Machine Safety Expert Witness

Automatic Door Expert

At first glance, it is not obvious why an automatic pedestrian door application would be appropriate for a robotics and automation expert. Consider, however, this definition of a robotics system: “a system with moving parts, a drive mechanism to drive the moving parts and a computer to choreograph and control the system.” With this understanding, an automatic pedestrian door is a robotic system with at least one degree of motion, typically sliding or revolving.

A case can be made that automatic doors are in fact a member of a modern branch of robotics known as “collaborative robotics.” Collaborative robotics describes applications where robotics and humans are working in the same space. This contrasts with more traditional robotics applications in factories where barriers keep robotics and humans separated. We have decades of experience with people operating in the same workspace as automatic doors. There are lessons to be learned from this collaborative robotics experience.

Early automatic door systems employed control mats. The control mats were connected to electrical contacts that opened the doors when there was weight on the mat and closed the doors when there was no weight on the mat. There was very limited computing involved and hence these early automatic doors did not fit the definition of a robotic system very well.

Later automatic door systems began replacing the control mats with non-contact sensors, typically mounted above the doors, to detect people. To analyze the signals from these sensors and determine whether (or not) to open or close the doors based on this sensor feedback the automatic door systems include computing elements. One of the sensor types that found wide deployment was a motion sensor based on the Doppler frequency shift. This type of motion sensor identifies the approach of people towards the doors. The computing element then commands the doors to open for the people to move through the door way. Unfortunately, there were numerous accidents where some people moved so slowly through the doorway that they effectively became invisible to the motion sensing. This danger is exacerbated when the person is directly under the sensor because there will be little or no component of that person’s velocity towards or away from the motion sensor.

To address this issue, the designers of automatic door systems added an infrared presence sensor intended to identify the presence, rather than the motion, of people that could get into the path of the moving door. This is an important lesson to the designers of safety systems. Thought should be given during the design process to possible future safety requirements that may evolve in response to lessons learned and changing safety standards. Supporting the ability to add additional safety sensors is a reasonable design goal, but the control system must also allow these new sensors to be added in a failsafe fashion. That is, the control system must be able to identify very quickly if the new sensor has failed or become disconnected and respond in an appropriate fashion.

By adding the second sensing element, the system designers were attempting to eliminate blind spots or weak areas in the sensor coverage pattern. This is important because when people get into the blind spots the door’s safety systems are not able to protect them and accidents can occur. Other causes of accidents can include improper set-up of the doors, component failures and failure to control the entrance and exit areas of the doors. Fortunately, there are safety standards that have evolved to provide guidance to automatic door designers to help them make their products safer.

I have substantial experience with automatic door cases as an expert witness. I have also published a peer-reviewed IEEE paper that discusses automatic door safety systems. Automatic door cases can be more complex than they first appear. They almost always require a consideration of design, installation, maintenance and adherence to safety standards..

I accept a small number of litigation support engagements to complement my regular work as a professional engineer designing automation and machine control systems.


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