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2DOF harness tensionner with Fly PTmover


My cockpit is static, I've build a pressure gSeat with bladders and RC servo.
I wanted to increase immersion by a harness tensionner.

I'd already tested a static harness tensionner on a 2 DOF plateform, and the feeling was great.
As my rig is static, I'll have to actuate the harness but the advantage is that I can make a 2 DOF tensionner.

2 DOF harness -> the strength on the right shoulder and left shoulder will be different

both will react along Surge (longitudinal acceleration)
and Sway (lateral acceleration) will tight one shoulder or the other (depending on left turn or right turn)
(maybe later add some heave information?)

harness : 5 points
the 5th will prevent the harness to move up, this will give actual tension (vs movement)!

prefer larger harness 3" (vs only 2")

RC servos 35kg.cm (5V to 8,3V)
PSU @7,3V

code for 4 servos as I want to combine the harness and the gSeat pressure bladders also RC servo driven.

FlyPT mover https://www.xsimulator.net/community/faq/flypt-mover.29/category



Here is a video showing step by step how I drive the 2 DOF belt with Fly PTmover:

The strength from 35kg.cm servo is enough.
Power up the servo, sit down, thight the belt as you wish, and start the sim. If you tight the belt before powering up the servos, they are loose and they could be pushed out of their range...

The only drawback, in my opinion is the noise of the servos: they are whinning...

here some infos gathered in this FAQ https://www.xsimulator.net/community/faq/harness-tensioner-simulation.361/

► shopping list
"HV high torque servo motor Robot servo 35kg RDS3235 Metal gear Coreless motor digital servo arduino servo for Robotic DIY"
I chose 270° range
You'll need a dedicated power supply (5V slower to 7,4V fastest)

Speed: 0.13sec/60 degree at(5v)
0.12sec/60 degree at(6v)
0.11sec/60 degree at(7.4v)

Torque: 29kg.cm.at(5v) -1.9A
32kg.cm.at(6v) -2.1A
35kg.cm.at(7.4v) -2.3A

or stronger but unecessary in my opinion
60kgcm 24€

speed is voltage related
torque is current related!

1/ choose speed AKA voltage
2/ check the spec which gives you the current at chosen voltage
3/ buy your PSU ;-)

if you choose 7,4V PSU, verify it'll be able to deliver up to 2.3A in order to give full torque (add a BIG margin ;-) )
11€ 7V 10A

here is my ball bearing support (12€ for 2 supports)
they are made of:
P000 https://www.aliexpress.com/item/32833812473.html
Zinc Alloy Diameter Bore Ball Bearing Pillow Block Mounted Support
they allow a big static disalignment as the bearing is mounted like a joint articulation :)

and 200mm Ø10 linear shaft Cylinder Chrome Plated Liner Rods




► budget:
2x 17€ for 35 kg.cm servo
11€ for 7V 10A PSU
(maybe consider a 12V 10A PSU + an Adjustable Power Module Constant Current 5A)
12€ for bearing rollers
15€ any arduino with a USB port (an original to support the community or a clone)

50€ for a used 3" width 5 points harness
total = 120€ all included

arduino code (for up to 4 servos)
// Multi Direct
// -> 4 servos
// <255><LeftBelt><127><127><RightBelt>
// Rig : Bit output -> 8 bits
// avec inversion
// PT Mover envoie de 0 à 255 par axe

/*Mover = output "Binary" et "10bits"
Arduino = Byte Data[2]
Data[0] = Serial.read();
Data[1] = Serial.read();
result = (Data[0] * 256 + Data[1]);


Mover = output "Binary" et "8bits"
Arduino = Byte Data
Data = Serial.read(); on obtient directement le résultat*/

#include <Servo.h>  // local library "Servo.h" vs library partagée <Servo.h>

const byte nbServos = 4;

// create servo objects to control any servo
Servo myServo[nbServos];
const byte servoPin[nbServos] = {2, 3, 4, 5};  // pins digitales (pas forcément ~pwm)
const byte inversion[nbServos] = {1, 1, 0, 0 }; // paramètre à changer si un servo part le mauvais sens
int OldSerialValue[nbServos] = {0, 0, 0, 0};
int NewSerialValue[nbServos] = {0, 0, 0, 0};

// servo span:
int servoHomeDegres[nbServos] = { 0, 0, 0, 0}; //sera mis à jour avec la mesure de pression initiale
int servoMaxDegres[nbServos] = { 90, 90, 90, 90}; // cuisseG, cuisseD, côtéG, côtéD
int servoPositionTarget[nbServos] = {0, 0, 0, 0};

const byte deadZone = 0;

