To raise the arm I needed a very powerful servo so I used a geared servo drive to boost the servo's output torque. These servo drives use an external potentiometer attached to the output shaft drive gear for positional control. A thick Aluminum arm was modified and attached to the output shaft. The lower cannon arm end was then bolted to a large 1/8" thick Aluminum base plate and the geared servo drive was mounted underneath the base plate. The swivel link from the cannon arm was then attached to the servo output arm. 

Later on during testing I found that while the cannon raised beautifully the weight of the cannon still caused it to drop too fast and it had a jerky motion to it that I didn't like. To solve this I added a brass arm to the servo output shaft and attached an extension spring to it to act as a counterbalance in order to reduce the load on the servo and smooth the lowering motion of the cannon.