Spacemonkey71

My wife asked me about the "digital proportional control" on my Tx the other day and I realized that I really didn't understand it myself. This is another one of those "Duh....." questions, I'm sure, so humor me

I know it has SOMETHING to do with the stick movement and how it relates to the servo movement, but EXACTLY what this means eludes me..... maybe I'm just sleep deprived from spending 2 500am nights finishing the repair job on the SuperBox Fly40 (I finally got the engine mounted, the electric switch, all the servos and connecting rods attached >>>> this old bird is once again READY FOR THE AIR!) I guarantee I can fly it better than the LAST guy! (see pic 2 - this is what happens when you bank too sharply while going too slow and too low!)

>>>>Incidentally, since then -in only 4 weeks!-, the SAME guy has sucessfully crashed a Mini Ultra Stik that wasn't his -and tried to give it back to the owner repaired with a hot glue gun!!! NOT A GOOD IDEA!!!! HOT GLUE IS NOT AIRPLANE REPAIR MATERIAL!!!!! (and his repair skills are NOT good either!) and his BRAND NEW trainer plane that he JUST got the day that he crashed the Super Box literally EXPLODED into the chunks no bigger than 10" long! Why? He over controlled the aileron in a turn, flipped it upside down and WHAM!!!! A 10 point Turf Dive! This guy just can't seem to LEARN and his instructor is really fed up with him because he doesn't listen either! (funny enough it was the instructor's Mini Stik he smashed! WHY he let him fly it is BEYOND me!!!) He's actually crashed THREE planes in FOUR weeks!!!! I think the only reason he hasn't gone 4 for 4 is that he's out of money and planes! I personally think he needs to stick to AirHogs and other $40-$50 foam fliers (not that there's ANYTHING wrong with AirHogs or other cheapie foam Micro fliers - I LOVE my Mini XPV and fly it almost every day (it's my guilty pleasure )

The repaired SuperBox is going to be my "beater" plane; I've got a Futaba SkySport 4 T4VF 4 channel radio for it (ebay $40 with 4 servos + 7 channel Rx + battery + charger + 2 Y-cables and 3 extension cables; not too bad of a haul [8D] -free shipping too! ) and a K&B .45 engine w/ 11x7 prop on it (that I also got off of ebay for $35 - it runs good and has great compression; I just have to remember to not use any juice hotter than 10% on that engine; apparently K&B makes a good engine, but it does NOT tolerate 15% and apparently it'll burn it up). I think it's got a little too much engine for that size/weight plane (56"ws, 5lbs) - it's going to be faster than a monkey with it's tail on fire!!!! [X(] so I'm going to have to be careful not to rip the wing off or break a spar (I saw this happen recently when a guy put a .61 on a plane built for a .40 and flew it WIDE OPEN for about 5 minutes. You could actually HEAR it when the spar snapped! He was greasing down the field @ ~20-25 ft and pull up steep; it made this loud muffled POP like snapping your fingers inside a snare drum)

Anyways, back to the original question: What is a good explanation of "digital proportional control" ? Sorry for the seg-way I'm really tired!

BarracudaHockey

My Feedback: (11)

It means roughly that the servo follows the stick movement proportionately. Move the stick half way and the servo moves half way. The phrase was coined before expo and computer radios.

Old style radios you held the stick and the control would pulse to full throw, how long you held it was how hard it turned or whatever.

bruce88123

Digital controls use signals that are either on or off or "1's" or "0's" in computer terms. There can be no ambiguity as there can be in analog controls where the signal can be anywhere between a baseline (typically zero or ground) and an upper voltage that is determined by the system. The on/off period or "pulse" is then manipulated by either varying its width or position or both via the system encoder. The RX must be able to decode the signal. The TX and RX must utilize the same "code" or algorithm or all you get is chaos. Next year all TVs are converting to digital format from their current analog format. Pictures should be sharper.

Proportional merely means that the amount of change in the signal or control movement is "proportional" or "similar" to the amount of stick movement. In the early days if you moved the control a little in one direction (or pushed a button) your only option was for full throw. There were a variety of other systems as well that we won't get into at this time.

