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Member postings for Nigel R

Here is a list of all the postings Nigel R has made in our forums. Click on a thread name to jump to the thread.

Thread: Theory of the operation of a servo
21/01/2020 15:02:11
Posted by Peter Christy on 21/01/2020 14:34:42:

Its actually not too difficult to apply a crude form of velocity damping to a conventional "analogue" servo by feeding some of the back emf from the motor (during the "off" period) to the pulse stretchers, and most amplifiers do this. It helps in controlling overshoots on light loads.

every day is a school day - fine points of analogue circuitry was/is not my strong suit

21/01/2020 13:52:44

"I stand to be corrected, but this is my interpretation of how a digital servo works"

You can see the digital PWM in effect quite clearly around 10:50 in the video linked above.

It is exactly like analogue PWM, but at a higher frequency.

As Pete has explained quite well, either analogue or digital can output anywhere between 0% and 100% power.

However, I would say this again, the most important difference is that the digital controller can drive the motor with more power and accuracy, when the error between desired position and actual position is very very small, the main reason for this is that the digital amp can implement more complex feedback processing, beyond just the simple 'proportional to error' that an analogue will do. It can also add rotation speed, and acceleration into the mix*. This gets better accuracy at low errors. You can't really see that stuff on the scope trace. All you can see is the PWM implementation.

* you can do it with analogue, but, more components needed, with tighter tolerances, more cost, etc.

Edited By Nigel R on 21/01/2020 13:59:48

21/01/2020 10:13:51

**LINK**

Bruce provides a pretty good overview of servo operation here.

21/01/2020 10:11:41

"Average these pulses out and the voltage would be much higher than the analogue equivalent"

I'm not sure you understand the control theory behind analogue and digital difference amplifiers. And nor does the guy in that video, it's 13 minutes of red herring talking about the PWM rate. Which is the least important aspect of the servo.

It would be perfectly possible to make an power equivalent digital version of an analogue servo.

It would be perfectly possible to make an analogue servo that had 300Hz PWM output.

Analogue servos ran at 50Hz because - traditionally - they were hung on the end of the 35mhz decoder, which (guess what) had a frame rate of 50H; the servo ran at the same speed to minimise complexity.

In a digital servo, the control signal is processed differently. This opens up some possibilities over and above the analogue servos.

The major difference in the digital servo is that the difference amp - being software - can be programmed however the manufacturer likes, and it can put out very high power much quicker than the analogue. In engineering terms, the digital amp can implement a higher order difference algorithm to figure out the drive voltage.

(none of this is visible from looking at a scope trace of the motor drive signal).

The implementation of the difference amp is what is responsible for determining how good a servo is at holding a position under load, any overshoot it suffers, and centering accuracy. An analogue circuit, there are less elements to play with to get this right, all the elements are physical components, so component tolerances affect its performance and that has to be factored into the design. With a digital amp, more accurate control is possible, their can be more simulated elements in the control algorithm, and none of them suffer from tolerance or lifespan issues.

It has little to none to do with the PWM frequency.

Thread: 7 Day challenge Nigel Hawes 30" Tucano...............
21/01/2020 09:49:10

well done!

Thread: DX9 Tx Battery Warning, Warning!
21/01/2020 06:47:25

It wasn't 95% discharged.

Figure on charging efficiency of 80% or so. I forget the exact figure.

Ie you put 3000mah back in the pack. Or thereabouts.

Gives 25% remaining. Assuming the 4000mah rating is honest.

The warning is there to avoid battery damage IIRC. Instant damage starts around 3v per cell. This is much lower than "flat" or fully discharged to 3.4v - which is a nominal figure trading off capacity per use against pack life of course.

During high discharge you could easily see 3v terminal voltage which instantly recovers when the discharge stops.

Thread: DB Hurricane paint and finish.
20/01/2020 17:10:35
Posted by Don Fry on 19/01/2020 17:36:49:

That point in a build where there is a months worth of bits to do. Nothing much to show. I am amused when someone is selling a bare airframe, not fitted together, no bits done, and the say 80% complete. That is still a nice colour scheme.

