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REMOTE CONTROL BY BOB YOUNG The development of digital proportional servos Last month, we discussed the difficulties that arose in the very early development of proportional control and examined some of the primitive systems which preceded the modern digital R/C system. The most primitive was the galloping ghost system which ultimately led to the early analog systems. It is difficult for modern modellers, who see only digital sets which are virtual clones of each other (even down to interchangable servos), to comprehend just how many different types of systems were in operation in the early sixties. A trip to the flying field in those days was really interesting, for you never knew just what would show up next. This was enhanced by the large numbers of modellers who scratchbuilt their own equipment. Thus it was possible, on one day, to see singlechannel valve sets operating alongside tuned reed, tuned filter, galloping ghost, Walter Good, analog proportional and digital proportional CLOCK RESET CH1 OUT CH2 OUT 17. . ._____________.I J7_________________ CH3 OUT CH4 OUT Fig.1: serial to parallel conversion is performed in the remote control receiver. In this process, a series of pulses from the transmitter are converted by the receiver to servo control pulses. 74 SILICON CHIP sets, both home made and commercial. All of these early attempts were useful but far from satisfactory, lacking accuracy, speed and power in the control actuators. They also lacked reliabilty, simultaneous operation and the required number of channels. Those days have long gone and while I do not miss the unreliability, I do miss the enthus iasm generated from our quest for perfection. However, in the very early 1960s there occurred one of those quantum leaps in technology that result in one system being adopted as an industry standard, due to the fact that it delivers exactly what the application calls for. Such was the case with the development of the digital proportional system as we now know it. It cured all of the above problems in one fell swoop and gave unlimited numbers of truly proportional and simultaneous controls, coupled with unheard of reliability. Thus did the new age of R/C modelling sweep away the old. At the very heart of this revolutionary discovery - and believe me it was truely revolutionary, breaking away completely from all lines of development to that date - was the proportional servo as we now know it. So comp lete was the development by Doug Spreng and Don Mathers (USA) in the early 1960s, that the concept remains virtually unchanged to this very day. Our article traces the changes in technology that have improved the operation and reliability of the concept, but the Spreng and Mathers touch is still easily identified in the most modern PCM equipment available today. The system they developed was the Pulse Position Modulation (PPM) system in which a series of pulses are transmitted in serial form and converted in the receiver (Rx) via a serial to parallel decoder (see Fig.1). This results in a number of output pulses, usually from 2 to 8, with the pulse width directly related to the width of the transmitted pulse. This in turn is directly related to the position of the control stick on the transmitter. Usually, pulse widths run fully counterclockwise (CCW), 1.0ms; neutral 1.5ms; and fully clockwise (CW) 2ms. Full control is available over this range in steps determined by the sensitivity of the servo amplifier. This minimum step is termed the "minimum impuls e" and is typically .005ms. Thus , approximately 100 steps are available each side of neutral (centre) on a good quality servo. These steps are so small that to all practical purposes they cannot be felt, and the servo appears to be slaved directly to the transmitter stick. This is the concept of "proportional control". Herein lies the secret of success of the digital system: fantastic servo performance. Fast, powerful and extremely accurate, it fulfilled all of our dreams. Here was the most clever development in model electronics and it has yet to be surpassed. For this reason, I have chosen to present the servo first in the following series of articles. Whilst it is traditional to start with the transmitter, I feel that delivering the control pulse to the servo input is the easy part. What takes place in the servo is where the magic resides. The question which must be dealt with first is just how do these servo amplifiers work? How do they convert the pulse width information into a clockwise (CW) or counter-clockwise) CCW instruction? i-•--5 5 0 I ---6 28 3 .•~rl 11:-, •,~rlO ---+-- - -1 0 4 1,5,24,26,27 screw 2 output wheel 3 output arm 4,6 case, top 7 right rack gear 8 left rack gear 9 pot drive gear 10 second intermediate gear 11 pot shaft 12 drive gear 13 first intermediate gear 14, 15 gear pins 16 case, centre 17 motor, 1OW 18 pot wiper contact 19 pot element, 1.5kQ 20 fibre washers 21 decoder PC board 22 grommet 23 connector assembly 25 case, bottom 28 output shaft cap 1 13 ~ . 15 - -- ---,----::---- · - - -- - - 14 ~~~ 16 -------i ' ----17 ~-®·------+-1--20 20------t--~. 