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Technique: Photos of Low-Speed Impacts
This describes how the photos of the formation of the sand impact craters were made. Check out the intro page first.
Overview: Why? What? How?
The goal is to freeze different stages of some fairly quick process (at least when compared to the time resulotion of the human eye). Since I don't have fancy toys like a high speed film camera, lack of good equipment has to be compensated with a smart setup applying just a an ordinary consumer-type digital camera and some self-made electronics.
Unfortunately, the camera's minimum exposure of about 1ms is too long for the purpose and furthermore the delay from pressing the button to actually opening the shutter is around a second - quite long. So, the basic idea is to make the complete room dark, open the shutter of the camera, drop the ball, ignite the photo flash after some time and close the shutter again. This way the real exposure time is given by the duration of the photo flash which was measured by me (using a DSO) to be less than 100us (better than 1/10,000 second) when set up accrodingly.
Dropping and sensing the ball
So, first we need to hold the ball and make sure it falls down each time at the same point, so using one's own hands is not an option. Instead, since steel balls are magnetic, we built a dropping device made of an induction coil. (In the image on the right, white paper is wrapped around the coil.) When switching off the current through the coil, the ball drops down. To prevent sparks and high voltage peaks, we were actually using a RC-damped MOSFET-based switch to slowly turn on/off the the current through the coil. Hence, using the switch time as start signal is not precise enough and a self-made light barrier was used. The light barrier consists of a orange LED and a photo diode. In front of the LED, a lens was introduced to focus the light onto the LED. Light interruption by the ball is the start signal. |
![dropping setup [13kb]](setup-dropping.jpg)
Flash delay trigger
After the start signal, some time passes during which the ball drops down and then finally touches the sand. This time is in the order of several 100ms for usual drop heights below 1m. I designed a delay trigger which will start running when the light barrier sends the start signal, wait some time and then fire the flash. The time can be varied in steps of half a millisecond (500us) using an impulse switch ("Drehimpulsgeber"). The intrinisc accuracy of the microcontroller (due to interrupt handling) is 4us and the base clock has a tolerance of less than 100ppm.
Below, you can see the circuit of the delay trigger including the light barrier and the required amplifier as well as the connection to the photo flash.
PNG image (717x527 as seen below): delay-circuit.png (17kb)
High-quality PDF: delay-circuit.pdf (96kb)
Permission to copy and use this schematic is hereby granted provided credit is given where it is due.
![delay stage circuit schematic [17kb]](delay-circuit.png)
The 3 LEDs connected to the microcontroller are a delay change indication for visual feedback when turning the switch. Additionally, the JP3 connector was connected to my self-made frequency counter to measure the actual delay time. There is some additional comfort like a switch which will enable auto increment mode where the microcontroller will increase the delay automatically by a fixed amount after each trigger event...
The microcontroller runs a self-developed firmware (written in C, compiled with avr-gcc) and was downloaded onto the controller using my USB-based Atmel programmer.
The complete setup
On the left, you can see an image of the complete setup. The camera is a Minolta DiMage7 with a remote trigger. Behind it is my self-made frequency counter. On the horizontal wood bars we mounted the flash, the MOSFED coil switch and the delay trigger board. The dropping device and light barrier are on the left. Since the experiment was made as a science summer school project in the small village Visnjan (Croatia), we had to live with what they had there. And since they did not have fancy equipment like metal bars or a good tripod, we had to be creative. So, as you can see, the rest of the setup is all made of pretty simple stuff like this blue-red-white string we found somewhere and used to fix nearly everything... Not on the picture are Quinn Burke and Slaven Glumac who operated the devices and produced all the photos. |
![complete setup [12kb]](setup-complete.jpg)
So, with the above setup, what was actually done: When all the testing is done successfully and everything is functional, the light is switched off. The cycle begins when the camera's shutter is opened and the ball is dropped by switching off the coil. As soon as the flash lights up, the shutter is closed again. Now, the delay is incresed by a fixed amount, the ball is put onto the dropping coil and the sand is levelled again by shaking the container. Putting the container back into its predefined position ends the cycle.
Additionally, the delay can be checked with the frequency counter (in time measurement mode). All that is done with lights switched off and it's best to have two people operating the devices.
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