Encoders Deliver Accurate Motion

Encoder suppliers have gone to great lengths to meet ever-growing system demands. Since customers’ requirements can be as varied as the applications, products from some suppliers have attributes considerably different from those of other suppliers. In some cases, there are similarities. “Smaller, higher resolution and less expensive summarizes the current trend in design engineering,” says Martin Gordinier, vice president of sales and marketing, Gurley Precision Instruments.

Some applications, especially machine tools, have increased safety requirements. “It’s been a requirement for a while now but it’s changing rapidly,” says Thomas Wyatt, national sales and product manager, Automation Div. HEIDENHAIN Corp. Earlier solutions involved two separate encoders. Today, users have alternatives. “Everybody is talking safety, so I can imagine in the near future, you’ll see these same specifications and standards apply to other industries, not just machine tools,” says Wyatt.

According to Wyatt, the trend to absolute feedback has taken hold in Europe but is just starting to gain acceptance in the U.S. “Absolute position is quicker; it’s more reliable because it’s in pure digital form,” he says. Focusing on just rotary encoders, the following six products demonstrate the effort encoder suppliers have taken to meet the diverse needs of customers.

ROTARY TO LINEAR MOTION

The BML linear product line from Balluff addresses applications that require a low-cost rotary encoder. An encoded magnet in a formed ring structure provides a precise position reference to the sensing electronics. Using quadrature technology, PLCs can be programmed to perform specific tasks based on multiple set points on the magnetic band. The sensor allows the translation of rotary motion directly into linear motion. Based on an inexpensive and easy installation, the sensor can replace systems that use multiple sensors, complex mounting solutions and cam switches, as well as rotary optical encoders. The sensor targets machine setup applications for reducing planned downtime in packing equipment, such as labelers, case sealers and over cappers.

HIGH PRECISION ENCODER

Designed for heavy-duty industrial applications, SICK STEGMANN Inc.’s DFS60 encoder uses a nickel code disc instead of a plastic or glass rotating disc. The approach provides exceptional concentricity, robustness and high resolution for an incremental hollow and blind hollow shaft encoder. In a 60-mm housing, the encoder achieves a maximum of 65,536 lines (pulses per revolution). A 30-mm spacing between the ball bearings that support the shaft provides excellent concentricity, lower encoder vibration and increased bearing life. Electrical isolation between the motor shaft and encoder increases both interference immunity and reliability. An integrated spring steel stator coupling also increases bearing life and allows error-free angle measurements, as well as the ability to adjust the shaft and axial movement of the motor shaft. The encoder is especially well-suited for machinery in packaging, woodworking, textile and printing industries.

DUAL-OUTPUT ENCODER

Using two sets of internal electronics, BEI’s dual-output HS35 hollow-shaft encoder sends identical isolated output signals to two separate connectors. The design provides users two interesting options. With redundant encoder signals, if one output is lost, attaching the cable to the other unit resumes operation in minutes, not hours or days. Alternatively, with the dual output, a single encoder can supply two separate controllers. The mechanically parallel and electrically isolated connectors simplify connecting to a second controller and avoid the cost and space of a second encoder. Powering each circuit independently enables the encoder to provide outputs at two different voltage levels. The hollow shaft encoder has a cast metal housing and dual preloaded bearings for increased ruggedness and reliability. In addition, rotating shaft seals and a sealed connector provide the unit NEMA 4 and 13, as well as IP65 ratings.

ENCODER WITH BI-DIRECTIONAL INTERFACE

An encoder with a digital, bi-directional interface called EnDat 2.2 from HEIDENHAIN Corp. provides a critical component of a safe drive. With serial data transfer, the ExN 425/437 series and ExN 1325/1337 series rotary encoders provide diagnostic, automatic possibilities, automatic commissioning and other possibilities for safety-related applications. A complete subsystem for a safe drive consist of an encoder with EnDat 2.2 transmission component, a data transfer line with EnDat 2.2 communication and an EnDat master — an EnDat 2.2 receiver component with a monitoring function. Based on RS-485 transmission technology, the maximum transmission frequency is 16 MHz, claimed to be the fastest purely serial interface for position encoders. To provide a safe interface, two position values generated independently from each other and error bits ensure reliable transmission of position data. For safety-related applications, the encoders are inspected for compliance with ISO 13 849-1 (SIL 2) and IEC 61 508 (performance level d). Among the requirements defined in these standards for assessing safety-related systems are failure probabilities of integrated components and subsystems.

MAGNETIC ABSOLUTE ENCODER

The Kübler by TURCK 3650 and 3670 encoder series measure absolute position magnetically, rather than optically. The magnetic sensing approach opens applications that had previously avoided absolute encoders. With a compact design and blind hollow shaft of up to 10 mm, the encoders have a resolution of 9 bits with SSI, 4 to 20 mA or 0 to 10V output. To meet measuring ranges of 45, 90, 180 or 360 degrees, the magnets can be configured as required for the application. Compared to potentiometers or commercial grade encoders, the magnetic encoders provide a longer service life, higher reliability and resistance humidity and condensation. An IP 69K protection rating and temperature range of -40 to 85C make outdoor use possible. The encoder’s die-cast housing withstands more than 500g shock and more than 30g vibration capability providing another ruggedness aspect for harsh environments.