Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Radio Equipment => Topic started by: Roger in France on September 10, 2006, 07:51:08 am
-
I need two high torque servos....around 80nmh. I have been offered some at a good price but have been told that "...they are digital and may not work with some radios...." I am using a Robbe F14 Navy. Will digital servos work with that radio?
Roger in France.
-
Roger
Can you quote that torque in Kgs-cm? My O-level physics is quite dusty these days and nmh to me only means Nickel Metal Hydride!
CAD2
-
Yes, these different units for torque are a b*$!&*!!
I think I may have got it wrong and it should be 85 Ncm or 8500cmg.
Any advice most welcome.
Roger in France.
-
Hi Roger,
Have a look at -
http://www.rccaraction.com/articles/digital_servo.asp (http://www.rccaraction.com/articles/digital_servo.asp)
http://www.futaba-rc.com/servos/digitalservos.pdf (http://www.futaba-rc.com/servos/digitalservos.pdf)
-
I could be wrong but I was under the impression that servos were servos, and therefore any servo would work with any radio, be it a new model transmitter or one thta was thirty years old, they always have for me...
Roy
-
Roger
I have one of these which I used for steering my Model Slipway Envoy. It's quick, powerful and very cheap - and it works with my old "steam" HiTech radio, too.
http://www.jperkinsdistribution.co.uk/detail.php?JPNO=7720220&activepage=1&Navmain=R/C%20equipment&subcatname=Servos
As for digital servos, I know very little about these except they are very expensive, very clever and probably designed for competition aerobatic model aircraft.
CAD2
-
I could be wrong but I was under the impression that servos were servos, and therefore any servo would work with any radio, be it a new model transmitter or one thta was thirty years old, they always have for me...
Roy
Noooo. That isnt the case. Digital servos need a radio that is capable of sending a digital signal. For example a radio which has 'servo neutral signal:1319 µs' would not operate a digital servo. These tend to be the following radios CONQUEST AM, FG Series (T4FG, T5FGK, T6FG,T6FGK, T7FGK), New Terratop, T3EGX, T3PGE,T2NL,T2LGX,T2FR,T2PK Series.**
Note its possible other digital servos may work with other digital enable radios.
-
All very interesting guys but my question still remains..."Will my Robbe F14 radio operate digital servos?
Martin, the articles appear not to address that issue. Are the mfgrs. being coy because they do not want to say "OK folk, here are these marveloous new servos - but, by the way, you need a new radio to use them".
Roger in France.
-
TO CAD2: The blurb does not say that your servo is digital. Maybe you didn't imply it was but you were simply saying, "Here is a high torque, cheap servo that works well for me".
Roger in France.
-
Roger
I try to make a point of not implying anything so, in case there is any doubt:
Here is a high-torque, cheap servo which works for me and which is not digital but might be a lot cheaper than the ones you have been offered and will certainly work with your F14. OK?
Now, if you want Chapter and Verse from one who seems to know, try this link:
http://www.dc-rc.org/PDF/Driving%20Motors%20and%20Servos.pdf#search=%22digital%20servos%22
CAD2
-
Hello all, a quick thank you to CAD2 for the link above. Even this idiot could understand it.
cheers
Jim
-
Jim
You are most welcome.
CAD2
-
TO CAD2: Sorry, I clearly annoyed you by using the word "implied". I simply meant that you had not been implicit and that I had had to assume that is what you meant. I would not, willingly, upset anyone on this forum as I find it such a valuable resource.
When I dislike or object to anything I keep quiet on the assumption that the other person's oppinion is probably as valuable as mine!
You were being helpful, I was trying to be certain, you continue to be helpful. Thank you.
Roger in France.
-
Having read all the material all of you have suggested and several major manufacturers catalogues I can still find nothing which clearly says "BEWARE: Digital servos may not work with your radio".
So I am working on the belief that my F14 radio (and the others I have) are analogue and will not drive digital servos.
The servo that CAD2 recommends seems ideal for my purposes and good value, I will go with that.
Roger in France.
