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REMOTE CONTROL By BOB YOUNG Transmitter encoders & black • wire syndrome All remote control transmitters use some sort of encoding which then must be decoded at the receiver. This month, we start on the subject of encoders and then pass on to practical matters such as the effect of battery fumes on PC boards. If we think of an RIG set in human terms, then the transmitter encoder is the "brain" that digests the incoming information from the control sticks and knobs. This raw information, in the form of parallel varying voltages, resistances, capacitances or inductances, must then be processed and converted into serial code format or "digital speak", if you will pardon the Orwellian jargon. In the PPM (pulse position modulation) system, the Rx and servo electronics are all passive elements and derive their timing from the master clock in the encoder, thereby making it possible to run as many receivers as you wish from the one transmitter. If we carry the human analogy further, the transmitter RF section then becomes the "voice", transmitting the processed serial control data over great distances. There is an interesting aside to this analogy, for in the "Book of Zohar", an ancient Jewish book said by some to contain the secrets of Moses, there are passages in which the "The Voice" is quite carefully distinguished from "The Speech". It appears that the ancient priests converted speech into "The Voice" . "The Voice" was then described as having an action lncoder Output similar to that of water in a pond when a stone is dropped into the middle of it. The stone disturbs the water and sends out waves which reach the distant shore. This is quite an up-todate description of the effect of Hertzian waves. So ends this month's sermon. First generation digital sets used a 2-transistor monostable multivibrator as the pulse generator. These very quickly gave way under the unrelenting pressure of cost to the "half shot" circuit - see Fig 1. This was the standard pulse generator circuit found in almost every set produced between 1969 and 1985. It is still very common and is very reliable and stable. This is an 8-channel halfshot encoder board which is actually half of the encoding circuitry used in the 16-channel transmitter shown in our December issue (page 15). This transmitter was developed for John Grant of Custom Model Cars. r------ ---, r--------, Audl1ary fl Auxiliary 12 I I ._ ________ J 14 SILICON CHIP L--------' Fig.1: this early R/C transmitter circuit uses a number of "half shots" (eg, Q3, Q4, Q5) to perform encoding of the control settings. Qt and Q2 provide the clock circuit while Q7 and Q8 provide the modulator stages. The half shot pulses are fed to the modulator via the associated diodes. Eventually, this circuit gave way to the unrelenting pressure for gadgetry and sales gimmicks. The half shot is not very flexible when it comes to the addition of ancilliary control effects such as servo reversing, half rate, exponential etc. Thus we saw the advent of the discrete symmetrical encoder, usually balanced around a midpoint reference voltage. This allowed the servo control voltages to be reversed without retrimming the neutrals. This development was eventually incorporated into standard encoder chips, the Signetics NE5044 being a good example. This type of chip is very flexible and allows all kinds of gadgets to be incorporated into a model Rf<:; transmitter. There are drawbacks, of course, and two of note are RF inter£erence from the Tx RF section and shifts in neutral due to contamination on the printed circuit board. This latter problem shows up in some older "half-shot" sets quite regularly, especially those using high impedance IC encoders. The most common source of contamination is the gas vented from overcharged batteries. This produces quite pronounced shifts in neutral positions with temperature or humidity changes. If this occurs, scrub the Tx PC board with methylated spirits and blow dry with a warm hairdryer (not hot). When the neutral shift is eliminated, spray the board with Electrolube Clear Protective Lacquer (CPL200) or something similar. It really is poor design procedure to include the Nicad battery inside the main electronics chamber or housing. In all of my new industrial designs, the battery is enclosed in a separate housing and is removable for charging. The gas from venting batteries does terrible things to electronic components and leaking electrolyte will eat its way through copper circuit boards and aluminium cases in very short order. As is always the case, do not leave batteries (dry or nicad) in elecronic equipment if it is not being used. If the transmitter has a removable battery, then take it out for charging. This procedure may be somewhat tedious but is well worth the effort in the long term. The "black wire" syndrome An associated problem is the dreaded "black wire" syndrome, the bane of all modellers and probably all users of nicad batteries. Nobody seems to know just what causes this problem but on the surface it appears to be a process by which gremlins, gradually and using great stealth, exchange the copper in the negative lead from the battery for some kind of black garbage. The transformed conductor is shiny blue-black in colour, devoid of any tensile strength and with a con- ductivity approaching that of air. The transmitter thus suffers, in human terms, coronary occlusion, with its supply of much needed electrons gradually strangled off. The results can be horrific but milder effects range from from low transmitter power to lack of decoupling. The latter can give rise to RF inter£erence in the encoder and this can cause shifting or jittery neutrals. Eventually, a complete breakdown will occur. If this happens during a flight, then it's a serious matter indeed. What to do Fortunately there are very simple solutions to this problem. Standard insulated hookup wire in which the conductors are tinned (ie, normal hookup wire in which the strands are bright and shiny rather than plain copper) will slow the process remarkably. The effect shows up mainly on plain untinned copper conductors. Keeping the battery ends moist with CRC 2-26 will also help. You should examine all battery wiring every 6 months or so. Pull back the insulation and examine the copper conductor. If there is "black wire", the lead will probably fall off in your hand. Re_Q_lace all leads showing the faintest traces of this black contamination. It usually stops at the switch, but not always. A word of advice here: use an old soldering iron tip to disconnect the lead. The black stuff will contaminate the tip and render it FEBRUARY1990 15 VIRTUALLY NO UP - ALL OOWN -UP 45' --.... OFFSET ZERO REFERENCE SERVO 0/P ARM PUSH ROD TD CONTflOL SURFACE Fig.2: the servos should be set up so that they give equal travel about the neutral point. If this is not done, the control will move faster in one direction than the other. useless for soldering thereafter. If the conductor is clean and bright but plain copper, then replace the lead anyway. If it is silver (plated) then you have few worries, but you should still check it periodically. A leaking battery can chew up good leads very rapidly. Battery damage in transmitters is a problem that frequenntly confronts the R/C serviceman. Care in this area will pay dividends. · ·One final word on the "black wire" business. It has been suggested that the black dye in the PVC insulation affects the copper. I have also seen this effect on the positive (Red) lead on rare occasions and on wires of all colours when used around earth terminals. It has also been suggested that overcharging plays some part. I have seen brand new sets which have been charged once or twice and left to stand for several years exhibit the problem. As stated previously, I have yet to see a satisfactory explanation for the problem. That does not mean that an explanation has not been published but if it has I have yet to read it. 16 SILICON CHIP Perhaps all I can say for sure on this subject is that leaving sets standing unused for any length of time is very poor practice. The batteries should be cycled regularly and left in the discharged condition. Control memory With the advent of LSI, the microprocessor eventually found its way into R/C equipment for models and thus we saw the development of the PCM (pulse code modulation) system. I am not a great fan of PCM for models and prefer the . old PPM system. However, the processor has revolutionised the PPM system and we now have transmitters offering some very nice features as a result. One of these features is the ability to store the trim and throttle settings for several different models in the transmitter memory. A PPM transmitter capable of storing trim locations for three models is a very useful item. But here again I must stress that before you rush out to buy such a transmitter, remember that this all costs money and is not really essential. A well trimmed model should fly with all controls set at neutral and the control throws set in such a manner that full throw gives just enough control response to accomplish the tasks required of that model. Smooth, precise flying begins with the correct setting up of the controls. There is a very practical reason for this and it has to do with mechanical advantage and servo flutter. Using less than full servo throw is poor engineering practice indeed. For many years I flew aerobatics, pylon and helicopters on the same day using the same transmitter without memory and without undue difficulty. However, I was very careful about the trimming and setting up of the controls in each model. Every model flew with the controls set at neutral. If they did not, then I landed and retrimmed the aircraft. At first glance this may seem