Boat electrics and power dips

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

A short while ago I completed a second conversion of the Severn lifeboat from the Chinese electronics to a twin motored 2.4GHz set-up. I used the new Mtroniks ESC “plug and play” for each motor to give tank steering, with counter rotating propellers. The transmitter was a Radiolink T4EU. The initial choice of motors were two JP 400, to give a bit more speed. Power was from 6 x 1.5V alkaline batteries.

The first “problem” encountered was having left and right hand propellers. The electrical set up for the ESC requires polarity to be followed. One BEC was used for the receiver, the other having its red lead decoupled. On switch on the two ESC powered up, they then recognise the stick positions on the transmitter as the neutral (centre) position and the first movement of the transmitter stick is memorised as ahead. Ah, with counter rotating propellers I discovered that one stick needs to go ahead, the other astern, so that the ESC then rotate the shafts in the desired direction when both pushed forward. This is NOT a problem as such, it just has to be remembered at each switch on, as the “plug and play” learn each time they are powered up. So centre sticks, switch on, move sticks in direction required, “problem” solved. It also means you don't have to reverse stick movement.

The second, real problem was discovered when on the water during first “sea trials”. For some reason the response of the sticks reversed after first full use, indeed it became almost uncontrollable, and it was obvious that the “failsafe” was operating in part during this episode.

NOTE: the failsafe on the Radiolink transmitter is on channel 3 [THR] and is factory set to full back on whichever stick is THR, remember it alters with Mode selection. Most 2.4GHz sets are designed for flying where the THR operates in one direction only, tick over is full back and full throttle is full ahead, so the failsafe is cutting the throttle (on a boat that's full astern!)

I was using both sticks up/down for throttles, and was in Mode 1, which meant that the righthand stick had the failsafe. This stick also had the boats rudder as left/right. My solution was to swap the channel stick connections inside the transmitter (re-soldering them) so that channel 3 became the left hand stick left/right and channel 4 now became the right hand up/down starboard throttle. On the bench it worked ok.

Sea trials again, a similar uncontrolled episode as before, but without any apparent failsafe full throttle cutting in. So I had dealt with that, but what was causing the confused controls. Why were the sticks changing settings and neutral positions being ignored? Reversing the stick settings back on shore and trying again did not help at all. Help!

Back on the bench. I decided to try and replicate what was happening in the water. The first thing was to put load on the motors to simulate the water effect, immediately the throttle controls were erratic and when centred, stabilised but sometimes reversed. Of course, on the bench when things get out of control one's instinct is to put the sticks to neutral. It was only by looking at the receiver and not the moving parts that I saw what was possibly occurring. With one motor running full all was ok, but powering on the second motor (regardless of direction) caused the receiver LED to blink, followed by electronic confusion. The power drain caused when the second motor started up was causing a dip in the BEC voltage to the receiver, hence a temporary OFF and no signal, so the failsafe kicked in, and the “plug and plays” established new stick positions for neutral and ahead as if the receiver had just been switched on again (which it had). And the sticks were every where but centred.

Sounded good to me. Checking the power on the JP 400, about 4A when both full on, so quite a pull when switched hard on. I swapped the motors for two MFA 360 3-pole. Slower motors but a lot less power consumption (0.5A @ 12V). Testing on the bench, all was smooth as a baby's. There was no blinking on the receiver LED.

On the water, perfect control, both motors working fine, tank steering no problems, no glitching or stalling or erratic behaviour.

I think I've found the cause, and provided a solution. I'm not saying it's a major problem but be aware that the plug and plays can reset if the power interrupts to them, and likewise, a lowered voltage to the receiver can cause the failsafe to cut in. My digital meter could not measure the voltage dip as it occurred, it was of short duration but was still significant.

Has anyone else experience of this or any comment to make, I'd really like confirmation of my findings. I have another tale to tell about failsafes and switch on for a later thread, concerning the Spektrum DX5e.

Kimosubby

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

A problem solved?

[Dave MilbournParticipant @davemilbourn48782]

Kim

This phenomenon is know as "brown out" i.e. not quite a full blackout. Have you tried disabling BOTH BECs and running the receiver from a separate 4-cell battery pack?

My vote for the most annoying aspect of modern radios is split 50:50 between failsafe and battery eliminators.

Dave M

[Malcolm FraryParticipant @malcolmfrary95515]

If the voltage from the battery is dipping under load, either the battery or its wiring is not up to the job. Either thicker wire or a battery capable of higher delivery or both. Trying to power a boat with any performance from alkaline batteries is doomed before you start. Almost anything rechargeable will be better. Their output voltage is much more stable over a wider range of current demands.

Early BEC chips liked a couple of volts to chew on over their output voltage, dip below that and the output voltage dipped as well. Modern ones need less, but the same characteristic is still there. On circuits that rely implicitly on a stable voltage supply for timing (e.g. servos, ESCs) rapidly changing supply, and therefore reference, voltages are certain to create havoc.

I really can't follow the rotation problem. If the motor driving the RH turning prop behaves OK, it usually means that the motor driving the LH prop needs to turn the other way for forward. Just reverse the wires between the motor and the ESC. The ESC coughs out one polarity for forward, one for reverse. It doesn't care which way the motor is turning. Or if there is a motor there in the first place.

