Specifications

98 Control and Automation Solutions Guide
This equipment and the main controller
are usually in a machine room located
at the top of the building. Alternatives
exist where there are machine roomless
designs where the motors are within
the elevator shaft. These reduce space
and equipment costs, but can be harder
to service. Some potential exists for
using linear induction motors and
linear switched reluctance machines
(LSRMs) for elevator applications.
It is critical that the elevator stop exactly
at oor level, so various sensors can be
used to assure this. To keep the elevator
at this level as passengers come and
go thus changing the weight, a brake
is usually used on the motor shaft.
This is a mechanical brake (drum type)
that needs to be energized to release.
Therefore in the event of a power failure,
the brake is applied preventing the
elevator from falling. Additional safety
mechanisms can also be added to
ensure that the elevator will not free-
fall due to cable breakage. These are
usually mechanical mechanisms that
respond to loss of pull from the cables.
There is a set of doors on each oor
and a set of doors that move with the
elevator. At the oor, these open and
close simultaneously so users hardly
notice that they are not one set. Optical
sensors and limit switches are used
to coordinate their movements. Both
doors’ movements are usually driven by
a motor mounted on the car roof. When
the car is not at that oor, that oor’s
doors are securely locked. Of course the
door closing process is also carefully
controlled to prevent pinching items
between the doors. Once all obstructions
are gone, after some delay, the doors
are closed gently. If the doors are kept
open too long, a buzzer sounds to hurry
people up. Some elevators (often freight
elevators), for example, have two sets
of doors on opposite ends of the car
for easier access to loading docks.
In addition to the standard array of
buttons that need to be sensed to
select the oors, other sensors and
input devices include card readers for
restricting access to authorized card
carriers only, elevator weight sensors,
and environmental sensors for air quality
(DS7505 thermostat, DS1923 humidity
sensor). Small heating/cooling units are
used on elevators to maintain occupant
comfort (MAX31785 fan controller).
There are various lighting needs for the
oor select buttons (MAX16814 LED
driver) current oor indicator display
(MAX6966 display driver), overhead
lighting and wall displays, speakers for
background music, buzzers, and other
needs. Emergency phones, reman
override controls, special “Code Blue”
controls in hospital elevators, and
security cameras (Maxim video products)
round out the systems in the car.
Signals from the input devices are
communicated to the elevator controller
by various means from analog signals to
binary levels to serial digital interfaces.
Overload sensing is one example of a
closed loop in the elevator system. If an
overload is detected, the controller does
not allow the elevator to move until the
weight is reduced below the limit. A
buzzer or some other audible indicator
of overload is activated in the car.
Commands from the elevator controller to
the lift motor VFDs are usually in the form
of RS-485, such as the MAX13448E RS-485
transceiver. The BAS communicates to the
elevator controller if there are emergency
override needs, such as from the building
re alarm system.
Escalators and Moving
Walkways
An escalator is a moving staircase and a
moving walkway is a horizontal moving
surface. Escalators have much higher
people-moving capacity than elevators,
but require ambulatory riders. They
generally connect oor to oor just
like staircases, and are most often seen
in department stores, transportation
terminals, hotels, convention centers,
and sports arenas. They can be installed
indoors or outdoors if weatherproofed.
The direction can be reversed either
manually or automatically.
Moving horizontal walkways are
made much like a conveyor belt
with a exible rubber or segmented
surface connected in a long loop. Both
escalators and moving walkways also
have moving handrails that move
(close to) the same rate as the stairs or
walkway. Many of these systems run
continuously during building operating
hours, but some are programmed to
stop if there are no users, thus saving
energy. The stopping and starting
must be gentle, and anticipating a
new rider is a benet so it is already
moving when they get on. This can be
accomplished with proximity sensing
(like the MAX44000 proximity sensor).
Drive is usually provided by AC
induction motors and VFDs. The use
of regenerative VFDs saves energy
from “down” escalators where the
users’ combined weight can play a
signicant role in feeding energy
back into the grid to oset the energy
needed for the “up” escalators.
Escalator oor openings present
a re propagation path risk so the
under sides of the escalator truss is
often protected with re sprinklers
or with reproof panels. The motors
themselves generate signicant heat,
especially for continuously moving
escalators and moving walkways, so
either dedicated HVAC systems or
adequate ventilation is needed.