User:Pranav Rathi/Notebook/OT/2011/03/01/Laser Shutter .2
The full detail is coming soon.
Motivation behind designing the shutter is speed, accuracy and variability (time). In our optical tweezers we need to center the trap over the tethered bead, to get the geometry right(because this affects the force measurement)and make our feed-back program run. To center the tether we need to turn the laser intensity on/off quickly for variable time intervals. To do this we used to use AOM, because it’s extremely quick (nsec) and variable. So I needed some-thing which can replace this AOM function. This shutter does it exact. It is fast with opening time of 4 and closing time 2 μsec. Since the shutter runs on/with the active voltage (controlled by the toggle foot switch), it remains active with the applied voltage, with the freedom to chose any active time. The speed of the shutter can also be controlled. In design the laser passes through an aperture, and the only moving part is the cylinder. No gears, and no electronics in the actual shutter, makes this design very stable and accurate, even under the heat produced by the laser beam. This shutter needs no special power supply, it can be run through a cell phone charge with an output of roughly 300mA/5V. Cost and construction time is also important. With this design, a shutter can be prepared under $40 with 10 hours of construction time (10 x 25 (hourly wage of a technician)=$250+40=$290). Still better than many commercially available shutter systems with same performance.
Design & Construction
There are three major parts of the system.
- Control box.
- Power supply.
The components used:
- 12V DC motor
- Wood rotation stage
- Spring with torque of .1 N m
- Rubber padding
- Pillar Post Extension, Length=1" from Thorlabs (shutter cylinder)
- 30mm Cage Plate Optic Mount from Thorlabs
- Post-holder, base-plate ext...
- 1 power jack M&F
- 1 1/4" mono Panel-Mount Audio Jack M&F
- 1 Foot Paddle
- 1 100Ω pot with, 1 220Ω resistor
- 1 on/off toggle switch
- 1 LED
- 1 box enclosure
- some connection wires, solder gun and solder wire
Any power-supply which can provide 300mA at 5V and above. The motor torque is power dependent and the shutter speed is resorting spring's stiffness (torque) dependent. So choose the spring carefully before decide on the power supply. I would recommend a variable power-supply which can be bought easily from any where.
The buildup is divided is the following categories. In the start make sure that you have a good set of screw driver, players, wire cutter & stripper and solder.
I will start with the choice of the motor. A 12V DC motor is a good choice because it can provide a wide rang of torques. The motor and the spring works against each other so it important to have the right set. So choose a 12V DC motor with a shaft length of at least 15mm. Mount the motor on the cage plate as shown in the picture, you will need 4/40 screws to tight it. once this is done, unscrew and take the motor out.
Now next task is to choose the right spring. The spring should be half the length of the shaft and the shaft should easily fit through the spring. I want to keep all this really simple, so there is a very easy way to choose the right spring. To choose the spring first need to know the torque of the motor. This is really simple, it can be mathematically calculate if the voltage and the current is known. But I like the experimental way; the setup is shown in the slide2. In the slide I have a DC motor with a leverage pivoted at the shaft and a free weight hanging with a string on the other side. In this, all I am doing is balancing the weigh at that point by giving the motor just enough power. so the torque is equal to the weight at the distance from the shaft:
Once this is know we can choose the spring with less torque than this. The restoring torque applied by the spring can also be measure in the same fashion (as motor) as shown in slide1, with hanging known weight. The spring stiffness and the torque can be related through:
Where K is the stiffness, r distance from the shaft center and n number of times spring twisted one complete 360 degrees circle before it was hacked. Since the torque is n dependent, it is a matter of great convenience, because now it is really easy to pick the right spring. All you have to do, is twist the required number of times before hacked to get the right restoring torque.
Now we are ready to put the the shutter together. Start with the motor, most of the motors have little holes for the screws to hold slide3. Choose a screw which can fit into that but still sticking out few millimeters slide4. Next is the rotation stage which joins the cylinder to the motor and holds the spring assembly. I choose the wood for this function, because its easy to machine and a good thermal-insulator slide5. I drilled two hole on both sides; to fit the shaft and 1/4 screw on either sides. To drill for 1/4 I used 15/64 size drill-bit slide6,7,8 & 9.