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RADIO CONTROL BY BOB YOUNG How servo pulses are transmitted This month we take a look at the method of transmitting servo pulses using pulse position modulation. This is another form of serial data transmission except that it is via a radio carrier instead of two wires, as used for computer data. Last month, we established the basic parameters for the input pulse used in a typical R/C servo. Fig.1, reprinted from last month, details these para­meters. This pulse must appear at the input for each servo used in the R/C system and hence an 8-channel system will have eight pulses in the data stream. Last month we also established that the servo works with a modulated width input pulse. If the pulse is wider than 1.5ms, the servo will move clockwise with respect to the neutral posi­tion and if it is narrower than 1.5ms it will move anticlockwise. More particularly, while it was not stated last month, the servo’s final position, after it has settled, is proportional to the position of the trailing edge of the input pulse. Now the problem with serial transmission of this form of pulse is that there must be some form of identification of the position of the leading and trailing edges of each pulse. This is at complete odds with normal serial data transmis­sion in which a sample is taken to establish whether the bit is high or low. The edges of the pulse play no part in the usual form of serial data transmission. Thus, for example, we could have a situation where all eight data bits are high and all we would see on an oscilloscope would be a solid high block (pulse) eight bits long with no gaps to identify the start and finish of each individual pulse (or bit). An additional complication is the fact that the trailing edge is not fixed and may vary between 1-2ms after the leading edge. Therefore if we are to serially transmit eight width modu­ lated pulses, we need to separate each pulse with a marker pulse. So how do we transmit this form of data quickly enough to keep the servo response Fig.1: typical input pulse parameters for an R/C servo. This pulse must appear at the input for each servo used in an R/C system. 76  Silicon Chip times as low as possible, so as not to intro­ duce delays in the control response? It is here that the cleverness of the two NASA engineers who designed the original digital proportional system really shows through. Doug Spreng and Don Mathers in the early 1960s not only designed a very clever servo system, they also designed a most efficient form of serial data transmission. There are no wasted pauses or periods in their system. Depending upon the number of channels in the system, one complete frame can be transmitted in as little as 14-25ms. The formula for frame rate is: FR = ((X x 2) + 6))ms where X = the number of channels while the “6” is the sync pause in milliseconds. Thus the frame rate for a 24-channel system would be 24 x 2 + 6 = 54ms. This is about as slow as the system can run because the pulse stretchers in the servos can not hold the charge for much longer. Also the delay in response time starts to become noticeable after this. It is difficult for the modern R/C flyer to appreciate just how revolutionary the original digital proportional system was when it was first introduced. Overnight we went from reeds with ON-OFF controls and perhaps two simultaneous controls, if we were lucky, to a rock-solid proportional system of unprecedented reliability with all controls simultaneous. It was a breathtaking development and a giant leap forward and now it is all taken for granted. True, there were analog simultaneous proportional systems but these were full of shortcomings and never really fulfilled the role required of them. Overnight the Mathers and Spreng system swept all before it and (PPM). Fig.2 shows the timing diagrams from an 8-channel transmitter using pulse position modulation. The bottom trace is the encoded pulse train, the serial data stream if you like, while the two traces above it are the width-modulated pulses for the first two channels. Note how the start of the channel 1 pulse (top trace) coincides with the start of the pulse train in the bottom trace. And note how the end of the channel 1 pulse coincides with the start of the channel 2 pulse (middle trace). You can also see how the start of the channel 2 pulse coincides with the start of the second pulse in the encoded pulse train. Marker pulses & sync pause Fig.2: these scope waveforms were taken from an 8-channel R/C transmitter. The bottom trace is the encoded pulse train, while the two traces above it are the width-modulated pulses for the first two channels. their system became the interna­tional standard for over 30 years. It is only now being rivalled but not replaced, by PCM, a standard bitstream form of serial data transmission. Even here though, the Mathers and Spreng servo system is still used, with the PCM data being converted