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REMOTE CONTROL BY BOB YOUNG Galloping ghost - the missing link in the evolution of proportional control These days, modellers take proportional control systems for granted, without realising just how much better they are than the systems they made obsolete. This month, we take a look at a system which was popular before digital electronics took over. . Over the past few months, we have discussed much in relation to the modern radio control system but we have yet to answer some vital questions: "what exactly is digital proportional control; why is it so good; and how did it evolve?" All of these questions will be answered in the following series of articles. However, before progressing further, I would suggest that the reader refresh his memory with the first article in this series, published in the October 1989 issue, for much of the history of the developments leading up to this system was covered there. For those who missed that article, it is sufficient to state that prior to proportional control, the typical multi-channel R/C system was usu- 1 • ally some sort of audio tone system using either filters or tuned reeds for decoding. Tuned reeds were by far the most popular and successful system, certainly in this country and in America. Now the point here is that these systems only gave two to three simultaneous controls (usually two) but, most important of all, they only gave neutral, full clockwise and full anticlockwise control positions. No intermediate positions were available. This was not really satisfactory for any form of modelling but despite these shortcomings, the best flyers could give demonstrations which would be hard to distinguish from those given by flyers today using · modern proportional control equipment. 01-lsec , I ~-A ~ ~ ~ ~ DOWN MAIN AERODYNAMIC FORCE= NEUTRAL Fig.1: the inertia of the model plus the elasticity of the air will average out a rapidly oscillating control to give smooth flight. If the control oscillates symmetrically about the neutral position as shown here, the result will be neutral control. 82 SILICON CHIP We learned to adapt and we learned all the tricks. A separate servo was used for elevator trim , one which could be nudged into the correct position and left there until retrimming was required. A similar type of servo was used for throttle as well. Pulsed controls Controls could be pulsed to average out the aerodynamic effects and short pulses only gave small control throws. Long pulses gave about half throw and the lever held full on gave full throw of course. The pulses never showed up in the manoeuvres and all looked perfectly smooth - in the hands of an expert that is. The average flyer, which included myself, never really knew what to make of all of these shortcomings and thus we never really learned to feel comfortable with this system. Believe me, I crashed many a good model trying to master this system. Something had to be done, and we all knew what it was. But the burning question was how? This problem, like all problems when solved, turned out to be easy so easy as a matter of fact, that I was stunned at the time at just how simple and effective the system really was. But let me tell you that the problem occupied many of the finest minds in the electronics world and baffled most of them until the early 1960s when two Americans, Don Mathers and Doug Spreng, developed the proportional system as we now know it. So complete was their approach that to this day little has been changed except the technology applied to solving the practical problems. However, we are now a little ahead of our story, I TONE OR CARRIER ONI CONTROL POTS REPETITION RATE ~1 J ONI I 0.1-1sec #I J I 50% MARK/S PACE RATIO _.,. B1 1 !.i... -T B2 , !.i... Fig.2: the "Galloping Ghost" system used mark-space/pulse rate encoding & decoding. In this scheme, the decoded pulses controlled a relay & this in turn was used to rapidly switch a small electric motor first in one direction & then the other. The model thus responded to the mean aerodynamic position of the control surface. having missed that important step mentioned earlier. The Galloping Ghost What on earth was the Galloping Ghost? I can hear the readers now: "this lad has finally flipped for real and is now seeing apparitions". Sorry to disappoint you, but "Galloping Ghost" is the name given to that missing step in the proportional control story. Earlier, I mentioned that the inertia of the model and the elasticity of air will average out rapidly oscillating controls and give a smooth result around the averaged control positions, as shown in Fig.1. Now this phenomenon has two very important uses: (1). If interference or weak signal areas are encountered, then random noise intruding into the servo amplifier will strike some sort of average which may be anywhere from full down to full up. This average is changing rapidly but, combined with the occasional snatch of uncorrupted control data, will often keep a model flying until full control is restored. This is the factor that made PPM such an effective system and which renders the concept of "fail-safe" (neutralised flying controls) invalid. (2). It's also useful in the Galloping Ghost type systems of constantly flapping servos, in which the rapid flutter is deliberately introduced and averaged out to a mean aerodynamic value by virtue of a controlled encoder. This is our missing link in the chain of development leading to true digital proportional control. The basis of the Galloping Ghost system was the concept