Hello Folks,

It has been a long time I posted anything to the forum. This time I am building a robotic cannon that shoots projectiles.

Although I figured out the structure of the arm that will support the assembly, I cannot figure out how to actuate the joints and how to make a reload mechanism.

The cannon takes a location input from the user and should point there and shoot on its own. I figured out the programming but cant get ideas for the above problems.

Any ideas suggestions will be of great help.

Please find attached the pics for reference. The two joints should be evident

Don't mind me leaving a comment here, I would love to see how this develops.

Cool! What kind of propulsion system will the projectiles use?

Have you tried using the linkage mechanism used in RC airplane control surfaces? Standard servos should easily be able to handle the load of your cannon.

some 41 or 40 gm servos should take care of that. . what would be the projectiles?

but this may be slow as i am doing this in my spare time which is almost infinitesimal

Yea i thought about this, but seems like it is not all that straight forward as i have to align the servo axis with that of the joint. But if you have any ideas about the same, I would be happy to know

Copy Swapnil as well:  I currently consider two ways to propel the projectiles : 1st using springs and the other using rubber bands. Re calibration would be an issue with rubber bands and hence i am more inclined towards a spring loaded system. Therefore I need ideas to design a reload mechanism.

@Tanmay: The projectiles are gonna be table tennis balls initially. but the long term plan is inspired by this small story that follows:

One of my professors of robotics at the university here in Darmstadt, Germany offers snickers to students who ask smart questions or answer to his questions. The thing is that he throws the snickers towards the participants and being human, misses most of the times. He therefore once playfully said that he would love to have a robot that can do it for him. I thought i will make one and gift it to him on Birthday next year. Hence the effort. Also it will be fun to make this. So the cannon will shoot snicker bars as an end goal. I understand the weight difference of the projectiles in case you have a doubt.

wow tasty project .
For reload mechanism look at electric airsoft guns . Or you could use gravity fed magzine  and rotary hook attached to a 10 rpm geared motor .

@rockmania.89,
Not hard to do.
Ignoring the firing mechanism for the moment, what the canon needs is traverse and elevate controls.
The structure is one vertical that can rotate horizontally, (call it the pillar), and
One horizontal that can pivot vertically (call it barrel, or barrel mount)
The direction the barrel points can be called 'forward'

In the pillar, you could cut two slots for servos each side of the forward direction.
Insert servos and secure with hot glue.
The pillar needs to be on a base that can rotate, which in turn is mounted on a fixed base.

One servo is for traverse. It's servo arm needs a pushrod, the other end of which is anchored to a point on the 'fixed' base. Call this the rudder or aileron servo.

The other servo is for elevation. It needs a pushrod connected to its servo arm at one end, and to the barrel/barrel arm at the other. Call this the elevator servo.

Now if you have a typical airplane RC system, connect the traverse servo to channel 1, and the elevate servo to channel 2 on the Rx.
The right stick on the Tx (assuming mode 2) will elevate/depress the barrel on being moved towards/away from yourself.
Moving the stick sideways will turn the barrel left/right.

Other channels on the RC. Can trigger the firing mechanism.

If your query is a genuine doubt, i hope this helps. And i'm available for further assistance.

If its just a joke to check out RCI members, please laugh off my attempts to help!
Regards

haha well this definitely is a genuine query. You can be sure of this.

I am not sure if I understood your description clearly. Can you add a sketch or something? Also I am not controlling this with a radio. I have made a front end application using wx python and using an arduino as the brain for the cannon. The code that i wrote takes a user input for the seat number in the class and points the cannon to the corresponding angles of azimuth and elevation and then boom!

having a precise control of the joint angles is key here. I was also thinking about stepper motors in the joints.

Please share your ideas on this and also the sketch explaining your solution in the previous post. Maybe it solves my problem of mounting servos.

the reload has to be automatic. that is the challenge. Can you elaborate the use of a 10 rpm geared motor that you mentioned?

