iPhone Accelerometer forward,back,left,right - ios

Question

I would like to know how to create code that would allow me to place an iPhone flat on a surface and move it forward to get positive y value, back to get -y, left to get -x and right to get -x. The code I am using only seems to measure positive acceleration and I can not generate negative values while the phone is laid flat on surface. If I lift the phone and then tilt it to the right or left, I will get positive x and negative x and same with y but I want the same affect while leaving it on the surface and simply shifting the phone in left and right directions while keeping it facing forward without any type of turning or rotation. I am using swift but will also be happy to use object c if there is a solution.
import UIKit
import CoreMotion
class ViewController: UIViewController {
var currentMaxAccelX: Double = 0.0
var currentMaxAccelY: Double = 0.0
var currentMaxAccelZ: Double = 0.0
let motionManager = CMMotionManager()
#IBOutlet var accX : UILabel? = nil
#IBOutlet var accY : UILabel? = nil
#IBOutlet var accZ : UILabel? = nil
#IBOutlet var maxAccX : UILabel? = nil
#IBOutlet var maxAccY : UILabel? = nil
#IBOutlet var maxAccZ : UILabel? = nil
override func viewDidLoad() {
super.viewDidLoad()
motionManager.accelerometerUpdateInterval = 0.1
//[self.motionManager startDeviceMotionUpdatesToQueue:[NSOperationQueue new] withHandler:^(CMDeviceMotion *motion, NSError *error)
motionManager.startAccelerometerUpdatesToQueue(NSOperationQueue.currentQueue(), withHandler: {(accelerometerData: CMAccelerometerData!, error:NSError!)in
self.outputAccelerationData(accelerometerData.acceleration)
if (error != nil)
{
println("\(error)")
}
})
}
func outputAccelerationData(acceleration:CMAcceleration)
{
// Swift does not have string formation yet
accX?.text = NSString(format:"%.4f", acceleration.x) as String
if fabs(acceleration.x) > fabs(currentMaxAccelX)
{
currentMaxAccelX = acceleration.x
}
accY?.text = NSString(format:"%.4f", acceleration.y) as String
if acceleration.y > currentMaxAccelY
{
currentMaxAccelY = acceleration.y
}
accZ?.text = NSString(format:"%.4f", acceleration.z) as String
if acceleration.z > currentMaxAccelZ
{
currentMaxAccelZ = acceleration.z
}
maxAccX?.text = NSString(format:"%.4f", currentMaxAccelX) as String
maxAccY?.text = NSString(format:"%.4f", currentMaxAccelY) as String
maxAccZ?.text = NSString(format:"%.4f", currentMaxAccelZ) as String
}