Digital Servos -- what's the advantage?
02-21-2004, 10:38 AM
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Digital Servos -- what's the advantage?
The title pretty much says it all... I was looking at the new Futaba 7C in my tower-talk yesterday, and was wondering what's the advantage of the digital servos? They're obviously more expensive, and they claim to have more "command pulses" sent to the motor, but what does that mean, really? Also, do they work with any old rx? Do they require special equipment (I've seen "servo synchronizers" that seem rather expensive...)
-Rick
-Rick
02-21-2004, 11:24 AM
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RE: Digital Servos -- what's the advantage?
Digital servos will give you more precise control of your airplane but only if you are careful with your linkages. The servos will work with any receiver. You will need synchronizers only if you plan to use multiple servos on one surface. If the servo is a Hitec you will need a programmer instead of a synchronizer, but again only if you plan to use multiple servos on one surface. Only you can decide if the extra money is worth it.
Digital servos can help prevent flutter if you are expecting that problem
Digital servos can help prevent flutter if you are expecting that problem
02-21-2004, 11:58 AM
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RE: Digital Servos -- what's the advantage?
Digitals are more precise, are faster (really fast), usually have more tourque.
What that means is that on a standard analog servo the RX send a "command pulse" to the servo, then the servo moves acording to the command. But for a digital servo the RX sends a 'command pulse" five times as fast. Meaning that by the time the analog servo receives its command the digital servo has alreaghty received five commands. By this the servo is faster.
I hope you can understand and that I was clear.
They're obviously more expensive, and they claim to have more "command pulses" sent to the motor, but what does that mean, really?
I hope you can understand and that I was clear.
02-21-2004, 03:53 PM
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RE: Digital Servos -- what's the advantage?
A lot of peoples perception of how good digital servos work is only true if they
are used with PCM radios. For the most part the advantage to digitals for most
of us is they are usually designed with higher torque motors and they have zero
or near zero deadband and better holding power. They don't react quicker and
some experts think they even react slower than analog servos. I have a digital
servo on the rudder on one of my planes and besides more torque there is not
a whole lot of difference. I'm no expert but I pay attention to what the experts say.
Here are some quotes from an article in Model Airplane News.(Sorry this is so long)
" Both PCM and FMreceive FM signals from the transmitter, so they are both FM radios.
The difference comes in how the information that goes to the servos is decoded. A
benefit of PCM is that you never get any erroneous servo movements, however, the
speed with which the signal is decoded and sent to the servos is very slightly delayed
due to having to be processed in the receivers decoder. PCM has better signal rejection
(that means if you get hit and lose control the plane will lock you out and not go wild all
over the sky like FM will). PCM has better noise immunity( not bothered by metel to
metel contact, etc). If using digital servos with a PPM(FM) radio and RF noise is present
this would lead to servo jiggling and a high power consumption from your flight batteries.
FM is generally stepless so it does have potentially greater resolution and generally better
speed." This means that analog is actually faster than digital although newer digitals are fast.
You have to pay more attention to control linkage binding with digitals because of the
zero deadband. They are always trying to seek neutral when at rest so if any binding is
present the servos jiggle and consume lots of battery power.
What this all means in my opinion is there are first rate digitals
and first rate analog servos just as there are crappy ones of both.
Buy high quality servos and both kinds work well.
Digitals are not faster because they are digital, they are faster if
they are designed that way.
tommy s
are used with PCM radios. For the most part the advantage to digitals for most
of us is they are usually designed with higher torque motors and they have zero
or near zero deadband and better holding power. They don't react quicker and
some experts think they even react slower than analog servos. I have a digital
servo on the rudder on one of my planes and besides more torque there is not
a whole lot of difference. I'm no expert but I pay attention to what the experts say.
Here are some quotes from an article in Model Airplane News.(Sorry this is so long)
" Both PCM and FMreceive FM signals from the transmitter, so they are both FM radios.
