1 """Tools for controlling atomic force microscopes.
3 Provides control of AFM postition using both short-range (piezo) and
4 long range (stepper) vertical positioning. There are separate modules
5 for controlling the piezo (`pypiezo`) and stepper (`stepper`), this
6 module only contains methods that require the capabilities of both.
9 import logging as _logging
11 from pypiezo.base import convert_volts_to_bits as _convert_volts_to_bits
12 from pypiezo.base import convert_meters_to_bits as _convert_meters_to_bits
13 from pypiezo.base import convert_bits_to_meters as _convert_bits_to_meters
14 from pypiezo.surface import SurfaceError as _SurfaceError
15 from pypiezo.surface import FlatFit as _FlatFit
21 LOG = _logging.getLogger('pyafm')
24 LOG.setLevel(_logging.DEBUG)
25 h = _logging.StreamHandler()
26 h.setLevel(_logging.WARN)
27 f = _logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
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
46 def __init__(self, piezo, stepper, axis_name='z'):
48 self.stepper = stepper
49 self.axis_name = axis_name
51 def move_just_onto_surface(self, depth=-50e-9, setpoint=2, far=200):
52 """Position the AFM tip close to the surface.
54 Uses `.piezo.get_surface_position()` to pinpoint the position
55 of the surface. Adjusts the stepper position as required via
56 `.stepper.step_relative()` to get within
57 `2*.stepper.step_size` meters of the surface. Then adjusts
58 the piezo to place the cantilever `depth` meters onto the
59 surface. Negative `depth`\s place the tip off the surface
61 If `.piezo.get_surface_position()` fails to find the surface,
62 backs off `far` half steps (for safety) and steps in (without
63 moving the zpiezo) until deflection voltage is greater than
66 LOG.info('moving to %g onto the surface' % depth)
68 stepper_tolerance = 2*self.stepper.step_size
70 axis = self.piezo.axis_by_name(self.axis_name)
72 zero = _convert_volts_to_bits(axis.axis_channel_config, 0)
73 target_def = _convert_volts_to_bits(axis.axis_channel_config, setpoint)
75 LOG.debug('zero the %s piezo output' % self.axis_name)
76 self.piezo.jump(axis_name=self.axis_name, position=zero)
78 LOG.debug("see if we're starting near the surface")
80 pos = self.piezo.get_surface_position(
81 axis_name=self.axis_name, max_deflection=target_def)
84 pos = self._stepper_approach_again(
85 target_deflection=target_def, far=far)
86 except _SurfaceError, e:
88 pos = self._stepper_approach_again(
89 target_deflection=target_def, far=far)
91 pos_m = _convert_bits_to_meters(
92 axis.axis_channel_config, axis.axis_config, pos)
93 LOG.debug('located surface at stepper %d, piezo %d (%g m)'
94 % (self.stepper.position, pos, pos_m))
96 LOG.debug('fine tune the stepper position')
97 while pos_m < -stepper_tolerance: # step back if we need to
98 LOG.debug('step back')
99 self.stepper.step_relative(-1, backlash_safe=True)
100 pos = self.piezo.get_surface_position(
101 axis_name=self.axis_name, max_deflection=target_def)
102 pos_m = _convert_bits_to_meters(
103 axis.axis_channel_config, axis.axis_config, pos)
104 LOG.debug('located surface at stepper %d, piezo %d (%g m)'
105 % (self.stepper.position, pos, pos_m))
106 while pos_m > stepper_tolerance: # step forward if we need to
107 LOG.debug('step forward')
108 self.stepper.step_relative(1)
109 pos = self.piezo.get_surface_position(
110 axis_name=self.axis_name, max_deflection=target_def)
111 pos_m = _convert_bits_to_meters(
112 axis.axis_channel_config, axis.axis_config, pos)
113 LOG.debug('located surface at stepper %d, piezo %d (%g m)'
114 % (self.stepper.position, pos, pos_m))
116 LOG.debug('adjust the %s piezo to place us just onto the surface'
118 target_m = pos_m + depth
119 target = _convert_meters_to_bits(
120 axis.axis_channel_config, axis.axis_config, target_m)
121 self.piezo.jump(self.axis_name, target)
124 'positioned %g m into the surface at stepper %d, piezo %d (%g m)'
125 % (depth, self.stepper.position, target, target_m))
128 def _stepper_approach_again(target_deflection, far):
129 LOG.info('back off %d half steps and approach until deflection > %g'
130 % (far, target_deflection))
133 self.stepper.step_relative(-far, backlash_safe=True)
135 cd = self.pyiezo.read_deflection() # cd = current deflection in bits
136 LOG.debug('single stepping approach')
137 while cd < target_deflection:
138 LOG.debug('deflection %g < setpoint %g. step closer'
139 % (cd, target_deflection))
140 self.stepper.single_step(1) # step in
141 cd = self.pyiezo.read_deflection()
143 for i in range(2*max(1, self.stepper.backlash)):
144 LOG.debug('additional surface location attempt')
146 pos = self.piezo.get_surface_position(
147 axis_name=self.axis_name, max_deflection=target_def)
149 except _SurfaceError, e:
151 self.stepper.single_step(-1) # step out
152 LOG.debug('giving up on finding the surface')