Long-length linear encoder addendum
Motion control: Successful implementation of linear encoders requires appropriate installation, calibration, and connectivity to the control system—in addition to an accurate encoder scale. Coverage of these areas here supplements the article, Long-length linear encoders. See the table of selected encoder manufacturers and products.
Linear encoders use requires appropriate installation, calibration, and connectivity to the control system—in addition to an accurate encoder scale. Below, see the table of selected encoder manufacturers and products. Separately, see the article, Long-length linear encoders, linked at the bottom of this article.
Tolerance of dirt and contaminants is one of the requirements placed on noncontact (exposed) optical linear encoders, but not a major concern for the sealed configuration of traditional rotary encoders—as discussed in the main article. Another differentiator between linear encoders and their rotary-encoder cousins is the need for generous setup tolerances.
“Wide setup tolerances are particularly important on long-length linear (LLL) encoders, for obvious reasons,” said Corrie Fearon, marketing manager for Encoder Products Div. of Renishaw plc. “Often there is a compromise between scale pitch and ease of installation, with encoders using coarser graduations typically being less ‘fussy’ than those reading finer pitches,” Fearon said.
Renishaw’s Resolute absolute encoders combine 30-micron (0.00012 in.) scale pitch with a scan head yaw tolerance specified at ±0.5 deg, Fearon explained. “In reality it’s about ±1.5 deg and even at those extremes the encoder doesn’t lose phase,” he stated. In contrast, yaw tolerance for a traditional dual-track optical encoder is much tighter—said to be about ±0.02 deg.
To ease installation, linear encoder manufacturers supply tools and guidance. Typically, the scale installation tool makes use of the machine’s motion to lay down the scale as it moves along the axis. “Consideration has to be made to support the scale reel throughout the installation process, and it’s crucial to ensure there’s safe, unrestricted access for an operator,” Fearon continued.
Optical encoders provide higher position accuracy than magnetic encoders (see main article). However, magnetic sensing is quite appropriate in many applications. One example cited by Fearon is a machine axis driven through a gearbox, where the encoder is used primarily for direct position feedback. On the other hand, he suggested the need to be aware of potential limitations of magnetic encoders, such as hysteresis and repeatability.
Optical encoder scales with total accuracy of ±4 micron (µm; 1 µm = 0.00004 in.) for lengths up to 5 meters are at the high-end of availability and come with a guarantee. Scale accuracy of ±5 µm/m is more common; other scales with accuracy certified to “better than ±25 µm” still represent products with good measurement quality. Actual scale accuracy delivered can often be better than the specified value—as verified in final product checking and illustrated in the main article. Error-mapping of a scale during installation on the machine can also be applied to compensate final axis accuracy.
When accuracy is paramount, Fearon recommended two approaches:
- Purchase an extremely accurate encoder
- Purchase a highly repeatable encoder and calibrate it with a laser interferometer system. Laser calibration tools are available from various encoder manufacturers.
With the second option the specification parameters to look at are hysteresis, jitter, and resolution, he suggested. Coarser pitch encoders in particular can have significant bi-directional hysteresis that might make them unsuitable for the application. “Jitter is the amount of noise coming from the fundamental detection method and the electronics, as signals are processed through the encoder; finer resolutions help reduce the quantization effects,” Fearon added.
Producing long-length optical encoder scales is limited, in part, by manufacturing economics, as discussed in the main article. Precise production machinery, measurements, and controls are requisite—along with a clean-room environment. Heidenhain Corp. has developed several photolithographic manufacturing processes to make optical scales with increasingly fine graduations that suit different application needs. Scales at one end of the spectrum have a typical graduation period of 40 μm, while scales targeting the most precise applications provide a graduation period of 2 μm or finer.
A substantial number of companies worldwide manufacture linear encoders. The table below lists some of those companies and presents product examples.
Selected long-length linear encoder manufacturers, products
(Encoders listed offer measurement length of 3 m or longer)
Representative encoder model(s)
BML S1A/S1F series
Newall Electronics Inc.
Pepperl + Fuchs
WCS position encoding system
Compiled by Control Engineering
* Noncontact (exposed) encoder
Connectivity to the motion control system or machine drive is an important aspect of encoder performance. Various interface protocols are available to transmit encoder status and position information from the encoder’s scan head to higher-level controls. Encoder manufacturers have either developed one or more in-house protocols, or licensed third-party products. Communication protocols fall into proprietary or open-source types. The latter are based on industry standards. Some well-known interface protocols include:
- Synchronous Serial Interface (SSI)—this serial protocol connects controllers and position sensors in an industrial network using a master/slave arrangement. A clock pulse train from the controller initiates a gated output from the sensor. Based on the iconic RS422 communication standard, SSI was developed in 1984 by Max Stegmann GmbH of Germany, specifically for transmitting absolute encoder position data.
- Hiperface—a proprietary serial protocol that can operate either point-to-point or use bus connections to link several encoders addressable from one master. It’s an asynchronous (non-synchronous) protocol that uses bidirectional RS485 communications for data transmission. Hiperface was also developed by Max Stegmann GmbH.
- EnDat (Encoder Data) interface—a proprietary serial protocol developed by Heidenhain provides synchronous data transmission and advanced functions intended for high-performance applications. This digital bidirectional interface enables encoder data storage in local memory, including diagnostics, identification, alarms, and information about connected components.
- BiSS interface—a more recent “open” synchronous, serial sensor/actuator interface developed by German company iC-Haus GmbH. This digital protocol runs on two unidirectional lines, but its continuous-mode (C-mode) implementation permits bidirectional parameter exchange. BiSS incorporates information storage, alarms, and diagnostic capabilities. Higher data transmission speed is another feature attributed to BiSS.
Absolute linear encoders from Sick Inc. presently apply SSI and Hiperface connectivity.
Heidenhain’s absolute linear encoders currently use EnDat 2.2, the latest version of the protocol. Encoders with EnDat 2.2 interface have the ability to transmit two independently derived position values to the controller for comparison and position error checking to ensure that the machine is in correct location. “Position values are calculated in different ways to ensure the encoder is at the right position. Comparison is done in the controller once it receives the position and error information in the data packet,” explained Nathan Mathiot, product specialist–machine tool marketing, at Heidenhain.
In a related item, Mathiot mentioned the European Union’s ongoing machine tool safety initiative, which includes certification of encoders and controls for functional safety. EnDat 2.2 interface reportedly accommodates provisions for implementing encoders in safety-related applications. Pertinent standards include International Electrotechnical Commission IEC 61508: Functional Safety and International Standardization Organization ISO 13849-1: Machinery Directive.
Besides EnDat 2.2, some Heidenhain absolute linear encoder models support third-party serial communication protocols from Fanuc (Fanuc Serial), Mitsubishi (High Speed Serial Interface), and Siemens (Drive-Cliq).
Renishaw uses BiSS-C communication protocol for absolute encoders. The company’s Resolute line of absolute optical encoders also supports third-party serial communication protocols from Fanuc, Panasonic, and Siemens.
In addition, a variety of industrial bus interfaces are available to interconnect encoders and the numerous other components that make up an automation system.
It should be noted that in many industrial applications linear encoders work together with rotary models, as described in online Ref. 2.
- Frank J. Bartos, PE, is a Control Engineering contributing content specialist. Reach him at braunbart(at)sbcglobal.net
Ref. 2 - Encoder application (See link at bottom.)
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