Low down on Digitals
12-01-2004, 03:58 PM
#1
Member
Thread Starter
My Feedback: (1)
Join Date: Dec 2003
Location: Justin,
TX
Posts: 88
Likes: 0
Received 0 Likes
on
0 Posts
Low down on Digitals
I looked around and I didn't find the answer I was looking for...So here goes.
What are the benefits to Digital and drawbacks?
I have a 1/3 scale Extra that I need new aileron servos for. I need at least 77 Oz of torque. I fly Futaba.
Right now it has Tower T-65's in it.....They guy that flew it said that the ailerons were slow.
I am looking to upgrade the servos, and the batterypack....I have a 6v in there, but I don't know much else about it.
What are the benefits to Digital and drawbacks?
I have a 1/3 scale Extra that I need new aileron servos for. I need at least 77 Oz of torque. I fly Futaba.
Right now it has Tower T-65's in it.....They guy that flew it said that the ailerons were slow.
I am looking to upgrade the servos, and the batterypack....I have a 6v in there, but I don't know much else about it.
12-01-2004, 04:34 PM
#2
Banned
Join Date: Mar 2003
Location: Tokoroa, , NEW ZEALAND
Posts: 3,848
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
If you do a search you'll find plenty of information on how Digitals compare to regular servos but to summarize:
In a normal RC transmitter/receiver pairing, servo positioning information is sent at a rate of up to 50 times per second.
Once this information is decoded by the receiver, it's passed on to the servos at the same rate.
In a *regular* servo, this means that the current servo position and the commanded position (which may be different if the stick has been moved) are only compared up to 50 times a second. Since the servo will only be commanded to move to a new position each time this comparison is performed, there will only be a maximum of 50 pulses of power sent to the servomotor each second when it is being commanded to move.
In a digital servo, a small processor in the servo itself makes a note of the signal that arrives from the receiver and constantly (well about 1,000 times per second) compares this to the actual servo position. The result is that the dgiital servo sends up to 1000 pulses of power to its motor every minute when it is being commanded to move.
Because the digital is effectively working much faster at the task of making sure the servo arm matches the position commanded by the transmitter stick, a digital servo will have better holding power and more torque when very small stick movements are requested. This will also manifest itself as vastly superior centering -- something that's often important on larger and 3D models where you're using large control surfaces and/or movements.
This much faster operation also explains why some digitals "sing" or whine even when at rest.
The upsides of this magic is that digitals have more constant torque (ie: the same torque whether they're 1 degree or 60 degrees away from the commanded position) and better holding power than their conventional equivalent.
The downside of this magic is that they will consume more power. You don't get more constant torque and better holding power for free you know :-)
Oh, and if you hadn't noticed -- they're also more expensive :-(
For most sport ships up to about 60-size, digital servos are a gross overkill (IMHO). A traditional standard sports servo will do just fine and few sports-fliers would notice the difference between a $12.50 budget servo and a $100+ titanium-geared digital.
However, for larger models or those where speed and precision are critical then digitals really do shine.
Even my 3D funfly really benefits from having some ultra-fast (0.08 second) HS5925 servos on the tail. It means I can do a *very* square wall and also pull out from vertical dives at ridiculously low altitudes with confidence.
In a normal RC transmitter/receiver pairing, servo positioning information is sent at a rate of up to 50 times per second.
Once this information is decoded by the receiver, it's passed on to the servos at the same rate.
In a *regular* servo, this means that the current servo position and the commanded position (which may be different if the stick has been moved) are only compared up to 50 times a second. Since the servo will only be commanded to move to a new position each time this comparison is performed, there will only be a maximum of 50 pulses of power sent to the servomotor each second when it is being commanded to move.
In a digital servo, a small processor in the servo itself makes a note of the signal that arrives from the receiver and constantly (well about 1,000 times per second) compares this to the actual servo position. The result is that the dgiital servo sends up to 1000 pulses of power to its motor every minute when it is being commanded to move.
