Acidity Tester Project 3 Engineers use robots to do tasks that might be harmful to humans. One common example is working with hazardous chemicals. A hazardous chemical is one that could cause a negative health effect, such as cancer, lung damage, skin irritation, or even death. If a mixture contains at least 1% of a known hazardous chemical, it is classified as a hazardous material. Consequently, many common household solutions, such as paints, cleaners, inks, and dyes can be considered hazardous. An acid is sometimes considered a hazardous chemical, because it can cause severe burns if it gets in your eyes, nose, or skin. Not all acids are hazardous, however. Some acids, such as vinegar and lemon juice, are used in cooking to give foods their distinctive flavor. In this project, your challenge is to build a sensor-controlled robot to determine the acidity of a liquid. Your robot should be mobile, so that hazardous chemicals can be kept away from human operators. You will use a Vernier ph Sensor and a Vernier NXT Sensor Adapter connected to your NXT to test the acidity of several common household solutions. Solutions that are acidic will have a ph value between 0 7. There are many ways to tackle this challenge. One idea described here is to have your robot move toward a cup of liquid, stop when a LEGO Touch Sensor contacts the cup, and then lower the ph Sensor into the cup. Once the ph of the liquid is determined, the robot should raise the ph Sensor from the cup and back away. You can play a sound file, such as a scream, to let people know the solution is hazardous. OBJECTIVES In this project, you will Build a robot to find a cup and lower a probe. Use the NXT to determine the ph value of a liquid. Play a sound based on sensor data. DIFFICULTY LEVEL Construction Intermediate Programming Intermediate STEM with Vernier and LEGO MINDSTORMS NXT 3-1
Project 3 MATERIALS computer coffee cup LEGO NXT Intelligent Brick household solutions (see Experiment 1) MINDSTORMS Edu NXT v2.0 software wash bottle filled with distilled water Vernier NXT Sensor Adapter waste cup Vernier ph Sensor* goggles LEGO MINDSTORMS NXT Educational Set *The ph Sensor should stand in ph 7 buffer solution when not in use to keep it from drying out. DESIGN REQUIREMENTS In this project, you will design and build a mobile robot for testing the ph of a solution. You should use a ceramic coffee cup or other relatively heavy container to hold the solution, since a lightweight container, like a Styrofoam cup, might tip over. The robot should approach the cup, stop when the touch sensor comes in contact with the cup, and then lower the ph Sensor. After a few seconds your robot should raise the ph Sensor, play a sound, and then back away from the cup. Your program should compare the ph value of the solution with a pre-defined limit. If the ph is less than 7, the robot should scream. If the ph is greater than 7, the robot should say good. We recommend using orange juice or cola for the acid in this project and water with a little baking soda added to it for the base. If you use any other acid solution, use caution as they can be toxic, corrosive, or irritating to the skin, eyes, lungs, or mucous membranes. Handle those solutions with care. Do not allow the solutions to contact your skin or clothing. You are strongly encouraged to use the Engineering Design Method and your creative imagination when tackling this challenge (see Appendix G). However, construction and programming instructions for one possible solution follow. CONSTRUCTION These directions were created using LEGO Digital Designer (LDD), a CAD-like software program for LEGO components. This program allows you to view the following building instructions interactively on your computer. A free download of LDD is available from www.lego.com. The Acidity Tester LDD file is provided on the accompanying CD. The base for this robotic device is a basic Four-Wheel Cart. (See Appendix E for detailed building instructions. The 4Wheel Cart LDD file is also provided on the accompanying CD.) The building instructions below are for two subassemblies that will be added to the basic cart: the Touch Sensor & Motor and the ph Sensor Holder. The Touch Sensor & Motor subassembly will be attached to the two motors on the Four-Wheel Cart. The ph Sensor Holder subassembly will be attached to the motor on the Touch Sensor & Motor subassembly. 3-2 STEM with Vernier and LEGO MINDSTORMS NXT
Acidity Tester Touch Sensor & Motor Step 1 Step 2 Step 3 Step 4 STEM with Vernier and LEGO MINDSTORMS NXT 3-3
Project 3 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 See Appendix E for cart building instructions 3-4 STEM with Vernier and LEGO MINDSTORMS NXT
Acidity Tester Note, the lower axle shown in Step 7 is the same axle shown in Step 3 of the Basic Cart instructions. Remove the 12M axle from the bottom of the Basic Cart, slide the long end of the angle beam down between the two motors, and reinsert the axle between all pieces. Secure on the ends with bushings. Use a cable to connect the upper motor to port A on the NXT. Connect the touch sensor to Port 2 and the cart s motors to Ports B and C. ph Sensor holder Step 1 Step 2 Step 3 Step 4 STEM with Vernier and LEGO MINDSTORMS NXT 3-5
Project 3 Step 5 Step 6 Step 7 Step 8 Step 9 Step 10 3-6 STEM with Vernier and LEGO MINDSTORMS NXT
Acidity Tester Step 11 Step 12 Step 13 Step 14 Step 15 Step 16 STEM with Vernier and LEGO MINDSTORMS NXT 3-7
Project 3 Step 17 Step 18 Attach the ph Sensor Holder to the motor that is part of the Touch Sensor & Motor subassembly; then position a Vernier ph Sensor between the four axles of the ph Sensor Holder. The bulb of the ph Sensor is easily breakable and should not extend below the end of the beam. The beam was intentionally made longer to protect the bulb of the ph Sensor from hitting the bottom of the cup. Secure the ph Sensor in place by wrapping two rubber bands from your MINDSTORMS NXT Educational Set around the upper and lower pair of axles. Connect the ph Sensor to the Vernier NXT Adapter; then connect the Adapter to Port 1 on the NXT. 3-8 STEM with Vernier and LEGO MINDSTORMS NXT
PROGRAMMING Acidity Tester The 03 Acidity Test.rbt program created with MINDSTORMS Edu NXT v2.0 software is shown below. (This program can be found on the accompanying CD.) 03 Acidity Test is a simple single pass program. When the program starts, it waits a second before executing to allow the user to get out of the way. The program enters a loop that runs the drive motors B and C forward until the touch sensor on the front of the robot contacts the cup of solution; then it stops. The robot then lowers the Vernier ph Sensor into the solution using motor A, waits four seconds, takes a ph reading, and displays it on the NXT screen. Next, a Switch command performs a ph reading and determines what sound file is played based on the value of the reading (see the figure below). If the ph is below 7 (the acid), the solution is considered hazardous and the robot will shout. If the ph is above 7, the solution is considered safe and the robot will say Good. (Settings for formatting the Sound block are shown in the figure below.) The robot then raises the ph Sensor and backs away from the cup. STEM with Vernier and LEGO MINDSTORMS NXT 3-9
Project 3 The figure below shows the settings for formatting the Switch block. EXTENSIONS 1. Add a LEGO Motion Sensor and Light Sensor to your robot to allow it to sort a set of liquidfilled cups into an Acid area and a Safe area. You can use black tape to outline the areas on the floor. The robot described above is a four-wheeled cart constrained to moving forward and backward. It is a good design when you require a lot of stability, but it is not good for turning. A suggestion would be to substitute the three-wheeled Driving Base robot is described in the Robot Educator of the NXT Programming software. 2. Liquids have other properties that can be considered potentially hazardous to humans, such as temperature, conductivity, or salinity. Repeat this project substituting a Vernier Stainless Steel Temperature Probe to test for scalding liquids, a Vernier Conductivity Probe to check for a heavy ion concentration in a liquid, or a Vernier Salinity Sensor to test the saltiness of a solution. 3-10 STEM with Vernier and LEGO MINDSTORMS NXT