1 # Copyright (C) 2009-2011 W. Trevor King <wking@drexel.edu>
3 # This file is part of pyafm.
5 # pyafm is free software; you can redistribute it and/or modify it
6 # under the terms of the GNU General Public License as published by the
7 # Free Software Foundation, either version 3 of the License, or (at your
8 # option) any later version.
10 # pyafm is distributed in the hope that it will be useful, but
11 # WITHOUT ANY WARRANTY; without even the implied warranty of
12 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 # General Public License for more details.
15 # You should have received a copy of the GNU General Public License
16 # along with pyafm. If not, see <http://www.gnu.org/licenses/>.
18 """Tools for controlling atomic force microscopes.
20 Provides control of AFM postition using both short-range (piezo) and
21 long range (stepper) vertical positioning. There are separate modules
22 for controlling the piezo (`pypiezo`) and stepper (`stepper`), this
23 module only contains methods that require the capabilities of both.
26 from pypiezo.base import convert_bits_to_meters as _convert_bits_to_meters
27 from pypiezo.base import convert_meters_to_bits as _convert_meters_to_bits
28 from pypiezo.base import convert_volts_to_bits as _convert_volts_to_bits
30 from . import LOG as _LOG
34 """Atomic force microscope positioning.
36 Uses a short range `piezo` and a long range `stepper` to position
37 an AFM tip relative to the surface.
41 piezo | pypiezo.afm.AFMpiezo instance
42 Fine positioning and deflection measurements.
43 stepper | stepper.Stepper instance
45 temperature | temperature.Controller instance or None
46 Optional temperature monitoring and control.
48 def __init__(self, piezo, stepper, temperature=None, axis_name='z'):
50 self.stepper = stepper
51 self.temperature = temperature
52 self.axis_name = axis_name
54 def get_temperature(self):
55 """Measure the sample temperature.
57 Return the sample temperature in Kelvin or `None` if such a
58 measurement is not possible.
60 if hasattr(self.temperature, 'get'):
61 return self.temperature.get_temperature()
63 def move_just_onto_surface(self, depth=-50e-9, setpoint=2, far=200):
64 """Position the AFM tip close to the surface.
66 Uses `.piezo.get_surface_position()` to pinpoint the position
67 of the surface. Adjusts the stepper position as required via
68 `.stepper.step_relative()` to get within
69 `2*.stepper.step_size` meters of the surface. Then adjusts
70 the piezo to place the cantilever `depth` meters onto the
71 surface. Negative `depth`\s place the tip off the surface
73 If `.piezo.get_surface_position()` fails to find the surface,
74 backs off `far` half steps (for safety) and steps in (without
75 moving the zpiezo) until deflection voltage is greater than
78 _LOG.info('moving to %g onto the surface' % depth)
80 stepper_tolerance = 2*self.stepper.step_size
82 axis = self.piezo.axis_by_name(self.axis_name)
84 zero = _convert_volts_to_bits(axis.config['channel'], 0)
85 target_def = _convert_volts_to_bits(axis.config['channel'], setpoint)
87 _LOG.debug('zero the %s piezo output' % self.axis_name)
88 self.piezo.jump(axis_name=self.axis_name, position=zero)
90 _LOG.debug("see if we're starting near the surface")
92 pos = self.piezo.get_surface_position(
93 axis_name=self.axis_name, max_deflection=target_def)
96 pos = self._stepper_approach_again(
97 target_deflection=target_def, far=far)
98 except _SurfaceError, e:
100 pos = self._stepper_approach_again(
101 target_deflection=target_def, far=far)
103 pos_m = _convert_bits_to_meters(axis.config, pos)
104 _LOG.debug('located surface at stepper %d, piezo %d (%g m)'
105 % (self.stepper.position, pos, pos_m))
107 _LOG.debug('fine tune the stepper position')
108 while pos_m < -stepper_tolerance: # step back if we need to
109 _LOG.debug('step back')
110 self.stepper.step_relative(-1, backlash_safe=True)
112 pos = self.piezo.get_surface_position(
113 axis_name=self.axis_name, max_deflection=target_def)
117 pos_m = _convert_bits_to_meters(axis.config, pos)
118 _LOG.debug('located surface at stepper %d, piezo %d (%g m)'
119 % (self.stepper.position, pos, pos_m))
120 while pos_m > stepper_tolerance: # step forward if we need to
121 _LOG.debug('step forward')
122 self.stepper.step_relative(1)
124 pos = self.piezo.get_surface_position(
125 axis_name=self.axis_name, max_deflection=target_def)
129 pos_m = _convert_bits_to_meters(axis.config, pos)
130 _LOG.debug('located surface at stepper %d, piezo %d (%g m)'
131 % (self.stepper.position, pos, pos_m))
133 _LOG.debug('adjust the %s piezo to place us just onto the surface'
135 target_m = pos_m + depth
136 target = _convert_meters_to_bits(axis.config, target_m)
137 self.piezo.jump(self.axis_name, target)
140 'positioned %g m into the surface at stepper %d, piezo %d (%g m)'
141 % (depth, self.stepper.position, target, target_m))
144 def _stepper_approach_again(self, target_deflection, far):
145 _LOG.info('back off %d half steps and approach until deflection > %g'
146 % (far, target_deflection))
149 self.stepper.step_relative(-far, backlash_safe=True)
151 cd = self.piezo.read_deflection() # cd = current deflection in bits
152 _LOG.debug('single stepping approach')
153 while cd < target_deflection:
154 _LOG.debug('deflection %g < setpoint %g. step closer'
155 % (cd, target_deflection))
156 self.stepper.single_step(1) # step in
157 cd = self.piezo.read_deflection()
159 for i in range(2*max(1, self.stepper.backlash)):
161 'additional surface location attempt (stepping backwards)')
163 pos = self.piezo.get_surface_position(
164 axis_name=self.axis_name, max_deflection=target_deflection)
166 except _SurfaceError, e:
168 self.stepper.single_step(-1) # step out
169 _LOG.debug('giving up on finding the surface')