference between fixed and relative thresholds in LabVIEW, that is the \u201cdarkness\u201d and the \u201cdarker\u201d settings in the Wait for Light Sensor block. Note: This article builds on \u201cExploring thresholds\u201d by Josep Fargas. It is recommended that you read that article first. The examples in this article were [&hellip;]<\/p>\n","protected":false},"author":449,"featured_media":10052,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[38,159],"tags":[444,143,443,177],"_links":{"self":[{"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/posts\/10049"}],"collection":[{"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/users\/449"}],"replies":[{"embeddable":true,"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/comments?post=10049"}],"version-history":[{"count":0,"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/posts\/10049\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/media\/10052"}],"wp:attachment":[{"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/media?parent=10049"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/categories?post=10049"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/legoeng.local\/wp-json\/wp\/v2\/tags?post=10049"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}