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Hitec Digital Servo Programmer vs Futab TX

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Old 01-14-2005, 03:09 PM
  #1  
hilleyja
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Default Hitec Digital Servo Programmer vs Futab TX

Most of you are probably aware of this but I get the impression there are some out there that aren't.

Hitec and JR TXs center servo signal is 1500. Futaba center servo signal is ~1520. The Hitec programmer assumes center to be 1500. What this means is if you program the Hitec digital servo's centering only using the programmer it will not be correct when you use a Futaba TX.

Solution: (centering servo; 90 degrees to case)

1) Connect your servo either to an airframe or a test stand (Test stand mounts a servo within a protracter scale).
2) Connect the servo to the Hitec programmer and run the reset function.
3) Connect the control arm to the servo as close to 90 degrees to the case as you can get it.
4) Run programmer center function until the arm is exactly 90 degrees to the case and set the endpoints to approximately 100 on either side of the center value.
5) Disconnect the servo from the programmer and connect it to a powered-up RX and TX; make sure TX subtrims and control trims are set to neutral.
6) Take note of the direction and offset of the servo control arm. Make an educated guess as to the programmer adjustment needed. E.G., if the arm is about 4 degrees off center I would try an adjustment of 10.
7) Reconnect the servo to the programmer and rerun the centering function. Add or subtract your adjustment and set center to it. Reset the endpoints again to approximately 100 on either side of the center value.
8) Reconnect the servo to the powered-up RX and TX. Your centering should now be closer. Note again the direction and offset needed to refine it.
9) Re-run steps 5-8 as many times as is needed to come to a perfect neutral point for the servo.
10) Your centering programming is now complete (make no more center adjustments when adjusting endpoints later on.) and you are ready to connect the control rods and adjust the servo for deflections. Make no adjustments to your TX's subtrims because you will throw-off the programming -- this is especially important if you want 2 matching servos to a control surface to stay in sequence.

Solution: (deflections -- set required deflections for control surface or, on a test stand; 60 degrees; I will describe setting control surface deflections)

1) Connect your servo to a powered-up RX and TX; make sure TX subtrims and control trims are set to neutral. (NOTE: With my Futaba 9C, I set the TX endpoint deflections to 125.)
2) Connect your pushrod to the servo arm and control horn and manually adjust for neutral deflection.
3) Deflect the control surface 100% with the TX control stick -- take note of deflection angle or distance from neutral. How does it compare with the maximums recommended by the airplane manufacturer? Do this from both directions. Make an educated guess as to the programmer adjustment needed for endpoints, just like you did during centering. E.G., if deflection is 1/2" to short I will initially add 10 to the endpoint in that direction.
4) Connect the servo to the programmer and go into the centering/endpoint function. Using the setting you derived during centering programmer reset the center to that value. Add your adjustments to the endpoints and reset them using the programmer.
5) Re-connect the servo to the power-up RX and TX and deflect the control surface 100% with the TX control stick. You should be closer to the deflection you want.
6) Repeat steps 3-5 until your maximum endpoints are set to the manufacturers's maximums.
7) Further adjustment of deflection should now be done only with the TX's DualRate adjustments.

Any comments and/or inputs to these procedures is greatly appreciated. They have worked well for me. I do have to give much of the credit to Joe Hunt at downonthedeck.com for steering me in this direction. I give Hitec no credit because their instruction manual is woefully inadequate.
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Old 01-14-2005, 03:39 PM
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Crash90
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Default RE: Hitec Digital Servo Programmer vs Futab TX

Why not program the servos for maximum throw, (rather than desired deflection) then mechanically set the deflection on the control surface? This would guarantee maximum mechanical advantage. No?
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Old 01-14-2005, 04:53 PM
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hilleyja
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Default RE: Hitec Digital Servo Programmer vs Futab TX

ORIGINAL: Crash90

Why not program the servos for maximum throw, (rather than desired deflection) then mechanically set the deflection on the control surface? This would guarantee maximum mechanical advantage. No?

That's what I described above -- it is the maximum deflection recommended by the airplane manufacturer. Use the Dual-rate function to reduce the throws. Of course there is no reason you can't program more than the recommended maximum and use the dual-rate function to reduce them.

