SPM Fundamentals for the MultiMode
Control Parameters and Feedback
38
MultiMode SPM Instruction Manual
Rev. B
comparator circuit through an analog-to-digital (A/D) converter. It is programmed to keep the two
inputs of the comparator circuit equal (0V). An output voltage generated by the computer
continuously moves the piezoelectric transducer in the Z direction to correct for differences read
into the A/D converter. This closed-loop feedback control is the heart of the imaging portion of the
control station.
2.4.2 General Description of Main Menu Items
This section is a general overview of various Realtime menu items. Refer to the
Command
Reference Manual
for more information about these settings.
Scan size
—Size of the scan along one side of a square. If the scan is non-square (as determined by
the
Aspect ratio
parameter), the value entered is the longer of the two sides.
Aspect ratio
—Determines whether the scan is to be square (
Aspect ration 1:1
), or non-square
(
Aspect ratio 2:1
,
4:1
,
8:1
,
16:1
,
32:1
,
69:1
,
128:1
or
256:1
). The number of lines limits the
Aspect ratio
parameter. You can scan a minimum of two lines.
X offset; Y offset
—These controls allow adjustment of the lateral scanned area and the center of
the scanned area.
Scan angle
—Combines X-axis and Y-axis drive voltages, causing the piezo to scan the sample at
varying X-Y angles.
Scan rate
—Sets the number of lines scanned per second in the fast scan (X-axis on display
monitor) direction.
Samples/Lines
—Set the number of pixels displayed per line.
Line
—Set the number of lines scanned per frame.
Slow scan axis
—Starts and stops the slow scan (Y-axis on display monitor). This control is used to
allow the user to check for lateral mechanical drift in the microscope or assist in tuning the
feedback gains. Always set to
Enable
unless checking for drift or tuning gains.
Z limit
—Limits the amount of drive voltage available to the Z piezo circuit. The Z control system
uses a 16-bit D/A converter which drives an ampli
fi
er capable of outputting voltages from +220V
to -220V. This means that the resolution of the control over the Z direction is approximately 6.7mV
per bit (440V divided by 65536). This setting defaults to 440V automatically. Reducing the Z limit
is useful, when using a “E” or “J” scanner, if scanning samples with relatively small Z features (less
than 10 nm peak-to-valley). For example, setting the Z limit to 55V means that 55 Volts is divided
by the same 16-bit digital control. This gives eight times
fi
ner control over the Z direction of the
scanner.
Integral gain
and
Proportional gain
—Controls the response time of the feedback loop. The
feedback loop tries to keep the output of the SPM equal to the setpoint reference chosen. It does
this by moving the piezo in Z to keep the SPM's output on track with the setpoint reference.
Piezoelectric transducers have a characteristic response time to the feedback voltage applied. The
gains are simply values that magnify the difference read at the A/D convertor. This causes the
