Linear encoders are often essential. Even where they’re not, benefits they offer can justify the additional cost they incur.
Linear encoders bypass the mechanical linkages of rotary-to-linear motion axes to directly measure positions of loads. That avoids the effect of backlash, pitch errors, vibration problems, and the way in which heat causes expansion and geometrical distortion … which is helpful in precision designs as well as a broadening array of automated machinery.
In the past, linear encoders were reserved mostly for advanced motion-system axes sporting linear motors, as linear encoders to get high enough resolution (to 5 µm or better) were costly glass-scale optical types.
Related: How do I pick between rotary and linear encoders?
But now new technologies employing magnetic operation or (in setups that are optical but designed to be more affordable) tape-scale sensor technologies are making linear encoders increasingly common on axes driven by rotary electric motors and ballscrews. Linear encoders with tape scales excel on long-stroke axes and applications needing easy installation. Linear-encoder variations that use magnetic operation include a linear scale that has permanent magnets or variable-reluctance strips. The read head travels with the guideway carriage with has sensors to track magnetic-field during strokes — to track position with resolutions of 25 to just a few µm in some cases.
In ballscrew-driven applications, a lower accuracy screw can be chosen if a linear encoder is used, because the encoder feedback lets the controller compensate for positioning errors introduced by the screw.
Still more cost-effective are linear-encoder variations that use magnetic induction through a steel strip covered in grooves and raised lines that a read head tracks. Elsewhere, very simple linear encoders use a read head to track the presence and absence of ball bearings within a guideway carriage.
Whether a linear system uses servo or stepper motors, adding a linear encoder can improve the performance of the machine and the quality of the process. In servo applications, the motor’s rotary encoder monitors its speed and direction, but a linear encoder monitors the load’s actual position. When stepper motors are used, position monitoring is especially important, as steppers typically run in open-loop configuration … making it difficult to unequivocally verify that the system moved to the correct position.
Absolute track coding essentially provides the read head with a unique pattern for every position. The read head contains two sensors — one for each track. One setup uses a Hall sensor to read the absolute track … and a magnetostrictive sensor to read the incremental track. On startup, the encoder reads the absolute track to determine its position … and then during movement, it reads the incremental track for position tracking and measurement.
The first consideration when choosing a linear encoder is whether the application requires incremental or absolute feedback. Here, design engineers should consider whether they need to know the actuator’s position after a power loss. If so, an absolute encoder is necessary — because an incremental encoder will lose its reference when the power supply is interrupted … requiring a rehoming sequence to determine the load’s actual position.
Another way to decide whether an application requires an incremental or an absolute encoder is to consider whether rehoming is feasible after a power loss. Knowing the actuator’s exact position may be noncritical, but if the travel distance is long relative to the machine’s speed (as is common with machine tools) an absolute encoder can help avoid downtime and productivity due to lengthy rehoming sequences.
Related: What types of linear encoders are there and how do I choose?
Whether incremental or absolute, the next factor to consider is what technology the application requires.
Proper airgap is key to reliable encoder operation
Regardless of encoder type, maintaining the correct gap between the sensor and the scale is key to maintaining read accuracy. Encoders employing magnetic modes of operation can allow gaps to several millimeters; in contrast, optical encoders can necessitate airgaps to within a fraction of a millimeter. Many manufacturers publish encoder accuracy at a minimum or maximum gap value. Using the latter value ensures that even with gap-distance variability, keeping the gap below the maximum allowable value essentially guarantees the encoder will deliver its published accuracy. In most cases, actual accuracy will exceed published values.
The two most common types of linear encoders are optical and magnetic. Traditionally, optical scales were the sole option for feedback resolutions below 5 mm. Today, improvements in magnetic-scale technology now allow their use on axes requiring feedback resolutions down to 1 mm as well.
Optical linear encoders for position tracking: Just as their rotary counterparts, optical linear encoders use a light source that shines through a linear scale and photodetectors on the scale’s other side to determine position. Optical linear encoders excel on motion axes requiring sub-µm resolution. Their use of light reflection or refraction does make them unforgiving of contaminants. Plus shock loads can knock this sensor gap out of specification and even damage the encoder — especially those with glass scales or delicate sensor ASICs.
Magnetic linear encoders for position tracking: Just as their rotary counterparts, magnetic linear encoders use a magnetic reader head and a magnetic scale to determine position. Consider the most common variation of magnetic linear encoders. These have a read-head sensing element that rides along a magnetically coded scale. The scale coding consists of regions of alternating polarity. These alternating north and south magnetic poles are spaced at a precise distance called the pole pitch. The read head of a magnetic linear encoder contains either Hall or magnetoresistive sensors. These two technologies offer similar strengths and drawbacks, and in fact both quantify magnetic fields as well. However:
• Magnetoresistive linear-encoder read heads track magnetic-field direction
• Hall effect linear-encoder read heads track magnetic-field strength
As the read head moves over the tape, it detects the magnetic poles on the scale through either a change in voltage or a change in magnetic resistance.
The linear scales of magnetic linear encoders are flexible multi-layered strips having an adhesive backing and the magnetic scale — topped off (in some cases) with a plastic or stainless-steel cover strip to protect the magnetic scale. Because it’s flexible and has an adhesive backing, the scale assembly is sometimes called a magnetic tape.
Related: How do magnetic encoders work?
One advantage of magnetic linear encoders is the way in which magnetic tape can be supplied in very long lengths. In fact, real-world application examples include magnetic scales upwards of 50 meters long. But for incremental encoding, this means the homing sequence to a single reference mark could require traversing the entire length of the encoder. Therefore, magnetic linear encoders often include distance-coded reference marks. These extra marks are magnetic poles on the scale in addition to the standard magnetic poles. The reference marks are individually spaced — in other words, in a distinctive irregular pattern along the length of the tape that’s independent of the standard magnetic poles.
After traversing two reference marks, linear encoders with such scales can report absolute position as well as the distance between the two marks and the direction of travel as well as the length of each magnetic pole and the basic increment — the distance between odd reference marks.
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