Adding plugin's stylesheet to wp_head

I am trying to add my plugin’s stylesheet into wp_head using this code

//adding link to plugin's stylesheet in wp-head

	function my_plugin_stylesheet() {
	$url = plugins_url();
	echo "<link rel='stylesheet' href='.$url./plugin_style.css' type='text/css' />";
	}
add_action('wp_head','my_plugin_stylesheet');

But this code is not working. Can someone please tell me what I am doing wrong?