3 # calibcant - tools for thermally calibrating AFM cantilevers
5 # Copyright (C) 2007-2009 William Trevor King
7 # This program is free software; you can redistribute it and/or
8 # modify it under the terms of the GNU General Public License as
9 # published by the Free Software Foundation; either version 3 of the
10 # License, or (at your option) any later version.
12 # This program is distributed in the hope that it will be useful, but
13 # WITHOUT ANY WARRANTY; without even the implied warranty of
14 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
15 # See the GNU General Public License for more details.
17 # You should have received a copy of the GNU General Public License
18 # along with this program; if not, write to the Free Software
19 # Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
22 # The author may be contacted at <wking@drexel.edu> on the Internet, or
23 # write to Trevor King, Drexel University, Physics Dept., 3141 Chestnut St.,
24 # Philadelphia PA 19104, USA.
27 Aquire and analyze cantilever calibration data.
29 W. Trevor King Dec. 2007-Jan. 2008
34 The relevent physical quantities are :
35 Vzp_out Output z-piezo voltage (what we generate)
36 Vzp Applied z-piezo voltage (after external ZPGAIN)
37 Zp The z-piezo position
38 Zcant The cantilever vertical deflection
39 Vphoto The photodiode vertical deflection voltage (what we measure)
40 Fcant The force on the cantilever
41 T The temperature of the cantilever and surrounding solution
42 (another thing we measure or guess)
43 k_b Boltzmann's constant
45 Which are related by the parameters :
47 zpSensitivity Zp / Vzp
48 photoSensitivity Vphoto / Zcant
51 Cantilever calibration assumes a pre-calibrated z-piezo
52 (zpSensitivity) and a amplifier (zpGain). In our lab, the z-piezo is
53 calibrated by imaging a calibration sample, which has features with
54 well defined sizes, and the gain is set with a knob on the Nanoscope.
56 photoSensitivity is measured by bumping the cantilever against the
57 surface, where Zp = Zcant (see bump_aquire() and the bump_analyze
60 k_cant is measured by watching the cantilever vibrate in free solution
61 (see the vib_aquire() and the vib_analyze submodule). The average
62 energy of the cantilever in the vertical direction is given by the
63 equipartition theorem.
64 1/2 k_b T = 1/2 k_cant <Zcant**2>
65 so k_cant = k_b T / Zcant**2
66 but Zcant = Vphoto / photoSensitivity
67 so k_cant = k_b T * photoSensitivty**2 / <Vphoto**2>
69 We measured photoSensitivity with the surface bumps. We can either
70 measure T using an external function (see temperature.py), or just
71 estimate it (see T_aquire() and the T_analyze submodule). Guessing
72 room temp ~22 deg C is actually fairly reasonable. Assuming the
73 actual fluid temperature is within +/- 5 deg, the error in the spring
74 constant k_cant is within 5/273.15 ~= 2%. A time series of Vphoto
75 while we're far from the surface and not changing Vzp_out will give us
76 the average variance <Vphoto**2>.
78 We do all these measurements a few times to estimate statistical
81 The functions are layed out in the families:
82 bump_*(), vib_*(), T_*(), and calib_*()
83 where calib_{save|load|analyze}() deal with derived data, not
86 For each family, * can be any of :
87 aquire get real-world data
88 save store real-world data to disk
89 load get real-world data from disk
90 analyze interperate the real-world data.
91 plot show a nice graphic to convince people we're working :p
93 read a file with a list of paths to previously saved
94 real world data load each file using *_load(), analyze
95 using *_analyze(), and optionally plot using *_plot().
96 Intended for re-processing old data.
97 A family name without any _* extension (e.g. bump()), runs *_aquire(),
98 *_save(), *_analyze(), *_plot().
100 We also define the two positioning functions:
101 move_just_onto_surface() and move_far_from_surface()
102 which make automating the calibration procedure more straightforward.