Because the digital is effectively working much faster at the task of making sure the servo arm matches the position commanded by the transmitter stick, a digital servo will have better holding power and more torque when very small stick movements are requested. This will also manifest itself as vastly superior centering -- something that's often important on larger and 3D models where you're using large control surfaces and/or movements.
This much faster operation also explains why some digitals "sing" or whine even when at rest.
The upsides of this magic is that digitals have more constant torque (ie: the same torque whether they're 1 degree or 60 degrees away from the commanded position) and better holding power than their conventional equivalent.
The downside of this magic is that they will consume more power. You don't get more constant torque and better holding power for free you know :-)
Oh, and if you hadn't noticed -- they're also more expensive :-(
For most sport ships up to about 60-size, digital servos are a gross overkill (IMHO). A traditional standard sports servo will do just fine and few sports-fliers would notice the difference between a $12.50 budget servo and a $100+ titanium-geared digital.
However, for larger models or those where speed and precision are critical then digitals really do shine.
Even my 3D funfly really benefits from having some ultra-fast (0.08 second) HS5925 servos on the tail. It means I can do a *very* square wall and also pull out from vertical dives at ridiculously low altitudes with confidence.
12-01-2004, 04:59 PM
#3
Member
Thread Starter
My Feedback: (1)
Join Date: Dec 2003
Location: Justin,
TX
Posts: 88
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
OK, so they are more powerful, respond faster, use more energy.
Are they faster? Not response faster, but move faster?
I need to make the ailerons move faster, so I can get stupid with my 1/3 extra
No one flies really big planes at my field, and no one knows about Digitals....I have read I need a device to center them. But I have also read I don't.
For everything there are two sides, and most times they oppose each other.
Thanks for your info......
Are they faster? Not response faster, but move faster?
I need to make the ailerons move faster, so I can get stupid with my 1/3 extra
No one flies really big planes at my field, and no one knows about Digitals....I have read I need a device to center them. But I have also read I don't.
For everything there are two sides, and most times they oppose each other.
Thanks for your info......
12-01-2004, 07:04 PM
#4
RE: Low down on Digitals
THe real advantage to digitals, in my opinion, is the fact that they generate very high torque values at very low error conditions.
Take a regular analog servo. Basically, it's torque ouput is proportional to the error (the angular difference between the current position of the output wheel, and the commanded postion).
So if you take this analog servo and try to twist the output wheel , you can move it quite a bit before the full torque of the servo tries to move it back to center. It may take 10 degrees of error (or more) to get full torque.
Now many of the digitals get full torque at only 1 or 2 degrees of error. If you try to turn them by hand, they feel almost like they are locked...big difference.
I think in another few years almost all servos will be digital, the ICs that make them work will be that cheap.
Take a regular analog servo. Basically, it's torque ouput is proportional to the error (the angular difference between the current position of the output wheel, and the commanded postion).
So if you take this analog servo and try to twist the output wheel , you can move it quite a bit before the full torque of the servo tries to move it back to center. It may take 10 degrees of error (or more) to get full torque.
Now many of the digitals get full torque at only 1 or 2 degrees of error. If you try to turn them by hand, they feel almost like they are locked...big difference.
I think in another few years almost all servos will be digital, the ICs that make them work will be that cheap.
12-01-2004, 07:24 PM
#5
Senior Member
Join Date: Sep 2002
Location: Rochester,
NY
Posts: 1,373
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
The short answer to that is yes. Under high load (getting stupid) given IDENTICAL mechanical advantage, a digital servo will give you what feels like more speed and power. I currently have something that can attach to a standard servo that brings them closer to digital's (at least half way). However the devices are not currently 'shock tested' on different servo's (It could burn out servo's faster than normal) and I don't have a wide variety of matched servos (one boosted one control) to test weather or not it is safe on any given servo just yet. I have about another months worth of testing (this is something I do in my spare time) before I have concrete results on their effectivness. I'm estimating a 10% increase in effective torque and 7% increase in effective speed under load.
