Cost Effective Encoding Solutions
Incremental encoders using magnetic technologies have been around a long time. Because of their robust nature, automotive designers have been using Low Resolution Magnetic Encoders for years in demanding applications that include antilock braking, transmission speed, ignition timing along with engine cam and crank position sensing. Non-automotive applications use magnetic encoders over other technologies such as optical to overcome problems associated with dust, moisture, shock, vibration, operating temperature, high speeds, wear, etc.
Past magnetic encoder technology, although accurate, cost effective and robust in design, was limited to low resolution applications. Today, magnetic Encoders have come of age. With the advent of new magnetic sensing and signal processing technologies, Medium and High Resolution Magnetic Encoders are able to meet the needs of most all applications with pulse counts as high as 2000 times past resolutions.
The unique advantage of magnetic encoder technologies over other encoder technologies comes from the fact that magnetic encoders do not require moving parts such as bearings or seals, the main cause of encoder failure. Magnetic encoders are comprised of only two major components that require no special compliant adaptation components such as coupling and straps for mounting.
The Magnet Target
The first component of a magnetic encoder is a permanent Magnet Target Wheel that is rigidly mounted on the rotating shaft of a motor or pulley system. The magnet target is magnetized with multiple poles typically on the periphery but can also be located on the face. The number of poles can vary with the pulse count needs of an application. The physical size and shape of the rotor can be manufactured to fit any shaft size available today.
Mounting magnetic target rotors can be accomplished in a number of different ways. The most cost effective requires a step in the shaft for the magnet to nest on. A screw on the end of the shaft is used to compress the magnet rotor onto the step of the shaft. As an alternate, adhesive can also be used. A second mounting method allows the magnet to be press fitted onto a shaft. Because magnetic materials are not well suited as press fit applications, magnet rotors will typically include a steel insert to accomplish the press fit. Another method involves the use of a special machined hub. The hub will either include a set-screw or clamp onto the shaft. Alternate methods are also available. Bearing tolerance rings, key ways and alignment pins have been shown to work equally well. For applications that have special requirements, proven designs are available to meet requirements such as high speed, high shock or stress loads, high temperature, etc.
Magnet targets do not need to be simple disc shapes but can also be configured to accomplish other needs of a design. A good example is a typical design for flow metering. The magnet target for this application is manufactured into a paddlewheel or turbine shape along with magnetic poles on the periphery for encoding purposes. This target then provides dual functionality for the application reducing overall cost. The other added benefit is the ability to function through non-magnetic barriers allowing a physical separation of the magnet target and the encoder electronics. For the flow meter application, there is no need to provide an opening into the flow path; consequently, eliminating the possibility of leakage.
The magnet target is not limited to rotating applications. The magnet target can be in the shape of long strips for use in linear moving applications. For long strip lengths, the magnetic strip or tape can be backed with a special steel backer for dimensional stability.
Magnetic material choices are numerous but considerations of magnetic uniformity, temperature performance, fluid compatibility, structural requirements and magnetic performance must be considered in any material selection. Most magnet targets are made using bonded magnet manufacturing methods due to performance and cost considerations.
The Encoder Module
The second and only other component of a Magnetic Encoder comprises the magnetic sensor and associated signal conditioning electronics. The encoder module is typically mounted directly on the end bell of a motor or flange of a pulley system. This module contains no moving parts such as seals and bearings; therefore, mechanical wear does not need to be considered. A real benefit for any application that requires a long life and reduction in field service.
Package configurations are endless allowing the designer to have an encoder module that can also address other needs such ease in installation, reduction in component counts and adaptation of other devices. Most applications can benefit from configuring the encoder housing as a guard for the moving components such as the shaft and magnet rotor. Magnetic encoder modules work well for through shaft application allowing the addition of other devices such as motor brakes, etc.
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