Absolute encoding disks create a unique code for each disk position. Multiple tracks with one sensor per track generate a special digital pattern known as Gray code. Gray code's uniqueness stems from only 1 data bit changing state with each count. This eliminates ambiguity in output values caused by multiple bits changing at slightly different times. Since only one bit changes, the reading has only two possible sequential values. Other codes such as binary or BCD can be derived from the Gray code value. Incremental disks are simple, having only one track shared by two sensors. The sensors are mounted so their signals are in quadrature, or 90° out of phase with each other. A special index slot provides a fixed reference for a starting shaft position.

Sine encoders offer high-level performance. Although more expensive than resolvers or incremental encoders, they are best in applications that need high accuracy coupled with high resolution. They are as rugged as a resolver and operate at speeds over 10,000 rpm.

Sine encoders resemble incremental encoders except the A and B data channels go to the controller as 1-V peak-topeak sine waves instead of square waves. The controller interpolates each complete sine wave as a means of increasing system resolution. This reduces truncation and quantization errors, allowing higher loop gains. Sine encoders can produce over 2 million counts/rev, or about 0.62 arc-sec of resolution. Such capability suits applications that handle high inertia or highmass loads. The greater resolution generates-more output pulses at slow speeds for finer control in accelerating, decelerating, and positioning these loads.

Like other encoders, sine encoders also may have commutation tracks, Hall emulation tracks, or auxiliary sinusoidal channels called C and D. These tracks provide absolute position within one revolution. The C and D channels resemble the sine and cosine signals used in resolvers.

An absolute, multiple-turn sine encoder is a variation of a sine encoder. These devices contain gears between the shaft and position wheel so the system knows the shaft position upon start up. They offer high precision, resolution, and accuracy for applications such as high-speed registration, film coating, and web control. Sine encoders also are candidates for low-speed operations where smooth rotation is critical. They help enable high gains, superior stiffness, and accurate positioning.

QUIETING NOISE

Feedback devices can generate electrical or optical signals. One advantage of using optical transmission lines for feedback signals is immunity to electrical noise or EMI/RFI environments. Noise can interfere with electrical feedback signals and distort data sent to the drive. It may be necessary to boost feedback signals out of the noise via amplifiers or signal conditioning devices. Newer feedback devices use ICs to convert and interpolate signals into more robust waveforms that overcome noise and propagation loss in connecting cables.