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RADIO CONTROL BY BOB YOUNG A fail-safe module for the throttle servo This month, we present a versatile in-line fail-safe module suitable for all brands of R/C equipment. It will provide preset servo pulses for the throttle in the event that all signal is lost. In this month’s column, we will look at what is probably best described as the first of the projected plug-in modules for the Mk.22 system. It is an in-line fail-safe module. It simply plugs into the line between the receiver and any positive pulse servo. In the event of a loss of signal from the receiver, the fail-safe automatically detects the signal loss and generates an output pulse of the correct voltage and pulse-width. The fail-safe pulse width can be preset via a potentiometer to any point between 1-2ms. It is mainly intended as a throttle fail-safe but could be used for any or all of the servos in a model. In the latter case, you will need a failsafe module for each servo. Particular attention has been paid to compatibility with imported radios as this module fills a very definite market need. The rationale behind a fail-safe throttle module is quite simple. Models travelling at 100km/h or more represent a serious risk to themselves and bystanders if control is lost. As kinetic energy or impact force is proportional to the square of the velocity, it is apparent that any reduction in the speed will reduce the impact. Halve the speed and quarter the impact. Halve it again and you have The fail-safe module is plugged in between the receiver and the servo. You need a fail-safe module for each servo you want to protect. 74  Silicon Chip cut the impact to one sixteenth of the original figure. Here we are talking very worthwhile savings. Motors and radios have much more chance of survival in crashes at greatly reduced speeds. History of fail-safes So a fail-safe throttle is a very good thing. In the past I have discussed PCM radios with their built-in failsafe systems and have stated that fail-safe as a concept was disproved back in 1964 by Phil Kraft. Allow me to elaborate on this contradiction. There are two forms of fail-safe. The first detects signal degradation and locks out all input when the signal falls below a predetermined level. At this point, all the servos run to preset positions, until usable signal levels are once more detected. The second system looks for a complete loss of signal and then, and only then, runs the servos to the preset positions, restoring control upon the receipt of any signal input. Now Phil Kraft’s great discovery, like all great discover­ies, was very simple and self-evident, once it had been made. Phil discovered that a snatch of control was better than no control at all! An occasional snatch of control has saved many an otherwise doomed model. Prior to Kraft’s discovery, all of the pioneer proportional systems were fitted with a lockout fail-safe. As soon as even mild interference was encountered the system went into lockout and control was lost until some nebulous time, the duration of which only the gods knew. Fail-safe very quickly became known as that circuit which neutralised the controls on the way to the crash. Digital propor­ tional systems began to smell a bit off to the astute R/C buff until Kraft realised the flaw in the design approach. His company produced a set which featured no fail-safe and the pilot was left to his own devices to fight his way through the effects of the interference. The effect was magical and the modern digital proportional system was born. The university graduates who designed the first generation PCM systems had either never heard of or had forgotten about Phil Kraft. Apparently, they could not be bothered reading the history of R/C devel­opment and rushed in with full lockout fail-safe systems. The first PCM systems were known as “Programmable Crash Mode” systems by astute R/C buffs and PCM began to smell too. PCM systems still feature fail-safe but at least it can now be activated or deactivated by the operator. Nevertheless, PCM still has a lingering air of decay about it. This is a shame really for the microprocessor has a great affinity for signal processing and error correction and the results should in theory be better than PPM. The Silvertone fail-safe module is, on the other hand, a signal loss detector. The fail-safe action is controlled by a pulse omission detector (POD) or missing pulse detector. This requires a complete absence of signal for a period of 500ms before triggering the fail-safe action. Control is restored immediately upon receipt of the incom­ing signal; no lockout, just good safe practice. Circuit description The circuit shown in Fig.1 is based on a single 4011 quad 2-input NAND gate package and while it looks fairly simple there are number of circuit functions with some NAND gates having more than one function. The first function has already been mentioned and is a POD or “pulse omission detector”. Other branches of electronics would refer to this as a “missing pulse detector. This function is performed by IC1b, diode D3 and capacitors C5a & C5b. Then there is the frame rate generator (an oscillator) involving IC1a & ICd and a monostable involving IC1c. Now let’s go through the circuit op- Fig.1: this circuit is essentially a “pulse omission detector”, otherwise referred to as a “missing pulse detector”. This function is performed by IC1b, diode D3 and capacitors C5a & C5b. If signal is missing, a preset servo signal is generated by the frame rate generator (an oscillator) involving IC1a & ICd and a monostable involving IC1c. eration. A 2-input NAND gate requires both of its inputs to be high for a low output. We use this characteristic to enable or disable