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Menu 3
Digitax ST Advanced User Guide 61
Issue Number: 1 www.controltechniques.com
Linear EnDat encoders
The comms resolution is set to the number of bits required for the whole position within the position data messages from the encoder. The linear
encoder comms to sine wave ratio is calculated from the sine wave period and LS comms bit length. The encoder does not give the equivalent lines
per rev directly, but gives the length of a sinewave period in nm. Therefore the drive uses the pole pitch (Pr 5.36 or 21.31) and the number of motor
poles (Pr 5.11 or 21.11) for the current active motor (defined by Pr 11.45) to calculate the equivalent lines per revolution.
ELPR = Pole pitch x Number of motor pole pairs / Length of a sinewave
Normally the Number of motor poles will be set to 2, and so
ELPR = Pole pitch / Length of a sinewave
It should be noted that the equivalent lines per rev parameter is only updated when auto-configuration occurs, i.e. when the encoder is initialized, and
that it uses the pole pitch for the currently active motor. The value for Pole pitch x Number of motor pole pairs is limited to 655.35mm by the drive. If
the pole pitch is left at its default value of zero which would give ELPR = 0, or the result of the calculation is over 50000, the drive will initiate an Enc15
trip.
Rotary hiperface encoders
The drive can recognize any of the following devices: SCS 60/70, SCM 60/70, SRS 50/60, SRM 50/60, SHS 170, SCS-KIT 101, SKS36, SKM36. If
the drive cannot recognize the encoder type it will initiate Enc12 trip.
Linear Hiperface encoders
The drive can recognize the LINCODER. The calculations used for linear EnDat encoders are also used to determine the equivalent lines per
revolution. However, the length of a sine wave is fixed at 5mm.
EnDat encoders
If the encoder type is EnDat comms only then after auto-configuration parameter 03.39 is set to zero if the number of turns bits is less than 6. This
automatically selects absolute mode and prevents over speed trips at the encoder roll-over points which would otherwise occur.
SSI, SC.SSI
SSI encoders normally use gray code data format. However, some encoders use binary format which may be selected by setting this parameter to
one.
0 = 0ms, 1 = 1ms, 2 = 2ms, 3 = 4ms, 4 = 8ms, 5 = 16ms
A sliding window filter may be applied to the feedback taken from the drive encoder. This is particularly useful in applications where the drive encoder
is used to give speed feedback for the speed controller and where the load includes a high inertia, and so the speed controller gains are very high.
Under these conditions, without a filter on the feedback, it is possible for the speed loop output to change constantly from one current limit to the other
and lock the integral term of the speed controller.
3.42
Drive encoder filter
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range 0 to 5 (0 to16 ms)
Default 0
Update rate Background read
3.43
Maximum drive encoder reference
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range 0 to 40,000 rpm
Default 3,000
Update rate Background read
3.44
Drive encoder reference scaling
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
3 111
Range 0.000 to 4.000
Default 1.000
Update rate Background read
3.45
Drive encoder reference
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
1111
Range ±100.0%
Update rate 4ms write
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