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3.4.3
Incremental plus commutation (absolute encoders) AB Servo, FD Servo,
FR Servo and SC Servo
Type
Incremental plus
commutation
(absolute encoders)
The incremental part of an incremental encoder with commutation works in exactly the same way
as an incremental encoder. The additional channels (U, V and W) give the absolute electrical
position to electrical 60°.
When operating the drive in RFC-S mode, the absolute position (to 60° electrical or better) of the
machine shaft is required to enable the drive to apply torque in the correct direction. The marker
signal is not effective until the shaft passes a particular position, so this cannot be used to
determine the absolute position. Therefore in RFC-S mode an encoder with additional commutation
is required.
NOTE
The U, V and W commutation signals should have a period that is one electrical revolution
as shown in Figure 3-4.
Therefore with a 6 pole machine the U, V and W commutation signals will repeat three times per
mechanical revolution, or with an 8 pole machine four times per mechanical revolution etc.
The U, V and W commutation signals are used when the drive is enabled to locate the position of
the machine shaft within 60° electrical so that the current vector can be applied within 30° electrical
either side of the correct position for maximum torque production. The torque capability of the drive
during this period can be reduced to as low as 0.866 of the nominal level.
Once the shaft has moved through a maximum of 60° electrical, one of the U, V or W signals will
have changed state. The location of the waveform edge is used to locate the machine position
exactly. This information is then stored by the option module and used until power-down to place
the current vector in the correct position for maximum torque. To ensure that this process is carried
out correctly the control algorithm waits for two changes of the state of the U, V and W waveforms,
after this point maximum torque is available.
Using this type of encoder does not result in any movement when the drive is first enabled after
power-up, only a small reduction in torque described above for the first 60° to 120° electrical of
movement.
NOTE
In AB Servo, FD Servo or FR Servo modes only, the value in Pr 1x.070 provides information on the
commutation signal inputs (UVW). Pr 1x.070 permits the user to determine the current segment and
status of the commutation signal inputs. For further details refer to Pr 1x.068.
12
Encoder
Quadrature incremental encoder with
AB Servo
commutation outputs.
With or without marker pulse.
Incremental encoder with frequency, direction
and commutation outputs.
FD Servo
With or without marker pulse.
Incremental encoder with forward, reverse and
FR Servo
commutation outputs.
With or without marker pulse.
Absolute SinCos encoder plus commutation
SC Servo
signals with or without marker pulse.
Description
SI-Universal Encoder User Guide
Pr 1x.038
3
4
5
12
Issue: 2
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