-
There may well be the start of an on-going problem here with manufactureres using the word "digital" in as sloppy a manner as possible to imply superiority where none exists because it is a new buzzword.
A fully digital system should be one where all of the signalling is in the form of coded numbers, and is handled at every stage between the control stick and the servo arm by devices which handle such numbers. Sadly, anything which uses chips from the "digital" part of the chip makers inventory tends to get called "digital". I suspect that digital in this case means that although the servo probably responds to a normal (i.e. on for a set length of time, off the rest of the time) signal, which is sort of binary, this control signal is then handled by a processor to control the motor.
I would be tempted to e-mail the manufacturer and ask. If you dont get an answer look elsewhere. Let the manufacturer know you have gone elsewhere, and why (it pays to be helpful).
I will stick with my standard servos which I have modified into winches. They work on the principle that when the stick is centred, nothing happens, when moved forward or back, the winch winds in the appropriate direction as long as I am shoving the stick. With a 12 or 15 mm double drum, there is plenty or torque for handling the sails on my 1M size yacht, and the speed is OK for my kind of non-competitive sailing.
-
Hi Roger, and all our readers
I have e-mailed you a good list,
http://www.prestwich.ndirect.co.uk/techbase.htm
courtesy of Prestwich Models [Liegh Lancashire]
Tug
-
To MALCOLMFRARY: Thanks for that. Yes, I guessed there was some "loose" use of the word "digital". I too will stick with standard servos until I have chance to experiment. However, I am certainly not experimenting at the prices currently demanded for "digital servos".
I guess the important thing in boat applications, with a servo which is required to have high torque, is a set of good metal gears and ballraces.
Roger in France.
-
TO TUG: Thanks for the list. I will look at it with interest.
Roger in France.
-
To continue my questions on servos....One hobby shop I talked to about high torque servos suggested that it was unwise to run two high torque servos on a Y lead "...because of the current draw....". Any comments, please?
Roger in France.
-
The man could well have a point. A normal servo takes about 1/2 amp nominal max.
A high torque one may take more if it has a similar or faster reaction time on the grounds that the energy has to come from somewhere. If that somewhere is from the receiver battery via the receiver and the servo lead, two acting together might well overstep the power handling of the wiring and connectors.
An interesting side-effect could be if the voltage drop in the ground line of the wiring was increased by the extra load, the controlloing signal could be modified, with unpredictable results for the servos.
-
This probably won't be of any direct help but I seem to remember that the original Futaba Mammoth servos had a separate pair of wires for their own power supply (i.e. in addition to the usual red/black/white wires from the Rx plug). I've checked the Ripmax website and there are no details on Futaba's current industrial servos, but if there are any more electronically savvy folk out there then maybe they could suggest how Roger might connect his two big servos to a separate NiCad pack.
It might also be interesting to see if there's a model R/C truck forum somewhere out there; I'd bet someone's already thought of this problem.
Fun hobby this, eh?
CAD2
-
Think of it like this.
There are three connections/wires from the servo's cable to the receiver, B+, Signal, and B- or ground. The two necessary for power are the B+ and B-. The two necessary for telling the servo what to do are the Signal and B- or ground. So, if the B+ to the receiver (where the servo's power comes from) is rerouted to another voltage/power source, a separate battery, then only the B- and Signal lines from the servo would be required at the receiver.
When a servo isn't doing anything it is still receiving power from the receiver, that part is never turned off except when the receiver's power source (battery or BEC) is disconnected. The servo doesn't 'use' any power except when it's told to do something by the Signal line. (That's not completely true, since the servo is drawing some current all the time, not much, sort of a tiny trickle. A lot like what a bank pays you compared to what you pay them.)
A separate battery for a servo is certainly possible. Just requires the proper lines/wires going to the proper place.
- 'Doc
[Getting the lines mixed up is how the first 'scale' electric cooking ranges were discovered by R/C modelers.]
