Control Reliability - Beyond The Wire

Picture this - your company has mandated that safety is priority one and that all new machinery must include the latest in safety equipment. Suppose the next purchase is a two-hand operator station for an automated assembly machine that incorporates a pneumatic cylinder that extends for parts hold down and that this hold down could put the operator's hands at risk. The electrical output of the two-hand unit is wired to a single solenoid directional valve that causes the cylinder to extend and clamp when the solenoid is powered and causes the cylinder to retract upon removal of power. If the operator does not maintain contact with both of the palm buttons the cylinder will retract. Does your system require additional optimization?

For example, assume that the operator has depressed both electronic palm buttons to initiate the machine cycle and then notices that the part is not positioned correctly. Attempting to reposition the part, he lets go of the buttons which should retract the cylinder away from the hold position. However, the directional valve for some unknown reason does not return to the normally-closed position. Even though the operator has let go of the button, the cylinder will continue to extend creating the potential that the operator's hand may be injured.