Pratt RC ejection system fails me twice at Black Rock!

Pratt RC ejection system fails me twice at Black Rock!

Post by The Silent Observe » Sun, 11 Aug 1996 04:00:00



Quote:


> >HELLO
> >ALL!!!!!

> >In regards
> >to the
> >following

> >>This type of thing is very common in other arenas of RC hobbies. Radios and
> recievers use pulse code modulation (PCM) to encode the signals so they aren't
> interfered with by other things on that freq. I've seen two airborne RC planes
> on the same freq., both with PCM.

> A little while ago I asked about the PCM systems at my local hobby shop and
> was told similar stories as well as stories about the remarkable air range of
> these systems.  For a FM PCM 2 channel system complete I was quoted
> approximately $80.  However I don't want or need the mechanical servos.
> Instead I wonder if someone makes an electric switch or relay for this system.
> Or maybe I could simply power up an electric match right off the receiver once
> I determined how the receiver powers the servos??????????

> If relyable this sounds like a great way to control the complex rockets like
> multiple stages.

Check back at the hobby shop, and pick up a copy of R/C Model magazine
while you're there -- look for the ads.  You should see several ads for
servoless switching banks that operate off a servo channel, electronic
speed controls that use an SCR circuit to chop DC for control of an
electric motor, R/C operated sound and light systems, and so forth; many
of these don't even require modification of the transmitter, though
others either involve a customized transmitter or an add-in circuit board
that only works with certain brands.

There might be safety issues, even with PCM radios, in using a direct R/C
control to fire motors, but for other rocket functions, including
ejection (at least as a backup) and stage separation, these might be one
way to go, and won't cost a great deal more than a Pratt ECS or
comparable system -- plus you'll have extra channels you can use to steer
a glide parachute or maneuver a boost glider.

--
+----------------------------------------------------------------------+
| It's easier to create chaos than order -- 2nd law of thermodynamics  |
+----------------------------------------------------------------------+

| TableTop Publications http://members.aol.com/silntobsvr/ttop_pub.htm |
+----------------------------------------------------------------------+
| All opinions expressed are my own, and should in no way be mistaken  |
| for those of anyone but a rabid libertarian.                         |
+----------------------------------------------------------------------+

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Fred Gru » Sun, 11 Aug 1996 04:00:00


Quote:

>HELLO
>ALL!!!!!

>In regards
>to the
>following
>>This type of thing is very common in other arenas of RC hobbies. Radios and

recievers use pulse code modulation (PCM) to encode the signals so they aren't
interfered with by other things on that freq. I've seen two airborne RC planes
on the same freq., both with PCM.

A little while ago I asked about the PCM systems at my local hobby shop and
was told similar stories as well as stories about the remarkable air range of
these systems.  For a FM PCM 2 channel system complete I was quoted
approximately $80.  However I don't want or need the mechanical servos.  
Instead I wonder if someone makes an electric switch or relay for this system.
Or maybe I could simply power up an electric match right off the receiver once
I determined how the receiver powers the servos??????????      

If relyable this sounds like a great way to control the complex rockets like
multiple stages.                

Anyone that uses understands these RC systems, please speak up and enlighten
those people as myself who have no clue about RC!                      

Thanks
Fred Gruis

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Bill Nels » Sun, 11 Aug 1996 04:00:00


:A little while ago I asked about the PCM systems at my local hobby shop and
:was told similar stories as well as stories about the remarkable air range of
:these systems.  For a FM PCM 2 channel system complete I was quoted
:approximately $80.  However I don't want or need the mechanical servos.  
:Instead I wonder if someone makes an electric switch or relay for this system.
:Or maybe I could simply power up an electric match right off the receiver once
:I determined how the receiver powers the servos??????????      

Won't work. The receiver is continuously supplying a signal to the servos.
In the earlier sets, you would often see some "jitter" in the servo, but
that has been greatly reduced in the past 5 years or so.

You could use the servo to close mechanical contacts, however.  If doing so,
I would use at least two servos, with one acting as a "safety". In other
words, it would have to be sent a "full deflection" (take your pick of the
direction) before the power would be passed to the second/third servo.

