Dual sensor encoder to counter eccentricity errors

Details Dual sensor encoder to counter eccentricity errors Dual sensor encoder to counter eccentricity errors

1999-01-19

2001-04-10

Evans, F. L. (Department: 2877)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C341S013000

Reexamination Certificate

active

06215119

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to a dual sensor encoder system that compensates for eccentricity errors that may be introduced during mounting. The dual sensor encoder system may be used to control positioning of a printing drum architecture.
2. Description of Related Art
Printing drum architectures require precise rotational alignment signaling to accurately align and time paper transport and image generation. This is particularly true for multiple head printing systems.
It was previously considered that low-cost encoders could not be used for systems requiring extremely high precision. Instead, if high precision was needed, more precise (and expensive) encoders had to be used. A particularly accurate known encoder is a Teledyne-Gurley encoder. However, such an encoder costs approximately $1000. In one particular drum architecture, it is desirable to have an encoder runout of about 0.5 mRad. However, conventional low-cost encoders typically have a runout error of about 3.0 mRad. As such, one would not expect that a low-cost encoder could work with an architecture having such high accuracy requirements. While a Teledyne-Gurley encoder would be suitable for such an application, it greatly increases the cost of the architecture.
SUMMARY OF THE INVENTION
The worst error from conventional low-cost wheel encoders is believed to be due to eccentric mounting of the codewheel. Thus, to ensure better accuracy, centering tolerances are believed to be critical. However, processes to carefully control the accuracy of codewheel placement on a drum architecture are time consuming, and thus add to the production costs of such drum architectures.
There is a need for a low-cost alternative that can achieve similar precision in accurately determining the location of the drum without the expense of a high cost encoder or extensive time and equipment to control the mounting eccentricity of a low-cost encoder.
One exemplary embodiment of the dual sensor encoder systems and methods of this invention overcome these problems by including two low-cost, lower precision wheel encoders sensors, a first encoder sensor and a second encoder sensor, that are mounted in a certain way on a codewheel to offset eccentric mounting errors. In particular, two low-cost readout sensors can be mounted at opposite sides of the encoder wheel, i.e., 180° apart, to form a dual sensor encoder system. With this arrangement, the effect of any eccentric mounting is calculated to be zero if the angle is taken to be the average of the sensor readings.
One exemplary embodiment of the systems and methods of this invention digitally synthesizes a signal that is phase locked half way in time between the two sensor signals to provide a correction signal. The method counts the time from the first encoder to the second encoder and uses half of the counted time as the next encoder pulse delay from the first encoder. When a leading edge of a pulse of the first encoder rises, an up-counter is cleared and both the up-counter and a down-counter begin to run. When the down-counter reaches zero, the colTected encoder pulse is formed. When the leading edge of a pulse of the second encoder finally rises, the elapsed time difference, as measured by the up-counter, is cut in half and the halved value from the up-counter is pre-loaded into the down-counter. Thus, the down-counter counts off half the previous elapsed time difference.
The up-counter always runs, as it is cleared by the leading edge of the pulse from the first encoder and captured by the leading edge of the pulse from the second encoder. The down-counter, however, only runs in the interval from the leading edge of the pulse from the first encoder until the down-counter reaches zero, which is roughly half of the time to the leading edge of the pulse from the second encoder. This is accomplished by having the next clock pulse after the down-counter reaches zero cause the down-counter to roll over to a maximum value and then disabling the down-counter awaiting reload.
By using this exemplary embodiment of the systems and methods of this invention, a corrected encoder sensor output signal having higher accuracy than that achieved using a single sensor encoder system is obtained.
A second exemplary embodiment of the systems and method of this invention uses the same codewheel and dual sensor encoder system. However, in the first exemplary embodiment, the rising edge of the second encoder sensor will occur after the rising edge of the pulse of the first encoder sensor and before the rising edge of the next first encoder sensor. That is, the rising edge of the pulse from the first encoder sensor will always lead the rising edge of the pulse from the second encoder sensor. However, this assumption may not always be correct if the alignment is substantially off. The second exemplary embodiment of the systems and methods of the invention resolves this problem.
Taking into consideration that the first and second encoder sensor signals can cross each other and assuming that a pair of pulses from the pair of encoder sensors will pass before the next pulse from either encoder sensor will arrive, the second exemplary embodiment of the systems and methods of this invention generates a corrected encoder pulse as follows: the time is counted from the first encoder pulse in the pair to the second encoder pulse in the pair, where the first encoder pulse is that which comes first from the first encoder sensor or the second encoder sensor. Half of this counted time is then used as the next corrected encoder pulse delay from the next encoder pulse that comes in, which can be generated by either the first encoder sensor or the second encoder sensor. This next encoder pulse that comes in is considered the leading signal for the next pair of pulses from the first encoder sensor and the second encoder sensor.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of the exemplary embodiments of the systems and methods according to this invention.


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patent: 5774074 (1998-06-01), Cooper et al.
patent: 0 665 116 (1995-08-01), None
HP Technical Data, “Three Channel Optical Incremental Encoder Modules,” 5965-5870E, pp. 2-52-2-62.
HP Technical Data, “Two and Three Channel Codewheels for use with HP Optical Encoder Modules,” 5965-6891E, pp. 2-127-2-138.

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