[TriEmbed] Question concerning a mechanical release

[Pete Soper]

Hi Lucas,
    Please "reply all" to this msg to let us know if the information so 
far has given you any ideas for a solution, and if so, what your 
implementation scheme will be. Also, if you're OK with sharing the 
details, we're eager to know what the string is connected to,  if the 
string release happens periodically, etc. If circumstances have changed 
or something else has made this project of yours moot, no worries, and I 
don't want you to feel any pressure to reply. It's just that some of us 
are very curious. :-)
    I'd like to summarize a few high points I remember from the msgs so 
far. Folks: I've only read a few msgs of this thread, so forgive me if 
this is partially or entirely redundant.
       1) Raspberry Pi gpio pins are *VERY* weak, only being able to 
source/sink a few milliamperes of current, in a strictly "3.3v power 
supply" context. That is, that pin cannot tolerate negative or >3.3v 
potentials, cannot be expected to hold a large current at ground, 
properly drive a high current load, etc. Part of the "0 < voltage < 3.3" 
requirement is that no inductive load can be allowed to dump it's back 
EMF from a magnetic field collapse into the Raspberry Pi pin: that will 
almost certainly render that part of your Raspberry Pi inoperable if it 
doesn't destroy the whole chip. So, for multiple reasons an RPI can only 
directly sink/source ("provide a ground for"/"drive") a few devices such 
as another integrated circuit or bare transistor or a (low voltage) LED 
with a suitably large value resistor for current limiting.
      2) Driving something like a solenoid, stepper or plain DC motor, 
relay, etc, requires a circuit that can "translate" the Raspberry Pi 
signal/sink into a relatively high current and usually higher voltage 
capable of running the device.
      3) Servo motors are special. In the context of this thread (i.e in 
regards to the "hobby-type" servos Rod Radford mentioned vs an 
industrial servo), they are self-contained motors that have a very low 
voltage, low current drive requirement easily met by a Raspberry Pi and 
conveniently works in conjunction with off the shelf software capable of 
causing the servo motor to hold position, move a fixed distance one way 
or the other, rotate continuously in one direction or another, etc.
    "String grabbing" would seem to be a special case of the "rope 
grabbing" someone described vis a vis sailboats, so emulating the 
sailboat device on a scale appropriate for your string and weight would 
seem sensible. Using a servo motor for the mechanical end of things 
would allow you to focus on your application and not get bogged down 
with hardware/software details.
   I look forward to seeing you and the others who have contributed to 
this thread at a future meeting. The next mtg at NCSU is Monday, October 
8th.

Best Regards,
Pete (home from UNC hospital and recovering from step one of a 
long-planned two part heart-remodeling project)

On 9/23/2013 6:57 PM, Lucas Rumney wrote:
> I am trying to make something that would achieve the following end:
>
> raspberry pi being able to release a weight on a string.
>
> My idea was that the pi could send a signal to disengage some kind of 
> clamp and release the weight, but I am not sure what kind of clamp or 
> what I should use here?
>
> Does anyone have any better ideas how to release a weight on a string 
> controlled by a pi?
>
> Thanks, I can't attend this meeting because of my courseload, but I 
> still want to participate in general discussions.
>
>
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