I I I I I Optimizing The Pid Controller; How The Control Process Works; Significance And Influence Of The Control Parameters - Danfoss VLT 5000 Series Manual

I I I I I Optimizing the PID controller

I I I I I How the control process works
The "TESTRUN" explained in the chapter
PC Software Interface can be used as a tool to
optimize the SyncPos controller settings, thereby
optimizing your system performance. To do that
you only need to know a few things about the con-
trol scheme of SyncPos:
The SyncPos position controller has two parts:
1. The Setpoint Generator interprets the various
positioning commands in SyncPos and gene-
rates a series of setpoint positions that even-
tually ends with the desired position.
Normally, all positioning processes have a tra-
peze-shaped velocity curve. That means that
after a phase of constant acceleration there is
a phase with constant velocity and finally a
phase with constant deceleration, which ends
at the desired target position.
2. The PID controller receives the setpoint posi-
tions from the Setpoint generator and in turn
calculates the speed reference needed for the
motor to follow the current setpoint position.
By setting the PID control parameters you can
directly influence to what degree and how
quickly a deviation from a theoretical set path
(as specified by the setpoint series) should be
counteracted.
The following behavior indicate that the control para-
meters are not optimally adjusted:
• • • • • Drive vibrates
• • • • • Drive is loud
• • • • • Frequent occurrence of position errors
• • • • • Poor control accuracy
NB!
The control parameters are load-dependent.
Thus the drive must be optimized under the
actual conditions of use.
In exceptional cases it may be necessary to deter-
mine various sets of control parameters while wor-
king with heavily changing load conditions and to
re-program them in subsequent application pro-
grams depending on the motion process.
MG.10.J4.02 – VLT is a registered Danfoss trade mark
®
VLT 5000 SyncPos option
I I I I I Significance and influence of the control param-
eters
The PID control unit of the SyncPos option trans-
fers the necessary output frequency via an internal
speed reference to the VLT. This set value is perio-
dically re-calculated with an interval of one milli-
second (interval is programmable by the TIMER
parameter). The SyncPos option is by default set
with soft 'fit for all' controller parameters.
The PID filter works according to the following
formula:
x = FFVEL * (Setpoint velocity)
y = FFACC * (Setpoint acceleration)
z = KPROP * (Position deviation)
{ = KINT * (Sum of all previous position devia-
tions) (limited by KILIM)
| = KDER * (Velocity of position deviation)
} = z + { + |
(limited by BANDWIDTH)
AV is the actual velocity
AP is the actual position (calculated from encoder
feedback) in qc (Quad Counts).
CP is the current Setpoint position in qc.
CV is the Setpoint velocity in qc/ms.
(Position deviation) is calculated by CP–AP.
NB!
In SYNCV mode the PID controller is working
with speed deviation instead of position
deviation. Speed deviation is calculated by CV–AV.
The controller in the SyncPos card utilizes two
control strategies at the same time:
1. An open-loop feed-forward control. Since the
asynchronous motor inherently has a good
open loop performance the feed-forward con-
trol is a very important part of the controller in
most applications. Benefits from using feed-
forwards control is a very fast and accurate
response to changes in the setpoint reference.
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