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Old May 17, 2012, 01:49 AM
Flying Zayin
Cathay Stray's Avatar
China, Guangdong, Guangzhou
Joined May 2012
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Question
Do Analog Servos brown out when used with a Digital Radio?

Hi everyone,
I've been in and out this hobby for a few years, mostly out, but making a return now.
I'm assembling 2 planes now, but it all came to a halt when I heard from a friend that analog servos don't live long when used with a digital radio and can easily kill a model by burning out in the air.

Well, now what?
I have a WFLY WFT09SII radio which is *supposed* to be digital, and a record of my skype talk with a lady from WFLY (who had been very helpful but there is still some uncertainty) - seems my radio can only be used with digi servos. No good.

My doubts are about whether the radio is really digital - is there a way to check it and get a simple answer yes/no (like, using an oscilloscope)?

And by the way, beware... Your ana servos might really be in danger, at least this is what I heard from WFLY.
Thanks to WFLY for being so willing to help. Seems they have done their homework well and now communicating with them is a pleasure.
______

Me
- I had an argument with a friend last night, he insisted that WFLY radios were NOT digital -
which is to say of course that the signal between the Tx and Rx is digital, but the signal given by Rx to the servos was analog....
Is this correct?

WFLY
- the former one is right, Tx and Rx is digital.
while between the rx and servos, it depends on the servos' signal

Me
- and the signal issued by Rx to the executive systems?
You mean the Rx is capable of giving BOTH ana AND digi signal?

WFLY
- yes, but I just checked, it had better the digital, much more match the tx
yo had better using the digital servo on wfly tx, it can avoid the problem you have mentioned
for example BURNING

Me
- Is there a way to ensure safety?
(like.... some sort of filter between the Rx and the servo or something like that)?

WFLY
- for this question, I have checked that there is no solution about it so far. the best way is to use the digital servos

________

Well well well.
The person I talked to was NOT an engineer. Knowing a thing or two about how Chinese companies work and how the signal can be distorted on the way from me to the engineer and back I still have some reservations and would like to make it totally sure...
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Old May 17, 2012, 02:53 AM
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United Arab Emirates, Dubai, Dubai
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All radios today and for last probably 40 years are digital. I.e. - servos are controlled via a pulse varying between 1ms and 2ms in length, repeated approx 50 times per second. Analog and digital servos of today accept it. They are some special purpose servos which need faster rate and shorter pulse but they are clearly marked as such.
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Old May 17, 2012, 03:01 AM
Flying Zayin
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China, Guangdong, Guangzhou
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Quote:
Originally Posted by Leszek_K View Post
Analog and digital servos of today accept it
If that was the case, why would the manufacturer's support team give me a special warning about "better this and not that"?

Anyway as long as I have already heard the alarm, and the fpv equipment on board of my plane is quite dear to me, I'd prefer to take no risks at all, so the question remains: HOW can one, using simple equipment, make sure whether his receiver issues digital or analog signal to the servos...
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Old May 17, 2012, 03:27 AM
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Originally Posted by Cathay Stray View Post
If that was the case, why would the manufacturer's support team give me a special warning about "better this and not that"?
As you already noted, the questions and answers were being translated imperfectly in both directions so I'm glad what you got was even close to the truth

Quote:
HOW can one, using simple equipment, make sure whether his receiver issues digital or analog signal to the servos...
As per the comment above, no current production receiver is using analogue outputs, what matters in terms of servo safety is the frame-rate the receiver is using. ( and pulse width in special cases for narrow width servos like the BLS251)

Those receivers producing a frame-rate around 45-50 Hz (20-22ms) are safe to use with both digital and analogue servos, but when you put them into 'fast' mode (8-11ms typically) you need to use digital servos.

Even in 'fast' mode most receivers only use that for a specific sub-set of channels and the other channels are still safe for analogue servos.

So what you need to check is the frame-rate that your receiver uses and which channels (if any) are in 'fast' mode, as that will tell you which servos are safe to use in which channels.