BTW, what do you mean when you say mechanically set the deflection? Once you program the servo you DON'T want to make any manual control adjustments (leave control horn and servo arm adjustments alone)-- you will throw off the programming. Likewise, you don't want to make any subtrim or endpont adjustments with your TX, again you will throw off the programming.
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Old 01-14-2005, 05:10 PM
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Crash90
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Default RE: Hitec Digital Servo Programmer vs Futab TX

Not so. The best way to set up a servo weather they are ganged or not is to program the servos for maximum throw. (making sure they are identical of corse) get the geometry the same then mechanically increase or decrease the control throws to the manufacturers specifications.
I you have 2 servos on say a single aileron. Using, say nelson control arms, both servos are perfectly matched. You can move the clevis closer or further from the control surface to achieve desired throws. As long as you move them both the same amount the geometry will not change.

This is ideal. The more the servo has to move to achieve a certain amount of deflection on the control surface, the more mechanical advantage you have.

If the ganged servos are on a Y running to a single channel on the rx. You can use the Rx to subtrim. (subtrimming them both at the same time) Once again, as long as the servos were properly matched first. However, this should not be necessary if you have the linkages set for neutral/neutral.
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Old 01-14-2005, 06:25 PM
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Default RE: Hitec Digital Servo Programmer vs Futab TX

Crash90, while this could/would/might be true with 3D type throws, what if you were setting up a pattern ship? You can program the servo for 180 degree throw with the programmer. Lots of throw but the resolution sorta goes to h&!!.

Ed M.
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Old 01-14-2005, 06:50 PM
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Crash90
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Default RE: Hitec Digital Servo Programmer vs Futab TX

Bentgear. While you are absolutely correct, (pattern ships would be set up differently you still have to pay attention to mechanical advantage/disadvantage. On the other side of things, you could program your servo to move 10 degrees while your control surfaces are moving 25 degrees. Not good.


Maybe we should have been clear on what kind of plane we are programming for. When hilleyja said he got his info from Joe Hunt, it was clear that he was referring to 3D because I saw the same video.
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Old 01-14-2005, 06:56 PM
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Default RE: Hitec Digital Servo Programmer vs Futab TX

I know, i have the same vid. Thouht I had put a smiley face on the end of last sentence, but dummy here forgot...........................

Ed M.
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Old 01-14-2005, 07:22 PM
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Crash90
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Default RE: Hitec Digital Servo Programmer vs Futab TX

I also think that rather than hooking and unhooking your servo to and from the Rx and programmer, then taking a "guess", once the center is set and the control surface is at neutral you can program your endpoints while the servo is still connected to the control surface. It is much easier and just as accurate as the method described in post 1.

With my H9 sukhoi, I installed the servo into the wing, set it to neutral, hooked up the linkage, then began turning the knob on the programmer until I reached my desired throw. Just make sure that the mechanical advantage is at least 1:1. . It saves a lot of steps.
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Old 01-14-2005, 08:01 PM
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hilleyja
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Default RE: Hitec Digital Servo Programmer vs Futab TX

ORIGINAL: Crash90

I also think that rather than hooking and unhooking your servo to and from the Rx and programmer, then taking a "guess", once the center is set and the control surface is at neutral you can program your endpoints while the servo is still connected to the control surface (Re-read my post, at no point do I say to connect and disconnect the control surface). It is much easier and just as accurate as the method described in post 1.

With my H9 sukhoi, I installed the servo into the wing, set it to neutral, hooked up the linkage, then began turning the knob on the programmer until I reached my desired throw. Just make sure that the mechanical advantage is at least 1:1. . I tried the same thing but when I connected it to the RX the endpoints where not set the same as the programmer. Just like the neutral signal, the endpoint signals are also different betwee the Hitec and Futaba. It saves a lot of steps.

I think I understand your point but do not agree with it. The reason I don't agree is that I set the servo arm and control horn connection as close as parallel I can get. Our goal is linearity of all servos connected to the same control surface. If we were talking about multiple servos connected to the same airlon and the wing was tapered from the root to the tip then some mechanical adjustments are needed to establish linearity. Describing that process was way beyond my intent. My comment aboout subtrims is correct when we are talking about two servos, on two RX channels, controlling one surface, e.g., Futaba's Ailevator function -- once you adjust the subtrim of one and not the other or differently then those servos are no longer matched.

