Innovation sometimes explodes onto the scene as a disruptive technology. More often, though, it arrives quietly in the form of continuous improvement that enhances performance and expands the scope of application capabilities. Sometimes evolutionary improvements are subtle, but once in a while they are game-changing.
When it comes to magnetic linear encoders, there have been steady improvements over the years in terms of resolution and linearity, enabling them to replace optical linear encoders in many applications at a fraction of the cost. One stubborn limitation, however, has been the trade-off between measuring performance and tape-to-sensor gap distance, sometimes called simply the gap distance or the ride height. Generally speaking, the higher the resolution and/or linearity specification, the smaller the allowable gap distance or ride height becomes. This reduction in ride height requires a corresponding tightening of machine tolerances in order to ensure that the maximum allowable gap distance is not exceeded.
Recent breakthroughs in magnetic encoder design and technology have resulted in a new class of linear encoder systems that offer greatly expanded ride height. For example, an incremental system with 1 μm resolution and a system accuracy of ± 10 μm required a typical maximum tape-to-sensor gap distance of 0.35 mm. Now, the new generation of encoder technology can deliver the same 1 μm resolution and a similar ± 12 μm system accuracy, but with a maximum gap distance of 1.0 mm, nearly a threefold increase in ride height. That means far better tolerance of variability in the gap distance as the machine goes through its motions.
What’s more, encoder functionality can be assured even when the gap distance increases to as much as 1.8 mm, albeit with some loss of accuracy at these extreme distances. The ability to tolerate expanded variation in ride height ensures that machine operation will not be disrupted by loss of the encoder signal, even when gap tolerances occasionally exceed design maximums. That translates directly into greater design freedom for the engineer, and more machine uptime with fewer nuisance stoppages for the end user.
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Incremental encoders are pretty simple and straightforward. They provide digital pulses, typically in A/B quadrature format, that represent relative position movement. The number of pulses the encoder sends out correspond to the amount of position movement. Count the pulses, do some simple math, you know how much movement has occurred from point A to point B. But, here’s the thing, you don’t actually know where you are exactly. You only know how far you’ve moved from where you started. You’ve counted an increment of movement. If you truly want to know where you are, you have to travel to a defined home or reference position and count continuously from that position.
Absolute encoders, on the other hand, provide a unique output value everywhere along the linear travel, usually in the form of a serial data “word”. Absolute encoders tell you exactly (absolutely) where they are at all times. There’s no need to go establish a home or reference position.
Tabletop automation is a trend that is gaining momentum, especially in the fields of medical laboratory automation and 3D printing. Both of these applications demand a level of linear positioning accuracy and speed that might suggest a servomotor as a solution, but market-driven cost constraints put most servos out of financial consideration. New advances in stepper motor design, including higher torque, higher power ratings, and the availability of closed-loop operation via integrated motor encoder feedback are enabling steppers to expand their application envelope to include many tasks that formerly demanded a servo system.
As tabletop automation development progresses, performance demands are increasing to the point that stepper systems may struggle to meet requirements. Fortunately, the addition of an external linear encoder for direct position feedback can enhance a stepper system to enable the expected level of reliable accuracy. An external linear encoder puts drive-mechanism non-linearity inside the control loop, meaning any deviations caused by drive component inaccuracy are automatically corrected and compensated by the overall closed-loop positioning system. In addition, the external linear encoder provides another level of assurance that the driven element has actually moved to the position indicated by the number of stepper pulses and/or the movement reported by the motor encoder. This prevents position errors due to stepper motor stalling, lost counts on the motor encoder, someone manually moving the mechanism against motor torque, or drive mechanism malfunction, i.e. broken drive belt or sheared/skipped gearing.