-
If you take the top of the reciever you will find the first two rows of pins are common,and only the signal pin goes to its own pin on the ic. so there is no power problem using a Y - lead .If you talk to any flyboy you will find they have been using them for years. can you not program two channels to work of one stick with that tx? you Can with a lot of tx from futaba.
http://www.futaba-rc.com/servos/digitalservos.pdf
Have a look at this if you want a big servo
http://www.sussex-model-centre.co.uk/shopexd.asp?id=19260
Peter
-
Reading the sheet that HS93's link turns up indicates that the digital servos will work with a standard radio outfit, the "digital" just refers to the internal control workings of the servo. It also indicates that power consumption can be expected to increase, and Futaba have provided thicker connection wire. If you wanted to run two of these, there might be a need for a heavier duty Y-lead, perhaps the flyboys know something more?
-
Yes Digital Servos will work with all radios and receivers. I use them with my Robbe Futaba FC-16 on the rear planes of my larger submarine. The Digital bit refers to the control circuit on the servo and how it interprets the signal from the receiver.
Digital servos draw more current and may have thicker servo wires to reflect this. you reciever should have no trouble supplying the current through it. The important thing is to make sure that you BEC is large enough. Most BEC's on ESC's are in the the 1.2-1Amp range. The stall current of most std servos are in the range of 200mAmps. So you should be able to power 4 standard servos and the receiver off an ESC like the Msonik range. For peace of mind I use an external switching BEC or even better a separate battery if space allows in my submarines. Not so important for scale boats where glitching doesn't have drastic efffects.
Remember that most (all?) BEC in ESC's are linear voltage regulators. They drop voltage by turning ecess voltage into heat and get pretty hot if you using a full complement of servos, and most ESC will now switch off if they overheat (ok for a scale boat but not a submarine!). Use a switching BEC like http://www.aurorra.co.uk/shop/19/index.htm if you have a high value model with more than 4 std servos or if your running digital servos. You cut the red wire on the ESC to disable the BEC on the ESC by the way.
-
Thanks all of you. Very, very helpful......as always on this forum.
Roger in France.
-
I followed the link to Prestwich Models, and contacted them. They very kindly replied (many thanks to Randy Carr), which I have pasted below -
"Malcolm, you can use digital servos anywhere you would normal servos, but you just have include other things with them (about calibrating them, etc.).
I think for boats, it might be a waste of money. The only benefit would be that the sheer accuracy of positioning the output arm and some performance benefits. For helicopters and other flying machines that need the best in return-to-center capability and other things like non-linear travel, digital servos are your best bet, if you can afford them.
I actually don't use them in any of my aircraft. I experimented with them, but couldn't find any direct benefits from using them in the type of flying that I've done."
-
All radios are I understand digital proportional, therefore will run any servo with no problems....
Roy
-
I am thinking somethings afoot here,
digital servo's? are they not the coreless motored ones
and being coreless they must be ac, derived from somewhere? Yes/ No?
-
Tug
No, sorry mate - they aren't the same thing at all ( and God knows, I've been all over the Internet researching this since Roger started this thread!). Think of a digital servo as like a high-frequency version of a "normal" servo, in much the same way as we can buy "normal" and "High-frequency" electronic speed controllers. The signal from the receiver is just the same for both types and so is the motor and gear train; it's the electronic bit in the middle that's different. A digital servo has an extra gubbins inside which takes the slow signal pulses from the receiver and ramps them up to about six times as fast. This makes the servo respond much more precisely to changes in the input signal. You can also program a digital servo to respond in the way you want it to e.g. total travel angle, linearity, centre position etc whereas you're stuck with what you're given with an analogue job. Overkill for model boats, in my humble opinion (if not then why do we have proportional controls?).
A coreless motor - as you say - needs a different input signal to one with an armature. This is a sexily "chopped-up" DC signal which is generated by another electronic gizzmo inside the servo and acts - as far as I can see - rather like an ignition distributor in pointing the current sequentially to different segments of the motor.
There may now be digital servos which also have coreless motors in them, but I'll stick with my cheapo Perkins analogue lump if that's OK with everybody.
CAD2
-
Thanks for that CAD2,
I was off frequency a bit there :-\ Tug