How to insert an image into excel using phpexcel in codeiniter

I am using phpexcel with codeigniter. I want to insert an image to excel file.and i am using below code

but it gives the error for file. It says file not found. But that file exists in correct folder. And I gave correct path. Please help me.The file exists in assets folder.

$objDrawing = new PHPExcel_Worksheet_Drawing();
				$objDrawing->setName('Logo');
				$objDrawing->setDescription('Logo');
				$logo = base_url() . 'images/logo.png'; // Provide path to your logo file
				$objDrawing->setPath($pr_file);
				$objDrawing->setOffsetX(8);    // setOffsetX works properly
				$objDrawing->setOffsetY(300);  //setOffsetY has no effect
				$objDrawing->setCoordinates('B1');
				$objDrawing->setHeight(75); // logo height
				$objDrawing->setWorksheet($this->excel->getActiveSheet());