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RADIO CONTROL BY BOB YOUNG A mixer module for F3B glider operations; Pt.1 Last month, we described the operation of the basic con­trols on a typical F3B international class sailplane and outlined some of the parameters affecting the design of such a sailplane. In this article, we will present the design for an F3B mixer module for the Silvertone Mk.22 transmitter. This month we will look at a simple F3B mixer module to illustrate some of the techniques used to control an F3B glider. This will highlight the complex nature of the task. At the same time, I will attempt to break the circuit operation down to the smallest sub-module so that these building blocks may be applied to any transmitter utilising half-rail encoders such as the NE5044. The half rail encoder makes possible servo reversing, mixing, etc. To begin with, this task is very definitely governed by the 90-10 rule, whereby 90% of the effort goes into achieving the last 10% of the result. Modern computer-based transmitters at­tempt to supply everything for everyone and in so doing, have often become burdensome to operate. Frustration during programming is quite common and often the desired configuration cannot be achieved due to system limitations. The module to be described will plug into the Silver­ tone Mk.22 mixer expansion port (TB11) to convert it into a fully config­ ured F3B transmitter with very little program­ming left for the operator. In fact, all that is usually required of the operator is to set up the chan- nel allocation and set the direction of travel on the servos. However, by long experience I realise that there will always be someone who will require an extra 10 widgets or a relocated emfanger. So a very high degree of flexibility has been built into the PC board to give virtually no limit upon the mixing possibilities. Multi-point mixing is available on all 24 channels with any channel able to be mixed with any other channel, if sufficient mixers are obtained. For those who take the trouble to understand the nature of the circuit presented and who are prepared to experiment, the possibilities are endless. The standard Silvertone Mk.22 eight channel encoder PC board carries four free mixers (two inverting and two non-invert­ing) while the F3B module features eight mixers and two end-point clamps, all of which may be operated in the pre-programmed or free mode. Thus there are twelve mixers available, which should be more than enough for the average F3B model. Keeping it simple The main difficulty facing the de- This is a single stick version of the Silvertone Mk.22 transmitter. It gives three-axis control (ailerons, elevator and rudder) via the knob on the stick. signer of any complex programmable system is keeping the programming simple and user friendly. This applies doubly to a discrete component system as the programming can very quickly become a nightmare of patch cords and wander leads. The module presented here overcomes this problem with an extension of the original wander lead programming system. To find out more about this concept, the interested reader should refer to the articles describing construction and programming of the Mk.22 transmitter, published in June 1995 and March, April, May, June, July, August and October 1996 issues of SILICON CHIP. This module is designed around a 28-pin socket that mates with the November 1998  63 Fig.1: the F3B module is constructed out of op amp mixers arranged in matched pairs, one inverting and one non-inverting. An inverting mixer will reverse the direction of rotation of the servo whereas a noninverting mixer will not. Two end point clamps are also provided (see text). mixer expansion port (TB11) on the standard Mk.22 trans­mitter encoder board. When used in the pre-programmed mode, simply plugging this module into TB11 converts the Mk.22 into a fully configured F3B transmitter. It features CROW landing con­figuration, launch camber and a novel knob-controlled camber vary facility that allows the wing section camber to be controlled from the front panel in flight. In this instance, one of the standard Mk.22 auxiliary con­trol knobs is programmed as a camber vary control. There is also a flap/elevator compensation mix and a “V” tail mixer set. This completes the basic pre-programmed instruc­tion set and all of these may be preset or switched in or out from the front panel. Sufficient free mixers are available to add in snap flap, ailerons mixed into flaps and coupled aileron/rudder, thus completing the full F3B complement of con­trols. The PC board is small enough to be hard wired into other brands of transmitters using a flexible lead. TB11 contains all mixing points that may be required for other Silvertone modules still in development. However the F3B module only uses about half of these so there are not a lot of connections to make. If more mixers are required, sufficient information will be given in this series to develop your own circuit board layout. The Mk.22 encoder is a voltage-driven unit using op amps and multiplexers, with the op amps referenced to a half rail (+2.5V) divider. Wander lead programming is used 64  Silicon Chip exclusively, with all controls fitted with identical 3-pin sockets to mate with 3-pin plugs on the encoder PC board. Programming is simple and once one channel is mastered, the rest is simple as all 24 channels follow the same layout and rules. Mixing can be achieved by simply coupling one channel to another with resistors but if this is done, reverse mixing will occur in proportion to the ratio of the series mixing resistor. Therefore it is necessary to insert a buffer amplifier in each mixer lead to isolate the stages. However there are applications where reverse mixing may be useful so keep the resistive mixing technique in mind. Likewise, channels may be run in parallel by using a “Y” lead on the input harness. In this case, variable gain is available on each channel, allowing servo travel to be matched precisely. Op amp mixers Essentially the F3B module is constructed out of op amp mixers arranged in matched pairs (one invert- ing and one non-inverting, as shown in Fig.1. An inverting mixer will reverse the direction of rotation of the servo