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Entry type: FAQ Entry ID: 36489093, Entry date: 06/04/2009
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How can you read in an SSI absolute encoder into a Technology CPU if you do not use an IM 174 or an ADI4?

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Description:
The IM 174 and the ADI4 are available for connecting hydraulic axes or third-party devices to the Technology CPU (reading in of encoder signals and output of analog setpoint values).
However, on the DP drive bus of Technology CPUs, it is also possible to use ET 200S (only IM 151-HF) and ET 200M (only IM 153-2) PROFIBUS slaves to read in digital and analog signals synchronously into the Technology CPU and output them accordingly. For example:

  • Cams and cam tracks
  • Evaluating hardware limit switches
  • Reading in analog absolute value encoders
  • Controlling hydraulic drives via analog outputs

You can use the 1SSI module (ET 200S) and the SM 338 PosInput (ET 200M) to read in SSI encoders and make the absolute value available as an IO input signal to a DP master (e.g. a Technology CPU) via PROFIBUS.
You can use this for connecting hydraulic axes, for example, which have an integrated SSI scale of length (not an analog linear scale) and which are to be controlled via an analog output of the ET 200S/ET 200M.

If you want to read in encoder values in S7 Technology via the ET 200M or ET 200S, only configuration masks for analog encoders are available.
Nevertheless, the encoder value provided by a 1SSI module or an SM 338 can be read in this way. Here, you must parameterize it like an analog encoder.
Below are recommendations for the correct parameterization.

Note:
The Technology CPU cannot recognize an encoder overflow (zero crossing) in this constellation!

13-bit rotary single-turn encoder, e.g. 6FX2001-5xx12 (Gray code):
 

ET200S (IM151-1 HF) - 1SSI Fast-Mode (6ES7 138-4DB03-0AB0 Fast) ET200M (IM 153-2) - SM 338 PosInput (6ES7 338-4BC01-0AB0 V2.0)
Hardware configuration - IM 151-1 HF:
  • Synchronize DP slave to equidistant DP cycle: activate.
  • Clock synchronization: activate clock-synchronized mode for 1SSI Fast-Mode.
  • Change time Ti/To.
Hardware configuration - IM 153-2:
  • Synchronize DP slave to equidistant DP cycle: activate.
  • Clock synchronization: activate clock-synchronized mode for SM338 POS-INPUT.
  • Change time Ti/To.
Hardware configuration - 1SSI Fast-Mode:
  • Input address: 284 (example)
  • Acquisition: synchronous
  • Encoder type: SSI 13-bit
  • Gray/binary converter: Gray
  • Baud rate: 125 kHz
  • Parity: none
  • Monoflop time: 64µs
  • Standardization: none
  • Number of trailing bits: 0
  • Reverse of direction of rotation: according to installation
  • Total number of pitches of the absolute encoder: 8192
  • Lifebeat: none
  • Encoder scanning frequency: no restrictions
Hardware configuration - SM 338 PosInput:
  • Input address: 268 (example)
  • Absolute encoder (SSI): 13-bit
  • Code type: Gray
  • Clock frequency: 125 kHz
  • Monoflop time: 64µs
  • Places [0..12]: 0
  • Steps/rotation: 8192
S7 Technology - axis/encoder configuration:
  • Log hardware addresses: Input: 286 (284+2 because under 16 bits)
  • Factor: Displacement per encoder increment
  • Offset: shift of encoder actual value if necessary
  • Encoder resolution: 13
  • Format: aligned right without sign
  • Minimum non-linearized value: 0
  • Maximum non-linearized value: 8192
  • Error tolerance time: 0.1s
S7 Technology - axis/encoder configuration:
  • Log hardware addresses: Input: 270 (268+2 because under 16 bits) for Channel 0, 274 for Channel 1, 278 for Channel 2
  • Factor: displacement per encoder increment
  • Offset: shift of encoder actual value if necessary
  • Encoder resolution: 13
  • Format: aligned right without sign
  • Minimum non-linearized value: 0
  • Maximum non-linearized value: 8192
  • Error tolerance time: 0.1s


