Window-wiper motion

[GeeTwo]

If you want to use a motor, look at window wiper linkages for inspiration. A four-bar linkage with uneven bar lengths can be made to do some interesting tricks, including this reciprocating motion - with the motor just going forwards.

Then again, in college I drove a 1970 Hornet with pneumatic (vacuum) wipers.

[MrBasse]

If it will only have 90 degrees of rotation, I would look at a potentiometer over an encoder. They are cheaper and can be used with any motor/gearbox combo pretty easily.

We have used single turn and ten turn pots pretty much every year to control simple systems.

How much force do you need? What kind of speed are you looking for? I like window motors for simple motion since they're cheap and easy to come by, but they are hard to interface with. You could also use a window wiper motor...

[MrCamilleri]

Think raising a flag--that sort of purpose. Although I should mention that there are two of these "flags"https://drive.google.com/file/d/0B7v...ew?usp=sharing