FEATURE SENSORS & SENSING SYSTEMS PRECISION WITHOUT TRADEOFFS
Rotary encoders are vital components in the motion control feedback loop in an increasing variety of applications. Encoders are required to provide long term reliability, durability, and high performance despite often working in severe conditions. Here, Jeff Smoot of CUI goes into more detail
eventually burn out. If the disk is made of plastic as a cost reduction measure, it will a have limited temperature range, and any distortions or warping will affect accuracy. The magnetic encoder’s
S
tandard encoders typically provide between 48 and 2,048 pulses per
revolution (ppr), with most applications requiring between 800 to 1,024 ppr. Higher ppr may seem to offer greater apparent precision, but is both more costly and complex, placing additional calculation and processing burdens on the system controller or digital processor that is closing the loop. In addition to being unnecessary, excess precision can actually be detrimental due to noise, vibration and jitter in shaft position. Most shaft encoders are based on
optical or magnetic principles. The optical method uses a glass or plastic disk with two sets of windows around the periphery - Figure 1. An LED light source and photodetectors are located on opposing sides of the disk - as the disk turns, the on/off passing of light through the windows provides the typical square wave A & B quadrature pulses. Factors such as dirt, oil and other
containments, which occur both during assembly and in the field, can easily interfere with the disk and slots. Traditionally, to mitigate exposure to contaminants, the encoder is placed in a bell housing. This approach does not completely eliminate exposure to environmental contaminants, and additionally introduces new challenges including elevated temperatures and higher application costs. LEDs in optical encoders have a limited lifetime and their brightness can dim by half within 10,000 to 20,000 hours (roughly one to two years), and they
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construction is similar to the optical encoder, except that it uses a magnetic field rather than beam of light. It has a magnetised disk that spins over an array of magneto- resistive sensors. Any rotation of the wheel produces a response in these sensors, which goes to a signal conditioning front end circuit to determine shaft position. While it offers a high level of durability, the magnetic encoder is not as accurate and is highly susceptible to magnetic interference produced by electric motors.
A NEW PLATFORM Given the need for accurate, precise, and rugged rotary position encoding, CUI has adapted the capacitive sensing operating principles of a standard linear position encoder. The result is a rotary encoder platform known as AMT. Capacitive sensing uses patterns of bars
or lines, with one set on the fixed element and the other set on the moving element - Figure 2. As the encoder rotates, an application specific integrated circuit (ASIC) counts the line changes and also interpolates to find the precise position of the encoder and direction of rotation. The encoder ASIC’s electrical output is
100% compatible with optical and magnetic encoders. This non-contact encoder implementation has several
significant user benefits: l
It is not affected by dust, dirt, or oil. l It is less sensitive to heat and cold. l It is less susceptible to vibration. l There is no LED.
l The encoder needs only 6 to 10mA of Above: Figure 1
operating current. Designers fine tuning the proportional
integral derivative (PID) control loop are able to adjust the encoder’s ppr count to optimise performance without needing to change encoders. The control engineer simply instructs a change in the line count parameter of the encoder, until the desired control loop result is obtained.
ADDED VALUE In installation and production, the capacitive encoder brings other benefits.
Mechanically, its mounting holes are also matched to the
other encoder types, making it a fit and function compatible unit - Figure 3. There is a possible concern with the capacitive-based encoder, as there is with any electronic transducer and associated circuitry - namely, susceptibility to electrical noise and interference (EMI). Careful design of the ASIC interface circuitry as well as fine tuning of the encoder
Above: Figure 2 demodulation algorithms has
mitigated these issues. The ASIC also offers future opportunities for designs to also include embedded, onboard diagnostics to verify the performance of the encoder mechanism and ASIC itself, as part of a more intelligent encoder and subsystem.
Above: Figure 3
Above left: the AMT11 in actual installation
Below right: with the availability of the field tested encoders based on capacitive sensing principles, there’s no longer a need for the design engineer to make the difficult choice between the attributes that optical and magnetic encoders force - short and long time reliability versus output accuracy
With the availability of the field tested encoders based on capacitive sensing principles, there’s no longer a need for the design engineer to make the difficult choice between the attributes that optical and magnetic encoders force - short and long time reliability versus output accuracy. The capacitive encoder can do both, and also brings benefits in mechanical mounting, inventory, ppr selection, readout zeroing, and power consumption, all with full compatibility to standard outputs.
CUI
www.cui.com T: +1 800 275 4899
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