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REMOTE CONTROL BY BOB YOUNG The development of digital proportional RC transmitters As we have noted in recent columns on this historical series, the development of the proportional control transmitter was a long, slow process which had its roots in the Galloping Ghost and similar analog systems. Galloping Ghost transmitt ers had only one stick, which gave rudd er and eleva tor control, and a lev er switch which gave positionable throttle via a pulse omission detector in th e decoder. This was quite primitive, as noted in previous articles on the Galloping Ghost system. Even more primitive by modern standards was the construction of the control stick assembly which was a very popular and reliable unit made commercially in the USA by "Protrol". The direct mounted potentiometers and the large completely open square hole is a far cry from the modern sealed gimbal in today's sets such as the Futaba Tx featured in this article. However, this type of stick construction died hard because it had one great advantage over the modern sealed gimbal: the excellent centring accuracy obtainable from th e direct mounted pots. The ·Americans fought hard to retain this typ e of stick assembly and retained it on their top-line competition sets almost until they were forced out of the commercial market. The customer is right The Japanese eventually got a stranglehold on the R/C transmitter market by producing sets with sealed gimbals, all moulded plastic contruction and lots of operating features. 90 SILICON CHIP Here we see a very practical and technically superior device giving way to customer pressure, for some very valid reasons secondary to performance. The "open gimbal", as it was known , was prone to dust ingress into the electronics and the large hole was quite ugly when compared to the slick finish of the modern Japanese units moulded in full plastic. The coup-de-grace was delivered to the open gimbal as the accuracy of manufacture of the sealed gimbal gradually improved. Today's sealed /, The interior of this Silvertone transmitter (made around 1969) shows a board using a half-shot encoder. The folded metal construction, while desirable from a technical point of view, was far less attractive to modellers than the modern moulded plastic sets. gimbals give nearly equal results and offer the above advantages as well. Thus died the "open gimbal" transmitter. It is interesting to note the rigid thinking of the American R/C manufacturing industry. Quite apart from the price disadvantage, they failed to recognise the importance of packaging and styling and thus they clung to the traditional methods for far too long. Their greatest failings were in staying with folded aluminium transmitter cases and in not recogmsmg the need for improved servo g\)artrains. That said, there were sound technical reasons for retaining the aluminium Tx case, even if it did look old fashioned, but their failure to improve their servo geartrains is totally inexplicable. As the Americans were the only reliable suppliers of OEM parts for small manufacturers, I can remember pleading with them, year after year, for more powerful and quieter geartrains and moulded transmitter cases. The pleas fell on deaf ears and my own sales suffered along with the Americans. In desperation and despite the cost, I finally began tooling for my own servo cases, but by this time the battle was lost. Thus died a major component of the Australian R/C industry. However, once again I digress and we must return to the main story. From the Galloping Ghost systems there developed full house analog proportional sets which featured two twin axis gimbal assemblies and looked for all the world like a typical early model digital system. The only problem was that they did not work anywhere near as well. Thus, we can see that by the early 1960s the mechanical form of the proportional transmitter was well established. Mathers & Spreng When Mathers and Spreng dev eloped their digital system in the early 1960s, there existed a sound mechanical layout to install their electronics into and there was nothing here to excite the fans. However, the revolutionary aspect of the Mathers and Spreng concept was in their use of the then almost unheard of digital techniques. In this , they turned the world of R/C electronics completely on its ear. Within a decade, their system had become the industry standard and completely swept aside all other systems. We have already examined th e digital servo and noted its need for a positive input pulse whose width is variable from 1-2ms. Spreng and Mathers pion eered the use of what was then called PDM (Pulse Duration Modulation) which was not quit e technically correct and which has since given way to the more correct PPM (Pulse Position Modulation). The actual servo input pulse varies in duration it is true, however the modulation system they used converted pulse duration into pulse position and it was this feature that gave the transmitt er its most powerful advantage. In essence, what they did was to transmit the control pulses in a serial form using marker pips at the start and finish of each pulse (s ee Fig.1 ). The marker for the trailing edge of pulse number 1 was the marker for the leading edge of pulse number 2 and so on. Thus , the data was carried in the position of the marker pulses. This had several advantages over prev'ious systems in that th e system was virtually a full carrier system with only a narrow spike of no transmission. This kept the receiver AGC clamped into low sensitivity and thus the best state for noise rejection. The frame or repetition rate was also very fast with all eight channels being updated every 22 milliseco nds, ]ULY1991 91 MASTER CLOCK ._____ _J I CHANNEL 1 ____.I CHANNEL 2 . _ _ _ _ I_ CHANNELJ _ _ __ _ _ _ __.n------------~' I ....,I CHANNEL 4 _ _ _ _ _ _ _ _ MARKER PULSE TR AIN 0 ~ ti1 i~~~fb~ --++--+]+----++-+[]+-----(.._____.-++--) (......________.