Theory of Operation

The AEDT-9140 is an emitter/detector module. Coupled with a codewheel, these modules translates rotary motion of a shaft into a three-channel digital output.

As seen in Figure 1, the modules contain a single Light Emitting Diode (LED) as its light source. The light is col-limated into a parallel beam by means of a single poly-carbonate lens located directly over the LED. Opposite the emitter is the integrated detector circuit. This IC consists of multiple sets of photodetectors and the signal process-ing circuitry necessary to produce the digital waveforms.The codewheel rotates between the emitter and detector, causing the light beam to be interrupted by the pattern of spaces and bars on the codewheel.

The photodiodes which detect these interruptions are arranged in a pattern that corresponds to the radius and design of the code-wheel. These detectors are also spaced such that a light period on one pair of detectors corre-sponds to a dark period on the adjacent pair of detectors. The photodiode outputs are then fed through the signal processing circuitry resulting in A, A-bar, B, B-bar, I and I-bar. Comparators receive these signals and produce the final output for channels A and B. Due to this integrated phasing technique, the digital output of channel A is in quadrature with that of channel B (90 degrees out of phase).

Definitions

Note: Refer to Figure 2

Cycles (N): The number of electrical cycles per revolution (CPR). Note: CPR refers to the raw signal from encoder, that is the cycles before 4x decode.One Cycle (C): 360 electrical degrees (°e).

One Shaft Rotation: 360 mechanical degrees, N cycles.Cycle Error ( C): An indication of cycle uniformity. The differ-ence between an observed shaft angle which gives rise to one electrical cycle, and the nominal angular increment of 1/N of a revolution.

Pulse Width (S): The number of electrical degrees that an output is high during 1 cycle. This value is nominally 180°e or 1/2 cycle.

Pulse Width Error ( P): The deviation, in electrical degrees, of the pulse width from its ideal value of 180°e.

State Width (S): The number of electrical degrees between a transition in the output of channel A and the neighboring transition in the output of channel B. There are 4 states per cycle, each nominally 90°e.

State Width Error ( S): The deviation, in electrical degrees, of each state width from its ideal value of 90°e.

Phase Error ( φ): The deviation of the phase from its ideal value of 90°e.

Direction of Rotation: When the codewheel rotates in the clockwise direction viewing from top of the module (di-rection from V to G), channel A will lead channel B. If the codewheel rotates in the opposite direction, channel B will lead channel A.

Optical Radius (Rop): The distance from the codewheel’s cen-ter of rotation to the optical center (O.C) of the encoder module.

Index Pulse Width (Po): The number of electrical degrees that an index is high during one full shaft rotation. This value is nominally 90°e or 1/4 cycle.