// =======================================
// Variables for info received from serial
// =======================================
int bufferPrevious = 0;      // To hold previous read fom serial command
int bufferCurrent = 0;       // To hold current read fom serial command
int bufferCount = 0;         // To hold current position in bufferCommand array
// byte bufferCommand[2*nbServos] = {0};  // (*2 if 10 bits) To hold received info from serial
int bufferCommand[4] = {0};  // To hold received info from serial

void setup()
  Serial.begin(115200); // opens serial port at specified baud rate

  // attach the Servos to the pins
  for (byte i = 0; i < nbServos; i++) {
    // pinMode(servoPin[i], OUTPUT); // done within the library
    myServo[i].attach(servoPin[i]);  // attaches the servo on servoPin pin
  // move the servos to signal startup
  MoveAllServos255toDegres(125); // mi-course
  // send all servos to home

void loop()
  // SerialValues contain the last order received (if there is no newer received, the last is kept)

  if (Serial.available())
    bufferPrevious = bufferCurrent; // Store previous byte
    bufferCurrent = Serial.read(); // Get the new byte
    bufferCommand[bufferCount] = bufferCurrent; // Put the new byte in the array
    bufferCount++; // Change to next position in the array
    if (bufferCurrent == 255) bufferCount = 0; // one 255 is the start of the position info
    if (bufferCount == nbServos) //si 8 bits, nbServos // si 10 bits nbServos*2
      //Having reach buffer count, means we have the new positions and that we can update the aimed position
      for (byte i = 0; i < nbServos; i++) {
        NewSerialValue[i] = bufferCommand[i];
        //NewSerialValue[i]= (bufferCommand[i*2] * 256) + bufferCommand[i*2+1]; // si 10 bits
      bufferCount = 0;
  // Update orders sent to motor driver
  for (byte i = 0; i < nbServos; i++) {
    if (abs(OldSerialValue[i] - NewSerialValue[i]) > deadZone) {
      if (inversion[i] == 1)
        envoiServoConsigne255toDegres(i, (255 - NewSerialValue[i]));
        envoiServoConsigne255toDegres(i, NewSerialValue[i]);
      OldSerialValue[i] = NewSerialValue[i];

void envoiServoConsigne255toDegres(byte servoID, int val )
  byte targetDegres;
  val = constrain(val, 0, 255); // constrain coupe au dessus et en dessous : écrêtage et pas mise à l'échelle (comme map)
  // sécurité pour éviter les cas où Simtools enverrait du négatif ou au-delà de 255
  targetDegres = map(val, 0, 255, servoHomeDegres[servoID], servoMaxDegres[servoID]);
  //  map(value, fromLow, fromHigh, toLow, toHigh)
  myServo[servoID].write(targetDegres);              // tell servo to go to position in variable : in steps of 1 degree
  // servo.write(angle)  -> angle: the value to write to the servo, from 0 to 180

void MoveAllServos255toDegres( int target)
  // send all servos to home
  for (byte i = 0; i < nbServos; i++) {
    envoiServoConsigne255toDegres(i, target);

here is the setup:
setup 2DOF harness.png

and the file itself: replace .txt extension by .Mover extension


  • 4Dof_Belt_255_multiDirect_v6bALPHA.txt
    94.9 KB · Views: 28
Last edited:


Replacement RDS3235 draws only about 0.01A no load,
compared with over 0.2A by its predecessor before failure
It is somehow too easy to damage an RDS3235
so that it draws 0.2A with no load, which I have done again.
That is enough to provoke eventual death by overheating.

One original and another replacement together draw only 0.02A no load.

Meanwhile, STM32 Blue Pill sim wind experiments begin:

FWIW, 4-wire fans run full speed with no connection to PWM pins,
and 100cfm PC fans become both loud and self-propelled;
inlet grilles are wanted.
Last edited:


it draws 0.2A with no load
One aggravating factor is stiction in RDS3235 servo bracket plastic bushing,
despite having been greased, effectively imposing loads
which resist servos fully homing to commanded angles.
Having worn its bushing and reduced stiction, the older RDS3235 runs cooler.

Perhaps RDS5160 servos are a better option,
having bearings instead of bushings for non-driven bracket legs.
RDS5160 bracket legs could be re-drilled and cut 10mm shorter
to match RDS3235 maximum strap excursions
while working at a smaller fraction of RDS5160 rating.

Otherwise, RDS3235 servos should probably be "broken in" by
  • initial sessions of 1 minute or so,
  • then power removed for 5 minutes or so
    • for heat to dissipate.
This is my solution, square aluminum profile and aluminum tube, drilled a hole for the tube and secured it with a bolt. Profile is monted to the seat with the top bolt and strong double sided tape. Simple and it works just fine.


  • 203004659_1250818362017284_1828046095295411777_n.jpg
    329.1 KB · Views: 33
@Henrikk thanks for posting a picture of your build! It's nice and neat
could you share with us what motor and controller you bought?
it looks like you 3Dprinted arms :)

maybe you would like to reduce friction between harness and seat with a roller?
@Henrikk thanks for posting a picture of your build! It's nice and neat
could you share with us what motor and controller you bought?
it looks like you 3Dprinted arms :)

maybe you would like to reduce friction between harness and seat with a roller?
First i did buy a couple of 35kg servos but then i saw @Wotevers solution so i went that route.

There really are no friction to talk about so i don't think i need rollers, the motors are plenty strong to squeeze the air out of me :D

After a bit of tweaking and getting used to it's a really cool addition to the rig, already feels strange when it's not turned on, a big thank you guys for the inspiration.