Feedback to 'Lock Up Your Software' - Murphy's Law Jan' 2001
to Murphy's Law
Mike Salish had a couple of interesting comments to make about
the elevator trade. I was amazed to read that it is the safest
mode of public transport around! His email follows:
Being named as you are just begged for this very good article
to be written. Much of my experience has been in elevator control
equipment which has proven to be about the safest public transport
on a per mile basis. This is accomplished by a generally accepted
ANSI code which in essence requires that a single failure of equipment
can not permit the elevator to move.
The generally accepted interpretation is that the "computer"
part of the design (usually microprocessor these days) can not
override ANY of the mechanical switches or sensors. And those
external components can independently prevent movement. It works
very well in preventing the unintentional motion (like with the
In practice in a normal office building with electronic controls
there are the following individual items that must be independently
operational for an elevator to move:
- Normal Computer Speed Control
- Terminal Motion Speed Control (as in last stop, often second
- Emergency Speed Control (if the first two didn't work, third
- Overspeed Governor Electrical Switch
- Mechanical Safety Governor
- End-of-Travel Electrical Switch
- All doors closed switches (in series)
Other than the first two items (which control and limit speeds)
the result of a fault condition is an immediate stop, regardless
of what the electronic controls determine. I also have seen designs
with the "fox and hens" problem where the software "protects"
the system and provides "redundancy". It is usually a cost-cutting
issue that drives such designs which provide little worst-case
Mike Salish Avanti, Systems, Inc.
From Morgan Jones:
Hi Niall, Saw your latest article in ESP - great stuff as usual.
I particularly liked the battle between the interlocks and the
paper cutters defeating the interlocks - I hadn't heard that one.
Anyone who references Donald Norman is OK with me - I love his
President MoJo Designs Inc