Granted Patent B2

US 7,643,525 · App. 11/930,481 · Granted Jan 5, 2010

Abstract

Claims (212)

1. A method of controlling a printhead comprising:

providing a laser source;

providing a plurality of current sources coupled to said laser source including at least a bias current source, a first drive current source and a second drive current source;

defining a set of operating states where each operating state corresponds to a desired laser output power level set by driving said laser source using a corresponding combination of said plurality of current sources;

setting a ratio that establishes a relationship between a first one of said desired laser output power levels and a second one of said desired laser output power levels; and

performing a calibration operation by:

sampling actual laser output power of said laser source for less than all of said operating states in said set;

computing adjustments to the current levels of said plurality of current sources such that the current level of said second drive current source relates to the current level of said first drive current source based upon a function of said ratio and for each operating state that is sampled, the sampled laser power level converges towards a corresponding one of said desired laser output power levels associated with that sampled operating state; and

adjusting said plurality of current sources including said bias current source, said first drive current source and said second drive current source to their corresponding computed current levels.

2. The method according to claim 1 , wherein said defining a set of operating states comprises defining said set as having three operating states including:

a first operating state corresponding to a first desired laser output power level where said combination of first and second drive current sources is set by said first drive current source on and said second drive current source off;

a second operating state corresponding to a second desired laser output power level where said combination of first and second drive current sources is set by said first drive current source off and said second drive current source on; and

a third operating state corresponding to a third desired laser output power level where said combination of first and second current sources is set by said first drive current source on and said second drive current source on.

3. The method according to claim 2 , wherein said sampling actual laser output power of said laser source for less than all of said operating states in said set comprises sampling only for said first and second operating states.

4. The method according to claim 1 , wherein said setting a ratio comprises:

defining said first one of said desired laser output power levels corresponding to setting said first drive current source on and said second drive current source off; and

defining said second one of said laser output power levels corresponding to setting said first drive current source off and said second drive current source on.

5. The method according to claim 1 , wherein said performing a calibration operation comprises:

iteratively repeating said sampling, computing and adjusting operations during a non-printing portion of a scan operation until an error measure characterizing a difference between said sampled laser power level and said corresponding desired laser output level for each sampled operating state converge substantially towards zero.

6. The method according to claim 1 , wherein said performing a calibration operation comprises:

adjusting said bias current level to correspond to a laser threshold current level of said laser source.

7. The method according to claim 1 , wherein said computing adjustments to the current levels of said plurality of current sources comprises:

computing adjustments to the current level of said bias current source by implementing the equation:

ⅆ

i

B

ⅆ

t

=

k

R

-

1

⁢

(

p

2

-

R

·

p

1

)

computing adjustments to the current level of said first drive current source by implementing the equation:

ⅆ

i

1

ⅆ

t

=

k

R

-

1

⁢

(

p

1

-

p

2

-

r

1

+

r

2

)

and

computing adjustments to the current level of said second drive current source by implementing the equation:

i 2 =Ri 1

wherein k is a scalar constant, p 1 is a first sample of laser output power corresponding to a first one of said operating states, p 2 is a second sample of laser output power corresponding to a second one of said operating states, R is said ratio, r 1 corresponds to said desired laser output power at said first operating state, r 2 corresponds to said desired laser output power at said second operating state, i 1 is the adjusted current level of said first drive current source and i 2 is the adjusted current level of said second drive current source.

8. A system for controlling a printhead comprising:

a laser source;

a plurality of current sources coupled to said laser source including at least a bias current source, a first drive current source and a second drive current source;

a controller that:

controls said plurality of current sources during imaging operations to write print elements corresponding to associated image data and a set of operating states where each operating state corresponds to a desired laser output power level set by driving said laser source using a corresponding combination of said plurality of current sources; and

performs a calibration operation during a non-printing portion of a scan operation that:

samples actual laser output power of said laser source for less than all of said operating states in said set;

computes adjustments to the current levels of said plurality of current sources such that the current level of said second drive current source is a ratiometric function of the current level of said first drive current source and for each operating state that is sampled, the sampled laser power level converges towards a corresponding one of said desired laser output power levels associated with that sampled operating state; and

adjusts said plurality of current sources including said bias current source, said first drive current source and said second drive current source to their corresponding computed current levels.

9. The system according to claim 8 , further comprising:

a first switch coupled between said laser source and said first drive current source, having a first switch control terminal;

a first control signal coupled between said controller and said first switch control terminal, said first control signal controlling said first switch to selectively couple said first drive current source to said laser source or a first dummy load;

a second switch coupled between said laser source and said second drive current source, having a second switch control terminal;

a second control signal coupled between said controller and said second switch control terminal, said second control signal controlling said second switch to selectively couple said second drive current source to said laser source or a second dummy load;

a third control signal coupled between said controller and said bias current source through which said controller programmably adjusts the current level of said bias current source;

a fourth control signal coupled between said controller and said first drive current source through which said controller programmably adjusts the current level of said first drive current source; and

a fifth control signal coupled between said controller and said second drive current source through which said controller programmably adjusts the current level of said second drive current source.

