codeigniter 3 : model in helper function getting undefined. How to access model in helper file function?

Question

Notice:

Undefined property: Lander::$Model_datalogger in /var/www/html/codeigniter/helpers/lead_helper.php on line 256

1. added in auto load

   $autoload ['model'] = array ("Model_datalogger");

2. trying to access

   ci =& get_instance();
   // $ci->load->model('Model_datalogger');
   $ci->Model_datalogger->newInstance();```