Category Archives: Making

The Great (Mini) Robot Race

Robotics – The Synthesis Project

The final project of the two year academy program requires that the students design, fabricate, program and test an autonomous robot. We have been doing this project since the inception of the program, but this year we have made some significant improvements on the project. In this post I will explain the project, and highlight those improvements.

This project is an extremely challenging task that requires successfully completing several “sub-projects”. We tell the students at the beginning of the school year, that this project is more difficult than any of the other projects – by a long shot!

“If you haven’t done everything, then you haven’t done a thing.” – Red Whittaker

We completely changed our robot competition parameters this year. In previous years we had the students design sumo wrestling battle bots. Although this was a fun project, we started to notice that some robots performed really well without really having to “think”. These robots generally lumbered around the ring, sometimes without even actually “seeing” their opponents. Through pure luck they just managed to push their lighter opponents out of the ring. We decided that we needed to change the project in order to force teams to be smarter and we also wanted to get away from a robot competition that seemed to focus on aggressive battle.

Ironically, I suppose, our robot project changes were inspired by the classic Nova film that documented the DARPA challenge known as the Great Robot Race. In this race, the autonomous vehicles raced through the Mohave dessert on a course that was revealed to the competitors only hours before the beginning of the race.

Bob set about building an impressive “maze” for the robots to navigate. The robots had to make their way through a series of 90 degree turns defined by a series of connecting corridors with vertical walls about 20 cm tall. The robots were then given two attempts to make it through the course as fast as they could.

The Brains and Braun

When we first started this project about twelve years ago, we first used a Java based board that we liked, but it was really expensive and we didn’t really need much of the hardware and software features that it offered. About eight years ago, the Arduino board was taking the Maker community by storm and we decided to hop on the Arduino bandwagon, and we have been very happy ever since. The simplicity, online community, the plethora of code examples and tutorials as well as the price have been key points in why we have decided to keep using the Arduino. This year we decided to incorporate DC motors from Sparkfun and Adafruit’s motor shield. The combination allowed the students more flexibility regarding drive train design, and also allowed for some interesting discussions around the advantages and disadvantages of servos vs dc motors. Our only complaint with the shield would be that it would be nice to have the headers pre soldered!

Designing The Circuitry

fritzing-preview-pcb

One of the major additions to the project was to require that students design and fabricate the circuitry for their robots. We did this by introducing two new skills to the project. Students had to learn how to use a printed circuit board (PCB) design software known as Fritzing, and then they had to learn how to fabricate their PCB boards using a CNC mill.

There are a number of amazing PCB design tools out there – and many are free! They all have their strengths and weaknesses. Here is what we found out as we did our research to find the right tool.

Autodesk’s Circuits is great because its web based, super easy to use and has an amazing feature where you can actually simulate the circuit. You can add a virtual voltmeter of ammeter to your virtual circuit and then with a push of a button, you can get virtual readings on the meters. I found this tool to be amazing for teaching circuitry and I allowed the students to use it as a “key” for their worksheets. It also has the ability to simulate an Arduino too! You can add an Arduino to your project, connect up LED’s, servos, etc. and actually see them light up or rotate as you change the code. It is a bit limited in that it doesn’t support most added libraries, but it is still amazing. We eventually decided not to use it for PCB design because it is unfortunately a bit clunky and doesn’t allow for much customization of the board.

Eagle is of course one of the most advanced and feature rich PCB design tools out there. It is also very complicated. IT of course offers the largest toolset, complete control of the design process and the free version is as close to a professional tool as one could hope for. The problem is that all these features come at a price – complexity. If we had an entire year to spend on this project, I might have decided to go with this tool, but we needed something that the students could learn quickly and weren’t going to get frustrated with…

Fritzing is a free, open source “beta” software that is very similar in look and feel to Autodesk’s Circuits – in fact I think Autodesk’s product must have been inspired by Fritzing? Although Fritzing lacks the amazing simulation tools that Autodesk Circuits has, it does offer a much better PCB design environment. The options that are available for editing the component foot prints, the PCB attributes, etc. make it really nice to work without making the tool too complex. This is the tool we decided to teach and use in class, and the students liked it.

pcb-milling-opener

Fabricating The Circuitry

Back in the fall we decided to invest in a small CNC mill produced by a local company out of San Francisco named Other Machine Co. This machine, called the OtherMill has been an amazing addition to the lab. With this machine, we have been able to teach the students how to fabricate their own PCB’s. The OtherMill is not just for PCB fabrication, in fact we have used it to mill small aluminum parts as well.

