doubt regarding acceleration values from phidgets IMU

edit

Hi all,

I am using Phidgets Spatial 3/3/3 IMU sensor along with rotary encoders to feed to the robot_pose_ekf. For IMU, I am using phidgets_imu and imu_filter_madgwick to get the sensor_msgs/Imu message. Now the problem is although the robot is stationary, the linear acceleration values in X and Y direction are not zero, both from phidgets_imu and imu_filter_madgwick. See below:

---
header: 
  seq: 12245
  stamp: 
    secs: 1435257458
    nsecs: 927698280
  frame_id: base_link
orientation: 
  x: 0.0
  y: 0.0
  z: 0.0
  w: 0.0
orientation_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
angular_velocity: 
  x: -0.00118647482551
  y: -0.000878773278379
  z: 0.0012882275209
angular_velocity_covariance: [1.2184696791468346e-07, 0.0, 0.0, 0.0, 1.2184696791468346e-07, 0.0, 0.0, 0.0, 1.2184696791468346e-07]
linear_acceleration: 
  x: 0.301461312895
  y: -0.283705212135
  z: 9.65745491309
linear_acceleration_covariance: [8.66124974095918e-06, 0.0, 0.0, 0.0, 8.66124974095918e-06, 0.0, 0.0, 0.0, 8.66124974095918e-06]
---

and I guess because of this there is lot of drift in the IMU (not sure). So, I would like to know why is this happening? Shouldn't the acceleration in X and Y direction be zero if the robot is stationary? Please let me know what is wrong here and how can it be corrected or am I misinterpreting something?

Thanks in advance.
Naman Kumar