Rotating
Mirror - Method
Tim Reibold and Brandon Winterling - 5/02
Method and Some Setup
Click
to Go to:
How
the Rotating Mirror Works
Measurements and Calculations
Setting up the Computer - Hardware, Software
Camera Setup
Using the Program and Taking pictures
For Pictures of our Setup Please see our Diagram
Page.
How
the Rotating Mirror Works
See source for a very
detailed description:
"Stroboscopic
Study of High Speed Projectiles in Water." Matthew Hinshaw, 1991
Overview
By aiming the camera at a rotating mirror one can get pictures that
separate an event both by time and space. This is the method we used to
get the progression of of our paintball exploding against the Plexiglas
and ring stand.
The mirror overlays images on the film as each of the flashes go off. It
has to be rotating so that the part one wants to see is reflected upon a different
part of the film to prevent the overlay of the event one is trying to
capture. To achieve this, everything has to be black except for the event
that is being captured.
The
camera is aimed at the rotating mirror so that one can see the event by
looking through the camera into the mirror. We referred to the angle where
the you can see the event through the camera as the effective mirror
angle. Once this angle is set, the rest of the setup is getting the timing
right. The mirror has to break a laser which will start the program 'ball
and mirror' running on the computer. As the mirror rotates to its
effective angle the gun is fired, the ball accelerates, and then comes into position. Just as the mirror is rotating to its effective angle the
ball has to be in the right position and the flashes go off. The below
timeline may help as a visual:
Laser is
broken Solenoid
is
Ball Flying and Mirror rotates
to
Complete;
on mirror
box activated, gun fires
Mirror
Rotating
position; first
flash fourth flash
|-------------------------------------|--------------------------------------------------------------------------------------|-------------------------------|
0
0.073 s
0.218 s
0.220 s
If any of the timing measurements are off by more than a couple milliseconds the ball will not even be in the picture. This presented us with a problem as the gun did not shoot at the same speed every time.
Measurements
and Calculations:
All measurements are given for our last photo shoot where we took our best
pictures.
Barrel
of Gun to Plexiglas:
1.0 m
Importance: Used to determine how long the
paintball would take to reach the Plexiglas.
Average
Speed of Paintball: 34
m/s
Importance: Used to calculate the time the
paintball took to come into position.
Average Acceleration Time: 81.8 s
Importance: We measured the average
velocity of the ball. However, that velocity does not apply to the first few
seconds where the ball is accelerating to that velocity. We measured
acceleration time by using the computer to time how long it took for the
ball to reach a photogate at the end of the barrel once the solenoid had
been pulled.
The acceleration time
and the time the paintball takes to go from the barrel to the Plexiglas
can be added together to find the time it takes the paintball to reach the Plexiglas once
the computer pulls the solenoid trigger on the gun.
Frequency
of Mirror: 4.8 rotations/second = 1 rotation every
214 ms
Importance: We have to know how long it
takes the mirror to rotate around to its 'effective angle'. In our case,
the effective angle was reached when the mirror rotated a little more than one rotation
once it had broken the photogate. So, we used the time it took the for
the mirror to rotate around once plus a few more milliseconds as the time
for the mirror to reach its effective angle.
Center
of Mirror to Plexiglas: 1.0
m
Importance: No calculations were done
with this measurement
Camera
to Mirror: As close as possible
(8 cm) so the image of the paintball
exploding would be as large as possible.
Computer
Setup
Hardware
A circuit diagram of the Blue Box can be found in Hinshaw's
paper (source at top of page). This box was
hooked to the computer and to the flash units through a ribbon cable. The
flashes could be turned on an off using switches 2 through 5 on the blue
annunciator box. Switch 1 controlled the laser input from the rotating
mirror box. The solenoid was plugged into the wall and hooked to the
safety button optoisolator. The safety button optoisolator input was connect to Pushbutton 1 (top-middle) of the black
Pushbutton Box. Four wires connected this Pushbutton box to the Blue
Annunciator box.
