Remove post fields for specific User Role

I’m trying to learn php, but am not very good with it yet. I’m trying to delete some post fileds for specific user roles. “Author” to be exact. This is what I have so far. It deletes the field for the “Author” role, but also for admin. I just want to delete the field for “Author’s”.

function remove_post_excerpt_field() {
	if($role=='author')
	remove_meta_box('postexcerpt', 'post', 'normal' );
}
add_action('admin_menu', 'remove_post_excerpt_field');