Error saving image with ajax in codeigniter

Good, I'm starting to use codeigniter with the upload library, and ajax to save an image, it is saved in a table image that has 3 attributes id_image (that is autoincrementable), route, and as foreign id_person. this part being clear my problem. is that when I submit, the next error in the console bounces:

  

POST link 500 (Internal Server   Error).

I attach my html code:

<form id="formImagen" action="<?php echo base_url();?>imagenes/guardar" method="POST">
 <input type="hidden" name="id_persona" value="<?php echo $getPersona->Id_persona; ?>">
 <input type="file" name="imagen">
 <button type="submit" class="btn btn-squared btn-info margin-inline">Subir Imagen</button>
</form>

in the input hidden I pick up the id of the person by a query.

the script:

$('document').ready(function(){

    $("#formImagen").submit(function (event){
        event.preventDefault();
        var formData = new FormData($("#formImagen")[0]);
        $.ajax({
            url:$("form").attr("action"),
            type:$("form").attr("method"),
            data:formData,
            cache:false,
            contentType:false,
            processData:false,
            success:function(respuesta){
                if (respuesta === 'exito'){
                    alert('guardado');
                }
                else{
                    console.log(respuesta);
                    alert('error al guardar');
                }
            }
        });
    });
});

and my controller:

public function guardar(){
      if($this->input->is_ajax_request()){
        $id = $this->input->post("id_persona");

        $config = [
          "upload_path" => "./assets/images/uploads",
          'allowed_types' => "png|jpg|jpeg" 
        ];

        $this->load->library("upload",$config);

          if($this->upload->do_upload('imagen')){
            $data array("upload_data" => $this->upload->data());
            $datos = array(
              "Ruta" => $data['upload_data']['file_name'],
              "Id_persona" => $id
            );
            if($this->Imagenes_model->guardar($datos)==true)
              echo "exito";
            else
              echo "error";
          }else{
            echo $this->upload->display_errors();
          }
      }
      else
        show_404();
    }