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-   -   Servo signal amplitude requirements (http://www.Wattflyer.com/forums/showthread.php?t=73639)

Rodneh 04-26-2014 11:08 PM

Servo signal amplitude requirements
 
Servo Signal Level


I've been curious of how critical the amplitude to the signal from the receiver to the servo is so I did the following checks:
I built an interface between a servo signal and it's source so I could control the amplitude of that pulse independent of the voltage on the servo from less than 0.5volts up to the voltage applied to the servo. Here are the results:
On all Futaba (and I checked many of their very old ones as well as the later versions) the amplitude required was only one volt no matter how high the voltage on the servo bus was. The same held true for a bunch of Tower Hobby servos, Hitec servos, and Appolo servos. All worked well with a signal amplitude of a little as one volt no matter what the voltage on the servos was as long as the servo voltage was above 4 volts. I tested from 4 volts to 6.2 volts on the servo bus.

Other brands of servos did not fare so well. The FMA all required at least 1.5 volts amplitude on the signal to reliably operate. The DAD servos required slightly over 2 volts on the signal. The AS-090 required 2,5 volts. The small GWS Micro Std's required 2 volts and the PowerUP's required 2.1 volts. Of all the miniature servos, the Hitec HS-55 worked very well and reliably worked with the signal as low as 1.0 volts all the time. The HS-55 seemed to be the most reliable small servo although my supply of them was limited so there may be other good ones.
The amplitude of the servo signal pulses were measured using an oscilloscope to be sure there was no inaccuracies due to form factor or pulse distortions.

Turner 04-27-2014 01:07 AM

What can one do with this information? Have you found any receivers that do not produce enough signal voltage?

kyleservicetech 04-27-2014 02:20 AM

Quote:

Originally Posted by Turner (Post 946598)
What can one do with this information? Have you found any receivers that do not produce enough signal voltage?

Yeah
I did this myself about a year ago. And, pretty much came up with the same exact results.

From checking the signal output voltage of my various Spektrum receivers, I found that they put out signal levels several times that of the requirement of the servos they were driving.

So, at least on the Spektrum line of receivers, there is little or no problems with compatibility with various servos.

solentlife 04-27-2014 07:17 AM

So surely the question is :

What servo / Rx combos do not work ? Assuming we can ignore plug / socket disparities ...

I used my old JR 1001, 5001, 4001 servos for a while with my FrSky gear and all seemed OK till one of the 1001 servos failed .. probably due to old age ... but anyway - I now relegate them to rudder / nose wheel use or throttle on wet-fuel.

Nigel

kyleservicetech 04-27-2014 07:40 AM

Quote:

Originally Posted by solentlife (Post 946628)
So surely the question is :

What servo / Rx combos do not work ? Assuming we can ignore plug / socket disparities ...

I used my old JR 1001, 5001, 4001 servos for a while with my FrSky gear and all seemed OK till one of the 1001 servos failed .. probably due to old age ... but anyway - I now relegate them to rudder / nose wheel use or throttle on wet-fuel.

Nigel

Good question.

From what's been tested so far, the servos don't appear to present any problems. What is important, is the servo output signals of the receivers.

Methinks most receivers that have been made in the past few dozen years use what is called CMOS (complementary metal-oxide semiconductor) technology, where the output circuitry signal goes "rail to rail", or from zero to the voltage supplied to the output integrated circuit. My Spektrum receivers servo output signals are all "rail to rail".

Before that, they used what's called "TTL" (Transistor Transistor Logic) integrated circuits, which could definitely cause problems with servo inputs. FYI, TTL circuitry output signals travel from about 0.7 volts as signal "low" to around 2.1 Volts DC as signal "high". Give or take a bit. Since some servos operate with a positive input of around 0.8 volts or more, that could definitely cause signal tolerance problems with a receiver using TTL outputs. That might be why Spektrum does not approve use of any of those "Servo Reversing Harness Cables" with any of their equipment. They could use TTL circuitry.

TTL circuits pull a bit of power, so I don't think they've been used in RC receivers for a long long time.

The only way to be certain is to hang an oscilloscope on the output of these old receivers. I think most, if not all current design 2.4 Ghz receivers use that CMOS technology, so I don't believe there would be a problem with the current 2.4 Ghz technology. I did check some 15 year old Futaba receivers some 10 years ago, they were also CMOS Rail to Rail signal outputs.

IMHO, it would be very unlikely that a receiver output signal could ever damage a servo input circuit. That would be assuming that the receiver battery voltage supply is within the rating of the servos electronics. Some old servos didn't like working with five cell Nicad battery packs.

solentlife 04-27-2014 07:49 AM

The only instance I know of incompatibility is the old plug Futaba servo's with other brand radios.

In the 60 - 70 - 80's each radio brand had own plug style ... and it was generally known that Futaba servos could not have plugs changed to connect to another ie JR Rx. But strange thing was that JR - which in those days used different Xtals and intermediate Freq in their gear could have plugs swapped and fit some other Rx's - not Futaba though.

