Usually when I’m searching YouTube to see what new old stuff has come up I type in Jansch or Renbourn or Thompson, but today just for a change I typed Mcshee, and this is what came up:
which lead to:
and:
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Dynamically Defined Dancing Pendulums
Following the previous post, I have re-created the animation using a non-linear dynamic analysis inside Strand7, rather than by feeding in the positions of each ball at each time step. The input using this approach is much simpler; just set each pendulum with an equal angular off-set, and specify the direction of gravity, and then let the program work out where they go. The first animation generated with this approach was visually nearly identical to the version generated with specified coordinates, which was encouraging, but didn’t make for a very interesting video. To add a bit of variety, and to replicate the behaviour of real pendulums more closely, I have also added some “viscous damping”, which gradually reduces the amplitude of each pendulums movement:
Details of the Strand7 animations settings:
- Analysis Type: Nonlinear transient dynamic analysis
- Non-linear options: Nonlinear geometry selected (nonlinear materials and creep not required)
- Time steps: 18,000 steps at 0.005 sec increments, save every 10 steps.
- Viscous damping selected
- Pendulum strings defined as steel wire with a diameter of 2 mm
- Pendulum viscosity set at 25 kN.s/m/m^3 (set by trial and error to reduce amplitude by about half over 90 seconds)
- Iteration limit = 20, Displacement Norm Tolerance = 1e-6, Residuals Norm Tolerance = 1e-5
First attempts had problems with the analysis becoming unstable after a few seconds, which were resolved by:
- Reducing the time step
- Reducing the displacement and residuals norms
- Reducing the stiffness of the pendulum cables
Posted in Finite Element Analysis, Newton
Tagged dynamic analysis, Pendulum animation, Strand7
2 Comments
Dancing Pendulums
This animation was produced in response to a video of real pendulums performing similar tricks:
Not having an animation program I thought I’d use the finite element program Strand7. Modelling the pendulums was easy, just 15 beam elements with a “translational mass” attribute on the end (more on the exact length of each pendulum later).
Strand7 Pendulum Model
Strand7 will perform a dynamic analysis, so I could have given each pendulum a displacement and then carried out the analysis of their movements over 90 seconds, but because I was only interested in generating the patterns in this case I decided to take the easy way out and generate the pendulum movement by inputting the displacements directly. The procedure was:
- Set up 30 “freedom cases”, each with the lower end of a different pendulum given a unit deflection in the X or Y direction
- Set up a series of 1800 increments, corresponding to time steps of 1/20 second, and factor each freedom case to replicate the pendulum motions.
- Run the analysis
- Create an animation of the resulting pendulum motions, using an end on view as in the original video
The only remaining problem was how to enter 1800 x 30 factors quickly and accurately. The solution (naturally) was to set up a table of the X and Y displacements of each pendulum in Excel:
Freedom case factor table
These factors were then transferred into Strand7 using the program’s API (the use of which I will describe in more detail in later posts).
The only remaining detail was to find the correct pendulum lengths to generate the patterns I was after. After a little experimentation it became clear that each pendulum much have a different integer number of cycles over some fixed period. I fixed the first pendulum length at 350 mm, which has a period of 1.187 seconds, or 51 cycles over 60.558 seconds. The other pendulum lengths were then adjusted to have 52 to 65 cycles over the same time period, giving a range from 336.7 mm to 215.5 mm.
Posted in Animation, Excel, Finite Element Analysis, Newton
Tagged animation, API, Pendulum patterns, Strand7
5 Comments
Android apps
Following yesterday’s post about my new toy, this one is about some of the applications to make it go. It will be fairly brief because I haven’t had a lot of time to explore, but the main aim is to provide a space for reader’s comments, so if you have an Android app you like, please leave a comment and let us know about it.
Office Stuff
- “Docs to Go” reads MS Office documents and pdf files. I upgraded to the “pro” version for about $14, which allows editing, jump to a page number in a pdf file, see more than the first sheet of a spreadsheet and various other enhancements. I’m happy with the pdf reader, but the Office readers are disappointing. Word files are wrapped to fill the screen for any zoom level, and many graphics appeared as black boxes. In Excel the text is displayed reasonably well, but it seems it doesn’t display charts at all.
- E-cell xls Pro: Display also disappointing with no charts, and the text formatting shown much less accurately than Docs to Go. Editing text does seem easier though. Both Excel programs offer no VBA support.
Other stuff:
- Gallery – the built-in image file viewer, works very nicely, fast and good resolution with a very intuitive interface.
- Barcode scanner – scans all sorts of barcodes, worth the free download if only to scan bar-code links to other applications.
- Cam scanner – scanned documents from camera images. Didn’t work well on the one I tried, but maybe the Acer camera just isn’t up to it. I’ll try it with some images from my SLR some time.
- Convert pad – nice unit conversion program. Very easy to use and fairly comprehensive. I just wish I could copy and paste into Excel easily.
- Autocad WS – this is very nice, you can upload Autocad drawings and view and edit them, all from your browser. Just requires a free account set up.
That’s all for now. Anyone with a good Android app, please leave a comment.
Acer A500 Tablet Review and Total Commander
Having got fed up waiting for supplies of the iPad2 to become available, I bought myself an Acer A500 Tablet about 3 weeks ago, so here are my first impressions. Remember that I’m very much a tablet newbie, who still uses dumb phone technology, so my first impressions may not be that accurate. (There is a more authoritative review here.)
The Acer A500 is an Android V3 based tablet computer with a 10″ screen and currently comes with 32 GB and wi-fi only. My main use for a tablet is to be able to read documents while travelling, and at meetings, without having to lug a heavy 17″ screen lap-top around everywhere. For that purpose the Acer seems a good choice, with the following benefits:
- The built in USB ports provide a convenient and fast way to transfer data from a Windows based PC.
- There is a (beta version) of the file manager Total Commander available, which makes file transfer/management even easier (see below for details).
- The screen is slightly higher resolution than iPad, and allows A4 documents to be read full screen in reasonable comfort.
- Applications for reading Office documents and pdf files are bundled and work well
- Other applications (picture browsers, music, video) also seem to work well.
- The screen is bright and has been easily readable everywhere I have tried it, including planes, trains and bright rooms (but it’s late Autumn here, window seats in the Summer could well be a problem).
- External view orientation lock and volume buttons are useful.
Aspects that I was not so happy with:
- I’m not that keen on the interface, but that’s probably unfamiliarity, and I’d probably like the iPad even less.
- The screen does seem to attract finger marks quickly, but more so than other screens? I don’t know.
- The formatting of Excel files in both of the applications I have tried is often way out.
- The battery life is allegedly less than the competition (but it was plenty for a 17 hour flight from Dubai, allowing for meals and a bit of sleeping).
- Selecting small links is often difficult (but again I have no benchmark to compare with other machines).
- When Internet browsing it goes into “mobile” mode by default, which is just silly for a 10″ high resolution screen. So far the only fix I have found reverts back to the mobile setting when the machine is switched off.
- In stand-by mode it seems to switch itself on every 15 minutes or so, so if you want to conserve the battery (or sleep in the same room!) you have to switch off.
In spite of the quibbles, I am generally happy with it, and it does what I bought it for well, especially with the Total Commander file manager.
I have been Total Commander user and fan since its introduction in 1993. It is a twin window file manager similar to the DOS based Norton Commander, and in its Windows version is a mature product that I am lost without. It is share-ware, with a not very annoying nag screen, but is well worth the modest asking price (especially with unlimited free upgrades). The Android version is available for beta testing only through this link. Please note that it is a beta version, and comes with a warning that if it eats all your data, don’t blame the author.
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