Imagine the smoothest elevator ride ever. That's what you'll get with Magnetek's new Pulsed Pre-Torque (PPT™) function control technique for PMAC drives.
The Pulsed Pre-Torque function senses needed pre-torque before the mechanical brake is released, reducing rollback, and resulting in the smoothest start on an elevator ride, ever. The new
PPT™ function requires no setup or tuning, nor an external load weighing device...It's just that easy.
THE PPT Function: The Technology Behind The Smoothest Elevator Ride, Ever.
How do we accomplish this innovative, yet deceivingly simple, motor control system for an elevator?
First, let’s look at a typical gearless traction elevator system...
The drive sheave is directly mounted to the shaft of a gearless elevator machine (motor). Five to eight lengths of wire ropes are attached to the top of the elevator cab and wrapped over the drive sheave in special grooves. A counterweight is attached to the other ends of the cables. This counterweight moves in the hoistway on its own guide rails. The traction necessary as the sheave turns is provided by the combined weight of the elevator car and the counterweight pressing the cables into the grooves on the drive sheave.
System safety and reduced motor strain is provided by a brake used in conjunction with the motor. The brake is able to hold the sheave, cables, cab, and counterweight stationary. The HPV 900 Series 2
drive is enabled to run when the car controller calls for the elevator to move to another floor. The brake is commanded to release before motion is initiated. The motor must provide a proper level of torque as the brake is disengaged (or "picked") while the elevator car transitions from stationary to moving. The motor then continues to move the cab according to the floor call from the car controller.
The Problem of Rollback
Depending on the system, configurations and weights involved, in certain circumstances the period between brake disengagement and motor activation may cause a brief rollback of the elevator cab. When the motor must hold the car to keep it stationary during this crossover period, auditory noises and/or noticeable vibrations may result. As the load of an elevator changes with each stop, the dynamics of this situation changes with every floor call. Magnetek identified a need for an improved counterbalancing system in which one or more of the drawbacks may be minimized in such circumstances.
Pulsed Pre-Torque™: Magnetek’s Anti Rollback Solution
Magnetek's years of experience and innovative thinking allowed us to develop our patent pending* Pulsed Pre-Torque™ (PPT), a control technique that addresses the rollback condition and delivers the smoothest elevator ride ever. We determine a pre-torque direction and magnitude during brake engagement prior to each elevator run. Prior to brake disengagement, our PPT™ function applies this measured pre-torque in the commanded rotational direction.
In this technique, Magnetek HPV 900 Series 2 drives use small amounts of feedback, as seen on the encoder mounted on a PMAC machine. With that feedback, incremental movements (movements smaller than a passenger in the car can detect) can be used to properly provide pre-torque feedback to the drive. The PPT function operates at the start of the elevator run before the brake is picked and generates an offset torque that matches the elevator load. The PPT control function operates without the use of an external load-weighing system. Synchronization with the brake is built into the system so this pre-torque value can be applied prior to the brake being opened, as seen in this overview of the PPT function.
*Patent “Method and Apparatus for Controlling Motion in a Counterbalancing System” by Garry Anderson, John Backman, Mark Kobiske, and Ed Butte of Magnetek.
Simplicity and Improved Ride Quality
It’s easy to use the PPT function. Simply disable the traditional ARB function and set the PPT parameter to “Enable.” No set up or tuning is needed. PPT set-up does not require the cost or hassle of an external load weighing device or field service equipment such as an oscillioscope either, eliminating the associated cost, setup and maintenance.
Magnetek’s Pulsed Pre-Torque means no more vibration or sensation underfoot. No more rollback, torque bump, or brake release sound. Just a smooth transition from stationary car to acceleration.