Analog and Digital Controllers PI manufactures a large variety ofanalog and digital nanopositioning controllers (see page 6-8 ff. ).State-of-the-art PI digital controlsystems offer several advantages over analog control systems: coor-dinate transformation, real-timelinearity compensation and elimi- nation of some types of drift. Digital controllers also allow virtu- ally instant changes of servo parameters for different load con- ditions, etc. However, not all digi-tal controllers are created equal.Poor implementations can add noise and lack certain capabilities of a well-designed analog imple-mentation, such as fast settling time, compatibility with advancedfeed-forward techniques, stabilityand robust operation. PI digital controllers can downloaddevice-specific parameters and cal- ibration information from ID-chip- equipped nanopositioning stages, facilitating interchangeability of nanomechanisms and controllers.All PI nanopositioning controllers(analog and digital) are equippedwith one or more user-tunablenotch filters. A controller with notch filter can be tuned to provide higher bandwidth because side- effects of system resonances can be suppressed before they affectsystem stability. For the mostdemanding step-and-settle appli- cations, PI’s exclusive Mach™ InputShaping

Parallel and Serial Designs, Controller Choice

Parallel and Serial KinematicsMechanisms There are two ways to achievemulti-axis motion: parallel and serial kinematics. Serial kinematics (nested or stacked systems) are simpler and less costly to imple- ment, but they have some limita-tions compared to parallel kine-matics systems (see p. 4-44for more information). In a multi-axis serial kinematicssystem, each actuator (and usuallyeach sensor) is assigned to exactlyone degree of freedom. In a paral- lel kinematics multi-axis system, all industrial actuators act directly on the same moving platform (relative to ground), enabling reduced size andinertia, and the elimination ofmicrofriction caused by moving cables (Fig. 4). This way, the same resonant frequency and dynamic behavior can be obtained for both the X and Y axes. The advantages are higher dynamics and scanningrates, better trajectory guidance aswell as better reproducibility and stability.
Fig. 5. Flatness ofan active-trajectory-controlled nano- positioning stage over 100 x 100 µm scanning range isabout 1 nm. Direct Parallel Metrology: Multi-Axis Measurements Relative to a Fixed Reference Parallel kinematics facilitatesimplementation of Direct ParallelMetrology—measurement of allcontrolled degrees of freedom rela- tive to ground. This is a more diffi-cult design to build but it leads to clear performance advantages. A parallel metrology sensor seesall motion in its measurementdirection, not just that of one actu-ator. This means that all motion is inside the servo-loop, no matter which actuator may have caused it, resulting in superior multi-axis pre- cision, repeatability and flatness, as shown in Fig. 5. Direct parallelmetrology also allows stiffer servosettings for faster response. Off- axis disturbances—external or internal, such as induced vibration caused by a fast step of one axis— can be damped by the servo.
® Fig. 4. Principle of a PI XY z , minimum-inertial-mass, monolithic, parallel kinemat-ics nanopositioning system. Accuracy, responsiveness and straightness/flatness are much better than in stacked multi-axis (serial kinematics) systems. See page 4-44for more details. implementation isavailable as an option.
© PI 1998-2005. Subject to change w/o notice. Cat 118 05/09.17 2-6