2323-00

This micro CNC desktop mill is super easy to setup, really easy to use and plays really nicely with Fusion 360 – our 3D CAD and CAM software. I was incredibly surprised and pleased by how easy it is to learn the operating software – known as OtherPlan. The company has a great support website full of great tutorials, and we were able to teach all the students how to use the machine in just a few days.

The PCB files generated by Fritzing (we exported them as Gerber files) worked flawlessly with the OtherMill, and within a very short period of time, all the students had designed and fabricated single sided PCB boards for their robots.

img_3283

The Final Results

As with any major changes to a project, there are lessons to be learned. We realized that the task was rather complicated and many of the students did not make it as far through the course as they had hoped. It was clear however that this competition proved more interesting from the perspective of getting students to see the importance of software design. Not only did we see very different software strategies, but the variance in hardware design was surprising. They really had to think about how the hardware and software had to work together, and they had to think about optimization. This was a clear advantage of this competition over the previous year’s competitions. Students spent far more time trying to figure out how they were going to shave time off their attempts, and how they were going to adjust software and hardware to better navigate the course. From our perspective, the changes to the project proved to be fantastic, and we are looking forward to improving on the project design for this year. Some of the things that we are going to do this year is introduce some different sensors for the students (like “feelers”) and also give them a price list and budget so that they have more hardware choices.

Our Custom Designed Bridge Tester

Our Goal – Make A Better Bridge Crusher!

This year Bob and I decided to upgrade our bridge project. We wanted to create an improved device for measuring the load on the model bridges created by the students. Specifically, we wanted to design a way for the students to collect meaningful performance data.

The problem with our old method (filling a bucket with sand until the bridge catastrophically failed) was that it didn’t allow for the students to collect evidence about where and why the bridge failed. In many cases, the bridge actually experienced a significant failure, but because the bridge collapsed around the loading plate, the loading plate actually acted as a support for the bridge.

We had seen other “bridge testers” from vendors (http://www.pitsco.com/Structures_Testing_Instrumenthttp://kelvin.com/kelvin-bridge-material-tester-w-cpad/http://www.vernier.com/products/sensors/vsmt/) but we had the idea that perhaps we could create one that might be better, or at least it might be fun to try. In this post, we are going to share with you what we created and why we think it actually turned out really well, but we also point out some room for improvement.

The Frame

The frame of the entire device is really based on the Vernier Structures and Materials Tester. We thought this frame was probably the best, and we wanted to make our tester from extruded aluminum tubing as well. We went about designing the bridge tester in our favorite CAD program, Fusion360 by Autodesk. We sent our CAD file to the company 8020 and they precut all our t-slotted aluminum frame members to size. This was awesome because it made assembly super easy and it saved us on shipping too! Our experience with this company was amazing – they were super helpful and even gave us some really helpful advice. If you are thinking about making anything requiring t-slotted aluminum, definitely order from them.

The Load Sensors

We wanted a a bridge tester that actually gave us data that allowed us to figure out how and why the student bridges failed. That meant that we needed more data and we needed data that could be connected to the design and fabrication of the bridge. We noticed that all the vendors’ designs had only one load sensor, and some also had a way to detect overall deflection. We suspected that we could get better data if we had four independent load sensors – one for each abutment where the bridges were supported. The four sensors would (theoretically) give the students a way to analyze how the load was being distributed, and thus tell us something about the torsional behavior of the bridge.

Sparkfun load sensor

Sparkfun load sensor

I set out to learn a bit about load sensors and I came across a fantastic tutorial at Sparkfun (https://learn.sparkfun.com/tutorials/getting-started-with-load-cells). We ordered four load sensors (see picture above) from Sparkfun. Our design (shown below) has the four load sensors  fitted with 3D printed “shoes” as the bridge abutments.

IMG_3048

The four load cells.

IMG_3044

Two load cells with abutments.

The sensors are mounted on custom fabricated aluminum plates that can be moved laterally to accommodate slightly different bridge widths. The load sensors had to be connected to load amplifiers that were then connected to an Arduino (more on that below). The load sensors were mounted to the frame of the bridge tester on custom laser cut plates:

IMG_3043

Load cell amplifiers for two load cells.

The load amplifiers have to be used with the load sensors in order to amplify the signal so that the Arduino can read the data correctly.

