Model Boat Mayhem

Technical, Techniques, Hints, and Tips => Other Technical Questions... => Topic started by: Angus64 on December 21, 2011, 10:32:27 AM

Title: Servo.
Post by: Angus64 on December 21, 2011, 10:32:27 AM
Hi. Can anyone tell me the difference between digital servos & analog servo or do they all work the same, I have a high speed 15Kg digital servo on order, they never said it was digital in the advertising , But the information I received says it. So I am hoping they are the same. I will get this sorted one day.
Angus.



Title: Re: Servo.
Post by: Martin [Admin] on December 21, 2011, 11:09:43 AM

Hi Angus,

 ... OK, I'm game to put my head above the parapet!

 Basically Analogue servos are the same Digital servos but process the signal from the receiver "differently"...

 Now I may be wrong here but an Analogue servo;
 1.  Receives a signal from the receiver,
 2.  Amplifies the signal,
 3.  Creates a "difference voltage" between the 'output arm' and the incoming signal (via a potentiometer),
 4.  Moves the 'output arm' in the "right" direction until the difference voltage is reduced to 0v.
 4a. As the 'output arm' moves toward a "0v", it slows down as it reaches 0v with a low "Q factor".

 A Digital servo;
 1.  Receives a signal from the receiver,
 2.  Amplifies the signal,
 3.  Creates a "set point" where the 'output arm' should be.
 4.  Computes the 'difference' between the "set point" and the current 'output arm' position via a potentiometer or encoder,
 4a. Moves the ' output arm' towards the set point with a high "Q factor".
 5.  I understand that a digital servo's motor is 'always on', holding the arm in position, (thus have a higher current drain).

 ( I have read somewhere that some digital servos don't like certain receivers, so watch out for that!)

 Have a look at:

http://www.futaba-rc.com/servos/digitalservos.pdf (http://www.futaba-rc.com/servos/digitalservos.pdf)
http://en.wikipedia.org/wiki/Servomechanism
www.youtube.com/watch?v=Q9wzfI1kgjk
www.youtube.com/watch?v=-OQF5xoXH2w

Title: Re: Servo.
Post by: MikeA on December 21, 2011, 11:38:23 AM
in think i actually understand that thanks martin :-))
Title: Re: Servo.
Post by: ACTion on December 21, 2011, 12:13:12 PM

thus have a higher current drain
You can add a coupe of "muches" to that last comment, coupled with a lot of RF interference from the brushless motor. If you have a sound system then I'd advise using a totally separate power supply for it. Also avoid BECs with just a 1A output and any Rx battery of less than about 2000mAH. You don't get this sort of power from a servo without putting in a lot of energy.
DM
Title: Re: Servo.
Post by: Subculture on December 21, 2011, 05:09:22 PM
Played with a couple of digital servos. I've not noticed them to be continuously 'on', but they do have a much tighter deadband than analogue models. On some models you can program a looser deadband if you desire. The main differences between analogue model and digital is that the motor is driven at a higher pulse rate, which tends to improve torque, and the implementation of an integrator, to give true PID (proportional, integrate, difference) in place of the PD characteristics of analogue counterparts.

Interestingly if you open up most modern servos (Hitec are one make ), you'll find they now use microcontrollers instead of dedicated IC's. The companies that made the old servo control chips seem to have discontinued making them, and i suspect that it's cheaper for a company to use microcontrollers anyway.

In essence, all servos are now digital, they're just flashed with different firmware so they behave differently. The 'analogue' models have software designed to emulate the old analogue chips like the NE544.
Title: Re: Servo.
Post by: Angus64 on December 22, 2011, 12:15:08 AM
Thanks Martin
                     You have explained servos pretty well, I can understand what I am working with now, I thought I may have jumped in to soon in buying the servo not knowing how  they worked, But I should be Ok with the one I have coming, the web address you included were quite good to. I have fitted the motor it's a quite large one from a auto heater fan 12 volt 300watt & have water cooling coil around it & fitted an electric pump to circulate the water, Also fan to help cool. The boat is a big one at 1.5 Mts I have gone for a 230 amp buggy ESC, with air cooling water should not be a problem things are well sealed up. :-))
Angus.
Title: Re: Servo.
Post by: Angus64 on December 22, 2011, 12:27:16 AM
Thanks to all who answered my post , I would be stumbling around in the dark if it were not for this forum, Electronics isn't one of my strong points , But I am becoming a little more enlightened on the subject now.
Angus
Title: Re: Servo.
Post by: Circlip on December 22, 2011, 11:06:26 AM
Hmm, The analogue servos I rember were the ones used  on the old reed gear. So since the late sixties, all the manufacturers stating "Digital" electronics were wrong? My good ole Remcon Quantum was obviously misslabeled.