to pulse width data before being fed to the servo. In other words, the PCM system is merely used to transmit the pulse width data. It is interesting to note that in theory PCM should give better results than PPM for two reasons. First, it is more difficult to transmit edges reliably than just to sample bits for high or low. The edges in a PCM system play no part in the carriage of information. Second, computers are very good at error detection and correction, yet in practice the PCM systems fail to live up to this promise. There is a flaw in the basic design philosophy of the modern PCM system it would appear. Pulse position modulation The system of data transmission devised by Mathers and Spreng is now known as Pulse Position Modulation In fact, the encoded pulse train is a series of “marker pulses” where each marker pulse identifies the end of one chan­nel’s pulse and the start of the next channel. There is one extra pulse in the system which is the start marker. This identifies the end of the sync pause and the start of the channel 1 pulse. Therefore, the bottom trace in Fig.2 shows the modulating waveform for an 8-channel PPM transmitter encoder and it has nine marker pulses. A 6-channel system would have seven marker pulses. To understand how this serial data stream is compiled, it is best to examine one of the early “half shot” encoders, which illustrates the principles involved more clearly than one of the modern IC encoders such as the NE­5044. Fig.3: the circuit of a half-shot encoder. Q1 & Q2 form a free-running multivibrator which is set at 25.4ms. This is the master clock for the encoder. Q3 to Q10 are eight identical half-shot multivibrators connected in a ripplethrough arrangement so that the trailing (falling) edge of one half-shot triggers the leading edge of the next. December 1997  77 shows the output of Q12. Note the location of the leading edges of the marker pulses relative to the leading edges of the channel control pulses. Here we see nine marker pulses whose position is relative to the width of each control pulse. Again the scope is confused and is trying to read the fre­ quency of the pulse train which is impossible because each pulse has a different period, with a sync pause thrown in the middle of the data stream for good measure. The sync pause, between the two sets of pulses in trace 3, allows the receiver decoder to reset before the next pulse train arrives. PWM to PPM Fig.4: these scope waveforms were taken from a 8-channel R/C receiver decoder. Trace 1 shows the output of the receiver detec­tor. Traces 2 & 3 are the decoded width-modulated pulses for channels 1 & 2 and are identical in form to the waveforms in Fig.2. Fig.3 is a circuit of a half-shot encoder similar to that used in the Silvertone transmitters from 1969 to 1974. Q1 and Q2 form a free running multivibrator which is set at 25.4ms. This multivibrator is the master clock for the encoder. The falling edge of the clock pulse triggers half-shot Q3 whose duration may vary between 1-2ms depending upon the setting of the 5kΩ potentiometer in the collector load of Q2. Follow the leader Transistors Q3-Q10 are eight identical half-shot mul­tivibrators connect­ ed in a ripple-through arrangement so that the trailing (falling) edge of one half-shot triggers the leading edge of the next. Again the width of the output pulse from these halfshots depends upon the position of the wiper in each of the 5kΩ control potentio­meters. These pots are located in the con­trols on the transmitter front panel. Q9 and Q10 are arranged a little differently as they are toggle switch auxiliary channels. Diodes D1-D10 form a mixing network which has all anodes coupled to a common line which in turn triggers the transistor pair Q11 & Q12. This pair of transistors is arranged as a one-shot multivibrator with a pulse output of 350µs. This one-shot acts as 78  Silicon Chip a marker pip generator. Referring again to Fig.2, the top trace shows the output of Q3 (channel 1) which is a positive-going pulse of about 10V amplitude and about 2ms in duration. In this case, the oscillo­ scope has measured the frame rate which is the period between the leading edge of each control pulse and is shown as 25.5ms. Trace 2 shows the output of Q4 which is the channel 2 pulse and in this case the scope has latched onto the pulse width which is shown as 1.77ms. The “unstable histogram” comment on each measurement indicates the difficulty the scope has in locking onto this form of pulse train. In the end we had to use an exter­nal trigger driven from the transmitter master clock to achieve reliable triggering. We have already noted that the trailing edge of channel 1 coincides with the leading edge of channel 2. If we were to serially transmit these two channels we would end up with a pulse approximately 3.77ms wide, with no way of knowing where pulse one stopped and pulse two began. Here is the really clever part of the system. The one-shot