Bill

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by AVin » Sun, 11 Aug 1996 04:00:00


Good thought Bill but don't just pick a direction. Find out which way the
servos drift as the receiver goes out of rang or the battery dies and make
an educated decision.

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by John H. Cato, Jr » Sun, 11 Aug 1996 04:00:00


Quote:

> Won't work. The receiver is continuously supplying a signal to the servos.
> In the earlier sets, you would often see some "jitter" in the servo, but
> that has been greatly reduced in the past 5 years or so.

> You could use the servo to close mechanical contacts, however.  If doing so,
> I would use at least two servos, with one acting as a "safety". In other
> words, it would have to be sent a "full deflection" (take your pick of the
> direction) before the power would be passed to the second/third servo.

> Bill

Bill, there was an article over a year ago (as I recall) in something like
Electronics Hobbyist (got it around here somewhere) that covered the design of an
electronic 'switch' that could be connected DIRECTLY to the outputs of the
receiver.  This was put forth as a better way to turn on and off these electric
motors that are used more and more in RC.  Used up to 6 MOSFETs to handle some
pretty high current.

Would seem a simple task to run the outputs of two of these switches thru an AND
gate - such that it would take BOTH of them to be activated to get a signal 'out'
(to drive, say, a high current final stage output switch).

I just don't like 'mechanical' when we're talking several G's of acceleration.

-- john.

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Bill Nels » Mon, 12 Aug 1996 04:00:00



: >
: > You could use the servo to close mechanical contacts, however.  If doing so,
: > I would use at least two servos, with one acting as a "safety". In other
: > words, it would have to be sent a "full deflection" (take your pick of the
: > direction) before the power would be passed to the second/third servo.
: >
: > Bill

:Bill, there was an article over a year ago (as I recall) in something like
:Electronics Hobbyist (got it around here somewhere) that covered the design of an
:electronic 'switch' that could be connected DIRECTLY to the outputs of the
:receiver.  This was put forth as a better way to turn on and off these electric
:motors that are used more and more in RC.  Used up to 6 MOSFETs to handle some
:pretty high current.

I suspect that there are commercial products that do this already. If you
were willing to pay for a tested product, then it would save some effort.

: I just don't like 'mechanical' when we're talking several G's of acceleration.

The servos seem to stand up to gee forces quite well.  I have seen ones that
have survived a vertical crash from an airplane. The deceleration forces of
such a crash are very high.

Bill

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by mas.. » Mon, 12 Aug 1996 04:00:00


Quote:

> : I just don't like 'mechanical' when we're talking several G's of acceleration.
> The servos seem to stand up to gee forces quite well.  I have seen ones that
> have survived a vertical crash from an airplane. The deceleration forces of
> such a crash are very high.

The servos may but the micro switches people tend to activate with the servos
don't! There are tiny G-sensitive parts inside them that do switch at the most
inappropriate times. You have to give consideration to both the positive G's
experienced at liftoff AND the negative G's experienced at burnout and
carefully orient the switch (or switches) so that it switch due to the G
forces. Using two is a good idea, just make sure that they are oriented in
opposite directions so that the G-load induced switching tendencies tend to
cancel between them. Or, save yourself some trouble and get electronic
switches.

Here's some parts of a recent thread from rec.models.rc.air courtesy of
WWW.DejaNews.Com:
==========

Quote:
>Is there such a thing as a PCM radio control which uses an electrical relay
>circuit in the place of a mechanical servo?  A simple on/off relay for the
>control of lights would be great.  I realize I could rig something up
>using a switch and a mechanical servo but would like to avoid this.  Also the
>transmitter would need not have mixing capabilities for this channel(s). Name
>of a company or contact would be great!

