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-   -   Digital Servo Noise (http://www.rcuniverse.com/forum/rc-radios-transmitters-receivers-servos-gyros-157/4978457-digital-servo-noise.html)

reyn3545 11-10-2006 09:27 AM

Digital Servo Noise
 
I'm building my first plane with digital servos, and I'm getting a high pitched whine from some of the servos, some of the time. If I move them with the radio they will quieten down sometimes, but not all the time. The noisiest is the 5955TG that I use for the rudder. I also have 5985MG's on the ailerons and elevators, and they have noise some of the time.

The servo arms aren't under any strain, and I'm not at the end of the throws... what could be at fault? I've read that some noise is to be expected, but not this much. Its a high pitched whining noise.

BarracudaHockey 11-10-2006 12:43 PM

RE: Digital Servo Noise
 
Digital servos buzz, don't sweat it.

JimDrew 11-10-2006 10:46 PM

RE: Digital Servo Noise
 
This is normal when using a radio system that has a dead band that is higher than the servo can handle. We don't get ANY noise when using our radio system... it's kind of freaky when you are using to it and it just goes away.

dirtybird 11-11-2006 03:36 PM

RE: Digital Servo Noise
 

Quote:

ORIGINAL: JimDrew

This is normal when using a radio system that has a dead band that is higher than the servo can handle. We don't get ANY noise when using our radio system... it's kind of freaky when you are using to it and it just goes away.
Most of the servos I have tested have a deadband of at least 1us. This is 10 times wider than the resolution of the 1024 PCS systems used.
Some JR servos have 0 us deadband and will hunt no mater what the resolution of the system is.
I don't think your system will improve the operation of any of these cases.

OhD 11-11-2006 06:08 PM

RE: Digital Servo Noise
 

Quote:

ORIGINAL: JimDrew

This is normal when using a radio system that has a dead band that is higher than the servo can handle. We don't get ANY noise when using our radio system... it's kind of freaky when you are using to it and it just goes away.

What is your definition of radio system deadband? Are you describing pulse width jitter? A PCM system typically has no jitter and the servos will not buzz unless they have very narrow deadband and/or poor damping.

ricassad 01-16-2007 11:27 PM

RE: Digital Servo Noise
 
I´ve just finished the radio installation in my brand new UcanDo 46. The radio is also a new one . A futaba 9PCMS , which came with a PCM receiver, 3 S9252 high torque digital servos servos and one 3151 digital servo ( sport) . I installed two 9252 in the alerons, one 9252 in the rudder, the 3151 for the throttle, and I had two 9001 ( non digital ) for the elevators. when I turned on the radio, the noise from the digitals was really very loud. I know digital servos make noise anytime you move the sticks( under strain ) . But just looking at the plane and hearing that '''melody" from the servos doen´t seem to me a big deal ....

It shouldn´t be this way. In the UCD all the rods are straight and short. There is no strain at all.

Ed_Moorman 01-17-2007 05:42 PM

RE: Digital Servo Noise
 
My JR digitals buzz a little. My Hitechs scream like I was strangling them. Don't let the noise bother you. They're fine.

rmh 01-17-2007 05:46 PM

RE: Digital Servo Noise
 
swapping from 1024 PCM to 2.4 DX7 system greatly reduces the hummin and hawing.
I mix digital and analogue of all types ( well- all modern types) and the system is extremely quiet . an I use 6 v unregulated NIMH exclusively ,as servo power

dirtybird 01-17-2007 07:04 PM

RE: Digital Servo Noise
 
Whats the frame rate on the DX-7?

pkoury 01-18-2007 11:16 AM

RE: Digital Servo Noise
 
Gravity, digital servos will buzz/ whine on elevator and ailerons because they are trying to hold a position while gravity is trying to pull them down. Disconnect all but one digital servo on a horizontal control surface (elevator or aileron) then turn on the radio and listen for the buzz, take your finger and lightly support the surface and see if the buzzing stops. My Futaba 9151s would will wake the dead.

dirtybird 01-18-2007 11:16 AM

RE: Digital Servo Noise
 

Quote:

ORIGINAL: dick Hanson

swapping from 1024 PCM to 2.4 DX7 system greatly reduces the hummin and hawing.
I mix digital and analogue of all types ( well- all modern types) and the system is extremely quiet . an I use 6 v unregulated NIMH exclusively ,as servo power
That's smart- using an unregulated supply. Some think using a regulator increases the capability of the battery. In truth it reduces the capability of the battery.

