Motion control: Long-length linear encoders look lively

Linear encoders provide direct position feedback to various machine tool and automation systems. Working in a linear format allows extreme length position measurement and control—critical for machine tool accuracy and production of precise large-scale parts. Focus here is linear encoders with 3-meter (about 10-ft) or greater measurement length.

10/28/2013


Diagram: The photoelectric sensing method shown applies to an optical linear encoder using a glass scale. With a steel scale, a slightly modified, reflective scanning method is used—where light shines on the scale tape and reflects back onto the scanningHigh dynamic operation characterizes CNC systems and precision assembly equipment where linear encoders work. Position measurements must be provided in tough environments and often under high traverse speeds and accelerations. Growing use of linear encoders with direct-drive linear motors underlines those requirements. For example, some encoder models offer traverse speeds of 20 meters/sec (65.6 ft/s) or greater.

Dirt, oil, chips, and contaminants are especially detrimental to optical encoders—requiring physical protection of the device or enhancement of the measurement process. This is one area where operation of linear encoders differs from that of their rotary-encoder cousins, which work in a sealed environment. Linear encoders can be categorized in different ways.

Optical or magnetic sensing

Encoders fall into two main technologies—optical and magnetic—which also apply to rotary encoders. Inductive encoders are not covered here. Linear encoders consist basically of a measuring scale containing extremely fine graduations (or magnetized information) that convey position and a scan head to read the scale.

In simplest terms, the measurement process of optical linear encoders entails photoelectric sensing of the scale and a scanning reticle that move relative to each other (see diagram). Projected light is modulated as it either passes through gaps in the gratings when the scale and reticle are aligned, or is blocked when lines of one grating coincide with gaps of the other. An array of photovoltaic cells converts these light-intensity variations into electrical signals. The specially structured grating of the scanning reticle filters the light to generate nearly sinusoidal output signals. Further signal processing at the scan head or the system controller yields position output.

Optical linear encoders use a glass or steel scale. Glass is generally limited to measurement lengths to about 4 m, with steel scales applicable for much longer lengths. Glass scale length is limited by the accuracy of machines that manufacture them, noted Nathan Mathiot, product specialist–machine tool marketing at Heidenhain Corp.

Main components of magnetic linear encoders are a magnetic tape and a scan head. Position information magnetized on the tape in a sequential code serves as the measurement scale. Tapes come in various lengths and can be extremely long. They’re typically laminated on a steel strip and have adhesive backing for ease of mounting on a machine. Position information is sensed as the scan head travels over the measuring tape.

Long-length linear (LLL) encoder applications don’t always require the precision of optical technology, explained Corrie Fearon, marketing manager for Encoder Products Div. of Renishaw plc. Magnetic encoders provide an alternative, with ±20 micron/meter (μm/m; 1 µm = 0.00004 in.) accuracy typical for high-end versions. “Magnetic encoders offer wider installation/running tolerances, immunity to dirt and contamination, and generally a cost saving compared to optical encoders,” Fearon said. (See more on accuracy and applications below; also at Refs. 1 and 2, online.)

Other encoder categories

Linear encoders are also designated as incremental or absolute type devices—terms that carry over from rotary encoders. Absolute encoders provide position essentially upon power up. Incremental encoders require axis movement to first establish a reference position. However, distance-coded reference marks (DCRMs) spaced along the scale minimize the need for machine movement. Encoder electronics establish the absolute reference after traversing two DCRMs, which typically means a movement of a few millimeters, according to Heidenhain.

“Reference marks are calculated by a formula, which leads eventually to a repeated unique position value and thereby becomes a length limit for incremental tape scales with DCRMs," Mathiot said. The longest distance-coded linear encoder made by Heidenhain has been 72 meters.

Photo: This Electroimpact Inc. wing-panel assembly machine at the Broughton, U.K., Airbus A380 production line uses Renishaw RGH41 optical linear incremental encoders. RGS40 tape scales are fitted in three working areas along the machine’s 160-m traverseAbsolute tape scales are also limited in measurement length by the pseudo-random code (PRC) placed on the track. As an example, Mathiot cited the LC281 linear encoder with 32 bit memory. “Each 10-nanometer step uses 1 of 4,294,967,296 possible position values stored in the memory. The PRC is unique for every position value and eventually it repeats,” he added.

Renishaw likewise noted a maximum length limit for incremental encoders in terms of “yield on making the scale that long,” but has supplied tape scales up to 100-m long. “For absolute encoders, maximum length of Renishaw Resolute optical encoder scales is currently 10 meters, limited by the code words selected for optimum dirt immunity and performance characteristics,” Fearon stated. Maximum length of the LMA-10 magnetic encoder scale is 16.2 m.

Optical linear encoders are further designated as sealed or exposed devices. In sealed units, a metallic housing and elastic sealing lips protect the scale, scan head, and guideway from ingress of contaminants found in industrial environments. Exposed (noncontact) linear encoders are physically simpler and often used in cleaner environments.

Harsh working environments

Noncontact optical linear encoders in particular face harsh conditions and need corrective enhancement of the optical scanning. Fearon noted two basic ways to mitigate effects of contamination: use optical filtering or average the scanning over a relatively larger area.