The Loading Mechanism

Bob designed and fabricated the loading mechanism that was based on many of the designs we had seen online. It consists of a block that is free to move vertically up or down a threaded rod which is then connected to a spoked wheel.

IMG_3047

Loading wheel.

When the wheel is turned, the block moves up or down the threaded rod. This bar is connected to a loading plate via a metal cable that hooks into the loading plate and the block:

IMG_3053

Load plate from below – revealing loading wire attachment.

IMG_3052

Loading wire attached to threaded block.

The loading plate is placed on the loading plane of the bridge (the “roadway”), and then the bridge is loaded by spinning the wheel, which lowers the block, which pulls the bridge downwards:

IMG_3050

Loading plate on bridge.

Collecting The Data: The Software

 The software responsible for collecting the data is made up of two programs – one that runs on the Arduino micro-controller that is connected to the load sensors, and the other is a Processing sketch that runs on a computer/laptop connected to the Arduino via USB. The code is pretty simple, and it was mostly written using code from other sources and then modified for our specific purposes.

The Arduino code just collects the data from the four load sensors and then sends the data serially as a comma delimited package. The Processing code reads the serial port and then essentially dumps the data into a csv file. It does have some flourishes like a graphical display of the individual sensor loads as well as a display of the total load and whether or not the bridge has met the minimum load requirement set in the project descriptor.

You can view and download all the code here on GitHub.

The Performance Report

When the data is displayed in a graphing program, it looks like this:

Sample bridge data

Sample bridge data

You can see that the four sensors do not read equal values, and that the bridge begins distributing the load unevenly. The blue and orange lines show that these two load sensors were equally loading and were taking on a larger load than the green and red values from the other two load sensors. Later inspection of the bridge showed that this bridge failed at the load sensors that were recording a higher load value. Also, these sensors were located diagonally from one another, and once again showed that the bridge was being twisted.

You can also see that around 80 seconds (the 800 data sample) that the bridge experienced a sudden decrease in load – this was the point of failure. The data clearly shows a point at which the bridge failed and thus gives us a clear metric for performance.

The student teams were each given the results and were asked to answer these questions:

  1. What was the maximum load that your bridge sustained before failure?
  2. Calculate the load to weight ratio of your bridge.
  3. What were the individual maximum force values on each load sensor before failure?
  4. Identify on your bridge where the bridge failed. Take a picture of this point of failure and note its location.
  5. Based on the load data for the four sensors, describe why your bridge may have failed.
  6. What could you have done to increase the performance of your bridge?

The data really allowed for some rich analysis and the students were able to make some really informed critiques of their design and fabrication quality. We have been very happy with the results!

Future Improvements

For version 2.0 we hope to add these improvements:

  • Add the ability to measure deflection. We think that this might be done by measuring the angular displacement of the loading wheel, but we aren’t sure just yet.
  • Some way for the software to detect a failure – perhaps a way to detect a significant decrease in the load data. It would have to allow for some downward movement of the load data because there is some settling and deforming that can occur that might not be catastrophic.
  • It would be nice to clean up the code – especially the Processing code. I’d like to add some fancy GUI elements too so that it is a bit more attractive.

If you would like to build your own advanced bridge tester for your classroom, we can send you all our CAD files, software files and even answer questions. Its not easy to build, but its fun, and we spent about $350 dollars on this project as opposed to the $1000 to $1300 that the vendors are selling theirs for.

3D Printing and STEM Education

As the Maker Movement spreads into the halls of nearly every school across the country, and with it the technologies that tend to be synonymous with that movement, I thought it might be useful for me to write a reflection about how we have used 3D printing in our program and some of the things that others might want to consider when thinking about investing in 3D printing for their school.

What is 3D Printing?

A 3D printer is any device that uses additive fabrication to essentially create some three dimensional object by building that object layer by layer. Currently the most popular way to do this in the educational sphere (because it is the least expensive) is to build the models by extruding melted plastic – similar to having a very precise hot glue gun. This has been the technology that we have used for the past five or six years in the Academy. Recently there has been an explosion in new inexpensive models coming to market that use light to harden photosensitive liquid polymers. These technologies (known as stereolithography  and digital light processing – DLP) use a focused light source or laser to harden the liquid layer by layer. The finished model in most cases is made of some kind of plastic – ABS, PLA, etc. Although the technology is moving forward with other materials, any kind of printer that would be used in a classroom environment is going to make plastic models.

