NORD Drivesystems BU0710 - Manual (Page 15)

3 Functional characteristics
BU 0710 GB 15
3.2.2 Position recognition with an absolute value encoder
The absolute encoder transfers the actual position value to the frequency inverter via the serial SSI interface in
Gray code. As the complete position information is stored in the absolute encoder all the time, it is still
available even if the axle has been moved while the inverter was off. That is why in this case the program
need not search for the reference point again.
If an absolute encoder is connected, the "distance calculation" parameter (P604) must be set to "absolute".
If the absolute encoder is not mounted on the motor shaft, the transmission ratio between motor and absolute
encoder must be specified in the relevant parameter. The inverter will convert the absolute revolutions to
revolutions of the motor by means of the multiplication and demultiplication parameters.
n
M
: number of revolutions of motor
n
M
= n
E
* mul / dev n
E:
: number of revolutions of absolute encoder
mul: multiplication
dem: demultiplication
Example: The absolute encoder is installed on the output face of the gearbox, the gear ratio of the gearbox is i = 26.34.
The following values are parameterised: multiplication 2634; demultiplication: 100
The sense of rotation of the absolute value encoder must be identical to that of the motor. When the phase
sequence of the output frequency is positive (clockwise sense of rotation), the actual position value is
supposed to grow. If the sense of rotation is found not to be the same, this condition can be corrected by
setting a negative value in P608 (demultiplication).
The zero point (point of origin) of the axle can be corrected by providing an offset value in the relevant
parameter. This offset value is added to the value obtained after the absolute encoder revolutions have been
converted into revolutions of the motor shaft. That is why a new offset value must be entered any time the
multiplication or demultiplication ratio has been changed.
The maximum allowable position value depends on the encoder resolution and on the multiplication or
demultiplication factors. The range of +/- 50000 revolutions cannot be exceeded however. A complete turn is
not allowed. With continuous axles turning in one direction most of the time, incremental encoders must be
used for position sensing. The inverter will limit position setpoints to the maximum allowable range of values
internally.
3.2.3 Encoder monitoring
When an absolute value encoder is used for position recognition, monitoring is ensured by activating the
voltage monitoring bit in the "Position detection monitoring" parameter (P617). The absolute encoder must
feature the corresponding option. If the bit succeeding the last data bit is not equal to 0, the value will be
ignored. If no value has been read by the end of a 500 ms period, the inverter puts out the "error E14 (E143)".
When both an absolute and an incremental encoder are involved, Parameter 631 can be used to check
whether any discrepancy occurs regarding the position information supplied by either of the two. The
maximum allowable deviation between the positions as sensed by the absolute and the incremental encoder
depends on the value set in parameter 631. A "0" setting will inactivate the monitoring function. When the
actual deviation exceeds the maximum allowable value, the "E14 (E146)“ error signal is activated. The
absolute and the incremental encoder are allowed to have different multiplication factors and to be installed in
different places. The system allows for individual multiplication (P607), demultiplication (P608), and offset
factors (P609) to be set for each of the two encoders.
If no second redundant encoder has been installed for position control, parameter P630 provides for defining a
position-related synchronization error. This approach is based on a comparison of the current position with the
position variation calculated from the current speed. Each time a destination position has been reached, the
estimated position is aligned with the actual value transmitted by the encoder in order to prevent the
synchronization error from accumulating with time. If the position difference exceeds the value set in P630, the
"Error 14 (145)" fault signal is activated. Where longer displacement distances are involved, higher values
should accordingly be provided in P630. The required value is best determined experimenting. Setting the
option to "0" will inactivate the monitoring function.