Important Info on Servo Travel

At it’s most basic description, the Taranis transmitter and receiver work together to send a message to a servo to rotate either clockwise or counterclockwise from a center resting position. Our class focuses on configuring the Taranis so that the message correctly reflects your intentions. If we’re to understand how that happens, we need to understand at least a little about the language of that message. The concepts are important and are woven throughout OpenTX. We need to deal with this before going any further into Basic Airplane Set Up on Taranis.

The radio sends 50 messages a second over each active channel. Each message commands servo movement to a position either clockwise or counterclockwise from its centered resting position. The message takes the form of a voltage pulse with a longer lasting pulse calling for rotation in one direction and a shorter duration pulse for rotation in the opposite direction. A pulse lasting 1500uS (microseconds) corresponds to the centered position.  A customary range of signals in RC radios is plus/minus 500uS on either side of the center position, so a typical operating range for servo signals could be 1000uS for full rotation on one side to 2000uS for full rotation on the other side. This form of messaging is called Pulse Width Modulation, or PWM for short. The “customary” part of that description is important to our Basic Airplane Set Up on Taranis class, so let’s consider industry standards and OpenTX defaults for a moment.

In PWM messages:

  • A 1500uS center resting signal is common in radios but not consistently so. Some radio manufacturers use a slightly off-set center closer to 1515uS. The Taranis uses 1500uS as the default center signal.
  • Some radios use 1500 +/- 500 for full travel as described above.  The default 100% travel on the Taranis is 1500 +/- 512uS, or 988uS to one side and 2012uS to the other side.
  • Most computer-based radios allow travel to be extended beyond 100%, some to 120%, others to 140%. If the “extended limits” box is checked in “Model Set Up” on the Taranis (screen #2 in the Model Setting Menu), servo travel limits can be extended to 150% of the default, or +/- 768uS.

In servos:

  • Full travel is plus/minus 45-degrees to either side in many servos but is only 40-degrees (or less) in some and a lot more in others.  Some can even rotate a full 360 degrees.
  • You would hope that an increasing PWM signal would at least produce a servo rotation in the same direction from servo to servo, but not so. Within a given brand, yes, but among different brands, there is no guarantee.

So… We have 50 messages per second in each channel and PWM coding as industry standards for radios and servos.  Beyond that, there are no industry standards for direction of servo rotation, default amount of rotation, center position signal, or extended PWM signal limits. This makes it important that we understand the defaults and limits included in the Taranis.

A quick note about units of measure: If you checked the “ppmus” (pulse position modulation in microseconds) option in your build/download of OpenTX firmware, your Taranis will display PWM microseconds in the Servos screen and the channel monitor. Otherwise you will see “%” for servo travel in those screens, and the percent shown is the percent of the default 512uS deflection from center. The following graphic summarizes the Taranis defaults and limits.

Servo Travel Graphic rev2

If you’re questioning “Why is this important?”, please read on.  A “weight” appears in three essential screens for our set up — Inputs, Mixer, and Servos. Each weight is expressed as a percent, and eventually all three weight percents are multiplied together to produce a combined weight that is the percent of the default servo travel limit of +/- 512uS to determine the signal to the servo:

net wt % in Inputs
(up to 100%)
x net wt % in Mixer
(up to 100%)
x % of Servo Travel Limit
(up to 100% or extended up to 150%)
= Percent of +/- 512uS travel signal to Servo

This construction of the servo signal is the basis of the “command chain” logic that we use in this class. If it seems complicated, please don’t panic. We construct the logic step by step in the Basic Airplane Set Up with Taranis class.  An example could use .75 as 75% weight in Inputs; 1.00 for 100% weight in Mixer; and 1.30 for 130% extended limit weight in Servos.  If those were the three values in order above, 0.75 * 1.00 * 1.30 = 0.975 or 97.5% of the default 512uS for commanded travel to the servo.  The resulting PWM signal would be 0.975* 512 = 499uS positive rotation from center or 1500+499uS = 1999uS.  Although we didn’t exceed the 100% (or 512uS)  final travel signal limit to the servo, we used 130% in the servo travel limit field, which would have required the Extended Limits box to have been checked.

It’s often said that there is no single “right” way to set up a model in OpenTX.  That is especially true with advanced set ups that use the extraordinary flexibility of OpenTX to achieve a special goal.  But in general there is a design to OpenTX that we can exploit to achieve predictable results and make troubleshooting easier. It’s that design that we will explore in Basic Airplane Set Up with Taranis.

