Automatic doors are ubiquitous. But this product that we all pretty much take for granted represents one of the most vexing engineering exercises on the planet. The challenge: Design something that is open for a pedestrian to walk through (but only then) and otherwise closed. Detailed engineering specs have been established to help engineers cope with the complexity, but what happens out here in the real world is a whole other story.
“What you need is a door that reliably opens and stays open every time you need it to open and that reliably closes and stays closed when you need it to,” says Roger Boyell. A forensic engineer with expertise in electronic control and communications systems, he’s been involved in numerous court cases involving automatic doors that didn’t always behave as expected. To assist engineers in meeting these objectives (and to cope with a litigious population), detailed engineering specifications for automatic doors (specifically ANSI/BHMA A156.10-2005) have evolved over the years in the U.S. Increasing in length and the amount of specifics with each revision, the spec calls out such exacting details as:
- The coefficient of friction of the sensing mat (not less than 0.66 when dry and clean)
- The closing speed of the door (one foot per second maximum for sliding doors weighing up to and including 71 kg)
- The force required to prevent a stopped power operated swinging door in the last 10 degrees of opening from moving in the direction of opening (not to exceed 40 lbf measured 1 inch from the lock edge of the door)
- Every automatic door for pedestrian traffic must be labelled “AUTOMATIC DOOR” in 13-mm type
Britan has its own door spec as well, British Standard BS 7036 But while these specs are intended to make the designer’s job easier, verifying that doors in the field always meet performance requirements is fraught with difficulty. In the early days of the technology, a floor mat was used as a switch to actuate the open-close cycle, and photo beams across the doorway were used as safety devices. Now, it’s common to have fully electronic sensors, which are far less maintenance-intensive. Typically pedestrians approaching or blocking the doorway are sensed by microwave radar or infrared devices, which send a signal to the door’s motor to open and close it as appropriate. Of course different people exhibit different electromagnetic properties, depending on their height, mass, speed, angle of approach, and even the color of their t-shirt There are so many variables involved, says Boyell, that it would be too expensive, not to mention impossible, to anticipate every possible scenario. (The good news is that it keeps all the lawyers busy.) Boyell points to a recent case involving an elderly woman who pushed her walker through an open sliding door. As she then attempted to enter herself, the doors closed shut on the walker, knocking her to the ground in the process. “A thousand fat people, slow people, and little people will walk up to that door and go through it with no problem,” says Boyell. “But here you had a case where the person just happened to stick an unexpected object that wasn’t detectable by the sensors into an open door and ran into a problem. It’s not a very satisfying outcome, but in the end it’s really no one’s fault.” While in many instances it was clear cut that an automatic door was the culprit, Boyell says that there have been plenty of cases where engineers working on opposite sides have entirely different opinions as to whether a door control system at fault. Part of the problem, he says, is that what constitutes “meeting the specs” is often a matter of interpretation. To wit, he recounts a case in which technicians had been called in almost on a monthly basis to make simple adjustments to the sensitivity of a door’s control system. “Somebody would complain that the door was “ghosting” or opening for pedestrians and vehicles simply passing by at a distance, so a technician would reduce the sensitivity or tilt the sensor downward,” he recalls. “Then somebody else would complain that it wasn’t opening every time a person wanted to pass through, and a different technician would come back and increase the sensitivity or tilt the sensor upward. The cycle would repeat all over again, until the adjustment was over-corrected and an individual was injured.” So what’s the solution? To ensure that an automatic door never, ever fails to work, Boyell says–not so jokingly–you have two options: Option one: Station a human next to the door and make sure it always operates as intended. Option two: Don’t have an automatic door.