Baldor MicroFlex, Installation Manual

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B-2 Control System MN1919
B.1.1 Current (Torque) control
Setting the control mode to Current Control configures the MicroFlex or MicroFlex as a torque
amplifier, as shown in Figure 51. Here, a torque reference is obtained from a specified source:
H
Mint WorkBench
H
A host using the ActiveX control
H
Analog input
The source provides a signal that is fed into the Torque profiler.
The profiler generates a torque demand signal that smoothly changes between successive
torque targets (reference values). This is achieved by specifying a rise time and fall time (see
the WorkBench v5 Parameters tool). The torque demand signal is fed into the torque controller
which determines the appropriate amount of current to apply to the windings of the motor.
This demand current is compared with the actual winding current measured from sensors, and
a suitable pulse width modulation (PWM) signal is generated. This PWM signal is fed to the
power electronics in the drive.
PWM
Measured current
Torque
controller
Power stage
+ motor
Torque
profiler
Torque
demand
Torque
reference
Host
Analog input
Figure 51 - Control structure in Current (Torque) control mode
The torque controller is a PI (Proportional Integral) controller. Gains are set using the Mint
keywords
KIPROP
and
KIINT
. The torque demand is scaled into a current demand. This is
compared with the measured current, obtained from the current sensors, and the error is fed
into the PI control calculation. The resulting value forms the PWM signal that is fed through
the power stage into the motor windings. The gain values
KIPROP
and
KINT
must be tuned
for a specific motor. This is performed automatically by the Commissioning Wizard.
The feedback device is used to determine motor position and speed. Motor speed can be
filtered to reduce measurement noise if necessary. The time constant of this filter is specified
using the keyword
KVTIME
. By default the filter is turned off (
KVTIME = 0
). Note that
introducing a filter on measured speed tends to reduce the stability of the speed controller.
This can make the tuning of the speed controller gains difficult if large values of
KVTIME
are
used.