What is the purpose of sum voltage monitor? I was looking at the design diagram of actuators. 
Every channel of the actuator is connected to a voltage differential transducer. The transducer outputs are fed to a sum voltage monitor. I could not understand its purpose. 

How can summed voltage exist if there is a single sensor? Or am I missing something here?
It compares the commanded position with the actual position, it is the standard feedback loop.