Codeigniter ajax form validation in modal not working - php

Question

I want to validate my form which is in bootstrap modal. But all time validation_errors() returning empty string. I am using Codeigniter 3 and HMVC.
here is my ajax code
$('#add_unload').on('click', function (e) {
e.preventDefault();
var carrier_type = $.trim($('#carrier').val());
var truck_plateno = $.trim($('#truck_plateno').val());
$.ajax({
url: site_url + 'pre_unload/add',
type: 'get',
dataType: 'json',
data: {
carrier_type : carrier_type,
truck_plateno:truck_plateno
}
}).done(function (response) {
console.log(response);
});
});
and here is my controller's add method
public function add() {
if (!$this->input->is_ajax_request()) {
die('wrong');
}
$carrier_type = $this->input->get('carrier_type', TRUE);
$this->form_validation->set_rules('truck_plateno', 'Truck Plate', 'required');
if ($carrier_type == 'O') {
$this->form_validation->set_rules('another_filed', 'XXXX', 'required');
} else {
$this->form_validation->set_rules('another_filed', 'XXXX', 'required');
}
if ($this->form_validation->run() === FALSE) {
$this->load->helper('form');
echo json_encode(array('err' => validation_errors()));
return;
}
}
var_dump($this->form_validation->run());return; always return false and validation_errors() return empty string. I have tried this too $this->form_validation->run($this) still no luck. My_form_validation.php also ok in library
** I have loaded form_validation library in construct
Any IDEA?