Fullscreen HTML5Med Res (??MB)HTML4

REMOTE CONTROL BY BOB YOUNG Practical applications of the low cost speed controller Since the first speed controller article was published in November 1992, many readers have contacted me with questions, hints, suggestions, criticisms & applications. This column is in response to those people & shows how the speed controller can be used in other ways. The very first thing that became obvious was the number of applications people were finding for what in my mind was purely an R/C project. This led immediately to the first problem they encountered: how do you drive the SpeedlB speed controller if you have no radio control outfit? The answer to this problem is simple and requires only a few components. Fig.1 shows the pulse input timing and voltage requirements for the SpeedlB speed controller. Fig.2 shows a simple variable pulse width generator using a single 4001 or 4011 CMOS quad gate package. VRl, R2 and Cl control the "ON" time which is continuously variable. VRl can be a simple potentiometer knob or the variable resistor in a joystick controller. Remember that when you use a joystick, the stick mechanism restricts the angular rotation of the pot to under 100°, so there will be less pulse width variation available than with a simple knob. To compensate for this, increase Cl Jll t?· I .:~:-------- and decrease R2. A small trimpot in series with VRl will provide a trim control for fine adjustment of neutral. R3 controls the "OFF" time, which increases with an increase in resistance. R3 is thus an effective frame rate control. Using this circuit, the timing conditions of any of the modern R/C sets can be simulated. I might add that when fitted with a servo socket, the above circuit will make a dandy little servo tester. If there is enough reader response, I may even be persuaded to do a project on a pulse width counter with 3-decimal place resolution, to allow the setting of transmitter and servo neutrals to precise limits. Built into a box with the servo tester and a meter to indicate servo current consumption (a sure guide to the health of servo motors), this unit would be a very useful tool for all R/C enthusiasts. PC parallel port control As an interesting alternative, the parallel port of a PC could be used to Ill .I • Fig.1: this diagram shows the pulse input timing & voltage requirements for the SpeedlB speed controller. generate a suitable pulse, with control coming from either a joystick or the UP-DOWN arrow keys. This port should provide enough output to drive the SpeedlB direct although it is probably a good idea to buffer the output of the computer for safety's sake. The timing of the "ON" and "OFF" periods could be adjusted quite simply by changing the values in FOR-NEXT loops in a BASIC program. In fact, all of the above are really quite unnecessary for in reality we are doing a double conversion. The simplest fix would be to make the high frequency (2kHz) pulse width generator of the speed controller into a free running circuit and do away with the 50Hz to 2kHz conversion completely. However, this would require a completely new PC board. Twin engine control Moving on now to a more R/C oriented question, one of our readers referred to the December 1991 photo of Wes Fisher's model of the Partenavia P61, which has twin engines. This model was featured again in the December 1992 issue. The question which arises is how are the twin throttles arranged and controlled? The answer to this is not so simple and opens up many questions concerning the advantages and disadvantages ·of twin-engined models. A brief discussion on internal combustion (IC) twin-engined models may help clarify some of the advantages of electric twin-engined models. To many modellers, the sound of two motors bellowing in harmony, overlaid with the characteristic audio beat note generated when the motors are almost perfectly in sync, is music APRIL 1993 53 to the ears. To me, it conjures up visions of changing two props, two plugs and filling two tanks every time one wants to fly or tune the engines for maximum performance. I hate filling fuel tanks and at least electrics do away with this chore. It also conjures up visions of my first near disastrous experiences with twin-engined power models. Notwithstanding all of the foregoing, they are very exciting models to build and fly and are great attention grabbers on any model field. The only thing that grabs more attention than a twin is a 4-engine model. And here, Dave Masterton topped the lot with his 6engined all-electric, B36 scale model. The big problem with IC motors is that they quit for all sorts of reasons and usually at the most inopportune times, such as during take-off and when you are flying low a long way The same considerations apply should one motor suddenly lose power or even suddenly