Q11 & Q12 generates a 350µs marker pip every time a falling edge is generated by transistors Q2-Q10. So the bottom trace of Fig.2 Thus we have now changed the system from a parallel pulse width system to a serial pulse position system, hence the name PPM or pulse position modulation. The data is carried in the position of each marker pulse. The output of Q12 is inverted in the modulator and the negative-going pulse train is used to modulate the transmitter, be it AM or FM. In the case of AM (amplitude modulation), the carrier is spiked or gated OFF for 350µs by each marker pip. Thus, as we have discussed previously, it is more correct to refer to the AM system as a “gated carrier” system as the carrier is not ampli­ tude modulated in the normal sense, merely switched ON or OFF. This form of modulation results in a very strong carrier for nearly 90% of the time and results in a solid relatively noise-free receiver signal. In the case of FM (frequency) modulation the carrier fre­quency is shifted by approximately 3kHz for 350µs upon the arri­ val of a marker pip. Once again the common term FM is incorrect as the system is in reality an NBFSK system (narrow band, fre­quency shift keying system) with the emphasis on the narrow bit. In other words the carrier is keyed or shifted 3kHz each time a marker pulse arrives. Hard-wired systems As stated previously, the top and middle traces of Fig.2 show the outputs of the pulse generators for channels 1 and 2. Compare these with Fig.1 and it is obvious that except for the amplitude, the two traces are exactly what we need to drive a servo. Fig.5: the circuit of a serial to parallel decoder. This was used in the Mk.22 receiver published in SILICON CHIP, April 1995. The serial pulse train is fed to IC1, a 74HC164 serial to parallel shift register. Its eight outputs become the width modulated pulses for the eight servo channels in the R/C car, boat or plane. Had the encoder been set up to run from 5V we could have hooked up servos to the collectors of Q2-Q10 and driven all eight servos direct from the encoder. For hard-wired systems this is quite feasible but for transmission over a twisted wire pair or radio link the data must be serially encoded as in Fig.2, trace 3. In the modern multiplexed encoder it is not possible to drive the servos direct from the encoder and a decoder must be used in this case with a twisted wire pair. The Silvertone Mk.22 encoder has a plug specifically built in for this purpose. Serial data decoding In the R/C receiver, the process is reversed. Fig.4 shows the timing diagrams for a receiver decoder and Fig.5 shows the circuit of a serial to parallel decoder. This was used in the Mk.22 receiver published in SILICON CHIP, April 1995. Fig.4, trace 1 shows the output of the receiver detector and is identical in form to the output of the transmitter one-shot. This signal is amplified and squared up through the pulse shaper Q1, IC2a, IC2b & IC2c. The cleaned up pulse train is fed to the appropriate pins on IC1. This is a 74HC164 serial to parallel shift register. The clock pulses are fed directly into pin 8 from IC2a. IC2b drives a sync separator consisting of diode D2, R9 & C10 which holds pins 1 and 2 of IC1 low as long as the 1-2ms pulses are present. During the long sync pause, pins 1 and 2 go high and the shift register is reset, ready to receive the channel 1 start pulse. IC2c, D1, R13 and C13 form a chip-enable driver which will hold pin 9 high so long as the clock pulses continue to arrive from the receiver. If these pulses disappear, then pin 9 will go low and the chip will be disabled. This protects the servo gears in the event of a transmission failure or the receiver being on when the transmitter is switched off. If the chip is not disa­bled, noise spikes may get through from the receiver and drive the servo up against the end stops, damaging the gear train. With the correct conditions on pins 1, 2 and 9, the pulses will be clocked through the shift register so that an exact copy of the encoder pulse appears at each of the output pins Q0-Q7. Referring again to Fig.4, trace 2 shows the output of chan­nel 1 which is an exact copy of the channel one pulse from the encoder except for amplitude. Likewise Fig.4, trace 3 shows the output of channel 2. Each of the output pins Q0-Q7 will mirror the transmitter encoder channels. Thus we have now converted the system back into a parallel, pulse width modulated system. Note that the output of the decoder is identical to the parameters pub- lished last month for the servo input. All we have to do now is to hook a servo to each of the channel output plugs and we have an 8-channel proportional radio control system. Even after working with this system for 32 years I still marvel at the magic of being able to maintain such complete and precise control over a model, at a distance, with no strings attached. SC SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. December 1997  79