Sensors in high-speed applications
Get it and go: Your motion system is tuned and ready to sprint. Can your sensors keep up? This article includes online-exclusive content.
| Kees/Allied Motion Technologies: To achieve, say, 12,000 rpm, encoder resolution would have to be lowered to 500 counts/rev; the low speed end would suffer even more. For a modest sample time of 500 microseconds and a one count change in the system, the minimum adequately controlled speed is about 240 rpm. A 0.1 rpm system, by contrast, would require at least 300,000 full quadrature counts per revolution which cannot be supplied by current encoder technology.
There is a solution, however, for both the high and low end of an encoder's speed range. Contrary to popular belief, the signal generated by an optical encoder is not digital, but mostly sinusoidal. In a "digital output" encoder, these signals are squared by means of a comparator thus yielding two state changes per full sine and cosine cycle. This generates a total of four measuring points or "clicks" per quadrature cycle. The rest of the position information contained in the sine/cosine is thrown out by the comparator. With the advent of fast a/d converters, DSPs, and such, it is possible to extract far more information from the sine/cosine than just four zero crossings. This process is called interpolation and is extremely effective in "multiplying" the physical linecount of an encoder. The interpolation process is well documented and usually follows the extraction of the angle of the rotating vector tracing a "Lissajous" circle and is obtained by dividing the sine into the cosine or vice versa. Specialized IC's are readily available to do just that and generate a "pseudo quadrature" with multiplication factors of up to 12 or 14 bits. With this approach, high speed (16,000 rpm) measurements are handled by just using the sine/cosine zero crossings of a low count encoder (typically with a linecount of 512 or 1,024 counts/rev). Low speed measurements, on the other hand, are served by the interpolator. Even a modest 12 bits of interpolation and 512 counts/rev will yield a resolution of 542,288 full quadrature cycles per revolution, more than enough to accommodate very low speeds. This kind of very high data rate functions like an "electronic flywheel." Changes in speed are immediately compensated with full system gain. Sine/cosine encoders must meet a few requirements — low distortion, good thermal characteristics, and a servo controlled light source — care to generate signals suitable for interpolation, but other than that, they are not inherently more expensive or complicated than conventional "digital output" versions, which are in essence identical. Sine/cosine encoders can achieve resolutions of up to 36 million full A and B cycles per revolution, or 144 million state changes/rev. Bob/Optek: The speed specification (count speed) of an optical encoder is affected by the optical properties of the sensor; the type, amount, and quality of the light source; the design of the electrical circuit and amplifier; the type and capability of the interface circuitry bus or protocol; and lastly, the design capability of the counting circuitry. Additionally, there are mechanical factors in an encoder design, such as bearings, seal integrity, couplings, wheel wobble, feature consistency, and concentricity, all of which affect the rotational characteristics and consistency of the encoder. Brian/Avtron: One of the key problems that encoders face is vibration from high speed shafts. A small misalignment of a coupled encoder or a slightly off-true stub shaft on a hollow-shaft mount can cause serious vibration problems. This vibration can cause the encoder disk to "crash," hitting the sensor head, or cause the optical disk to crack. In optical and magnetic encoders, these problems can be addressed by spacing the sensor farther from the disk and using shatter-proof encoder disks. A second problem that comes from vibration is bearing failure. Locked-in, multiple bearings with higher holding force (up to 1,500 lb) avoid most problems, however. Another serious concern is pushing the high-speed signal through long wires. The answer here is high current drivers (3,000 mA vs. 40 mA). These powerful line drivers can push high frequency signals through less-than-perfect wires over 1,000 ft in length. A final concern is sealing. Some encoders do away with seals to reduce friction and heat at high speeds. But this invites dust and dirt which can accumulate inside the encoder, possibly preventing sensors from seeing the lines on the disk. Some types of magnetic encoders, however, don't need seals; the magnetic sensor can "see through" dirt, water, and other contaminants that may enter the encoder. Chuck/Danaher Industrial Controls: Communicating position data represents a major bottleneck in the speed of an absolute encoder system. A controller can only respond to data it has received, so the faster it can receive data, the more frequently it can make a position or speed-based control decision. Network speed varies greatly depending on the type. An encoder communicating to a controller over DeviceNet or Profibus, which can support multiple types of devices, can update position every 1 to 50 msec, depending on the network length and traffic. An encoder on SSI can typically communicate a position every 5 msec. A new, open protocol dedicated to encoder communications, called BiSS, can provide position updates to the controller every 56 microseconds. Martin/Gurley Precision: The maximum operating speed for a given application is determined either mechanically — the so-called slew speed limit — or by electronic frequency response limitations imposed by the encoder electronics or the control system's encoder interface. In rotary encoders, bearings and shaft couplings are sometimes the limiting factors. Obviously, top speed can be limited by particular bearing and coupling designs. Typical design considerations involve choosing load capacity, size, construction, lubricant, and bearing life to match the intended encoder end use. Flexible couplings have maximum speed limitations as well. Similar limitations occur in enclosed linear encoders. Just as with rotary encoders, common construction methods involve the use of friction wear pads or small bearings as rollers in order to maintain component alignment. These are subject to the same high and low speed problems as rotaries. One of the primary advantages of modular, non-contacting encoders, both rotary and linear, is that they impose no practical mechanical speed limit. The electronic limit in a rotary incremental application is obtained by the equation: Vmax = Fmax x 1,000 x 60 (rpm) / (L x K), where Fmax is either the maximum permissible encoder square wave frequency, or the maximum input frequency of connected external electronics in kHz (whichever is more limiting); L is the number of lines on the disc; and K is the interpolation factor. The way to overcome application speed limits is by providing the correct encoder (modular vs. enclosed) for a given application; choosing the best code disk/scale material, diameter, and cross section; managing combined disk/scale resolution and electronic interpolation; using electronic components and circuits with acceptable frequency response; using optimum interface circuitry and counter/controller clock frequency; documenting correct steps for installation and alignment; and recommending the correct coupling and fastening methods. Howard/Renishaw: Some of these transmission bottlenecks can be overcome by switching from RS-422 output to a serial system. Using an analog system and interpolating the signals in the drive/control electronics eliminates transmission imposed limitations altogether. Similarly, it can be difficult to wire sensors to two sides of a conveyor. In these situations, opposed-mode sensors — those with the emitter and receiver in separate units -- are inconvenient. "Here, sensors that include the emitter and receiver in one unit are a better choice," says Kielblock. |
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© 2012 Penton Media Inc.
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