Machine Design

Sensor Sense: Optical Absolute-Position Linear Encoders

Rotary encoders are indispensable tools for providing position feedback in many motion-control and automation processes. Lifts, overhead monorails, automated storage and retrieval systems, and cranes are only a few of the many devices reliant on them.

However, traditional rotary encoders depend on physical contact to convert circular motion to a PLC or drive-friendly signal. Consistent with any contact-based system, mechanical wear, coping with imperfect alignment, and tolerance errors often lead to inaccuracies, encoder damage, and downtime.

Optic-based linear encoders use a multi-LED array and uniquely coded reference rail to determine position of a carrier mechanism. They seamlessly emulate a 512-turn, 1,024-pulse/rev encoder with outputs that offer industry-standard control protocols such as SSI, DeviceNet, Profibus DP, Ethernet IP, and parallel interfaces using binary or Gray code. There are no wear problems because scanning uses optics and is contact-free. Fraction-of-a-millimeter accuracy and 1-msec response time minimizes any sacrifices in accuracy or performance associated with converting encoder signals.

Positioning is absolute, so there is no need for home reference points. Units know their position immediately when energized. Detailed diagnostic codes are also available, so a controller is never more than a few milliseconds away from verifying a scan head’s health and alignment.

The coded reference rail comes in fiber laminate form for general-purpose use, or stainless steel for environmentally challenging installations that can see physical abuse, excessive heat, aggressive chemicals, or moisture. Rugged yet pliable, the code rail can flex vertically to follow sloping track systems, or horizontally to follow the curves of monorail tracks or rotary index tables. MD

Pepperl+Fuchs ( supplied information for this column.

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