Thread: Question Is EPA = ATV
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Old Oct 03, 2007, 04:23 AM
A.T. is offline
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EPA, ATV, Dual & Triple Rates, Expo Differences & Caution + ESC / BEC Set Up

Originally Posted by Thermalin
Does end point adjustment mean the same as adjustable travel volume.
I thought ATV was where you would get increased servo movement the farther the stick moved egfrom center...
so as to assist in avoiding over control by beginners?? and EPA was setting the max amount of movement
each side of center on the servo to assist with throttle hookups, etc??
I was reading in a towertalk were it said ATV also known as EPA????
"did I have it wrong all this time. Mike
Expanding upon my post #6 21Sept07 above, answers to further questions asked direct by readers.

"E.P.A - A.T.V - Dual Rates differences, cautions and arming ESC."

ATV/EPA is the master setting for servo travel to prevent binding (e.g. servo trying to drive car steering
beyond capability of steering rack or throttle travel) or other model specific limitations.
The object is to maintain system resolution by working with the maximum control signal pulse
width and corresponding servo pulse width range.
Limiting ATV and poor linkage design and or ratios degrades resolution by absorbing or deleting the available finite steps.
Optimum is to maximise Transmitter Pulse Swing together with MAX Servo Angle.
Typically servo angle is limited to 60 degrees either side of centre, thus poorly engineered linkage
ratios that do not allow maximum servo angle must be avoided.

. Use TX or Servo Programmer for Digital Servo
- Always use servo programmer in preference to a transmitter when adjusting digital servo..

Pre computerised transmitters:
ATV = Adjustable Travel Volume on Aileron, Elevator & Rudder.
This function adjusts the servo left and right throws setting range 30% to 110% via one pot only.
. Both ends adjust proportionally, this function usefull when a model is to sensitive to the full servo movement
(or the mechanical connection causes servo to overthrow or bind and EPA not available).
Dual Rates = A lower setting (fraction) of either ATV or EPA.
. Dual/triple rates effectively limit ATV or the control signal pulse width to limit or govern the surfaces travel volume.
Note: 3D setup with massive throw is not desirable for precision aerobatics because resolution is sacrificed with dual rate limits.
EPA = End Point Adjustment adjusts a servo's left and right throws via two separate pots.
. The rate setting range is typically 30% to 110% and used to adjust the servo left and right throws
on throttle individually for linkage correction on engine.

Post computerised transmitters:
. Once ATV/EPA & Sub Trim values are setup, NEVER try to reset them again later without a complete reset
of that model memory and starting the set up from the beginning.
The reason being is subsequent ATV/EPA changes also alter and affect mixes and more.
Always set dual/triple rate and mix values in the specific screen thereof to prevent problems later on.

Servo Pulse duration is now typically from 0.9mS to 2.1mS with 1.5mS as center.
NB Hitec 7*** series Digital servo have high 2048 resolution set at max 120 degrees,
these must be reduced to lower 1024 resolution for 180 degree operation.
Old-time (Futaba & Sanwa aka Airtronics) had a different pulse set-up:
Min 0.9 max 1.8 mSec and midpoint/neutral at 1.4 mSec

Most current transmitters work with 1200us pulse swing
(us=usec=micro-seconds pulse width or control signal).
The actual range is 900-1500-2100, neutral is 1500usec.
1200us equates to 1200 finite steps, 600 either side of centre.

ATV/EPA sets the MAX pulse swing the servo can realize; accordingly this limits the servos angle or
travel arc as well as system resolution.
Accordingly, EPA in computer transmitters can be adjusted individually for each direction from
usually 0% (no deflection from centre) to 125%.