I still wonder what the frame rate of the DX7 is. If it is high enough you won't need digital servos.

rmh 01-18-2007 06:00 PM

RE: Digital Servo Noise
 
I don't knowthe specs on the frame rate but-----------
My 33% EDGE has JR sport 125 servos on the elevators (140 in ounces at 6 v)
These servos are 3 pole NON coreless and NON digital. and sell for 32 bucks .
ready?
using my 10X on PCM set at aprox 125% throw - and a 4300 ma NiMh very low impedance 5 cell pack- I can rapidly jiggle the tx stick and the servos follow quite well -If I let the stick "fly" the servos go to center -no overshoot.
I setup the DX7 -same throws and same batt and repeat the test
the servos track slightly tigher around center and better at extreme ends (no binding)
th real difference is that if I let the stick "fly" the servos will follow the stick overshoot movements -probably 3-4 cycles -
the sound at max stick up/down speed- movement is higher pitched - which says to me the rate is higher .
With no instrumentation - I am certain the info rate is higher than the 1024PCM -tho how much - I don't know- as I don't have a scope and really don't really need that info as the observed test provides the answer I wanted to see.
If you find the actual rate -- let me know -just as a FWIW thing
On another note- my digitals are all quieter using this DX7 system

tkilwein 01-19-2007 09:07 PM

RE: Digital Servo Noise
 
I have HS5955TG servos and had my 9Z & 309rx in the plane and had servo buzz from all of them.

Purchased a 14MZ setup and the servo buzz is completly gone!
No matchboxes, setting up dual aileron, dual elevator, dual rudder servos is a piece of cake with the icing.
And talk about fast!

NdFrSpeed 01-20-2007 02:15 PM

RE: Digital Servo Noise
 
Not sure about other peoples HS 5955,,but i use 16 of them,,if mine are buzzing i find out why and fix the problem,,sometimes you will get just a very minor sound out of a Elevator servor from the weight of the of it,,,if youve got a rudder servo buzzing,,probably have the cables to tight and putting undo strain against the servo arm,,Ail sevors,,if you running 2 per Ail and theres buzzing,there fighting one an another,,the Geometry is off,,you should be able to go to full deflection and they should not make a sound either way,or the rudder servos.

NdFrSpeed

NJRCFLYER2 01-21-2007 07:32 PM

RE: Digital Servo Noise
 
Quote:

That's smart- using an unregulated supply. Some think using a regulator increases the capability of the battery. In truth it reduces the capability of the battery.
Not so. A properly designed linear regulator will in fact have the effect of lowering the internal resistance of the battery, from the receiver and servo load perspective. I can explain why if you like, but what it translates to is that a proper match of battery/regulator and load is always superior to an unregulated battery, in several ways. If reliability is the concern, there are simple methods to increase reliability over a single battery setup.

rmh 01-21-2007 08:55 PM

RE: Digital Servo Noise
 
I can't buy that argument -except that the reg may act as a high current restriction.
To be blunt - I have seen way too much trouble with those setups
maybe because those I have seen, were poorly designed/poorly made / over rated/ undersized /improperly tested - whatever .

tkilwein 01-21-2007 09:36 PM

RE: Digital Servo Noise
 
I cant resist this one. Normally don't do this but.

The internal resistance of the battery is in series with the linear regulator which also has some internal resistance.
R1= resistance of battery.
R2=resistance of linear regulator

SO total resistance = R1+R2 this does not lower resistance but increases it. OHMS law here, physics is physics.
We can also talk about the internal components of the battery, regulator and the loading device, (resistive, inductive, capacitive or combinations of) and any surrounding glue logic-components around the regulator, battery and the load.
Linear regulators dissapate heat as a by product by its nature-design. This is a lossy product.

Switchers are a different animal.

I deal with power designs-systems all the time, both at work and at home.
I use both at work and in my rc stuff at home, pros and cons for each, everyone has there own opinions.

rmh 01-21-2007 09:49 PM

RE: Digital Servo Noise
 
I also designed logic control systems for power systems -for years -they were pneumatic -but the same really as electrical controls when it all comes down to it - both on/off and some delayed / accumlative responses etc....
one hard cold fact: If you can avoid another part ----- do it.
I am admittedly a dinasaur:
I seenew cars which feature devices to distract the driver from his task of driving safely-- marketed as safety devices!!
worse yet -- the sh-t sells.

NJRCFLYER2 01-22-2007 06:25 AM

RE: Digital Servo Noise
 
Except that the linear regulator is an active component that has a near instantaneous response to changes in the output voltage, via a sense input. With the right decoupling capacitance on the output to supply current during brief transients as the regulator reacts to change, the effect is a very smooth and constant voltage output. Think about what has to happen to make this occur. Apply an ohms law example. We've got a battery supply voltage with a piddly 100 mA load and an internal resistance of 1 ohm, which is showing 8V to our meter under that 100 mA load. Right there we know that it's really supplying 8.1V internally before that loss from internal resistance is factored in.