  Regards  Ian.
Title: Re: Servo.
Post by: malcolmfrary on December 22, 2011, 11:42:13 AM
Hmm, The analogue servos I rember were the ones used  on the old reed gear. So since the late sixties, all the manufacturers stating "Digital" electronics were wrong? My good ole Remcon Quantum was obviously misslabeled.

  Regards  Ian.
"Digital" is one of those words that have been badly debased by marketing people, who feel the need to make their product sound more exciting. 
Back in the days of tuned reeds, pretty much everything was analogue.  With the advent of integrated circuits, the processing of the signals could be done using digital techniques, albeit that the core of the system, the pulse timing, was very analogue.  When the patent/copyright on the original servo chips ran out, PICs became a viable alternative, and allowed the analogue timing parts to be replaced with a bit of program. 
Thus these are more digital than the original "digital" devices, but depending on design can still use a conventional motor or can drive a stepper or brushless motor.  Then there are the differing techniques for determining what position the servo output arm is in.  The usual variable resistor is a device providing an analogue voltage that can be interpreted into a digital value for the benefit of the PIC program. 
Until manufacturers start laying more of their cards face up, we are stuck with trying to interpret the blatherings of marketing people who I suspect couldn't tell the difference between a brushless motor, a piece of knotted string and a verbal blind alley when they run into the wall at the other end.
Title: Re: Servo.
Post by: RaaArtyGunner on December 22, 2011, 12:37:19 PM
"Digital" is one of those words that have been badly debased by marketing people, who feel the need to make their product sound more exciting. 
Back in the days of tuned reeds, pretty much everything was analogue.  With the advent of integrated circuits, the processing of the signals could be done using digital techniques, albeit that the core of the system, the pulse timing, was very analogue.  When the patent/copyright on the original servo chips ran out, PICs became a viable alternative, and allowed the analogue timing parts to be replaced with a bit of program. 
Thus these are more digital than the original "digital" devices, but depending on design can still use a conventional motor or can drive a stepper or brushless motor.  Then there are the differing techniques for determining what position the servo output arm is in.  The usual variable resistor is a device providing an analogue voltage that can be interpreted into a digital value for the benefit of the PIC program. 
Until manufacturers start laying more of their cards face up, we are stuck with trying to interpret the blatherings of marketing people who I suspect couldn't tell the difference between a brushless motor, a piece of knotted string and a verbal blind alley when they run into the wall at the other end.


 :-)) :-)) :-)) :-))
Title: Re: Servo.
Post by: Subculture on December 22, 2011, 12:38:12 PM
I've only seen one stepper motor based servo to date suitable for hobby use, was called 'openstepper' and based on 'openservo' which was/is an open source project for creating high end servo control, but using normal servo mechanics. http://hackedgadgets.com/2008/11/19/open-stepper-project/

I see one or two manufacturers are now offering brushless motor servos, at a premium price. Hobbyking supply some very neat servos which use a magnetic encoder instead of a carbon pot. I remember seeing some advertising for early proportional servos from the late 60's/early 70's which showed servos with induction feedback. I wonder why they were discontinued- cost perhaps, or did they drift?
Title: Re: Servo.
Post by: Netleyned on December 22, 2011, 05:28:15 PM
I ask myself 'Do model boaters need this cutting edge technology?
Smaller, lighter more accurate equipment are all good points for
airy planes, but at the end of the pier the water is non scale,
the wind is non scale and the model moves to these variables.
A skilled guy with a tank steering tug will probably be as good as
one with all singing all dancing mixing and the same applies to rag
and stick. The operator's skills still count for most of the performance.

