Encoder inputs lead to good outputs

ControlDesign.com

Through advances in design, encoders have evolved into a more popular option for applications that once used resolvers. Field Editor Kevin Russelburg reports in this edition of Specmate.

By Kevin Russelburg, Field Editor

MACHINE DESIGNERS have a wide range of feedback devices available to accommodate an equally wide range of applications. Sorting through the options can be a daunting task, but staying focused on a few fundamentals can ease the process.

Encoders are a common solution for speed control, acting as key motion sensors for measuring, cutting and positioning applications. In basic terms, an encoder translates angular position into an electronic signal. They are typically characterized as rotary or linear, incremental or absolute, and have either optical or magnetic signal generation.

Optical rotary encoders are the most common, are comprised of a housing, light source, code disc, photodetector, and embedded electronics. The two main types are incremental and absolute position, with incremental being the most common.

Through advances in design, encoders have evolved into a more popular option for applications that once used resolvers. "In general, encoders are often more specified than resolvers for new applications,” says Scott Orlosky, marketing manager, BEI. "Their higher precision and industrial robustness make them suitable for a wide range of applications.”

ADVERTISEMENT "By contrast to incremental encoders, an absolute encoder provides an output with a unique code pattern for each position of the shaft."

Optical rotary encoders use a light source (typically an LED), a rotating disc to shutter the light, and a photodetector. The disc contains a specific number of lines that determine the pulses per revolution (PPR). "Incremental encoders are usually supplied with two channels (A and B) that are offset from one another by a quarter of a cycle (90º). This type of signal is referred to as ‘quadrature’ and allows the user to determine not only the speed of rotation, but its direction as well," explains Orlosky. The square wave output is inherently easy for digital signal processing techniques to handle. “Generally, incremental encoders provide more resolution at a lower cost than their absolute encoder cousins,” he adds. “They also have a simpler interface because they have fewer output lines. Typically, an incremental encoder would have four lines: two quadrature (A and B) signals, and power and ground lines. A 12-bit absolute encoder, by contrast, would use 12 output wires plus a power and ground line.

Incremental encoders send a quadrature output signal with a specific number of pulses for each shaft revolution." Powered conveyors, CNC equipment, and cut-to-length machine applications are well-suited for incremental devices. Resolutions of 50–5,000 PPR are standard, but counts to 100,000 are possible by using an ASIC microprocessor.

"By contrast to incremental encoders, an absolute encoder provides an output with a unique code pattern for each position of the shaft,” says Jerry Morelli, product manager, Pepperl+Fuchs. This code is derived from independent tracks on the encoder disc corresponding to individual photodetectors. "Absolute encoders are typically used in systems that must retain precise position, rather than speed,” adds Morelli.

One of the key attributes of absolute encoders is that their position always is precisely known, even after a power-down/power-up cycle. “This makes them more suitable for situations where power can be interrupted, is difficult or costly to supply, or in situations where exact, absolute position is critical,” states Orlosky. Examples include indexing tool changers, slitters, rewinders, and 3D CMM inspection equipment where re-initialization is costly or impractical.

"Magnetic sensing technology is very resistant to dust, grease, moisture and other contaminants common in industrial environments, and to shock and vibration,” says Rick Armstrong, marketing manager, Danaher Motion.

There are several types of magnetic sensors. Variable-reluctance sensors detect changes in the magnetic field caused by the presence or movement of a ferromagnetic object. The simplest variable-reluctance rotary sensor, often called a magnetic pickup, consists of a coil wound around a permanent magnet. This generates a voltage pulse when a gear tooth moves past it. Rugged, reliable, and inexpensive, this sensor is used mostly to measure speed.

Another type of sensor uses a permanent magnet and a Hall Effect or magnetoresistive device to produce a change in either voltage or electrical resistance in the presence of ferromagnetic material, which can be in the form of a gear tooth (in a rotary encoder) or a metal band with slots (in a linear encoder). This type of sensor will work down to zero speed, and is available in both rotary and linear forms.