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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 01-03-2019, 12:04 PM
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Default How to determine servo compatibility with radio modes?

Suspect this is a dumb question, but here we go...

I'm considering upgrading the electronics in my 4wd buggy. I'm currently using pretty basic stuff, a Spektrum DX5C, 310 receiver, & Hitec 5565MH servo. The thing is, for how basic my setup is, it all works & feels fine. The published servo specs are actually remarkably good, given the price. Still, one of those "don't know what I'm missing" things, I'm sure...

So, if I upgrade the servo, but want something that will be compatible with a higher end radio used in whatever high speed mode it offers down the road, how do I tell if it will work from the published servo specs?
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Old 01-03-2019, 12:34 PM
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Analog servo's need a slow framerate which normally is 20msec or 50Hz. The slowest setting on an advanced Spektrum transmitter is 16msec I believe which should work with analog servo's. The pulse width of a centered servo is 1.5msec

Normal digital servo's still have a center pulse width of 1.5msec but can handle frame rates up to 3msec or 333Hz

The so called SR and SSR modes from Futaba and Sanwa require special servo's. Futaba's SR mode wil work with so called 760usec servo's, the cente pulse width is at 760usec, half the width of a normal servo. Futaba has mede a list but I believe most Heli tail servo's with such a spec will work as well.
Sanwa its SSR is not that well known to mee but I believe there are 2 modes, 1 like the Futaba SR and I have read of something twice as fast. I believe Sanwa servo's working on these modes are capable to work on normal speed as wel.

Regarding Futaba, I have taken snapshot from my handheld digial scope to show the speeds:
Bought myself a Futaba T7XC transmitter.
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Old 01-04-2019, 08:00 AM
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Having a "fast" servo is relative to the skill of the driver and the reflexes that the driver can handle, there comes a point of diminishing return where having too fast of a servo can make a car very difficult to control within reason. Many pro level drivers will recommend investing in a servo that is rated for 0.08 to 0.12 sec/60 as considered a "fast" servo. I have some 0.08 servos that are still too fast for me and I use both steering and throttle speed settings to slow the response to make cars significantly easier to drive. If you decide to invest in a new radio system, I would make sure it offers speed settings in addition to expo rate settings.
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Old 01-04-2019, 05:15 PM
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Originally Posted by big ted View Post
Suspect this is a dumb question, but here we go...
Not dumb at all, in fact your question peaked my interest. I know a lot more about helicopters than cars/buggies/trucks but I now see that the surface products offer various high frame rate options. Here is the thing about checking compatibility. Seems most radios and receivers present their specs in time units (16millisecs aka 16ms, 11 ms, 5.5ms, etc.). On the other hand, servos tend to present their frame rate limits in frequency. Look at this servo for example:
xpert-rc WR-7701-HV Standard Size Waterproof Servo
I have three of these in use. The "frequency" spec is 200-300 Hertz or HZ or cycles per second. It is easy to convert between the time (aka period) spec and frequency. They are just 1 over each other. So to see if a servo will work with a high frame rate RX/TX, take the lowest number (fastest) frame rate and invert it. So for example Spektrum's higher end stuff goes down to 5.5ms. Written in decimal form that is 0.0055. So to find the equivalent frequency f=1/rate or f=1/.0055=181 Hz. The servos ratings are for maximum (fastest) frame-rates so anything less is OK. 181Hz is less than 200 Hz, so it is fine. If the servo has a 300Hz rating 1/300=.0033 or 3.3 ms so it will be ok with an TX/RX running down to 3.3ms frame rate.

I have a DX4S which only goes down to 11ms or 91 Hz.

I have been using this RX which I like because it has no wire sticking out. The antenna is internal like a cell phone.
https://www.spektrumrc.com/Products/...odID=SPMSR2100
Note that it supports 5.5ms frame rate.

So if you got the Xpert servo and the SR2100, you would need to get at least a DX5R to run at 5.5ms. Your DX5C only goes down to 11ms
https://www.spektrumrc.com/Content/M...Comparison.pdf

Roelf mentioned the 760us (narrow band) type servos. Interesting to see they are showing up in the surface world. Like he said, they are also used for the rudder or tail on helicopters. The narrow center pulse width allows very high frame rates, up to 560 Hz. Futaba started this back before flybarless helicopters when they sold tail gyros and servos that ran at 560 Hz. Standard 1500us servos (most of the ones out there) can only run up to 333 Hz. Otherwise the pulses would overlap each other. I don't know how these very fast frame rates are viewed in the surface world but my personal opinion is that it is/was mostly a marketing ploy by Futaba. You had to buy their servo to work with their gyro. Now others make 760us servos at least for helicopters (MKS for example). MKS sells a lot of 760us tail servos but if you looks at their most expensive Car servo, it is 1500us type and runs up to 333Hz.
MKS Servos USA :: Car Servo :: HBL599 (0.08 sec/60° - 583.27 oz/in @8.2V)

