List the CMS SQL Server Instances using Powershell

I need to run a set of Powershell commands from my Central Management Server onto all of the SQL Server Instances that it contains. Can someone let me know how this could be achieved? I need to loop each server instance from the central management and calculate/get the free space. I am using SQL Server 2012. I need to try out the following things: 1. Search particular extension files on Server Instances 2. Check the space occupied by them 3. Check Free Space 4. Store the above info. in a file on the current system I am simply aiming for getting the size of backup files (.bak files) only. I have a solution already where we query Invoke-Command to find the files for us on remote machine. I wanted to know whether there exist a better solution to get the task done.