Digital Q control for enhanced measurement capability in cantilever-based instruments
View Patent ↗A digital system for controlling the quality factor in a resonant device. The resonant device can be a a device that includes a cantilever within its system, such as an atomic force microscope. The quality factor can be digitally controlled to avoid noise effect in the analog components. A direct digital synthesizer implemented in a way that provides access to the output of the phase accumulator. That output is a number which usually drives a lookup table to produce a cosine or sine output wave. The output wave is created, but the number is also adjusted to form a second number that drives a second lookup table to create an adjustment factor to adjust the output from the cosine table. The adjusted digital signal than drives a DA converter which produces an output drive for the cantilever.
1. A system for driving a cantilever of an atomic force microscope, comprising:
a first lookup table, driven by a value indicative of a phase of said atomic force microscope, to create a first sinusoidal output driving signal for said atomic force microscope;
a second lookup table, to create a second sinusoidal output driving signal for said atomic force microscope, driven by a second signal;
a first adder, configured to receive said value indicative of the phase of said atomic force microscope, and to add a cantilever phase adjustment value to said value to create said second signal which drives said second lookup table, wherein said first adder operates to create said second signal without any vector operations; and
a digital adder, creating a drive signal for driving the cantilever, by adding said first and second sinusoidal outputs.
2. The system as in claim 1 , further comprising an accumulator maintaining information indicative of a current phase of the cantilever, to create said value.
3. The system as in claim 2 , wherein said first and second sinusoidal output driving signals are cosine signals that are determined based on the value.
4. The system as in claim 2 , wherein said first lookup table and said accumulator are part of a direct digital synthesizer.
5. The system as in claim 1 , further comprising a digital multiplier, receiving said second sinusoidal output driving signal, and multiplying said second sinusoidal output driving signal by a gain adjustment value, to create a gain adjusted signal, and wherein said gain adjusted signal is sent to said digital adder.
6. The system as in claim 1 , further comprising a D/A converter, receiving an output of said digital adder, and creating an analog drive signal for driving the cantilever based on said output of said digital adder.
7. The system as in claim 6 , further comprising the cantilever, receiving said analog drive signal, wherein said cantilever is moved based on said analog drive signal.
8. The system as in claim 1 , wherein said second lookup table has substantially the same content as said first lookup table.
9. The system as in claim 1 , wherein said first and second lookup tables and said digital adder are part of a direct digital synthesizer that includes a phase accumulator that creates said value indicative of phase, and said direct digital synthesizer is implemented such that output access to said phase accumulator is available.
10. The system as in claim 9 , wherein said direct digital synthesizer is implemented in programmable logic.
11. A method for driving a cantilever of an atomic force microscope, comprising:
creating a first sinusoidal output driving signal for said atomic force microscope by applying a value indicative of a phase of the atomic force microscope to a first lookup table;
adding a cantilever phase adjustment value to said value to create a second signal, said adding carried out without any vector operations;
creating a second sinusoidal output driving signal for said atomic force microscope, by applying said second signal to a second lookup table; and
creating a drive signal for driving the cantilever, by digitally adding said first and second sinusoidal outputs.
12. The method as in claim 11 , further maintaining information indicative of the current phase of the cantilever, to create said value.
13. The method as in claim 12 , wherein said first and second sinusoidal output driving signals are cosines based on the value.
14. The method as in claim 11 , further comprising digitally multiplying said second sinusoidal output driving signal by a gain adjustment value, to create a gain adjusted signal, and wherein said gain adjusted signal is used for said creating a drive signal.
15. The method as in claim 11 , further comprising D/A converting an output of said creating a drive signal, and creating an analog drive signal for driving the cantilever based on said D/A converting.
16. The method as in claim 11 , wherein said first and second lookup tables are each part of a direct digital synthesizer.
17. The method as in claim 11 , wherein said second lookup table has substantially the same content as said first lookup table.
18. The method as in claim 11 , further comprising driving the cantilever, using said analog drive signal, wherein said cantilever is driven to move moved based on said analog drive signal.