Spacemonkey71

Now that makes sense. This is a DUH moment (again! [:@]). Basically put, it's like a button, either it IS pressed (ON) it is NOT pressed (OFF) no matter how hard or softly you press the button. The "Proportional" effect gives it the ability to recognize different "levels of input" (ie how hard the button is being pushed) in order to output varying degrees of whatever (ex hitting the button hard may = 100% activation while hitting it "soft"may yield 20% of full activation)

Thanks for the help! I knew it was something ridiculously simple, I'm just having "Brain Lockups" courtesy of no sleep and a WILD MONKEY of a 19 month old son who finds it easier to RUN UNCONTROLLABLY around the house rather than walk (usually while carrying something important, expensive, or pointy...)= [X(]

JohnW

My Feedback: (6)

Digital Proportional Control - The stick movements are proportional, i.e. they move back and forth. The stick position is measured by the radio, and represented by a digital number. The control bit, I'm sure there is a joke in there, but I'll leave it alone.

I’m not going to get into A/D converters and all that stuff. But as an example, my radio has 2048 resolutions steps from stick to servo. So, each stick position is represented by a number between 0 and 2048. I.E., center would be stick position 1024, and so on. This isn’t exactly how it all works, but close enough.
.

MinnFlyer

My Feedback: (4)

A good way to visualize "Digital" vs "Analog" is to think of a clock. A digital clock give a much more accurate measurement.

If you look at the clock below, you might tell someone that the time is 8:30, but a digital readout is more exact. It's basically the same thing with signals.

BarracudaHockey

My Feedback: (11)

The term "digital proportional" was probably more marketing that digital. I seem to remember it being around before PCM which, if I recall correctly, was the first real "digital" control system.

alfredbmor

Is the difference between Analog and Digital the use of an electric potentiometer (analog) and a simple chip (Digital) in our radios?

BarracudaHockey

My Feedback: (11)

To be truly "digital" it needs to be PCM or SS (2.4) even at that the signal going to the servo is converted back to analog at the receiver end, even with "digital" servos.

bruce88123

Not really. PPM is still fully digital but uses a different encoding method. PPM or PWM are still digital. The amount the sticks are moved effects the amount the pulse widths are changed or how much their positions are shifted. Yes, PCM is a pulse encoding technique that requires yet another encode/decode method and so far no 2 manufacturers use the same code.

beau0090_99

Yeah,
If anyone knows about serial communications, you can think about the difference between PCM and PCM as the difference between Modbus RTU or Modbus ASCII. They are both digital serial drivers, but one uses a diferent way of finishing the string of data(End of Transmission EOT), and the way the data is verified (checksum) is different. The TX and the RX speak digitally to each other. The Tx generally would wither use pots(analog) or encoders(digital) to encode the stick positions, and the RX's job is to decode these values per channel and send it to the Servos, which now translate an analog (probably 4-20mA) signal on the line to movement of the motor. Loop closure (making sure the motor is where it is told) is done internal to the servo either analog (internal pot), or digitally (encoder). The Digital in Digital Proportional means the communications method used via Radio Frequency.
Any experts out there? Was this summarized correctly?
Thanks,
Curtis

alfredbmor

Most modern transmitters use different types of chip sets, how can you tell the difference between an analog and a digital transmission?
By the use of a singular chip set?
(Which decodes the steps of the sticks and send it via PPM, PCM or 2.4 Mhz to a receiver)
If I use an old receiver (analog), with digital servos and a Digital TX, How can I tell the eficiency of the Digital servos?
Are there digital receivers already?

The main question would be, how to explain the difference between analog and digital and how they work?

beau0090_99

This is where most of my knowledge starts to break down. It is my understanding that our simple digital proportional radios, like my 72MHz 6EXAP will use 8 bit encoding of the sticks, which gives about 256 counts of resolution (it may be 9 bits or 512 counts, but I am not sure). The PCM's were advertised as 1024, which is 10 bits. Now, I am not sure how they encode the stick signal in the Tx. I have a feeling that it is done with Pots and a multi-channel A/D converter and then the signal is simply sent as a serial packet on the 72Mhz or 2.4 GHz carrier frequency.

Now to be honest, I am not sure how an analog signal is carried over RF. I can understand how you would filter out the signal, but to process that using analog hardware, i am not sure what would be involved.

What I think does happen for continuity between a PCM and PPM system is that he signal is probably encoded as 12-bits, and the last 2 or 3 bits are insignificant if they are not used. Therefor if you have a CM 1024 Rx and digital servos with that resolution, you may see the added precision. I come from the industrial Servo world where we use large scale brushless servo motors to create machines for packaging foods and such things and understand what the resolution does to repeatability, but I am not sure it makes a tremendous difference in this hobby. I don't think you are going to notice a change of res from 256, to 512, or 1024.