12-02-2004, 12:19 PM
#6
My Feedback: (5)
Join Date: Jan 2002
Location: San Tan Valley,
AZ
Posts: 5,768
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
ORIGINAL: XJet
In a digital servo, a small processor in the servo itself makes a note of the signal that arrives from the receiver and constantly (well about 1,000 times per second) compares this to the actual servo position. The result is that the dgiital servo sends up to 1000 pulses of power to its motor every minute when it is being commanded to move.
Because the digital is effectively working much faster at the task of making sure the servo arm matches the position commanded by the transmitter stick, a digital servo will have better holding power and more torque when very small stick movements are requested. This will also manifest itself as vastly superior centering -- something that's often important on larger and 3D models where you're using large control surfaces and/or movements.
This much faster operation also explains why some digitals "sing" or whine even when at rest.
The upsides of this magic is that digitals have more constant torque (ie: the same torque whether they're 1 degree or 60 degrees away from the commanded position) and better holding power than their conventional equivalent.
The downside of this magic is that they will consume more power. You don't get more constant torque and better holding power for free you know :-)
In a digital servo, a small processor in the servo itself makes a note of the signal that arrives from the receiver and constantly (well about 1,000 times per second) compares this to the actual servo position. The result is that the dgiital servo sends up to 1000 pulses of power to its motor every minute when it is being commanded to move.
Because the digital is effectively working much faster at the task of making sure the servo arm matches the position commanded by the transmitter stick, a digital servo will have better holding power and more torque when very small stick movements are requested. This will also manifest itself as vastly superior centering -- something that's often important on larger and 3D models where you're using large control surfaces and/or movements.
This much faster operation also explains why some digitals "sing" or whine even when at rest.
The upsides of this magic is that digitals have more constant torque (ie: the same torque whether they're 1 degree or 60 degrees away from the commanded position) and better holding power than their conventional equivalent.
The downside of this magic is that they will consume more power. You don't get more constant torque and better holding power for free you know :-)
I have found the digital I have (Futaba 9250) takes an error signal of 100 usec to develop its maximum torque. Not a great deal different from the 9206s I have. And the 9206's use a lot less current.
I don't know who came up with the term "holding power" . If you test it, you will find a standard servo has a mechanical advantage of nearly 400 to one on the output arm. I nearly broke my test rig trying to test it. The standard servo has all the holding power you are ever likely to need
12-02-2004, 04:25 PM
#7
Banned
Join Date: Mar 2003
Location: Tokoroa, , NEW ZEALAND
Posts: 3,848
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
I think the difference in "holding power" is very dependent on the brand/model of servo being tested.
There are some very good regular servos that have great holding power and centering -- but there are also a number which are very poor in both departments.
I've yet to see *any* digital servo that has inferior centering and holding power when compared to its conventional equivalent however.
As for mechanical advantage -- this is also something that varies considerably from one servo type to another.
For example -- I've had some hi-torque servos that got all their power through a significant amount of gearing. They were quite difficult to turn by hand with the power off so you could say that at least some of their hold ing power was attributable to this mechanical advantage.
On the other hand, I have some HS5925 servos which also have excellent torque (over 100 oz-ins) yet, when not powered up, will move simply as a result of the weight of the control surface to which they're connected or if the wind blows against that surface. This ease of movement is due at least in part to the fact that they don't gear the motor down anywhere near as much as other servos do.
However, once powered up and fed with a steady stream of control pulses, the 5925 (digital) has more holding power than the conventional hi-torque servo -- despite the diifferences in mechanical advantage.
I recognise that much of the stuff published by manufacturers is hype, designed to justify the price of their expensive digital offerings and, as I've already said, the average sport flier will never notice the difference.
However, a good digital does definitely offer some valuable benefits if you're doing more than circuits and landings with your sunday-special 40-60 powered sport model.
There are some very good regular servos that have great holding power and centering -- but there are also a number which are very poor in both departments.
I've yet to see *any* digital servo that has inferior centering and holding power when compared to its conventional equivalent however.
As for mechanical advantage -- this is also something that varies considerably from one servo type to another.
For example -- I've had some hi-torque servos that got all their power through a significant amount of gearing. They were quite difficult to turn by hand with the power off so you could say that at least some of their hold ing power was attributable to this mechanical advantage.