And here i was, thinking you'll provide GPS coordinates for the target...
 I'm just an analog monster from the past.
Thinking of mechanical/elec solutions.
What you want is digital youngsters.
Come on, RCI guys. Offer a solution.

You can also consider actuating a pvc air cannon:
http://m.instructables.com/id/Launch-supersonic-spuds-with-a-PVC-Air-Cannon/

Though it wont be self sufficient and after few uses you will have to add a new air canister.
The spring system can be reloaded by making a continuos rotation servo and then linking it to the spring via the launch plate. So you can make a mini servo winch that pulls back the launch plate and engages/disengages using another servo.

@ KKIyer: well firstly, since i forgot to do this in my previous post, thank you for taking your time out to type all that.
the GPS thing sounds like a good idea, since i can then fire gifts all around the world then and become the first modern day brown santa.

I actually figured out the digitals of the project, Where I am actually stuck is the mechanics. In a nutshell, I need ideas to actuate my cannon joints. I think this is a problem of mechanics than software since i am sure i will be able to make whatever moves my joints via my code. The problem is getting the joint moving. This is where i need help from the forum considering folks here must have faced similar challenges.

In the meanwhile i changed the assembly by simply combining the same pipes in a different arrangement.
Actuated the elevation of the boom and have a similar idea for the azimuth.

Please refer to the pics below. However the design is still open for suggestions in case you guys have one. I also need ideas for a auto reload mechanism as the spring tension will be variable based on the distance dialed in by the user. hence i need to figure out how to set variable tension and the lock and fire mechanism so that a human is not needed for that.

Once again ideas, suggestions and inspirational pointers will be cherished!!

i thought of this winch system as well. just sounded a lot of work to me. but it is on my list if i find nothing else.

Firstly how is the firing mechanism of the cannon.complete the cannon with it, you can then think of auto reloading...it would mean getting the trigger lever to its original position.

By the ways which part of thane do you live...i am in Vasant Vihar

My family is in Kopri. I am in Germany since some time now.

surely after seeing you're size of the lever arms only a 10 or 5  rpm geared down motor will work . servo would not withstand that large force of long arms . you'll need motor driver's and an interface for that

use metal or cf / glass fibre.  joints and hinges.  some CNC aluminum Wii do

You are in Germany and want to gift a German guy a cannon! Is it just me who finds this funny?
I'm not being a douche. I know you want to gift your professor, which is really great. It's just that the wacky center of my brain finds it funny giving a german guy a cannon! Aren't those guys all born tank commanders?   

Hi Rock. Quite Tedious but interesting project. I came up with one solution in my mind. maybe it ll help you out. But if this concept works then your loading and firing mechanism ll be quite simple. All you need to do is to find a high torque or say high pressure solenoid coil. Connect the Solenoid at one end of the barrel. Once you power it the shaft of the solenoid coil will move with a kick, and the kick produced will be good enough to shoot your snicker. Provided you ve to search for such a solenoid that should produce enough torque to kick the snicker out of the barrel upto a certain distance. More distance can be covered by adjusting the angle of the barrel. The loading mechanism is more simple, as it works on Gravity Feed. You need to design a Magazine similar to that of a Rifle Magazine, wherein the Snickers are going to be your bullets. cut a hole into the barrel similar to the size of the magazine. fill the magazine with snickers and place it onto the hole of the barrel such that the last snicker has to be inside the barrel. Once the solenoid shoots the snicker inside the barrel, the snicker residing over the last snicker will take its place. This seems a bit simple, but it ll be a tricky job in itself, coz the precision level has to be very high. But I guess this ll work out if carried out properly. And for the Barrel rotation mechanism its better to use Stepper motors, coz you can set precise angles. Also keep updating the process. Thanks.

@rockmania.89,
As an alternative, you could consider a trebuchet...
Intended exactly for your kind of objective!
Regards.

Or a catapult  .........

may be that works too but the cannon looks cooler