Failure detection in lighting system

1. A method of detecting a failed luminaire and/or light sensor in a lighting system comprising M luminaires and N light sensors, wherein the elements D sl of an N×M transfer matrix D expresses how each luminaire l affect a light sensor s with respect to sensed light intensity, wherein s=1, 2, . . . , N, and l=1, 2, . . . , M, and wherein each luminaire is controllable with respect to intensity of light emitted by the luminaire and each light sensor is adapted to sense intensity of light, the method comprising the steps of:

setting the intensity of light emitted by the respective luminaires in the lighting system to a first intensity setting according to the respective elements of a first intensity vector I (1) ,

determining a first sensor intensity vector E (1) , each element of E (1) corresponding to the intensity of light measured by the respective light sensors when the luminaires emit light according to the first intensity setting,

setting the intensity of light emitted by the respective luminaires to at least a second intensity setting according to the respective elements of a second intensity vector I (2) , wherein at least one element of I (2) differs from a corresponding element or elements in I (1) ,

determining a second sensor intensity vector E (2) , each element of E (2) corresponding to the intensity of light measured by the respective light sensors when the luminaires emit light according to the second intensity setting,

determining fault detection vectors from a difference between

E(1) and D·I (1) , and between

E(2) and D·I (2) , respectively, and

identifying a failed luminaire and/or light sensor on basis of a difference between corresponding elements of the respective fault detection vectors.

2. A method according to claim 1 , wherein the determination of the second sensor intensity vector E (2) is performed successive to the determination of the first sensor intensity vector E (1) .

3. A method according to claim 1 , wherein the step of identifying a failed luminaire and/or light sensor comprises determining a fail vector F based on the determined sensor intensity vectors E (1) , E (2) , the light intensity vectors I (1) , I (2) and the transfer matrix D, wherein the fail vector F indicates if a failed luminaire and/or light sensor is present in the lighting system, wherein the fail vector F is determined from a difference between a first fail vector F (1) and a second fail vector F (2) , wherein

F (1)= E (1)− D·I (1) , and

F (2)= E (2)− D·I (2) ),

wherein a non-zero fail vector F indicates the presence of a failed luminaire and/or light sensor in the lighting system.

4. A method according to claim 3 , wherein a threshold level for the fail vector F is set, and wherein values of elements in the fail vector above said threshold indicates the presence of a failed luminaire and/or light sensor in the lighting system.

5. A method according to claim 3 , wherein the step of identifying a failed luminaire and/or light sensor comprises identifying the largest element in the fail vector F.

6. A method according to claim 5 , wherein if the fail vector F is non-zero, and on a condition that D is invertible, failed luminaires are identified by non-zero elements in

d=D− 1· F.

7. A method according to claim 5 , wherein the step of identifying a failed luminaire comprises defining vectors x j for all luminaires, wherein x j j=I j (2) −I j (1) , where x l j =0 for 1≠j, and a failed luminaire j* is identified by j*=argmin j ∥F−D·x j ∥ 2 , where ∥···∥ denotes the norm of F−D·x j .

8. A method according to claim 7 , wherein the step of identifying a failed luminaire and/or light sensor comprises a step of comparing two hypotheses, wherein a first hypothesis is based on identifying a most likely to have failed light sensor s* as the largest element of the fail vector F, and defining a second fail vector F s* representing the fail vector F with the element s* removed, and wherein a second hypothesis is based on identifying a most likely to have failed luminaire j* by determining j*=argmin j ∥F−D·x j ∥ 2 .

9. A method according to claim 8 , wherein the error after the first hypothesis is determined as ∥F s* ∥ 2 , and the error after the second hypothesis is determined as ∥F−D·x j* ∥ 2 , where ∥···∥ denotes the norm of F s* and F−D·x j* , respectively, and wherein x j* is determined from x j for j*.

10. A method according to claim 8 , wherein the light sensor s* is concluded to have failed if the error after the first hypothesis is smaller than the error after the second hypothesis, and wherein the luminaire j* is concluded to have failed if the error after the first hypothesis is larger than the error after the second hypothesis.

11. A method according to claim 10 , further comprising eliminating the failed sensor and/or luminaire from consideration in further detection of a failed luminaire and/or light sensor in the lighting system according to the steps of a method.

12. A method according to claim 11 , further comprising a step of sending information about an identified failed luminaire and/or light sensor to an external receiver.

13. A failure resistant lighting system comprising M luminaires (l) and N light sensors (s) and a control unit, wherein each luminaire is controllable with respect to intensity of light emitted by the luminaire and each light sensor is adapted to sense intensity of light, and the control unit is adapted to control operation of the luminaires and light sensors, wherein the elements D sl of an N×M transfer matrix D expresses how each luminaire l affect a light sensor s with respect to sensed light intensity, wherein s=1, 2, . . . , N, and l=1, 2, . . . , M, and wherein the control unit further is adapted to:

set the intensity of light emitted by the respective luminaires in the lighting system to a first intensity setting according to the respective elements of a first intensity vector I(1) I(1),

determine a first sensor intensity vector E (1) , each element of E (1) corresponding to the intensity of light measured by the respective light sensors when the luminaires emit light according to the first intensity setting,

set the intensity of light emitted by the respective luminaires to at least a second intensity setting according to the respective elements of a second intensity vector I (2) , wherein at least one element of I (2) differs from a corresponding element or elements in I (1) ,

determine a second sensor intensity vector E (2) , each element of E (2) corresponding to the intensity of light measured by the respective light sensors when the luminaires emit light according to the second intensity setting,

determine fault detection vectors from a difference between

E(1) and D·I (1) , and between

E(2) and D·I (2) , respectively, and

identify a failed luminaire and/or light sensor on basis of a difference between corresponding elements of the respective fault detection vectors.

14. A failure resistant lighting system according to claim 13 , further comprising a communication unit adapted to communicate information about an identified failed luminaire or light sensor to an external receiver.

15. A non-transitory computer program product adapted to, when executed in a processing unit, perform a method according to claim 12 .

16. A non-transitory computer-readable storage medium on which there is stored a computer program product adapted to, when executed in a processing unit, perform a method according to claim 12 .