Know your position

Dec. 14, 2007
Encoders can handle the environment, the application and size limitations

Encoders are being engineered to meet increasingly specific needs. Space considerations, environmental factors and applications come into play when determining which encoder is best suited for a given job. When properly matched, plants can achieve their operational objectives while avoiding unnecessary and costly downtime.

Physical environment: Encoders can withstand dust, dirt and explosion hazards. For instance, magnetic encoders and digital tachometers by Dynapar are more rugged than optical rotary encoders and resistant to dust, moisture, shock and vibration. They use magneto-resistive (MR) technology to track machine speed and provide digital shaft velocity feedback. A magnetic pulse wheel, imprinted with small magnetic poles and an integrated MR sensor produce magnetic fields that penetrate dust, dirt, oil and contaminants. The wheel rotates with the shaft, and the sensor’s resistors change value with the magnetic field.

In harsh areas, where caustic chemicals and corrosive agents threaten equipment reliability, stainless steel encoders offer a rugged alternative to aluminum or plastic. Encoder Products’ Model 865T, designed to mount on NEMA C-face motors, has a housing milled from solid piece of 316 stainless steel. An optional stainless steel cover protects internal bearings and electronics from dust and machine wash-downs.

Another example is BEI’s H25 and H20 incremental rotary encoders, available with optional stainless steel housing, encoder body and bearing housing. Applications are chemical or petrochemical, food packaging, medical and similarly harsh industrial environments.

Space constraints: Industrial machinery and equipment is getting smaller, and so are the electronic components. For space-constrained applications, an array of small encoders is available. An example is the surface-mounted encoder series by Avago Technologies. Measuring 6.5 millimeters by 4.2 millimeters by 1.69 millimeters, the AEDR-8320 is useful in robotic equipment, pick-and-place machines and other applications. The reflective analog optical encoder offers precise, real-time positioning and directional sensing. It uses a reflective code wheel or linear code strip to sense position and velocity. Two-channel analog signal output offers 180 lines per inch resolution that can be increased through interpolation. The lead-free package is compliant with the European Union’s Restriction of Hazardous Substances (RoHS) directive.

Targeted applications: Dynapar’s Hengstler Acuro AX71 absolute encoder was designed with the help of a drilling services firm to withstand downhole conditions. Its stainless steel construction, IP67 sealing and bearings withstand shock loads to 100 g. An ATEX-certified housing allows it to be used in explosive environments without electrical barriers. The unit’s electronics use heat-stable substrates and diodes that ensure reliable operation to 140°F. In addition, the encoder is self-grounding to prevent shorting.

Heidenhain’s LS 688C linear encoder is for manually operated machine tools with digital readout. It’s scanning carriage rides on ball bearings, providing rigidity and tight backlash and mounting tolerances. By comparison, in machine tools without linear encoders rely on the pitch of the ball screw, which is subject to thermal expansion, resulting in inaccurate output. Linear encoders compensate for thermal expansion, thereby keeping machine tools thermally stable and producing tight manufacturing tolerances.

Built-in encoders are available for applications such as conveyor belts and door openers that require low-resolution speed and position feedback. Bodine’s custom-made Incodermotors with built-in magnetic encoders are available as permanent magnet DC, brushless DC or gearmotors. The encoder is pre-wired and dropped into the OEM’s design, reducing assembly time and protecting it from the environment.

E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at [email protected].

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