10. The system according to claim 8 , wherein said controller comprises a programmed microcontroller.

11. The system according to claim 8 , wherein said controller further comprises:

a sample and hold circuit to sample and store said samples of actual laser output power of said laser source for less than all of said operating states in said set;

sample and hold decode logic to select a particular operating state to collect sample data; and

a compensation circuit that computes said adjustments to the current levels of said plurality of current sources.

12. The system according to claim 11 , wherein said compensation circuit comprises:

a first circuit module that calculates the adjusted current level for said first drive current source;

a second circuit module that calculates the adjusted current level for said bias current source; and

a third circuit module that provides the adjusted current level for said second drive current source by scaling the adjusted current level of said first laser source by a predetermined ratio.

13. The system according to claim 12 , wherein said third circuit module comprises a voltage divider that divides a value of a control signal coupled to said first drive current source.

14. The system according to claim 8 , wherein said controller iteratively samples, computes and adjusts during said calibration operation until an error measure characterizing a difference between said sampled laser power level and said corresponding desired laser output power level for each sampled operating state converge substantially towards zero.

15. The system according to claim 8 , wherein said bias current level is adjusted during said calibration operation to correspond to a laser threshold current level of said laser source.

16. The system according to claim 8 , wherein:

adjustments to the current level of said bias current source are implemented by the equation:

ⅆ

i

B

ⅆ

t

=

k

R

-

1

⁢

(

p

2

-

R

·

p

1

)

adjustments to the current level of said first drive current source are implemented by the equation:

ⅆ

i

1

ⅆ

t

=

k

R

-

1

⁢

(

p

1

-

p

2

-

r

1

+

r

2

)

and

adjustments to the current level of said second drive current source are implemented by the equation:

i 2 =Ri 1

wherein k is a scalar constant, p 1 is a first sample of laser output power corresponding to a first one of said operating states, p 2 is a second sample of laser output power corresponding to a second one of said operating states, R is said ratio, r 1 corresponds to said desired laser output power at said first operating state, r 2 corresponds to said desired laser output power at said second operating state, i 1 is the adjusted current level of said first drive current source and i 2 is the adjusted current level of said second drive current source.

17. A control system for controlling laser power in a printhead comprising:

a first memory for storing a set of operating states where each operating state corresponds to a stored value designating a desired laser output power level set by driving a laser source using a corresponding combination of current sources selected from a plurality of current sources including at least a bias current source, a first drive current source and a second drive current source;

a processor that performs a calibration operation that:

requests samples of actual laser output power of said laser source for less than all of said operating states in said set;

stores received sample data in a second memory;

computes adjustments to the current levels of said plurality of current sources such that the current level of said second drive current source is a ratiometric function of the current level of said first drive current source and for each operating state that is sampled, the stored sample data in said second memory converges towards a corresponding one of said desired laser output power levels associated with that sampled operating state; and

outputs a control signal to adjust said plurality of current sources including said bias current source, said first drive current source and said second drive current source to their corresponding computed current levels.

18. The control system according to claim 17 , wherein said processor iteratively samples, computes and adjusts during said calibration operation until an error measure characterizing a difference between said sampled laser power level and said corresponding desired laser output power level for each sampled operating state converge substantially towards zero.

19. The control system according to claim 17 , wherein said bias current level is adjusted during said calibration operation to correspond to a laser threshold current level of said laser source.

20. The system according to claim 8 , wherein:

adjustments to the current level of said bias current source are implemented by the equation:

ⅆ

i

B

ⅆ

t

=

k

R

-

1

⁢

(

p

2

-

R

·

p

1

)

adjustments to the current level of said first drive current source are implemented by the equation:

ⅆ

i

1

ⅆ

t

=

k

R

-

1

⁢

(

p

1

-

p

2

-

r

1

+

r

2

)

and

adjustments to the current level of said second drive current source are implemented by the equation:

i 2 =Ri 1

wherein k is a scalar constant, p 1 is a first sample of laser output power corresponding to a first one of said operating states, p 2 is a second sample of laser output power corresponding to a second one of said operating states, R is said ratio, r 1 corresponds to said desired laser output power at said first operating state, r 2 corresponds to said desired laser output power at said second operating state, i 1 is the adjusted current level of said first drive current source and i 2 is the adjusted current level of said second drive current source.

Assignments (7)