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Old Jan 26, 2005, 01:34 PM
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Servo jitter

How to solve the problem of jittering servos in case of no signal (TX off) for a receiver design without using microcontroller?

Servos should be silent when TX is OFF but RX is ON. Is there any simpler solution?
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Old Jan 26, 2005, 02:33 PM
"Simplify, then add lightness"
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Quote:
Is there any simpler solution?
Yes. Just make sure you turn on the transmitter before turning on the receiver/servos/motors, and turn off the receiver/servos/motor before turning off the transmitter.
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Old Jan 26, 2005, 03:36 PM
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I don't think its possible..(but I stand to be corrected). The reason the servos will jitter is because they are not receiving the control signals to keep them in one place and thus "hunt". If you wanted to centre all servos on signal loss you would need to send a continious 1.5ms pulse every 20 ms to simulate the signal the receiver would normally send. This could be done using two dual NE555 timers, for 4 channels, or one NE555 and some other logic circuits to provide as many outputs as you have servos. However the simplest way would probably be PIC based
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Old Jan 26, 2005, 03:50 PM
RIP Ric
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The problem with that approach is that the servos would then return to some electrical 'center' position, depending on what you set with the timer. This may not be desirable.
My understand is that the intelligent Rx's that are "quiet" when they lose a TX signal actually maintain the last position. They keep pushing out the same signal they last saw, as it's all digital anyway, they look for a good signal, store it, and one part of the algorithm is generating a clean signal from that stored data, while the rest of it now goes looking for the next good signal.

My feeling is - no, you cannot do this with simple analog technology.
..a
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Old Jan 26, 2005, 09:34 PM
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The real fault lies within the Rx. In absence of the RF signal, it just demodulates background noise. The decoder circuit then does its thing and turns the noise into random servo pulses. Although a good deal of the noise based pulses are garbage, the servo still does what it can to satisfy its need to move the control horn.

A servo would need a bit of intelligence to minimize this. It would be a VERY complex analog solution, but a simple task for a microcontroller. It is best done in the Rx where the full PPM frame can be analyzed for the most reliable noise detection.

RC-CAM
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Old Jan 26, 2005, 10:24 PM
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Guys: have you experience this, I am using a FUTABA Single Convertion PCM Receiver 72.350 MHZ with Futaba 9CHP transmitter, it work perfectly, but when I put the transmitter near the receiver about 1 foot away, there are 3 servo on the plane, one servo nearest to transmitter start to JITTER only when I position the Receiver in specific position, while the servo is Jittering, the 2 other servo work perfectly.......next I unplug the Jittering servo and plug it to other channel, same thing happen still Jittering.........The Jittering only happen when I position the Receiver to a specific position........I guess the Radiation from the Transmitter might have cause the Jittering to servo.....Its a PCM Reciever it should have not Jittering, its should be in last command Hold-position or in Preset fail-safe mode.

Have another experience with Futaba Dual Convertion PCM Receiver 36.470 MHZ, I am using this Receiver on Bell 222 Helicopter, while setting up the Helicopter the transmitter is 1 foot away from the receiver, but when I move the transmitter just below the table (about 2 feet away), the receiver have stop responding, and I move back to 1 foot away then the Receiver start to respond again.........I do not know whats causing the problem, just to play safe, I just stop using that Receiver.....I guess the Receiver might have overloaded due to strong Radiation coming from the Transmitter, but the Receiver work pecfectly when perform a standard range check.

Charles0198: Hows your Synthesize Receiver project? did you able to make the 72 MHZ Frequecy Scanner/Monitoring?
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Old Jan 27, 2005, 02:55 AM
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Hi, charles

Have a look to F.Thobois's French site ( ...Home Nordnet ...I do not remember exactly ) he explains the choice of the decoder's circuit ( CD 4015 instead of NE 5045 ) not to have servo jitter without TX emission.
Also page 212 on his "construction d'ensembles R/C" book

See Also http://www.rcgroups.com/forums/showthread.php?t=298197 for "deluxe" solution ...

Alain
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Old Jan 27, 2005, 03:58 AM
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Need PIC16F876

I need some PIC16F876A in 28/SOIC and 28/PDIP package for this purpose. If somebody have extra/spare parts or want to exchange for some other part please contact me.
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Old Jan 27, 2005, 04:24 AM
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Good question: Where are you ???

Alain
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Old Jan 27, 2005, 05:23 AM
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You may be able to adjust the reciever 'squelch' cicuitry. Most of the chips used have this featire - its desined to suppress background noise until a carrier of a certain strength is recieved. You can hear it in many walkie/ talkie type rados where you get a burst of noise after the remote finishes transmitting, followed by a muting.

lso in FM broacast sets where its refereed to as 'muting' so when tuning between stations yu do not get a huge hiss.

You would need to ID teh IF /discriminator chips and probably adjust a resistor value to optimise.

Or buy berg recievers, which do this - and more - in a different way.
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Old Jan 27, 2005, 05:24 AM
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There is no carrier when the Tx is off, which is the condition he's concerned with.
..a
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Old Jan 27, 2005, 02:25 PM
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vintage1

seems reasonable solution... how could it be done with MC3362 design, any idea?
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Old Jan 28, 2005, 12:56 AM
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mute

I have checked MC3362 but it doesn't have the mute option. It is available in MC3363. Any other helpful idea...
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Old Jan 28, 2005, 04:54 AM
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Quote:
Originally Posted by Acetronics
Have a look to F.Thobois's French site ( ...Home Nordnet ...I do not remember exactly ) he explains the choice of the decoder's circuit ( CD 4015 instead of NE 5045 ) not to have servo jitter without TX emission.
Also page 212 on his "construction d'ensembles R/C" book
This is how the receivers of Francis Thobois are designed:

When the transmitter is off, the receiver demodulator output is noise. This is squared into pulses which are, close enough together (typically less than 1 ms) that there is never a sufficient time interval for the decoder to see a synchronisation time (typically 4 ms). In this condition, PROVIDED the decoder is of the shift register type (i.e. 4015) and NOT of the Johnson counter type (i.e. 4017) then the outputs remain inactive, the servos see no pulses and remain quiet.
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Old Jan 28, 2005, 07:09 AM
RIP Ric
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very cool, but how is this accomplished in an analog circuit?
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