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issue: July 2008 APPLIANCE Magazine

Motor Technology
Affordable Reliability


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Noncontact solid-state encoders provide frictionless, zero-wear, “fit and forget” reliability.

The new OnAxis magnetic rotary encoders from Hoffman Estates, IL, U.S.–based Renishaw (www.renishaw.com) are designed to offer engineers cost-effective motor positioning and speed control with long-term absolute-position feedback. According to Eirik Scott, magnetic sensor product manager, the reliability stems from the encoder’s solid-state design. “With no parts to wear, there is no need to service these encoders—just fit and forget,” he says.

The design is based on what Scott calls “a simple yet robust sensing principle.” That, combined with the ability to put all of the required circuitry into a single chip, keeps the devices small and cost-effective. “By using some of the latest silicon processes, we are able to manufacture very small chips containing all of the processing needed to produce a wide range of outputs from just one design,” Scott explains. “Therefore, the silicon design costs are minimized as well as production costs. Automated final optimization and test equipment also keeps down the total production cost.”

The encoders are built around two core components—a small diametrically polarized magnet and a custom Hall sensor ASIC. The ASIC contains a circular array of Hall effect sensors that generate a voltage when exposed to a magnetic flux field. The device detects the change in magnetic flux as the magnet rotates above it. Sine and cosine voltage outputs from the sensor array vary with magnetic position and are converted to absolute angle position by a fast flash interpolator. The absolute angle position value is output through a parallel binary interface or a serial SSI interface. Relative changes of angle position are output through A QUAD B encoder signals.

To ensure reliability, the ASIC is epoxy-embedded inside a metal housing to deliver IP68 sealing, a temperature range of –40° to 125°C, and high resistance to shock, vibration, G forces, and pressures. The IC is also designed to cancel magnetic interference, which enables it to operate in areas of high external magnetic fields and cope with imperfections in the shape of the flux field of the actuating magnetic.

The RoHS-compliant encoders can be used in motor control, shaft position, and velocity measurement applications. The encoder’s reliable feedback makes it ideal for medical applications with fluid dispensing. “The two-part encoder design combined with reliability provides a perfect solution,” Scott says. “A solid wall can be placed between the rotating shaft containing the magnet and the sensor, providing complete isolation of the fluid from the encoder. For other medical or food applications, the body of the encoder can be produced in a stainless steel to allow for washing down the machine.”

The devices are available in analog, incremental, and absolute formats with resolutions up to 13 bit (8192 counts per revolution), speeds to more than 30,000 rpm, measurement accuracies to 0.5 degrees, and 5- or 24-V power supply. Versions for immersion applications are also available. 

 

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