Aerotech UNIDEX 600 Series, User Manual

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Parameters U600 User’s Guide
C-26
Aerotech, Inc.
Version 1.4
using a dual feedback loop, the velocity and position loop must be in phase (with each
other). Otherwise, toggling this parameter would correct one feedback element and
improperly phase the other.
Additionally, a unipolar command may be output by the servo loop for a unidirectional
spindle motor (if required), by setting this parameter to -1. The status of this command is
indicated by the " invert polarity" bit of the ALT_STATUS axis parameter. A value of 1
indicates that the ICMDPOLARITY value is 1.
C.2.53. IMAX
This axis parameter sets the peak commanded output current, when the axis is operating
in the current (torque) mode. This is done by limiting the maximum output voltage of the
current command signal that is in turn translated into a proportional motor current by the
drive module. 10 volts is represented by 32,767. To set this parameter, determine the
maximum input command voltage that the amplifier requires to produce the maximum
desired motor current. The default value of this parameter is 32,767, producing a 10-volt
current command signal that would command the maximum current from the drive
module.
For brushless motors this parameter is set to the peak value of the desired sinusoidal
output current.
This parameter could also be used to limit the maximum commanded velocity in the
velocity command mode, but is not normally used for this purpose due to the VELTRAP
and VELCMDTRAP axis parameters.
See the IAVGLIMIT axis parameter for an example on calculating the instantaneous
torque produced by the motor.
C.2.53.1. Computing Torque (Closed-Loop Torque Mode)
The torque applied to the motor (in torque mode) may be easily calculated for brush or
brushless motors if you know a few parameter values and the K
t
(motor torque constant).
The UNIDEX 600/650 Controllers have a 16 bit Digital-to-Analog converter used to
convert a signed 16 bit number (+32,767 through –32,767) to an analog voltage in the
range of +10 volts through –10 volts. This voltage is applied to the command input of the
servo amplifier (knowing the G
m
(transconductance) value of the servo amplifier allows
the current (Amps) output to be calculated). Aerotech’s amplifiers typically (model
dependant) have a peak output of 20 or 30 amps. Simply meaning, a +/- 10 volts in to the
amplifier will be equal to +/- x amps, where x is the peak output current of the servo
amplifier. This current produces a torque generated by the motor, equal to the current
multiplied by the K
t
of the motor. For example, if the peak output command (or
instantaneous output command) from DAC was 16,385 (+5 volts), the amplifier produced
30 amps for an input of 10 volts, and the motor had a K
t
of 16 oz-in (ounce-inches) per
amp, then:
(5 volt command / 10 volt, max amps input command) * 30 amps (max
I output) * 16 oz-in K
t
= 240
or
5/10 * 30 *16 = 240