Yes, sort of.  Relays are pretty much relics of a bygone era for small
signals, but solid-state switches are readily available and more
reliable.  Jomar makes one that plugs in place of a servo, and should
handle a few watts worth of lights without a problem.  I think Joe's
company was absorbed by EMS.  Try calling them at (800) 845-8978.
Ian
San diego
=========
A company called JOMAR makes an optically isolated switch (about $35 I
think) that plugs in like a servo.  You can adjust direction of
actuation and it can handle 10 amps for short time.  I use then for
onboard glow on my 4 strokes.
Ted Brindle
=========
Hobby Lobby sells the switch you're looking for.
Rich Hollyday
=========
Check this ftp site for plans for an R/C switch that is, I think,
exactly what you're looking for (among other R/C related things).
I haven't built one of these yet, so I can't testify as to the
suitability of this switch for any applications.
  ftp://ftp.luth.se/pub/misc/rc
  Hope this helps,
  erl
--


Lawrence Livermore National Laboratory   7000 East Ave., P.O. Box 808, L-571
Livermore, Calif. 94550                  (510)422-1958    FAX: (510)424-3317

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by John H. Cato, Jr » Mon, 12 Aug 1996 04:00:00


  (much worthwhile content snipped)

Quote:

> Lawrence Livermore National Laboratory   7000 East Ave., P.O. Box 808, L-571
> Livermore, Calif. 94550                  (510)422-1958    FAX: (510)424-3317

Warren, thanks so ever much for that good information.

It is stuff like this that truly makes 'networking' the wonderful thing
that it is.

-- john.

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Jim Ker » Tue, 13 Aug 1996 04:00:00


: >>This type of thing is very common in other arenas of RC hobbies. Radios and
: recievers use pulse code modulation (PCM) to encode the signals so they aren't
: interfered with by other things on that freq. I've seen two airborne RC planes
: on the same freq., both with PCM.

Ahhh.... No.

That's why RC fields have elaborate systems involving inpounding
transmitters and/or "frequency pin boards" to make sure that only one
person is transmitting on a particular frequency at a time.

Now, there are some new radios that will scan and find unused channels
and/or have more elaborate data encoding to avoid glitches, but they are
the exception - not to mention BIG bucks.

: A little while ago I asked about the PCM systems at my local hobby shop and
: was told similar stories as well as stories about the remarkable air range of

Sounds like some of these stories were just that - stories. (The air
range is pretty good though).

: these systems.  For a FM PCM 2 channel system complete I was quoted
: approximately $80.  

$80 for a PCM system? Where do you shop?  I've never seen PCM for less
than $200...

However I don't want or need the mechanical servos.  

Correct, you don't need mechanical servos.

: Instead I wonder if someone makes an electric switch or relay for this system.

Yea - they are used all the time for electric RC. They should have (or
can get) these at your hobby shop.

: Or maybe I could simply power up an electric match right off the receiver once
: I determined how the receiver powers the servos??????????    

No. Well, you could, but it would ignite the moment that you turn the
transmitter power on...

The typical servo is connected via 3 wires - power, ground, and
position. The position output from the reciever is a variable width
pulse (don't remember the numbers off the top of my head). At one
end of the travel, it's a short pulse. At the other the pulse is longer.
But the transmitter is constantly sending pulses that are echoed to the
servo by the reciever. You need an interface between the reciever and
your ignitor.

jim    ! Spell checking left as an exercise for the reader.

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Fred Gru » Tue, 13 Aug 1996 04:00:00


Quote:



>: >
>: > You could use the servo to close mechanical contacts, however.  If doing so,
>: > I would use at least two servos, with one acting as a "safety". In other
>: > words, it would have to be sent a "full deflection" (take your pick of the
>: > direction) before the power would be passed to the second/third servo.
>: >
>: > Bill
>:Bill, there was an article over a year ago (as I recall) in something like
>:Electronics Hobbyist (got it around here somewhere) that covered the design of an
>:electronic 'switch' that could be connected DIRECTLY to the outputs of the
>:receiver.  This was put forth as a better way to turn on and off these electric
>:motors that are used more and more in RC.  Used up to 6 MOSFETs to handle some
>:pretty high current.
>I suspect that there are commercial products that do this already. If you
>were willing to pay for a tested product, then it would save some effort.
>: I just don't like 'mechanical' when we're talking several G's of acceleration.
>The servos seem to stand up to gee forces quite well.  I have seen ones that
>have survived a vertical crash from an airplane. The deceleration forces of
>such a crash are very high.
>Bill

Thank you for the feedback guys!  Even though the servo idea (especially when
using two opposing servos) would probably work fine, I would prefer to use an
electronic switch(s) for simplicity.  Pointing me to some specific contacts,
companies, resources (issue, volume #'s) would be greatly appreciated.