On the other hand, I have some HS5925 servos which also have excellent torque (over 100 oz-ins) yet, when not powered up, will move simply as a result of the weight of the control surface to which they're connected or if the wind blows against that surface. This ease of movement is due at least in part to the fact that they don't gear the motor down anywhere near as much as other servos do.
However, once powered up and fed with a steady stream of control pulses, the 5925 (digital) has more holding power than the conventional hi-torque servo -- despite the diifferences in mechanical advantage.
I recognise that much of the stuff published by manufacturers is hype, designed to justify the price of their expensive digital offerings and, as I've already said, the average sport flier will never notice the difference.
However, a good digital does definitely offer some valuable benefits if you're doing more than circuits and landings with your sunday-special 40-60 powered sport model.
12-02-2004, 05:02 PM
#8
Senior Member
My Feedback: (3)
Join Date: Jul 2002
Location: Fairfax,
VA
Posts: 2,071
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
Look at the servo specs of one of the manufactures. Hitecrcd lists a table with some specs about all there servos. You will find that coreless servos have faster transit times. Combine that with a 5 cell and its very quick. Coreless are very pricey though. I use the 225mg hitec with 5 cell pack on ailerons and have had others tell me that they move very fast.
Carl
Carl
12-02-2004, 06:43 PM
#9
My Feedback: (5)
Join Date: Jan 2002
Location: San Tan Valley,
AZ
Posts: 5,768
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
ORIGINAL: XJet
On the other hand, I have some HS5925 servos which also have excellent torque (over 100 oz-ins) yet, when not powered up, will move simply as a result of the weight of the control surface to which they're connected or if the wind blows against that surface. This ease of movement is due at least in part to the fact that they don't gear the motor down anywhere near as much as other servos do.
However, once powered up and fed with a steady stream of control pulses, the 5925 (digital) has more holding power than the conventional hi-torque servo -- despite the diifferences in mechanical advantage.
On the other hand, I have some HS5925 servos which also have excellent torque (over 100 oz-ins) yet, when not powered up, will move simply as a result of the weight of the control surface to which they're connected or if the wind blows against that surface. This ease of movement is due at least in part to the fact that they don't gear the motor down anywhere near as much as other servos do.
However, once powered up and fed with a steady stream of control pulses, the 5925 (digital) has more holding power than the conventional hi-torque servo -- despite the diifferences in mechanical advantage.
I am not saying the digital is not a better servo- it is.
I am also not saying the digital servo does not have better holding power. I am just saying the standard servo has all the holding power you are ever going to need. Try it. Get a fish scale and connect it to the output arm and try to move it while it is powered up.
You will probably break something before you get it to move. A gear most likely.
12-03-2004, 04:36 PM
#10
Senior Member
Join Date: Sep 2002
Location: Rochester,
NY
Posts: 1,373
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
Stall torque (holding power) is a poor method of judging a servo's performance. I'm not sure why that's the prefered method in use. Put a servo in a vice, find a handfull of objects that weigh 5 ounces each, attach it with a string to the servo horn hole that is 1 inch away from the output shaft. Measure the transit time from far left to far right, each time you measure the transit time (starting with no weight) add another 5 ounces to the string untill the transit time becomes longer than you think is acceptable for a control surface movement. This will give you a very good idea of how the servo's perform, I'm sure you'll notice different curves for different types and brands of servo's. If anyone happens to have a handfull of miscelanous servo's floating around I'd be interested in seeing what kind of numbers people come up with. It's a relativly easy test to perform. You don't even really need something that weights exactly 5 ounces, just as long as all the objects weigh the same. It may be out of most people's league but ideally you want to use a wallwart style power supply instead of a battery (as the results will be scewed as the battery warms up and drains)