Whatever the process of the 3D printer, these technologies are different from subtractive manufacturing which starts with a “chunk of stuff” and then carves the material away, leaving the 3D model. These always require some kind of cutting instrument like a hardened metal drill bit, or even possibly a laser or high pressure water jet. These methods are still preferred for the actual manufacturing of things like airplane parts, high precision medical instruments and all sorts of other machinery because these methods are highly precise and can be used to create parts from almost any material – metal, stone, plastic, etc. There are two issues with subtractive manufacturing though. The machinery is generally very expensive and learning how to use it properly is quite challenging.

If you would like to learn more about 3D printing in general, I suggest this great website:

http://3dprintingindustry.com/3d-printing-basics-free-beginners-guide/processes/

Our Printers

We currently have three operational 3D printers and one new DLP printer that we are currently assembling. The first 3D printer that we purchased was a Stratasys uPrint:

This printer has served us very well. It builds very precise models using really solid ABS plastic. Its precision comes at a cost though – it is quite slow. OK, its really slow! A nose cone for a rocket can take upwards of six hours to build! The other drawback of this printer is the cost and availability of build/support material. We just bought a complete restock of material and it cost us nearly $1700! Keep in mind that this should last us about six years to eight years.

The other two models that we have are the ubiquitous MakerBot Replicator 2’s. These are much simpler to operate (when they aren’t clogged) and they are much faster. They are also much less expensive. The uPrint cost us about $20,000 dollars including the rinse tank, while each MakerBot Replicator 2 cost us about $2200. Actually one of the MakerBots was part of a DonorsChoose/Autodesk program that cost us nothing (thank you donors and Autodesk!). The material for these machines is much cheaper – about $90 to $50 dollars per spool, as opposed to about $200+ per spool for the uPrint. The drawbacks of these machines is that they need constant maintenance, manual calibration, and the models that they build are not as accurate nor as precise.

We recently were very honored to be the recipients of a new 3D printer, donated by our high school’s parent organization – WeAreSR. Although we haven’t yet been able to use our new 3D printer from Kudo3D, we are excited by its potential. This DLP printer is said to have a much higher resolution, a much faster build speed, and a very large build volume. We will be posting an update once we get it running – which should be soon!

Rapid Prototyping = Rapid Learning

The really rewarding educational aspect of 3D printing, from a teacher’s perspective, is the acceleration of the learning cycle. Students can quickly identify weaknesses in their designs because they can have a part in their hands in literally hours, then make adjustments and have a new version fabricated, sometimes in a single class period. This would be nearly impossible ten years ago.

Now some might argue that it relieves students from the importance of having to think more carefully about their work, but I think this is outweighed by the advantage of allowing students to more quickly assess their spatial reasoning, and as long as we teachers force the students to also reflect on why their design failed or needed revision, then I think ultimately the students will learn more quickly.

This does not mean that we always allow the students to print whatever they want. We do act as “gatekeepers” to the printers so that we aren’t wasting student time, our time and resources. The models must pass a few minimal requirements, such as double checking dimensions, seeing if the model could be made more efficiently as multiple parts, etc.

3D Printing = 3D Spacial Problem Solving

The 3D printers have acted as a great arena for students to learn and develop their 3D spatial problem solving skills. To be clear, its actually the combination of 3D CAD software used to design the models and 3D printing to create the actualized models that helps students visualize, navigate and anticipate interesting three dimensional problems. Generally, the printers are used to create parts that are then used in more complex assemblies. The interface of these parts is where we see students encountering and having to solve complex spatial puzzles.

One of the things that I have witnessed is the advancing complexities of student designs as they become more familiar with the software and also develop their ability to mentally construct the spatial relationships between assembly components. At some point, I’d like to document this process and perhaps develop an assessment tool for measuring the development of these cognitive skills.

The Limitations (Not Star Trek Yet…)

The really amazing aspect of 3D printing is the ability to create real objects from imaginary ones with an almost perfect translation. I do think that it is important to realize that there are some limitations and also some things to consider before you run out and buy one of these things. Here are few things to consider:

All Those Plastic Things

One of my biggest complaints to the 3D printing industry is the lack of any clear and clean way to take 3D printed models that were unsuccessful and break them down back into raw materials for use in the printer. At the end of the school year, we have a fairly large bin of unwanted models that we collect for recycling. Some of the models are indeed recyclable while others are not depending on the material used. I think the manufacturers need to come up with a clear “cradle to cradle” solution for their printers that allow users to throw their models back into the machine to be re-extruded. It is theoretically possible and at least one company is offering a product called the Fillabot for addressing this issue.