To be fair (and I have kept some fairly OCD logs to back this up), if I have a bare airframe, ready to cover, I'm about three quarters done.

Note, my 'ready to cover' also means I have installed (and removed) all the electrical and motive power already. Other definitions of 'ready to cover' are possible!

The above may apply very badly to scale masterpieces with a fistful of finishing work and detail stuff.

Thread: Recommendations please
20/01/2020 16:04:43

If you're looking at charging at the field in that kind of time, I'd use a single charger, do them one at a time at a 'one hour' charge rate. Any of the 4 button jobs should do it.

20/01/2020 13:42:45

If you have a 12V supply capable of a few hundred mA, a 5W 40Ohm dropper resistor will get you around 150mA current into a nimh pack for trickle charging. Couldn't be simpler and is all I regularly use on nimhs*.

Overnight trickle charging also has the handy benefit of balancing the cells - something a one hour charger will not do.

* Barring an occasional capacity check done via a standard four button charger.

Thread: Synthetic instead of caster oil?
20/01/2020 10:30:44

ok - thanks for clarifying jon

20/01/2020 09:38:41

" 100/1 oil/ petrol....try that with your laser ( substituting methonal obviously)"

how much lubrication does the petrol itself count for?

methanol isn't lubricating at all, is it?

so would presume any glow fuel will always need more oil than an equivalent petrol engine?

Thread: Trendsetters and Iconic Models over the Years?
20/01/2020 09:32:37

To add to the list, if its not on it already -

Aeromaster.

20/01/2020 09:32:08
Posted by Robin Colbourne on 19/01/2020 23:25:27:


The Tauri and Ugly Stick are definitely classics; the Ugly Stick, in particular, being much copied through the years, and even influencing such classics in their own right as the GWS Pico Stick; I bet that would have surprised Phil Kraft!

I think the Stik was something that would have been discovered eventually - no matter who discovered it.

It's the logical answer to "how simple can an all-wood RC airplane be?"

For simple, also read "quick to make".

Kraft's particular genius with the Stik was styling the tail end to look like a Fokker and plonking some big iron crosses on it smiley

Thread: Dereck Woodward Bigga Bit
20/01/2020 09:13:21

As far as balsa builds go, this must be one of the simplest 20 size airframes I've ever been near. I've taken only three complete days at the workbench to get this far.

Major woodwork jobs left - fin/rudder, top rear sheeting, lipo tray and hatch. Then it is time for covering.

I'm going to leave the rudder servo until after covering, then I can figure where my balance is coming out and use the servo to adjust it a bit before fixing it in place. It is, like most of this build, out of the spares box, a mighty Hitec 300 - with ball bearings no less - in this case. I have an offcut of lightweight red sullivan snake, which is long enough (if needed) to get the servo right up against where the lipo will sit.

I keep thinking that one or other of the 40 (ish) two strokes I have, would go very nicely in a 48" version...

19/01/2020 23:08:35

Wing bolts drilled and tapped. Reinforced the threads with thin cyano as ever after tapping.

19/01/2020 23:07:31

20200119_225514.jpg

19/01/2020 18:06:48

This session saw the wing saddle fettled , the wing dowel fixed in place and the ply plate to spread the load from the bolts all sorted.

More to come...

19/01/2020 18:05:28

20200119_110939.jpg

18/01/2020 23:09:00

Got a few jobs done tonight.

First one, added in some soft 1/4 to support the fin. Quite easy to fit when the rear fuselage is exactly a triangle without any slight bend in the sides.

Next up, blocks for the wing bolts, a chance to use up some more offcuts of ply (2 layers) and some balsa (1 layer) so solid that it has little other use in life. Also put in some more soft 1/4 wing saddle doublers as is common. 

Last, the tailskid is finished. Some 14g offcut stitched on to a piece of 4mm ply.

Not so very much more to do on the fuselage ☺

Edited By Nigel R on 18/01/2020 23:14:55

18/01/2020 23:02:06

20200118_224352.jpg

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