odd and dated, but the concept is as valid today with the processor memory encoder as it was then with the old half shot encoder. The reason is very simple. Servo output arms are a rotary device and thus will only give equal throw about neutral if the neutral reference point is goo to the control push rod. If this angle is not goo, then some amount of differential control throw will be introduced into the system and the model will respond faster in one direction than the other. Fig.2 illustrates this point. Now this can be a useful feature and is used quite often in setting up control throws, particularly on the ailerons where large amounts of differential control are desirable at times. It is not in the least bit desirable if it gives an unwanted faster roll to the left than the right or more down than up. Control geometry is a very complex subject and will form a column of its own in due course. For the moment I will just confine myself to pointing out that everything is not as simple as it looks and that hi-tech gimmickry is useful but no substitute for the careful and studied application of basic principles. All of the foregoing aside however, the new encoders do have Problems? .. .and you don't have our NEW 1990/91 This is a 7-channel symmetrical encoder developed by Proportional Systems Australia (PSA). some very nice features and the most important are listed below: Servo reversing: a slide switch is provided on the Tx to invert the pulse width on each channel, thereby reversing the direction of travel of the servo. The alternative is to rewire the servo by reversing the two outside pot wires and the two motor leads. This feature calls for a deal of caution on the part of the user in case take-off is made with the servos reversed. Let me stress right now that this is all too easy. All control throws must be checked before the first flight of the day for correct direction of travel. This applies on any Tx, even those without servo reversing. It only takes the pushrod to be accidentally replaced after adjustment on the wrong side of the servo to wreak havoc. With servo reversing, it is even easier to come undone, especially if two models are being used on the one transmitter and reversal is required for only one of the models. I have seen the odd pilot who is clever enough to fly with reversed controls but they are rare indeed. Dual rate: this is somewhat dated in my mind by the exponential system. A switch is provided for one or more channels on the front of the Tx with an associated potentiometer. The pot is adjusted to set the overall percentage of servo travel available (O - 100%) with full stick throw. On half rate, full stick throw will only deliver 50% of the available servo travel. Returning the dual rate switch to the off position restores 100% of the servo travel. This is useful for high speed flight where the controls become very sensitive around neutral. It must be remembered where this switch is before commencing any manoeuvre, particularly outside loops. I have seen models crashed by pilots starting too low to the ground in the belief that they were in "high rate" when in fact they were in "low rate" . It is very awkward to get to the rate switch in time if this error is made. Another drawback is that two distinct sets of control reflexes must be developed, one for high rate and one for low rate. Learning to fly is hard enough without added handicaps such as this. Exponential control: often switched in by an external or internal switch and gives electronic damping of the servo throw 1;1round neutral. As the name implies, the control throw follows an exponential curve, with less throw at neutral and increased pace as the stick moves to extremes. The advantage of this scheme is that the control response of the aircraft is alwavs constant whereas with dual rate, two sets of reflexive responses must be developed. Servo end point adjustment (EPA): a very useful feature and quite safe to use. It is especially useful for throttle adjustment where it is not desirable for the servo to run up against the end stops. If this happens, it increases servo drain and can burn out servo motors and amplifiers. It can also lead to Rx failure due to flat batteries in the model. A potentiometer is included in the channel for fine adjustment of one end of the servo travel. This is a very important point and sets not fitted with EPA must be set up very carefully to avoid these problems. ~ 148 page electronic parts and accessories catalogue ... Its our latest TRADE catalogue for the consumer ARISTA ... Your one-stop problem solver. ...Stylus ... ... Plugs, Jacks and Sockets ... ...Batteries .. . ... Cable .. . ...Tools and Technical Aids ... ...Plug and Power Packs ... ... Car/Auto Accessories .. . ... Boxed Hi Fi Speakers ... ... Raw Replacement Speakers ... ...Speaker Accessories ... ... 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