The difference comes in how the information that goes to the servos is decoded. A
benefit of PCM is that you never get any erroneous servo movements, however, the
speed with which the signal is decoded and sent to the servos is very slightly delayed
due to having to be processed in the receivers decoder. PCM has better signal rejection
(that means if you get hit and lose control the plane will lock you out and not go wild all
over the sky like FM will). PCM has better noise immunity( not bothered by metel to
metel contact, etc). If using digital servos with a PPM(FM) radio and RF noise is present
this would lead to servo jiggling and a high power consumption from your flight batteries.
FM is generally stepless so it does have potentially greater resolution and generally better
speed." This means that analog is actually faster than digital although newer digitals are fast.
You have to pay more attention to control linkage binding with digitals because of the
zero deadband. They are always trying to seek neutral when at rest so if any binding is
present the servos jiggle and consume lots of battery power.
What this all means in my opinion is there are first rate digitals
and first rate analog servos just as there are crappy ones of both.
Buy high quality servos and both kinds work well.
Digitals are not faster because they are digital, they are faster if
they are designed that way.
tommy s
02-21-2004, 10:34 PM
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RE: Digital Servos -- what's the advantage?
Sorry Tommy, but you are reading the article wrong. The author does make it confusing, but basically he is saying that ppm is faster than pcm, which is true, since ppm doesn't have to be decoded. Switching servos from a good analog to a good digital will show a difference if a pilot is good enough to notice the difference. There are a lot of sport flyers out there, and while they are good flyers, they wouldn't notice the difference. There are also a lot of competition flyers that would certainly feel the difference. A digital servo is not necessarily faster or slower, that depends on a lot of factors, but generally since a digital servo has a MUCH tighter deadband, it will make a surface much more sensitive. This can be compensated for with exponential if you want, but it is certainly different.
HTH
Jim
HTH
Jim
02-21-2004, 11:56 PM
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RE: Digital Servos -- what's the advantage?
Digital servos are the same as there analog counterparts, except for the digital micro-processor. This is used to analyze the incoming RX signals and controls the servo motor. Digital servos use the same case, motors and gear-trains as standard servos. They also have the same feedback potentiometer [POT] as standard servos.
Digital amps are comprised of a quartz crystal controlled micro-processor, field effect transistors [FET] and heavier wire pigtails.
Analog amps are comprised of custom logic chips, timing components and standard pigtails.
Digital servos differ in how they process the incoming RX control signals and subsequently control power to the servo motor. Reduced deadband is realized, the servo reacts faster, accelerates and decelerates smoother and faster, resolution is greatly increased and holding power is incredible.
Cored motors:
Cored electric motors are incredibly common; you'll see this design concept every time you turn around if you’re looking. Auto starters, power tools, RC car motors and lots more... The heavy rotating mass [core] of these electric motors known as an armature is comprised of metal plates [poles] sandwiched around a metal shaft, the metal shaft is supported at both ends by bearings, and each pole is wrapped with copper wire [windings]. More poles, equals more windings which in turn creates smoother less notchy operation. The armature spins at high rpm encompassed by a permanent hollow center magnet which is located within and lines the inside diameter of the metal can. The armature spins within the inside diameter of the magnet. Power is introduced to the windings, which in turn creates an electro-magnetic field. This field is opposed by the magnet which in turn causes the armature to rotate.
Coreless motors:
Coreless motors operate within the same design concepts, but are assembled in a different manner. The coreless armature is lightweight the windings are formed into a cylindrical shape [no metal plates or poles, typically a wire mesh screen]. The armature is fixed to a metal shaft at one extreme and the armature is supported at one end [imagine a hole saw with an arbor attached]. The armature rotates around the outside diameter of a permanent hollow center magnet within a space between the inside diameter of the metal can.