oscillators or to gate signal through the circuit. The signal input from TB2 is derived from any normal R/C receiver (positive pulse output) in either AM or FM, PPM or PCM format. TB2 is a normal servo plug and simply plugs into the receiver channel desired. NAND gates IC1b and IC1c provide the normal straight-through path for the positive servo input pulse. As pin 5 of IC1b, is tied high, the gate inverts the positive input pulses and thereby discharges capacitors C5a & C5b via diode D3. This is the “pulse omission detector”. C5a & C5b are charged via the 470kΩ resistor R4 and need to be continually discharged via D3 for normal servo operation to be maintained. Since C5a & C5b are normally kept discharged by diode D3, they also hold pin 13 of IC1d low and thus keep it disabled. The master clock is thus rendered inoperative. IC1c inverts the signal from IC1b and the normal positive-going pulse appears at the signal out pin of TB1. The servo is plugged into this socket. Master clock generator Gates IC1a and IC1d form a free-running multivibrator which generates the frame rate master clock. Kit Availability The fail-safe module is available as follows: Fully assembled module complete with servo leads.........................$47.50 Complete kit with PC board and servo leads....................................$32.50 PC board only.....................................................................................$5.50 When ordering, purchasers should nominate the R/C system they are using. Postage & packing for the above kits is $3.00. Payment may be made by Bankcard, cheque or money order to Silver­tone Electronics. Send orders to Silvertone Electronics, PO Box 580, Riverwood, NSW, 2210. Phone/fax (02) 9533 3517. June 1997  75 Fig.2 (left): the component overlay diagram for the PC board. Most of the parts are surface mount types. Note: board shown approximately 170% actual size. Right: this larger than life-size view shows one of the prototype fail-safe modules. Normally it would be fitted with heatshrink sleeving before being installed in the model. This is set by resistors R6 & R7 and capacitor C3 to approximately 20ms. If the incoming pulse at TB2 disappears, capacitors C5a & C5b charge via R4 and pin 13 of IC1d goes high. This allows the master clock to start running. IC1c, VR1, R5 and C4 form a halfshot or monostable pulse generator. This generates a positive pulse which may be set anywhere between 1 - 2ms with VR1. Thus with no input at TB2, the output of IC1b will be high and IC1c’s output will be the internal generated signal. This is a perfectly normal positive servo driving pulse with a width between 1-2ms, set by VR1. Diode D1 serves a triple purpose. First, it protects against reverse voltage on the supply rail. Second, it serves to drop the supply rail to the IC by 0.6V. This is a very important point when using some imported receivers. These receivers can have an output pulse as low as 2.5V which means that the 4011 may not switch reliably because the input pulse never reaches half rail. The 0.6V across D1 eliminates this possibility. Third, it can serve to isolate a backup battery, a point we will examine later. This version is known as Mode 1 and is the preferred op­tion. It is simple to build and simple to install and operate. The kit is all surface mount and comes with the PC board and all the components. The component overlay for the PC board is shown in Fig.2. If you have not assembled a surface mount PC board before, I suggest that you refer to the article on “Working with Surface Mount Components” in the January 1995 issue of SILICON CHIP. When you have assembled the board, just plug it into the servo lead, set the desired fail safe point on the servo and go and have fun. Other versions The above version is simple and uncomplicated. At least, the design was simple before the “what if?” Fig.3: this diagram illustrates a modification which has been made to the Silvertone keyboard to cope with the problem of paired slots. It involves the use of an additional key. 76  Silicon Chip brigade got hold of it! As is my usual practice with any new design, I give prototypes to various people for testing and evaluation and such was the case with the prototype fail-safe modules. No sooner had the first prototypes gone out than the phone rang and the wail went thus. “It doesn’t work if the battery falls out of the model!” I had no sooner put the phone down and the next wail came in: “what happens if the battery shorts out to the car chassis and the car catches on fire and the battery goes flat?” Looking back on the whole affair, I guess it serves me right for calling it a fail-safe module. I should have given it another name like throttle shut-off or something equally simplis­tic. Now we come to the messy bit. To begin I must say that no circuit designer can protect people against their own stupidity. Batteries should not short out to the car chassis or leads become disconnected. Correct installation requires leads to be taped and batteries and receivers to be wrapped in foam. However, cells do fail and batteries do go flat so the criticism does have some validity. The solution was the provision of points P1 and P2 on the board. This lets diode D1 serve its third purpose, which is to act as an isolation diode for a second battery. In this case, the positive lead of TB1 is taken to P2. Thus, if a “Y” or dual socket lead is plugged into the servo socket, the servo uses one socket and a second 4.8V battery pack (any capacity) is plugged into the spare socket. This calls for another switch harness to stop the second pack going flat when the set is not in use. Diode D2 was added for the same reasons as D1. Again it’s simple and easy to manage. Using a standard receiver