109 import piezo.z_piezo_utils as z_piezo_utils
110 from splittable_kwargs import splittableKwargsFunction, \
111 make_splittable_kwargs_function
123 @splittableKwargsFunction()
124 def bump_aquire(zpiezo, push_depth=200, npoints=1024, push_speed=1000) :
126 Ramps closer push_depth and returns to the original position.
128 zpiezo an opened zpiezo.zpiezo instance
129 push_depth distance to approach, in nm
130 npoints number points during the approach and during the retreat
131 push_speed piezo speed during approach and retreat, in nm/s
132 Returns the aquired ramp data dictionary, with data in DAC/ADC bits.
134 # generate the bump output
135 nm_per_step = float(push_depth) / npoints
136 freq = push_speed / nm_per_step # freq is sample frequency in Hz
137 start_pos = zpiezo.curPos()
138 pos_dist = zpiezo.pos_nm2out(push_depth) - zpiezo.pos_nm2out(0)
139 close_pos = start_pos + pos_dist
140 appr = numpy.linspace(start_pos, close_pos, npoints)
141 retr = numpy.linspace(close_pos, start_pos, npoints)
142 out = numpy.concatenate((appr, retr))
143 # run the bump, and measure deflection
144 if config.TEXT_VERBOSE :
145 print "Bump %g nm at %g nm/s" % (push_depth, push_speed)
146 data = zpiezo.ramp(out, freq)
149 @splittableKwargsFunction(bump_aquire,
150 (bump_analyze.bump_save, 'data'),
151 (bump_analyze.bump_analyze, 'data'))
154 Wrapper around bump_aquire(), bump_save(), bump_analyze()
156 bump_aquire_kwargs,bump_save_kwargs,bump_analyze_kwargs = \
157 bump._splitargs(bump, kwargs)
158 data = bump_aquire(**bump_aquire_kwargs)
159 bump_analyze.bump_save(data, **bump_save_kwargs)
160 photoSensitivity = bump_analyze.bump_analyze(data, **bump_analyze_kwargs)
161 return photoSensitivity
164 # Fairly stubby, since a one shot Temp measurement is a common thing.
165 # We just wrap that to provide a consistent interface.
167 @splittableKwargsFunction()
168 def T_aquire(get_T=None) :
170 Measure the current temperature of the sample,
171 or, if get_T == None, fake it by returning config.DEFAULT_TEMP
174 if config.TEXT_VERBOSE :
175 print "Fake temperature %g" % config.DEFAULT_TEMP
176 return config.DEFAULT_TEMP
178 if config.TEXT_VERBOSE :
179 print "Measure temperature"
182 @splittableKwargsFunction(T_aquire,
183 (T_analyze.T_save, 'T'),
184 (T_analyze.T_analyze, 'T'))
187 Wrapper around T_aquire(), T_save(), T_analyze(), T_plot()
189 T_aquire_kwargs,T_save_kwargs,T_analyze_kwargs = \
190 T._splitargs(T, kwargs)
191 T_raw = T_aquire(**T_aquire_kwargs)
192 T_analyze.T_save(T_raw, **T_save_kwargs)
193 T_ret = T_analyze.T_analyze(T_raw, **T_analyze_kwargs) # returns array
198 @splittableKwargsFunction()
199 def vib_aquire(zpiezo, time=1, freq=50e3) :
201 Record data for TIME seconds at FREQ Hz from ZPIEZO at it's current position.
203 # round up to the nearest power of two, for efficient FFT-ing
204 nsamps = FFT_tools.ceil_pow_of_two(time*freq)
206 # take some data, keeping the position voltage at it's current value
207 out = numpy.ones((nsamps,), dtype=numpy.uint16) * zpiezo.curPos()
208 if config.TEXT_VERBOSE :
209 print "get %g seconds of data" % time
210 data = zpiezo.ramp(out, freq)
211 data['sample frequency Hz'] = numpy.array([freq])
214 @splittableKwargsFunction(vib_aquire,
215 (vib_analyze.vib_save, 'data'),
216 (vib_analyze.vib_analyze, 'deflection_bits', 'freq'))
219 Wrapper around vib_aquire(), vib_save(), vib_analyze()
221 vib_aquire_kwargs,vib_save_kwargs,vib_analyze_kwargs = \
222 vib._splitargs(vib, kwargs)
223 data = vib_aquire(**vib_aquire_kwargs)
224 vib_analyze.vib_save(data, **vib_save_kwargs)
225 freq = data['sample frequency Hz']
226 deflection_bits = data['Deflection input']
227 Vphoto_var = vib_analyze.vib_analyze(deflection_bits=deflection_bits,
228 freq=freq, **vib_analyze_kwargs)
231 # A few positioning functions, so we can run bump_aquire() and vib_aquire()
232 # with proper spacing relative to the surface.