12-03-2004, 08:47 PM
#11
My Feedback: (5)
Join Date: Jan 2002
Location: San Tan Valley,
AZ
Posts: 5,768
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
ORIGINAL: Lynx
Stall torque (holding power) is a poor method of judging a servo's performance. I'm not sure why that's the prefered method in use. Put a servo in a vice, find a handfull of objects that weigh 5 ounces each, attach it with a string to the servo horn hole that is 1 inch away from the output shaft. Measure the transit time from far left to far right, each time you measure the transit time (starting with no weight) add another 5 ounces to the string untill the transit time becomes longer than you think is acceptable for a control surface movement. This will give you a very good idea of how the servo's perform, I'm sure you'll notice different curves for different types and brands of servo's. If anyone happens to have a handfull of miscelanous servo's floating around I'd be interested in seeing what kind of numbers people come up with. It's a relativly easy test to perform. You don't even really need something that weights exactly 5 ounces, just as long as all the objects weigh the same. It may be out of most people's league but ideally you want to use a wallwart style power supply instead of a battery (as the results will be scewed as the battery warms up and drains)
Stall torque (holding power) is a poor method of judging a servo's performance. I'm not sure why that's the prefered method in use. Put a servo in a vice, find a handfull of objects that weigh 5 ounces each, attach it with a string to the servo horn hole that is 1 inch away from the output shaft. Measure the transit time from far left to far right, each time you measure the transit time (starting with no weight) add another 5 ounces to the string untill the transit time becomes longer than you think is acceptable for a control surface movement. This will give you a very good idea of how the servo's perform, I'm sure you'll notice different curves for different types and brands of servo's. If anyone happens to have a handfull of miscelanous servo's floating around I'd be interested in seeing what kind of numbers people come up with. It's a relativly easy test to perform. You don't even really need something that weights exactly 5 ounces, just as long as all the objects weigh the same. It may be out of most people's league but ideally you want to use a wallwart style power supply instead of a battery (as the results will be scewed as the battery warms up and drains)
I have completed load tests vs the applied error signal and constructed graphs but I have trouble getting people to look at them. There is something about a plot on a graph that scares people. You have to look at the plots to understand the difference between the digital and the standard servo though.
12-04-2004, 02:49 AM
#12
Senior Member
Join Date: Sep 2002
Location: Rochester,
NY
Posts: 1,373
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
I know exactly what you're talking about dirty, please send me said graph's. I'll chew them up with glee
love this kind of data. I'm not in a position to purchase a large amount of equipment in order to test, my disposable income is very low and more of than not goes to other things I should buy. Which is probably best but I have a thousand ideas that I can't test because of it[sm=disappointed.gif]
love this kind of data. I'm not in a position to purchase a large amount of equipment in order to test, my disposable income is very low and more of than not goes to other things I should buy. Which is probably best but I have a thousand ideas that I can't test because of it[sm=disappointed.gif]
12-04-2004, 10:19 AM
#13
RE: Low down on Digitals
Go ahead and post a graph or two here. Some of us nerds like that stuff. [8D]
ORIGINAL: dirtybird
If you do any servo load testing you will find there is a big difference between stall torque and holding power. But you are correct either one is a poor indicator of servo performance. On the other hand I don't think your method is any better.
I have completed load tests vs the applied error signal and constructed graphs but I have trouble getting people to look at them. There is something about a plot on a graph that scares people. You have to look at the plots to understand the difference between the digital and the standard servo though.
