NERF OR NUTHIN'
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Toy mechanism hacking
There are several approaches to creating the weapon system for the NERFBOT contest, from using an unmodified NERF gun, modifying a NERF gun, or even creating your own design.
This part will describe the modifications required to change a standard NERF gun into a robot weapon system. I have noticed a lot of the newer NERF guns are spring loaded. The modifications here apply only to the guns that are the air pumped types.
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This is a standard 4 round, single shot, and manually rotated NERF gun. Pretty much the lowest variant. The rules allow for 6 shots, but the techniques shown here can be applied on other NERF weapons as well. |
Fig 1: 4 Round, single shot NERF gun
Disassembly:
Typically, the NERF guns are put together with screws, but most seem to have at least one or two caps that are glued or epoxied on. The gun above, the red cap at the rear must be pried off. Don't worry about damaging it, as it is not used.
A dremel tool makes fairly short work of the frame leaving only the minimum necessary frame left behind. You really only need to keep the barrel assembly, and the portion of the gun to mount the barrel. (I need a better picture here of the assembly)
Once that is done, we can begin to put things back together. This will result in a fairly small profile gun that can be fitted onto a platform of choice.
A typical platform is made out of extruded plastic used in sign shops under a product name of Syntra. It is light, easy to work with and fairly durable. It is quickly becoming the material of choice for robot makers.
Now, we need to decide on how to turn the barrel, and fire the weapon. The barrel is normally manually rotated until all positions are used. One or two ideas can be used here, either a solenoid or servo to pull the barrel through a mechanical linkage, or a motor and some sort of position sensor.
This particular model has no mechanical linkage, so a motor and some sort of belt seems to be the easiest way to do this. The motor must be mounted in parallel with the grooved area at the rear of the barrel, and a belt wrapped around to the motor assembly. A stepper motor would be a good choice, as once you determine the number of steps required, no sensor or any other interface is required.
Laser Detector Design:
The key to detection of other Nerfbots isn't the laser but the reflective strip. The reflective strip has the property of being retro-reflective. What this means for the detection is that when the laser hits upon one of these strips much of the light will be reflected nearly back to the source. The laser is a convenient source of collimated light. Other sources could be used but it would require lenses to bring the light into a focused beam.
Reflective strip: Scotchlite Reflective Strips by 3M (Conspicuity sheeting)
These next two pictures show how the light is being reflected back close to the source. The retroreflective strip is about 3 meters away. The laser can be seen at the bottom. The white tube in the middle is made of paper and is to help demonstrate the reflection pattern. There are two black marks on the tube to show where the phototransistors in the detection circuit are placed. The laser dot on the wall only moved less than 1cm from OFF the strip to ON the strip. At the detector the change in reflected light is intense.
Laser OFF target strip:
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Laser ON target strip:
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Detection schemes:
- Place a phototransistor close to the laser (1cm) and amplify its output. Compare the output to a threshold. Adjust threshold to distinguish when reflective strip is being hit.
Pro: Simple
Con: Threshold would vary with room lighting. On a robot the view is moving and the threshold would need to be varied depending on what is in the view. The flicker of overhead lighting would also interfere.
- Place a phototransistor close to the laser (1cm) and amplify and high-pass filter its output. Modulate the laser at an high enough frequency so it will no be blocked by the filter. Compare the output to a threshold. Adjust threshold to distinguish when reflective strip is being hit.
Pro: Hi-pass filter removes sensitivity to lights flickering and variations in the scene as the robot roves.
Con: Sensitive to higher frequency modulated light sources such as infrared remote controls.
Con: Sensitive to reflection from other objects such as white walls, but to a much lesser extent than the reflective strips. The effect is that a very close range the detector starts to pick up light coloured objects that are illuminated by the laser.
- Place a phototransistor close to the laser (1cm) and a second phototransistor a short distance away (4.5cm). Amplify and high-pass filter the difference in output of the two phototransistors. Compare the output to a threshold. Adjust threshold to distinguish when reflective strip is being hit.
Pro: Not sensitive to non-retroreflective objects such as white walls.
Pro: Not sensitive to higher frequency modulated light sources such as infrared remote controls since the light is going to hit both phototransistors.
Circuit of detector using scheme 3:
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This section is under construction
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