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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: OT: Repeatably lobbing "projectiles"
Date: Thu, 21 Nov 2024 19:44:57 -0700
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On 11/21/2024 8:04 AM, Liz Tuddenham wrote:
>> The mechanism must be able to be loaded, "cocked" and released
>> without humans being involved.
> 
> Have you seen the devices for throwing clay pigeons that are 'cocked' by
> a crank on a lorry windscreen wiper motor tensioning a chest expander
> spring.  All very off-the-shelf stuff but not particularly accurate or
> efficient. (but bloody frightening when you are working on one and it
> goes off unexpectedly).

(Mechanical) efficiency isn't particularly important.  I can't
imagine the projectiles ever having any significant mass (as if
they DID, they would pose a risk to the environment)

>> I think re-SETTING that would be difficult.
> 
> In an air rifle it is allre-set by a mechanical linkage that operates
> from 'breaking' the barrel to reload, with small auxiliary springs that
> pull all the components into place automatically.  The user is unaware
> of any complication.

Yes, but a machine is going to have to do this "to itself"; load
the projectile, cock the mechanism (for a specific "throw range"),
then "fire" the projectile.  Lather, rinse, repeat.

>> The spring approach was relatively easy to mock up (despite being
>> a poor performer).  I used a linear actuator to move the spring
>> (effectively shortening -- cocking -- it or "resetting" to the
>> uncocked position).  The release mechanism could then stay stationary;
>> the spring pressing up against it while cocked and being retracted
>> (reset) once it had delivered its energy to the load.
> 
> Not exactly off-the-shelf, but have you thought about a linear motor?
> That would be eminently controllable as the speed of the projectile
> could be measured and corrected as it accelerates.

But that places constraints on the types of materials that can be used
in the projectile.

I keep coming back to pneumatics.  There are several children toys that
"fire" lightweight projectiles.  They, of course, aren't particularly
concerned with repeatability.  And, their projectiles are often so
flimsy that air currents can affect their travel.

I recall making "cannons" by taping together tin cans whose tops and bottoms
had been removed.  Squirting some lighter fluid in the bottom-most. Dropping
a tennis ball in and touching a match to a "firing hole".

Of course, that's an explosive launch but it is really just an expanding
volume of air doing the work.

I figure if I could pressurize a container to a specific pressure
(based on desired range) and then discharge that stored energy into
a chamber behind the projectile, it should eject it.  Then, repressurize
the container for the next "shot".  Select the projectile for the
most uniform flight characteristics.

This puts the key moving parts in the "release valve" -- hopefully
something that can be purchased with somewhat repeatable performance.

Then "calibration" is simply empirically building a table of
(pressure,distance) tuples (assuming elevation is constant; else add
a third member to the tuples).