Control techniques for motor driven systems utilizing back-EMF measurement techniques

1. A method for determining a resonant frequency of a motor-driven mechanical system, the method comprising:

driving a current step into the motor-driven mechanical system;

during a sample period:

sampling a back-EMF signal of the motor-driven mechanical system twice in succession, and

generating a binary signal from the two successive samples;

identifying first and second occurrences of a same type of predetermined event from the generated binary signal; and

determining the resonant frequency of the mechanical system based on a time that elapses between the first and second occurrences of the same type of predetermined event in the back-EMF signal.

2. The method of claim 1 , wherein the predetermined event is a polarity change in the back-EMF signal.

3. The method of claim 1 , wherein the determination of the resonant frequency is further based on the time that elapses between a first type of polarity change in the back-EMF signal and a subsequent occurrence of the same type of polarity change in the back-EMF signal.

4. The method of claim 1 , wherein the back-EMF signal is detected from a node joining the motor-driven mechanical system and a driver of the motor-driven mechanical system.

5. The method of claim 1 , wherein the predetermined event is a transition in the back-EMF signal.

6. An automatic resonance detection and driving system for a motor-driven mechanical system, comprising:

a driver, including:

a register to store a resonant frequency of the motor-driven mechanical system; and

a drive-signal generator that generates a drive signal based on the resonant frequency;

a back-EMF detector, including:

a pair of sampling capacitors having inputs each coupled to a common node of the driver by respective sampling switches and coupled to each other by a respective drive switch,

a comparator, having inputs coupled to outputs of the sampling capacitors, and

a controller to generate sampling control signals to the respective sampling switches in succession with each other in a sampling phase and to generate an output control signal to the drive switch in an output phase; and

an analyzer that determines the resonant frequency of the motor-driven mechanical system based on timing of successive occurrences of a same type of predetermined event from the sampling control signals generated during the sampling phase.

7. The system of claim 6 , wherein the predetermined event is a polarity change in the back-EMF signal.

8. The system of claim 6 , further comprising a slope detector that detects a slope of the back-EMF signal.

9. The system of claim 8 , wherein the predetermined event is a polarity change in the slope of the back-EMF signal.

10. The system of claim 8 , wherein the slope detector is a comparator having an offset less than a maximum change in voltage of the back-EMF signal.

11. The system of claim 8 , wherein the slope detector comprises:

a comparator;

a first switch connected to an input of the slope detector;

a second switch connected to the input and in parallel with the first switch;

a first capacitor connected between the first switch and a first input to the comparator;

a second capacitor connected between the second switch a second input to the comparator; and

a third switch connected between input terminals of the first capacitor and input terminals of the second capacitor;

a fourth switch connecting an output terminal of the first capacitor to a reference voltage; and

a fifth switch connecting an output terminal of the second capacitor to the reference voltage.

12. An automatic resonance detection and driving system for a motor-driven mechanical system, comprising:

a driver including:

a register to store a resonant frequency of the motor-driven mechanical system; and

a drive-signal generator that generates a drive signal to a motor based on the resonant frequency;

a resonance detector, including:

a pair of sampling capacitors having inputs, each coupled to a separate node of a circuit occupied by the driver and the motor by respective sampling switches, and coupled to each other by a respective drive switch, and

a comparator, having inputs coupled to outputs of the sampling capacitors, and

a controller to generate sampling control signals to the respective sampling switches in a sampling phase and to generate an output control signal to the drive switch in an output phase; and

an analyzer that determines the resonant frequency of the motor-driven mechanical system based on timing of successive occurrences of a same type of predetermined event from the sampling control signals generated during the sampling phase.

13. The system of claim 12 , wherein the resonance detector comprises:

a comparator;

a first capacitor having an input terminal connected to a first input of the resonance detector and an output terminal connected to a first input to the comparator;

a second capacitor having an input terminal connected to a second input of the resonance detector and an output terminal connected to a second input to the comparator;

a first switch connecting the output terminal of the first capacitor to a reference voltage; and

a second switch connecting the output terminal of the second capacitor to the reference voltage.

14. The system of claim 13 , further comprising:

an input filter including:

a first resistor having a first end connected to a first input;

a second resistor having a first end connected to a second input; and

a first capacitor connecting a second end of the first resistor to a second end of the second resistor; and

an input sampler including:

a third capacitor having a first end connected to the second end of the first resistor of the input filter;

a fourth capacitor having a first end connected to the second end of the second resistor of the input filter;

a first switch connecting a second end of the third capacitor to a first reference voltage; and

a second switch connecting a second end of the fourth capacitor to a second reference voltage;

wherein a second end of the third capacitor is coupled to a first input of the comparator of the first stage of the slope detector and a second end of the fourth capacitor is coupled to a second input of the comparator of the first stage of the slope detector.

15. The system of claim 12 , wherein the resonance detector comprises, a multi-stage detector, each stage having:

a comparator;

a first capacitor with a first end connected to a first output of the comparator;

a second capacitor with a first end connected to a second output of the comparator;

a first switch connecting the first end of the first capacitor to a first reference voltage;

a second switch connecting the first end of the second capacitor to a second reference voltage;

wherein the second end of the first capacitor of the first stage is a first input to the comparator of the second stage and the second end of the second capacitor of the first stage is a second input to the comparator of the second stage.

16. The system of claim 15 , wherein the slope detector has four stages.

17. A resonance detector, comprising:

a counter driven by a clock signal,

a detector coupled to a source of a back-EMF signal generated by an oscillating system, the detector configured to, during a sampling period:

sample the back-EMF signal generated by the oscillating system twice in succession, and

generate a binary signal from the two successive samples, and a controller to:

update the counter based the generated binary signal of each sample period,

detect first and second occurrences of a same type of predetermined event from the counter,

wherein a time that elapses between the first and second occurrences of a same type of predetermined event represents a resonant period of the oscillating system.

18. The resonance detector of claim 17 , wherein the detector comprises:

a comparator;

a first switch connected to an input of the slope detector;

a second switch connected to the input and in parallel with the first switch;

a first capacitor connected between the first switch and a first input to the comparator;

a second capacitor connected between the second switch a second input to the comparator; and

a third switch connecting input terminals of the first capacitor and the second capacitor together.

19. The resonance detector of claim 17 , wherein the detector comprises:

a comparator;

a first capacitor having an input terminal connected to a first input of the slope detector and an output terminal connected to a first input to the comparator;

a second capacitor having an input terminal connected to a second input of the slope detector and an output terminal connected to a second input to the comparator;

a first switch connecting the output terminal of the first capacitor to a reference voltage; and

a second switch connecting the output terminal of the second capacitor to the reference voltage.

20. A method for determining the resonant frequency of a motor-driven mechanical system, the method comprising:

driving a current step into the motor-driven mechanical system;

during a sample period:

sampling a back-EMF signal from two separate nodes of the motor-driven mechanical system;

generating a binary signal from the two samples;

identifying first and second occurrences of a same type of predetermined event from the generated binary signal; and

determining the resonant frequency of the mechanical system based on a time that elapses between the first and second occurrences of the same type of predetermined event in the back-EMF signal.

21. The method of claim 20 , wherein the back-EMF signal is represented by a pair of voltages sampled from the two nodes.

22. The method of claim 20 , wherein the back-EMF signal is represented by a pair of voltages sampled from the two nodes, then filtered to obtain a signal representing a slope of the back-EMF.