Its Not That “Rapid”

Now, in relative terms when compared to milling, 3D printing is pretty fast, but it is actually slow in the context of the classroom. Even though its called rapid prototyping, it can seem really slow for some folks who are new to the world of manufacturing prototypes. You see, in the past, modelers would make a prototype out of clay, create a mold, cast the mold, make refinements, etc. Or one would calibrate and setup the CNC mill, have to change out bits, run test cuts, etc. In this context, 3D printing seems rapid. But its still not Star Trek.

The time can vary significantly based on the type of the printer, the complexity of the design, and the size of the model. This can be really frustrating for some teachers who want to be able to print an entire class’ models and have them ready for the next class period – that won’t happen. It can take hours to print just one model. You have to design your course in a way that allows the students to work on parallel tasks and then you need to have some way of keeping track of the printing queue.

It Won’t Make Everything

These machines are amazing, and we really love our array of 3D Printers, but experience has taught us that there are limitations to what they can make. Because all of these printers essentially work with liquified plastic, there are limits to the geometry of what you can build. As the models are built, they can “sag” or deform under their own weight. This can lead to small deformities, or catastrophic failures. Calibration can also be an issue with some of the less expensive or older printers. If the build plate is not properly leveled and calibrated, the entire build process can fail. Models with significant “overhang” can collapse, ruining the model.

Different printers deal with this slightly differently. Our MakerBots, for example, add “supports” to the model. These are little posts that act to hold up arching forms. The problem with this is that these posts then need to be removed from the model, and we have found this to be less than ideal. It adds extra time to the process because you have to do some post finishing work which can include filing and some sanding. Our uPrint actually adds a soluble support to the model that can be removed using a mild (but still toxic) solution. Again, this post processing adds a significant amount of time to the entire fabrication process.

Size is also a limitation. Don’t think for a minute that just because the build plate is 8 inches by 6 inches, that you can build a model with that footprint. You can’t. Once again, because you are dealing with liquified plastic that cools, it also shrinks. The larger the volume of the model, the greater the chance that the model will curl, buckle, and deform. Read the fine print from the manufacturer to get the real build size limit.

Easier, But Not Easy

The last point we want to make is that working with a 3D printing is certainly easier than running a 5 axis CNC mill, but they are not as easy to work with as an actual 2D printer. Adding that extra dimension has its challenges. Plan on spending quite a bit of time learning how to maintain your printer. Just like 2D printers, 3D printers “jam” all the time. The extruded plastic can get stuck in the nozzle and you can come back after several hours of printing and find that the very last cm of the model never printed because the nozzle is completely gummed up! Be aware that these can be infuriating moments that take significant amounts of time to fix. I have spoken to some teachers that got so frustrated that their printers ended up just sitting in a corner of the classroom, tragically unused.

Our Recommendations

Our recommendations are simple. Before you go out and buy one of these things, you have to be willing to put in quite a bit of time to maintain it and learn how to optimize your printer’s performance. There are tricks to optimizing each printer out there, and it will require that you watch some YouTube videos, dig through online support forums and be patient.

There are clear and obvious reasons to get one of these if you are running a STEM program, especially one focused on engineering or design. What might not be obvious is that these machines can also be incorporated into mathematics education, and definitely into a 3D art course or sculpture course.

There are so many models out there now, and they all claim to be the very best value. Each will obviously have advantages and disadvantages. Ease of use and less expensive generally means that your models will not be as precise or accurate. Inexpensive models can also be difficult to maintain. DLP printers are looking promising. They are coming down in price, they are faster and they are very precise. They can print models using different materials (like castable resin, or flexible resin). On the other hand, keep in mind that they are still more expensive, and they use a somewhat toxic resin that can be a non starter for some teachers.

A Modernized Bridge Design Contest

IMG_0283

Modernizing An Old Classic

We have just completed the second project in the Academy for the 2014-15 school year. It was a huge success! This project takes a classic physics project and “upgrades” it by incorporating modern engineering design technology and fabrication techniques.