Prior to setting EPA/ATV/Dual rates:
. Recheck geometry as well as radio end points & D/R are all at factory default settings.
Make sure that you are using full travel in the radio and have not already preset any EPA or D/R down
to get the desired travel.
To obtain the best centering and resolution possible, maximise mechanical linkage by using maximum EPA and
Dual rate settings with the shortest servo arm possible.
[If possible, the control horn length on the surface should 1:1 with the servo horn].
Check both halves travel full distances equally on a Y lead in one channel, so as to maximize mechanical
and electronic advantage for the servo, and then try with your separate pmix.
Low EPA /D/R do affect the centering capabilities of any servo.
{Subtrim moves the EPA's so they are equal on both sides in relation to the new center point.}

Sub Trim:
. Use sparingly to center servos once only, when arm cannot be centered using splines on shaft, during setup.
If both EPA and D/R settings are at 125% and the sub trim is at or near 100% full deflection in either direction,
you will exceed the tolerance of the receiver/servo.
Some receivers are set to accept a specific pulse range and when many computerised TX are set up as mentioned,
you will exceed that range. Beside receiver not responding correctly, servo can be driven against mechanical stops leading to Burnout
It is not that the receiver is defective or that the TX is defective, it is just that in this unlikely situation they will be incompatible.
Later use of sub trims will also impact on the user mixes of most TX.[(NB: see note under glossary).

NB: Any binding at end points will cause excess current drain and drop battery voltage to low levels
which may induce jitters, burn out servo, Brown out
or other adverse results, see also:
Travel vs Limit as some high end Tx do have a "T.Limit" facility:
"Travel" is the amount movement you get when you move a control from one extreme to the other.
So this is the full movement from one end of stick travel to the other, plus trims; full movement from
knob rotation's one end to the other; or a switch from one side to the other.
When you add mixes, such as V-Tail, you can add the travel amount of two channels together.
This can cause problems because the servos may try to drive too far for the linkage, control surface,
or the servo itself. "Limit" is the maximum amount a channel can move when it is driven above the travel
amount by a mixer. Normally, the system defaults are all you need.
You can make the Travel and Limit amounts larger or even the same, but it all depends upon how
your model needs to be set up. ""

Throttle Setting for Electronic Speed Controllers
[Includes other electronic control boards & switches such as Multirotor control boards]:
Many ESC will not arm unless the throttle trim on a computerised TX is lowered to 60<80% or even minimum,
alternatively EPA is adjusted almost to maximum because, in simple terms, the maximum servo travel displayed
on a computer TX is about 80% of that of a non computerised TX to provide for the trim and sub trim adjustments.
For the tech minded, 125% max servo throw for majority of computerised transmitters, equates to 100% of actual channel spacing.
At 125% the min is about .5ms (-45degrees) and the max is 2.5ms (+45degrees) and 1.5ms is nominal center.
Since an ESC wants to see the low setting (~.5ms) at low throttle computerised Tx owners must lower trim in order the ESC will arm and
provide reasonable range of speed.
Majority of ESC manufacturers state throttle trim on standard Tx must also be reduced to 50% or less
(or minimum as with Skyartec and their clones).

NB: Never set up an ESC with trim at absolute minimum or EPA at maximum
because some ESC will lock out if overheated or other cause and can only be reprogrammed/reset by
starting with a lower than previously set value.
During the subsequent reset process the minimum trim must be increased back to say 20% to provide a margin
for future resets. (e.g. Graupner Pico Mos series).
see also
. Electric Power - Every Thing You wanted To Know about EP Flight pdf by Ken Myers
. Electric Power Basics .pdf by Ken Myers - January 2011

Unexplained problems with Computerised Transmitters ?
. Make sure battery is fully charged when programming.
. If Low Battery Warning sounds when flying - perform full range check, testing all mixes and trims before next flight.
This because a low battery frequently does not have the capacity to flash the trim and other adjustments to
memory since TX was last turned on.
Programming and other trim adjustments are only saved at time or after transmitter is turned off.
. If TX is turned off/on again to quickly, previous adjustments may not have been fully saved,
same as any other Flash (USB) device.
. Memory chips have a limited storage life and will last only a few hundred thousand write cycles.
Accordingly it cannot continuously write all data to the memory chip and so as to prolong the life of the flash memory,
data is held in memory cache of the processor until switch off.