of markspace, pulse-rate decoding. In this system, a small electric motor was connected directly to the terminals of a relay which was switched by an encoder. This encoder was controlled. through gimballed joysticks driving potentiometers, suitably arranged to vary the mark-space, pulse-rate ratios of the relay switching. Oscillating control surfaces The name "Galloping Ghost" came from the franti c appearance of the constantly thrashing control surfac es. To hear a model gliding overhead controlled by this system was a real experience - the noise was unbelievable. Of course, the motor armatures and brushes didn't take too well to this sort of treatment. Nevertheless, the system was remarkably reliable considering the strain on the motors and relays. The block diagrams in Fig.2 show the theory behind this truly incredible and very primitive system. Fig.3 gives the timing diagrams for various combinations of mark-space ratio and pulse-rate. For a modern modeller, versed in the art of digital electronics, the following system will seem almost incomprehensible, yet it formed a major link in the chain of development and some really satisfying flying was done using this system. It also serves to indicate just how difficult the prob- lem of achieving proportional control really was and just how powerful the techniques of digital electronics are today The electronics revolution has largely been possible due to digital concepts an d it is difficult for modern readers to realise that in 1960 very few of us had even heard of the term , let alone had any knowledge of the techniques involved. Fig.4 shows the way the two separate control outputs are taken from the servo drive disc, one for the elevator and one for the rudder. The series of diagrams in Fig.5 shows the way in which the mean aerodynam ic force is generated for the two controls; in other words, the decoded output. The system worked as follows : · The transmitter was a conventional single-channel type using either tone modulation or straight carrier switching. A mark-space, pulse-rate encoder controlled the transmitter and delivered to the receiver a coded signal carrying two streams of data. One stream cons isted of a signal with a mark-space ratio varying from 20-80% to 80-20% Is Your Product Or Service Getting The Exposure It Deserves? 25,000 consumers will read this page. They need to see your advertisement if you want them to buy your product. Contact Paul (018) 28 5532 or Ian (03) 696 5411 to reserve this space. FEBRUARY1991 83 50-50 0.5sec 50-50 0.25sec 50-50 1sec n n 25-75 0.5sec LJ LJ 75-25 0.5sec 40-60 1sec Fig.3: these timing diagrams show various combinations of mark-space ratio and pulse-rate for a proportional control system. The second stream was carried in the coded signal generated by the variation of speed at which this markspace signal was repeated, commonly termed the pulse rate. This rate was usually varied from approximately 0.ls to 0.8s. The exact rate depended upon the transit time of the servo motor. Both these signals were of course superimposed upon one another and needed to be separated at the receiver end. Now the logical and modern thing to do would be to use , solid state, mark-space, pulse-rate detectors and this was eventually done in some systems, but once this constantly variable DC voltage is generated, what do you do with it? Remember, this is in the days long before ICs and those nice little op amps. This was in the electronic dark ages and electronic solutions were prohibitively expensive. I can remember buying my first transistor around this time; it cost me 25 shillings and I bled for a week. However, we are ahead of our story again. Motor decoding Modellers, being by nature a tight fisted lot, came up with a primitive 84 SILICON CHIP but effective system using a single electric motor for decoding. Fig.4 shows the mechanical arrangement of the basic mark-space, pulse- rate decoder. The mark-space decoder is simply a slotted yoke mounted and pivoted at a suitable point to allow the control pin to accomplish almost one complete revolution (80% of a revolution to be exact). This yoke is usually connected to the rudder of the model, neutral being at the 12 o'clock position. If the motor rotates clockwise, the rudder will turn right, for example, and if the motor rotates anticlockwise, it turns left. In practice, the relay is switching constantly between open and closed at a mark-space ratio determined by the transmitter encoder. Thus, we now have a yoke which will deliver to the control surface a deflection which is rapidly and equally varying around neutral if the mark-space ratio is exactly 50%. If the mark-space ratio is varied, then the avtJrage position will shift from neutral in an amount directly proportional to the mark-space variation (Fig.5d & 5e) . Fig.4 also shows the arrangement for the pulse rate detector, which is simply another yoke set at right angles to the first and again pivoted suitably to all9w the control pin to rotate freely over 80% of a revolution. If the pulse rate is taken to its highest, the motor oscillates quickly about the 12 o'clock position and the averTO ELEVATOR DRIVE PIN age deflection is considered to be "up" elevator. (The rate control was usually connected to the elevator in the model). As the rate was slowed, the motor had more time to rotate and the average position moved towards the "down" position. At the slowest rate, the elevator was in the full "down" position. Note here that, in effect, full down is really neutral elevator. Fig.5c shows the timing of the