Coreless motors respond faster to the electro-magnetic field due to their light-weight. The coreless armatures accelerate and decelerate much faster and smoother resulting in less overshoot of the commanded position, cause and effect is more precession. Additionally more force is generated with like amounts of power, on a side by side comparison to a cored armature with a smaller diameter. The larger diameter coreless armature emulates a longer lever or arm when pushing the motor shaft, this equates to a higher torque rating. The lack of poles opposed by electro-motive force [EMF] allows the coreless armature to center more accurately while maintaining or holding position with increased authority [there is no space between poles, thus more positions to rest at].
Analog and digital servos share the same internal components less the servo amplifier [the device which commands the servos position].
Analog amplifiers interpret RX command and pulse power [on/off] to the armature at say 50 cycles per second, the space in between the on/off cycles is known as dead-band [db]. No power is consumed at idle. Power is delivered at full available voltage. The potentiometer [pot] feeds position info to the amplifier. If a signal is received from the RX or the servo arm is deflected the amplifier pulses power to either move too or resist the opposing force. The amplifier compares position to commands and reacts to the need by alternating the duration of the pulse to speed up or slow down the servo motor, thus moving too or holding the commanded position.
Digital amplifiers via the micro-processor interpret RX commands or signals and operate within fixed parameters; the preset commands together with commanded position are then delivered to the servo motor. Center, end points and maximum speed are preset parameters. The duration of the power pulse and the amount of power utilized to activate the servo motor varies dependent on the need. Servo performance is greatly enhanced with this method of motor control. Digital amp's also send power pulses to the servo motor at 300 cycles per second much more frequent than the analog amp [power is consumed at idle]. Known result [servo buzzing]... The increased pulse cycles command the servo motor to react and perform with more precision simply because of the increased pulse frequency or interval. Net results are faster response to control command signals, lower dead-band numbers, increased holding power and much better resolution.
Programmable Digital amplifiers, in addition to the aforementioned digital amplifier operating description have a programmable memory which can be altered via a programmer. The servo operation parameters therefore can be manipulated within programmable parameters to the user's individual needs. Center and end-point positions, speed, dead-band, rotation, failsafe and more are programmable.
Digital amps are comprised of a quartz crystal controlled micro-processor, field effect transistors [FET] and heavier wire pigtails.
Analog amps are comprised of custom logic chips, timing components and standard pigtails.
Digital servos differ in how they process the incoming RX control signals and subsequently control power to the servo motor. Reduced deadband is realized, the servo reacts faster, accelerates and decelerates smoother and faster, resolution is greatly increased and holding power is incredible.
Cored motors:
Cored electric motors are incredibly common; you'll see this design concept every time you turn around if you’re looking. Auto starters, power tools, RC car motors and lots more... The heavy rotating mass [core] of these electric motors known as an armature is comprised of metal plates [poles] sandwiched around a metal shaft, the metal shaft is supported at both ends by bearings, and each pole is wrapped with copper wire [windings]. More poles, equals more windings which in turn creates smoother less notchy operation. The armature spins at high rpm encompassed by a permanent hollow center magnet which is located within and lines the inside diameter of the metal can. The armature spins within the inside diameter of the magnet. Power is introduced to the windings, which in turn creates an electro-magnetic field. This field is opposed by the magnet which in turn causes the armature to rotate.
Coreless motors:
Coreless motors operate within the same design concepts, but are assembled in a different manner. The coreless armature is lightweight the windings are formed into a cylindrical shape [no metal plates or poles, typically a wire mesh screen]. The armature is fixed to a metal shaft at one extreme and the armature is supported at one end [imagine a hole saw with an arbor attached]. The armature rotates around the outside diameter of a permanent hollow center magnet within a space between the inside diameter of the metal can.
Coreless motors respond faster to the electro-magnetic field due to their light-weight. The coreless armatures accelerate and decelerate much faster and smoother resulting in less overshoot of the commanded position, cause and effect is more precession. Additionally more force is generated with like amounts of power, on a side by side comparison to a cored armature with a smaller diameter. The larger diameter coreless armature emulates a longer lever or arm when pushing the motor shaft, this equates to a higher torque rating. The lack of poles opposed by electro-motive force [EMF] allows the coreless armature to center more accurately while maintaining or holding position with increased authority [there is no space between poles, thus more positions to rest at].
