The primary mirror control system (M1CS) stabilizes the 492 segments of the Thirty Meter Telescope primary mirror in
the presence of disturbances. Each Primary Segment Assembly (PSA) has three actuators and position sensors that
control the piston, tip, and tilt of the mirror segment. Requirements for the PSA position controller are presented, with
the main requirements being 10 Newton per micron stiffness below one Hertz, where wind is the primary disturbance.
Bandwidths of the PSA position controller of about twenty Hertz, assuming a soft actuator, are needed to meet this
requirement. A finite element model of the PSA was developed and used for a preliminary control design. PSA structural
modes at 40, 90, and 120 impact the control design. We have studied control designs with different actuators, sensors,
and structural filters in order to assess disturbance rejection properties and interactions with the PSA structural modes.
The performance requirements are achieved using voice coil actuators with modal control architecture for piston, tip, and
tilt. Force interactions with the underlying mirror cell are important, and we present the status of our studies of the
control structure interaction effect (CSIE). A related paper presents further analysis of the CSIE and MICS global
position control loop.