Need help ramping the motors on my h bride driver.

Hi all
First of all I know a little about arduino but i am pretty much a noob. I am trying to learn, but there is something that I want to do and I can't figure out how. I built an rc tank that runs from two half h bride drivers and an arduino nano. The nano is coded to take the input from a rc receiver and output it to the h bride driver. The issue is my little tank is so powerful that I want to add some ramping to the motors instead of them just going full power as soon as I give the full power signal and I don't want them to slow down so fast either. I am having trouble figuring out how to do this so I thought I would ask for help here. Here is the code I am working with.
I did not code most of this. I found it online and me and a friend did a couple changes.

//read PPM signals from 2 channels of an RC reciever and convert the values to PWM in either direction.
// digital pins 5 & 9 control motor1, digital pins 6 & 10 control motor2. DP 12 and 13 are neutral indicator lights. DP 2 and 3 are inputs from the R/C receiver. All analog pins are open. When motor pin is HIGH, bridge is open. All mosfets are pulled up/down to stay closed unless told to open.

int motor1_a = 5;
int motor1_b = 9;

int motor2_a = 6;
int motor2_b = 10;

//Neutral indicator LED's (FWD and REV LED's are on the motor driver board.
int ledPin1 = 12;
int ledPin2 = 13;

int ppm1 = 2; // connect the desired channel (PPM signal) from your RC receiver to digital pin 2 on Arduino.
int ppm2 = 3;

unsigned long rcval1 = 1500;
unsigned long rcval2 = 1500;

int deadband_high = 265; // set the desired deadband ceiling (ie. if adj_val is above this, you go forward)
int deadband_low = 245; // set deadband floor (ie. below this, go backward)

int pwm_ceiling = 256; // adjusts speed of PWM signal at turn-on
int pwm_floor = 254;

void setup() {

Serial.begin(9600);

// set all the other pins you're using as outputs:
//motor1 pins
pinMode(motor1_a, OUTPUT);
pinMode(motor1_b, OUTPUT);

pinMode(motor2_a, OUTPUT);
pinMode(motor2_b, OUTPUT);

//led's
pinMode(ledPin1, OUTPUT);
pinMode(ledPin2, OUTPUT);

//PPM inputs from RC receiver
pinMode(ppm1, INPUT); //Pin 2 as input
pinMode(ppm2, INPUT); //Pin 3 as input

}

void driveMotor(int thisMotor, int driveVal)
{
//Convert RC signal from 1000 range to 512 range (256 for each direction).
driveVal = map(constrain(driveVal, 1000, 2000), 1000, 2000, 0, 511);
// Default to motor 2
int motor_a = motor2_a;
int motor_b = motor2_b;
int ledPin = ledPin2;

// if motor is 1 change pins
if(thisMotor == 1) {
motor_a = motor1_a;
motor_b = motor1_b;
ledPin = ledPin1;
}

if (driveVal > deadband_high) {
//Forward
digitalWrite(motor_b, LOW);
analogWrite(motor_a, driveVal - pwm_ceiling);
digitalWrite(ledPin, LOW);
}
else if (driveVal < deadband_low) {
//REVERSE
digitalWrite(motor_a, LOW);
analogWrite(motor_b, pwm_floor - driveVal);
digitalWrite(ledPin, LOW);
}
else {
//STOP
digitalWrite(motor_a, LOW);
digitalWrite(motor_b, LOW);
digitalWrite(ledPin, HIGH); // turn on neutral light, turn motor pins off
}
}

void loop() {
rcval1 = pulseIn(ppm1, HIGH); // Listen for pulse on PPM1
rcval2 = pulseIn(ppm2, HIGH); // Listen for pulse on PPM2
driveMotor(1,rcval1);
driveMotor(2,rcval2);

/*
//print values
Serial.print("channel 1: ");
Serial.print(adj_val1);
Serial.print(" ");
Serial.print("rc1_val raw: ");
Serial.print(rc1_val);
Serial.print(" ");
Serial.print("channel 2: ");
Serial.print(adj_val2);
Serial.print(" ");
Serial.print("rc2_val raw: ");
Serial.print(rc2_val);
Serial.println(" ");
*/

}

I built an rc tank that runs from two half h bride drivers

Does that bear any relationship to a half bridge driver?

I don't understand your question. I have two half h bridge drivers connected together to form an H bride driver.

I don't understand your question.

Well, I don't understand what an "h bride driver" is, either, so I guess we are even.

(I know what an H bridge driver is.)

ok haha Well they are really simple actually. Check this out.

Ramping is pretty easy. Instead of using the RC pulse to set the speed directly, use it to set a target_speed. Then using a timer, increment or decrement the actual speed to move towards the target. Set the PWM output to the actual speed.