
776 Brief Paper
makes it meaningful, and they will not remember much of it as it
will not be associated with retrieval strategies which are
integrated with the rest of the task. There is some concern that the
present generation of automated systems, which are monitored
by former manual operators, are riding on their skills, which later
generations of operators cannot be expected to have.
Working storage:
The other important aspect of cognitive
skills in on-line decision making is that decisions are made within
the context of the operator's knowledge of the current state of the
process. This is a more complex form of running memory than the
notion of a limited capacity short-term store used for items such
as telephone numbers. The operator has in his head (Bainbridge,
1975) not raw data about the process state, but results of making
predictions and decisions about the process which will be useful
in future situations, including his future actions. This information
takes time to build up. Manual operators may come into the
control room quarter to half an hour before they are due to take
over control, so they can get this feel for what the process is doing.
The implication of this for manual take-over from automatically
controlled plant is that the operator who has to do something
quickly can only do so on the basis of minimum information, he
will not be able to make decisions based on wide knowledge of the
plant state until he has had time to check and think about it.
1.1.3
Monitoring.
It may seem that the operator who is expected
solely to monitor that the automatics are acting correctly, and to
call the supervisor if they are not, has a relatively simple task
which does not raise the above complexities. One complexity
which it does raise of course is that the supervisor too will not be
able to take-over if he has not been reviewing his relevant
knowledge, or practising a crucial manual skill. Another problem
arises when one asks whether monitoring can be done by an
unskilled operator.
We know from many 'vigilance" studies (Mackworth, 1950)
that it is impossible for even a highly motivated human being to
maintain effective visual attention towards a source of
information on which very little happens, for more than about
half an hour. This means that it is humanly impossible to carry
out the basic function of monitoring for unlikely abnormalities~
which therefore has to be done by an automatic alarm system
connected to sound signals. (Manual operators will notice
abnormal behaviour of variables which they look at as part of
their control task, but may be equally poor at noticing changes
on others.) This raises the question of who notices when the alarm
system is not working properly. Again, the operator will not
monitor the automatics effectively if they have been operating
acceptably for a long period. A classic method of enforcing
operator attention to a steady-state system is to require him to
make a log, Unfortunately people can write down numbers
without noticing what they are.
A more serious irony is that the automatic control system has
been put in because it can do the job better than the operator, but
yet the operator is being asked to monitor that it is working
effectively. There are two types of problem with this. In complex
modes of operation the monitor needs to know what the corrcct
behaviour of the process should be, for example in batch
processes where the variables have to follow a particular
trajectory in time. Such knowledge requires either special
training or special displays.
The second problem is that if the decisions can be fully
specified then a computer can make them more quickly, taking
into account more dimensions and using more accurately
specified criteria than a human operator can. There is therefore
no way in which the human operator can check in real-time that
the computer is following its rules correctly. One can therefore
only expect the operator to monitor the computer's decisions at
some meta-level, to decide whether the computer's decisions are
'acceptable', If the computer is being used to make the decisions
because human judgement and intuitive reasoning are not
adequate in this context, then which of the decisions is to be
accepted ? The human monitor has been given an impossible task.
1.2. Operator attitudes.
I know of one automated plant where
the management had to be present during the night shift, or the
operators switched the process to 'manual'. This raises general
issues about the importance of skill to the individual. One result
of skill is that the operator knows he can take-over adequately if
required. Otherwise the job is one of the worst types, it is very
boring but very responsible, yet there is no opportunity to aquire
or maintain the qualities required to handle the responsibility.
The level of skill that a worker has is also a major aspect of his
status, both within and outside the working community. If the job
is 'deskilled' by being reduced to monitoring, this is difficult for
the individuals involved to come to terms with. It also leads to the
ironies of incongruous pay differentials, when the deskilled
workers insist on a high pay level as the remaining symbol of a
status which is no longer justified by the job content.
Ekkers and colleagues (1979) have published a preliminary
study of the correlations between control system characteristics
and the operators' subjective health and feeling of achievement.
To greatly simplify: high coherence of process information, high
process complexity and high process controllability (whether
manual or by adequate automatics) were all associated with low
levels of stress and workload and good health, and the inverse,
while fast process dynamics and a high frequency of actions
which cannot be made directly on the interface were associated
with high stress and workload and poor health. High process
controllability, good interface ergonomics and a rich pattern of
activities were all associated with high feeling of achievement.
Many studies show that high levels of stress lead to errors, whitc
poor health and low job satisfaction lead to the high indirect costs
of absenteeism, etc. (e.g. Mobley and colleagues, 1979i.
2. Approaches to solutions
One might state these problems as a paradox, that by
automating the process the human operator is given a task which
is only possible for someone who is in on-line control. This
section will discuss some possible solutions to problems of
maintaining the efficiency and skills of the operator if he is
expected to monitor and take over control; the next section will
introduce recent proposals for keeping the human operator on-
line with computer support.
Solving these problems involves very multi-dimensional
decision making: suggestions for discussion will be made here.
The recommendations in any particular case will depend on such
factors as process size and complexity, the rate of process change,
the speed and frequency of process or automatic control failure,
the variability of the product and the environment, the simplicity
and cost of shut down, and the qualities of the operator.
2.1.
Monitoring.
In any situation where a low probability
event must be noticed quickly then the operator must be given
artificial assistance, if necessary even alarms on alarms. In a
process with a large number of loops there is no way in which the
human operator can get quickly to the correct part of the plant
without alarms, preferably also some form of alarm analysis.
Unfortunately a proliferation of flashing red lights will confuse
rather than help. There are major problems and ironies in the
design of large alarm systems for the human operator
(Rasmussen and Rouse, 1981).
Displays can help the operator to monitor automatic control
performance, by showing the target values. This is simple for
single tolerance bands, but becomes more complex if tolerances
change throughout batch processing. One possible solution is to
show the currently appropriate tolerances on a VDU by software
generation. This does not actually get round the problems, but
only raises the same ones in a different form. The operator will
not watch the VDU if there is a very low probability of the
computer control failing. If the computer can generate the
required values then it should also be able to do the monitoring
and alarms. And how does the operator monitor that the
computer is working correctly, or take over if it obviously is not'?
Major problems may be raised for an operator who is highly
practised at using computer generated displays if these are no
longer available in an emergency. One ironic but sensible
suggestion is that direct wired displays should be used for the
main process information, and software displays for quantitative
detail (Jervis and Pope, 1977).
'Catastrophic' breaks to failure are relatively easy to identify.
Unfortunately automatic control can 'camouflage' system failure
by controlling against the variable changes, so that trends do not
become apparent until they are beyond control. This implies that
the automatics should also monitor unusual variable movement.
"Graceful degradation' of performance is quoted in 'Fitts List's of