Analog and digital servos share the same internal components less the servo amplifier [the device which commands the servos position].
Analog amplifiers interpret RX command and pulse power [on/off] to the armature at say 50 cycles per second, the space in between the on/off cycles is known as dead-band [db]. No power is consumed at idle. Power is delivered at full available voltage. The potentiometer [pot] feeds position info to the amplifier. If a signal is received from the RX or the servo arm is deflected the amplifier pulses power to either move too or resist the opposing force. The amplifier compares position to commands and reacts to the need by alternating the duration of the pulse to speed up or slow down the servo motor, thus moving too or holding the commanded position.
Digital amplifiers via the micro-processor interpret RX commands or signals and operate within fixed parameters; the preset commands together with commanded position are then delivered to the servo motor. Center, end points and maximum speed are preset parameters. The duration of the power pulse and the amount of power utilized to activate the servo motor varies dependent on the need. Servo performance is greatly enhanced with this method of motor control. Digital amp's also send power pulses to the servo motor at 300 cycles per second much more frequent than the analog amp [power is consumed at idle]. Known result [servo buzzing]... The increased pulse cycles command the servo motor to react and perform with more precision simply because of the increased pulse frequency or interval. Net results are faster response to control command signals, lower dead-band numbers, increased holding power and much better resolution.
Programmable Digital amplifiers, in addition to the aforementioned digital amplifier operating description have a programmable memory which can be altered via a programmer. The servo operation parameters therefore can be manipulated within programmable parameters to the user's individual needs. Center and end-point positions, speed, dead-band, rotation, failsafe and more are programmable.
02-22-2004, 02:09 AM
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RE: Digital Servos -- what's the advantage?
ORIGINAL: BaronSchwab
Digitals are more precise, are faster (really fast), usually have more tourque.
Digitals are more precise, are faster (really fast), usually have more tourque.
02-22-2004, 09:37 AM
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RE: Digital Servos -- what's the advantage?
TOmmy, you are not correct.
If you want to prove it, contact Hitec or go to their site.
THey will show their servos and the digitals are their best analog servos with a different amplifier.
If you want to prove it, contact Hitec or go to their site.
THey will show their servos and the digitals are their best analog servos with a different amplifier.
ORIGINAL: tommy s
A lot of peoples perception of how good digital servos work is only true if they
are used with PCM radios. For the most part the advantage to digitals for most
of us is they are usually designed with higher torque motors and they have zero
or near zero deadband and better holding power. They don't react quicker and
some experts think they even react slower than analog servos. I have a digital
servo on the rudder on one of my planes and besides more torque there is not
a whole lot of difference. I'm no expert but I pay attention to what the experts say.
Here are some quotes from an article in Model Airplane News.(Sorry this is so long)
tommy s
A lot of peoples perception of how good digital servos work is only true if they
are used with PCM radios. For the most part the advantage to digitals for most
of us is they are usually designed with higher torque motors and they have zero
or near zero deadband and better holding power. They don't react quicker and
some experts think they even react slower than analog servos. I have a digital
servo on the rudder on one of my planes and besides more torque there is not
a whole lot of difference. I'm no expert but I pay attention to what the experts say.
Here are some quotes from an article in Model Airplane News.(Sorry this is so long)
tommy s
02-23-2004, 09:29 AM
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RE: Digital Servos -- what's the advantage?
This is not me being right or wrong, This article was in Model Airplane News.
As far as my own personal experience, I have always used premium servos
on ships for competition, 4131's and 4721's and until JR came out with the 8611
digital I would use premium analogs over cheap digitals anytime.