There is a more detailed explanation here that may help : http://www.helifreak.com/showthread.php?t=263175
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Old May 17, 2012, 04:18 AM
Flying Zayin
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China, Guangdong, Guangzhou
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Originally Posted by Telrin View Post
So what you need to check is the frame-rate that your receiver uses and which channels (if any) are in 'fast' mode, as that will tell you which servos are safe to use in which channels.
Thanks a lot, this explains pretty much everything. So what I need to measure is the duration of impulses, right?
OK, not sure how I will do it but I'm a fast learner... I will do it.
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Old May 17, 2012, 07:38 AM
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Bonnie Scotland
Joined Jun 2004
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Correct me if I am wrong but you are concerned about the signal between the receiver and the servos?

The term digital has been bandied about in relation to radio control for many years. It is used inaccurately and in a misleading way, rather like FM when meaning VHF, etc. The two are entirely different and not interchangeable. The problem when things get a little technical is that many people do not understand what they are hearing. Most times they do not need to know but it makes for difficulties in trying to explain when that basic understanding is either wrong or completely missing. It is like trying to have a conversation with someone in a foreign language when one of the parties learned everything he knew about the language from reading a sales brochure. He has all the words but has no idea what they mean.

It is not really correct to say that all radios today and for last probably 40 years are digital. Variable pulse length is more like analogue than digital. Consequently fast or normal frame rates actually make no difference, the signal from the receiver to the servo is still analogue.
The difference between digital and analogue servos lies within the servo itself. As explanations go this one is hard to beat or misunderstand.

http://www.rchelicopterfun.com/rc-servos.html
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Old May 17, 2012, 11:50 AM
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Yes, this analog vs. digital stuff often gets mixed up.

The signal from the RX to the servo is analog. There is a pulse length of around 0.8 - 2.2 microseconds, with 1.5ms being the center.
The only exception I know of are the Fasst BUS Recievers and Servos; here, the signal is truly digital.

These analog pulses occur with a framerate of 50Hz normally, meaning each 20ms there is a new pulse coming from the RX to the servo.
"Digital" servos also work with these analog pulses, but the servo motor is controlled by digital circuitry. The pulses to the motor have a much higher PWM frequency than analog servos. This is why they whine more annoyingly than analog servos.
The PWM frequency of analog servos depends on the framerate of the signals from the RX. For each pulse from the RX, the servo motor gets one pulse.
With the usual 50Hz used for decades, the electronics work within specs.

Some newer 2.4GHz systems use a higher frequency on the RX pulse output, to achive a quicker response on the servos. For digital servos, this is no problem, as their internal PWM frequency is much higher anyways.
But analog servos will adjust to that higher RX output framerate, by suddenly having a higher internal PWM frequency. And this might kill the electronics and motor prematurely, just like overclocking a CPU too hard.

The Fasst system has a 16ms framerate. Analog servos still work, but I would not go any faster. My Jeti Duplex system lets me conveniently choose the RX framerate.
If your system has less then 16ms, I would not chose analog servos.
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Old May 17, 2012, 12:30 PM
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Quote:
Originally Posted by electrotor View Post
Consequently fast or normal frame rates actually make no difference...
That simply isn't true, most analogue servos will not tolerate frame rates above 50Hz.

Point taken on the signal being analogue at the point of delivery to the servo though.
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Old May 17, 2012, 12:45 PM
AndyKunz's Avatar
Illinois
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Quote:
Originally Posted by Julez View Post
Yes, this analog vs. digital stuff often gets mixed up.

The signal from the RX to the servo is analog.
Yes, you DID get that mixed up.

The signal from a receiver to a servo is digital - it is either ON or OFF. The duration of the signal is proportional - NOT analog - with the time indicating the position that the servo should move to. This is called Pulse Width Modulation, or PWM.

PWM (varying the duration of a digital pulse) is a digital communication method.

This is why it was very easy to use transistor-based electronics in the late 1960's when PPM became the standard for RC systems (Signetics produced a 3-chip set for encoding and decoding the digital signals (NE5044 and NE5045), and for driving servos (NE544).

You can find decoder schematics which use a timer circuit (often a 555) and a shift register (both CMOS and LS versions). The timer generates a reset after 6ms of no activity on the incoming data, and the shift register moves the actions through its outputs directly to servos.

