[Boom Master]

Building a belt fed, autoloading Nerf cannon for my tank scenario play.

Use of a step motor seems an obvious way to align the cartridge with the barrel bore. Have to lift 4.1 pound of belt 36 inches vertically onto a sprocket with a 3.5 inch radius. Calculates I hope to be 2020.66 Oz, inch
Operates off a 12 volt DC system.

The sprocket has to rotate at a rate of 30 rpm intermittently to reach the rate of fire goal of 3 Nerf rounds per second...

So far, the cost is prohibitive. 5-6 HUNDRED dollars for the motor and controller. And I have no idea how to put it altogether and make it work electronically and program wise.

So I need a reality check for cost. Anybody KNOW how to do it and would for a fee?

If I can reach the same performance goals for less cost, and within my skill list, I would prefer to go there. A geared motor winding up a torsion or spiral spring comes to mind as that was the principal of my 6 shot autoloading magazine Nerf cannon which was manually wound up. I already know HOW to index it mechanically.

Any thoughts?

[GhostJedai]

Use a standard DC motor with a rotary shaft encoder. Same thing, 1/10 the price.

You might have to opt for something with Low RPM/High Torque, but it'll get the job done.

[Boom Master]

I gave up on step motors and settled on a 12 volt motorcycle starter.

So I have a high torque and supposedly it is a geared down motor. When I did a trial just to see if it had enough power to lift 5 pounds of belt, it zipped 5 feet right through it in a second. Plenty of power and too much speed.

I suspect if I had the indexing system engaged, it would have torn itself apart.

So what IS a "rotary shaft encoder" ? Internet worked fine. First glance, they cost the same or more than step motors. Have no clue in reading so far how to control it, how it works, Does it draw current continuously?

I will have to turn the motor off and on to rewind the torque spring. Thinking some sort of sensor on the sprocket that will turn the motor on and when it passes it again, turns it off. It will rewind only one revolution at a time.

To slow it down, I have to find out the amps it draws and figure out the resistance needed to lower the voltage. That would work Right?

[*knock-out05*]

Do you have the right shoes on it?

[GhostJedai]

Originally Posted by Boom Master

So what IS a "rotary shaft encoder" ? Internet worked fine. First glance, they cost the same or more than step motors. Have no clue in reading so far how to control it, how it works, Does it draw current continuously?

Well, the Shaft encoders I'm used to are small visual discs that are attached to the shaft itself, with another portion anchored to a static space (be it a bushing or chassis). It simply reads the rotational velocity of the shaft, and typically, will have an output to the motor that you can drive with PWM signals, similar to a small stepper. Here's a decent example: http://www.dfrobot.com/index.php?rou...4#.UbtkjfmG3nE


Okay, I've done some calculations (guided by this: http://www.leadshine.com/Pdf/Calculation.pdf ) and I've figured that if the total load is around 1lb, you should be able to get by with a motor rated at 51oz*in, or .3575N*m.

As such, this motor should work for you: http://www.phidgets.com/products.php...uct_id=3260E_0