increase power. The result is an unwanted turn whose intensity will be proportional to the difference in power between the two motors. Designers (full size and model) over the years have gone to considerable trouble to produce aircraft with sufficient safety margins to overcome the problems of asymmetric flight. These measures include such devices as twin fins and rudders, lifting fins, outthrust, swept wing leading edges and so on. As a result, twin-engine aircraft today are much safer than they ever were. But caution is still required and the best fix is still good pilot training in emergency procedures. The emergency procedure for loss of power in one engine is to first reduce power if possible until you have +5V 4001 OUTPUT R1 R3 1.8M 150k out and cannot see or hear which motor has quit. Now the golden rule with multiengined aircraft is that you must never turn into the dead motor. For this reason, it is absolutely vital that you immediately identify which motor has quit, in order to take the corrective action required. The problem is, if the model is a long way away and/or out of earshot, the first indication of trouble comes when the model turns into the dead motor due to the asymmetric forces generated when only one motor is functioning. These forces are considerable and the resulting turn can be quite violent. It can also be outside the range of the normal flight controls to rectify. In this case, the model will go into a spiral dive and eventually crash if the throttles are not pulled back quickly enough. Thus, an engine failure in a multi-engined model can present real problems, even to experienced pilots. 54 SILICON CHIP Fig.2: this simple variable pulse width generator can be used to drive the Speed 1B controller. It uses a single 4001 or 4011 quad gate package. the aircraft flying straight and level. You must then identify which motor has cut and begin a turn back towards the landing area, this turn being towards the side with the functioning motor. Once the turn is initiated, you then gradually increase the throttle until enough power is established to bring the aircraft home. The last thing you want is to have to go around again with one dead motor. The other golden rule is never increase the throttle suddenly. Instead, the correct procedure is to adopt a "gently does it at all times" approach. Some models fly quite well on one engine, while some will not fly at all. In the latter case, all you can do is cut the good engine and put down as safely as possible. Electric advantages One distinct advantage of electric power is that the motors do not cut out unless something very unusual happens. The worst that happens is that one motor loses power if two separate batteries are used to supply the drive power and one goes flat ahead of the other. If a single battery is used to supply both motors, then their RPM should track reasonably well across the entire flight time. An interesting approach in regard to twin batteries would be to use a phototacho to control the RPM balance between the two motors. This could also be applied to IC motors with good effect. From the foregoing, it becomes obvious that engine management in multi-engined aircraft is a most important function. Even small variations in RPM between motors can become annoying because you constantly need to alter the trim of the aircraft. Believe me, there is nothing more annoying to a pilot than to be constantly altering the trim of his aircraft during flight. I can well remember when I was in the "Biscuit Bombers" in National Service. After the load was dropped, we used to delight in all moving down to the tail at once, giving the pilot time to retrim, and then all moving up to the front. We'd give him time to retrim again and then move down the back again. After 10 minutes of this, the pilot would burst out of the cockpit roaring "if you lot don't sit still I will chuck you all out of the back door"! After that, we would all be as meek as lambs; until the next flight. The situation for engine management in models is further complicated by the fact that we do not have tachometers on models and the transmitter stick layout makes the use of twin throttles difficult, if not impossible. One method is to use a system of bellcranks and rods to allow a single servo to drive both throttles. This is a very rigid approach and does not allow any in-flight trimming. It also requires careful planning in the building stage to get the linkages in without fouling aileron and undercarriage components. The easier, albeit more expensive approach, is to use a split lead ("Y" harness) from the throttle channel and feed two independent servos. Most modern receivers have sufficient output drive capability to do this safely. Again this system does not allow any in-flight trimming of the motors. Ideally, we would