Many Allen Bradley servo motors use Stegmann encoders.12/28/2017Many Allen Bradley servo motors use Stegmann encoders as feedback devices as opposed to a standard sinusoidal encoder. Sinusoidal encoders encode position information by providing a pair of quadrature sine and cosine signals as the shaft is rotated. The signals may be generated by optical or magnetic means and typically produce 512 or 1024 cycles per mechanical revolution. For noise immunity the signals are typically transmitted differentially from the encoder to the sensor interface electronics. In order to extract reliable position and speed information from the sinusoidal encoder signals, a certain amount of pre-conditioning of the analogue signals must be implemented. As a first stage, the differential SIN and COS signals (typically 1Vpp input signal range) from the sinusoidal encoder must be applied to input differential amplifiers. This ensures maximum noise immunity and may also be used to appropriately amplify and level shift the resultant single-ended SIN and COS signals for later input to the Analogue to Digital Converter (ADC) stage. Next, the SIN and COS signals are applied to comparators that generate square-wave, TTL-level signals (EIA & EIB) that are synchronized to the SIN and COS signals respectively. The analogue SIN and COS signals are also fed to dual sample and hold amplifiers (SHA) for subsequent conversion to digital and post-processing in the digital signal processor (DSP). The signals EIA and EIB are typically applied to the internal quadrature counter of a dedicated digital Encoder Interface Unit (EIU). The resultant parallel word in the quadrature counter provides the crude position estimate from the sinusoidal encoder. There are four counts per cycle of the sinusoidal waveforms (SIN and COS). And so for a 512 line encoder the maximum count value is 2047 (4*512-1) which provides 11 bits of crude position information. In the case of a 1024-line encoder the maximum count is 4095 which provides 12 bits of resolution. In the case where the EIB signal leads the EIA signal, the encoder is determined to be rotating in the reverse direction and the quadrature count value (EIUCNT) is decremented at each edge of the EIA and EIB signals. Naturally, when EIA leads EIB, the quadrature count value is incremented at each event. Fine position resolution is obtained by further processing the digitized SIN and COS signals to provide much finer granularity between the EIA and EIB events. In some applications, it is necessary to know the initial position of the rotor following power up. There are different techniques used to obtain this information depending on the particular encoder design. Some encoder designs provide an alternative pair of sine and cosine signals that provide one cycle per mechanical revolution, from which it is possible to derive an initial position estimate. Alternatively, some modern sinusoidal encoders now provide a dedicated serial interface that can be used to extract the initial position following power up. During normal operation, the complete position information must be constructed from both the crude and fine position information. The crude information contains the cycle identification information, which, depending on the encoder used, can be 11 or 12 bits. The fine information is the result of the calculation from the sinusoidal signals. With high-resolution analogue to digital conversion of 12 bits on the SIN and COS, position information to 23/24 bit can be achieved. Since the crude position information is a quadrature value, the 2 least significant bits (LSB) are the same as the 2 most significant bits (MSB) of the fine data. Thus, only the 9 MSBs of the crude information are used in the case of a 512-line encoder. In the case of a 1024 line encoder the first 10 bits are used. Stegmann’s Hiperface compatible encoders differ slightly from this configuration as they only output the 512/1024 SINCOS waveforms and transmit the other waveform information as digital position instead. Depending on the encoder, it is possible to obtain up to 15 bits of digital position information. In the case of the multi-turn encoders it is possible to get 12 bits of multi-turn information, i.e. the number of complete revolutions. Back To Blog
Many Allen Bradley servo motors use Stegmann encoders.12/28/2017Many Allen Bradley servo motors use Stegmann encoders as feedback devices as opposed to a standard sinusoidal encoder. Sinusoidal encoders encode position information by providing a pair of quadrature sine and cosine signals as the shaft is rotated. The signals may be generated by optical or magnetic means and typically produce 512 or 1024 cycles per mechanical revolution. For noise immunity the signals are typically transmitted differentially from the encoder to the sensor interface electronics. In order to extract reliable position and speed information from the sinusoidal encoder signals, a certain amount of pre-conditioning of the analogue signals must be implemented. As a first stage, the differential SIN and COS signals (typically 1Vpp input signal range) from the sinusoidal encoder must be applied to input differential amplifiers. This ensures maximum noise immunity and may also be used to appropriately amplify and level shift the resultant single-ended SIN and COS signals for later input to the Analogue to Digital Converter (ADC) stage. Next, the SIN and COS signals are applied to comparators that generate square-wave, TTL-level signals (EIA & EIB) that are synchronized to the SIN and COS signals respectively. The analogue SIN and COS signals are also fed to dual sample and hold amplifiers (SHA) for subsequent conversion to digital and post-processing in the digital signal processor (DSP). The signals EIA and EIB are typically applied to the internal quadrature counter of a dedicated digital Encoder Interface Unit (EIU). The resultant parallel word in the quadrature counter provides the crude position estimate from the sinusoidal encoder. There are four counts per cycle of the sinusoidal waveforms (SIN and COS). And so for a 512 line encoder the maximum count value is 2047 (4*512-1) which provides 11 bits of crude position information. In the case of a 1024-line encoder the maximum count is 4095 which provides 12 bits of resolution. In the case where the EIB signal leads the EIA signal, the encoder is determined to be rotating in the reverse direction and the quadrature count value (EIUCNT) is decremented at each edge of the EIA and EIB signals. Naturally, when EIA leads EIB, the quadrature count value is incremented at each event. Fine position resolution is obtained by further processing the digitized SIN and COS signals to provide much finer granularity between the EIA and EIB events. In some applications, it is necessary to know the initial position of the rotor following power up. There are different techniques used to obtain this information depending on the particular encoder design. Some encoder designs provide an alternative pair of sine and cosine signals that provide one cycle per mechanical revolution, from which it is possible to derive an initial position estimate. Alternatively, some modern sinusoidal encoders now provide a dedicated serial interface that can be used to extract the initial position following power up. During normal operation, the complete position information must be constructed from both the crude and fine position information. The crude information contains the cycle identification information, which, depending on the encoder used, can be 11 or 12 bits. The fine information is the result of the calculation from the sinusoidal signals. With high-resolution analogue to digital conversion of 12 bits on the SIN and COS, position information to 23/24 bit can be achieved. Since the crude position information is a quadrature value, the 2 least significant bits (LSB) are the same as the 2 most significant bits (MSB) of the fine data. Thus, only the 9 MSBs of the crude information are used in the case of a 512-line encoder. In the case of a 1024 line encoder the first 10 bits are used. Stegmann’s Hiperface compatible encoders differ slightly from this configuration as they only output the 512/1024 SINCOS waveforms and transmit the other waveform information as digital position instead. Depending on the encoder, it is possible to obtain up to 15 bits of digital position information. In the case of the multi-turn encoders it is possible to get 12 bits of multi-turn information, i.e. the number of complete revolutions.