Door operators and door motion

The door operator is the motor and mechanism on the car top that opens and closes the car door, and through the clutch, the hoistway door with it. Priya started Tamsin on operators by putting her on the car top of Tillicum car 1 and then, the same afternoon, on the car top of Quayside car 2. "Same job, sixty years apart," she said. On Tillicum, a DC motor drove a reduction belt and a crank arm, and the door's speed was set by resistors and a set of cam-operated switches that shorted them out at different points of travel. On Quayside, a small permanent-magnet motor drove a toothed belt directly, and a controller read an encoder on the motor and followed a speed profile set in its parameters. Both did the same thing: open the door quickly, slow it down before it hit the stop, close it at a speed the code allows and finish the last few centimeters gently so the door locks without slamming.

What an operator must do

Every door operator has the same jobs, however it is built. It must move the doors through a full cycle, open, hold and close, in a time short enough that the elevator does not lose capacity, but with speed and force limited so that a person struck by the door is not hurt. It must slow the doors before they reach the end of travel in each direction so that panels, hangers and stops are not hammered. It must hold the door fully closed while the car runs, and on most cars hold it fully open while passengers load. It must respond instantly when the reopening device sees an obstruction, reversing the door. And it must tell the controller where the door is: fully open, fully closed and, on modern cars, everywhere in between.

The controller decides when the doors open and close; the operator decides how. On Tillicum, the relays DO and DC from ELV-310 energize the operator's open and close circuits, and limit switches in the operator report door open and door closed. On Quayside, the main controller sends open and close commands to the door controller over a serial link, and the door controller returns the door's position and its own faults. Either way, the door operator is a machine the mechanic adjusts, and it is the source of most door complaints.

Older DC operators with resistors and cams

Tillicum's operator is typical of operators built from the 1950s to the 1980s and still found in many buildings. A small DC motor, often shunt or permanent-magnet, drives a reduction, by belt or gears, to a main shaft. A crank arm on the shaft drives a linkage to the car door hanger, so the door's speed follows the crank's rotation naturally: slow at the start, fastest in the middle and slow near the end, like a piston. Speed is adjusted with resistors: a series resistor in the armature circuit sets the overall speed, and a resistor across the armature, switched by a cam, brakes the motor and slows the door near the ends of travel. The cams are set on the shaft so that their switches operate at the right points. Adjusting such an operator means setting resistor bands and cam positions in a particular order, and the order matters, because each adjustment affects the others.

These operators are reliable and simple but have no feedback. If the door becomes heavier or tighter, because a hoistway door hanger wears or a sill fills with grit, the door simply moves more slowly, or faster if the load lightens. A DC operator that was adjusted to a perfect door on the day it was installed may be quite wrong after twenty years of wear at one landing, which is the floor-3 problem from lesson 1.

# DC door operator: armature voltage and door speed (illustrative)
supply = 110            # operator supply voltage, V DC
r_arm = 6.0            # armature resistance, ohm
k = 0.0138                 # door speed per volt of back-EMF, ft/s per V
current = 1.6           # running current while moving the door, A
for r_series in (10, 20, 30):
    back_emf = supply - current * (r_series + r_arm)
    speed = k * back_emf
    print(f"series resistance {r_series:5.1f} ohm -> back-EMF {back_emf:5.1f} V -> door speed about {speed:4.2f} ft/s")

series resistance  10.0 ohm -> back-EMF  84.4 V -> door speed about 1.16 ft/s
series resistance  20.0 ohm -> back-EMF  68.4 V -> door speed about 0.94 ft/s
series resistance  30.0 ohm -> back-EMF  52.4 V -> door speed about 0.72 ft/s

More series resistance drops more voltage, leaves less back-EMF across the motor, and slows the door. The numbers are illustrative, but the pattern is what Ray uses when he sets a band on an old operator: move the band a little, time the door, and move it again.

Modern closed-loop operators

Most operators installed in the last twenty-five years are closed-loop: an AC or permanent-magnet motor is run by its own small variable-frequency drive or motor controller, with an encoder or position sensor on the motor or the belt. The door controller follows a programmed speed profile for opening and closing, measures the actual position and speed and corrects the motor current to stay on the profile. Because it measures the load, it can compensate for a heavier or stiffer door, detect when the door is obstructed by the rise in current or loss of speed, learn the length of travel automatically and limit the force and kinetic energy within set bounds. Quayside's operator learns its door width on a learn run, stores separate profiles for the car door alone and for each landing, and logs faults such as door obstructed, door slow to close and encoder fault.