Again thanks,
Fred Gruis

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Frank J. Bur » Tue, 13 Aug 1996 04:00:00


The output of the receiver is a pulse width from 1-2ms wide that is decoded by
the servo as the amount of travel to produce.  You would have to make a circuit
that would decode the pulse width to determine when to activate or not.  There
are people who make electronic switches, like jo-mar that are around $40, but they
glitch when turned on, and are very sensitive to glitching.  I would not recommend
them for this use.  The recent articles in HPR about using the Radio Shack pager
sounds like the most economical think going.  You could buy two for the same price as
a two channel system, and one switch.

They also have a much higher output power level, to avoid being flooded out
like the Pratt systems seem to do.

Frank


Quote:


>>HELLO
>>ALL!!!!!

>>In regards
>>to the
>>following

>>>This type of thing is very common in other arenas of RC hobbies. Radios and
>recievers use pulse code modulation (PCM) to encode the signals so they aren't
>interfered with by other things on that freq. I've seen two airborne RC planes
>on the same freq., both with PCM.

>A little while ago I asked about the PCM systems at my local hobby shop and
>was told similar stories as well as stories about the remarkable air range of
>these systems.  For a FM PCM 2 channel system complete I was quoted
>approximately $80.  However I don't want or need the mechanical servos.  
>Instead I wonder if someone makes an electric switch or relay for this system.
>Or maybe I could simply power up an electric match right off the receiver once
>I determined how the receiver powers the servos??????????  

>If relyable this sounds like a great way to control the complex rockets like
>multiple stages.            

>Anyone that uses understands these RC systems, please speak up and enlighten
>those people as myself who have no clue about RC!                  

>Thanks
>Fred Gruis

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Henry Spence » Tue, 13 Aug 1996 04:00:00


Quote:

>You could use the servo to close mechanical contacts, however.  If doing so,
>I would use at least two servos, with one acting as a "safety". In other
>words, it would have to be sent a "full deflection" (take your pick of the
>direction) before the power would be passed to the second/third servo.

Bear in mind that that is how the pros do it -- "arm" and "fire" are
always separate commands.  In the modern versions, such as the one that
triggers the shuttle's destruct charges, each command is a multi-bit
digital code, and random trash that just happens to be on the right
frequency is ignored.
--
 ...the truly fundamental discoveries seldom       |       Henry Spencer

 
 
 

Pratt RC ejection system fails me twice at Black Rock!

Post by Marcus Lee » Wed, 14 Aug 1996 04:00:00




Quote:

>Bear in mind that that is how the pros do it -- "arm" and "fire" are
>always separate commands.  In the modern versions, such as the one that
>triggers the shuttle's destruct charges, each command is a multi-bit
>digital code, and random trash that just happens to be on the right
>frequency is ignored.

This is how amateur radio operators have been doing remote control of (for
  example) repeater controllers for decades.  The simpler systems use
  DTMF, while the more sophisticated ones use packet radio, with
  funky remote control hardware off of the TNC.

A DTMF-based system would be a vast improvement over a carrier-sense system,
  which the Pratt system seems to be.  There are single-chip DTMF decoders.
  You could simply pick one tone for "ARM" and the other for "FIRE".
  The user picks a pair just prior to launch, thus being both simple,
  and rather safer that the described Pratt system.
--
----------------------------------------------------------------------
Marcus Leech                   Mail: Dept 4C16, MS 238, CAR
Systems Security Architect     Phone:    (ESN) 393-9145  +1 613 763 9145
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