. Electronic Speed Control - Why BEC and LVC cause most EP RX problems

. ESC - Low Voltage Cutoff Reset - warning re different detection settings between brands.
(Never rely on a LVC actually working without immediate or built up damage to any battery.).

. Peak Servo Current Tests - Shows why good servos cost more than than cheap ones and why they cause less problems.

. Latency and Resolution What they are and what they do - good brief explanation.

Alan T.
Alan's Hobby, Model & RC FAQ Web Links

AFR, AST, ATL, ATV, D/R, EPA, Expo and Triple Rates.

AFR is the normal High end (primary) setting of a transmitter/servo position to differentiate from a lower positiion as set in D/R.

AST is ATV where you get just one setting that affects both ends of the travel.
If you turn it down to 90% both right aileron and left aileron will move only 90% of the total distance the servo is able to move on its own.

ATL is a Throttle trim function only.
Throttle trim only works at 50% or less of throttle stick travel.
Thus adjusting your idle does not affect your wide open engine performance.

ATV Adjustable Travel Value is specific to each individual servo.
Adjusting ATV on aileron channel will not, for example, adjust the servo throw of the second aileron
servo if you have flaperons set up.
ATV is the 'master' control with priority over all other items.
Therefore, adjusting ATV will affect anything else that is a percentage from that number, including rates, exponential, and mixes.

Dual Rate (D/R) provides user with a means to set one rate per switch position
or total distance the servo/surface will travel...lets say it is 1/3" on an elevator.
A 50% dual rate would mean that when you throw the switch, the servo moves exactly half as
far per stick position as it did on the first rate.
So the elevator stick all the way up is now on 1/6" travel instead of 1/3".
Similarly, at half stick it is now 1/12" instead of 1/6".
Triple Rates (T/R) is similar to D/R except using a three position switch, see below.

End Point Adjustment (EPA) provides for separate adjustment at each end of servo travel. e.g.
A throttle servo movement can be adjusted at bottom end to set cut off without effecting top end movement.

Expo: Exponential Rate provides servo travel that is not directly proportional to stick travel. e.g.
30% movement during first half of the stick throw, and the other 70 percent during the second half,
thus making the center less sensitive.
(% used may vary from model/user to model).
NB: set as (-) expo on Hitec/Futaba whereas (+) expo on JR.
(When using D/R, Expo must be set for both High & Low rates separately.)

Subtrim : Moves the EPA's so they are equal on both sides in relation to the new center point.
Exception : "Sub Trim is the point the servo will be at when there is 0% input from the input
(stick/slider/switch/etc.) driving the control of Spektrum - Andy (Spektrum programmer)".

Triple Rates: An extension of dual rates where a three position switch is used to set Low, Medium and Full rates
for competitive flying or Takeoff/Launch mode, Normal Flight and Aerobatics.
11th September 2013 added:
Servo Resolution
At the servo, a pulse of a given width is received that determines the amount of movement.
Regardless of the transmitter setting, you need the same pulse to give the servo a given movement.
Another way to think of it is this: For the full end-to-end travel with ATV all the way up to 150%, you have 150 "ticks".
You want it to be 1/3 smaller, or 100 "ticks", so you adjust the Dual Rate to the setting that gives you that movement.
If you set the ATV to 100%, you have 100 "ticks. So you get the same amount of travel.
Where the best resolution is needed, you set the ATV to the maximum possible,
and then reduce the amount of control surface travel by moving the pushrod in at the servo horn.
This reduces the amount of control surface travel and increases torque.
Now, instead of 100 "ticks" of travel, you have 150 "ticks", which is higher resolution at the control surface.
You should be able to resolve this mechanically.
If you have too much throw at the control surface, you shorten the arm at the servo and/or lengthen the control horn at the control surface.
This is ALWAYS doable. If your control horn isn't long enough, then you can make or buy longer ones.
If you think it cannot be resolved mechanically, look at the problem carefully to see why you think that's the case.

Bill Baxter, Manager Hobby Services 11Sep2013
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