25-bit rotary single-turn encoder, e.g. 6FX2001-5xx24 (Gray code):
 

ET200S (IM151-1 HF) - 1SSI Fast-Mode (6ES7 138-4DB03-0AB0 Fast) ET200M (IM 153-2) - SM 338 PosInput (6ES7 338-4BC01-0AB0 V2.0)
Hardware configuration - IM 151-1 HF:
  • Synchronize DP slave to equidistant DP cycle: activate.
  • Clock synchronization: activate clock-synchronized mode for 1SSI Fast-Mode.
  • Change time Ti/To.
Hardware configuration - IM 153-2:
  • Synchronize DP slave to equidistant DP cycle: activate.
  • Clock synchronization: activate clock-synchronized mode for SM338 POS-INPUT.
  • Change time Ti/To.
Hardware configuration - 1SSI Fast-Mode:
  • Input address: 284 (example)
  • Acquisition: synchronous
  • Encoder type: SSI 25-bit
  • Gray/binary converter: Gray
  • Baud rate: 125 kHz
  • Parity: none
  • Monoflop time: 64µs
  • Standardization: none
  • Number of trailing bits: 0
  • Reverse of direction of rotation: according to installation
  • Total number of pitches of the absolute encoder: 33554432
  • Lifebeat: none
  • Encoder scanning frequency: no restrictions
Hardware configuration - SM 338 PosInput:
  • Input address: 268 (example)
  • Absolute encoder (SSI): 25-bit
  • Code type: Gray
  • Clock frequency: 125 kHz
  • Monoflop time: 64µs
  • Places [0..12]: 0
  • Steps/rotation: 8192
S7 Technology - axis/encoder configuration:
  • Log hardware addresses: Input: 284 (because over 16 bits)
  • Factor: displacement per encoder increment
  • Offset: shift of encoder actual value if necessary
  • Encoder resolution: 25
  • Format: aligned right without sign
  • Minimum non-linearized value: 0
  • Maximum non-linearized value: 33554432
  • Error tolerance time: 0.1s
S7 Technology - axis/encoder configuration:
  • Log hardware addresses: Input: 268 (268+2 because over 16 bits) for Channel 0, 272 for Channel 1, 276 for Channel 2
  • Factor: displacement per encoder increment
  • Offset: shift of encoder actual value if necessary
  • Encoder resolution: 25
  • Format: aligned right without sign
  • Minimum non-linearized value: 0
  • Maximum non-linearized value: 33554432
  • Error tolerance time: 0.1s

25-bit linear scale; for instance, BTL5-S172B-M0250-P-S32 (5µm / 25-bit / Gray code / 250mm length):
 

