Transmitter range & Frame rate

[Tony Bell 2Participant @tonybell2]

In writing my article "Before Buying" I have two problems which I would like your practical help with.

"Transmitter range is never quoted but it does not actually matter as when sailing, your boat never goes beyond 100m as you will loose sight/controlability before then." Is this true?

Is 100m a practical limit and do all Tx/Rx work within 100m? Any problems with "Park Flyers"?

The second problem is one I have only just discovered.

"Frame Rate is now a problem as you cannot use a computer transmitter outputting a Frame Rate of 11ms (90Hz) with a servo expecting 20ms (50Hz), it will burnout" Is this correct? Does any member have experience of this?

Tony – Theory is no sustitute for experience.

[Tony Bell 2Participant @tonybell2]

[Colin BishopModerator @colinbishop34627]

Transmitter range can vary according to the ambient conditions, local interference and the way the RX has been instralled so it is difficult to give it with precision. But most 'proper' radios will meet the normal requirements of model boats in normal conditions and assuming that the RX aerial is in the recommended orientation.

Generally speaking, the more expensive brands such as Futaba and Spektrum will be more reliable that the cheaper stuff.

Colin

[ashley needhamParticipant @ashleyneedham69188]

100m is a VERY long way away for a model boat and to honest…why would you be that far away??

The range is ok for all practical purposes. As colin says, a boat set up up as per will meet all your range requirements. So saying, a full size sticky up aeriel will confer more range that something hidden in a mast or superstructure, but even then again, you really dont need to go that far out.

Frame rate??? no idea what you are talking about. Radio gear is all matched up and so I suspect that this theoretical hypothosiss is just that, hypo whatsist. Do you actually have a problem?? or is it only a possible problem; not heard, or seen any posts regarding burnt out servos.

Ashley

[Dave MilbournParticipant @davemilbourn48782]

"Frame Rate is now a problem as you cannot use a computer transmitter outputting a Frame Rate of 11ms (90Hz) with a servo expecting 20ms (50Hz), it will burnout" Is this correct? Does any member have experience of this?

No. A standard servo is expecting a +5v signal pulse of between 1 and 2mS. It has no way of measuring the interval between pulses as there is no need for it to do so. Typically this signal is part of a sequence of such signals (or a frame) – one signal per channel – which takes 20mS for the whole sequence before repeating. A faster frame rate will issue a signal to each servo rather more frequently so it should respond quicker to stick movements. This is why you don't see special servos advertised for sets with higher frame rates (e.g. the Futaba 2.4GHz sets). The problem only materialises when you have a device which needs to read two signals e.g. a mixer.

Hope that clarifes things for you.

Dave M

[shipwrightParticipant @shipwright]

Tony,

I have only used "analog" servos. I decided to do a bit of research on the internet – I apologise in advance if this is teaching grandmother to suck eggs. I think that the terms "analog" and "digital" in this context are slightly misleading. In the analog servo the receiver sends a pulsestream to the servo at the rate of 50 pulses per second (pps). The "width" of each pulse is varied by the position of the joystick. So the pulses can be nearly "on" all the time down to nearly "off" all the time. The average voltage of this pwm pulse stream will depend upon the pulse width. Inside the servo this average voltage is compared with a reference voltage derived from a potentiometer attached to the output shaft of the servo and the 2 voltages are compared and the difference is used to drive the servo shaft until the difference (also known as the error signal) is nearly zero – the servo output shaft is now at the position signalled/requested by the receiver. The digital servo differs in one parameter – the rate of the pulsestream sent to the motor. In the digital servo the 50pps pulsestream is converted into a higher rate pulsestream – I have seen 300 pps quoted but I don't know whether that is universal. The main advantage of the digital servo is the it responds more quickly (in terms of angular velocity and acceleration) than an equivalent analog servo. The main application is in the model aircraft world where fast responses of control surfaces is important.

