SSI Encoders
- Thread starterchawalang
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porkchop
Well-Known Member
Two very different technologies that can both go into the same product. First off, I would say that although both are advantages over more simplistic technologies I have worked on large automation systems with huge budgets that operate safely and efficiently for years using neither technology.
Grey Code is a way of rejiggering the standard binary number representation so that only a single digit changes at any given transition on the encoder disk. In automation it is unique to absolute encoders although it has other uses elsewhere. As an example, if you're counting from 0 to 3 in binary normally the sequence would be 00, 01, 10, 11, 00, ... but that sequence has two times that multiple bits change at the same transition (from 1 to 2 and 3 to 0). However, if you swap 11 and 10 then the sequence is 00, 01, 11, 10, 00, ... and then the sequence only has one digit change at each transition. Looking at the number sequence is a little more confusing, but this is almost always handled directly by the hardware. Grey Code is helpful in automation because it removes a lot of potential timing issues. When an electric motor is spinning a 12 digit absolute encoder at 5000 RPM you don't want your control system to have to guess if the 1 μs time difference between digit changes was a poling/timing issue or actual shaft movement. It is also a lot more noise tolerant.
SSI referrers to how the encoder passes its positional information on to the control system. It is mostly used for absolute encoders, but can be used for other styles as well. Instead of parallel communications where you have a single conductor for each digit of the absolute encoder, the encoder itself (or a converter nearby) keeps track of the position and transmits that digitally to the control system. This allows for position information to be transmitted accurately over a much longer distance. It also is less effected by EMI and the significant reduction in conductor count makes budgets happy. In my experience in entertainment automation, actual SSI is not particularly common, but the EnDat encoders that I do see operate on a very similar principle. In either case if you're talking about a large installation (like Vegas showrooms) often the serial encoder signal will go to another device that will send the position information over a industrial bus or network protocol (TCP/IP, DriveCliq, ProfiNet, DeviceNet, Profibus DP, ...) to the automation system to further increase transmission speed and reliability.
I feel like the Danaher Encoder Handbook is a pretty decent reference to use as a jumping off point. If you dig around on most encoder manufacturer's websites most of them have some pretty good technical information on the physical details and differences between different encoder technologies.
Grey Code is a way of rejiggering the standard binary number representation so that only a single digit changes at any given transition on the encoder disk. In automation it is unique to absolute encoders although it has other uses elsewhere. As an example, if you're counting from 0 to 3 in binary normally the sequence would be 00, 01, 10, 11, 00, ... but that sequence has two times that multiple bits change at the same transition (from 1 to 2 and 3 to 0). However, if you swap 11 and 10 then the sequence is 00, 01, 11, 10, 00, ... and then the sequence only has one digit change at each transition. Looking at the number sequence is a little more confusing, but this is almost always handled directly by the hardware. Grey Code is helpful in automation because it removes a lot of potential timing issues. When an electric motor is spinning a 12 digit absolute encoder at 5000 RPM you don't want your control system to have to guess if the 1 μs time difference between digit changes was a poling/timing issue or actual shaft movement. It is also a lot more noise tolerant.
SSI referrers to how the encoder passes its positional information on to the control system. It is mostly used for absolute encoders, but can be used for other styles as well. Instead of parallel communications where you have a single conductor for each digit of the absolute encoder, the encoder itself (or a converter nearby) keeps track of the position and transmits that digitally to the control system. This allows for position information to be transmitted accurately over a much longer distance. It also is less effected by EMI and the significant reduction in conductor count makes budgets happy. In my experience in entertainment automation, actual SSI is not particularly common, but the EnDat encoders that I do see operate on a very similar principle. In either case if you're talking about a large installation (like Vegas showrooms) often the serial encoder signal will go to another device that will send the position information over a industrial bus or network protocol (TCP/IP, DriveCliq, ProfiNet, DeviceNet, Profibus DP, ...) to the automation system to further increase transmission speed and reliability.
I feel like the Danaher Encoder Handbook is a pretty decent reference to use as a jumping off point. If you dig around on most encoder manufacturer's websites most of them have some pretty good technical information on the physical details and differences between different encoder technologies.
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