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 any mechanically driven resonant device, but more particularly can be 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. One of the controls can use 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 eight lookup table to produce a cosine or sign output wave. The output wave is created, but the number is also adjusted to form a second number of the drives a second lookup table to create an adjustment factor. The adjustment factor is used to adjusts the output from the cosine table, to create an adjusted digital signal. The adjusted digital signal than drives a DA converter which produces an output drive for the cantilever.
1. A system, comprising:
a driving part, that includes an accumulator therein which accumulator has an output signal indicative of a number representing a current phase of driving of a mechanically driven resonant system;
a first lookup table, driven by said number to produce an output signal indicative of a current voltage of driving value associated with said number; and
a phase adjustment circuit, also driven by said number, to create an adjustment to said output signal, based on a desired phase adjustment amount.
2. A system as in claim 1 , wherein said phase adjustment circuit includes digital circuitry.
3. A system as in claim 1 , wherein said phase adjustment circuit includes a first look up table, which is used to change said number by an amount related to a desired phase adjustment to create a phase adjustment signal.
4. A system as in claim 3 , further comprising a second lookup table, driven by said phase adjustment signal to create an output signal.
5. A system as in claim 4 , wherein said second lookup table has substantially the same content as said first lookup table, and creates an output signal indicative of a voltage that corresponds to said phase adjustment signal.
6. A system as in claim 4 , further comprising a gain adjustment part, which adjusts a gain of said output signal of said second lookup table to create a gain adjusted signal.
7. A system as in claim 6 , further comprising an adder which adds said gain adjusted signal to said output signal from said first lookup table.
8. A system as in claim 7 , wherein said adder is a digital adder.
9. A system as in claim 8 , further comprising a D/A converter, receiving an output from said adder, and producing an analog drive signal.
10. A system as in claim 9 , further comprising the mechanically driven resonant system, receiving said analog drive signal.
11. A system as in claim 10 , wherein said mechanically driven resonant system includes a movable cantilever.
12. A system as in claim 11 , wherein said movable cantilever is part of an atomic force microscope.
13. A system as in claim 1 , further comprising an adder, which adds together said output signal from said first lookup table, and said adjustment created by said phase adjustment circuit, to produce a compensated output signal.
14. A system as in claim 13 , wherein said compensated output signal is digital, and further comprising a D/A converter, converting said signal to analog.
15. A system as in claim 13 , further comprising a mechanically driven resonant system, receiving said compensated output signal.
16. A system as in claim 15 , wherein the mechanically driven resonant system includes a cantilever.
17. A system as in claim 15 , wherein the mechanically driven resonant system includes an atomic force microscope, and a cantilever portion of the atomic force microscope.
18. A system as in claim 1 , wherein said driving part and said first lookup table are part of a direct digital synthesizer implemented such that output access to said accumulator is available.
19. A system as in claim 18 , wherein said direct digital synthesizer is implemented in programmable logic.