Embodiments relate to current difference sensors, systems and methods. The LED based lighting systems may include fixtures with one or more LED light bars, integrated sensors, onboard intelligence to send and receive signals and control the LED light bars, and network connectivity to other fixtures. Workpiece clamping units are carried by the guide units, and drive means moves the guide units on the carrier. The control arm is adapted to be mounted to the trunnion arm in any one of a plurality of different positions and the return arm is capable of being mounted to the casing in a corresponding position such that the return arm can be aligned with the control arm to establish the neutral position.