Measured sensor distance correction

1. A distortion correction device for a web printing press, the device comprising:

a first sensor and a second sensor separated by a calibrated sensor distance, the first and second sensors to detect marks on a substrate as the substrate advances through the press;

an encoder to detect advancement of the substrate; and

a scaling measurement correction module to:

detect a mark on the substrate with the first sensor;

detect the mark on the substrate with the second sensor;

using an output from the encoder, determine a measured distance between the sensors based on detection of the mark at each sensor;

compare the measured distance to the calibrated sensor distance;

determine a correction factor based on the comparison of the measured distance to the calibrated sensor distance; and

adjust operation of the web printing press based on the correction factor.

2. The apparatus of claim 1 , further comprising a temperature sensor placed between the first sensor and the second sensor, wherein the scaling measurement correction module is to adjust the correction factor based on a temperature measured by the temperature sensor.

3. The apparatus of claim 1 , wherein the mark is among a plurality of marks printed on the substrate, and the scaling measurement correction module is to update the correction factor for each mark of the plurality of marks as each mark passes both the first sensor and the second sensor.

4. The apparatus of claim 1 , wherein the encoder is a rotary encoder coupled to a nip roller of the web printing press.

5. The apparatus of claim 1 , wherein the correction factor is the measured distance divided by the calibrated sensor distance.

6. The apparatus of claim 1 , further comprising a web tension controller, the scaling measurement correction module to control the web tension controller using the correction factor for adjustment of tension on the substrate.

7. The apparatus of claim 1 , wherein the scaling measurement correction module is to:

determine a scaling measurement based on an amount of substrate advancement, as detected by the encoder, between detection of the mark by the first sensor and detection of the mark by the second sensor,

convert the scaling measurement to a corrected scaling measurement using the correction factor, and

transmit the corrected scaling measurement to a web tension controller of the web printing press.

8. An apparatus comprising:

a first sensor and a second sensor mounted in a web printing press and separated by a calibrated sensor distance, the first and second sensors to detect marks on a substrate as the substrate advances through the press;

an encoder to detect advancement of the substrate; and

a scaling measurement correction module to:

determine a measured sensor distance based on an amount of substrate advancement, as detected by the encoder, between detection of a mark by the first sensor and detection of the mark by the second sensor,

determine a correction factor to convert the measured sensor distance to the calibrated sensor distance, and

adjust operation of the web printing press based on the correction factor.

9. The apparatus of claim 8 , wherein the scaling measurement correction module is to:

transmit a corrected scaling measurement based on the correction factor to a web tension controller of the web printing press.

10. The apparatus of claim 9 , wherein the scaling measurement correction module is to:

determine the scaling measurement based on an amount of substrate advancement, as detected by the encoder, between detection of the mark by the first sensor and detection of the mark by the second sensor, and

convert the scaling measurement to a corrected scaling measurement using the correction factor.

11. The apparatus of claim 8 , further comprising a temperature sensor placed between the first sensor and the second sensor, wherein the scaling measurement correction module is to adjust the correction factor based on a temperature measured by the temperature sensor.

12. The apparatus of claim 8 , wherein the calibrated sensor distance is accurate to at least seven microns.

13. The apparatus of claim 8 , wherein the mark is among a plurality of marks printed on the substrate, and the scaling measurement correction module is to update the correction factor for each mark of the plurality of marks as each mark passes the first sensor and the second sensor.

14. The apparatus of claim 8 , wherein the encoder is a rotary encoder coupled to a nip roller of the web printing press.

15. The apparatus of claim 8 , wherein the correction factor is the measured sensor distance divided by the calibrated sensor distance.

16. The apparatus of claim 8 , further comprising a web tension controller, the scaling measurement correction module to controller the web tension controller using the correction factor for adjustment of tension on the substrate.

17. A method of distortion correction for a web printing press, the method comprising:

with a first sensor and a second sensor separated by a calibrated sensor distance, the first and second sensors to detect marks on a substrate as the substrate advances through the press, detecting a mark on the substrate with the first sensor;

detecting the mark on the substrate with the second sensor;

with an encoder to detect advancement of the substrate and a scaling measurement correction module, determining a measured distance between the sensors based on detection of the mark at each sensor;

with the measurement correction module, comparing the measured distance to the calibrated sensor distance and determining a correction factor based on the comparison of the measured distance to the calibrated sensor distance; and

with the measurement correction module, adjusting operation of the web printing press based on the correction factor.

18. The method of claim 17 , further comprising adjust the correction factor based on a temperature measured by a temperature sensor placed between the first sensor and the second sensor.

19. The method of claim 17 , wherein the mark is among a plurality of marks printed on the substrate, the method further comprising updating the correction factor for each mark of the plurality of marks as each mark passes both the first sensor and the second sensor.

20. The method of claim 17 , wherein the correction factor is the measured distance divided by the calibrated sensor distance.