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I. Introduction
The Automatic Passenger Counting system manufactured by PerMetrics Technologies has a lineage that dates back about 15 years. Established originally as APC Systems Ltd., and owned by London Mat Industries Ltd. and Microtronix Vehicle Technologies Ltd. PMT's mainstay APC products are based on the placement of treadle switches in vehicle stepwells. While some might argue that non-contact technologies are preferable, we continue to manufacture a treadle-based technology. Why?
The reason is simple: accuracy. In our experience, there is no more
accurate technology available.
II. Commonly Asked Questions About Treadle Switch-based APC
Q: How does a treadle switch-based system count?
A: The treadle switch-based APC technology offered by PMT utilizes a finite-state method, analyzing passenger stepping patterns to count passengers entering or exiting the doorway of a vehicle, typically a transit bus. This counting "engine" receives relatively little input data - just the transitions of switches closing and switches opening. It takes these transitions and the times between them to determine passenger flow. This is a complex task that the microprocessor in the Passenger Counting Unit (PCU) handles in real time.
Q: Can the PMT APC system make mistakes?
A: Of course, but maybe not in the ways
you might think. Since the feet of a passenger, or, rarely, an object such
as a cane, create the switch closures, the PCU will not mistake a swinging
arm, umbrella or parcel for a passenger. The switch closure patterns created
by a person with crutches or a cane are known, and do not generally fool
the system.
Q: What does fool the system?
A: People behave in consistent ways when
entering and exiting a vehicle. However, exuberant young people, or just
harried commuters running to catch the bus, will sometimes skip a step
in the stepwell; a confused traveler will sometimes walk part way up the
steps to ask the driver for information, then back off the bus. In some
cases, the system will count a passenger on when they never completely
entered the bus. In some cases, the system will misunderstand in which
direction the passenger is moving and will create a "split".
Q: What is a split?
A: A split is a situation where the system
misunderstands the direction of passenger movement; entering passengers
may be counted as exiting, or exiting passengers may be counted as entering.
Q: What causes a split?
A: The most obvious cause of a split is
the occasional unusual behavior of a passenger, where the system misunderstands
the direction of motion. A less obvious situation occurs when, particularly
during rush hours, the passengers crowding the stepwells create stepping
patterns that the system misinterprets. In a case like this, several passengers
may be counted in the wrong direction.
Q: Does this mean that all the passengers are miss-counted?
A: Absolutely not! The timing of the passenger
stepping patterns is very important. A short pause in passenger motion
is enough to allow the system to recover.
Q: How does this splitting affect the counting accuracy?
A: We find that most splitting occurs when
passengers are entering the bus - when the stepwells are crowded and passengers
are shuffling around. Splits arising from this situation cause the system
to count some "on" passengers as "off" passengers. As a result, the system,
over time, will generate slightly more off counts than on counts. This
tends to be statistically consistent. Analysis of data by our customers
and ourselves indicates that statistical corrections can be applied in
post-processing to improve counting accuracy even further.
Q: What is the counting accuracy of PMT's APC system?
A: The answer depends somewhat on the analysis
performed, but overall accuracy, under all conditions, on the order of
95% is achieved.
Q: How do passengers react to PMT's treadle-based APC system?
A: The APC equipment is hidden from view,
as are all cables and other wiring. The treadle switches are designed to
look like the normal rubber mats. The passengers, and often the drivers,
are not aware that the bus is APC-equipped.
III. Treadle Switch-based APC versus Infrared Beams - Pros and Cons
At present, the main competition for PMT's treadle switch-based APC
technology are systems employing beams of near-infrared light. We will
restrict this comparison to physical, electrical and operation characteristics,
since cost is heavily dependent on the particular installation and the
features required of the Passenger Counting Unit itself.
