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How to determine servo compatibility with radio modes?

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How to determine servo compatibility with radio modes?

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Old 01-07-2019, 12:21 PM
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That HiTEC servo is fairly common, seen many racers use that servo with all different brands of luxury radios systems in my area.... I would keep the servo and upgrade your radio, it should work just fine. Chances are, if you upgrade to a fancy $200 servo, I doubt you will feel any difference in smoothness, you probably won't notice a difference in smoothness when you get the new radio either, give it a year with the new radio, then go back to the old system and then you'll feel how much more smooth the high end radio is
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Old 01-07-2019, 02:47 PM
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In the search if my Savox SV-1257TG can handle 333hz I came to a site where it was listed as 200Hz. Funny thing was that the Align DS620 is a rebranded 1257TG and was listed as a 333hz servo. So I mailed Savox Europe And got a direct aswer this morning that it can handle 333hz. On my second question if they have a 720us - 800hz servo they were clear that they do not make 720us servo's.

But regarding that servo data list there are a bunch of 200hz servo's so I think that is why Spektrum does not go faster than 5.5ms.
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Old 01-07-2019, 03:07 PM
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Originally Posted by Roelof View Post
But regarding that servo data list there are a bunch of 200hz servo's so I think that is why Spektrum does not go faster than 5.5ms.
To implement a 760us centerpulse servo, the TX has to transmit an entirely different range of pulses. I know that in setting up helicopter flight controllers, you have to be sure the you choose the right servo type in the setup software and the controllers all support both pulse-width ranges. But the TX is still sending at 11ms frame rate. The controller is running closed feedback loop so it makes some sense that it can interact with the servos faster than the operator is changing servo positions. Maybe some surface TX's (Sanwa Futaba ?) have TX's that can send both pulse width ranges but I have not seen that in any brand of air TX.

I mentioned before that the limit for 1500us servos is about 333Hz whether the electronics can go faster or not. At 333Hz the pulse repeats every 3ms. A 1500us centerpulse servo goes up to about 2100us (2.1ms). So a 2.1ms pulse repeating every 3ms has 900us between pulses. This gives the servo some time to respond to that last pulse. You could go up to about 470 Hz without actual pulse overlap, but it seems that just isn't done. 333Hz is pretty much the limit for 1500us type servos.
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Old 01-07-2019, 04:22 PM
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Of the 3 different brands of luxury radio systems I have used (Radiopost, KO Propo and Graupner), each gives you the ability to set the frame rate for each channel, here is a screen shot from the Graupner manual where they also offer a proprietary frame rate for their servos. Anyway, this gives you the flexibility to run any brand of servo on the market with just about any brand of luxury radio system, I suspect your HiTEC servo will work just fine on the default 6ms with this radio example here:
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Old 01-07-2019, 06:20 PM
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Makes sense. However, now if I refer to Nezil's excellent post in the MT-44 thread:
  • Sanwa MT-44 (FH4T Normal Mode): 95Hz / 10.5ms
  • Sanwa MT-44 (FH4T SHR Mode): 383Hz / 2.6ms
  • Sanwa MT-44 (FH4T SSR Mode - Narrow Pulse): 383Hz / 2.6ms
  • Sanwa MT-44 (FH3 Normal Mode): 83Hz / 12.0ms
  • Sanwa MT-44 (FH3 SHR Mode): 332Hz / 3.0ms
Seems that if I go for this particular radio, and my servo doesn't work with a 3 ms frame rate, I'm stuck running at a 10.5 ms frame rate, which is no better than I have currently...

I've reached out to Hitec. We'll see what they say.
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Old 01-07-2019, 07:28 PM
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Originally Posted by big ted View Post
Thanks for the input everyone. I think what's confusing me a bit is that the datasheet for my current servo doesn't specify a maximum operating frequency, but instead calls out a 'deadband' of 2us. Not sure the relevance of this since the servo clearly cannot operate at 1/2us = 500kHz!

Ultimately, I'm trying to figure out where to spend my money first. A decent servo or a better radio.
Get a Sanwa radio first. They are more consistent than spectrum. Sanwa also has the pgs cl and cx which are probably the two cheapest programmable, low latency servos on the market. They will work in ssr mode and the m12s can program them from the radio if you have an rx482 or 472 receiver. One of the cheaper Sanwa radios can also do this. I believe it is the mt44. Looks like the mts has the same capability.
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Old 01-07-2019, 08:10 PM
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Originally Posted by big ted View Post
...my servo doesn't work with a 3 ms frame rate, I'm stuck running at a 10.5 ms frame rate, which is no better than I have currently...
I think you're trying to compare the wrong features... when you select a luxury radio system, you are paying for advanced ergos such as adjustable drop down steering, swivel, tension, throttle position/angle and general comfort. Touch screen menus to minimize time swapping model memories, and flexibility for expanded model naming or custom LED colors for each model memory, perhaps a lap counter with data export, etc... the list goes on.

For me, the deal breaker with Spektrum is the servo speed feature is dicey, I have come across 2 local racers who could not get the feature to properly work with their systems and I feel that is the single most important feature to be competitive.

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Old 01-07-2019, 10:07 PM
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This is great information so i cant resist offering a little something. My opinion is that that the mechanical response frequency of ssr (sanwa) is almost connected. .07 seconds on a low cost pgs.

The specification that the US military provides to fighter simulators is 400hz and above of mechanical response is close enough to realtime that a trainer will feel like the real thing.

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Old 01-08-2019, 12:01 AM
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Originally Posted by rhodesengr View Post
To implement a 760us centerpulse servo, the TX has to transmit an entirely different range of pulses. I know that in setting up helicopter flight controllers, you have to be sure the you choose the right servo type in the setup software and the controllers all support both pulse-width ranges. But the TX is still sending at 11ms frame rate. The controller is running closed feedback loop so it makes some sense that it can interact with the servos faster than the operator is changing servo positions. Maybe some surface TX's (Sanwa Futaba ?) have TX's that can send both pulse width ranges but I have not seen that in any brand of air TX.

I mentioned before that the limit for 1500us servos is about 333Hz whether the electronics can go faster or not. At 333Hz the pulse repeats every 3ms. A 1500us centerpulse servo goes up to about 2100us (2.1ms). So a 2.1ms pulse repeating every 3ms has 900us between pulses. This gives the servo some time to respond to that last pulse. You could go up to about 470 Hz without actual pulse overlap, but it seems that just isn't done. 333Hz is pretty much the limit for 1500us type servos.
Yes, but with heli's the tail servo is controled through a gyro and the gyro is sending out the 720us pulse for a faster correction on the tail. Take the gyro out and you need a 1.5ms servo.

And yes, I know the limitation of 333hz with 1.5ms servo's. OK, Sanwas is pushing it up to 385hz

The Futaba 7PX/T7XC can control 720us servo's



Aand I believe the Sanwa SSR mode is something simulair or even the same.
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