The C-arc contains two motors. One motor that determines the wire feed speed and one motor that determines the wire tension. The wire tension is controlled by measuring the motor torque and closed loop control of the wire tension motor. The torque measurement is explained here.
The wire speed is controlled by measurement of the rotation speed of the speed motor with the help of a quadrature encoder on the back of the motor, and of course closed loop control.
Next to the two motors in the C-arc, the wire handling also requires two motors on the fixed world, in the wire spool setup. One motor is required to generate the spooling torque and one DC servo makes sure the wire is spooled evenly.
The 4 motors that make up the wire handling are controlled by a single custom design controller. Next to motor control this control box also has a feature to track how much wire has been spent. It also provides some flexibility in how the wire is re-reeled. You can select the distance between the wire coils on the re-reel spool and setup the start position of the re-reel action.