Method for monitoring a linear compressor

1. A method for monitoring a linear compressor, comprising:

measuring a current and a voltage though a driving coil of the linear compressor;

determining a velocity dependent induced voltage in the driving coil based at least in part on the current and voltage through the driving coil;

extracting a higher order harmonic from the velocity dependent induced voltage, extracting the higher order harmonic comprising

multiplying a signal of the velocity dependent induced voltage in the driving coil by a sinusoidal function having a frequency corresponding to the higher order harmonic and

integrating a product from said step of multiplying over a period of a fundamental frequency of the signal of the velocity dependent induced voltage in the driving coil; and

establishing that a piston of the linear compressor is crashing if the higher order harmonic is greater than a reference value.

2. The method of claim 1 , wherein the higher order harmonic is at least a third order harmonic.

3. The method of claim 1 , wherein the higher order harmonic is at least a fifth order harmonic.

4. The method of claim 1 , wherein said step of determining comprises determining the velocity dependent induced voltage in the driving coil with the following:

V

i

⁡

(

ⅆ

x

ⅆ

t

)

=

V

-

i

⁢

⁢

R

-

L

⁢

ⅆ

i

ⅆ

t

where

V i (dx/dt) is the velocity dependent induced voltage in the driving coil,

V is the voltage through the driving coil,

i is the current through the driving coil,

R is a resistance of the driving coil,

L is an inductance of the driving coil, and

di/dt is a change in the current through the driving coil with respect to time.

5. The method of claim 1 , further comprising reducing a current supplied to the driving coil if the higher order harmonic is greater than the reference value at said step of establishing.

6. The method of claim 5 , further comprising repeating said steps of measuring, determining, extracting and establishing after said step of reducing.

7. The method of claim 1 , wherein said step of establishing comprises establishing that the piston of the linear compressor is crashing if the higher order harmonic is greater than the reference value or that the piston of the linear compressor is not crashing if the higher order harmonic is less than the reference value.

8. The method of claim 1 , further comprising

reducing a current supplied to the driving coil until the piston of the linear compressor is not crashing; and

adjusting the reference value based at least in part on the current supplied to the driving coil at said step of reducing.

9. A linear compressor, comprising:

a cylinder assembly defining a chamber;

a piston assembly having a piston head slidably received within the chamber of the cylinder assembly, the piston assembly also having a magnet;

a driving coil positioned adjacent the magnet of the piston assembly, a magnetic field of the driving coil engaging the magnet of the piston assembly in order to move the piston within the chamber of the cylinder during operation of the driving coil; and

a controller in operative communication with the driving coil, the controller programmed for

ascertaining a current and a voltage though the driving coil;

determining a velocity dependent induced voltage in the driving coil based at least in part on the current and voltage through the driving coil;

extracting a higher order harmonic from the velocity dependent induced voltage, extracting the higher order harmonic comprising

multiplying a signal of the velocity dependent induced voltage in the driving coil by a sinusoidal function having a frequency corresponding to the higher order harmonic and

integrating a product from said step of multiplying over a period of fundamental frequency of the signal of the velocity dependent induced voltage in the driving coil; and

establishing that the piston is crashing if the higher order harmonic is greater than a reference value.

10. The linear compressor of claim 9 , wherein the higher order harmonic is at least a third order harmonic.

11. The linear compressor of claim 9 , wherein the higher order harmonic is at least a fifth order harmonic.

12. The linear compressor of claim 9 , wherein said step of determining comprises determining the velocity dependent induced voltage in the driving coil with the following:

V

i

⁡

(

ⅆ

x

ⅆ

t

)

=

V

-

i

⁢

⁢

R

-

L

⁢

ⅆ

i

ⅆ

t

where

V i (dx/dt) is the velocity dependent induced voltage in the driving coil,

V is the voltage through the driving coil,

i is the current through the driving coil,

R is a resistance of the driving coil,

L is an inductance of the driving coil, and

di/dt is a change in the current through the driving coil with respect to time.

13. The linear compressor of claim 9 , wherein the controller is further programmed for reducing a current supplied to the driving coil if the higher order harmonic is greater than the reference value at said step of establishing.

14. The linear compressor of claim 13 , wherein the controller is further programmed for repeating said steps of measuring, determining, extracting and establishing after said step of reducing.

15. The linear compressor of claim 9 , wherein said step of establishing comprises establishing that the piston is crashing if the higher order harmonic is greater than the reference value or that the piston is not crashing if the higher order harmonic is less than the reference value.

16. The linear compressor of claim 9 , wherein the controller is further programmed for

reducing a current supplied to the driving coil until the piston is not crashing; and

adjusting the reference value based at least in part on the current supplied to the driving coil at said step of reducing.