Pros - Treadle Switches
* Very accurate - around 95% overall
* No preventive maintenance required
* Invisible to passengers and drivers
* Not susceptible to damage from electrostatic discharge (ESD)
* Not affected by arm motion or items being carried
* Able to easily handle doorways with two passenger streams
* Accuracy can be improved through statistical processing
Pros - IR beams
* No moving parts
* Simpler installation
* Simple replacement of defective equipment
* Probably not susceptible to "splitting"
Cons - Treadle Switches
* Front treadle switches have 3 to 5 year lifetime
* Installation is more difficult
* Replacement is more difficult
Cons - IR beams
* Accuracy believed to be 80 to 90%
* Daily preventive maintenance required (cleaning sensor windows and door glass)
* Sensors visible to passengers, inviting vandalism
* Potentially susceptible to ESD damage
* Affected by spurious beam interruption
* More difficult to install in doorways with two passenger streams
IV. Expected Treadle Switch Lifetimes
London Mat Industries MATEX (TM) treadle switches are used world-wide in transit vehicles, especially for rear door interlocks and safety circuits. In rear door applications, lifetimes are typically 10 years or more. Toronto Transit Commission recently retired trolley buses with 15 year-old LMI treadles, still working properly.
Front door treadle switch lifetimes are shorter. The life of a mat in the front door is not an easy one, especially in climates where salt is used in the wintertime. In general, front door treadle switches fail due to punctures (rocks, grit, glass, etc.) and the resulting introduction of water, salt and so on, into the switches. We feel the typical lifetime of a front door treadle switch is 3 to 5 years in adverse climates. Communities enjoying better weather also experience longer treadle switch lifetimes.
We recommend that transit companies consider the front door treadle switches to be life-limited parts and institute a replacement program aimed at a 5 year maximum life, modified up or down as experience dictates. Otherwise, failures can be detected through analysis of trip-to-trip load balance (on counts divided by off counts).
We also recommend installation and wiring practices that minimize the
effort required for treadle switch replacement.
V. Factors Affecting Treadle Switch Reliability
We are constantly looking for ways to improve treadle switch lifetimes.
Recent advances include offsetting the switch element toward the bottom
of the mat (thicker rubber on top), and the introduction of stainless steel
switch plates. However, factors beyond our control are the usual causes
of treadle switch failure.
Installation
It is critical that the treadle switches be installed properly. Important points include:
* Inspection of the stepwell for damage, dents and protrusions
* Proper cleaning of the steps prior to treadle switch installation
* Careful hole drilling and deburring
* Adequate caulking of treadle switch edges, and wire exit points using approved silicone caulk
* Neat wiring under bus so that wires are not stressed or subject to being snagged
* Post-installation inspection, critical to making sure that all criteria
are met
Stepwell Cleaning
The treadle switches require no particular preventive maintenance. Although it is important that they be cleaned regularly to avoid buildup of dirt, stones, or glass that could cut into the mats, generally the bus cleaning performed the garages is adequate to ensure clean stepwells.
On-going Inspections
Other bus conditions can affect treadle switch lifetimes. These include:
* Accident damage
* Damaged or misaligned doors, rubbing on the mats
* Loose or damaged handrails
Alert garage personnel, familiar with the APC system, can detect and
correct problems before they affect the treadle switches. Sometimes, however,
garage personnel are the cause of treadle switch failures. The stepwells
make convenient work benches, allowing for damage by drilling, hammering,
welding, soldering, and so on. Adequate training of garage personnel is
important to prevent unnecessary damage and shortening of treadle switch
lifetimes.
VI. Conclusion
We at PerMetrics Technologies are proud of our APC technology. We are dedicated to providing reliable equipment of the highest possible accuracy, and we believe that treadle switch-based technology meets those goals.
OK, one last question: if treadle-switches are so great, then why is PMT offering non-contact technology? Some end users are not convinced that treadle switches have the reliability for their application, and this attitude is reflected in their requirements. To meet these requirements, PMT is offering a treadle switch replacement technology - our new Zone Sensors. Essentially, the overhead, near-infrared sensors replace treadle switch elements, allowing us to use the same, proven, algorithm for determining passenger counts.
There are also those level-entry applications, such as low-floor buses, subways, some light rail vehicles, and buildings, for which the treadle switch solution is not feasible. These challenging situations may involve wide doorways (double or multi-stream), where bi-directional traffic is likely. For this type of application, PMT is soon to offer an exciting new alternative - the Optical Passenger Sensor.
Updated 80 Feb 2002