You have to keep unplugging the servo between the programmer and RX until you establish the servo neutral point. Once you have the servo neutral point, loosely connect the control surface, max deflect the TX, and make sure the servo endpoint is not in excess of the control surface deflection capability. If it isn't then complete your control surface connection and leave it connected while programming for the control surface neutral point and subsequent endpoints. Just like the neutral programming you will still have to alternate between connections to the RX and the programmer.

BTW, we are talking about Hitec digital servos and Futaba TX. You cannot program your servo strictly with the programmer because Futaba's neutral is not the same as the programmer's.
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Old 01-14-2005, 08:27 PM
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Default RE: Hitec Digital Servo Programmer vs Futab TX

I agree. However, it has been my experience that all of my hitec servos were off the same amount when going from the programmer to the Rx. Therefore, you can simply compensate for that while setting the center and end points on the second servo.

There is definately more than one way to program these servos. I just believe that some methods are much more time consuming than others.
I just don't agree , with a 3d plane, that you should be setting your control surface deflection by limiting the servo throw. Keep the servos throw at around 60 degrees and Mechanically turn down the amount of throw.
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Old 01-14-2005, 11:38 PM
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Default RE: Hitec Digital Servo Programmer vs Futab TX

This problem with TX’s not providing the prerequisite 1500us (micro-seconds pulse-width) at neutral is common to any TX. It’s an electro-mechanical thing, together with the OEM’s acceptable design tolerances. Several factors are at play; mechanical linkage (gimbals) and component tolerances as well as acceptable mechanical set-up tolerances and good old wear and tear. All TX’s run the gamete with this stuff at neutral and end-points. Having the capability to offset these deficiencies is when the Hitec programmable Digitals come to play.

One cannot assume the linkage and or offset from the surface is identical from one control horn to the next, in fact it’s nearly impossible IMO. A mechanical advantage only exists if the ratio is greater than the servo arm driving the control arm. 1:1 denotes equal lengths, mechanical advantage is not in play. Once the control arm pivot is extended longer or past the servo arms pivot point a “mechanical advantage” is in play. Generally a mechanical advantage exists in GS aircraft; it’s simply due to the thickness of the control surface together with the offset from the control surface to the control arm pivot. Most of us are using 1.25” arms, while an elevator of a GS model for example maybe 2” thick, the hinge line would be at 1” or half the thickness. As a rule I maintain around a ¾” offset from the surface to the control arm pivot point to maintain linkage clearance 1” + ¾” = 1.75”. The ratio is 1.25:1.75 a mechanical advantage is present.

Another factor is the accuracy of the control arm placement and it being square to the center of the surface, these are BIG areas of compromise as it’s very difficult to accurately place the arm and maintain any consistency from one control arm to the next try as you may. To correct these common deficiencies we can use a protractor and relocate the control arm pivots as required to maintain equal travel volume of the servo arm and control surface. This is paramount when matching servos as this is commonly overlooked and contributes to surface and or linkage binding regardless to how well the servos are matched to one another.

I believe the best method for utilizing the unique features of the Hitec programmable Digitals is to set them up in the model. Match the transmitted centers of the TX at neutral as described by others above. MAX out ATV or end-points less 5-10%. This will provide the best overall system resolution. Use the programmer to achieve the desired throws at these MAX settings as mentioned again by others above. Once properly programmed, you’ll have the best system resolution plausible in this scenario. There is simply nothing else on the market that can broach the accuracy and linear system resolution afforded this combination.

Utilizing a servo jig to program center and end-point has little to know advantage IMO. As I alluded to previously TX and model deficiencies need to be addressed, the jig does nothing for these venues.

One more thought; digitals servos are capable of reaching MAX power with VERY little error correction or factor as compared to their analog cousins. They do not need to wind up or reach MAX travel arcs to produce MAX power ala the analogs.
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Old 01-15-2005, 11:06 AM
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Default RE: Hitec Digital Servo Programmer vs Futab TX

Thanks for breaking it down Mglavin, that makes total sense.
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