Appendix – Anti-Rollback Setup
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Appendix
Anti-Rollback
Using Anti-Rollback (ARB)
Elevator rollback occurs when an elevator motor drive is started and the brake is released but the hoist
motor has not yet developed enough torque to prevent gravity from moving the car. The car may
move up or down depending on the overall balance of equipment and payload. Velocity regulators
normally used for speed regulation will eventually detect unwanted movement and react to halt the
car. But there will be a position error accumulated during that process that can represent many inches
of unwanted car movement relative to the landing. This effect is most noticeable with low friction gear-
less elevators. It may be totally masked by the friction of an elevator driven through worm gears.
Rollback by itself does not pose any hazards, but it does give an uneasy, out of control feeling to
passengers. In many installations brake release timing is adjusted so that the brake is released just
as the car begins to accelerate toward the next landing to mask the rollback effect. This often results
in jerky starts as the brake linings release. The correct countermeasure is to weigh the car just as the
doors close to determine the degree of gravity unbalance, then pre-torque the motor so that when the
brake is released all forces are balanced. This method is very effective, but does require expensive
calibrated load weighing equipment.
The purpose of the Magnetek Anti-rollback feature is to help prevent rollback on elevators that do not
use load weighing or do not use the motor pre-torque capability provided by the Magnetek drive. It
uses a double integrator (type 2) regulator when operating at zero speed to hold the elevator car at an
average speed of zero and to regulate a constant position as the brake is released. When the velocity
reference leaves zero speed to accelerate the car toward the next landing, the active velocity regulator
is switched to be E-Reg to precisely track (follow) the velocity reference profile. Be aware that this
anti-rollback feature works from encoder/tachometer signals. So there must and will be some
movement in order for the feature to function, but the position error generated by elevator movement
will recover. The bandwidth gain of the system will determine how much movement will occur.
Several new operating options and adjustments are provided.
Set-Up And Tuning Of Anti-Rollback (ARB)
1. The ARB function uses adjustment settings, Inertia(A1), response(A1). These settings and others
are critical for good performance of the E-Reg velocity regulator. The first step for good ARB
performance is to disable ARB by setting ARB MODE to 0 (zero) and to tune all other E-Reg
adjustments for a smooth ride and good floor-to-floor elevator performance. Be sure that the car
weight and counterweights have been adjusted to be at the final values. Follow the suggested
procedures for tuning E-Reg as listed in this Tech Manual. Ignore elevator rollback while adjusting
primary elevator performance features. If rope resonance exists, also adjust the notch filter for
minimum interference. Then tune up ARB last as necessary to prevent elevator rollback with an
unbalanced payload.
2. After all other adjustments are satisfactory, ARB MODE to a
1 to enable ARB when starting an
elevator run. Set up the following initial ARB adjustment values. Be sure to read ADJUSTMENT
HINTS and CAUTIONS listed below.
a) Set ARB Bandwidth, to 2 times the response setting (A1).
b) Set the initial value of ARB damping, to 0.5.
3. If the drive will be using an internally generated velocity reference or serial link commands set ARB
Speed Threshold to 0.0%, otherwise if an analog reference is used set to 0.5.
4. With the car empty at a convenient landing, prepare to start the elevator drive, call for zero velocity
from the car controller via the normal way, release the elevator brake, and observe any car motion.
When the car comes to a stop, the observation is complete, set the brake and stop the drive, again
through normal control channels. Do so and observe that...
a) The contactor picks, and the drive starts.
b) There may be some initial upward motion, but the car should return to its original position
within a second or so.
c) When motion is halted, there will be motor armature current producing torque holding the car.
This can be observed at armature current (D2) on the local display or a separate DC clamp-
on ammeter, if available.
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