Like I said they're not automatically better just because they're digital.
tommy s
As far as my own personal experience, I have always used premium servos
on ships for competition, 4131's and 4721's and until JR came out with the 8611
digital I would use premium analogs over cheap digitals anytime.
Like I said they're not automatically better just because they're digital.
tommy s
02-23-2004, 11:24 AM
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RE: Digital Servos -- what's the advantage?
ORIGINAL: tommy s
This is not me being right or wrong, This article was in Model Airplane News.
As far as my own personal experience, I have always used premium servos
on ships for competition, 4131's and 4721's and until JR came out with the 8611
digital I would use premium analogs over cheap digitals anytime.
Like I said they're not automatically better just because they're digital.
tommy s
This is not me being right or wrong, This article was in Model Airplane News.
As far as my own personal experience, I have always used premium servos
on ships for competition, 4131's and 4721's and until JR came out with the 8611
digital I would use premium analogs over cheap digitals anytime.
Like I said they're not automatically better just because they're digital.
tommy s
02-23-2004, 01:03 PM
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RE: Digital Servos -- what's the advantage?
I'm not interested in the ins and outs of how they work. I just know how
they react in my airplanes. Comparing JR's standard 811 digital servos
versus 4131's on ailerons and throttle and 4721's on rudder and elevator
in a 60 size Top Cat pattern plane, the airplane feels more "locked in" with
the 4131's and 4721's.
I have a Widebody 60 and the 8611 digital servo on rudder is better than any
I have ever used. When doing maneuvers such as knife-edge loops etc it
holds position better requiring far less corrections and adjustments.
I can't say the same for the 811's.
I will use 8611's on all surfaces on my next airplane as I know the
advantage, especialy in pattern, of the airplane staying where you put it.
tommy s
they react in my airplanes. Comparing JR's standard 811 digital servos
versus 4131's on ailerons and throttle and 4721's on rudder and elevator
in a 60 size Top Cat pattern plane, the airplane feels more "locked in" with
the 4131's and 4721's.
I have a Widebody 60 and the 8611 digital servo on rudder is better than any
I have ever used. When doing maneuvers such as knife-edge loops etc it
holds position better requiring far less corrections and adjustments.
I can't say the same for the 811's.
I will use 8611's on all surfaces on my next airplane as I know the
advantage, especialy in pattern, of the airplane staying where you put it.
tommy s
02-23-2004, 11:26 PM
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RE: Digital Servos -- what's the advantage?
No one can argue with you on the way you prefer the way the aircraft to feel. But did you consider that the reason the digital servo might need more correction is because the greater precision causes you to have to fiddle with it more to get it exactly to the right position? It also gives you more control when you learn to use it. I found in the begining I had to program in a small amount of exponential to soften the feel of the digital simply because I was used to the sloppy way the standard servo responds.
02-24-2004, 09:07 AM
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RE: Digital Servos -- what's the advantage?
Why would I want to change to a servo that made the plane fly worse, Just
so I could brag to everyone "I have digital servos" ? I don't think so.
I've tried 'em. Premium digitals are a major improvement in performance
but cheap digitals are not.
I can only tell you how they perform in my pattern plane. Obviously you have
a different view. I do not have a problem with digital servos , I just think if
you are going to invest in more expensive servos get the best ones.
tommy s
so I could brag to everyone "I have digital servos" ? I don't think so.
I've tried 'em. Premium digitals are a major improvement in performance
but cheap digitals are not.
I can only tell you how they perform in my pattern plane. Obviously you have
a different view. I do not have a problem with digital servos , I just think if
you are going to invest in more expensive servos get the best ones.
tommy s
02-24-2004, 09:55 AM
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RE: Digital Servos -- what's the advantage?
Well, just to weigh in with a few comments here . . .
One guy said that a digital get's it's signal from the RX 5 times faster than an analog servo. Sorry, but not true - the frame rate of a system is the frame rate of a system, and the servo CANNOT change that. Yes, there are some special application digitals that CAN run a higher frame rate, but they are typically for Helicopter rudder applications where there is a special gyro also which will drive them at that rate.