I have big, 700 size helicopters and some have 760us tail servos and some have 1500us tail servos. I can't notice a difference. Maybe the best of the best can but that is debatable. So you have to decide for yourself or read the opinion of very high end drivers to see if they can notice a real difference in narrow band servos.

IMHO, when looking at servos, torgue is probably more important than speed or frame rate. In bouncing down an off road track, the wheels are getting smashed around pretty hard. Its the servo torque that is going to keep the wheel in place. However, any of the higher end servos are going to have plenty of torque and speed and run frequencies that allow 5.5ms or even 3.3ms.

As usual, I have "over-answered" but hopefully this helps and I actually learned something about surface electronics in writing it.
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Old 01-05-2019, 01:59 AM
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Originally Posted by billdelong View Post
Having a "fast" servo is relative to the skill of the driver and the reflexes that the driver can handle, there comes a point of diminishing return where having too fast of a servo can make a car very difficult to control within reason. Many pro level drivers will recommend investing in a servo that is rated for 0.08 to 0.12 sec/60 as considered a "fast" servo. I have some 0.08 servos that are still too fast for me and I use both steering and throttle speed settings to slow the response to make cars significantly easier to drive. If you decide to invest in a new radio system, I would make sure it offers speed settings in addition to expo rate settings.
I would discuss with your statement. Even the fastest servo will not move faster then your hand. So having the slow one will result that you have adjust your hand to the delayed reaction of the servo. I would recommend to use the fastest one on steering. In my case I use 0,07/60deg.

If you find out it is easier to drive I believe that you have get used and adjust your hand to slow action. Having fast servo it seems at the start that you i.e. turn earlier than needed. I would suggest to numb a bit setup of your car (change toe, springs) rather then slowing down servo.

It also IMHO applies to beginners. I have changed servo in my son's car (beginner) to the fast one and I have noticed that he drives much more precise and he reduced number of wall hits
As for original question some companies like Futaba publish their servo compatibility.
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Old 01-05-2019, 02:36 AM
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Originally Posted by billdelong View Post
. I have some 0.08 servos that are still too fast for me and I use both steering and throttle speed settings to slow the response to make cars significantly easier to drive. If you decide to invest in a new radio system, I would make sure it offers speed settings in addition to expo rate settings.
With fast onroad racing like 1/8GP and 1/12 or 1/10 modified the servo can never be too fast. Any gain of speed, latency and accurately will give a much better and precice placement of your car.
If you have steady problems with a too fast servo then it is within your hands/fingers.A slow servo will only damp the unwanted small movements of your fingers. And yes, with slower classes like stock racing and offroad on larger tracks speed is not that important.
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Old 01-05-2019, 06:26 AM
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Just to be clear, I did not pull some random numbers out of the air when I posted 0.08 to 0.12 sec/60 as considered a recommended "fast" servo range for most drivers, I got this information a couple years ago while attending a "Race Clinic" that was hosted by Ryan Lutz and Joe Bornhorst for a large race event at the local track in my area. They spent the first half of the day giving us a treasure trove full of pro tips in the pits, then the second half of the day they took turns driving each participants car and would make recommendations to help each respective driver improve their skills. This was the first time I had been introduced to the concept of using "steering speed" to settle down my over responsive car. Yes, there are times where it makes more sense to properly tune a car, but I was already using a team drivers setup and Ryan could see that I was still struggling with it. After making the minor adjustment (reducing my steering speed to about 90%) it was like driving a completely different car, something that was just so easy to control, just point and shoot, I think I heard a faint choir singing in the background... "Ahhhhh"

My average consistency jumped up from roughly 90%-93% on average to around 95%-98% average since then. Just so you know I'm not blowing smoke, here's one of my more recent race results. That said, I used to race on-road back in the day, and I distinctly recall test driving a friends car with a 0.04 sec/60 servo, and it was insanely too fast for my skill at the time... while I do agree that you can train yourself to handle faster servos, and if your reflexes are fast enough it can be beneficial for some people, I can assure you that I am definitely not one of those who have insane fast reflex... I actually have a 0.14 sec/60 servo in my 1/8 buggy and keep it at 100% steering speed... it's perfect for me, I had a 0.12 servo in it originally and had thought that was slow enough, but I actually got faster lap times with the 0.14 servo!

