Fullscreen HTML5Med Res (??MB)HTML4

REMOTE CONTROL By BOB YOUNG Modern radio control systems This month, we will be taking a look at the main features offered by modern radio control systems for model aircraft. Among the most useful of these is the "buddy box" system of dual control used when an instructor is training a modeller to fly. Modern radio remote control systems for models are based on a transmitter (Tx} with built in control levers or steering wheel, a telescopic antenna and a battery pack (Fig.1}. The electronic circuitry includes an RF section of less than 1W (typically 500mW) and an encoder which converts the control stick · positions to a serial data stream. The modulation can be AM (amplitude modulation) or NBFSK (narrow band frequency shift keying) with a typical shift of ± 2.5kHz. The data stream may be encoded in PPM (pulse position modulation) or PCM (pulse code modulation) format. Typical figures for a PPM system are: frame rate 20ms; neutral 1.5ms; control 1-Zms; and identification interval Bms (Fig.2}. The receiver (Rx) has a separate battery pack and sockets to accommodate one or more servos (Fig.3}. The antenna is .usually a piece of flexible hook-up wire about 1 metre long. The Rx usually includes an RF section, a mixer/oscillator stage, a single conversion IF, an audio amplifier and a serial to parallel decoder. Adequate DC filtering and regulation is very important to eliminate servo noise. The servos are the muscles of the system and supply the power required to move the control surfaces or wheels etc. These items are marvels of modern technological achievement and can respond to the decoded signals with errors of less than ± 0.25° at the servo disc (or arm) with a high degree of reliability. Each servo consists of a small electric motor, an electronic amplifier, a gear train, a plastic housing and a feedback potentiometer, which is driven by the output shaft of the servo. This potentiometer provides the positional information to the servo amplifier. Each servo is fitted with a disc or arm (often called a "horn") which couples to the control pushrod. Batteries This modern transmitter boasts a range of features, including channel mixing, dual rate control, trim adjustment and servo end point adjustment. It employs frequency modulation and can control up to seven channels. 40 SILICON CHIP The batteries are the heart of the modern R/C (radio control) system but these items are also the number one killer of models. Modellers using cheap nicads in an airplane do so at their own peril (and that of everyone around them). In use, Rx nicads are called upon to deliver anything up to four amps for very brief periods in a 4-servo system. The situation is even worse in helicopters because all four servos are running continuously. .1/I "/ /4) AUX 2 (IF FITTED) OH TOP FACE 3 <at>AUXt(•RTTED) ~ / Transmitter Fig.1 illustrates a typical layout of a radio control Tx. However, there are as many transmitter layouts as there are manufacturers these days, so Fig.1 should be taken as a guide only. The technological explosion common to the electronics field in general has also hit the model business. Consequently, transmitters have suddenly sprouted a great profusion of knobs, dials, switches, FUNCTION 1 2 3 4 5 6 MOTOR AILERON/RUDDER/STEERING ELEVATOR RUDDER AUX 1 (RETRACTS ETC) AUX 2 (FLAPS ETC) Fig.1: this diagram shows a typical transmitter control layout, together with the stick allocations. The gimballed stick assemblies drive two potentiometers to provide control of aileron, throttle, rudder and elevator. 140 A 500 milliamp-hour (mA.h) battery will last for approximately 2½ hours in an aircraft and 45 minutes in a helicopter. For this reason, the minimum safe battery requirement for helicopters is 1.2 amp hours (A.h). When I see nicads designed for calculators in a receiver battery box, my heart sinks. The situation in a Tx is quite different, the typical current consumption being a constant 150mA. Calculator batteries may be OK but there are many considerations to take into account in this very important section and all will be dealt with fully in due course. CHANNEL digital displays and the like. Most of it, from my observations, appears never to be used by the consumer. Likewise, the internal electronics have undergone the same revolution, with AM (amplitude modulation) being displaced to some extent by FM (Frequency Modulation) and PPM (Pulse Position Modulation) being displaced by PCM (Pulse Code Modulation). To simplify the explanations in this series I will confine the discussion wherever possible to the still very common, reliable and inexpen~ sive AM PPM system which served us so well for 20 years. These systems use two gimbal stick assemblies as the primary mechanic- al controls (Fig 1). The principles applied in this system are still used in the others to a large extent, servos, for example, being interchangeable between all three systems. The minimum number of channels required for powered aircraft use is three: rudder, elevator and throttle. Model cars and boats can be operated quite successfully on two channels (steering and throttle) but even here the modern trend to sophistication is calling for gear shift and 4-wheel drive in cars, and mixture control and trim tabs on boats. The table in the corner of Fig.1 shows the typical channel number- CARRERf MODULATION FRAME C1 C2 C3 C4 1-2ms FRAME: 16111s 4CH 20ms &CH NTIFICATION: 8ms Fig.2: the modulation frame for a 4-channel PPM system. Control is affected by altering the positions of the pulses with respect to the start of the frame. NOVEMBER 1989 41 AERIAL 27 !,, PLUG IN CRYSTAL DDfRIF ND AILERON FITTED) ~p TWO MULTI CONNECTORS FOR 8 CHANNELS- (J l ,, BOTH THE SWITCH AND ...