ORIGINAL: Lynx
Stall torque (holding power) is a poor method of judging a servo's performance. I'm not sure why that's the prefered method in use. Put a servo in a vice, find a handfull of objects that weigh 5 ounces each, attach it with a string to the servo horn hole that is 1 inch away from the output shaft. Measure the transit time from far left to far right, each time you measure the transit time (starting with no weight) add another 5 ounces to the string untill the transit time becomes longer than you think is acceptable for a control surface movement. This will give you a very good idea of how the servo's perform, I'm sure you'll notice different curves for different types and brands of servo's. If anyone happens to have a handfull of miscelanous servo's floating around I'd be interested in seeing what kind of numbers people come up with. It's a relativly easy test to perform. You don't even really need something that weights exactly 5 ounces, just as long as all the objects weigh the same. It may be out of most people's league but ideally you want to use a wallwart style power supply instead of a battery (as the results will be scewed as the battery warms up and drains)
Stall torque (holding power) is a poor method of judging a servo's performance. I'm not sure why that's the prefered method in use. Put a servo in a vice, find a handfull of objects that weigh 5 ounces each, attach it with a string to the servo horn hole that is 1 inch away from the output shaft. Measure the transit time from far left to far right, each time you measure the transit time (starting with no weight) add another 5 ounces to the string untill the transit time becomes longer than you think is acceptable for a control surface movement. This will give you a very good idea of how the servo's perform, I'm sure you'll notice different curves for different types and brands of servo's. If anyone happens to have a handfull of miscelanous servo's floating around I'd be interested in seeing what kind of numbers people come up with. It's a relativly easy test to perform. You don't even really need something that weights exactly 5 ounces, just as long as all the objects weigh the same. It may be out of most people's league but ideally you want to use a wallwart style power supply instead of a battery (as the results will be scewed as the battery warms up and drains)
I have completed load tests vs the applied error signal and constructed graphs but I have trouble getting people to look at them. There is something about a plot on a graph that scares people. You have to look at the plots to understand the difference between the digital and the standard servo though.
12-04-2004, 01:23 PM
#14
My Feedback: (1)
RE: Low down on Digitals
I agree that a 5 cell battery pack is the cheapest method to achieve higher servo performance. The speed of the motor is proportional to the driving voltage. However your battery will now be sourcing high currents.
An interesting simple method of testing servo response is to use a pulse generator with a variable rate. As you increase the frame rate, the servo response get better and better until the servo starts to buzz. That is when the frame stretcher in the servo starts to overlap from fram to fram and the current used will rise alot.
Since servos are used with many different radio systems, they don't have time constanst optimized for a particular system, so performance suffers.
I built an industrial servo that the drive voltage was simply proportional to the error signal. Worked OK, but was slow to center as the motor torque drooped as the error decreased. Then I add a pair of comparitors, and an integrater that took the error voltage, converted it to a current to charge the intergrator capacitor. When the voltage passed a threshhold, the charge on the cap was dumped, and a pulse was sent to the motor driver. Since it was a full current drive pulse, the motor would move toward the null position to decrease the error. As the error was reduced, the time to recharge the intergrator capacitor increased, and the pulse rate to the motor reduced. To achieve a deadband (to keep the motor from buzzing and heating), I placed a large valve resistor across the integrator. Result, a servo system with incredible speed and power. A few more components.
But with IC's, it is possible to do the entire "digital" servo for no additional costs. We are being ripped off by the RC companies, IMHO.
An interesting simple method of testing servo response is to use a pulse generator with a variable rate. As you increase the frame rate, the servo response get better and better until the servo starts to buzz. That is when the frame stretcher in the servo starts to overlap from fram to fram and the current used will rise alot.
Since servos are used with many different radio systems, they don't have time constanst optimized for a particular system, so performance suffers.
I built an industrial servo that the drive voltage was simply proportional to the error signal. Worked OK, but was slow to center as the motor torque drooped as the error decreased. Then I add a pair of comparitors, and an integrater that took the error voltage, converted it to a current to charge the intergrator capacitor. When the voltage passed a threshhold, the charge on the cap was dumped, and a pulse was sent to the motor driver. Since it was a full current drive pulse, the motor would move toward the null position to decrease the error. As the error was reduced, the time to recharge the intergrator capacitor increased, and the pulse rate to the motor reduced. To achieve a deadband (to keep the motor from buzzing and heating), I placed a large valve resistor across the integrator. Result, a servo system with incredible speed and power. A few more components.
But with IC's, it is possible to do the entire "digital" servo for no additional costs. We are being ripped off by the RC companies, IMHO.
12-04-2004, 04:22 PM
#15
Senior Member
Join Date: Sep 2002
Location: Rochester,
NY
Posts: 1,373
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
A good comparison to realize just how bad they're ripping us off would be the following.
2 joysticks: About 100 dollars.
0-1ghz handheld transciever: About 200 dollars.
Palm 1 Zire (PDA) about 150 dollars.
The falling out of the chair laughter I got when I heard the new Futaba transmiter pricetag of $2200... Priceless
2 joysticks: About 100 dollars.
