Position encoder with scale calibration

Details Position encoder with scale calibration Position encoder with scale calibration

2001-09-19

2004-02-03

Pyo, Kevin (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C324S202000

Reexamination Certificate

active

06686585

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a position sensing and/or displacement sensing system such as scale-based encoders, having a signal processor that corrects for scale inaccuracy. More particularly, the correction is based on a limited set of correction coefficients.
2. Related Art
Generally, this invention applies to the field of position sensing and/or displacement sensing systems, specifically scale based encoders or sensors. Scale based sensors are typically optical or magnetic and are characterized by having a scale (a component with “markings” of some sort), and a sensing head to read the markings on the scale. Other types of sensors, such as capacitive probes, measure displacement by the change in strength of some physical parameter, whereas scale based sensors measure displacement by observing the movement of the scale.
In a conventional system, the marks on the scale are periodic, thereby creating a periodic pattern that is observable by the sensor. The measured displacement of the periodic scale is proportional to the number of cycles of the observable pattern that the sensor observes during the displacement.
The accuracy of any particular individual scale-based sensor is affected by the specific scale. The accuracy over ranges which are medium to long relative to the period of the markings is closely related to the accuracy with which the markings are placed on the scale and the flatness of the scale. While the average accuracy of any scale is usually quite good, that is, the total number of marks over the length of the scale is well known, the accumulated error about the average is difficult to maintain at an acceptably low level.
Conventionally, long sensor scales have been calibrated with great care and the calibration data for a particular scale delivered to customers either as certification that the scale meets certain performance levels or as a means for the customer to back out the calibrated errors.
A simplified schematic diagram of an optical sensor
10
is shown in
FIG. 1
for reference. The sensor includes a glass scale
100
on which there is a periodic array
110
of transmissive and opaque regions, a source of illumination
210
that illuminates the scale
100
, an optical detector
250
with detecting elements
220
to sense the position of a fringe pattern
150
created by light passing through the periodic array
110
, and a processor
300
that operates on the signals generated by the detecting elements
220
. When the scale
100
moves relative to the light source/optical detector combination, the fringe pattern
150
moves proportionately. It is therefore motion of the fringe pattern that the sensor
10
uses to estimate displacement.
In addition to the periodic array
110
, the scale
100
may include an indexing or reference mark
125
. This mark identifies a specific known, fixed location along the scale
100
and allows the sensor to uniquely identify one cycle of the periodic array
110
. The presence or passing of this mark is detected, typically by one or more detecting elements which form the index mark sensor
225
, in the detector
250
.
The processor
300
converts the detected signals from the detector
250
into estimates of scale displacement. Various processing algorithms can be used, depending on the specific form of the signals.
According to one method shown in the block diagram in
FIG. 3
, referring to systems such as shown in
FIG. 1
, the processor inputs the fringe data
305
. At block
310
, the algorithm first estimates the location of the scale modulo one period of the periodic array
110
, known as the fractional cycle. At block
320
, it determines if the scale has moved from one period of the array to an adjacent period. At block
330
, it increments or decrements, as appropriate, an accumulator holding the number of periods of the periodic array that have passed since initialization. At block
340
, the processor then adds the fraction cycle calculated in block
310
to the number of full cycles in the accumulator from block
330
. The resulting scale displacement
355
is output.
Preferably, the processor
300
also accepts index mark sensor data signals
325
from the index mark sensor
325
, and at block
315
, it determines whether the index mark is present. Then, at block
350
it uses those signals to generate an initialization pulse to reset the accumulator.
For sensors in which the scale does not have an indexing mark, the accumulator is typically initialized by an external command
315
a
, generally when the scale is positioned at one end
101
of its range of travel, perhaps against a hard stop.
Although a linear displacement sensor is depicted in
FIG. 1
, it will be apparent to one skilled in the art that the same sensing and processing approach and principles are known in connection with rotary motion detection systems, as has been described in the literature.
SUMMARY OF THE INVENTION
The present invention provides a measurement system in which the calibration data is easily incorporated into or obtained by the position sensing and/or displacement sensing system's processing unit, so that the system provides the customer with highly accurate measurements without that customer's active intervention, even where the scale has known inaccuracy. The invention provides not only field replacement of a scale, but also sales of scales independently of sensors, with easy incorporation of the scale's calibration data into the processor.
The invention is a scale-based encoder with a signal processor or other processer that corrects or adjusts for scale inaccuracy based on a limited set of correction coefficients or other adjustment data. The correction coefficients are initially calculated, for example at the factory, but can be loaded subsequently into the encoder, such as when it is in the field. The correction coefficients are the slopes and offsets that provide a piecewise linear correction curve. Other data characteristic of adjustment data may alternatively be used. Several ways for communicating the coefficients to the processor are envisioned. Correction is applicable to linear or rotary encoders. The invention is applicable to alternative position sensors such as capactive encoders, magnetic encoders, inductive encoders, image processing encoders, etc.
In accordance with the invention, there is provided a method and system for detecting relative movement and correcting for scale inaccuracy. A scale is relatively movable with respect to a source with at least one detectable property. A periodic detector includes a sensing region positioned thereon, positioned relative to the scale to detect the detectable property, wherein the periodic detector detects and transmits a measure of displacement of the scale in response to a movement of the scale. A processor is operatively connected to the periodic detector, receiving the measure of displacement from the periodic detector, receiving calibration data corresponding to the scale indicative of an approximation correlating to the scale, the approximation including a plurality of segments, each of the segments corresponding to a portion of the scale, and converting the measure of displacement into a calibrated displacement using the correlation data.
In at least one embodiment, the approximation is linear and piecewise. The approximation may be a higher order approximation.
In at least one embodiment, the detectable property may include a periodic array of alternating regions, and the periodic array is linear.
In at least one embodiment, the detectable property may include a periodic array of alternating regions, and the periodic array is radial.
In at least one embodiment, a center of the radial array and a center of rotation are not coincident.
In at least one embodiment, the periodic detector transmits an analog signal representative of the measure of displacement, and the processor receives the analog signal as the measure of displacement.
In at least one embodiment, the processor further transmits an o

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