A General Note of Caution:  The Taranis with OpenTX generally offers more servo travel than other radios, and the settings on other radios are unlikely to be the settings you need on the Taranis.  Please follow the instructions in this class carefully as though you were setting up a new model and do not try to simply transfer your existing settings to the Taranis.

Click to move on to Startup Warnings, Main Screens, and Model Selection (1/13)






Important Info on Servo Travel — 11 Comments

  1. Can you explain how trims propagate? There is a trims setting in inputs, mixes and servos.

    For instance, trim on aileron input with respect to trim on aileron servos left and right??

    This is not clear to me, and should be included, since trimming is the next thing to do when basic mixing is right.


    • I’ll at least take a stab at it.

      If you select trims on the Inputs window, the trim settings will propagate through all uses of that trim assuming that you’ve selected the bracketed input ([I1], [I2], etc.). If you select trims in Mixes, I believe that the trim settings will only be used by that particular mix. The setting in Servos is actually “subtrim”, which is somewhat akin to centering, and thereby affects only that servo.

      Hope this helps!

      Leonard (mac44mag)

      • I Shems — I haven’t tested all the possible combinations and permutations to see if there are any exceptions to Leonard’s explanation, but I believe that he’s right on all counts. If you’re up for a little knowledge building, you can test a change in trims and/or subtrims in various places and look at the result in the channel monitor on the Tx itself, or it’s even easier to test changes in Companion. Hope these explanations help. Thanks for asking. Best regards………… Bob R. (modbuilder on RCG)

  2. This is really helpful, but there’s one point that should be clarified. The mainstream radios I have dealt with, specifically JR and Spektrum, are set up so that 100% travel is approximately 400us on each side of 1500. What is referred to here as full travel, 500us each way, is considered 125% throw.

    This is critically important for anyone trying to set up Taranis to emulate a Spektrum transmitter in order to fly any of the BNF UMX models. There are dire warnings about not exceeding 100% throw, as doing so will damage the linear servos in these models. It this case, 100% refers to Spektrum standards and means 1100 to 1900us. On Taranis this is approximately 80% travel. So the first step is to set limits on all channels to +/-80%.

    It’s also important for in setting up for AS3X models with SAFE technology, as they don’t take well to excessive travel either.

    Finally, for anyone converting a model from one of the traditional radios to Taranis will find control throws and sensitivity considerably increased if they just set everything at 100%. They need to understand this 80% factor if they are to set up Taranis to give comparable results to their old radio.

    • Hello Daedaluss66 — I suspect that you’ve not read the whole class content yet. No where do I suggest translating Tx settings directly from another radio system into settings in the Taranis. The class describes how to set up the Taranis starting from scratch as though you were setting up a new model. This segment on “Important Info on Servo Travel” emphasizes the differences among radio systems for the very reasons that you cite. There are just too many opportunities to mess up a direct translation for those same reasons. Far better to just play like you’re setting up a new model from scratch, and follow the set up instructions. If you’ve followed the instructions in the class, and then had trouble, please submit another comment with specifics and I’ll do what I can to help. Regards……..R

  3. Robert re UG Courses..Basic Airplane Set Up etc..Model Set Up 2/13 12 fields pg 2
    I have refreshed rebooted re logged etc but still cant get to the “details” links
    I get “Sorry, no such page”
    I can probably work these out but the missing details are the essence of that section.
    Thanks and best wishes, Barts

    • Hello Barts. Sorry you’re having trouble pulling up the links. I’ve made some changes to how the web site handles those documents that I hope will solve the problem you’re having. The downside is that the headings for those documents may also show up at the bottom of each page, so the page will be more cluttered. So be it. Let’s see if that helps. Please let me know if the links are working for you now. Regards……….. Bob R.

  4. Excellently explained Bob…
    From your example, I think in the Servo Travel Limit you meant to say 130%, but inadvertently put 120%.
    Thanks for the great work…
    Best regards…
    Anindya ( andytransonic)

  5. Sorry Bob to come up with that point.

    You write here:
    “Each weight is expressed as a percent, and eventually all three weight percents are multiplied together to produce a combined weight that is the percent of the default servo travel limit of +/- 512uS to determine the signal to the servo…”

    For you in US it might be clear, but here in Germany we would calculate like this:

    50% * 50% * 100% = 250000%³ (But there is no such thing like %³!)

    What you mean here is

    0.5 * 0.5 * 1.0 = 0.25

    You might add an example, to clear things up.

    br KH

    • Thanks for the feedback and info, Karl. I wasn’t aware that it might be interpreted that way. I’ll see what I can do to make it clear for everyone. Best regards………… R

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