Filtering, the more powerful method, amplifies good signals at the fundamental frequency to which the optics are tuned, while rejecting other harmonics caused by contamination. This method is suitable to incremental encoders. “Adding auto gain control (AGC) to the system helps boost dirt immunity still further, but adding AGC to a weak optical scheme simply amplifies poor signals and will not overcome phase-shift, or other issues,” Fearon said.

Renishaw’s Resolute line of absolute, noncontact linear encoders applies different, advanced techniques to obtain dirt immunity. After capturing an image of the measurement scale, the encoder performs cross-checking and error-rejection via an on-board digital signal processor—counteracting chaotic light back-scatter onto the scan head from grease, oil, or particulate contaminants on the scale. High system redundancy allows the correct position to be determined even with large parts of the image obscured, according to Fearon.

“To provide further safety and diagnostic coverage, Resolute encoders include a position-checking algorithm that tracks position, ensuring that only correct data are sent to the system drive/controller,” Fearon added. Actually, two independent calculation methods are used and checked in the scan head. Then, one correct result is sent, relieving the drive/controller from that task (see more at Ref. 3).

Sealed optical linear encoders minimize or eliminate the need for extensive dirt immunity measures. Heidenhain offers both exposed and sealed encoders but applies the latter exclusively in the environment of numerically controlled machine tools.

“A sealed linear encoder is the correct encoder for harsh environments, where coolant, oil, and chips are present,” Mathiot said.

Examples of Heidenhain sealed LLL encoders include LC211 (absolute) for measurements up to 28 m and LB382 (incremental) available for up to 30 m and 72 m on special order. Both models use a steel scale tape. These encoders offer ±5 micron (µm) accuracy grade—defined as position error tolerance over any 1-m measurement length. Some other models offer ±3 μm accuracy. Sealed encoders intended for non-CNC machine applications have up to 3 m measuring range and typically ±10 μm accuracy grade.

Heidenhain likewise noted methods that “lower sensitivity to contamination” for its exposed LLL encoders, which find application in high-accuracy production/assembly machines, measuring devices, and direct drives. One method mentioned is single-field (rather than four-field) scanning to generate position signals. While output signals with single-field sensing experience amplitude change due to contamination, offset and phase position are said to remain unchanged.

“Signals remain highly interpolable and position error within one signal period remains small,” according to Heidenhain. Representative exposed encoders include LIC series (absolute) and LIDA series (incremental)—with position error per signal period of up to ±0.08 μm and ±0.2 μm, respectively.

Accuracy of signals

Graph: Actual encoder scale accuracy can often be significantly better than the specified value. An example shown is Renishaw’s RTLC optical scale—with ±4 micron total accuracy over the entire 11.9-m length. Courtesy: Renishaw plc.Use of a large scanning field relative to the scale grating period also reduces contamination sensitivity. Reportedly, high-quality signals are output in the presence of contaminants up to 3-mm diameter and position error stays well under scale accuracy grade specs.

Some of the scale accuracies cited above refer to certified values. In practice, encoder scales achieve substantially better results (see diagram).

Sick Inc.—a subsidiary of German company Sick AG—offers several LLL magnetic encoders. TTK70 noncontact absolute encoder has ±10 μm accuracy grade, applicable up to the unit’s full 4-m length. LinCoder L320, another noncontact absolute device, comes in lengths up to 40 m and with a ±0.3 mm/meter accuracy spec (at 20 C).

Another encoder product from Sick is particularly noteworthy for its extreme-length measurement capability. Pomux KH53 provides absolute position measurement for up to 1.7 km (that’s 1,700 m).

“With its IP 66 rating and an aluminum housing, Pomux KH53 is a very rugged solution for those extreme length and harsh environment applications,” said Mandee Liberty, absolute and linear encoders product manager at Sick.

Long-length linear encoders range over wide applications. Sick cited applications for KH53 in overhead cranes and storage/conveyor systems, among others. Liberty noted success for KH53 in those applications with measuring lengths in the 15 to 800 meter range.

In Renishaw’s experience, long-length encoder applications include large bearing grinding machines, LCD screen manufacturing stages, wide format printing, and machines for laser/water-jet cutting and aircraft wing production (see photo).

Heidenhain noted application examples in the manufacture of wind turbine components and automotive parts on a transfer line. In a related market trend, the company sees its customers moving toward use of encoders with linear motors to obtain faster machining of parts.

- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at braunbart(at)sbcglobal.net  

Key concepts

  • Use linear encoders, optical or magnetic, for direct position feedback.
  • Extreme length position measurement and control aid machine tool accuracy and production of precise large-scale parts.
  • Linear encoders >3 m are covered here.   

Consider this

Linear encoders can provide high accuracy measurement over long distances; what application could you improve with better feedback?

ONLINE

Ref. 1 - LLL encoder addendum (See link at bottom.)

Ref. 2 - Encoder application (See link at bottom.)

Ref. 3 - Position determination/checking algorithms

http://www.renishaw.com/en/white-paper-examines-the-position-determination-and-checking-algorithms-of-resolute-absolute-encoder--18764 

www.heidenhain.com 

www.renishaw.com 

www.sickusa.com 

Further reading: Electromagnetic linear encoders to the rescue



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