We started with a great project that is now available online through Engineering Encounters. This was a project that was originally published by Stephen J. Ressler of the United States Military Academy. It is a rigorous approach to designing and building bridges from file folders:

https://bridgecontest.org/resources/file-folder-bridges/

Its a great project with an incredible set of resources, background information, and step by step instructions. Unlike less rigorous and involved bridge design projects (using toothpicks for example), this project has the students building compression members (beams) and tension members (cords) and gussets to better model real world designs and to give the students the opportunity to learn and make decisions about which members to use in different parts of their own designs.

The only issue that we had with this project is that it requires the rather tedious process of having students trace out the unfolded beam designs onto file folder material and then use scissors and  blades to cut out each beam and cord. But we have a laser cutter! There had to be a way to incorporate both 3D CAD design and our laser cutter in order to modernize this process. We also knew that Autodesk Inventor had some really amazing tools for analyzing design structures.

From Sheet Metal To Manila Folders

Autodesk Inventor has an amazing set of tools for designing sheet metal parts. Using these tools, an engineer can construct 3D models made of folded metal parts made from just about any thickness of metal stock. Once you have designed the folded metal part, Inventor will create a flat pattern design for you that you could then send to a CNC plasma cutter to cut from sheet metal stock. You would then fold the part up manually and you would have your folded part.

Inventor gives you the ability to custom define the thickness of your stock, and some of the parameters around how it can be bent. We defined our stock to be as thick as manila folder paper. The next step is a bit tricky, but with the help of a great video I came across from Rob Cohee, we were able to define custom folded paper beam stock that the students could then use to build out their frames. Once again, Inventor has an amazing set of tools for defining structural frames (called The Frame Generator) that can then be populated with any kind of structural beam. You can also define your own structural beams that can be used to populate your frame.

I have included a video below that we use with the students to help guide them through this process:

Using the frame generator tool in Inventor also allows the student to miter and trim the beam members, which allows the students to focus on design rather than getting lost in the time consuming process of calculating the cut angles. The following video shows you how this can be done:

Once the students had designed the bridges, it was time to prepare the flat patterns and have the laser cutter do the work of cutting them out.

Fold, Glue, Repeat. (Some Assembly Required)

IMG_0353

The students prepare their flat pattern cut-outs for the laser cutter and then you let the laser “rip”! Its awesome to sit back and watch this machine cut. I never get sick of watching it! Having the students do this would take SO much longer, the cut parts would be less accurate, and as all CTE teachers know, one of the most dangerous tools in the shop is an Exacto blade.

IMG_0355

Some might argue that the “manual” process of cutting all these beams out by hand is “good for the students”, but we feel that saving time here allows us to use that time in other areas, such as virtual testing.  Before the students get to build their design, we ask them to use Inventor’s frame analysis tools to help them analyze potential weaknesses in their designs. The following video shows just how amazing this tool is:

Once the students have done their analysis and cut their construction members, its time for folding and gluing, and folding, and gluing, and … At this point our project does not differ from the Engineering Encounters project. The students use a sheet of paper (actually two 11 x 17 sheets) with an elevation view (printed from Inventor as a CAD drawing) glued to a board as a guide for assembling the beams, cords and gussets:

IMG_0351

IMG_0362

This process goes relatively quickly as the students have done all the prep work to make sure that the pieces all fit together. Once again, this really demonstrates how modern technology can allow the students to focus their attention on design.

To Break Or Not To Break

Once the bridges are assembled, its time to test them out. The performance metrics for the contest are not actually based on the strongest bridge but rather a more realistic approach. We have attached a monetary value to each beam, gusset and cord. The bridges are then tested to a set value – the required load. The bridge that holds that load and is “manufactured” least amount of money is then given the highest marks.

Once the bridge has been tested at the required load, we then give the students the choice to see just how much the bridges can hold before catastrophic failure. Most students (encouraged by both peers and staff!) decide to take their bridge to the limit.

Its always a fun way to end the project!

We Have Lift Off!

15414703973_b0258a08ca_h  15846865828_b08d8515cd_h 16033503742_be978fd9ca_h

This is a follow up post to Modeling A Rocket’s Journey – A Synthesis where I described how the students in the first year program were engaged in creating a predictive report for their model rockets. I want to emphasize that these model rockets were not kits. Each rocket was designed using 3D CAD software, and each component was either fabricated from raw material, or was created from material that was not intended for use in model rocketry. The only exception to this is the actual rocket motor.

The next step was to launch the rockets and have the altimeter payload collect altitude data.