4 27 27---__,__; : _ _-i-----· e , 22----•o 23✓ F 24 24 Closed loop feedback Essentially, the modern R/C servo is a closed loop feedback servo in which a pulse is fed to the servo amplifier and compared to the output pulse from a reference generator on the servo amplifier board. This reference pulse is controlled by the position of the servo output arm via a potentiometer. The input pulse and the reference pulse are applied to a summing junction and the resultant 26---& KPS11-11A Fig.2: this exploded diagram shows all the parts used in a typical servo control. The key elements include the decoder PC board (21), the motor (17), a servo feedback pot (18,19), various gears & the output wheel or arm (2,3). Modern servos are built around dedicated IC servo chips (eg, the NE544 from Signetics) & are very compact & reliable. MARCH 1991 75 ,---------- -- ----- --- ----- - -- ---, \ \ I 10011 \ +5V ' I I I 22k 2.2k GNO I I I I l .,. .,. Rl 4.7k 22k .,. Cl Fig.3: an early servo decoder and drive amplifier, using discrete components throughout. Effectively, the motor drives the pot (VRl) until the pulses produced by Ql and Q2 match those at the input. error signal is then available to control the power and direction of the servo motor's rotation. The motor drive circuitry is arranged in such a manner that the servo always attempts to cancel any error (zero output to the summing junction), at which point the servo comes to rest until another error appears. Fig.2 shows an exploded diagram of a typical servo, in this case one using linear and rotary output arms. 100U As we have control over the input pulse width from the transmitter, we therefore have control over the position of the servo output arm. The accuracy of the servo is dependent upon the servo amplifier sensitivity which is termed the minimum impulse power. As noted previously, a good servo will deliver up to 100 steps each side of neutral and so we have complete control over the servo output arm. It is slaved precisely to the REFERENCE GENERATOR OUT PULSE IN SUM APPLIED TO PULSE STRETCHER ____ n'------------- .., MINIMUM IMPULSE NEGATIVE SUM WHEN INPUT SHORTER THAN REFERENCE GENERATOR Li Fig.4: this diagram shows what happens in the decoder circuitry of Fig.3. Pulses from the input and reference generator are compared to generate an error pulse which is applied to the servo motor. 76 SILICON CHIP .,. transmitter control stick, hence the name proportional control. Servo circuit Fig.3 is a circuit diagram of a very early American servo, the Orbit PS4D, manufactured by one of the pioneers, Bob Dunham. Orbit lead the way for many years in high quality radio control systems and Bob Dunham was a top contest flyer in his early years. Transistors Q1 & Q2 form a one shot multivibrator which is triggered by the leading edge of the incoming pulse from the decoder output. Interestingly enough, this servo worked on a negative input pulse, whereas the industry standard is now positive. This one shot will generate a pulse of opposite polarity to the incoming pulse from the decoder and is called the "reference generator". The width of this pulse is controlled by the position of servo feedback potentiometer VR1, which is in turn related to the position of the output arm on the servo. This pot is usually driven by the output gear of the servo mechanism. Both of these pulses are applied to the summing junction R1, R2 . Fig.4 shows the effect at this summing junction. Briefly the output of the sum- INFRA RED NIGHT VIEWER A limited purchase of some 6032A tubes which were removed from new, and near new equipment allows us to offer this Incredibly priced IR NIGHT VIEWER KIT BARGAIN! You could payover$2000foraviewer which uses a similar tube. All the tubes are "AS NEW", and are GUARANTEED not to have any blemishes! THE PRICE OF THIS UNREPEATABLE BARGAIN??: ONLY $259.00 Fig.5: Several servos are shown in this photo, with the one on the right being a currently available model. ming junction will be a pulse of either negative or positive polarity, depending upon which pulse is the longer. This pulse is then applied to the bases of Q3 & Q4, a PNP/NPN pair. Depending upon polarity, one of these transistors triggers and the output is applied to the following pulse stretcher/Schmitt trigger. The output of this network is ap plied to motor drive transistors Q7 & QlO which in turn drive the motor CW or CCW, depending upon the polarity of the longer pulse at the summing junction. The motor is driven until the potentiometer causes the Ql/Q2 one shot to deliver a pulse to the summing junction of equal length to the incoming pulse from the decoder. At this point, the motor will switch off and the servo will take up its new position until there is a change in the width of the incoming pulse, whereupon the process will start all over again. Diode pair D3 & D4 prevent both sides of the servo amplifier from switching on simultaneously which would instantly destroy the output transistors. C2 is a noise suppression filter. Notice that one side of the armature is connected to the motor case for additional shielding. Feedback resistor R3 changes the pulse width of the reference generator order to shut the motor down ahead of time, so that the servo does not overshoot and go into oscillation about neutral. This is a critical function, for if there is too much damping, the servo shuts down early and the centring accuracy is badly affected; too little damping, and the servo hunts or oscillates and servo current consumption shoots up and the output transistors start to run hot. The ideal result is called "dead beat" damping in which the servo runs to the point and stops instantly. In practice, this is very difficult to achieve and I have found that it is best for centring accuracy if the servo overshoots and moves back just once. This gives a very accurate neutral. Another problem with closed loop servo amplifiers is that a certain amount of deadband must be introduced into the system if the servo is ever to come to rest and not sit there