服务帐户的 Google 云端硬盘访问权限

I am trying to access my Google Drive files from a php script that will be run in a cron job, but I can't see my files. I am using a Service account set up in Google Developpers console, which do not need authorisation interaction with the user (me). This is my code:

require_once 'Google/Client.php';
require_once 'Google/Service/Drive.php';
$client_id = '260186860844-kg9uiqe7pjms3s54gabqfnph0iamjdjn.apps.googleusercontent.com';
$service_account_name = '260186860844-kg9uiqe7pjms3s54gabqfnph0iamjdjn@developer.gserviceaccount.com';
$key_file_location = 'privatekey.p12';

$client = new Google_Client();
$client->setApplicationName("Whatever");
if (isset($_SESSION['service_token'])) {
  $client->setAccessToken($_SESSION['service_token']);
}
$key = file_get_contents($key_file_location);
$cred = new Google_Auth_AssertionCredentials(
    $service_account_name,
    array('https://www.googleapis.com/auth/drive'),
    $key
);
$client->setAssertionCredentials($cred);
if($client->getAuth()->isAccessTokenExpired()) {
  $client->getAuth()->refreshTokenWithAssertion($cred);
}
$_SESSION['service_token'] = $client->getAccessToken();
$service = new Google_Service_Drive($client);

$parameters = array(
    //    'maxResults' => 10,
    );
$dr_results = $service->files->listFiles($parameters);
foreach ($dr_results as $item) {
    echo $item->title, "<br /> 
";
}

似乎我已正确连接(获得令牌，没有提示错误)，但我只列出了一个文件，该文件不属于我的云端硬盘，名为如何开始使用云端硬盘"！