Position Encoders for Servo Drives - DR. JOHANNES HEIDENHAIN GmbH - #19

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       19 Diagram 2: Shaft speed and resulting output frequency as a function of the number of signal periods per revolution Shaft speed [min–1] .. Signal periods per revolution Output frequency [kHz] .. HEIDENHAIN absolute encoders for “digital” drives also supply additional sinusoidal incremental signals with the same characteristics as those described above. Absolute encoders from HEIDENHAIN use the EnDat interface (for Encoder Data) for the serial data transmission of absolute position values and other information for automatic selfconfi guration, monitoring and diagnosis. (See Absolute Position Values – EnDat.) This makes it possible to use the same subsequent electronics and cabling technology for all HEIDENHAIN encoders. Important encoder specifi cations can be read from the memory of the EnDat encoder for automatic self-confi guration, and motor-specifi c parameters can be saved in the OEM memory area of the encoder. The usable size of the OEM memory on the rotary encoders in the current catalogs is at least 1.4 KB (ƒ 704 EnDat words); for the ATEX encoders it is 0.44 KB (ƒ 224 EnDat words). Most absolute encoders themselves already subdivide the sinusoidal scanning signals by a factor of 4 096 or greater. If the transmission of absolute positions is fast enough (for example, EnDat 2.1 with 2 MHz or EnDat 2.2 with 8 MHz clock frequency), these systems can do without incremental signal evaluation. Benefi ts of this data transmission technology include greater noise immunity of the transmission path and less expensive connectors and cables. Encoders with EnDat2.2 interface offer the additional feature of being able to evaluate an external temperature sensor, located in the motor coil, for example. The digitized temperature values are transmitted as part of the EnDat 2.2 protocol without an additional line. Bandwidth The attainable gain for the position and speed control loops, and therefore the bandwidth of the drives for command response and control reliability, are sometimes limited by the rigidity of the coupling between the motor shaft and encoder shaft as well as by the natural frequency of the coupling. HEIDENHAIN therefore offers rotary and angular encoders for high-rigidity shaft coupling. The stator couplings mounted on the encoders have a high natural frequency up to 2 kHz. For the inductive rotary encoders, the stator and rotor are fi rmly screwed to the motor housing and to the shaft. This means that the rigidity of the motor shaft is of the most signifi cance for the attainable natural frequency. (See alsoMechanical Design and Installation.) Size A higher permissible operating temperature permits a smaller motor size for a specifi c rated torque. Since the temperature of the motor also affects the temperature of the encoder, HEIDENHAIN offers encoders for permissible operating temperatures up to 120 °C. These encoders make it possible to design machines with smaller motors. Power loss and quietness The power loss of the motor, the accompanying heat generation, and the acoustic noise during operation are infl uenced by the position error of the encoder within one signal period. For this reason, rotary encoders with a high signal quality of better than ± 1 % of the signal period are preferred. (See also Measuring Accuracy.) Properties and Mounting