Electronics
It is difficult to connect components from different suppliers
and achieve the required specifications for high-performance
systems. Aerotech understands the interaction between the
various elements and designs control electronics to achieve
the desired results.
When operating at the nanometer level, any disturbance on
the system can lead to position errors. These disturbances can
be external (ground floor, structural) or internal (noise on
feedback signals, poor power electronics). Aerotech goes to
great lengths to minimize self-induced electrical noise when
designing the electronics. At a minimum the power amplifiers
of the drive must be linear rather than the more common
PWM (Pulse Width Modulated) amplifiers.
 Compare the waveforms from PWM and linear amplifiers
shown in Figures 5 and 6. Figure 5 shows an example of a
PWM voltage and subsequent current for a sine wave. As can
be seen from the figure, the voltage to the motor is a square
wave. While inductance will filter this square wave, the resulting current will still contain a high-frequency component
known as ripple current. When controlling systems with
resolutions greater than 50 nm, the effect of this ripple current
is negligible. When controlling systems with resolutions less
than 50 nm, and more specifically, at resolutions less than 5
nm, the ripple can cause disturbances in the system. The end
result is poor in-position stability.
Figure 6 shows the output voltage and current from a linear
amplifier. As can be seen from this plot, the voltage and
current waveform have no ripple current. Ultimately, this will
lead to better in-position stability.
Linear amplifiers are not without their own inherent
drawbacks. Due to their inefficiency, linear amplifiers are
large and generate a significant amount of heat. Due to the
transistor topology they can suffer from cross-over distortion.
Cross-over distortion occurs when the current/voltage move
through 0. Instead of the voltage/current having a smooth
transition, a discontinuity occurs that can cause problems for
the servo loop as it tries to hold the stage in position.
Assuming no external disturbance on the system, the amount
of force, and hence current, needed to hold position will be
low. This means that the current will be in the cross-over
distortion zone which makes it difficult for the servo system to
maintain stability. Figure 7 shows this situation. Many currently
available linear power amplifiers have this problem, but
Aerotech has eliminated cross-over instability in our linear
amplifiers, as can be seen from Figure 8.
Aerotech electronics support the different feedback devices
mentioned. Because all of these feedback devices are
inherently analog, there is some amount of noise on the
signals. Of great importance is the ability of the control
electronics to remove the unwanted noise and operate on the
true signal. Aerotech has spent significant R&D on the
development of electronics to optimize the data from the
feedback devices. |
