EPOS4: SSI configuration features

Topic:

• Is it possible to use a SSI encoder which has more than 32 bit data for an EPOS4?

Situation:

Most of the position controllers are based on 32 bit data length processing for position acquisition and control. Nevertheless there are some SSI multi-turn encoders present which even have a total resolution of more than 32 Bit and might even have additional leading or trailing status bits.

The quite high resolution of such SSI encoders offers the opportunity to use this encoder types in a very flexible way focused on the concrete demands of an application. One application might demand for a high resolution within one turn of a encoder shaft turn but the total positioning range is mechanically quite limited anyway. For most other applications a single-turn resolution of 4096 incr. per encoder shaft turn (= 12 bits) is already more than sufficient but it is quite important that a quite higher number of encoder shaft turns (e.g. 20 bits = 1048576) can be counted due to large positioning range. If a SSI encoder offers a high multi-turn and high single-turn resolution, it can be used for a wide variety of different application demands if the motor control is able to process the data.

Solution:

The EPOS4 based on firmware version 0x160 (or higher) can be adapted in a very flexible way to the features and data handling of SSI encoders of different suppliers.  It is possible to define what part of the position data of a SSI encoder with more than 32 position bits position (out of a maximum 62 bit data stream) is actually relevant for the application. It is possible ...

• ... to ignore one or more of the LSB (= lowest significant bits) of the "single-turn" data which means that the resolution within one turn of the encoder shaft is reduced. If a very high "single-turn" resolution of a SSI encoder is not required, this is a possibility to reduce the total length of position data bits in use by control to 32 bit.
  
  
• ... to ignore one or more of the MSB (= most significant bits) of the "multi-turn" position data if the total positioning distance is less than the theoretical maximum possible number of SSI encoder shaft turns.

EPOS Studio's "Startup" wizard can be used to define quite easily but in a flexible way how the SSI data has to be evaluated and what part of the position data has to be taken into account by the controller depending on concrete application requirements.

SSI encoder example 1:

There is a multi-turn SSI encoder in use with the following specification in use:

• Special bits leading: none
• Multi-turn resolution: 16 bits
• Single-turn resolution: 18 bits
• Special bits trailing: 10 bits

The SSI encoder's total position data length of 34 position data bits (= 16 "multi-turn" bits + 18 "single-turn" bits) can be reduced to 32 position data bits by the controller in different ways:

Configuration A:
If the total motion distance by an application will be less than 14 bits (= 16384 encoder shaft turns), it is possible to ignore the top 2 bits of the 16 bits "multi-turn" data.

Configuration B:
If the "single-turn" resolution of 18 bits (= 262144 incr. per encoder shaft turn) is not required by an application, it is possible to reduce it to 16 bits (= 65536 incr. per encoder shaft turn) in use by the controller. Actually a resolution of 10 up to 12 bits per encoder shaft turn is sufficient by most applications.

SSI encoder example 2:

There is a single-turn SSI encoder in use with the following specification in use:

• Special bits leading: 2 bits
• Multi-turn resolution: 20 bits
  but filled up to a length of 22 bits by additional leading '0' bits.
• Single-turn resolution: 16 bits
• Special bits trailing: 8 bits

The total position data length of 36 position data bits (= 20 "multi-turn" bits + 16 "single-turn" bits) can be reduced to 32 position data bits in different ways.
This very specific SSI encoder type fills the number of "multi-turn" data up to a fixed length of 22 bits by adding leading '0' bits in front of the actual "multi-turn" bits. The actual number of "multi-turn" data bits is still 20. This value has to be configured as the SSI encoder's "multi-turn" position data length. The leading '0' bits count as part of the "Special bits leading" in this case. The number of "Special bits leading" sums up to 4 (= 2 actual leading bits + 2 x '0' bits) due to this.

Possible configuration:
The following configuration is just one which might be commonly in use out of other possibilities. The SSI encoder's "single-turn" resolution of 16 bits (= 65536 incr. per encoder shaft turn) is reduced to 12 bits (= 4096 incr. per encoder shaft turn) for the controller which is still acceptable by most applications.

Additional notes:

• If a master demands for the complete position data provided by the SSI encoder, it can be retrieved as a 64-bit position data value by EPOS4's object "SSI position raw value complete" (0x3012/0x0D).
• The content of the configured "SSI bits leading" can be accessed by EPOS4's object "SSI special bits leading data" (0x3012/0x0C).
• The content of the configured "SSI bits trailing" can be accessed by EPOS4's object "SSI special bits trailing data" (0x3012/0x06).
  
  

Please refer to chapter "SSI absolute encoder" of the "EPOS4 Firmware Specification" to learn more details about the configuration and accessible process data of a SSI encoder.