0-1ghz handheld transciever: About 200 dollars.
Palm 1 Zire (PDA) about 150 dollars.
The falling out of the chair laughter I got when I heard the new Futaba transmiter pricetag of $2200... Priceless
12-04-2004, 08:04 PM
#16
My Feedback: (5)
Join Date: Jan 2002
Location: San Tan Valley,
AZ
Posts: 5,768
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
I have tried to upload the charts I have but I get an error message that the type of file is not supported. If you would like a copy send 1$ to:
Richard H. Kelly
1452 S Ellsworth #2036
Mesa Az 85208
I have torque vs. error signal plots for:
Futaba 3001,3151,9202, 9204, 9206,9250
Hitec 605,615, 5625
Airtronics 94102
I can also tell you the deadband, resolution, linearity, and transit times for these servos as per my test.
I also have a low cost JR digital that I will soon test.
Generally I have found that The Futaba's test about 30% less than their spec.(stall torque) This is due to the different way of testing. I think my test is more representative of the way the servo is used.
Hitecs usually exceed their spec. Say what you want about Hitec. At least they are honest about their torque spec
Richard H. Kelly
1452 S Ellsworth #2036
Mesa Az 85208
I have torque vs. error signal plots for:
Futaba 3001,3151,9202, 9204, 9206,9250
Hitec 605,615, 5625
Airtronics 94102
I can also tell you the deadband, resolution, linearity, and transit times for these servos as per my test.
I also have a low cost JR digital that I will soon test.
Generally I have found that The Futaba's test about 30% less than their spec.(stall torque) This is due to the different way of testing. I think my test is more representative of the way the servo is used.
Hitecs usually exceed their spec. Say what you want about Hitec. At least they are honest about their torque spec
12-04-2004, 10:26 PM
#18
Senior Member
My Feedback: (15)
Join Date: Feb 2002
Location: Goldsboro,
NC
Posts: 1,960
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
Its interesting to note that everyone who has flown my planes with digitals says " man thats rock steady".
Futaba is sending out Sport digitals with some of the new 7 channel radios.
I can tell the difference between like servos when flying. The digitals have a definate edge even when sport flying!
I believe Don Lowe was commenting on that in one of the magazines.......some of its your skill but a lot of it can be equipment.
Try flying with a poor centering hs605 and swap it out for a 5625 or better yet a 5925
Futaba is sending out Sport digitals with some of the new 7 channel radios.
I can tell the difference between like servos when flying. The digitals have a definate edge even when sport flying!
I believe Don Lowe was commenting on that in one of the magazines.......some of its your skill but a lot of it can be equipment.
Try flying with a poor centering hs605 and swap it out for a 5625 or better yet a 5925
12-05-2004, 11:29 AM
#19
My Feedback: (5)
Join Date: Jan 2002
Location: San Tan Valley,
AZ
Posts: 5,768
Likes: 0
Received 0 Likes
on
0 Posts
RE: Low down on Digitals
ORIGINAL: Lynx
It's only a buck but before I send it to you, how exactly do you test them?
It's only a buck but before I send it to you, how exactly do you test them?
The Transit test rig was built for me by a retired professional engineer, Bob Dougless. Bob and I used to work together at Boeing. A wheel is attached to the servo wheel and drives a steel belt. An adjustable block rides on the belt and moves the dial indicator and also trips the microswitch mounted on the wood block below the belt. The size of the drive wheel is such that 1 degree rotation of the wheel results in 0.01 in movement of the block as measured by the dial indicator. The unit is made from plexiglas .
The test procedure for transit time is to determine where the microswitch opens as measured by the dial indicator then adjust the servo 60 degrees from that point(0.6 in on the dial indicator) then disconnect the servo pulse.
The pulse width is then adjusted to the maximum then reapplied to the servo thru a DPDT switch. The other pole of the DPDT sw simultaneously applies a 1kc sinewave from the function generator to the counter though the microswitch. The block is driven over by the servo to the microswitch opening it after 60 degrees of travel. Thus the transit time of the servo appears on the counter for 60 degrees of travel in milliseconds.
