Things That Go Wrong ..........A New Appreciation: Problems with R/C!!!

August 14, 2020

 

 

This is a tale that probably plays out many times.  R/C model boats and in particular model yacht racing seems to be continually plagued by glitches of many sorts, and servo problems can be infuriatingly difficult to solve.  Most can be resolved once you understand the core of the problem but getting to it can drive you “nuts”.  Many years ago, one of our members (John H.) gave some very wise advice, “use alkaline batteries and buy good quality when they come on special”; he was right!

 

Recently, I had to deal with servo jitters in three different model boats, two sail and one power.  In one sail boat, sheeting would suddenly change (usually at distance) and then recover; in the other sail boat it would would fail and sometimes steering as well (again, often at distance).  In each boat the batteries had been charged and voltage was good.  Since both boats were using 2.4 GHz Tx/Rx control and antenna position is critical (to ensure signal is not lost at or near water level), that was a concern.  One boat used Spektrum and the other a FlySky radio.  One boat continued to jitter, out of the water and the other did not.

 

I started to check the boat with the Fly Sky system and of course ……. no jitters!  to eliminate the servos as a problem, I plugged a working (test) servo into each receiver channel in turn, they all worked.  I plugged the original servos back in, one at a time and they worked.  the verdict …… not a servo problem and not a receiver problem.  OK, was it the batteries?  Both boats used NiMh batteries and all showed normal voltage ……… not there!!!!  The “Spektrum boat” had had an intermittent sheeting problem for a couple of seasons but with new batteries there had been no change ……… seemingly not a battery problem.  Many years previously, I had used FM radio control and found it was affected when used near a microwave tower, I had wrapped aluminum foil around the Rx and that seemed to have solved the problem.  I wondered if some source of interference was affecting these two boats but since others sailing in the same area had no problem, likely not.  I had previously raised the Rx in the “Spektrum boat” as high as possible in the hull but that had no affect on the intermittent sheeting problem which occurred just as often under strong as light wind conditions.

 

I know, get to the point!  I assumed, because the batteries showed normal voltage and that the radio systems worked on the bench, that the batteries were not a problem and I did not put a boat in the water with replacement alkaline or NiCd batteries (which I could have done).  The FlySky radio provides bi-directional transmission and can display voltage at both the Tx and Rx.  It was after I had been working on the “FlySky boat” for some time and with both the sail servo and the rudder servo plugged in that the servo jitter started again and I could see that the Rx voltage was going crazy!  Yes the light came on, it was a battery problem after all!!!!!  NiMh batteries can develop considerable internal resistance as a result of age and the way that they are charged.  Different makes have different characteristics.  Although these batteries have good voltage, if the internal resistance has become too great, the current draw demanded by powerful servos cannot be met and sometimes this will cause a jitter and sometimes an intermittent servo response.  Given time without use, the batteries may recover enough to allow the servos to start working again (masking the cause of failure).

 

Alkaline batteries will work just fine in all three boats and as soon as their voltage drops by about 20% it’s time to replace them (to avoid an on-water loss of power).   Good quality alkalines are reliable and a well known product; but they can be expensive if you need to cover a lot of model activity.  NiCd batteries have low internal resistance and offer some excellent characteristics for model use but, because of their potential toxicity, they are no longer widely available.  The recently developed LiFe batteries offer useful application in model boats as well but they are relatively expensive.  Although high capacity, good charge cycle and safety  characteristics are a very much in their favour, experience with long term use remains limited.

 

From what I understand, the 4th generation, 2100 mah Eneloop batteries (produced now by Panasonic) are as good as we are likely to get if we want to use NiMH rechargables in model boats.  But just because we can do the wiring to allow recharging in a boat, don’t do it!  take the batteries out and keep connections dry and clean to avoid corrosion.

 

Now just to add a final note.  The power boat also developed servo jitter, but that was easier to identify, though not to solve.  The jitter changed with rpm and was clearly related to the motor. This time, I used a 11.4v LiPo battery for power, and current draw was not an issue but my brushed motor was clearly creating a lot of r/f noise and the use of capacitors across the terminals did little to stop the noise.  Twisting together the wiring didn’t help much either.  Fortunately, I had some toroidal magnets but to completely stop the problem of electromagnetic interference I had to place a magnet on each of the wirings to active receiver channels.  Because the model is quite small, it was extremely difficult to make this installation.  Electronically, that’s one noisy motor but the good thing is that it makes little sound!

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