The solenoid was mounted behind the gun so that it could pull the trigger. We built a stand and clamps so that the solenoid did not move when it pulled back on the trigger. The plunger of the solenoid (the part that pulls) was connected to the trigger by tying a piece of wire to the trigger and attaching it to the plunger. This made it possible to pull the trigger of the gun using the computer.
Software
Program: Ball
and Mirror on the Multiplex + Demo Cal.Prg. Disk
Ball and mirror is a program that simply delays the flashes and pull of
the solenoid based on user input. The program begins sequencing once
Pushbutton 1 has been hit and the mirror breaks the laser beam.
Inputs
Mirror Delay:
214 ms
What does it mean: Time it takes for the
mirror to rotate to the start of its effective angle. Essentially, this is
how long after the mirror breaks the laser and triggers the photogate to the time the first flash
goes off.
Flight Time: 111 ms
What does it mean: Time it takes for the
paintball to reach the Plexiglas once the computer pulls the solenoid
trigger. This is the Acceleration time plus the Time for the ball to go
from the barrel to the Plexiglas.
Interval: 0.85 ms
What does it mean: Time between flashes. You
must make sure that all the flashes go off while the mirror is rotating
through its effective angle.
Synch Time: Mirror Delay (-) Flight time. This will tell the computer how long after the mirror breaks the laser beam to pull the solenoid trigger. YOU MUST MAKE SURE MIRROR DELAY IS MORE THAN FLIGHT TIME. IF YOU GET A NEGATIVE SYNCH TIME THE PROGRAM WILL SHUT DOWN.
| Camera | Nikon D1 |
| Flashes | 4 Vivitar 283s |
| Flash Duration | Minimum (approx. 1/30,000 s) |
| Lens | 55 mm Micro-Nikkor |
| F-Stop | 2.8 |
| Shutter Speed | 1 s |
| Focal Length | 55 mm |
| ISO | 200 |
| Resolution | 2000x1312 |
We used a remote trigger and monitor while taking our pictures. We used the remote trigger to open the shutter of the camera because pushing the button on top would make the camera bounce while the picture was taken. We used the monitor to give us a bigger, brighter preview of the picture we had just taken. When using a monitor for this is tends to make the images appear brighter than they actually are.
Using
the Program and Taking Pictures
Preparing
1) Let the mirror spin for a few minutes so that it reaches a constant
speed.
2) Make sure the gun is loaded and the laser on the mirror box is aimed at
the photogate.
3) Input all values into the computer and hit "[4] Call timer."
This prepares the computer for input from the laser on the mirror box.
When PB1 is depressed on the input box it will start the program the next
time the laser is broken by the mirror.
Taking a Picture
1) One person turns off the lights and opens the shutter on the camera.
2) When the person at the computer hears the shutter open they depress PB1
to start the program and hold down the safety switch on the solenoid
optoisolator (For people doing this at S&M it is a little black box
referred to as the Madman Optoisolator; named after its creator, Madman).
The computer will not be able to fire the gun if the optoisolator is not
enabled, so it is important that this is pressed when PB1 is pressed.
3) If everything goes right the gun will fire
and the flashes will go off at the right time.
Troubleshooting
1) If the event is not captured but the right background for the
picture is visible (i.e., you see what the bullet is supposed to hit but the bullet is
not there), then the flight time simply needs to be adjusted.
2) If the background where the event is supposed to take
place is not visible, then mirror delay needs to be adjusted so that the flashes go off
while the mirror is traveling through its effective angle.
3) If only one piece of the action is captured on the film or the images
are too close together, then the interval needs to be adjusted.
Good luck with your pictures!!
All
photos on this site are copyrighted by Brandon Winterling and Timothy
Reibold, 2002.
To inquire about picture use please contact
Loren Winters at winters@ncssm.edu