Control Techniques Digitax ST Advanced User's Manual page 58

Parameter
Menu 3
structure
If Pr 3.39 is set to 0 the encoder is a linear encoder and the following apply:
1. Pr 3.33 defines the ratio between the length of a sine wave period and the length of the least significant comms bit.
2. No mask is applied to the turns displayed in Pr 3.28.
3. Pr 3.35 defines the number of comms bits used to give the whole position value.
If the position feedback device is SC.Hiper or SC.EnDat it is possible for the drive to set up this parameter automatically from information obtained
from the encoder (see Pr 3.41).
EnDat, SSI - Comms only encoder mode
If this parameter is set to 1 or 2 the drive always takes the complete absolute position for these comms only type encoders. The turns (Pr 3.28),
position (Pr 3.29) and fine position (Pr 3.30) will be an exact representation of the position from the encoder. If the encoder does not provide 16bits of
turns information, the internal representation of the turns used by the position controller in Menu 13 and functions within the SM-Applications Module
such as the Advanced Position Controller, rolls over at the maximum position value from the encoder. This jump in position is likely to cause unwanted
effects. As the SSI format does not include any error checking it is not possible for the drive to detect if the position data has been corrupted by noise
on the encoder signals. The benefit of using the absolute position directly from an SSI encoder is that even if the encoder communications are
disturbed by noise and position errors occur, the position will always recover the correct position after the disturbance has ceased. The EnDat format
includes a CRC that is used by the drive to detect corrupted data, and so if the position data has been corrupted the drive uses the previous correct
data until new uncorrupted data is received.
It should be noted that if the encoder does not provide at least 6 bits of turns information the speed feedback used to generate the drive over speed
trip is incorrect when the position rolls over or under the maximum value. Therefore this mode should not be used with encoders that do not provide
this turns information unless the encoder is not going to pass through the maximum position.
If this parameter is set to 0 the drive only takes the absolute position directly from the encoder during initialisation. The change of position over each
sample is then used to determine the current position. This method always gives 16 bits of turns information that can be used without jumps in
position by the position controller in Menu13 and SM-Applications modules etc. This method will only operate correctly if the change of position over
any 250μs period is less than 0.5 of a turn, or else the turns information will be incorrect. The turns can then only be corrected by re-initializing the
encoder. Under normal operating conditions and at a maximum speed of 40,000rpm the maximum change of position is less than 0.5 turns, however,
if noise corrupts the data from an SSI encoder it is possible to have apparent large change of position, and this can result in the turns information
becoming and remaining corrupted until the encoder is re-initialized. This problem should not occur with EnDat encoders because three consecutive
corrupted messages at the slowest sample rate (i.e. 25μs) would be required even at the maximum speed of 40,000rpm before the change of position
would be the required 0.5 turns to give possible corruption of the turns information. If three consecutive messages with CRC errors occur this will
cause the drive to produce an EnC5 trip. The drive can only be re-enabled after the trip is reset which will re-initialize the encoder and correct the
absolute turns.
If an SSI encoder is used, but is not powered from the drive, and the encoder is powered up after the drive, it is possible that the first change of
position detected could be large enough to cause the problem described above. This can be avoided if the encoder interface is initialized via Pr 3.47
after the encoder has powered up. If the encoder includes a bit that indicates the status of the power supply the power supply monitor should be
enabled (see Pr 3.40). This will ensure that the drive remains tripped until the encoder is powered up and the action of resetting the trip will reinitialise
the encoder interface.
If the position feedback device is EnDat it is possible for the drive to set up this parameter automatically from information obtained from the encoder
(see Pr 3.41).
Drive encoder error detection level
3.40
Bit SP
Coding
Range 0 to 7
Default 1
Update rate Background read
Trips can be enabled/disabled using Pr 3.40 as follows.
Bit Function
0 Wire break detect
1 Phase error detect
2 SSI power supply bit monitor
58
Keypad and
Parameter x.00
display
FI DE Txt VM DP ND
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Parameter Advanced parameter
description format descriptions
RA NC NV PT US RW BU
1 1
Serial comms Electronic
protocol nameplate
PS
1
Digitax ST Advanced User Guide
Issue Number: 1
Performance