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Diagnostics Optimization CT MODBUS RTU Technical Data
E300 Design Guide 355
Issue Number: 1
The speed feedback ripple seen by the speed controller and the position feedback reference is given by the following equations when the filter set up
value Drive Encoder Feedback Filter (C09) = 0.
Ripple for a rotary system in rpm = 60 / Speed controller sample time / Position resolution
Ripple for a linear system in mm/s = Pole pitch in mm / Speed controller sample time / Position resolution
The speed controller sample time is 250 µs. If the filter set up value is non-zero the ripple is given by:
Ripple for a rotary system in rpm = 60 / Filter time / Position resolution
Ripple for a linear system in mm/s = Pole pitch in mm / Filter time / Position resolution
The description so far covers the Drive position feedback interface.
It is not advisable to use the speed feedback filter unless it is specifically required for high inertia applications with high controller gains, or if
commutation signals alone are used for feedback, because the filter has a non-linear transfer function. It is preferable to use the current demand
filters (Final Current Filter Time Constant (J28) or Run Current Loop Filter (I10)) as these are linear first order filters that provide filtering on noise
generated from both the speed reference and the speed feedback. It should be noted that any filtering included within the speed controller feedback
loop, either on the speed feedback or the current demand, introduces a delay and limits the maximum bandwidth of the controller for stable operation.
The speed ripple seen by the speed controller can be quite high in some cases, for example with a 4096 line encoder the speed ripple is 14.6 rpm
with a sample time of 250 µs. This causes high frequency torque ripple and acoustic motor noise. These effects increase with the level of speed
feedback ripple and with the gains used in the speed controller. Therefore high speed feedback ripple usually limits the maximum possible gain
settings for the speed controller, and so a position feedback device with high position resolution is usually required for a system with high dynamic
performance or stiffness. It should be noted that the ripple caused by feedback quantisation and does not define speed feedback resolution.
The speed controller accumulates all pulses from the position feedback, and so the speed controller resolution is not limited by the
feedback, but by the resolution of the speed reference.
Drive Encoder Revolutions (J52), Drive Encoder Position (J53) and Drive Encoder Fine Position (J54) combined give the encoder position with a
resolution of 1/2
32
of a revolution/pole pitch as a 48 bit number.
Provided the position feedback interface set-up parameters are correct, the position is always converted to units of 1/2
32
of a revolution/pole pitch, but
some parts of the value may not be relevant depending on the resolution of the feedback device. For example a 1024 line digital encoder produces
4096 counts per revolution, and so the position is represented by the bits in the shaded area only.
When the position feedback moves by more than one revolution or pole pitch the Drive Encoder Revolutions (J52) increments or decrements in the
form of a sixteen bit roll-over counter. If an absolute position feedback device (except AB Servo, FD Servo, FR Servo, SC Servo) is used the position
Filter defined by
Drive Encoder
Feedback Filter
(C09)
16ms filter
Speed feedback
from Drive position
feedback interface
Speed controller
Speed Feedback(J51)
Speed feedback filtering for Drive position feedback interface
J52 Drive Encoder Revolutions
Mode RFC-A, RFC-S
Minimum 0 Maximum 65535
Default Units
Type 16 Bit Power Down Save Update Rate 4 ms write
Display Format Standard Decimal Places 0
Coding RO, ND, NC, PT, BU
Revolutions/Pole Pitches Position Fine Position
01547 1632 31
Revolutions/Pole Pitches Fine Position
01547 1632 31
Position
20 19
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