Because there are more pulse edges per second in the digital servo, the switching power loss is likely to be greater and therefore the digital servo might consume more current than the analog servo. But I cannot comment on burning of the wires connecting the servo to the receiver – I would have thought that the servo manufacturer would use wire thickness/strand number to suit the maximum current drawn. Hope this is of some use. Futaba have published a note on this topic – "digitalservos.pdf"

Ian

[shipwrightParticipant @shipwright]

Tony,

After I posted the above I realised that it did not cover the frame transmission (from transmitter to receiver) in a multichannel system. This is quite a complex topic and I suggest that you look at the technical note section of the following link :

**LINK**

Ian

[shipwrightParticipant @shipwright]

Tony,

Apologies for posting yet again ! I have just found another website with useful information about servos and frame rates :

**LINK** – this article says that you could use a digital servo at a frame rate of 50 fps but you couldn't use an analog servo at frame rates much higher than 50 fps (whereas the digital servo is designed for higher frame rates)

My initial explanation (above) is somewhat flawed because it ignored the framing of the servo pulses. The explanation in the Futaba note was misleading because I assumed that the 20 msec was available for one servo whereas the timeslot per channel in most RC systems is about 2 msec with a repetition rate of about 50 frames per second. The more you investigate the more complex it seems ! I see from your website that you are well aware of frame rates etc so I was teaching grandmother to suck eggs – sorry !

Ian

[The Long BuildParticipant @thelongbuild]

No Idea about frame rates , Never hear of them until now, but as regards distance, I have used a Radio Link 2.4 on a boat in Coniston Lake, I could just about make out which way it was going before I chickened out and turned it around and brought it back.

[Dave MilbournParticipant @davemilbourn48782]

Forgot to say that the shorter frame rates are achieved by overlapping the individual signals within the frame. That's what confuses my poor P94!
So-called digital servos are a totally different beast, about which I know little and am prepared to say even less.
For those of a nervous disposition, it doesn't matter a whole lot as long as the servo you fit works.
DM

[Malcolm FraryParticipant @malcolmfrary95515]

Normal (i.e. those that I have) servos measure the pulse length by ramping up a voltage in the internal circuitry, then comparing that voltage with the voltage coming off the position sensing pot. If they disagree, it drives the motor to make them agree. The voltage developed from the pulse length has to be got rid of before the next pulse turns up, or a wrong voltage will be compared, so a long enough gap to allow discharge is needed. 18mS (20mS frame minus the 2mS pulse) is plenty of time, and most servos will probably be happy with much less.

There are manufacturers out there who will do their best to scare impressionable customers with statements like "Use only our approved equipment else your house will burn down" when the truth is that using a faster frame rate MIGHT cause misoperation. The latter, though more accurate, is less dramatic.

[Dave MilbournParticipant @davemilbourn48782]

If it helps at all, I have the following sets ALL operating bog-standard, cooking servos quite happily:

Futaba 6EXA 2.4GHz; Futaba 6J 2.4GHz; Spektrum Dx5e 2.4GHz; Planet 5 2.4GHz; Hitec Focus 6 and Ranger 3 (both 40MHz).

Reading it up, the only likely expensive conflict might occur if you try to use a special wizzy hi-frame rate servo with a standard receiver. If you're rich enough to have paid over a hundred quid for a servo you'll likely be better informed about it. Those shown in the Servo Shop all seem to be for connecting to hi-res helicopter gyros for controlling the tail rotor. Makes sense to me, so I'll not be investing in one for the rudder on Sea Nymph………………….

Dave M

[Tony Bell 2Participant @tonybell2]

Thanks Shipwright for the link to "Helifreak"

It filled in a lot of gaps and explains many of the things I have written about.

People may be getting confused with Frame Rates as there are two different situations where it is used.

20ms/50Hz frame rate used to send the signal from the receiver to the servo.

"Drive Frame Rate" 3.3ms/333Hz is the frequency of the pulse used to power the motor.

RedBird300@Helifreak did make me think about a problem that beautifully brings me back to my opening statement ending in "with a degree of accuracy"

Latency – meaning the time taken from moving the joystick to when the servo moves. ie Accuracy or Precision. I found a website showing a list of transmitters and receivers with their Latency.

Spektrum

Dx8/Ar8000 – Frame Rate 11ms =19ms FR 22ms = 25ms

Dx7/AR7000 = 45ms

Dx6/AR7000 = 36ms

Shows Frame Rate does matter as well as chosing the correct receiver.

Thanks again, all I have to do now is test my receiver antennas in different directions and heights above the waterline

Edited By Tony Bell 2 on 05/04/2013 20:36:50

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