___ THE CHARGING CONNECTION CAN BE MOUNTED IN THE ( SIDE OF THE PLANE, CAR ETC, BUT NOT A BOAT,,_,....,_ _ ~ ~~~~~f.!!~ ' TO CHARGER CiNNECTIDN Fig.3: typical arrangement for a multi-channel airborne system showing the servo allocation for each channel number. The on/off switch and the charging connection are best mounted on the side of the fuselage. The battery pack should be rated at 500mA.h for aircraft and 1.2A.h for helicopters. ing and allocation of stick movements. The control gimbals are arranged to give a complete 360° of movement, thus enabling accurate mixing of two controls simultaneously. These gimballed stick assemblies are used to drive two potentiometers, one for each control channel. In operation, the output from each stick pot in the transmitter is slaved to the feedback pot in the servo via the encoder, decoder and servo amplifier electronics. This is termed "proportional control" and is the magic ingredient in modern radio control systems. It now means that if 7.5° of control deflection is called for, then that is precisely what we get (with an error of possibly ± 0.25°). In practice we do not fly like that but merely use the sticks to point 42 SILICON CHIP the model in the direction required. In other words, we fly by feel. But it all comes back to having that delightful, highly accurate coupling between the control stick and the servo. Of course, there are differences of opinion over which is the best way to combine the primary controls of the aircraft on the Tx gimbals. To understand this, consider that a model aircraft usually requires a minimum of four channels for successful operation (I am sidestepping the 3 channel argument for simplicity). These four channels control the ailerons (roll axis), elevators (pitch axis), rudder (yaw axis) and throttle (speed). There are two popular configurations for the control gimbals: (1) aileron/throttle on the right hand stick (Mode 1); and (2) aileron/ elevators on the right hand stick (Mode 2). The rudder is always on the left hand stick, combined with either throttle or elevator (depending on which mode is used). Now much ink has been spilled in bitter arguments by the experts on which mode is the best and I have no intention of opening this debate again. I prefer Mode 1, having started on Mode 2 and changed. The purists prefer Mode 2, arguing that full size aircraft have the aileron/elevator controls combined and therefore so should models. The big problem is that, in a full size aircraft, you use a fully articulated wrist, elbow and shoulder; in models you have only a thumb planted firmly on top of a small control stick. In the end, it is all a matter of personal preference but an important choice nonetheless. If you pick a mode that doesn't suit you, your ability to learn to fly may be seriously impaired - my years of instructing taught me that. Often, the deciding factor is the mode used by the instructor at your club. For this reason, there is often a predominance of one mode or the other in certain clubs. However, please remember this point: it is your ability to learn to fly that is at stake here and the final decision should be yours. While on the subject of learning to fly, I feel a bit of good advice is in order. Join a club and take advantage of the available instructional program. It will save you much heartache and unnecessary expense. Model aircraft are very difficult to fly and it takes almost as much time to learn to fly them as it does for a full size aircraft. Six hours of instruction from beginning to solo is a common figure. The big problem here is the complete lack of feel for the aircraft by the pilot (apart from the visual feedback). This interestingly enough is shortly to be overcome in some new sets about to hit the market. In these sets, a down-link transmission from the aircraft servos on a separate frequency will be used to provide feel for the control sticks. Incredible! Trim controls Two trim controls (one for each axis) are adjacent to each gimbal assembly to provide fine trim for the controls. These are usually called trim levers and provide about 15 % of the full range of movement. An aircraft can change trim for various reasons during a flight and some in-flight retrimming may be required. This eliminates the need to hold the stick off-centre during flight. If only three channels are used, the rudder servo is usually plugged into the aileron channel, so that the primary steering control is under the right thumb. There are many very interesting Tx layouts provided for cars, the most interesting being those with a steering wheel and throttle trigger in place of the control sticks. These are very popular and provide quite a natural feel. The photo on the following page shows Tx development taken to its logical conclusion. Here, the •••""h •• .:. • ..:>, The servos are the 'muscles' of a radio control system. These three units are from a model aircraft and plug directly into the 7-channel FM receiver at bottom left. A plug-in crystal sets the receiver frequency. transmitter circuit and the controls are built into a chair in which the pilot sits (the picture shows the author in his younger days). It's very strange at first but quite interesting once you are used to it. Transporting the chair is a problem, though. Auxiliary channels The auxiliary channels are usually very simple. Typically, they include a toggle switch for a retractable undercarriage (if fitted) and slide controls for the flaps