Newport New Focus Picomotor 8743-CL, User Manual

Page: 28 / 133

28
3 Theory of Operation
3.1 How the Picomotor Actuator Works
The patented design of the Picomotor actuator relies on the basic difference between
dynamic and static friction. A graphic example of this is the “tablecloth trick”, in which a
quick pull of the cloth leaves the dishes on the table, while a slow pull of the tablecloth
ends up pulling the dishes off the table (high static friction).
The Picomotor actuator uses the same principle with a threaded jaw, similar to two halves
of a split nut, clamped around a precision 80-pitch screw. One jaw is connected to one end
of a piezoelectric transducer, and the other jaw is connected to the other end of the
transducer. A slow electrical signal applied to the piezo slowly changes the length,
causing the two jaws to slide in opposite direction. This slow sliding motion makes the
screw turn (static friction). At the end of the transducer motion, a fast electrical signal
quickly returns the jaws to their starting positions. Because of the screw’s inertia and low
dynamic friction, it remains motionless, holding its position. Simply reversing the order of
the fast and slow signals reverses the direction of rotation.
3.2 Stepping the Picomotor with the 8743-CL Controller/Driver
Although the Picomotor is not a stepper, per se, it can be driven step-by-step. Each step
size depends on the Picomotor model, but it is usually < 30nm, allowing for very small,
controlled movements.
8743-CL can be programmed for single step control, as well as a predetermined number of
steps. Moreover, 8743-CL can drive the Picomotor in a continuous motion, in either
direction at variable speeds.
As opposed to magnetic motors, the Picomotor actuator has a set-and-forget stability.
When no power is applied, the actuator does not move. The user can feel confident that the
Picomotor will not change its position, even when the 8743-CL Controller/Driver is
powered off. Therefore, the 8743-CL has a power ON/OFF switch for convenience. If a
user has a 2-Picomotor system, once the system is set, the user can turn the power OFF, to
assure that the Picomotors will not be inadvertently moved through computer control. In
addition, the 8743-CL Controller/Driver can be programmed with the outputs
disconnected, with the same effect as in the power OFF state.