--) [ Fig.1: the Spreng & Mathers technique transmitted the control pulses in a serial form using marker pips at the start and finish of each pulse. The marker for the trailing edge of pulse number 1 was the marker for the leading edge of pulse number 2 and so on. spent in range checking, tuning and chasing fly-away models. You never went to the flying field without your name and address inscribed indelibly on your model. How long is it since anyone has done that? I haven't seen a name on a model for years. How times have changed. After the advent of crystal-locked superhets and digital proportional systems , we had nothing to do all day but fly the models and drink coffee; really boring stuff. We all very quickly began to put on weight from lack of exercise and I have not lost it since. Who said progress was all good? I can so there was no time lag in control response . In addition, the system was inherently stable, using as it did full monostable multi vibrators as the master clock and pulse-width generators. There was absolutely no tuning requ ired in the entire system. This fact absolutely floored us old timers, brought up as we were on tu ni ng direct coupled super-regen trans istorised rece ivers, tuned reed aud io tones and free running transmitter oscillators. Tuning and tweaking was an important part of life on the flying fi eld for us and a good part of the day was MARKER PULSE GENERATOR BUS VR1 50k TO ~ - - FOLLOWING CHANNELS _rL_ ..,. 92 S ILICON CHIP Fig.2: this simple "half-shot" circuit formed the backbone of transmitter encoders for many years. Any number of these circuits could be strung in a row, depending on the number of channels needed. remember once running after a model from the Cooks River up into the main street of Earl wood shopping centre, a distanc e of 3-4km. R/C modellers had to be fit in thos e days. They also had to be insensitive to the "village idiot" label inevitably hung upon them. Believe me, nothing looks sillier than a grown man chasing a runaway model whilst waving a transmitter aerial ineffectually at it. You also had to have a heart as big as a lion to walk into a house with a model sticking out of a broken. win dow or the tile roof and for ask it back. However I digress yet again, so back to the story. In 1964, Howard Bonner brought out the Digimite 8-channel proportional system which was very professional in approach and appearance. Featuring such novel features as failsafe and full y wired servos with plugs for instant interchangeability, this system set the pace for several years. The most remarkable feature was, however, the sea led control gimbals, a first for the industry. The Bonner system suffered several drawbacks as we have already seen in past articles, and the main criticism of the Bonner sealed sticks was the mechanical trim. Modern gimbals use a trim lever which rotates the body of the control potentiometer and thus gives about 15% additional range to the stick travel, leaving the stick still mechanically centred. Bonner, on the other hand, used a mechanical trim which again gave about 15% of the travel for trim but instead shifted the control pot by shifting th e mechanical neutral position of the control stick. The disadvantage of this system was that if the model was flying in a trim that required (say) full up trim, then the available up elevator travel was less than the down elevator travel. I personally felt this criticism was unjustified for the simple reason that a good fly er trimmed his model correctly so that the trim is always in the qmtre. This applies even today with the modern microprocessor encoders and I have stated this previously, on many occasions. There was one very big advantage in the Bonner sticks and that was that all of the servo travel was available from the stick regardless of the trim position, whereas in the electrical trim system, 15% of the servo travel is These photographs show the open gimbal construction used in Galloping Ghost transmitters. kept in reserve and not available from the stick. The microprocessor systems at least have cured this problem. This advantage to me completely outweighed the disadvantage of the stick centre moving and I used Bonner Screws ,... ·r•g . -. +l Fig.3: this exploded view shows the main components inside a modern sealed gimbal assembly. Note the yokes which operate the two pots & also the mechanical trim levers. <at>) No.2 xl/8" L- -<at>- Transmitter encoders To finish the discussion on transmitter development, we need to talk about transmitter encoders which were also partly covered in the January 1990 article. Although the full 2-transistor multivibrator used for producing PPM signals was very stable, it was also very heavy on component cost and the relent- No.2x5/16" l sticks in my own equipment for many years. I also continued to fly with these sticks long after I had changed my production sets to electric trim due to customer pressure. ~7ifflj1 f No.2x3/16" •lli.! . Side plate centring tab on other side 11(3 .2x1/8" Side plate/ ,,,,;,, ,/. I ' I J\ Allen key 1·5 mm Pot carrier keys Yoke ~Yoke,.,:,,.;• 4/40x1/8" Set screw ' "---, / Allen Bradley type J Threaded bushing Fig. 3 less pressure for cost savings produced the clever little circuit featured in Fig.2 . Known as the "half-shot", it very quickly also became industry standard and was the backbone of transmitter encoders for many years. You just simply strung as many of these things in a row as you needed channels. and you had a simple and very reliable transmitter encoder. We built 32-channel transmitters for our robotic puppets from these pulse width generators and they were very successful. Their big asset was their voltage stability while thir chief disadvantage was again component count when compared to the new IC encoders. Another disadvantage was that they were not flexible enough for modern demands in regard to servo reversing, dual rate and exponential control configurations. Thus they gave way to the balanced rail encoders which used a stable reference voltage and which allowed symmetrical operation for servo reversing at the transmitter end. The Signetics NE5044 is good example of this type of encoder. However, nothing beats the microprocessor for flexibility and they are gradually finding their way into more and more R/C transmitter encoders. Finally, while this electronic race for improvement was in progress, there was a relentless quest for improved appearence and accuracy in the transmitter mechanicals. The transmitter of today, loaded to the gills with microprocessors and liquid crystal displays, is a far cry from the bent tin jobs that us old timers called the answers to our prayers. SC ]UL Y 1991 93