One guy said that PCM is slower. Once again, false. The data frame rate is the same, what there is is a very slight delay in signal transmission. This is not a slowdown, but a delay. Kinda like getting hit with the stream off a 5" firehose. If one is five feet away, and the other 10, and you get hit with it, can you really tell the difference, since the same volume of water is hitting you? The delay is so small as to be humany imperceptible, but the advantages are obvious.
Lastly, the biggest advantage of a digital is holding torque. The motors in the digitals are pretty much the same as the analogs, but the analog servo will put out a signal to the motor that is proportional in strength to how far off the commanded position that servo is. So, the farther away, the faster the recovery. The problems is, that around center, the errors are so small, that the corrections are extremely weak, and the servo can be deflected off-center and cannot correct exactly. In the digital, the digital amp
can send full power at any level of error - no proportion to the error - and as such, will hold a position or make smaller corrections much more authoritatively. This is partly why digitals buzz - it's not a bug, but they are holding center tighter than any analog is capable of, and thus the buzz.
Comparing the JR811 to a premium servo is not a fair comparison. Lets face it - the 811 is basically a 531/537 with a digital amp. Same single bearing, same gears. The digital amp should make it center better, but all else is dependent on the mechanics that have not changed. Kinda like putting a $5000 stereo receiver on $20.00 speakers - at some point, the limiting factor moves elsewhere, and in the 811, it is the lower end mechanicals. Try an 8231, 8411, or such and you will see the difference.
As for personal experience, I fly helicopters, and I have not found an analog servo at any level that feels as tight as a good premium digital. With the heli, it is probably because centering is so much more critical thatn in planes, at lest for stability in the hover.
- Tim
One guy said that a digital get's it's signal from the RX 5 times faster than an analog servo. Sorry, but not true - the frame rate of a system is the frame rate of a system, and the servo CANNOT change that. Yes, there are some special application digitals that CAN run a higher frame rate, but they are typically for Helicopter rudder applications where there is a special gyro also which will drive them at that rate.
One guy said that PCM is slower. Once again, false. The data frame rate is the same, what there is is a very slight delay in signal transmission. This is not a slowdown, but a delay. Kinda like getting hit with the stream off a 5" firehose. If one is five feet away, and the other 10, and you get hit with it, can you really tell the difference, since the same volume of water is hitting you? The delay is so small as to be humany imperceptible, but the advantages are obvious.
Lastly, the biggest advantage of a digital is holding torque. The motors in the digitals are pretty much the same as the analogs, but the analog servo will put out a signal to the motor that is proportional in strength to how far off the commanded position that servo is. So, the farther away, the faster the recovery. The problems is, that around center, the errors are so small, that the corrections are extremely weak, and the servo can be deflected off-center and cannot correct exactly. In the digital, the digital amp
can send full power at any level of error - no proportion to the error - and as such, will hold a position or make smaller corrections much more authoritatively. This is partly why digitals buzz - it's not a bug, but they are holding center tighter than any analog is capable of, and thus the buzz.
Comparing the JR811 to a premium servo is not a fair comparison. Lets face it - the 811 is basically a 531/537 with a digital amp. Same single bearing, same gears. The digital amp should make it center better, but all else is dependent on the mechanics that have not changed. Kinda like putting a $5000 stereo receiver on $20.00 speakers - at some point, the limiting factor moves elsewhere, and in the 811, it is the lower end mechanicals. Try an 8231, 8411, or such and you will see the difference.
As for personal experience, I fly helicopters, and I have not found an analog servo at any level that feels as tight as a good premium digital. With the heli, it is probably because centering is so much more critical thatn in planes, at lest for stability in the hover.
- Tim
02-24-2004, 12:20 PM
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RE: Digital Servos -- what's the advantage?
