An apparatus comprises a movable member, first and second rigid members
positioned on opposite sides of the movable member, a first suspension
assembly connected between a base and the first and second rigid members,
a second suspension assembly connected between the first and second rigid
members and the movable member, first and second actuators coupled to the
first and second rigid members along a first central axis, and third and
fourth actuators coupled to the movable member along a second central
axis.