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hi, I am using the Multimode SPM with Nanoscope V controller in the tapping and contact mode. I tune the P and I gains in the range suggested in the manual, however I could not find the maximum permissible values for these gains which can be used safely. By safe I mean avoiding damage to the SPM or the Nanoscope controller. Can you please tell me what these are. In addition is it always necessary use an I gain lesser than P?
Umar Khan.
Hi Umar Khan,
In general yes. The feedback loop is programmed in a way that you basically always use a P gain > I gain. Before causing damage to the SPM itself I would more concerned to cause damage to my AFM tip. The gains are too high when you start to see oscillations appear. These will be rather obvious in the error signals. If that appears you will have to reduce your gain settings. If these gain settings do not provide suficient tracking you will have to reduce your scan rate.
Best,
Stefan
hi Stefan, Right, but before I can bring the oscillations out, I need a range in which I can experiment with the P and I gains. Can I use gains as high as 30 without damaging the SPM? I would like to minimize the chances of damaging the tip or cantilever, but in case this happens I can replace these. Also I would like to experiment with I gains higher than P. Can I do this safely?
Umar,
When you start seeing oscillations in your error sigal your gains are too high as your feedback loop is now starting to be unstable. If you continue to increase these gains significantly after you observe oscillations you may actually hear the piezo. This is NOT where you want to be as you may now start to damage your AFM. When you observe that, please back off your gains immediately.
The actual P and I values will strongly depend on which mode and in which medium your are runing your experiment, i.e. contact, tapping, tapping in fluid etc. You can try turning the PI loop compleltly off and you can try to use I values higher than P. Just remember, if you observe instabilities in the feedback loop, back off.
The default settings for gains in Nanoscope's experiments selector are usually quite close to the values necessary for good tracking. So if you goal is to take great AFM images, there is typically very little if any need to deviate from the default parameters by a lot.