arcs involved. However, the dwell time at servo reversal added to the down elevator effect and the trimming of the model biased the system to account for the rest. In this way, we achieved two proportional controls from a single channel receiver - quite a step forward but well short of the requirements for a model aircraft, in which four simultaneous controls are tL~ minimum required for full control. Note also that the decoding was far from perfect, with a great deal of mixing between controls occuring (Fig.5 illustrates this quite clearly). Theoretically, the transmitter joystick should have been positioned in the centre of a square hole, but in order to overcome some of the problems of the control mixing, which occured mainly at control extremes, there were some strange configurations at the borders of this no longer square hole. In fact, you had to learn to fly not to the spring centre of the sticks but to the feel of the model. If it was going where you wanted it, then it really did not matter much where the stick was. You never had time look at that anyway. Believe me , it was all you could manage to keep the model airborne. It was ingenious, simple, cheap and diabolically primitive. However it worked, and worked well and gave many of us a taste for a much more professional system, if only we could work out how to do it, that is. New developments RUO.OER NEUTRAL DRIVE DISK ON MOTOR OUTPUT SHAFT Fig.4: this diagram shows the arrangement of a mechanical markspace/pulse rate decoder. The markspace decoder consisted of a slotted yoke connected to the rudder, while the pulse-rate detector used a second yoke (connected at right angles to the first) to control the elevator. There were several important developments that followed on from this basic system. One was the system marketed by Dr Walter Good in which two audio tones were transmitted simultaneously, each modulated for mark-space, pulse rate. This gave four simultaneous proportional (galloping) channels. Motor control was achieved through a trimable (positionable) servo driven ~ t ,_ 0 " ~ ™ O o~: +~J, - ~:,H" """'""'""'""''" 01.~i ~ ~ ol~. 1 ~ 0 RUDDER {/ \) I.~ 0.25sec ~h\/ EFFECTIVE --Fi~,;::;,;:;,_,_EL_EV-AT_O_R 't'.f" ~ ,Jj - ~,--- lsec NEUTRAL MIXING OR~:RFECT DECODING EFFECT Fig.5: this series of diagrams shows the way in which the mean aerodynamic force is generated for the two controls; in other words, the decoded output. Note how the rudder moves away from the neutral position for mark-space ratios other than 50:50, while the elevator position varies according to the pulse rate. from a pulse om1ss10n detector on each of the two tone channels - one for high throttle and one for low. This was a very sophisticated and complex system for its time and held us all in awe whenever the very occasional example showed up on one of our fields. Here was a taste of what was soon to come. The feel of the transmitter was marvellous and those twin proportional sticks felt just right, compared with the five cumbersome lever switches on our reed transmitters. Never mind that the performance of the rest of the set was as primitive as it is possible to imagine. A model fitted with three flapping controls was a sight to behold but at the time, we thought it was great. They were exciting days and it is impossible for modern modellers to comprehend the degree of yearning that the dream of true, simultaneous control generated in us. One must remember that at that time, a transistorised anything was rare on our fields, for many of us were still flying single channel valve sets. As a matter of fact, I did not stop producing valve super-regenerative receivers until around 1969, long after I began producing a solid state superhet 6-channel digital system. Even then, the only thing that stopped me was the difficulty in obtaining valves and 22.5V batteries. The demand for a valve system was always there. In this respect, a trip to the flying field was always very interesting, for one would see operating, side by side, valve super-regen single channel systems, superhet reed, 8channel audio tone, Galloping Ghost, Walter Good dual Galloping Ghost, and a host of home-made systems using many and varied approaches. The final step in the mark-space systems came with the development of solid state mark-space, pulse rate decoders. These delivered a DC output proportional to the input. From here the difficult part began. Just what could you do with this sort of output? These days, a DC-coupled, closed loop feedback servo would be possible but this was well before op amps. The answer in those days was to feed this power into a magnetic actuator. This device consisted of a circular magnet inside a coil wound at right angles to the magnetic field. This was lightly spring-loaded for return to neutral. Thus, with the polarity aligned in one direction, the magnet would begin to deflect clockwise and with the current flow reversed, in the counter clockwise direction. The greater the power fed into the coil, the more deflection obtained. They were not very powerful, giving about 3-5 oun,ces on a spring balance, but this was enough for a lightlyloaded, slow-flying aircraft. They were quite accurate and gave true and smooth proportional control, free of the gallop and decoding mixing associated with the "galloping ghost" systems. They were, however, not the answer we were looking for. That had to wait until two very gifted Americans made their contribution to the field of radio control and that story will be told in next month's issue. See you then. SC FEBRUARY1991 85