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Thanks both, I appreciate that the power unit itself is small – but thats the maximum I could get in because of weight. If it was my boat, I'd have gone brushless and used Li-Po batteries. Six re-chargeables means less voltage (7.2) as against six throw aways (9V).

I tried, as an experiment, a separate receiver battery – the set-up worked just fine with the JP motors. So the problem is a voltage dip under load. Again, had to save weight so avoided carrying another battery.

Malcolm, you may not have used the Plug and play yet, but no matter what way around you wire up, the ESC will always sense the polarity, and assume that the first stick movement is ahead (cos that's what it's been programmed to receive.) The instructions with them require the polarity be maintained from the ESC to the motor too. My explanation does read odd and seems odd, but believe me, the way I got it all to work is as written above.

Having used these "plug and play" I'll revert back to the manual setting ESC, at least you can then depend on them being the same as last time used (most of the time).

At least it gets the grey matter working.

Cheers, Kim

[Colin BishopModerator @colinbishop34627]

I tend to agree with Dave that wherever possible it is best to fit a separate RX power supply but of course this is not always practical.

In principle 'bleeding' power off the main motor supply must always entail a degree of vulnerability to voltage fluctuations and this seems to be what has happened here.

Colin

[Gareth JonesParticipant @garethjones79649]

Hi Kim,

I am interested to see what you have to say about the Spektrum DX5E. I bought a couple of their MR200 marine receivers last year and had lots of problems trying to get them to work in yachts. I will try and remember the symptoms when you start the thread.

Gareth

[Malcolm FraryParticipant @malcolmfrary95515]

Plug n play does set up the ESCs to assume that center is off, forward is forward, and reverse is reverse. It also assumes that the reverse switches on the transmitter are set to normal otherwise total confusion ensues. The output from the ESC just offers voltage to the motor by connecting the two motor wires to the power leads internally under control of the logic circuits. By connecting the motor wires one way or the other it is you who determines which way the motors will turn in response to the stick movements. Unless you have done something strange, unlikely and non standard with the wiring. The only times I have run into problems is when I have scored an own goal by forgetting the reverse switches on the TX.

Polarity. ESCs are always connected to the power observing the correct polarity, else they die. Power polarity between ESC and receiver is always correct because the plug layout prevents anything else. The actual signal telling the ESC what to do does not have polarity. It is a series of pulses, and it is the timed length of these pulses that determines what the ESC is about to do. Normally, 1.5mS is center off, longer (up to 2mS normally) forward, shorter (down to 1mS) reverse. As standard, this repeats every 20mS or so. The bigger the difference, the more power is offered to the output. The polarity of the power leaving the ESC is determined by the ESC interpreting the signal from the RX The polarity arriving at the motor, and thus direction of rotation of the motor, is determined by which way round the two wires are connected.

Check the battery voltage under load – it's a reasonable bet that 9 volts off a small alkaline battery will drop enormously to less than the 7.2 volts that the rechargeable battery will maintain. As a result, the boat will probably perform better off 7.2v rechargeable than an alleged 9v from an alkaline. Be prepared for a surprise or two – just one Speed 400 in a Lotse (pilot launch, 21" long) gets it up on the plane and scuttling along very nicely. Losing the battery box is a good way to improve performance. It is a big, heavy lump of plastic whose weight is better gone.

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

I understand all that you are saying Malcolm.

What I was getting across was that when the receiver lost signal out on the water (due to the power drop) and then found it again, the ESC then had new neutral points and probably stick movements not necessarily forwards, hence I then had a vessel "not under control" as I had no idea where the new neutrals were and what direction the ESC had assumed was forward. And then the cycle repeated again several times before I could get the vessel back. Amongst that was the failsafe acting with each drop on the starboard motor as well. It was the combination of events I was describing.

It's no longer my boat, and with the smaller motors suits the newcomer well. He will, no doubt, upgrade to Li-Po shortly, the motors being capable of taking 15V. I'll also, if he does that, suggest he get a small 4.8V pack for the receiver (as he'll have some weight to play with.) I could then swap over the 360's for the JP 400's.

Have you any experience with the Spektrum DX5e? I'm struggling to set one up for sail winch control.

Thanks again, Kim

[Malcolm FraryParticipant @malcolmfrary95515]

For my sailboat sailing I just use an old 27MHz set. Very basic 2 channel, that transmitter has had the left stick center spring removed. Simple, reliable, and when the boat takes it upon itself to make a landfall somewhere inconvenient, it has a handy long stick for reaching and poking with, a lost feature with 2.4GHz.

Winches are simple enough, really just a servo with a round arm that turns a lot. The difficulty usually appears with the mechanical aspect – keeping the string where it belongs and not having it fall off and create either a Gordian knot or a tight squidge under the drum.

