Model

<?php

class Imc_model extends CI_Model{
    protected $table = array(
        'imc'=>'imc'
    );
    public function __construct(){
        parent::__construct();
    }

    function insertar($params){

        $fecha = date('Y-m-d H:i:s');
        $opt = array(
            'calculo'=>$params['calculo'],
            'fecha'=>$fecha
        );

        $this->db->insert($this->table['imc'],$opt);
    }

}