234 @splittableKwargsFunction()
235 def move_just_onto_surface(stepper, zpiezo, Depth_nm=-50, setpoint=2) :
237 Uses z_piezo_utils.getSurfPos() to pinpoint the position of the
238 surface. Adjusts the stepper position as required to get within
239 stepper_tol nm of the surface. Then set Vzp to place the
240 cantilever Depth_nm onto the surface. Negative Depth_nm values
241 will place the cantilever that many nm _off_ the surface.
243 If getSurfPos() fails to find the surface, backs off (for safety)
244 and steps in (without moving the zpiezo) until Vphoto > setpoint.
246 stepper_tol = 250 # nm, generous estimate of the fullstep stepsize
248 if config.TEXT_VERBOSE :
249 print "moving just onto surface"
251 if config.TEXT_VERBOSE :
252 print "zero the z piezo output"
253 zpiezo.jumpToPos(zpiezo.pos_nm2out(0))
254 # See if we're near the surface already
255 if config.TEXT_VERBOSE :
256 print "See if we're starting near the surface"
258 dist = zpiezo.pos_out2nm( \
259 z_piezo_utils.getSurfPos(zpiezo, zpiezo.def_V2in(setpoint))
261 except (z_piezo_utils.tooClose, z_piezo_utils.poorFit), string :
262 if config.TEXT_VERBOSE :
263 print "distance failed with: ", string
264 print "Back off 200 half steps"
265 # Back away 200 steps
266 stepper.step_rel(-400)
267 stepper.step_rel(200)
268 sp = zpiezo.def_V2in(setpoint) # sp = setpoint in bits
269 zpiezo.updateInputs()
270 cd = zpiezo.curDef() # cd = current deflection in bits
271 if config.TEXT_VERBOSE :
272 print "Single stepping approach"
274 if config.TEXT_VERBOSE :
275 print "deflection %g < setpoint %g. step closer" % (cd, sp)
276 stepper.step_rel(2) # Full step in
277 zpiezo.updateInputs()
279 # Back off two steps (protecting against backlash)
280 if config.TEXT_VERBOSE :
281 print "Step back 4 half steps to get off the setpoint"
282 stepper.step_rel(-200)
283 stepper.step_rel(196)
284 # get the distance to the surface
285 zpiezo.updateInputs()
286 if config.TEXT_VERBOSE :
287 print "get surf pos, with setpoint %g (%d)" % (setpoint, zpiezo.def_V2in(setpoint))
288 for i in range(20) : # HACK, keep stepping back until we get a distance
290 dist = zpiezo.pos_out2nm( \
291 z_piezo_utils.getSurfPos(zpiezo,zpiezo.def_V2in(setpoint)))
292 except (z_piezo_utils.tooClose, z_piezo_utils.poorFit), string :
293 stepper.step_rel(-200)
294 stepper.step_rel(198)
298 print "tried %d times, still too close! bailing" % i
299 print "probably an invalid setpoint."
300 raise Exception, "weirdness"
301 if config.TEXT_VERBOSE :
302 print 'distance to surface ', dist, ' nm'
303 # fine tune the stepper position
304 while dist < -stepper_tol : # step back if we need to
305 stepper.step_rel(-200)
306 stepper.step_rel(198)
307 dist = zpiezo.pos_out2nm( \
308 z_piezo_utils.getSurfPos(zpiezo, zpiezo.def_V2in(setpoint)))
309 if config.TEXT_VERBOSE :
310 print 'distance to surface ', dist, ' nm, step back'
311 while dist > stepper_tol : # and step forward if we need to
313 dist = zpiezo.pos_out2nm( \
314 z_piezo_utils.getSurfPos(zpiezo, zpiezo.def_V2in(setpoint)))
315 if config.TEXT_VERBOSE :
316 print 'distance to surface ', dist, ' nm, step closer'
317 # now adjust the zpiezo to place us just onto the surface
318 target = dist + Depth_nm
319 zpiezo.jumpToPos(zpiezo.pos_nm2out(target))
321 if config.TEXT_VERBOSE :
322 print "We're %g nm into the surface" % Depth_nm
324 @splittableKwargsFunction()
325 def move_far_from_surface(stepper, um_back=50) :
