Differences

This shows you the differences between two versions of the page.

--- qm [2025/03/28 12:38] – pklapetek
+++ qm [2025/03/28 13:34] (current) – pklapetek
@@ Line 4: / Line 4: @@
 Goal of the quantitative mechanics measurement regime is to evaluate local mechanical properties using off-resonance tapping (ORT) technique for feedback and evaluating the individual force-distance curves obtained during this process.
-To make it work, a periodic excitation in the range of hundreds of nanometers has to be added to the cantilever motion, which allows the probe to snap in contact, measure a force-distance dependence in the contact, determine the maximum value of force for feedback loop, leave the contact providing an adhesion value and then to repeate this process again and again. To make it work we need to use an additional shaker piezo or to combine internally the signal generator 1 (from lock-in 1) with 20-bit z-piezo signal and shake with the whole z-piezo.
+To make it work, a periodic excitation in the range of hundreds of nanometers has to be added to the cantilever motion, which allows the probe to snap in contact, measure a force-distance dependence in the contact, determine the maximum value of force for the feedback loop, leave the contact providing an adhesion value and then to repeat this process again and again. To make it work we need to use an additional shaker z-piezo or to combine internally the signal generator 1 (from lock-in 1) with 20-bit z-piezo signal and shake with the whole z-piezo.
-To use this mode with Gwyscope GUI, first adjust the probe signal like you would do it in contact mode. Then switch to “QM” regime (while still being out of contact). Use "Adjust sensor" button to call the QM settings dialogue, which should look like this:
+To use this mode with Gwyscope GUI, first adjust the probe signal the same way as you would do it in the [[contact|contact mode]]. Cross-check in the [[settings|Settings]] (Read and set raw signals dialogue) that a low pass filter is set to some really low frequency, just to remove a mean value from our deflection signal, not to distort it. Then switch to “QM” regime (while still being out of contact). Use "Adjust sensor" button to call the QM settings dialogue, which should look like this:
+{{ :qm.png?direct&400 |}}
 The dialogue shows you a dynamic view on the force-distance curves collected and evaluated. The curves are downsampled to much smaller value than how many data points DSP could provide, so their size is in hundreds of points at maximum. They are shown in time (bottom graph, normalized to show both the shaking piezo and force) and as a force-distance plot (top graph). When you are not in feedback, you will probably see only the sine signal from the piezo and some noise. To see more,
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   * Shaking z-piezo offset (useful e.g. when you need a positive voltage only for it)
   * Shift between the force and shaker z-piezo signal. The maximum values of the curves should match and this parameter can control it.
-  * Deflection sensitivity for the cantilever.
+  * Deflection sensitivity for the cantilever. Note that this parameter significantly affects the shape of the force-distance curve as it is used to convert the probe z-position to probe-sample separation.
   * Stiffness of the cantilever.
   * Tip radius estimate.
@@ Line 26: / Line 27: @@
 The dialogue also shows some of the evaluation results, which can be used to tune the settings. Detected maximum and minimum force is shown in one of the graphs.
-If all of this seems to work and curves are repeatable, include both the part in the contact and out of the contact, you can close the dialogue and start scanning. If the QM mode is selected, your scan list will automatically include the mechanical response signals, like Modulus, Adhesion, etc.
+If all of this seems to work and curves are repeatable, include both the part in the contact and out of the contact, you can close the dialogue and start scanning. If the QM mode is selected, your scan list will automatically include the mechanical response signals, like Modulus, Adhesion, etc. as shown here:
+{{ :qmrun.png?direct&600 |}}
+Here are some troubleshooting tips:
+  * start from larger shaking z-piezo amplitudes to make sure that probe gets out of contact between individual approaches. If it does, you can decrease the amplitude.
+  * Start from lower frequencies and larger forces.
+  * Even if the setpoint is entered in Volts, you can see the force on the graph.
+  * Cross-check if end of your evaluation limit is enough beyond the maximum position, otherwise you will not find the maximum. Check where the evaluated maxima and minima are on the lower graph.
+  * Remember that your feedback loop gets only single number per one period, so it is slower than in contact mode.
+  * To set-up the regime for the first time, use softer cantilever, stiff sample (e.g. any calibration grating) and care only for the feedback and adhesion, not for modulus.
+  * To get a reasonable modulus result, probe should elastically deform the sample, so stiffer cantilever and higher force will be usually better. When you will operate with these settings on a stiff sample, you might easily destroy the probe apex.