Drive and Safety Methods for
New Generation Elevator Systems

This work is funded by a prominent elevator company in Japan.

High rise buildings cannot benefit from conventional elevators anymore.

This research has been going on since 2007, under four projects, spanning 2007-2008, 2008-2009, 2010-2012 and 2011-2013. Active work is going on in several areas. Please see the work program outline where core competencies of graduate students that are required or to be developed are also shown. Shortly, the following are the milestones:

  1. New magnetostatic design for speeds up to 150meters/minute (mpm).
  2. Construction of a 12m high linear motor elevator for high speed experiments.
  3. A Communication method for real-time communication of 8000 motor driver units in real-time.
  4. Shared load carrying method for two linear motor elevators working together.
  5. Design of dedicated embedded controller for linear motors.
  6. and many other areas.

The project as explained below was completed in September 2009. The final product, a 3m long linear motor with 40kg payload capacity per 25cm of mover has been delivered. The current design linear motor is seen in the pictures below.

Two new projects with the same partner with a much larger budget has been started in fall 2010. We need 4 graduate students to work on the new project. If you are interested, please see the call for students at the bottom of the page.

Linear motor 3 with me, Mutsuno and Aoki.

Motor 3 general view.

Motor 3 with the whole team: Ayhan, Deniz, me, Cagri, Mutsuno, Ender, Aoki..

A video of the motor. First normal speed, then fast. (AVI file 5.2MB)

Introduction

Recent trends in increasingly taller buildings have lead to a bottleneck of transportation of goods and services to the higher floors. Since larger and larger number of people work on the same footprint, the percentage of floor space that must be reserved for elevators, and energy, communication, water and sewer connections is increasing to the point that taller buildings may no longer be profitable.

The percentage of floor space reserved for elevators can be reduced if more than one elevator could run on the same elevator shaft. However, classical roped elevators severely limit the number of elevators sharing the same shaft because the mechanical systems consisting of ropes, pulleys and counterweights get in the way of each other.

We were approached by a Japanese company to develop an elevator, directly driven by a linear motor. In this system there are no ropes, no counterweights, but only the linear motor generating enough force to lift an elevator cab. Thus it becomes possible to install any number of elevator cabs on the same shaft. Another benefit is that since there is no mechanical limit, the elevator can be made as tall as necessary, which makes it a strong candidate for kilometer-high building, which is one of the goals of architecture.

In the proposed installation, it is not possible for the elevator cabs to bypass each other; the lowest cab will never reach the highest floor, or if one cab needs to turn back, it will block the way of the one above it. This problem can be efficiently solved by using group control of elevators in which a floor call does not simply stop the nearest elevator (an especially poor technique), but a suitable elevator moving in the direction is reserved. Thus it is possible to implement a smoothly functioning system. The book "Control of traffic systems in buildings" by Sandor Markon et al., Springer, 2006 details the methods that can be used.

There are many problems to be solved before the system can be put into service such as safety, generation of enough force, drive methods, efficiency and cost to name a few. We have attacked several of them, and have built a functioning motor, capable at this point of lifting one person.

The linear motor general view.

Design Phase

The motor is a coreless permanent magnet synchronous type, where the mover contains the magnets and the stator contains the coils(opposite of general practice). The decision was made on the basis of compactness, simplicity of production, ease of position control, and lack of wires connected to the moving part. Initial phase was the design of stator, followed by the magnetic design of the complete motor. We came up with the design shown in the picture below.

Stator design.

The magnetic design was completed with the analysis of the complete motor with mover. Initial design was made by prof Takahashi of Okayama Univ. Japan, and the final design was completed at Sabanci University using the finite element magnetics design program FEMM. An example analysis of one of the design candidates is shown below.

Snapshot of magnetic analysis..

Construction

The design was finalized and the actual motor was produced by a local company specializing in high power DC motor production Femsan. Motor prototype length is 2m, usable height is 200mm. It can stand 3A of continuous current. Whole stator is impregnated in epoxy for rigidity. This simplifies design, but any burnt coil cannot be repaired. Below is the prototype shortly after arriving from the factory, with a very simple test mover. It could deliver 22kgf of thrust, and weighed about 25kg. Due to full force of magnets resting on the bearings creating friction and heavy weight, it was not expected to lift itself. However it was used for initial testing and setup. It can be seen below. At that time the coils on each phase were connected in series and high voltage was used to proved enough current. A single motor controller was used in open loop.

The linear motor general view as it arrived. A simple test mover was constructed in a hurry to test it. .

Later the motor was placed vertical, and the control system was changed into a distributed one. Four motor controllers were used, essentially making it four motors connected end to end. A new cantilever mover was designed and built which did not put any stress on the linear bearings. We could at this stage control the movers separately and smoothly between adjacent motor segments. The movers generated net 8kgf of lift force.

Motor standing up with two of the new movers. Each could be driven spearately..

The new mover is very light in construction, has a minimum thickness iron backing found by analysis, and can give a net upward force of 8kgf using N38 magnets..

Control Implementation

In the begining we controlled the motor open loop. This obviously is not the right way to do it, since the motion is not smooth and it is very inefficient. We later attached a straight linear encoder and implemented a sensor based feedback controller. This reduced the currents and increased the stiffness of the motor considerably. Also several motor controllers shared the same mover position information. Since our motor controllers were not designed for this task, we had to modify their firmware. The controllers use a Hitachi SH2 16 bit microcontroller and 10A inverter (derated to 5A for safety). The firmware programming is done in C language. Evenually we will progress into a new sensorless motor control method which allows position control with arbitrary speed.

Motor controllers currently on the motor..

Results

Currently we have the system running with closed loop sensor based control. We have installed six movers, three with N45 magnets and three with N38 Magnets. It is now possible to lift one person with this configuration. The vibration is not percievable.

Lifting our grad student Nese. Prof S. Markon from Japan also visible, the initiator of the project and of many ideas used.

Safety Devices

The motor should safely be stopped from falling in case of several failure scenarios. Simply shorting the motor terminals together allows the movers to fall slowly using the back EMF brake. However, it is necessary to stop the movers before hitting the bottom. We have a novel idea of using the magnetic properties of the stator to control a mechanical brake, with the contribution of Prof Komatsu from Ritsumeikan Univ. Japan. This way, the mover can be stopped from falling in case anything goes wrong with the motor. So at this point we have two safety devices(similar to an actual elevator) in which we can stop the cab from free fall. Others will follow.

Future Work

There are a lot of things to be done. Here are some ideas(in no particular order):

  1. Redesign of the stator for better maintainability
  2. Redesign of the mover with Halbach array magnetic structure for boosting lift.
  3. Introduction of a new sensorless drive method
  4. Design of a computer network for communication and synchronization between multiple controllers
  5. Design of a proper elevator system with at least 3m height, double stators and 200kg load capacity.
  6. Method of coupling the motor to the elevator cab to eliminate side forces.
  7. Design of a totally new stator which is more efficient compared to the current one.

Call for Students

We need four masters students for the new project starting from September 2010 to work on one specific topic of the new design of linear motor. Please see the work program outline .

Tuition vaiwer is provided (i.e. you do not pay tuition fee), and monthly salary equivalent to TUBITAK BIDEB support (1250TL/month as of 2010) will be paid.

We are always looking for enthusiastic undergraduate students who are willing to work on the project also. As you can see there is work to be done in many disciplines. It is best if you have skills and can help in a specific area. If you are interested, please contact me; grad or undergrad students welcome. There is an opportunity of summer internship at the Japanese company for the hardworking students.

Last updated 20 Jul 2011