ORIGINAL: tadawson
Well, just to weigh in with a few comments here . . .
One guy said that a digital get's it's signal from the RX 5 times faster than an analog servo. Sorry, but not true - the frame rate of a system is the frame rate of a system, and the servo CANNOT change that. Yes, there are some special application digitals that CAN run a higher frame rate, but they are typically for Helicopter rudder applications where there is a special gyro also which will drive them at that rate.
One guy said that PCM is slower. Once again, false. The data frame rate is the same, what there is is a very slight delay in signal transmission. This is not a slowdown, but a delay. Kinda like getting hit with the stream off a 5" firehose. If one is five feet away, and the other 10, and you get hit with it, can you really tell the difference, since the same volume of water is hitting you? The delay is so small as to be humany imperceptible, but the advantages are obvious.
Lastly, the biggest advantage of a digital is holding torque. The motors in the digitals are pretty much the same as the analogs, but the analog servo will put out a signal to the motor that is proportional in strength to how far off the commanded position that servo is. So, the farther away, the faster the recovery. The problems is, that around center, the errors are so small, that the corrections are extremely weak, and the servo can be deflected off-center and cannot correct exactly. In the digital, the digital amp
can send full power at any level of error - no proportion to the error - and as such, will hold a position or make smaller corrections much more authoritatively. This is partly why digitals buzz - it's not a bug, but they are holding center tighter than any analog is capable of, and thus the buzz.
Comparing the JR811 to a premium servo is not a fair comparison. Lets face it - the 811 is basically a 531/537 with a digital amp. Same single bearing, same gears. The digital amp should make it center better, but all else is dependent on the mechanics that have not changed. Kinda like putting a $5000 stereo receiver on $20.00 speakers - at some point, the limiting factor moves elsewhere, and in the 811, it is the lower end mechanicals. Try an 8231, 8411, or such and you will see the difference.
As for personal experience, I fly helicopters, and I have not found an analog servo at any level that feels as tight as a good premium digital. With the heli, it is probably because centering is so much more critical thatn in planes, at lest for stability in the hover.
- Tim
Well, just to weigh in with a few comments here . . .
One guy said that a digital get's it's signal from the RX 5 times faster than an analog servo. Sorry, but not true - the frame rate of a system is the frame rate of a system, and the servo CANNOT change that. Yes, there are some special application digitals that CAN run a higher frame rate, but they are typically for Helicopter rudder applications where there is a special gyro also which will drive them at that rate.
One guy said that PCM is slower. Once again, false. The data frame rate is the same, what there is is a very slight delay in signal transmission. This is not a slowdown, but a delay. Kinda like getting hit with the stream off a 5" firehose. If one is five feet away, and the other 10, and you get hit with it, can you really tell the difference, since the same volume of water is hitting you? The delay is so small as to be humany imperceptible, but the advantages are obvious.
Lastly, the biggest advantage of a digital is holding torque. The motors in the digitals are pretty much the same as the analogs, but the analog servo will put out a signal to the motor that is proportional in strength to how far off the commanded position that servo is. So, the farther away, the faster the recovery. The problems is, that around center, the errors are so small, that the corrections are extremely weak, and the servo can be deflected off-center and cannot correct exactly. In the digital, the digital amp
can send full power at any level of error - no proportion to the error - and as such, will hold a position or make smaller corrections much more authoritatively. This is partly why digitals buzz - it's not a bug, but they are holding center tighter than any analog is capable of, and thus the buzz.
Comparing the JR811 to a premium servo is not a fair comparison. Lets face it - the 811 is basically a 531/537 with a digital amp. Same single bearing, same gears. The digital amp should make it center better, but all else is dependent on the mechanics that have not changed. Kinda like putting a $5000 stereo receiver on $20.00 speakers - at some point, the limiting factor moves elsewhere, and in the 811, it is the lower end mechanicals. Try an 8231, 8411, or such and you will see the difference.
