WO2014030177A1 - Apparatus and method for the stabilization of linear encoders - Google Patents

Abstract

The apparatus and method are able to concentrate the metrological core in a limited-size single piece of equipment, a bislider, for locally interpolated differential error recovery. A bislider is a low CTE spacer keeping two read-heads (2, 4) at fixed and invariant relative distance; when the bislider slides over the scale (1), two signals are produced by the read-heads. If required, thermometers may be applied to connections (6) for the compensation of the bislider expansion. The goal of the apparatus is to stabilize the scale measurements over time, in spite of possible thermo-mechanical (local or general) expansion / contraction of the linear encoder line grating spacing, by a method according to which the differences are recorded of simultaneous readings yielded by the two read-heads when the bislider is at positions where both readings are nominally multiple of the bislider length. These values permit the performing of a recovery procedure periodically, or when significant variations due to e.g. thermal drift are suspected. The recorded X2-X1 values, the resulting recovery values, and the difference between recorded and recovery values, are registered in suitable look-up tables.

Description

TITLE

Apparatus and method for the stabilization of linear encoders

FIELD OF INVENTION

This invention relates to linear encoders that are widely used for numerically controlled machine tools.

A linear encoder is a sensor, transducer or read-head paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a digital readout (DRO) or motion controller. The encoder can be either incremental or absolute.

There are two main areas of application for linear encoders: measurement and motion systems. Measurement applications include coordinate-measuring machines (CMM), laser scanners, calipers, gear measurement, tension testers, and digital read outs (DROs).

Servo controlled motion systems employ linear encoder so as to provide accurate, high-speed movement.

Typical applications include robotics, machine tools, presses and bending machines, pick-and- place PCB assembly equipment, automation and production equipment, semiconductors handling and test equipment, wire bonders, printers and digital presses.

Linear encoder technologies include optical, magnetic, inductive, capacitive and eddy current. Optical linear encoders dominate the high resolution market and may employ shuttering/Moire, diffraction or holographic principles; typical incremental scale periods vary from hundreds down to sub-micrometer and following interpolation can provide resolutions as fine as a nanometer. Magnetic linear encoders employ either active (magnetized) or passive (variable reluctance) scales and position may be sensed using sense-coils, Hall effect or magneto-resistive read-heads; with coarser scale periods than optical encoders (typically a few hundred micrometers to several millimeters) resolutions in the order of a micrometer are the norm.

Capacitive linear encoders work by sensing the capacitance between a reader and scale; typical applications are digital calipers.

Inductive technology is robust to contaminants, allowing calipers and other measurement tools that are coolant-proof; a popular application of the inductive measuring principle is the Inductosyn, a resolver unwound into a linear system and the Spherosyn encoder, based on the principle of electromagnetic induction and uses coils to sense nickel-chrome ball-bearings mounted within a tube.

Eddy current uses a scale coded with high and low permeability, non-magnetic materials, which is detected and decoded by monitoring changes in inductance of an AC circuit that includes an inductive coil sensor.

The linear encoders are ideal for machines and other equipment whose feed axes are controlled in a closed loop, such as milling machines, machining centers, boring machines, lathes and grinding machines.

The excellent dynamic behavior of linear encoders, their sound reliability even at high traversing speed and accelerations, make them a natural choice for highly-dynamic conventional axes as well as for direct drives.

BACKGROUND OF THE INVENTION

Linear encoders are made of a scale (or ruler) and of a read-head (or slider).

The measurement occurs between the scale and the read-head, the former being typically mounted onto a machine fixed component and the latter onto a moving component.

The scale is typically provided with spaced very regular line grating and the slider is typically provided with means capable to detect the relative position between the slider and the grating lines.

Such relative position is made available by the slider in the form of analogue or digital signals that are converted, by known appropriate electronics, into data that represent the relative position between the moving component and the stationary component of the axis of linear motion.

The accuracy in measuring the position of the stationary component relative to the moving component depends on the quality of the line gratings, on the actual installation on a machine, and on the thermal state.

Encoder manufacturers either state conformity to specified Maximum Permissible Errors (MPE's), or provide calibration data of local errors as measured by comparison to a length reference standard.

According to the ISO 1 , calibration values are referred to the standard reference temperature of 20 °C.

For low accuracy applications, the stated MPE is accepted and no compensation is performed. For medium accuracy applications, linear encoders calibration values are used to populate a look-up compensation table: these values are subtracted real-time from the encoder readings. Intermediate values between two consecutive table entries are derived by interpolation (typically linear).

For high accuracy applications, values measured in situ after installation on the actual machines are taken to populate the look-up table, rather than the calibration values provided by the manufacturer. This accounts for the actual stress imposed on the ruler and for any misalignment.

Measurements are made by comparison to a length reference standard (e.g. a laser interferometer).

For top accuracy applications (e.g. laboratory CMMs), thermal compensation is superimposed to the above compensation; its magnitude is evaluated by predicting the encoder thermal expansion, based on a known CTE (Coefficient of Thermal Expansion) and a measured mean temperature. Depending on the actual set up, the relevant CTE and temperature may be of the encoder itself, or of the machine component underneath.

