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HEDS-6511 Datasheet(PDF) 4 Page - Agilent(Hewlett-Packard) |
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HEDS-6511 Datasheet(HTML) 4 Page - Agilent(Hewlett-Packard) |
4 / 15 page 4 Theory of Operation The HEDS-65xx / HEDL-65xx translate the rotary motion of a shaft into either a two or three channel digital output. The HEDS-65xx uses one of the standard HEDS-9000 or HEDS-9040 modules for encoding purposes. The HEDL-654x uses the standard HEDL-9040 for encoding purposes. As seen in the block diagram, these modules contain a single Light Emitting Diode (LED) as their light source (emitter). The light is collimated into a single parallel beam by means of a plastic lens located directly over the LED. Opposite the emitter is the integrated detector circuit (detector). This circuit consists of multiple sets of photodetectors and the signal processing circuitry necessary to produce the digital waveforms. The codewheel rotates between the emitter and detector, causing the light beam to be interrupted by a 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 codewheel. These detectors are also spaced such that a light period on one pair of detectors corresponds to a dark period on the adjacent pair of detectors. The photodiode outputs are then fed into the signal processing circuitry resulting in A, – A, B, and – B (I and – I also in the three channel encoders). Comparators receive these signals and produce the final outputs 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). In the HEDS-6540 / HEDL-6540 the output of the comparator for the index pulse is combined with that of the outputs of channel A and channel B to produce the final index pulse. The index pulse is generated once every rotation of the codewheel and is a one state width (nominally 90 electrical degrees), true high index pulse. It is coincident with the low states on channels A and B. Definitions Count (N): The number of bar and window pairs or counts per revolution (CPR) of the codewheel. One Cycle (C): 360 electrical degrees (e), 1 bar and window pair. One Shaft Rotation: 360 mechanical degrees, N cycles. Position Error ( ∆Θ): The normal- ized angular difference between the actual shaft position and the position indicated by the encoder cycle count. Cycle Error ( ∆C): An indication of cycle uniformity. The difference 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 (P): The number of electrical degrees that an output is high during one 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 ( Φ): the number of electrical degrees between the center of high state on channel A and the center of the high state on channel B. This value is nominally 90 e for quadrature output. Phase Error ( ∆Φ): The deviation of the phase from its ideal value of 90 e. Direction of Rotation: When the codewheel rotates in a counter- clockwise direction (when viewed from the encoder end of the motor) channel A will lead channel B. If the codewheel rotates in the clockwise direction channel B will lead channel A. Index Pulse Width (P0): The number of electrical degrees that an index output is high during one full shaft rotation. This value is nominally 90 e or 1/4 cycle. |
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