As for personal experience, I fly helicopters, and I have not found an analog servo at any level that feels as tight as a good premium digital. With the heli, it is probably because centering is so much more critical thatn in planes, at lest for stability in the hover.
- Tim
First- A digital servo does not get more pulses from the receiver - but for every pulse it gets from the receiver it puts out from five to ten times more power pulses to the motor than the analog servo does.
Second - A PCM system is slower - but by not enough that you would notice anything.
Third - - Holding power is actually another way to refer to the deadband. The deadband is tighter on a digital servo.
Lastly- The bearings, gears, and mechanics are the same on the cheaper servos as used on the premium servos. The difference is the motor.
02-24-2004, 01:52 PM
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RE: Digital Servos -- what's the advantage?
ORIGINAL: dirtybird
This argument just goes on and on. Fun isn't it?
First- A digital servo does not get more pulses from the receiver - but for every pulse it gets from the receiver it puts out from five to ten times more power pulses to the motor than the analog servo does.
Second - A PCM system is slower - but by not enough that you would notice anything.
Third - - Holding power is actually another way to refer to the deadband. The deadband is tighter on a digital servo.
Lastly- The bearings, gears, and mechanics are the same on the cheaper servos as used on the premium servos. The difference is the motor.
This argument just goes on and on. Fun isn't it?
First- A digital servo does not get more pulses from the receiver - but for every pulse it gets from the receiver it puts out from five to ten times more power pulses to the motor than the analog servo does.
Second - A PCM system is slower - but by not enough that you would notice anything.
Third - - Holding power is actually another way to refer to the deadband. The deadband is tighter on a digital servo.
Lastly- The bearings, gears, and mechanics are the same on the cheaper servos as used on the premium servos. The difference is the motor.
- Tim
02-24-2004, 05:42 PM
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RE: Digital Servos -- what's the advantage?
ORIGINAL: tadawson
So, in your world, the bearings and gears are the same in a $15.00 Analog (likely NO bearings) and a hight end digital? Somehow, I don't think so . . . . not even close!
Take a look at the Futaba web site. You will find a cross reference chart showing which gears can be used in what servos. This is so you can buy the cheaper plastic gears if you distroy one of the first gears and don't want to buy the expensive metal gear set. It also shows that the parts are the same for the cheaper servos that are used in the more expensive units.
You can see for yourself the cases & output arms are the same. That leaves only the bearings. I don't believe the bearings in the S3001 are any different than the ones in the 9150's that I have
- Tim
ORIGINAL: dirtybird
This argument just goes on and on. Fun isn't it?
First- A digital servo does not get more pulses from the receiver - but for every pulse it gets from the receiver it puts out from five to ten times more power pulses to the motor than the analog servo does.
Second - A PCM system is slower - but by not enough that you would notice anything.
Third - - Holding power is actually another way to refer to the deadband. The deadband is tighter on a digital servo.
Lastly- The bearings, gears, and mechanics are the same on the cheaper servos as used on the premium servos. The difference is the motor.
This argument just goes on and on. Fun isn't it?
First- A digital servo does not get more pulses from the receiver - but for every pulse it gets from the receiver it puts out from five to ten times more power pulses to the motor than the analog servo does.
Second - A PCM system is slower - but by not enough that you would notice anything.
Third - - Holding power is actually another way to refer to the deadband. The deadband is tighter on a digital servo.
Lastly- The bearings, gears, and mechanics are the same on the cheaper servos as used on the premium servos. The difference is the motor.
Take a look at the Futaba web site. You will find a cross reference chart showing which gears can be used in what servos. This is so you can buy the cheaper plastic gears if you distroy one of the first gears and don't want to buy the expensive metal gear set. It also shows that the parts are the same for the cheaper servos that are used in the more expensive units.
You can see for yourself the cases & output arms are the same. That leaves only the bearings. I don't believe the bearings in the S3001 are any different than the ones in the 9150's that I have
- Tim