Ned
Title: Re: Servo.
Post by: Subculture on December 22, 2011, 06:37:28 PM
For most boat modellers, you don't need anything beyond bog standard equipment, but it's nice to have it there if you have an unusual application.
Title: Re: Servo.
Post by: Martin [Admin] on December 22, 2011, 10:11:45 PM

 I would recommend a metal gear or high quality servo for the main ballast control in a model submarine though!
Title: Re: Servo.
Post by: Subculture on December 23, 2011, 11:29:27 AM
If you take a look inside some model submarines you'll see that the vast majority are fitted with bog standard servos e.g. plastic geared and non ballraced.

Some people like to use a ballraced servo if they're using it for a pinch valve. Compared to the hammering servos get in model cars and helicopters, a submarine presents relatively modest demands provided the linkages aren't binding.

The one piece of radio gear in a sub that should be the best you can afford, is the receiver.
Title: Re: Servo.
Post by: nhp651 on December 23, 2011, 01:45:44 PM
hope you don't mind me hijacking this thread for another question about servos,please, but I think it might be relative.
I read in a recent model magazine on servos that they are (what I would call) "none denominational"..ie the servo will work, so long as the three wires are wired into the plug in the correct order they will work with any receiver/tranny combo, however when I was wiring up a futaba servo into my clyde earlier this year, it wouldn't work...no movement at all, and I was using my Hitec transmiter/reciever combo, yet if I tried it with an acoms tranny receiver combo it would ( but that was only a 2 channel and no  good for the clyde).
then I wired in a sanwa servo to my hitec and it worked fine.
what was I doing wrong, or is it a phalacy that servos will work with all trannies/recever combo's.
cheers, for any help, as IO now have more futaba servos than sanwa's, lol
Title: Re: Servo.
Post by: Subculture on December 23, 2011, 01:53:06 PM
I've never had any problem with using Futaba servos with Hitec receivers.

The problem stuff was very old servos which sometimes used four wires- they can't be used with modern 3-wire receivers. Also need to be careful with old Sanwa servos, and Fleet equipment, as they had to be awkward and didn't conform to the Futaba/JR standard of wiring, so you needed to swap the pins around in the plugs.
Title: Re: Servo.
Post by: nhp651 on December 23, 2011, 07:01:01 PM
Strange then.....haven't a clue what could be wrong in that case.
neil.
Title: Re: Servo.
Post by: RaaArtyGunner on December 23, 2011, 09:03:01 PM
Can't the 'plugs' be installed back to front so that only the centre terminal is common to all makes but the other two pins are opposites.
Seem to recall something on here about turning the 'plug' around and making sure "x coloured wire" is nearest the receiver.

But hey the Black art is still Black to me.
Title: Re: Servo.
Post by: malcolmfrary on December 23, 2011, 09:15:12 PM
Most go black to the outside edge of the RX board, red in the middle and white to the inside.  White is the signal, and is usually blocked to DC, so plugging in reversed safely does nothing.  Sanwa had to be different.  Its a long time since I looked at one, and I can't remember. 
Some manufacturers try different shaped plugs, but we are modelers and possess knives and sanding blocks.  Those that are unkeyed can be inserted back to front, reversing sorts that.  My favoured trick to prevent it working is to insert the plug between the rows of pins.  Works every time.  Total inaction.
Title: Re: Servo.
Post by: Subculture on December 23, 2011, 09:55:38 PM
That can blow some of the cheaper receivers.
Title: Re: Servo.
Post by: RaaArtyGunner on December 23, 2011, 10:23:54 PM
Most go black to the outside edge of the RX board, red in the middle and white to the inside.  White is the signal, and is usually blocked to DC, so plugging in reversed safely does nothing.  Sanwa had to be different.  Its a long time since I looked at one, and I can't remember. 
Some manufacturers try different shaped plugs, but we are modelers and possess knives and sanding blocks.  Those that are unkeyed can be inserted back to front, reversing sorts that.  My favoured trick to prevent it working is to insert the plug between the rows of pins.  Works every time.  Total inaction.

Thank you Malcolm, but with my 'expertise' some of those tricks come naturally so I don't need to learn anymore  {-) {-) {-)
Title: Re: Servo.
Post by: malcolmfrary on December 24, 2011, 10:12:14 AM
That can blow some of the cheaper receivers.
I didn't say it was a good trick. {:-{
Title: Re: Servo.
Post by: Circlip on December 24, 2011, 11:11:32 AM
And don't forget, Furtaba servos had to be different in the good old days with a centre point of 1.7 when everone else had 1.5

   Regards   Ian.