Technology Option F01
08.2012
6SE7087-6QX70 (Version AN) Siemens AG
9-54
Compendium Motion Control
SIMOVERT MASTERDRIVES
9.4.12
Position control system [340]
The position controller is shown in [340]. The method used to connect
the position controller to the technology is described in [801 + 817] and
in the section entitled "Commissioning the technology".
The position control system [340] is implemented using a PI controller
with a deactivatable I component.
If the actual position signal is very unstable, it can be stabilized using
the actual value smoothing parameter. It should be remembered,
however, that smoothing the signal reduces the potential dynamic
response of the system. The set input is used to synchronize the output
of the smoothing element with the actual value of the position sensing
system during setting or correction processes, e.g. on a rotary axis with
tool compensation. Synchronization is only necessary if a smoothing
time constant has been entered in P195.
Position setpoint smoothing is only appropriate if the speed precontrol
feature of the position control system is used. In this case, the
smoothing time constant should be set to the equivalent time constant
of the speed control loop. Position setpoint smoothing is not usually
required. Position setpoint smoothing must also be set when the
position setpoint is set.
Position difference smoothing is preferred for rotary axes and angular
synchronization, since it overcomes the problems associated with
setting events. Like actual position smoothing, position difference
smoothing reduces the potential dynamic response of the drive.
If the position setpoint (e.g. for synchronization or positioning) is
generated in a slower sampling time than that of the position controller,
the consequence is jumps in the setpoint for the position controller. This
results in unstable operation and reduces the level of accuracy that can
be achieved. In order to optimize the transition between sampling
times, it is possible to convert the rough graduation of the setpoint
commands into a fine graduation for the position controller. This task is
performed by the position setpoint interpolator, the operation of which is
defined by two parameters:
P770 defines the ratio between the sampling time of the position
setpoint generator to the sampling time of the position controller in
steps of 2
P770
. Example: time slot of the position setpoint generator =
T4, sampling time of the position controller = T1, P770 = 3.
If P770 is set to a positive value, the position setpoint is extrapolated
(lookahead calculation). If P770 is negative, the setpoint is interpolated.
The extrapolation option must be used when no speed precontrol is
active on the position controller. If the precontrol function is active,
interpolation of the position setpoint should be selected instead.
P771 defines the limit for the setpoint change, referred to the sampling
time of the prior position setpoint detection.
The following setting rule applies for P771:
60
[ms]
detection
setpoint
position
prior
the
of
time
Scanning
LU/min]
[1000
P205
2
]
[LU
P771
⋅
⋅
=
Actual position
smoothing P195
Position setpoint
smoothing P191
Position difference
smoothing P199
Position setpoint
interpolator
P770/P771
