Motion control: Statue of Liberty rescue elevator
Driving smooth elevator operation
An integral component of the rescue elevator is a 25 hp variable frequency drive (VFD), featuring a patented torque-control technology for motor control. The drive controls the two 10 hp motors that move the elevator and collaborates with a programmable logic controller (PLC) in controlling the overall elevator system. While the PLC is in primary control of the elevator, it is the drive that provides the ultra-smooth ride, managing the acceleration and deceleration through programmable speed ramps, torque proving, and dynamic braking.
The motor drive “is the heart and soul of the drive system,” said Brian Trapani, the lead project engineer for TESI on the project. “It controls the two motors simultaneously and makes sure the elevator rides smoothly.”
A laser positioning system feeds information to the PLC. Once a rider selects a command to go up or down, the PLC processes the information and relays it to the drive, which then instructs the motors to build enough torque to move the elevator. The drive then sends back the verification to the PLC that the torque has reached the minimum required level, which then triggers the PLC to release the brakes. This “torque proving” ensures that each motor is online and generating enough torque before instructing the PLC to release the brakes.
The smooth ride when decelerating is achieved primarily through the dynamic braking feature of the drive. The elevator has spring set, electrically released fail-safe motor brakes, which act in normal operation more as parking brakes. Rather than using the brakes to stop the elevator, the drive slows down the motors to a gradual stop, and then the PLC sets the brakes to hold the elevator in place.
As the elevator ascends or descends and nears its destination, the drive transitions the motors from moving the elevator at a set speed to gradual braking. This dynamic braking is achieved by converting the kinetic energy of motion to electrical energy inside the drive.
The communication between the PLC and drive is not complex. There are three hardwired speed inputs into the drive, eliminating the need for a full serial interface. The PLC controls the position and provides the appropriate speed reference to the drive.
The advanced drive selected “can run with open-loop control with no issues, and the control structure is very tight,” said Trapani. “Other drives use closed-loop control, where you have to echo an encoder loop signal back through so it knows the positioning, which requires a lot of handshaking between the PLC and drive.”
An ancillary benefit to the smooth ride and dynamic braking is that the brakes and other components of the elevator system have a longer life. The brakes are not stressed by constantly stopping the elevator, and the mechanical systems benefit from the smooth ride throughout all motion cycles—critical factors in establishing the 20- to 30-year life expectancy of the system.
“The system features a fail-safe PLC with a fail-safe drive,” said Trapani. “When people are 100 feet in the air, you want to make sure that the brakes are working properly.”
“All of our systems use variable frequency drives,” said Grovatt. A few years ago, TESI switched to this “advanced technology that other drives don’t feature. We now have four or five systems in place,” he said, calling it “a real workhorse.”
TESI was hired by Liberty Elevator of Patterson, N.J., to design and manufacture the rescue elevator. Liberty was the subcontractor overseeing the installation of all elevators on the project. The other new elevators are in the pedestal of the statue: one main passenger elevator travels from the ground floor to the top of the pedestal, and one with wheelchair access goes from an intermediate level to the pedestal viewing level.
The 9/11 events were sobering to New York City and all of the United States. Public venues, through this tragedy, asked if it is possible to evacuate visitors quickly and efficiently, including the Statue of Liberty site. With the renovation and new rescue elevator, a much more efficient, rapid evacuation of visitors now is possible. Also, the new design helps day-to-day operations and access for maintenance personnel and visitors who experience emergencies from any level between the pedestal and crown to the pedestal and ground level. This elevator, working successfully in the Statue of Liberty, makes a pivotal difference in safety and usage conditions.
- Information provided by ABB and Tower Elevator Systems; edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer, firstname.lastname@example.org.
Also see the Control Engineering Machine Safety blog, with more safety advice about codes, standards, and best practices related to machine safety.
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