The results of my transit time testing are as follows:
(Average of five test runs)
Servo type Test time Published specification
S148 0.2436sec 0.280sec
9206 0.214 0.190
9250 0.1564 0.110
9204 0.1942 0.19
Hitec 615 0.255 ?
The torque tester.
It is constructed on a wooden base and contains a servo mounting device, a dial indicator, and a digital fish scale. The dial indicator is firmly mounted , but the fish scale is mounted on a short piece of 2x4 that is not firmly attached to the board. The dial indicator and the fish scale are attached to the servo output wheel with Kevlar string. The 2x4 fish scale mount is attached to a dowel with Kevlar string in a manner that turning the dowel clockwise draws the 2x4 toward it and loads the servo. The original idea was to test holding power of the servo by turning the dowel forcing the servo to resist. This idea was abandoned as I was afraid I would break something. A servo motor has a mechanical advantage of about 400 to 1 on the output arm. You would most likely break a gear rather than move the servo arm. I believe the term “holding power” quoted on the internet RC forums is meaningless. The cheapest servo you can find has all the holding power you need.
For my torque tests the fish scale is held in one place and the servo input pulse is varied . The force exerted on the fish scale is recorded and plotted vs the pulse width for the torque plots.
The test pulse is generated by the Vexacontrol ServoXciter. A readout of the pulse width in microseconds is provided on the ServoXciter.
The SerovXciter has a small internal 9V battery that is not adequate for long term testing. A heavy duty Y harness is used to connect an external 1500mah NI-CD to provide power for the servo. The plus lead to the ServoXciter from the Y harness is disconnected and only the pulse is connected to the servo. Thus the ServoXciter supplies the drive pulse and the battery supplies the power.
The torque test procedure is as follows:
1) With the NI-CD battery disconnected a milliammeter is connected in series with the disconnected plus lead between the ServoXciter and the servo. The servo input pulse is adjusted for minimum current on the ammeter, the pulse width recorded, and the ammeter disconnected.
2) The NI-CD battery is reconnected and the pulse width adjusted to the to the extreme servo end position that results in maximum torque recorded on the fish scale. The load is recorded along with the pulse width.
3) The pulse width is reduced in steps, recording the pulse width and servo load until the pulse width recorded in step 1 is reached.
The fish scale reads in pounds and ounces. This must be converted to oz and corrected to oz-in by multiplying by the size of the servo wheel. This is accomplished with the help of a spread sheet.
The pulse width value is normalized by subtracting the value recorded in step one from all pulse readings.
The results are plotted using plotting software Curveexpert 1.3. A smooth curve is drawn thru the points using Richards Model of the sinusoidal methods.
Analysis:
The Torque plots for Standard and digital are as expected. The maximum torque for each servo occurs at an error signal of about 100 micro seconds. The maximum torque recorded is about 35 oz-in which agrees with the specification for the two servos. Note that for most error signals less than 100 us the digital produces about twice torque of the standard servo. That’s what you get for the extra $20 the digital amplifier costs. It produces the same ultimate torque the non digital amplifier servo does but it does it quicker.
The torque curves show that the 9206 servo is a much stronger servo. If the 9206 plot is compare with the plot for the 9250 it shows the two servos produce almost the same torque for the same error signal up until the 9250 poops out. This says that the digital amplifier means nothing extra for this class of servo. Granted the 9206 in a much stronger servo but throughout the test range it uses considerably less current than the 9250. It is clearly the better choice in the high torque range. On the other hand the 9206 is rated for 133 oz-in and the best I could get out of it is 100 oz-in.
Torque curve plots take a bit of study to understand them but there is no simple comparison of the servos like there is for engines. (RPM vs Prop size). The maximum torque produced by the servo is rarely seen by the average user because it take an error signal of nearly 100 microseconds for the servo to develop it. That’s almost 10 degrees. That means the servo position must lag the transmitter control position by 10 degrees for the servo to develop the maximum torque.
With the introduction of the digital servo it is even harder to understand as the torque curve is altered.