pack, multiple fail-safes (for other channels) could be run in parallel with no problems. This ar­rangement is known as Mode 2. The “what if?” brigade were aghast at this solution! Anoth­er battery and another switch! All that weight and two switches to switch on and off. What if you forget to charge the battery or switch the switch? Here we come to the main objection to these people. They expect others to look after them and will not face the conse­quences of their own actions. How did they think I was going to move the servo when the main receiver battery has fallen out of the model or caught fire or disappeared in a puff of smoke? By now the reader has begun to realise that there is no end to this game but I had to have one more try just out of cussed­ness. In this case, the solution is to add R1 & C2 and change the back-up battery to a 3-cell button pack of anywhere between 50-500mA.h capacity. As there is not enough vol­tage to tolerate the diode voltage drop through diode D2, the positive servo socket lead must be taken directly to P1. This is known as Mode 3. Now the back-up battery charges automatically from the main receiver battery at a rate set by R1. This rate can be very low and I have found 3-5mA quite adequate. I cannot do anything about the second on/off switch which incidentally should go into the servo lead in this mode. This allows the Rx battery to charge the back-up battery without the drain from the servo. Just switch the Rx on a few minutes before the fail-safe. However, there are a few catches to this system too. As I said there is no end to this game once you start. The smaller the battery, the less number of servo actions possible before the battery goes flat. As all throttle movements come from the back-up battery it is possible to exhaust this battery and leave yourself without a throttle. Actually, the battery recovers quickly and 20-30 seconds is usually enough to get another servo movement. If the back-up battery is too large it will take too much power from the main battery to charge it, so compromise is the order of the day. A 100mA.h button cell pack is a good compro­mise. There is one more problem in that the servo current drain will also influence the number of movements available. A rough servo with a poor motor will require a larger current than a good servo. “But what if . . . ?” I rapidly became tired of this game. I recommend the Mode 1 version of this fail-safe. No, it won’t save the model if it is attacked by a cruise missile or a demented sparrow hawk but it will give you extra insurance against total loss of a model if there is a serious loss of signal. SC Feedback On Previous Articles The February 1997 article evoked an unusually large amount of comment, most of which was favourable. However, some people (mostly trade) still refuse to believe that transmitter intermod­ulation presents a real problem and have commissioned further testing by independent organisations which is fine by me. The series of articles presented in February, March and May 1997 will stand or fall on their own merit in light of further testing. On another level, Wal Gill from Coff’s Harbour (NSW) sent down a worthwhile suggestion for an added safety feature for the keyboard described in the February issue of SILICON CHIP. Wal found my description of the function of the paired slots (601-614) a little ambiguous so he suggested making available a spe­cial key with the window moved 14mm higher for use in the paired slots. These keys are to be reserved for the exclusive use of the 646-659 frequencies. An additional row of numbers from 646-659 should be printed on the keyboard 14mm above row 601-630 which coincides with the existing key window. Thus, when a normal key is inserted in 608 for example, the number 608 appears in the window. If, howev­er, a special key carrying the number 651 is inserted, then the figure 606 is masked off and the correct number (651) appears in the window, thereby eliminating the ambiguity. Fig.3 illustrates the concept. Well done Wal. I love constructive stuff like this. Com­plain about the shortcomings and then present the solution. The modified keys will be available by the time this column appears in print. Another reader, Renee Jackson from Deniliquin, NSW, has sent in the story of her latest creation along with the pictures. The model is a “363” Delta with modified control surfaces and a cockpit and fairing added. It is powered by a “rather tired” O.S. 40H motor. The model is fitted with a Mk.22 Tx and Rx with Hitec servos and a prototype Silvertone fail-safe module on the throt­tle. The Tx setup is for “delta-mix” on elevons with a standard rudder and throttle. I am told that it flies a gentle as a lamb, with a very docile stall, and is quite forgiving to fly. Nice to see someone using some of the more advanced features of the Mk.22 to full advantage. Another reader, Anthony Mott of Black­burn (Vic), is using one of the very advanced (or more unusual) features of the Mk.22 system. Anthony is building a submersible with a twisted pair umbilical cord in place of the RF modules. To date he is success­fully running with 40 metres of cable with no problems. So as you can see, the Mk.22 has found its place in the R/C field. The hard-wired encoder/decoder feature is a big hit with the non-modelling fraternity. Mk.22 encoder/decoder modules have found their way into a myriad of control systems in a wide varie­ty of forms. This model from Renee Jackson of Deniliquin, NSW, is a “363” Delta with modified control surfaces and a cockpit and fairing added. It is powered by an O.S. 40H motor and is controlled by a Silvertone Mk.22 Tx and Rx, with Hitec servos and a prototype Silvertone fail-safe module on the throttle. June 1997  77