Tham khảo tài liệu 'industrial brushless servomotors episode 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Brushless commutation 75 Absolute encoders One form of absolute encoder is a development of the optical incremental type. Unlike the incremental type however the signals generated represent discrete shaft positions. Each position is given its own binary number and each digit of the number must have a separate track on the glass disc. Each track has its own scanning the scans being checked with respect to each other so that reading errors are avoided. The basic principle is shown in Figure b . If the disc starts in an anticlockwise direction from the position shown the shaft positions are defined by the signals 1111 0000 0001 . . . 1110. The simple four-track encoder shown would be capable of defining only 16 positions of the shaft. A 10-track disc defines 210 or 1024 positions and so on. Absolute encoders clearly have great complexity and are relatively expensive to produce. The design must combine the photoelectric requirements with a resistance to adverse conditions such as mechanical shock and vibration 4 . There are other designs of absolute encoder. The Sincos type produces sine and cosine signals in analogue form which can be passed directly to the drive unlike the resolver where a signal from the drive is amplitude-modulated and passed back to the drive. More significantly the signals from the Sincos device can also be encoded locally and passed to the drive in digital form. Although many times more expensive than a resolver the Sincos device has clear advantages in the digital control of interconnected servo systems. Power electronics In Figure current is supplied to the brushless motor from a power electronic inverter which must repeatedly change the direction of the stator current. The direction cannot be changed without the interruption and reinstatement of a current which may be tens or even hundreds of amps. There 76 Industrial Brushless Servomotors are at least four semiconductor devices which can be used as switches for controlling .
