Intro: Fast Shutter Foot-switch for Lasers
Introduction and Motivation
Motivation behind designing this shutter is speed, accuracy and variability (activation-time). In our optical tweezers we need to center the trap over a DNA-tethered bead, to get the DNA-overstretching geometry right (this affects the force measurement) and make our feedback program run. To center the tether we need to turn the laser intensity (trap) on/off quickly for various time intervals. To do this we used to use AOM (acoustooptic module), because it’s extremely quick (nano second on/off time). But AOM has some inherent oscillation problem which makes it use as a switch, unsuccessful. So I needed something else, (a shutter) which can replace this AOM function and can be controlled through a foot-switch (paddle).
This shutter does it exact and it is fast (activation-time) with an opening time of 4 and closing time of 2 μsec. The shutter runs on a +5 volts voltage. It moves to an on-position with an active voltage (controlled by the toggle foot switch) working against a restore-spring and remains on when the voltage is applied and turns to off when the voltage is removed. This gives a freedom of choice for the duration shutter is active with very simple electronics used to control the speed (activation time.)
In the design the laser passes through an aperture on the only moving part a cylinder. No gears and no electronics in the design make it very simple, stable and accurate, even under the heat produced by a laser beam. The shutter needs no special power supply and can be run on 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.
Step 1: Design & Construction
There are three major parts of the system.
The components used:
12V DC motor
Wood rotation stage
Spring with torque of .1 N m
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 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.
Step 2: Construction
The buildup is divided is the following categories. In the start make sure that you have a good set of screw driver, pliers, 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 range 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 or platform as shown in the picture, you will need 4/40 screws to tight it (in present setup). 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 the spring-choice procedure really simple and easy. To choose the spring first need to know the torque of the motor. This is really simple; it can be mathematically calculated if the voltage and the current are known. But I like the experimental way; the setup is shown in the pic 1&2. In the picture 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:
torque= radius X force = radius X mass*g
Once this is know we can choose the spring with less torque. The restoring torque applied by the spring can also be measure in the same fashion (as motor) as shown in pic 3&4, 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.
There is even easier way to deal with this mambo-jumbo; get a motor get a spring and put them together with trial and error.
Now we are ready to put the shutter together. Start with the motor; most of the motors have little holes for the screws to hold pic 4&5. Choose a screw which can fit into that but still sticking out few millimeters. 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 it’s easy to machine and a good thermal-insulator . I drilled two hole on both sides; to fit the shaft and 1/4 screw on either pic 6-10. To drill for 1/4 I used 15/64 size drill-bit. Now the motor can be attached to one side of the stage and cylinder to other pic 11-13.
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. These 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 pic14-16. This shutter is little different of what is shown in the video/pictures but follows the same concept. Put the shorter end of the spring in the hole on the stage like pic 17. Now the motor shaft will go through the spring into the stage pic 18&19. The other side of the spring is hold against the screw pic20. 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 . In end chop/wrap the left over part of spring on the screw and mount the motor.
A fully assembled shutter is ready. Now the next task is to prepare a control box.
I used a 2X3 inch aluminium box to house the electronics pic 21 & 22. The electronics is really simple. The circuit is shown in the pic 23. The foot switch is used to connect the motor ground to the power-supply hence activates the shutter pic 24. 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.
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.