Another thing that I did wrong before was that I would reduce the travel (throw) of my steering to make my cars easier to drive, such that I'd drive full lock from corner to corner and use throttle to control my steering. This strategy works well for wide sweeping corners, but I was losing precious time in tight chicanes, it was Tyler Hooks (current 13.5T National Champ) who convinced me to go with max possible travel to get more steering and force myself to properly learn steering control and this has helped me start to get a little more faster, not that I'm running at the pro level just yet, but I'm able to get on pace with some local pro's in my area.

I also run on a turf track and I feel the characteristics are very similar to racing on-road asphalt tracks, reducing steering speed has helped me more on turf than on dirt
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Old 01-05-2019, 01:36 PM
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Well, I cannot consider my self as a person who posses insane fast reflexes. Anyway in on road racing my experience is different than yours and I consider precision given by ultra fast servo as a benefit in racing.
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Old 01-06-2019, 11:54 PM
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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.
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Old 01-07-2019, 12:02 AM
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I think the deadband is the low pulse part after the PPM pulse.
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Old 01-07-2019, 06:47 AM
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Originally Posted by big ted View Post
Ultimately, I'm trying to figure out where to spend my money first. A decent servo or a better radio.
The resolution of the servo will be limited to what the radio has to offer. I recently had to switch back to my backup system (Radiolink) after sending in my Graupner system in for service. While my Radiolink still provided the same fast lap and overall consistency for the 6 weeks I ran it, I could feel the difference in resolution where it felt "notchy". It's one of those things that you would never tell any difference when running a budget system with lower resolution in the first place, but after spending a year or more with a quality system and then going back to lower resolution, you will feel it. Same with servos to a point, most modern servos already offer 12 bit resolution or more anyway and I think this is the point where most folks won't be able to tell any difference even if their radio offered a higher resolution to support a servo for it.

I would pick a $300 radio and $100 servo to get a decent balance between speed/resolution, where a $700 radio and $200 servo is not worth the extra expense for what you get IMO.
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Old 01-07-2019, 07:29 AM
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Originally Posted by big ted View Post
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!
Deadband is the amount of change required in the input pulse width before the servo does anything. Think of it as the electrical equivalent of play or slop in a mechanical system. Deadband is not related to the frame rate.
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Old 01-07-2019, 09:04 AM
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Originally Posted by big ted View Post
I think what's confusing me a bit is that the datasheet for my current servo doesn't specify a maximum operating frequency.
What servo do you have?

Not all servos post a max frequency. We ran into this a lot a few years back in the heli world as FBL controllers were hitting the market, Some of the controller companies posted servo lists where they had tested popular servos to determine reasonable operating frequencies. Sometimes, the specs are there if you dig enough or contact the manufacturer. For one thing, if its not a digital servo, it for sure won't work at high frame rates.

The safe approach is to switch to a servo that posts a frequency spec.
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Old 01-07-2019, 10:07 AM
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Originally Posted by rhodesengr View Post
What servo do you have?

Not all servos post a max frequency. We ran into this a lot a few years back in the heli world as FBL controllers were hitting the market, Some of the controller companies posted servo lists where they had tested popular servos to determine reasonable operating frequencies. Sometimes, the specs are there if you dig enough or contact the manufacturer. For one thing, if its not a digital servo, it for sure won't work at high frame rates.

The safe approach is to switch to a servo that posts a frequency spec.
It's a Hitec 5565MH. As I say, it's a pretty budget servo, but the specs look remarkably good for the price: 194 oz/in torque & 0.09 s/60 @ 7.4V. I'm not expecting miracles, but it's digital so I also think it'll work fine with a faster radio.
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Old 01-07-2019, 12:03 PM
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Originally Posted by big ted View Post
It's a Hitec 5565MH. As I say, it's a pretty budget servo, but the specs look remarkably good for the price: 194 oz/in torque & 0.09 s/60 @ 7.4V. I'm not expecting miracles, but it's digital so I also think it'll work fine with a faster radio.
yeah, Hitec is one of those companies that doesn't seem to include a frequency spec on their data sheets. If it starts to get warm just sitting there driven by a high frame rate then that is not good. If it were me, I would contact Hitec and ask them. Actually, if it were me, I would use a servo that lists a frequency rating.
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