Quote:
The only exception I know of are the Fasst BUS Recievers and Servos; here, the signal is truly digital.
Digital in the binary data sense.

Quote:
These analog pulses occur with a framerate of 50Hz normally, meaning each 20ms there is a new pulse coming from the RX to the servo.
Correction: These digital pulses occur at different frame rates, as short as 5.5ms for Spektrum surface radios, although as you pointed out they can vary by manufacturer, with JR and Spektrum using 11ms and 22ms frame rates.

Andy
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Old May 17, 2012, 02:00 PM
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Thanks for the info Andy, now I know why my 4 channel futaba conquest radio from the early '90s was labeled a "digital propotional" system.
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Old May 17, 2012, 02:28 PM
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Exactly. The term was coined in the '60's, actually, and quite accurately describes the technology.

Andy
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Old May 17, 2012, 06:20 PM
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Bonnie Scotland
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I guess we have to agree on a definition of digital then. I was always taught that digital = binary and analogue = proportional. I do understand your explanation Andy but how do you then differentiate between digital in the PWM sense and digital in the digital audio or radio sense? I am not trying to catch you out or be argumentative as I would like to better understand the whole subject and you seem to have a better grasp than me, especially if you are on the Spektrum Dev Team.
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Old May 17, 2012, 08:20 PM
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Stop trying to define the terms digital and analog and look at how an RC system actually works. You'll find it's a bit of both. The sticks produce analog output, with is turned by the encoder into a PPM signal that carries the information in the form of varying pulse width. The RF section sends that to the receiver, which sorts out the pulses and sends them to the appropriate servos. The servo gets a signal that carries the information in analog form (pulse width) and transforms it into analog output (arm position) proportional to the position of the transmitter stick.

So while there may be quite a bit of digital processing in the encoding, decoding and servo positioning, the basic information in a PPM system is analog (i.e., its not binary digits). In the case of PCM, by contrast, the signal is sent in digital form (bits).

Since the mid-60s, the PPM system has remained essentially unchanged and has always been called digital. This distinguishes it from the analog systems of that time, which sent te information in the form of varying audio tones (three for the AER channels, with throttle controlled by the rate of switching among the tones). Analog died out fairly quickly and by the late 60s digital proportional had taken over. I have servos from that era which still work with a modern radio.

The distinction between analog and digital servos concerns how they process the incoming analog pulse data and turn it into analog output. Digital servos can be faster and more precise.

As explained above, the issue of digital servo use with certain radios that was raised originally applies to ("digital") radios with high frame rates. This is an explicit feature of certain radios and is very easy to identify -- just look at the manual or specifications. No need to measure anything yourself. Just make sure you don't use the radio in high frame rate mode with analog servos. In normal frame rate mode there's no problem.
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Old May 17, 2012, 09:02 PM
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Illinois
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Originally Posted by electrotor View Post
I guess we have to agree on a definition of digital then. I was always taught that digital = binary and analogue = proportional. I do understand your explanation Andy but how do you then differentiate between digital in the PWM sense and digital in the digital audio or radio sense? I am not trying to catch you out or be argumentative as I would like to better understand the whole subject and you seem to have a better grasp than me, especially if you are on the Spektrum Dev Team.
Those servos are really better called "serial" because they use a form of serial communication (not unlike an RS-232 port, just real fast).

In a radio, the proper term for what you're asking about is really Pulse Code Modulation, or PCM. In Spektrum radios, a packet of information goes out in a way not unlike how your computer works on the Internet, that is, data is sent in packets. This is also a form of digital communication, but with the coding gains and extra things that go out on the airwaves, it's using a lot of analog magic to do it.

Andy
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Old May 17, 2012, 09:03 PM
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Originally Posted by Daedalus66 View Post
Stop trying to define the terms digital and analog and look at how an RC system actually works. You'll find it's a bit of both.
Good advice! And at every level it's a little of each. And just when you think you got it as one, go down another level and it changes again Kind of like "is light a wave or a particle?"

Andy
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