User:Pranav Rathi/Notebook/OT/2011/03/01/Laser Shutter .2
Motivation
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
Components
There are three major parts of the system.
- Shutter.
- Control box.
- Power supply.
The components used:
Shutter
- 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...
Control Box
- 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
Power Supply
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.
Construction
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.
Shutter
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:
- [math]\displaystyle{ \mathbf{\tau}=r \times F = r \times m .g }[/math]
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:
- [math]\displaystyle{ \mathbf{\tau}=2{\pi}n.K.r^2 }[/math]
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. Now the motor can be attached to one side of the stage and cylinder to other slide10 & 11.
Now we can machine the spring assembly. Slide 12 shows the parts of spring assembly. We need two metal strips of .75 and 1 cm long with a hole in the end for the screws. This strips with the screw on the motor will define the boundaries of the movement. Cylinder will rotate end to end between these strips. To screw the strips directly on the stage we need to drill two holes for the screws. The holes are 135 degree apart over the face of the stage as shown in the slide13. We also drill a small hole for the spring attachment near the edge.
Now all parts are ready to get together slide 14. This shutter is little different of what is shown in the video but follows the same concept. Put the shorter end of the spring in the hole on the stage like slide 15. Now the motor shaft will go through the spring into the stage. The other side of the spring is hold against the screw. You can twist the required number of turns before it holds to get the right restoring torque. Now the spring wants to move the stage clockwise but the strip against the screw does not let it (this is position 1). As the voltage is applied the motor turns counterclockwise working against the spring until the second strip stops it. When the voltage is applied the stage remains in the second position. As soon as the voltage is turned down the spring restores the stage into the previous position. These two positions can be chosen for the shutter to be on/off slide 16,17 & 18.
A fully assembled shutter is ready. Now the next task is to prepare a control box.
Control Box
I used a 2X3 inch aluminium box to house the electronics slide20. The electronics is really simple. The circuit is shown in the slide21. The foot switch is used to connect the motor ground to the power-supply hence activates the shutter. Since the shutter works on active voltage, the voltage is always on, when the foot paddle is active. So it is very important to give the right voltage to the motor to avoid damage to the motor over the long duration of active voltage. To do this i use a 100 ohm POT. POT let me choose the right voltage and current. This can be done once the shutter is ready.
Now connect the shutter to the shutter input and foot-paddle to its input.
Power Supply
The good thing about the system is that it can use any power supply which can give enough power to operate the motor. The POT inside the control box let choose the appropriate voltage and current and hence makes easy to choose a power-supply.
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