Anyone else care to add to this?
Curtis
P.S. More to your point, I am not sure that there are fully digital Rx's that actually tell the Servo where to be using some sort of Serial network. For noise immunity, you would probably need a few more wires like an RS-485/422 network.

Campgems

A simple explanation would be your neighbor across the street and you exchanging information. Your comunication method is his front porch light. The signal you have aggreed to is that if he wants you to stand still, he will turn the light on for a second, then turn it off. If he wants you to move to the right, he will turn the light on for 1.5 seconds then off, or if he wants you to move to the left, he will turn the light on for 0.5 seconds then off. Every 20 seconds he will do this. This would translate to a data packet sent out my your transmitter. The actual signal is binary, on or off, but the duration of the signal is variable or analog. This packet contains the information for one channel, you moving right or left.

So if he want you to step forward or backward, turn to the righ or left, you need two more channels. He could go the the front room and use that light for a second channel and then the upsairs bedroom for the third. His packet is repeated every 20 seconds, so running around the house isn't a good idea. What he sent three flashes of the light for three channels. The problem is, if you happened to blink for the first, you might try to use Chan 2 blink for Chan 1, or chan 3. To solve this, He flipps on the light and imeadiatly turns it off, it is only on for say 0.1 seconds. We now know that the next time the light comes on, it will have the data for which way Chan 1, me moving to the right or to the left . After it turns off , then back on, he is now signaling Chan 2 forward or backward. Off goes the light and then after a pause it comes on again, this is Chan 3.

This is the basick comunication from TX to RX. The RX breaks down the data stream and sends the information for Chan1 out to the Chan1 servo and so on. The servo waits for this signal to decide which direction to go. The problem though is where is he at to start with. For this, we'll decide that the servo make a pluse internally equal to 1 second when it is centered. The it compairs its pulse to the light the neighbor sends. One second light and servo is at 1 second. nothing to do . If the pluse and the light don't equal each other in length though then the servo determines wheater the light is longer that 1 second or shorter. It then starts a motor that starts the output arm turning, When this arm turns, it changes the the width of the pulse the servo creates and the motor will keep turning until the generated pulse is equal to the light. Then it stops.

This is the basic way all of our RC radios work, starting back with the very first proportional sets like my first, the Royal Clasic 69 which I built. It was 27mhz so any citizen band radio screwed it up, It was AM modulation, it was awful, by todays standard. The decoding of the amount of stick movement was by a RC (Resistor/Capicator)new input to a digital switch. When the signal came along to start that pulse, the RC net would blead down to turn off the pulse. Each RC net had to be ballanced with an O scope when you set up the radio. There were counters to tell which pluse to place on the data stream, and again, it was tuned by who made the set, in my case, me. The reciever was quite a bit easier, just a tuner and a counter to direct the pulses to the right channel. THe servos though were a nightmare to tune. You had mechanically turn the little pot and tighten it down put the gears back in turn on the set and see where you centered. Tear down the case again and make your adjustments. The servo you adjusted for Chan 1 probably needed a lot of adjustment if you moved it to Chan 2.

The later sets, with PPM, PCM 524, PCM 1028, etc, were all improvements on how the stick movement was changed into a pluse set to the servo. There were limitations on how fast the RC nets could change state, so optic decoders came into play. The frame rate, how many times a second each servo got refreshed data also improved. I haven't followed these improvements other than to know you need a TX and a RX on the same frequency, talking the same language, IE PPM or PCM, and that as long as the servo could understand the pulse from the receiver ( positive or negitive shift), all was well. Knowing how the coding and decoding is done isn't necessary anylonger. Even if you knew the details, the is no way you can "tune" your equipment, it's all burnt on a chip now. No little trim pots to tweak. Take it out of the box. Plug it in, charge the battery and away you go. Life is good.

