Compare a date and insert in the parameters array?

Controller

if ($data_inicial != "") {
    $parametros = array_merge($parametros, array(('data_cheque' ,'>=', $data_inicial));
    //print_r($parametros);
}

if ($data_final != "") {
   $parametros = array_merge($parametros, array(('data_cheque' ,'>=', $data_final));
   //print_r($parametros);
}

Why this comparison does not happen, and the error in the SQL query, which parameters would you use for this comparison or how to compare?