Local strain of the machine component underneath a linear encoder induces variations of the local grating spacing, resulting in measurement errors, unpredicted and hence not compensated for.

Changes of the mean temperature induce linear expansion all along the scale, which may or may not be compensated for (depending on whether one or more thermometers are available and used for this).

Local thermal gradients induce local scale expansion, resulting in scale non linearity.

Unless a dense arrays of thermometers are active all along the scale, this effect is unpredicted and hence not compensated for.

To be able to compensate for thermo-mechanical deformations occurring during the intended use of the axis of linear motion to which the linear encoder is applied, new measurements would need to be performed at any new deformation state in order to update the calibration data and the corresponding compensation data.

To perform new measurements aiming to the determination of new calibration and compensation data, the normal operation of the machine would need to be interrupted with very significant costs deriving from the interruption of the relevant process.

Furthermore, the newly obtained calibration and compensation data would be valid only if the process mechanical conditions and thermal conditions will be kept stable during subsequent operation.

Loss of accuracy deriving from thermo-mechanical variations occurring between subsequent re- calibration operations cannot be avoided.

The calibration values populating the look-up table and driving the compensation, were taken at initial thermo-mechanical conditions, which may be significantly different from the actual ones of use.

Even in top accuracy prior art applications, the in-situ compensation cannot be repeated very often because this operation is time and money consuming: not only because skilled metrologists and expensive measuring equipment are required, but also - and above all - because of the very high costs incurred in interrupting machining and production.

Thermo-mechanical variations occurring between subsequent re-calibrations cannot be compensated for, resulting in spoiled accuracy.

SUMMARY OF THE INVENTION

The present invention addresses the problems above and, in particular, provides an apparatus and a method that are able to concentrate the metrological core in a limited-size single piece of equipment, hereafter referred to as a bi-sensor or bislider for locally interpolated differential error recovery.

The bislider goal is to stabilize the scale measurements over time, in spite of possible thermo- mechanical (local or general) expansion / contraction of the linear encoder line grating spacing. If good error compensation is achieved by other means (e.g. by comparison with an interferometer) - possibly providing traceability - prior to the application of the bislider concept, then the bislider stabilization extends the benefit of this compensation over time.

The bislider can be designed to be calibrated easily - either in laboratory or on field - and to be largely insensitive to the environment, e.g. by choosing a low CTE material.

A limited effort in the bislider design and implementation would benefit the whole line scale, possibly meters long.

A bislider is a low CTE spacer keeping two read-heads at fixed and invariant relative distance. When the bislider slides over the scale, two signals are produced by the read-heads.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the bislider according to the present invention is described hereinafter with reference to the figure of enclosure drawing tables that illustrate respectively:

Figure 1 - a state of the art application of a linear encoder;

Figure 2 - the preferred embodiment of the bislider according to the invention;

Figure 3 - the data flow chart of the operations under working conditions;

Figure 4 - the measuring principle of a spacer equipped with two read-heads.

DETAILED DESCRIPTION OF THE INVENTION

In Figure 1 a prior art application of a linear encoder is depicted where 1 is a commercially available encoder scale that is typically encapsulated and connected to the non-moving component of a linear axis assembly (not shown), 2 is a commercially available read-head (slider) and 3 depicts a bracket that typically connects the read-head to the moving component of said linear axis assembly.

In Figure 2 the preferred embodiment of the invention is depicted where 1 is said commercially available encoder scale; 2 is said commercially available read-head (slider); 4 is an additional read-head, identical to read-head 2; 5 depicts the bislider spacer that interconnects read-heads 2 and 4 and that is fixed or integer part to the bracket 3; 6 identify the connection to possible thermometers (not shown) that may be applied, if required, for the accurate determination of the bislider temperature.

The bislider spacer 5 can either be made of a low CTE material or, alternatively, be made of known CTE material and its expansion/contraction is stabilized by known compensation techniques based on the determination of the spacer temperature as measured by thermometers (not shown).

In figure 4 a schematic representation of the bislider concept is depicted; XI and X2 are the counts based on two read-heads; b is the length of the bislider; thick marks delimit encoder segments of nominal length b.

At start up, a preliminary reference procedure is performed.

After homing both read-heads at the same reference mark to initialize the counters in a coordinated way, the bislider is moved to scan a full stroke.

In spite of a possibly continuous movement, the stroke is conceptually divided in contiguous segments of nominal length b, each explored in a separate procedural step.

Each time the carriage carrying the read-heads is at a position nominally multiple of the bislider length b, the difference of readings X2-X1 is recorded.

At the end of the stroke, a table of the recorded X2-X1 values is kept for future reference; this reference table portraits the scale in reference state. Recording all readings over individual segments allows to capture scale local behaviors, e.g. non linearity due to non-uniform mounting strain, or to local interaction between read-head and scale induced by parasitic movements of the carriage.

At will (e.g. periodically, or when significant variations due to e.g. thermal drift are suspected), a recovery procedure is performed.