The recent ones with a shrouded drum as sold by Component Shop, and, if I have read the advert properly, Howes, are as simple as you can get and very effective for my kind of sailing. Just a question of figuring out the distance the line winds, and tweaking the line geometry between hull and sail boom to suit. The exact number of turns, and thus the winding distance, does vary according to the individual transmitter. My best bet would be to leave it "natural" at the transmitter, and do the tweaking mechanically at the boat end. It will save accidents later on when it gets a wrong profile and tries to wind the winch through its own mounting.

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

I've been building and sailing yachts for several years, both free sailers with Braine gear and radio controlled too. I agree the old 27 and 40 MHz sets were ideal with just the two channels, but with bigger fields of entrants there became a need for owning several additional crystal pairs. There are still only 13 frequency slots for 27MHz although the band is now split to 32 possible with only 10kHz gaps but old gear is not that well tuned.

Of course, the advent of 2.4 does away with the need for crystals, and far more yachts can sail together. It's now a question of which sets are best use for sailing, and strangely enough it's the cheaper sets, although end point adjustment does appeal. The Radiolink and the Planet 5 can both be simply used as 2 channel units. I have a spare Spektrum so gave it a try as it does have marine receivers available, and it is attempting to train it to what I want it to do that is annoying. Currently I cannot change the failsafe position.

We use large powerful sail winches, as you know, with continuous loops under deck, driving sheets on deck held taut with elastic. We do tweak on deck as you put it, but mostly with 2.4 sets we set end points for the sticks to control winch travel. That facility is not present with the cheaper sets though, so we rely on a repeatable end point of winch travel.

I am just finishing a "footy" Vintage boats "Bob-A-Bout" and have incorporated one of the enclosed-cup sail winches as supplied by Component Shop. I'm curious as to how that will perform.

Aye, Kim

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Malcolm, breaking news – I've discovered the problem. My apologies to Spektrum.

This Spektrum is "used" and on opening and looking within I found a good repair on the THR stick potentiometer mounting, only the position was about 3mm out from the end point, hence the stick end point and the failsafe end point overlapped. Drilled a new hole and now they don't (end points coincide exactly) and more importantly won't as I'm moving the failsafe to a stick function not in use. (Just like the previous owner did!)

I'm in full agreement with DM, failsafes and BEC's are the most annoying contrivances on these modern sets.

Kim

[Dave MilbournParticipant @davemilbourn48782]

Don't be too kind to Spektrum, Kim. It was their lousy manufacture and QC which set the LH stick neutral signal at about 1.3mS and meant that it couldn't be adjusted to the standard 1.5mS even with full trim. These floating neutral values are fine when all you have to connect is a rotary servo, but anything electronic which requires a set value is a headache. As for failsafes which you can't turn on/off or reallocate…………
I'll have more to say about "budget" sets in my forthcoming MB article (Winter Special 2014). At least you didn't pay the full price for it!
DM

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Hi Dave, I'm not into the actual electronics side so didn't realise that the neutral points are also different for Spektrum sets. I'm starting the Spektrum thread now, under Failsafes and the DX5e. I've had other contacts that have had very similar "events" as I tried to describe, and some have caused damage. As yet none has been able to let me know how to re-set the failsafe either, and we've all tried.

It's still a usable set once you know its quirks, but as you will read in that new thread, don't get the MR200 receiver if you hate failsafes.

Aye, Kim

[Dave MilbournParticipant @davemilbourn48782]

Nah – I got so fed up with the set that I flogged it……………..

DM

[Malcolm FraryParticipant @malcolmfrary95515]

For my size of boat (Victoria) its very adequate. I can sail in conditions that would have pulled the sail arm out by its roots. I am using it as a single end winch, rather than a continuous loop for simplicity's sake. The thing to watch is the type and size of line. Too thick, it can over-fill the space in the shroud and tangle under light air conditions, when there is not enough pull, too thin, it might find a way past the shroud. It also needs a smooth surface for free running. I have been using 30lb braided Salmon backing, and through dumb luck, this seems to work very well, ticking all the boxes. Its a bit thinner than the thinnest venetian blind cord that B&Q sell.

Any failsafe would be a potential disaster on my lake – when taking the boat is taking me for a walk round the lake, there are a few patches where I have to pass behind stands of trees. I can easily get away with sailing the boat into an open area, setting it to circle, and switching off while out of sight. The old fashioned gear simply picks up where it left off when I switch back on after passing behind the trees. There just isn't a real failsafe condition that is safe in any given condition. A plane, with signal loss, is normally required to descend in a fairly tidy manner, a truck is safest stopped. A boat could do anything given an inappropriate command by a "smart" radio. Unwanted full reverse on a power boat is unhelpful, so is unexpected close hauled in the wrong conditions.

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Hi Malcolm,

is that salmon backing braid the yellow coloured one? I found it and its excellent, no stretch at all. I use it as the main loop and then a slightly lighter strength white braid for the sheets. 100m for £11, as apposed to the exorbitant £5 for 5m from some suppliers. In fact a lot of my sailing/yachting gear is sourced from fishing tackle shops.

Happy sailing, Kim

OK Dave, I think they know who you flogged it to now……….

[Malcolm FraryParticipant @malcolmfrary95515]

The stuff that I have is a fetching shade of brown.

Edited By Malcolm Frary on 27/04/2014 11:30:56

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