Launch Conditions – A Bit Soggy

Unfortunately the week of our scheduled launch happened to be a week of some pretty hefty rains. We rescheduled the launch twice before finally accepting the soggy launch conditions. With umbrellas and rain jackets, we trudged out to the baseball diamond and got to work setting up for the launch. We had some minor difficulties in the wet weather, but eventually had a very successful launch day.

Most of the rockets were able to launch and deploy their valuable payload – the Pnut Altimeter.

15414689283_8197c70584_h

The students seemed very excited to finally see the rockets launch, and to see the successful deployment of the parachutes. Although we all got a little wet and muddy, we had a great time!

The Altimeter Data

The altimeters use a small barometric pressure sensor to collect altitude data (the altimeters also contain a small temperature sensor and voltage sensor). The altitude is recorded in feet every .05 seconds. Here is an example of one rocket’s recorded flight data:

altitude_vs_time

https://plot.ly/~stemples/9

The students were then asked to use the data to create a comparative analysis report. I will detail how the assignment was set up and also discuss how the students performed on this assignment. That will be for another post.

I want to also thank Mr. Kainz for his amazing photos that are displayed here.

Modeling A Rocket’s Journey – A Synthesis

A Synthesis Of All The Models (Thus Far)

In this post I will describe a culminating activity for the first year students in the Academy. This is really the destination that the students have been headed towards since the beginning of the course. Everything they have learned is synthesized in this activity where the students gather data from various observations/experiments and then use the data to predict their own model rocket’s journey.

Note: There were two significant simplifications that we had to make based on the ability level of the students and the physics content covered in class. We had to assume that there was no air resistance force acting on the rocket during the thrust and cruise phases. We also assumed that the mass of the rocket did not change. I intend to have the students reflect on how this might affect the predictions and then analyze the actual performance data. More on this later…

Measuring The Rocket Engine Thrust

We first needed to figure out the average force exerted by the rocket motor on the rockets and the time interval during which that force would be applied. This would give the students both the thrust force and the length of time of the thrust phase. We needed to collect force measurements for the rocket motors that we were using (C6-5). You can actually download this from many different websites, but it was much more fun to actually do it ourselves! Mr. Holt made a neat little rocket motor holder that was attached to a force meter and we went out into the rain to test the motor (see video below – thank’s Gary!):

The force data was then shared out to the students – here is what the graph looked like:

rocket-engine-test

And this is the force vs time graph one retailer posted on their website:

Although the students had not been introduced to the concept of Impulse-Momentum transfer, we can use the average force, and that seems to work out really well. Just to make sure we could do this, I used the Integral tool in LoggerPro to measure the impulse, and it came out to 8.83 N s – really close to what Estes states – 8.8 N s.

A Mini Wind Tunnel Test

The students then needed to measure the drag force on their parachutes (all cut on our new laser cutter) as a function of air speed so that they could estimate the terminal velocity of their rocket during the descent phase.  Next step was to test the parachutes. Luckily, Mr. Holt and I had helped two of our previous students create a really nice wind tunnel. We used a force meter attached to a vertical post inside the tunnel…

parachute_test

…and then we used a little Kestrel anemometer to measure the air speed…

air_speed

Students were able to increase the air speed in the tunnel by turning a rheostat that controlled the fan speed. They then measured the wind velocity and graphed that against the measured force – just like NASA!

Here is some sample data to show how the results came out – not bad!

parachute-test-data

Students now had a way to estimate the descent velocity because they could calculate the gravitational force on their rocket, using the measured mass of their rockets, and then they could use their data to find the corresponding wind speed.

Putting It All Together

As part of their final (50%), the students were asked to then take this data, measure the mass of their model rocket and construct a prediction. The prediction was to include these five elements:

  1. A set of force diagrams for the different phases – thrust, cruise, and descent. The diagrams also had to include accompanying net force equations.
  2. An acceleration vs time graph.
  3. A velocity vs time graph.
  4. A position vs time graph.
  5. Finally a calculation sheet that includes all calculations required to create the motion graphs.

The students have been asked to turn this in before the actual launch.

As we collected the data above, I never explicitly reveal how the data should be used to make these predictions, but I do give them some guiding questions that orients them. They work with their partner’s on this report, but I warn them that they will both be held responsible for understanding the process of creating the prediction report.

Testing the Predictions

Each student rocket will be equipped with a small altimeter (from Apogee Rocketry – love this thing!).