whereas a non-inverting mixer will not. Two end point clamps are provided and these provide a special feature that we will look at later. A high level of consistency has been achieved in the physi­cal layout with the original Mk.22 encoder and as already men­tioned, the 3-pin programming plug has been retained. A novel touch in this module is the way these programming pins are ar­ranged. Each pair of mixers share a common 3-pin input and output plug pair arranged as shown on Fig.1. Not only is this arrangement simple to program but by rotating the wander lead by 180 degrees, each mixer is available for independent use; a novel touch. As indicated on Fig.1, the pre-programmed input and output leads are linked to the centre pin of each 3-pin plug. For the sake of simplicity, the lefthand 3-pin plug is always the input and the righthand 3-pin plug is always the output. This is shown The lefthand 3-pin plug is always the input, while the righthand 3-pin plug is always the output. This is shown in Fig.2(a), with the inverting mixer on the lower half. Thus, to reverse the servo direction, all that is required is for the micro-shunts to be placed on the inverting or noninverting pins, as shown in Fig.2(b) and Fig.2(c). Fig.2(d) and Fig.2(e) show the patch plug options. full travel end-point is used as the flaps-up position, some unwanted mixing will appear in the flaps. To prevent this, the end-point clamp is applied to the flap control. This limits the voltage swing at TB11 to the mid-rail voltage, which makes the “flaps up” position servo neutral. Thus when moving the flap lever past neutral, the servo will stop at neutral as the lever travels to the full position. In other words, the last half of the flap lever travel is lost. This provides a very interesting feature in the Mk.22 for if we plug the auxiliary potentiometer on the front panel onto TB2 of the endpoint module, we now have a very effective camber control. This may be adjusted in flight to optimise the wing camber to the conditions of the day. Again, this is a very novel feature and something which cannot be obtained in a computer-programmed setup. This view (taken with a digital camera) shows the completed module plugged into the Silvertone Mk.22 mixer expansion port (TB11). in Fig.2(a), with the inverting mixer on the lower half. Thus to reverse the servo direction all that is required is for the micro-shunts to be placed on the inverting or non-inverting pins, as shown in Fig.2(b) and Fig.2(c). The micro-shunts may also be replaced with a DPDT switch for remote switching. The micro-shunts are the same as shorting links commonly used in personal computers. You will note that in Fig.2(b) and Fig.2(c), one input and one output pin are left free and by using one of the patch leads described in the October 1996 issue this free mixer may be used for other tasks if required – see Fig.2(d). Alternatively, the pre-programming can be completely disabled by using both mixers as independent units, as in Fig.2(e). If the application calls for a dedicated installation, the header pins can be dispensed with and all programming points may be hard-wired to remote switches. Referring back now to Fig.1, op amps IC3a & IC3b are two sections of an LM324. IC3a is connected as a non-inverting mixer while IC3b is an inverting mixer. One of the problems with this arrangement is the fact that the gain (servo travel) control on the non-inverting mixer is not as flexible as that of the inverting mixer. The inverting mixer gives excellent control from zero to full travel whereas it is not possible to reach zero gain on the non-inverting mixer. Also the gains of the two mixers are not matched and the input and output voltages must be adjusted with series resistors. Even so, the end result is a matched pair over most of the useful range of servo travel. VR2 and VR3 are the master gain controls and provide the servo travel adjustments (ATV – Adjustable Travel Volume). TB4 and TB5 are the input/ output connectors and are physically ar­ranged as in Fig.2(a). That as is all there is to the basic mixer module. In the full circuit to be presented next month, you will find this module repeated four times with slight variations to suit the programming requirements. End-point adjustment The end point adjustment performs a special function in that it acts as a clamp or brake upon the servo, stopping it at a preset point in its travel. In the full module, this is used to clamp servo travel at somewhere around neutral and performs the camber control. One of the problems encountered in the discrete encoder is that mixing is referenced to neutral which is the half-rail position. As the servo travels further away from neutral, the mixing becomes more noticeable. Now with flaps in an F3B module, mixing is applied both to and from the flap control which is usually the throttle lever on the transmitter. Thus if the flap lever Auxiliary pot setting The setting on the auxiliary pot will define the flaps up position and this may be varied both above and below the neutral flap location. This will provide reflex or camber to the wing airfoil to the deflection best suited to the day. Alter­natively, the camber may be switched in preset amounts by arranging the correct voltages to pin 3 of IC2a. R2 and R4 set the poten­tiometer sensitivity; the larger the value, the less sensitive the potentiometer. TB3 provides a simple reverse for the endpoint adjustment. By moving the micro-shunt on TB3, the polarity of the diode is reversed and thus the endpoint adjustment is applied to either the high or low end as required. The biggest problem in designing a flexible system is that the designer must allow for the placement of servos in the model. There is absolutely no way of knowing which direction the servo will travel in, so allowance must be made for servo reversing in all modules. This virtually doubles the complexity of any design and can be quite a nuisance at times. Lots of early computer transmitters insisted on defined servo placements and were somewhat restrictive as a result. These modules can be used with most brands of transmitters featuring the half-rail encoder. Next month we will present the full circuit and conSC struction of the module. November 1998  65