and fuel mixture (needle valve on the motor) etc. Most transmitters will also have some sort of meter and this can serve one of two functions. The more common but less useful type functions as a battery voltage indicator while the more useful type functions as a Tx output meter. An output meter does have one drawback, though - it will change reading according to hand position and extension of the antenna, which leaves the user unsure of the true reading. However, when used correctly (ie, antenna fully extended and vertical, and both hands on the Tx case), they give a good indication of both Tx output and battery voltage. Buddy box One very useful feature in a model aircraft Tx is a "buddy box" or dual control system. This is not very common these days, which is a shame for it really does make learning to fly much less of a chore; In this system, two transmitters are joined with a plug-in cord. A pushbutton switch on the master Tx is then used to select modulation output from one transmitter or the other. In operation, the instructor holds down the momentary thumb switch, thereby passing control of the aircraft to the pupil. If there is an emergency, he simply releases the switch and transfers control to his own transmitter. This system saves the instructor from having to wrestle the Tx away from the pupil if there are problems. Indeed, some pupils will NOVEMBER 1989 43 In this novel arrangement, the transmitter circuit and the controls are built into a chair in which the pilot sits. It's very strange at first hut quite an interesting way to fly once you are used to it. withhold the Tx, insisting that they have everything under control right up until the moment the model starts digging a hole. It's very annoying for the instructor when this happens. I recall one incident when my son returned the Tx to me one microse- · cond before the model hit the ground and then complained for the next 15 years that "Dad crashed the model". Still, that's not quite as bad as my first multi-channel flight. I pulled the wings off the model during a steep turn and then, as the fuselage screamed down like an arrow, handed the Tx to MY instructor and said "Here, it's all yours". As stated previously, learning to fly is not easy and some instruction is a great help. Encoding features The old half-shot encoder which formed the basis of R/C sets for 15 years (circuit included in next mouth's column) was not very flexible electronically. It has now been replaced by modern multiplexed encoder ICs (eg, the NE5044), allowing a whole host of new features to be added. These include: • Servo reversing a slide switch is provided on the Tx to invert the pulses on each channel, 44 SILICON CHIP thus reversing the direction of travel of the servo. This feature calls for a deal of caution on the part of the user in case take-off is made with the servos reversed. For this reason, all control throws should be checked for correct direction of travel before the first flight of each day. This advice applies even if you are using a Tx without servo reversing. It only takes a pushrod to be accidentally replaced on the wrong side of a servo to wreak havoc. With servo reversing, it is even easier to come undone, especially if two models are used with the one transmitter. I have seen the odd pilot who is clever enough to fly with reversed controls but they are rare indeed. • Servo end point adjustment (EPA) - this is a very useful feature and quite safe to use. It is especially useful for throttle adjustment where it is undesirable for the servo to run up against the end stops. Running a servo against the end stops increases current drain and can burn out the servo motor and amplifier. This in turn can flatten the batteries and lead to Rx failure in the model. A small potentiometer (one for each channel) is used to adjust the servo travel to overcome this problem. If the Tx does not have EPA, the system must be set up carefully to avoid these problems. • Dual rate - this feature involves a switch and an associated pot on the front panel of the Tx for one or more channels. The pot is adjusted to set the overall percentage of servo travel available (0-100%) 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 the servo travel to 100%. This feature is useful for high speed flight where the controls become very sensitive around neutral. It does, however, require some care on the part of the pilot. In particular, the position of the dual rate switch should be checked before commencing any manoevre, especially outside loops. I have seen models crash because the pilots started a manoevre 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. To my mind, the dual rate feature has been dated by the introduction of the exponential system. • Exponential control - often switched in by an external or internal switch, this feature gives electronic damping of the servo throw around neutral. As the name implies, the control throw follows an exponential curve, with less throw close to neutral and greater throw as the stick moves to the extremes. The advantage here is that the control response of the aircraft is always constant whereas with dual rate, two sets of reflexive 'responses must be developed. • Battery pack - all transmitters use a built in battery pack made up of either conventional or nickel cadmium cells. Because the Tx places few demands on the battery, with only about 150mA of current consumption, low-cost batteries may be used with comparative safety. Well that's it for this month. Next month, we'll look at the electronic considerations that go to make a good Tx. •§;]