Don

bruce88123

All transmitters made in the past approx 20 years or so have been one form of digital or another. The stick positions generate an analog signal that is then multiplexed and converted in the TX to a digital format of choice and then sent to the RF section for transmission. A digitally modulated RF signal is what comes out of the antenna. All RX's of the same time period have also been digital RX's of some type. The raw RF enters and is then processed in a manner appropriate to the system in use. In ALL cases the signal out of the RX that goes to the servo is analog. It is a DC signal voltage typically between Ground and 5 volts. A digital servo re-converts this to a digital value for use inside the servo whereas a std servo just uses the analog value.

jetmech05

listen it's simple..It's all FM...that's right it's FM...You move a stick and the airplane moves the way it's supposed to.......as long as the magic smoke is still in all the componets then the FM (fu**ing magic) usually works.....You don't believe in magic smoke...Ok hook something up wrong and you see smoke.....usually that componet doesn't work any more...because you let the magic smoke out.....
It's my lie and I'm sticking to it
Good Flyin to all

bruce88123

I prefer using PFM.

Spacemonkey71

Yeah, but you're supposed to yell "Diagon Alley" when that happens

I used to let the magic smoke out of my PC REGULARLY! One time I blew the main circuit breaker that connected all 6 apartments in my building - incidentally that was also when I fused/arc-welded a screwdriver to the motherboard! Talk about "don't touch that!" My fingernails turned BLACK! I'm kinda suprised I didn't get fried!!! (yeah I know the "local" circuit breaker should have caught it, but these were OLD apartments and I have a feeling that they were wired by someone's semi-mongoloid half-ape brother in law!!!! ) When Ga Power came up and told us what happend, all I could say was "Wow! That's really interesting! Do you know who did it?"

Arceenut

I would like to correct many people's understanding of the signal between the receiver and the servo. Regardless of the method of transmission of signal between the transmitter and the receiver (PPM, PCM, AM, FM, 27MHz,54MHz,72MHz, 2.4GHz) , the signal between the receiver and the servo is a pulse width, usually between ~1millisecond to ~2 milliseconds, with ~1.5 millisecond being the servo centered.. Also this is the signal whether the servo is analog or digital. otherwise a different receiver would be needed between "analog" servos and "digital" servos. It is NOT a varying voltage level as some people seem to understand. If it were a varying voltage level, the servo position would vary all over the place with changes in supply battery condition.

HighPlains

My Feedback: (1)

It is very entertaining reading all of this thread. It would take several hours to correct all the misconceptions. Perhaps this is due to the evolution of the technology over the last 50 years. Even with an engineering background, the changes are difficult to keep up with. Without one, people rely on tribal knowledge, and over the top advertisements written by marketing pukes.

B.L.E.

Just let me add that the pulse width is infinately variable and thus it is an analog signal. It is not a binary number string which is what digital is. Most common voltmeters will integrate the pulse width into a dc voltage reading and show a varying dc voltage as you change the pulse width and this is the reason lots of folks believe that the servo is controlled by a varying dc voltage on the signal wire.

Campgems

The varrying voltage is due to the lag time of the needle responding to the on/off of the signal. There is no circuit in the volt meter to account for pulse width. With a digital meter, you will get some unreliable numbers as a digital meter will take a sample of the voltage every now and then. The frequency of this sample is different between meters but ussally in the several milisecond range. Otherwise, the display would be changing to fast to read.

The AC portoin of the meter though measures RMS Root Mean Square voltage, not peat voltage. This is the only adjustments to read voltage.

To really see the signal strength, shape, and timing, you need an oscilloscope.

Don

bruce88123

Unfortunately ARCEENUT is incorrect. A PCM signal as used in our RC is NOT a varying pulse width system, also known as PWM. What is transmitted is a digital word. In this word there are a number of possible pulse positions for each "function" and the signal is ENCODED into a series of pulses which are either present or missing but ALWAYS the same width. http://www.irig106.org/docs/106-05/chapter4.pdf The RX then decodes the pulse "word".

aerowoof

digital porportional dates back to about 1967 or a bit earlier,you do not need a computer radio to have digital porportional.The sticks analog movement is encoded to a digital code in the pulse width that is the decoded by the reciever and the servo moves.this is the basic principle still in use today .

Campgems

Bruce you are getting the internal handeling of the servo data confused with how the servo handels it. In you explanation, you would need to have PCM servos, PPM servo, PWM servos etc. THe fact is that any servo will work with any of the modulation methods.

Internally in the receiver, you are correct, the data is a word, packet, what ever terminology you want to use, but when the output is finally set to the servo , it is pulse width. How else would a servo work with several generations of transmitters on any RF frequency????

You should read Arceenut's post again. I think you missed the point he was making.

Don