DA40 Diamond Star
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Conflict Airborne Conflict
Air Traffic Control Issued New Clearance
Human Factors
I was working the D-Side; and unplugged for a short duration of time to tend to some administrative duties. Prior to me leaving; both the R-Side and I identified a couple of VFR targets which were likely to penetrate the sector southbound over the lake; and were potential traffic for the primary sector traffic flow; inbound Class B arrivals from the northeast. After completing these duties; I plugged back in on the D-Side. As I was reassuming the position; the R-Side Controller was issuing a traffic advisory to a DA40; as one of these VFR targets had begun a slow descent; and had leveled at 080 MSL; southbound. The DA40 was level at 080 MSL; northbound; and it was clear the targets were likely to merge. The R-Side; a long time professional continued to issue this traffic on multiple occasions; as the DA40 reported haze and had yet to get the VFR traffic in sight. The R-Side called an adjacent facility (Approach) to gain control; as the DA40 had yet to penetrate our airspace.  The R-Side then assigned the DA40 a block altitude of 080B090 MSL. I personally thought the DA40 would climb to eliminate the conflict; as the R-Side continued to issue the traffic with increasing urgency. The DA40 did not climb; and the aircraft continued to get closer; opposite direction; at the same indicated altitude. Finally; when the aircraft came to what the R-Side considered unsafe proximity and still closing; the R-Side instructed an emergency climb to the DA40 to 090. As the DA40 reported his climb; he indicated a Twin Cessna passed beneath him. I instructed the R-Side; after a short time; to ask the DA40 how close; laterally; the aircraft came to one another. The DA40 indicated the Twin Cessna passed 'Directly Beneath' him. I feel the R-Side saved a potential mid-air collision; and I am simply writing this report to record this data of a job well done by one of our finest. A recurrence of this event; under current VFR rules; is most likely unavoidable. However; I feel this incident should be recorded and disseminated as a training tool to demonstrate to others that our job; first and foremost; is to separate traffic. Where ambiguity of separation exists; as in this case; take all necessary measures to reestablish a safe situation.
Enroute Controller witnessed a developing conflict event between an IFR and VFR aircraft prompting the RADAR Controller to take positive action to separate the aircraft.