Some operators use a motor directly on a toothed belt that carries the door hangers; others use a gearmotor with arms; a few use linear motors. Whatever the mechanism, the parameters typically include opening speed, closing speed, acceleration, the length of the slow zones at each end, the force limit on closing, the hold force while open and closed, and the nudging speed used when the doors have been held open too long. The maker's manual gives the allowed ranges, and the kinetic energy and force limits of lesson 5 set the outer bounds.

The door motion profile

A good door cycle has a recognizable shape. On opening, the door starts slowly while the clutch engages and the interlock releases, accelerates to opening speed, runs, then slows down over the last part of travel and stops against its open stop without a bump. It holds open for the dwell time set by the controller. On closing, it accelerates to closing speed, which is slower than opening speed because closing is when people are struck, runs, slows over the last part of travel, creeps through a short check zone where the clutch releases and the hoistway door picks up its lock, and closes fully with enough force to make the door contacts and interlock reliably. If the door reverses on an obstruction, it stops and reopens at once.

# Opening and closing times for a trapezoidal door profile
width = 1.8              # door travel for the opening, ft
v_open, v_close = 2.0, 1.0     # top speeds, ft/s
a = 4.0                  # acceleration and deceleration, ft/s2
slow = 0.16               # length of the slow zone at the end of travel, ft
v_slow = 0.26            # creep speed in the slow zone, ft/s

def cycle_time(v):
    d_acc = v * v / (2 * a)            # distance to accelerate (and to decelerate)
    run = width - slow - 2 * d_acc     # distance at top speed
    return 2 * v / a + run / v + slow / v_slow

print(f"opening time {cycle_time(v_open):.1f} s, closing time {cycle_time(v_close):.1f} s")

opening time 1.9 s, closing time 2.5 s

These are the figures for one panel of Quayside's 42 in center-opening door, each panel traveling half the opening. The closing time is longer than the opening time because the closing speed is half the opening speed, and both times include the slow zone at the end. A door that opens in under two seconds and closes in about three is typical for a busy passenger car; the closing time must never be shorter than the minimum on the door's data plate, which comes from the kinetic energy limits.

Dwell time, nudging and accessibility

How long the doors stay open is set by the controller, not the operator, but mechanics adjust it. Most controllers use a longer dwell when a car stops for a hall call, so that a waiting passenger has time to see the lantern and reach the car, and a shorter dwell for a car call, when passengers are already inside. Accessibility rules set minimums for both. In the US, the 2010 ADA Standards set a minimum time from the hall lantern signal to the doors starting to close that grows with the walking distance from the call button to the car, with a floor of a few seconds, and a minimum door dwell for a car call. The controller's dwell parameters must meet these, and many buildings choose longer times than the minimum.

If the doors are held open, by a passenger blocking the light curtain or a cart in the entrance, many controllers start nudging after a set time: a buzzer sounds and the doors close at reduced speed with the light curtain bypassed, relying on a mechanical edge or force limit. The code allows nudging only at a lower kinetic energy than normal closing, the figure for a door without an operative reopening device, which lesson 5 gives. Nudging keeps a car from being held out of service by one blocked door, but it must be set gently and never on doors whose only protection is the light curtain.

# Hall call notification time: minimum time from lantern to doors closing
# T = distance / walking speed, with a minimum (accessibility rule, illustrative form)
walk_speed = 1.5            # ft/s
minimum = 5.0                # s
for name, d in [("button beside the car", 5.0), ("button at end of lobby", 10.0), ("across a wide lobby", 20.0)]:
    t = max(minimum, d / walk_speed)
    print(f"{name:<24} distance {d:4.1f} ft -> at least {t:4.1f} s")

button beside the car    distance  5.0 ft -> at least  5.0 s
button at end of lobby   distance 10.0 ft -> at least  6.7 s
across a wide lobby      distance 20.0 ft -> at least 13.3 s

The walking speed in the formula is slow on purpose, so that a person using a cane or a wheelchair can reach the car. At Quayside, with destination dispatch, the indicated car can be at the far end of the lobby, and its dwell parameters were set from the longest walk, not the shortest.

Clutches, vanes and how the car door drives the hoistway door

The car door must pick up the hoistway door at every floor. The device that does it is the clutch, also called the door coupler or vane. On many cars it is a pair of vanes on the car door that close together, or expand apart, at the start of the opening stroke to grip two rollers on the hoistway door's interlock. As the vanes grip, they first lift or push the interlock roller to unlock the hoistway door, then drag it open with the car door. On closing, the car door pushes the hoistway door closed, and in the last part of travel the vanes release the rollers; the hoistway door's closer finishes the job and the interlock locks. The clutch is also what makes the car door restrictor and the interlock's unlocking zone work, as lesson 4 explains. A clutch that is out of alignment with the hoistway door rollers by a few millimeters at one floor causes noisy, jerky or failed door operation at that floor only.