There were even 3rd party servos .. (today it's norm as the plug etc. has pretty well standardised on the JR style)... but how they bridged that gap - I have no clue .. I wouldn't use them, I stayed with OEM in those days.

Nigel

kyleservicetech 04-27-2014 07:55 AM

Quote:

Originally Posted by solentlife (Post 946633)
The only instance I know of incompatibility is the old plug Futaba servo's with other brand radios.

In the 60 - 70 - 80's each radio brand had own plug style ... and it was generally known that Futaba servos could not have plugs changed to connect to another ie JR Rx. But strange thing was that JR - which in those days used different Xtals and intermediate Freq in their gear could have plugs swapped and fit some other Rx's - not Futaba though.

There were even 3rd party servos .. (today it's norm as the plug etc. has pretty well standardised on the JR style)... but how they bridged that gap - I have no clue .. I wouldn't use them, I stayed with OEM in those days.

Nigel

Yeah, I've still got some servo hardware from an old Microavionics radio built in the 1960's. Those servos used a four wire connector plug. Those servos also were not worth a crap either. I got about 20 flights between failures on them.

solentlife 04-27-2014 08:04 AM

I still have a box of Futaba servos and gear .. the old Futaba M series ... 3 pin plugs :

http://i6.photobucket.com/albums/y24...a/DSCF0837.jpg

Here's the plug ... OK corrosion over the years has destroyed the pins ..

http://i6.photobucket.com/albums/y24...a/DSCF0831.jpg

I've used the old extensions I had with those plugs to produce plugs to fit balance leads individual cell. The pins are perfect fit to balanced plug.... just remove the signal one and the remaining two are spaced correct.

Nigel

Rodneh 04-27-2014 01:57 PM

What can you do with this info? Depends on your needs, if you have any servos with a high input impedance (I have not checked to see what any servos have a low input impedance and the receiver output has a somewhat high output impedance, the signal seen by the servo is severely reduced. If servo input impedance equals the receiver output impedance, the servo sees only one half of the amplitude of the pulse. Now all the 2.4GHz receivers I've checked put out about a 2.7 volt amplitude regardless of the voltage driving the receiver bus. If there is any impedance in the line connecting the servo to the receiver (like an extension with a bit of dirt or corrosion on the connector) the servo may not see an adequate signal. This is especially true if you are using some of the older servos such a one of the DAD's. While generally not a problem, if you find yourself having some intermittent or faulty servo operation it is just another item to check if all else fails.

solentlife 04-27-2014 02:34 PM

isn't it a case that vast majority of people .. myself included ... just assume that radio manufacturers have done their homework and all current servos will basically work with current systems ?

The chances of an idiot like me - wanting to use old servos is pretty remote .. I just happened to have the old servos from my JR Propo days that were used in the exact same model 30yrs ago !

I can tell you this ... those servos were reckoned to be good mid-range quality servos in their day. Put one up against a cheap one today .. boy are they slow !! My tg9e $2 servos can do multiple cycles in time one of those does ONE side to side ! Only thing I can say for the old ones ... the gear trains were substantial !

I replaced with Bluebird budget jobs and the difference is just out of this world. I wish I had BB's back in the 80's. I paid a small fortune for those old JR jobbies.

Nigel

kyleservicetech 04-27-2014 09:10 PM

Quote:

Originally Posted by solentlife (Post 946635)
I still have a box of Futaba servos and gear .. the old Futaba M series ... 3 pin plugs :
http://i6.photobucket.com/albums/y24...a/DSCF0831.jpg[/URL]


Nigel

Hey
I had some of those old servos with these connectors. I just re-terminated them with the newer Hitec type connectors. They worked just fine with the old Futaba receivers, as well as the various Spektrum receivers now in use.

They were installed in a model that I gave to my son several years ago.

solentlife 04-27-2014 09:20 PM

I was told that they would not work if just plug changed ...

They probably don't now anyway .. the M series radio hasn't been fired up for ages .. the plug in the picture is actually the Tx NiCD connector - you can see it's just corroded away .. the NiCD's were all corroded and had to be extracted with chemical gloves on !!

My ex in UK put my radios in the shed when I left .. and I was not able to collect for significant time later ... it shot my JR and the Futaba ...

Nigel

kyleservicetech 04-27-2014 10:24 PM

Quote:

Originally Posted by solentlife (Post 946681)
I was told that they would not work if just plug changed ...

They probably don't now anyway .. the M series radio hasn't been fired up for ages .. the plug in the picture is actually the Tx NiCD connector - you can see it's just corroded away .. the NiCD's were all corroded and had to be extracted with chemical gloves on !!

My ex in UK put my radios in the shed when I left .. and I was not able to collect for significant time later ... it shot my JR and the Futaba ...

Nigel

Agreed:

At the present cost of servos, trying to use units that are 30 plus years old just simply isn't worth it.


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