like independent throttle control to bring both motors into sync and since the mechanical arrangement of the transmitter makes this almost impossible, how can it be done? Mixed channels The answer lies in a concept known as "mixing", in which two channels are mixed to allow a composite output to be applied to two separate servos. Mixing can take two forms. The most popular these days is mixing at the transmitter (encoder) end of the tion of this device in some detail. Essentially, the mixer is an active "Y" harness with one extra lead fitted which provides the control signal for the ratio of mix. Thus, the device is fitted with two servo sockets which connect with two completely normal (unmodified) servos. If the two input leads are plugged into the throttle channel and one of the auxiliary channels, then moving the throttle lever on the Tx will move both servos in the same direction, thus applying throttle changes to both motors simultaneously and in equal pro- "For model boats, particularly electric powered boats fitted with reversing speed controllers, steering achieved by differential control of the throttles is quite useful". R/C link. The older and less popular method is to fit a mixer to the receiver output. Both systems work equally well and for those modellers who do not have modern systems with mixers in the transmitter, the receiver mixer provides quite a satisfactory solution. A typical receiver mixer provides a mix ratio over the range of 25:75 to 75:25 using a single pot, as well as a fine trimpot control for each servo neutral adjustment. The most difficult concept to grasp is the receiver mixer, so I will now concentrate on explaining the opera- portion. So far we have just a normal "Y" lead operation. The cunning part is in the operation of the second lead. This applies a differential output to each channel, thus advancing one throttle and retarding the other, again in equal proportions. The really clever part, however, is that this ratio of mix is adjustable from 25:75 to 75:25. Thus, the auxiliary lever now becomes a throttle balance control, allowing one throttle to be advanced and one retarded; just what the doctor ordered! It takes little imagination to see the uses for such a device, the most corn- man being the mixing of ailerons, elevators and flaps for trim compensation in fixed wing aircraft. Another very popular use is mixing of the collective pitch for tail rotor control in helicopters. For model boats, particularly electric-powered boats fitted with reversing speed controllers, steering achieved by differential throttle control is quite useful. In this case, the two input leads are fitted into the rudder and throttle channels. The rudder channel controls the differential input and the throttle the simultaneous input. Typically, one motor can be put into reverse and the other into forward and the boat spun on its own axis. Modern R/C equipment has developed this concept into the mixing encoder, thus doing away with the model mounted mixer. However, the concept is similar in operation. One important point to keep in mind when using any mixer is that each channel can only supply 50% of the servo throw, in order to allow the second servo to provide the last 50% of the throw. Therefore, some compensation in the mechanical linkages is required to keep the controls as effective as with non mixer use. This effect is minimised in the modern mixing transmitter, by allowing the use of 100% plus of servo travel. Keep in mind here that there are stops in the servo gear box housings and it is very easy to remove servo gear teeth if the output gear is rammed hard against these stops. SC YOU AN NOW AFFORD SATELLITE TV SYST For many years you have probably looked at sate II ite TV systems and thought "one day". You can now purchase the following K-band system for only: $995 Here's what you get: • A 1.6 metre prime focus dish antenna, complete with all the mounting hardware. • One super low-noise LNB (1.4dB or better). • One Ku-band feedhorn and a magnetic signal polariser. • 30 metres of low-loss coaxial cable with a single pair control line. • lnfrared remote control pre programmed satellite receiver with selectable IF & audio bandwidth, polarity & digital readout. Your receiver is pre-programmed to the popular OPTUS transponders via the internal memory. AV-COMM PtJ Ltd, PO Box 225, Balgowlah NSW 2093, Ph: (02) 949 7417. Fax: (02) 949 7095. All items are available separately. Ask about our C-band LNBs, NTSC-to-PAL converters, video time date generators, FM2 & EPAL &· Pay TV hardware. r--------------x I YES GARRY, please send me more information on K-band I I satellite systems. Name: _ _ _ _ _ _ _ _ _ _ _ __ I Address: _ _ _ _ _ _ _ _ _ _ _ __ : _ _ _ _ _ _ _ _ _ P'code: _ _ __ 1 Phone _ _ __ _ _ _ _ _ _ __ _ I ACN 002 174 478 10192 APRIL 1993 55