This is identical to the reference procedure, but the resulting table is separately recorded as a recovery table.

A third table is then calculated as the difference of the reference to the recovery table.

Any non-null difference is attributed to the scale end, and quantifies the amount of apparent shift occurring at the right end of the segment.

Finally, a dynamic compensation table is evaluated by numerical integration: the sum of all values in rows 1 to i-1 of the previous difference table is recorded as entry value in the i-th row of the dynamic compensation table (see figure 3).

In usual encoder operation, the second read-head only is observed.

Its readings are corrected by a value derived from the dynamic correction table, used as a lookup table.

For each position, the actual correction value is evaluated by interpolation of the two values at the extremes of the relevant segment, very much like ordinarily done for corrected encoders.

After correction, the variations occurred since the reference state are filtered out.

This stabilization technique can lead to enhanced traceability when the encoder is calibrated at reference state by means of external calibrated equipment, e.g. a laser interferometer.

In this case, a further look-up table is needed, the static correction table, as ordinarily done for linear encoders. The final correction will be the sum of the static and dynamic corrections.

An attractive feature of the bislider is that it is mostly based on available and relatively cheap components: linear encoders and reading read-heads.

In addition, machine tool CNC's are often equipped with extra linear encoders channels, ready to accommodate the bislider second read-head.

Also the interpolation of look up tables is likely to be an available feature of existing CNC's. As a result, this technique might be used to retrofit existing machines with minimum impact; the one new component is the bislider.

While the invention has been described with reference to the preferred embodiment thereof, it will be appreciated by those of ordinary skill in the art that various modifications can be made to the system and method of the invention without departing from the spirit and scope of the invention as a whole.

The work leading to this invention has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° CP-FP 229112-2.

Claims

1. "Apparatus for the stabilization of linear encoders" comprising:

an encoder scale (1), encapsulated and typically connected to the non-moving component of a linear axis assembly,

a read-head (2),

a bracket (3) that connects the read-head (2) to the moving component of said linear axis assembly,

characterized by an additional read-head (4), identical to read-head (2), and by the application of a spacer (5), keeping and interconnecting said two read-heads (2, 4) at fixed and invariant relative distance so as to form a bislider, said spacer (5) being fixed, or integer part, to the bracket (3).

2. Apparatus according to claim 1 characterized in that, when the bislider slides over the scale (1), two signals are produced by the read-heads (2, 4).

3. Apparatus according to any of the previous claims comprising connections (6) for the application of thermometers permitting the accurate measurement of the bislider spacer temperature and its material expansion/contraction stabilized by known compensation techniques.

4. Apparatus according to any of the previous claims characterized in that the bislider spacer (5) is made of a low CTE - Coefficient of Thermal Expansion - material or, alternatively, of known CTE material.

5. Apparatus according to any of the previous claims comprising available and relatively cheap components applicable, with minimum impact, to new or, alternatively, retrofit existing machines.

6. Method for the stabilization of linear encoders using the apparatus according to any of the previous claims comprising the following steps:

- initializing the counters in a coordinated way, consisting in homing both read-heads (2, 4) at the same reference mark on count bases, respectively (XI) and (X2);

- moving of the bislider spacer (5) to scan a full stroke; the stroke being conceptually divided in contiguous segments of nominal length (b), each explored in a separate procedural step;

- recording of the difference of readings (X2-X1); each time the carriage carrying the read-heads is at a position nominally multiple of the bislider length (b);

- keeping of the recorded (X2-X1) values at the end of the stroke for future reference;

- performing of a recovery procedure at will: periodically, or when significant variations due to e.g. thermal drift are suspected.

7. Method according to claim 6 characterized in that the recorded (X2-X1) values are registered in a reference table portraying the scale in reference state.

8. Method according to claim 6 characterized in that the recovery procedure is identical to the reference procedure, but the resulting table is separately recorded as a recovery table.

9. Method according to claims 6, 7 and 8 characterized by a table calculated as the difference of the reference to the recovery table, any non-null difference being attributed to the scale end, and quantifying the amount of apparent shift occurring at the right end of the segment.

10. Method according to any of the previous claims characterized by a dynamic compensation table evaluated by numerical integration, the sum of all values in rows 1 to i-1 of the difference table being recorded as entry value in the i-th row of the dynamic compensation table.

1 1. Method according to any of the previous claims characterized in that recording all readings over individual segments allows to capture scale local behaviors, e.g. non linearity due to non-uniform mounting strain, or to local interaction between read-head and scale induced by parasitic movements of the carriage.

12. Method according to any of the previous claims characterized in that the second read- head readings are corrected by a value derived from the dynamic correction table, used as a look-up table, the correction value being evaluated, for each position, by interpolation of the two values at the extremes of the relevant segment.

13. Method according to any of the previous claims characterized in that, when the encoder is calibrated at reference state by means of external calibrated equipment, e.g. a laser interferometer, since it is necessary a further static correction table, as ordinarily done for linear encoders, a final correction is given by the sum of the static and dynamic corrections.