oscillating. Capacitor Cl across the summing junction performs this function. The value of this capacitor is critical in establishing the centring accuracy of the servo. If it is too big, the deadband is too wide and the servo will be sloppy around neutral. If Cl is too small, the servo will jitter, causing excessive servo current drain, and probably damaging the output transistors and motor. As you can see from Fig.3, this amplifier had quite a large component count and could only be fitted into fairly large servos. Several such servos are shown in the photo of Fig.5. The photo of Fig. 7 shows a servo made by Silvertone Electronics. Note the double deck PC board in the Silvertone unit. This was quite a ...... includes a 6032A tube (as per sketch) electronics kit, ample plastics for the case and a 75mm round I.R. filter: Can be cut to suit your torch. All you need to finish off this kit is a good torch, and any old camera lens, small magnifying glass or an eye piece: The lenses can be obtained from camera repairers, camera shops, or can be recovered from old cameras. VISIBLE LASER DIODE POINTER - KIT Based on a "State of the art" 3mW Visible Laser diode, and a matching heatsink/collimatorassembly. The circuit even has provision for digital switching: Communications, security, etc. This complete kit includes everything you need to make the pointer illustrated except for the batteries (3 AA cells). Our SPECIAL price for the VISIBLE LASER POINTER KIT?? ONLY 239.00 Also available is a kit with the same PCB and all onboard .components, but using an Infra-Red laser diode and it's matching heatsink/ collimator assembly: This can be used for communications, security etc!! ONLY$99.00 LOOK AT THESE BARGAINS Some of these are in limited supply so be quick. All are NEW. Hall Effect IC's. data supplied ............ 1o for $20 Stepper Motors ............ ... ... ........... 2 for $20 Small 3 12VDC motors..... ................. 2 for $ 5 20VDC to 1SKVDC (S00uA) converters .......$30each 0.BmW HE-NE Laser tubes ............. .... ...... $120each Stereo (Dual) VU meters .........................•.•...$4each 150ohm 25W wirewound pots............ 2 for $ 5 [Z][B OATLEY ELECTRONICS PO BOX 89, OATLEY, NSW 2223 Telephone: (02) 579 4985 Fax No: (02) 570 7910 Certified p&p: $5 inAusl. NZ (Airmail):$1 D Fax orders are accepted with credit card payments. MARCH 1991 77 ,--- -- - --- - -- - -- ---- ~I I I I I I 330!l I I 10k - 1 Q6 AT188 I VR1 + 2.2k .,. 01 1N914 +4.BV I I 100!l 0.1 I I I I \ I I +GND 0.1 3.9k 47k100k 27k .,. .,. ... .,. 1.5I BP * ADJUST TO SET DAMPING t ADJUST TO SET DEADBANO problem to produce and service. To compound rny problems, I always used an emitter follower stage on the input to buffer the servo from the decoder, giving a transistor count of 11 compared to the 10-transistor Orbit servo . The relentless demand for lower cost and smaller servos eventually forced us to use the circuit in Fig.6 which is simpler in construction. It was used by many manufacturers but it never worked as well as the Schmitt trigger amplifier of Fig.3. It was prone to several problems, amongst which were non-linearity and changes of deadband and damping with servo position. One thing both of these discrete amplifiers shared in cornrnon how- ever was the fact that both required a centre tap on the battery pack and this type of servo was cornrnonly referred to as a "4-wire servo". Motor resistance was typically 3Q as against the 11Q motors used in the modern IC servo. The four wires were signal, usually a colour; positive 4.8V, usually red or orange; centre tap +2.4V, usually white; and ground or zero volts, usually black or brown. Note that reversing the direction of one of these servos requires the two end leads on the feedback pot to be reversed, and the two armature wires on the motor to be reversed. Do not reverse the red and black wires, for all that will achieve is a burnt out servo. One point here is that 3-wire servos can be used in 4-wire systems Fig.7: this photo shows a servo made by Silvertone Electronics. Note the doubledeck PC board with the parts crammed in to save space. Also visible is the drive motor and feedback pot. 78 SILICON CHIP Fig.6: this was a later and simpler proportional control receiver which was the first to use a DTL (diodetransistor-logic) to reduce the component count. but not the other way around. Technology finally came to the rescue of the servo manufacturer in the form of the IC servo amplifier. There were many early versions of these chips and most of them suffered serious defects of one kind or the other. Sarne of these were voltage instability, output drive latch-up in which the chip just simply melted down, non linearity and a host of other small problems, not the least of which was loss of drive voltage across the output transistors. Time and perseverence finally rid us of these little challenges and the present generation of IC servo amplifiers are immaculate in their operation. Reliable, accurate and extremely small, especially when made in surface mount form, these amplifiers have given the modeller true proportional control. I wonder how many truly appreciate the incredible cleverness of the human minds that conceived these devices? One very popular version of the IC servo is the Signetics NE544. Here in one small package are all of the features that we dreamed of for many, many years. If you ever have occasion to use one of these little devices, please take the time to marvel at the wonder of it all and spare a small thought for Don Mathers and Doug Spreng, two people who helped to make it all possible. SC