This altimeter records altitude data in 1/10 of a second intervals, and we have found it to be very accurate and reliable. We will be launching next week, so tune in soon for an exciting update on how the launches went!

Building The Net Torque Model – Part 2

Investigating An Unbalanced Net Torque

We started by looking at the fact that a disk experiencing a net unbalanced torque also experienced a change in rotational or angular velocity. The students used a rotary motion sensor to measure the angular position and the angular velocity of a disk experiencing a constant torque and the students immediately recognized the similarity between a particle experiencing constant linear acceleration and a rigid body experiencing constant angular acceleration.

IMG_0704

So, the natural next step in our investigation was to determine the causal relationship between a net unbalanced torque and the angular motion state of a rigid body. The students discussed how we might set up an investigation that would help us understand this relationship, and I helped guide them towards a final investigation design where we used a rotary motion sensor attached to the wooden disk we had used in a previous investigation.

The rotary sensors from Vernier are rather expensive, but also quite nice. They come with a plastic spindle with three different pulley radii. The only issue that I have with these is that they do not include a useful screw for attaching objects to the pulley – they expect you to purchase their completely over priced accessory kit. I don’t suggest this. Instead, you can create your own and then use a 6/32 screw to attach them to the pulley – just be careful not strip the threads!

IMG_0703

Working off a similar investigation when studying the causal relationship between a net unbalanced force and linear acceleration, the students recognized that we needed to include a force sensor for measuring the applied force. The students then had LoggerPro plot the points for the calculated torque (using a calculated column) and the measured angular acceleration.  The data isn’t super clean, but its good enough for the students to conclude that the relationship is most likely linear.

Dimensional Analysis – Inferring Rotational Inertia

I was amazed to discover that some students in the class set off to understand what the units of the slope could be reduced to. They immediately saw that the proportional constant (slope) included kg (mass) but that wasn’t all. After some work, the class had determined that the units for the slope were kg * m^2.

This led to a qualitative discussion about the inertia of rotating objects. We discussed hoops and disks primarily, and the class seemed to agree that the units made sense. Although I didn’t have any hoop-disk sets like the ones you can buy from various vendors, we did perform some thought experiments around mass distribution and rotation, but we needed to be sure that we were on the right track.

A Better Variable Inertia Disk

So last year I was looking for an investigation that would really help the students discover the importance of mass distribution for a rotating object. Reading the material on the AMTA website regarding the unit on rotational motion, the researches stressed the importance of connecting mass distribution to the rotational inertia. I found this variable inertia disk from Fischer Scientific and decided to purchase it. I commend these guys on making this, but frankly I decided that I could make a better one.

My colleague and I set about redesigning these disks. The improvements we made included a) better compartments for the metal marbles so that they didn’t move around, b) an index and lip so that the two sides fit more securely together, and c) more compartments so that we can test more mass configurations. After we made our designs, we used our Makerbot 3D printer to print out ten copies of the disks.

IMG_0707 IMG_0708

With these disks attached to the rotary motion sensors, the students were able to confirm that although the total mass of the disk did not change, the angular acceleration declined as the mass was moved outward from the radius of rotation. This was more of a qualitative investigation, but I think the students were able to clearly see how moving the mass farther from the axis affected the rotational inertia of the disk.

Deploying The Model (So far)

Now that the students had built a predictive model that described the quantitative relationship between rotational inertia, angular acceleration and net torque, they were ready to test it.

We returned to the investigation setup with the disk, but this time attached a hanging mass to the string and they attempted to predict the angular acceleration. The students immediately made the mistake (as I thought they might) in considering only the disk in the system. This gave me a chance to review with them the following process:

  1. Identify the system schema (now it includes things that can rotate!)
  2. Draw your force vector (free body/particle) diagrams and now also your force-moment arm (rigid body) diagrams.
  3. Write the summation of forces AND the summation of torques for the system.
  4. Do some algebra.

The trickiest part to figuring this out is getting the signs right due to interaction pairs and making sure that they agree. I ask the students to start with a diagram and then go through and label each force + or – by picking a force and then finding its partner and then making sure that the rest of the force directions agree.

Once they were able to get a prediction that we all agreed seemed correct, they ran the experiment. The class was able to predict the angular acceleration within about 10% error. We discussed the possible reason for discrepancy which led to some interesting discussions around modeling the rotational inertia of the attachment screw and washer, and also the frictional forces at work.

The students were pretty convinced that the model worked, and so next stop – energy and momentum in systems with rotational motion.