多个上传编码器无法验证上传路径

我有多个上传文件的控制器代码，像这样:

public function submit_add() {
        $this->load->library('upload');
        $article = new Article_mdl();
        $article->artikel_title = $this->input->post('artikel_title');
        $article->artikel_date = $this->input->post('artikel_date');
        $article->artikel_text = $this->input->post('artikel_text');
        $article->arkat_id = $this->input->post('arkat_id');
        if (!empty($_FILES['artikel_image']['name'])) {
            if (!$this->upload->do_upload('artikel_image')) {
                $error = array('error' => $this->upload->display_errors());
                print_r($error);
                exit();
            } else {
                $article->user_id = $this->session->userdata('user_id');
                $article->artikel_is_active = 1;
                $idartikel = $article->insert();
                $files = $_FILES;
                $cpt = count($_FILES['artikel_image']['name']);
                $dataPhoto = array();
                for($i=0; $i<$cpt; $i++){
                    $_FILES['artikel_image']['name']= $files['artikel_image']['name'][$i];
                    $_FILES['artikel_image']['type']= $files['artikel_image']['type'][$i];
                    $_FILES['artikel_image']['tmp_name']= $files['artikel_image']['tmp_name'][$i];
                    $_FILES['artikel_image']['error']= $files['artikel_image']['error'][$i];
                    $_FILES['artikel_image']['size']= $files['artikel_image']['size'][$i];    
                $this->upload->initialize($this->set_upload_options());
                $this->upload->do_upload();
                //echo "<pre>";print_r($xxx);die;
                $dataPhoto[] = array('artikel_photo_artikel_id' => $idartikel, 'artikel_photo_images' => $_FILES['artikel_image']['name'], 'artikel_photo_status' => 1);
            }
            $article->insertArtikelPhoto($dataPhoto);
            }
        }
        redirect('admin/article/');
    }
private function set_upload_options(){   
    //  upload an image options
        $config = array();
        $config['upload_path'] = './upload/article/';
        $config['allowed_types'] = 'gif|jpg|png';
        $config['max_size']      = '0';
        $config['overwrite']     = FALSE;
        $config['encrypt_name']  = TRUE;
        return $config;
    }

我的问题是为什么当我提交数据时，我得到了错误"上传路径似乎无效。"目录路径config存在。怎么了?请帮助……谢谢你之前的帮助，我会很感激:)