Salut fdufnews,
J'ai un peu progressé depuis...
Ça marche comme je le souhaite désormais 
Merci quand même pour ta réponse
#define UP_TIME 6800 // Time after which speed should decreases
#define DELTA_SPEED 0.95 // The delta decreasing speed.
// Motor state:
#define STOP 0 // 0: motor stopped
#define GO_DOWN 1 // 1: motor goes down.
#define GO_UP 2 // 2: motor goes up.
const int buttonPin = 2;
const int contactPin = 3;
const int buttonPin2 = 4;
const int motorPin1 = 13;
const int motorPin2 = 12;
const int motorPwm= 11;
// variables will change:
int buttonState = 0;
int contactState = 0;
int buttonState2 = 0;
long startTime; // Start time for a movement.
float deltaSpeed; // Decreasing float.
int inMove;
int motorPower;
void setup() {
pinMode(motorPin1, OUTPUT);
pinMode(motorPin2, OUTPUT);
pinMode(motorPwm, OUTPUT);
pinMode(contactPin, INPUT);
pinMode(buttonPin, INPUT);
pinMode(buttonPin2, INPUT);
Serial.begin(9600);
inMove=STOP; // Not in movement at init.
deltaSpeed=DELTA_SPEED; // Set the delta decreasing speed.
digitalWrite(motorPin1, LOW); // No movement.
digitalWrite(motorPin2, LOW); // No movement.
}
void loop() {
// read the state of pushbuttons values:
buttonState = digitalRead(buttonPin);
contactState = digitalRead(contactPin);
buttonState2 = digitalRead(buttonPin2);
if (!inMove) { // Check if arm is in movement.
if (buttonState == HIGH) { // Is the main button pressed ?
Serial.print(65, BYTE);
delay(300);
startTime=millis(); // Set motor Power. // Wait for 300 ms... // Go down.
analogWrite(motorPwm, 255);
digitalWrite(motorPin1, HIGH);// Set motor Power.
inMove=GO_DOWN;
}
}
else {
switch (inMove) {
case GO_DOWN:
if (buttonState == HIGH && buttonState2 == HIGH) { // System stops itself.
Serial.print(66, BYTE);
delay(10);
analogWrite(motorPwm, 0);
digitalWrite(motorPin1, LOW); // Full speed for next time.
motorPower=255; // Set new power (full power to UP).
delay(250); // Set state to GO-UP.
analogWrite(motorPwm, motorPower);
digitalWrite(motorPin2, HIGH); // Go up.
inMove=GO_UP;
}
break;
case GO_UP:
if (millis()-startTime>=UP_TIME) { // If near contactor...
motorPower=(long) ((float) motorPower*deltaSpeed); // .. decrease motor speed.
analogWrite(motorPwm, motorPower+160); // Set new power.
digitalWrite(motorPin2, HIGH);
}
if (contactState == HIGH) { // No movement.
Serial.print(67,BYTE); // System is returned to original state.
analogWrite(motorPwm, 0); // Set power to 0 (stop engine ;o) ).
digitalWrite(motorPin2, LOW);
inMove=STOP;
}
break;
}
}
}