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Project a object at variable distances/trajectory

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If you can fix the object weight, this will be simpler. If you can say "object will weigh 45 grams plus minus 3 grams" for example, then you don't need to include scales in the launcher.

If the object is ferromagnetic, you can build a railgun-like launcher. If not, you will need to either build a slingshot-like launcher with a spring of some sort, an actuator to pull it, and a release to release it, or you will need to do something pneumatic. Pneumatic might actually be simpler. You'll need to measure the kinds of force that can be produced, and calibrate your control appropriately.

Another kind of actuator that might work is one or more rubber wheels that give something an initial velocity; this is used by some auto-firing Nerf guns for example. It depends on the shape of the object. Or you might be able to use some kind of sled that mates properly with the actuator.

The pivoting is simple with two servos (heading and pitch.) Make sure to get reliable, strong servos with good control, such as Dynamixel AX-12 or MX-28. The math for how to change the firing force based on angle and distance is very simple trigonometry and calculus, which is easy to put into the coding microcontroller.

To build a system like this and make it robust, expect to spend a lot of time doing trial-and-error and go back to re-work things based on learnings. Also, if the cost of the final components is $X, expect to spend at least 3 * $X in various research/testing.

The easiest way to do is would be to mount a compressed air cannon on a turret.
Have an MCU controlling everything.
A light sensor would tell it when the object is loaded. You place a infrared or laser emitter to one side of tube then the sensor is on other side. When object is placed in tube it breaks the signal letting the MCU know object has been loaded.
You will need an air compressor actuated by relay, pressure sensor, and a solenoid valve. The relay and solenoid valve are controlled by the MCU and the pressure sensor tells the MCU when the right pressure is achieved.
Turret position could be achieved by servo or stepper depending on how large your cannon will be.
The air cannon assembly can be made of PVC tubing or metal pipe. KNOW your PSI safety limits. DO NOT EXCEED!
Google DIY air cannon or just go to instructables.

Getting the object to go the distance you want, you will either have to do the math or just keep firing your object and then changing your programming to get the desired PSI and inclination of the barrel.

Your programming would look something like this:
1. Object Loaded?
2. True. Turn on air pump. Pump. Receive PSI from sensor. Pressure reached? True- Turn off Pump
3. Move turret to random position within specified scope.
4. Fire. Activate solenoid valve.
5. Repeat.

This sounds fun, might actually build one myself.

This is awesome guys! Thanks for the advice.

@jwatte - Yes the weight can be 45 grams plus minus 3 grams.

@Azraels - The whole "Unit" I expect to be no more than 8" x 8" and height not an issue....would this be possible with the components you describe?

I dont see why not. Just take what designs you find on Google and scale down.  But I would go with the metal piping instead of PVC for something that small. Make sure you check the rated PSI. Also do not allow your programming to turn the turret to face you. Allow for a safe zone where it cannot point.  :P

An air cannon works great if you have reasonable uniformity in the object size/shape. If not, then you can use an air cannon driving a sled that you load the object in -- more like a piston at this point.


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