There's also a fixed 10 ohm resistor in series with a honking strong servo, which is growling a bit at rest and drawing that 100 mA. Ignoring all the other wire and connector losses, there's actually 7V present at the servo at this load, due to the drop across the 10 ohm resistor. Now Cletus, our servo operator, twists the servo arm like a madman and it starts to draw 500mA. Assume the battery stays at 1 ohm resistance under more load, now it is supplying only 7.6V and there is also a 5V drop across the series resistor, so the servo now sees only 2.6V. Pretend a servo will still work at 2.6V

Now make it variable series resistor instead. As Cletus starts his servo arm twisting, the current begins to rise and the voltage drop across the variable resistor rises too. We have an exceptionally quick operator (Clyde) at the controls of the variable resistor though. This dude sees the dip in voltage in a wink and cranks the knob to lower the resistance and keep the voltage to the servo where it belongs. Clyde changed the variable resistor to only 1.2 ohms to keep the servo voltage output at 7V, right where it was when the load was only 100 mA.

The Clyde-O-Tron is what a linear regulator amounts to (a darned fast responding and well behaved one at that). Best part is that the servo doesn't know a thing about the variations that are happening at the battery, unless the battery drops below the regulated voltage value minus the dropout voltage of the regulator (a good one for our purposes is usually about .5V under mucho load, or around .2V to .3V under a moderate (normal) load. Example - a regulator with a .5V dropout under load X and a 6V regulated output will completely absorb battery voltage output variations down to 6.5V at the battery. As the battery goes below that, the regulator output finally begins to drop in a fairly linear fashion that correspnds to the battery voltage drop. Things keep working though, right down to the minimum level the servos will work at.

NJRCFLYER2 01-22-2007 06:32 AM

RE: Digital Servo Noise
 
Oh yeah, the linear reg is lossy etc. But it's simple (less stuff to break) and quiet (doesn't annoy the other stuff in the airplane).

dirtybird 01-22-2007 12:19 PM

RE: Digital Servo Noise
 
There is simply no way a device added in series with a battery will increase its capability. A capacitor in parallel can help, but one that can be any good would be far bigger than than the battery. More battery capacity is cheaper and lighter.

NJRCFLYER2 01-22-2007 09:21 PM

RE: Digital Servo Noise
 
No dispute that there are limits to what a series regulator can do. The key is that when you match battery/regulator properly to the load, it outperforms a battery by itself, unless your main performance criteria is running hot & hard while the battery is freshly charged. Don't match them properly, yeah, you get bad performance. If you walk yourself through a few examples, you may actually be surprised to see how. Use Ohms law...

Another poster made the point that switchers are more efficient, which is definitely true. You actually can get an increase in battery efficiency and deliver quite a decent amount of current into the load with the right design. There are pluses and minuses to everything. Switchers work, I've used them, but it always bothered me that I had a higher than necessary parts count and was adding noise to the mix. I've flown tons with no regulators too. I don't add stuff that doesn't have a benefit. Since the odds of a new failure do increase with one regulator in series, I use a redundant pair of batteries and a dual redundant regulator. So a failure is obviously now survivable and also detectable. Plus I eliminate the mechanical switch as a failure point etc. It's a darned light solution too.

rmh 01-22-2007 09:24 PM

RE: Digital Servo Noise
 
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

NJRCFLYER2 01-22-2007 09:35 PM

RE: Digital Servo Noise
 
Airframe & noisemaker up front heap big $$$ too! Hey, I've had a battery failure, so I do what I do with a good reason. Others may have been more fortunate so far.

tkilwein 01-22-2007 09:57 PM

RE: Digital Servo Noise
 
The efficiency of any regulator is less than 100%. Specs are online for people to check details for each one.
Perputual energy if more than 100%.
There is always a loss in the current technologies. The amount can be very small to very large, depending on the design.

The right cap as said earlier in a can make or break a design. ceramic, oscon, Alum etc.
The surge current on caps has to be greater than the design requires for longevity
If the cap has to low of a surge current rating for the design then bad things can happen to it.

The new Linear tech LT46xx series are killers and have very high efficiency.
And they only require a input and output cap or a few caps depending on what is required.
They are a complete switcher design minus the in-out decoupling caps all in one small footprint.
They are very nice in most applications.

Super caps another story.


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