ET200S (IM151-1 HF) - 1SSI Fast-Mode (6ES7 138-4DB03-0AB0 Fast) ET200M (IM 153-2) - SM 338 PosInput (6ES7 338-4BC01-0AB0 V2.0)
Hardware configuration - IM 151-1 HF:
  • Synchronize DP slave to equidistant DP cycle: activate.
  • Clock synchronization: activate clock-synchronized mode for 1SSI Fast-Mode.
  • Change time Ti/To.
Hardware configuration - IM 153-2:
  • Synchronize DP slave to equidistant DP cycle: activate.
  • Clock synchronization: activate clock-synchronized mode for SM338 POS-INPUT.
  • Change time Ti/To.
Hardware configuration - 1SSI Fast-Mode:
  • Input address: 284 (example)
  • Acquisition: synchronous
  • Encoder type: SSI 25-bit
  • Gray/binary converter: Gray
  • Baud rate: 125 kHz
  • Parity: none
  • Monoflop time: 64µs
  • Standardization: none
  • Number of trailing bits: 0
  • Reverse of direction of rotation: according to installation
  • Total number of pitches of the absolute encoder: 33554432
  • Lifebeat: none
  • Encoder scanning frequency: no restrictions
Hardware configuration - SM 338 PosInput:
  • Input address: 268 (example)
  • Absolute encoder (SSI): 25-bit
  • Code type: Gray
  • Clock frequency: 125 kHz
  • Monoflop time: 64µs
  • Places [0..12]: 0
  • Steps/rotation: 8192
S7 Technology - axis/encoder configuration:
  • Log hardware addresses: Input: 284 (because over 16 bits)
  • Factor: 5.e-003 (displacement per encoder increment)
  • Offset: shift of encoder actual value if necessary
  • Encoder resolution: 25
  • Format: aligned right without sign
  • Minimum non-linearized value: 0
  • Maximum non-linearized value: 33554432 or (250mm length / 0.005mm resolution = min. 50000 encoder increments)
  • Error tolerance time: 0.1s
S7 Technology - axis/encoder configuration:
  • Log hardware addresses: Input: 268 (268+2 because over 16 bits) for Channel 0, 272 for Channel 1, 276 for Channel 2
  • Factor: 5.e-003 (displacement per encoder increment)
  • Offset: shift of encoder actual value if necessary
  • Encoder resolution: 25
  • Format: aligned right without sign
  • Minimum non-linearized value: 0
  • Maximum non-linearized value 33554432 or (250mm length / 0.005mm resolution = min. 50000 encoder increments)
  • Error tolerance time: 0.1s

Example:
A 25-bit linear scale (for instance, BTL5-S172B-M0250-P-S32 with 5µm / Gray code / 250mm length) is to be read in by 1SSI module.

For this, on the DP drive you must add an ET 200S (IM151-1 HF) with a 1SSI FastMode module. The synchronicity must be activated for the IM 151-1 HF and for the 1SSI FastMode module, and the Ti and To times must be changed accordingly.


Fig. 01: Setting the DP slave properties

In the hardware configuration you must configure the 1SSI FastMode module so that the encoder value can be read in. The above-mentioned encoder is a 25-bit encoder with Gray code. The total number of pitches of the encoder is 225 = 33554432. Since this linear encoder stores the position value "aligned right" in the SSI message, you do not need to set a standardization. If the encoder did not provide the message value aligned right, but did not use the 2 lower places in the message, then you would need to set a standardization of 2. Acquisition of the encoder values is to be synchronous with the clock-synchronous PROFIBUS. The baud rate of 125 kHz is sufficient to read out 25 bits within the monoflop time of 200µs+64µs.


Fig. 02: Setting the properties of the 1SSI FastMode module

Usually, a linear axis type is chosen for hydraulic axes.


Fig. 03: Setting of the axis type

In our example we are working with a 25-bit encoder. Here, you should use the channel address of the encoder from the hardware configuration for the input address of the encoder value.
If the encoder has less than 16 bits, you must increase the channel address of the encoder from the hardware configuration by 2 (e.g. 286 = 284+2).


Fig. 04: Settings the encoder assignment

The above-mentioned linear scale has an encoder resolution of 5µm. This must be entered as factor and if necessary, also an offset to correct the position of the scale. The encoder resolution corresponds to the encoders maximum bit number of 25 bits. The 1SSI module and the SM 338 module stores an encoder value right aligned as integer value in the IO area. For this reason you must set the format "Aligned right without sign". The encoder actual value always starts with 0 and the minimum non-linearized value is also to be set to 0. The maximum non-linearized value can be set by default to the maximum encoder resolution (225) or to the maximum travel distance of the linear scale (this can never exceed the maximum encoder resolution!). The above-mentioned linear scale has a length of 250mm and a resolution of 5µm. Therefore, a value of 50000 (250mm/5µm) would also be possible as maximum non-linearized value.


Fig. 05: Setting the position values from the IO

Keywords:
T-CPU

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