327 Step back a specified number of microns.
328 (uses very rough estimate of step distance at the moment)
331 steps = int(um_back*1000/step_nm)
332 print "step back %d steps" % steps
333 stepper.step_rel(-steps)
336 # and finally, the calib family
338 @splittableKwargsFunction((move_just_onto_surface, 'stepper', 'zpiezo'),
339 (bump, 'zpiezo', 'log_dir', 'Vphoto_in2V'),
340 (move_far_from_surface, 'stepper'),
342 (vib, 'zpiezo', 'log_dir', 'Vphoto_in2V'),
343 (analyze.calib_save, 'bumps','Ts','vibs','log_dir'))
344 def calib_aquire(stepper, zpiezo, num_bumps=10, num_Ts=10, num_vibs=20,
345 log_dir=config.LOG_DIR, Vphoto_in2V=config.Vphoto_in2V,
348 Aquire data for calibrating a cantilever in one function.
349 return (bump, T, vib), each of which is an array.
351 stepper a stepper.stepper_obj for coarse Z positioning
352 zpiezo a z_piezo.z_piezo for fine positioning and deflection readin
353 num_bumps number of 'bumps' (see Outputs)
354 num_temps number of 'Ts' (see Outputs)
355 num_vibs number of 'vib's (see Outputs)
356 log_dir directory to log data to. Default 'None' disables logging.
357 Vphoto_in2V function to convert photodiode input bits to Volts
359 + other kwargs. Run calib_aquire._kwargs(calib_aquire) to see
360 all options. Run calib_aquire._childSplittables to see a list
361 of kwarg functions that this function calls.
363 Outputs (all are arrays of recorded data) :
364 bumps measured (V_photodiode / nm_tip) proportionality constant
365 Ts measured temperature (K)
366 vibs measured V_photodiode variance in free solution
368 move_just_onto_surface_kwargs,bump_kwargs,move_far_from_surface_kwargs, \
369 T_kwargs,vib_kwargs,calib_save_kwargs = \
370 calib_aquire._splitargs(calib_aquire, kwargs)
372 bumps = numpy.zeros((num_bumps,), dtype=numpy.float)
373 for i in range(num_bumps) :
374 move_just_onto_surface(stepper, zpiezo, **move_just_onto_surface_kwargs)
375 bumps[i] = bump(zpiezo=zpiezo, log_dir=log_dir,
376 Vphoto_in2V=Vphoto_in2V, **bump_kwargs)
377 if config.TEXT_VERBOSE :
380 move_far_from_surface(stepper, **move_far_from_surface_kwargs)
383 Ts = numpy.zeros((num_Ts,), dtype=numpy.float)
384 for i in range(num_Ts) :
385 Ts[i] = T(**T_kwargs)
386 time.sleep(1) # wait a bit to get an independent temperature measure
390 vibs = numpy.zeros((num_vibs,), dtype=numpy.float)
391 for i in range(num_vibs) :
392 vibs[i] = vib(zpiezo=zpiezo, log_dir=log_dir, Vphoto_in2V=Vphoto_in2V,
396 analyze.calib_save(bumps, Ts, vibs, log_dir, **calib_save_kwargs)
398 return (bumps, Ts, vibs)
401 @splittableKwargsFunction( \
402 (calib_aquire, 'log_dir'),
403 (analyze.calib_analyze, 'bumps','Ts','vibs'))
404 def calib(log_dir=config.LOG_DIR, **kwargs) :
406 Calibrate a cantilever in one function.
407 The I-don't-care-about-the-details black box version :p.
412 k cantilever spring constant (in N/m, or equivalently nN/nm)
413 k_s standard deviation in our estimate of k
415 See get_calibration_data() for the data aquisition code
416 See analyze_calibration_data() for the analysis code
418 calib_aquire_kwargs,calib_analyze_kwargs = \
419 calib._splitargs(calib, kwargs)
420 a, T, vib = calib_aquire(**calib_aquire_kwargs)
421 k,k_s,ps2_m, ps2_s,T_m,T_s,one_o_Vp2_m,one_o_Vp2_s = \
422 analyze.calib_analyze(a, T, vib, **calib_analyze_kwargs)
423 analyze.calib_save_analysis(k, k_s, ps2_m, ps2_s, T_m, T_s,
424 one_o_Vp2_m, one_o_Vp2_s, log_dir)