U511 User’s Manual Tuning Servo Loops
Version 1.1 Aerotech, Inc. 8-25
8.7. Tuning With Tachometer Feedback
The UNIDEX 511 servo is easily configured for compatibility with external tachometers
providing negative velocity feedback. To adapt to an external tachometer based Velocity
Loop, the inherent digital Velocity Loop operation within the controller needs to be
disabled. This is done by setting the digital servo loop proportional gain (“Kp”) to zero.
The servo system's Velocity Loop then needs compensated by the tachometer/amplifier
combination.
When configured this way, the analog outputs of the UNIDEX 511, which normally
deliver current commands to amplifiers, will now deliver velocity commands to amplifiers
accepting tachometer feedback.
In this configuration, the servo system has the following characteristics:
• The amplifier is configured to accept tachometer based (negative) velocity
feedback
• The amplifier controls the Velocity Loop of the servo system. Tuning for the
Velocity Loop is accomplished in the pre-amp section of the amplifier
• The proportional gain parameter (“Kp”) in the UNIDEX 511 controller’s
servo loop parameter set has been set to zero (0) for that axis, disabling its
digital Velocity Loop functionality
• The controller is now commanding velocity to the amplifier instead of
commanding torque to the amplifier
8.7.1. In-Position Integrator
Setting “Ki”≠0 can enable an in-position integrator. After “Ki” is set to a nonzero value,
the UNIDEX 511 will attempt to remove steady-state position errors. This function also
helps to reduce the effects of tachometer loop drift. In-position integration is
accomplished at a rate that is directly proportional to the integral gain value that is set in
“Ki.”
If a value for “Ki” is too large, it will induce oscillation into the position error and
increase the settling time.
8.7.2. Velocity Feed Forward
The following error (position error) that occurs while the axis is moving may be reduced
significantly by setting the “Vff” to a non-zero value. When the velocity feed forward
function is enabled (i.e., “Vff”≠0), an added voltage is sent to the tachometer loop. This
signal is proportional to the Velocity Command and the value of “Vff.” “Vff” is adjusted
to minimize position error of the servo system.
E
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