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Home✈️Aircraft🚁HelicopterHelicopter Main Discussion › A question about tx/rx resolution on ccpm.
08-23-2008 03:03 PM  12 years ago
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baby_zyklon

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A question about tx/rx resolution on ccpm.
Recently, I jumped over to the ccpm bandwagon and started flying on ccpm. However, I observe that my ccpm setup seems to have a bit less resolution than a normal mechanical setup. For instance, with mechanical setup, I can get very distinct servo movement(steps) with a change of 0.5% on the pitch curve, but with ccpm, it seems it only have similar effect with 1% changes. I tried it with the exact same servos(the very same 3 pieces of servos)and I was able to replicate it.

I am wondering if this is normal on a PCM1024 system? Will changing to a 2048 resolution tx/rx increase the resolution? I am currently using +/-100 on my endpoints and 50% for pitch and cyclic in the swash afr, with 12%/85% on the min/max pitch curve point. Does it mean my resolution has drop to 1024 x 100/140 x 50/100 x (85-12)/100= 266(which imho is still a lot)? Or is there a special forumla to calculate it? Or am I still getting 1024?

Or is all this a servo limitation showing up due to me using a servo horn too big?
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08-23-2008 08:08 PM  12 years ago
reckless loony

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The loss of resolution is due to the increased length of the servo horn. Basically if you double the length of the servo horn you will have halved the resolution at the end of the servo horn.

You can’t just use a 2048 step resolution radio system to increase the resolution you also have to use 2048 step resolution servos.

An example of 2048 step resolution servos are the new Hitec HS79xx G2 series of servos. All other Hitec servos are 1024 step resolution.
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08-24-2008 03:22 PM  12 years ago
baby_zyklon

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hi reckless loony, do you have a link to info about the 2048 resolution of the hitec 79xx series? How about the futaba BLS451/9452? Are those 1024 or 2048?
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09-08-2008 02:12 PM  12 years ago
reckless loony

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http://www.hitecrcd.com/servos/show?name=HS-7965MG

“new G2 second generation programmable digital circuit. This circuit has twice the resolution of our original circuit”

The BLS451 is a new servo therefore it will be 2048 even though Futaba don’t actually state it.
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09-08-2008 02:45 PM  12 years ago
baby_zyklon

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Thanks for the response. I have seen that before, but it doesn't actually says that its 2048, it just says it is twice the resolution. How do we know wheather the old one is 512, 1024 or maybe even some weird number?
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09-24-2008 03:35 AM  12 years ago
reckless loony

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Dead band of a servo gives indication of servo resolution.

http://www.hitecrcd.com/product_fil...1/HS-6975HB.pdf

Dead band for the HS6975 is 2 microseconds

http://www.hitecrcd.com/product_fil...inal_071019.pdf

Dead band for the HS7965 is 1 microsecond, which uses the G2 double resolution circuitry.

Speed and torque of both servos at 6v is pretty much the same. Do note the dead band of Hitec other standard circuitry digital servos including their sport series all have 2 microsecond dead band. This is independent of speed and torque and purely a function of how good the electronic circuitry is.

Here is a comparison of the HS6975 and the new HS7965 G2 series servo.

Both servos are connected to the same channel via Y lead and are powered by a Fromeco voltage regulator set at 6V. Each click you hear is the sub trim. Note that when the sub trim is clicked back and forth you can then clearly see the difference.

http://www.youtube.com/watch?v=FBKFvA3VcVc
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09-24-2008 04:36 AM  12 years ago
baby_zyklon

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Great to know that hitec has increased the resolution for their digitals. Have they improve the centering ability when loaded?

I appreciate the links you have posted, but they only show that the deadband has been reduced by half without showing any resolution figures. Btw, I see that each 40degree movement each side is 400microseconds. Does it mean the resolution of a servo with 1 micro second deadband for a total of 80degree travel(both side combined) is 800steps? Or does it not work that way?
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09-27-2008 07:37 AM  11 years ago
reckless loony

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Did you not watch the video?

http://www.youtube.com/watch?v=FBKFvA3VcVc

It clearly shows the improvement with a G2 servo over the one that still uses the lower resolution circuitry.

The dead band figure is a resolution figure. The smaller it is the smaller the movement you will get of out the servo arm and better centering. In the video where the sub-trim is clicked back and forth the G2 circuitry equipped servo HS7965 moves while the other servo with the older circuitry doesn’t move at all.

You are not going to find any mention of a digital resolution number because that only applies to digital servos and doesn’t really give you any idea as to what it means with regards to the smallest amount of movement of the servo arm. Only a dead band figure can be used to compare across all servo types including Analog servos.

But since you want proof of the older digital servo being 1024 step resolution then here are pics.

You can see that the HS6975 servo uses an Atmel 8 bit Microcontroller. The data sheet being.

http://www.datasheetcatalog.org/dat...atmel/2486S.pdf

If you read the features you see that it has 8 ADC (Analog to Digital Converters) of which 6 are 10-bit accuracy naturally only one of which is used to connect to the servo potentiometer. 10-bits gives 1,024 steps of resolution.

Now with the new HS7965 digital servo with the G2 circuitry (marketing crap) it is going to be using a Microcontroller that is going to have an ADC that is 11-bit accuracy because that gives 2,048 steps of resolution and hence why the dead band for the HS7965 is half that of the HS6975.

The signal out of the receiver going to the servos is PWM (Pulse Width Modulation). How wide the pulse is determines the servo arm position.

Futaba servos respond to a PWM signal namely pulse width from 1.07ms wide to 1.93ms wide with 1.5ms wide being servo centre.

JR servos respond to a PWM signal that goes from 1.1ms wide to 1.9ms wide with 1.5ms wide being servo centre.

Hitec servos respond to a PWM signal that goes from 1.1ms wide to 1.9ms wide with 1.5ms wide being servo centre in other words the same as JR servos.

The PWM has nothing to do with servo resolution the data sheet there is simply telling you that you get 40 deg of rotation of the servo horn for every 0.4ms pulse width change naturally you get only 80 degrees total rotation for a 1.1ms to 1.9ms PWM signal change.
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09-27-2008 08:24 AM  11 years ago
baby_zyklon

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wow. You took the servo apart.

Yeah, I saw the part in the middle where the 7975 moved back and forth while the 6965 just stayed there.

I see, so deadband and resolution has nothing to do with the pwm.
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09-27-2008 12:49 PM  11 years ago
BarracudaHockey

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I think you will find that if you shorten the servo arms and run up your ccpm numbers you will find what you're looking for.Andy
AMA 77227
http://www.jaxrc.com
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09-27-2008 04:12 PM  11 years ago
baby_zyklon

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On a push-pull setup, wouldn't shortening the servo arm cause "unequal throw" issue?
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09-27-2008 07:50 PM  11 years ago
BarracudaHockey

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Yea thats true, I have been messing with a bunch of 450 stuff lately so thats what I had in my mind.Andy
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http://www.jaxrc.com
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