to the beams 50a and 50b the beams 50a and 50b are closely spaced and adapted to operate normally with the vertical edge of the 34 t 34 bar 40b adjusted to interrupt the lower beam 50a while the upper beam remains intact whereby a signal may be continuously beamed from transmitter 56 to receiver 58 and controller 62 thus while an infrared beam is normally being projected through the transmitting cable 52b beam 50b and receiving cable 54b to receiver 58 the end edge of the rotor 34 t 34 bar flange 40b normally interrupts the beam 50a being transmitted across the gap by fiber optic cable 52a so that no signal is being transmitted therethrough as the matrix of the rotor deforms downwardly flange 40b moves out of the beam 50a between lower fiber optic cable 52a and receiving cable 54a and a corresponding part of receiver 58 starts receiving a signal from transmitter 56 upon receipt of the signal from transmitter 56 through fiber optic cable 52a beam 50a and cable 54a a signal is relayed to controller 62 so that it in turn may actuate the drive unit 47 to move the end of the sector plate 42 down to neutralize the leakage path formed by the downward movement of the rotor if the rotor should deform upwardly whereby flange 40b would interrupt upper beam 50b a signal would be sent from receiver to controller 58 and the controller 62 would reversely actuate motor drive 44 to drive the.