The buttons and indicator lamps on the control cabinet panel are usually placed one by one following the electrical schematic: whatever supplies, whatever is controlled and whatever is interlocked are put next to each other. That works well for the electrician during commissioning, but not necessarily for the person who operates the machine every day. The operator looks at the panel several dozen times a day, and the questions asked are concentrated on a few: what is the state now, is the flow sufficient, has the vacuum reached its level, should the machine be stopped.
The first screen carries the running status and the key readings
The starting point for rearranging the panel is to sort the operator's actions into two groups: the ones done every day and the ones done twice or three times a year. There are only four actions done every day: check the current status, check the vacuum, check the temperature, and start and stop the machine. The readings and indications these four need are concentrated on the first screen, directly in the line of sight, so that standing in front of the machine the operator does not have to look down or lean sideways, and the arrangement of the gauges and the indicator lamps lets the eye settle at one distance.
The second screen carries the settings: the temperature setting, heating on and off, the manual and automatic changeover. Such a parameter may be adjusted once a month, or not at all over a whole operating cycle. Once they are pushed down, the first screen is no longer broken up by a large number of buttons that are rarely touched, and the indicator lamps can no longer be mistaken for one another because of which one they are. When a parameter does have to be changed, the operator knows clearly that there are two more levels to go down to find it.
Why the stop button has to be within reach of the right hand
The alarm and stop buttons are placed within reach of the right hand, and that is not a matter of habit but of reaction time. When something goes wrong the first reaction is to stop, and how fast the hand can do so depends on which direction it has to move. Placing the button where the operator has to reach across the panel, or shift the body to reach it, adds a deliberate delay at the moment the alarm sounds, and for a machine that is processing oil that delay is not short.
Space has to be left around the stop button as well: it should not sit right next to buttons that are pressed often, and it should not be placed in a corner of the panel where it can be struck by the back of a hand or by a sleeve, because an unintended stop also affects production. The red stop button and the green start button have to be distinguishable at a glance in both position and colour, and that is part of the protection against misoperation. The use of red and green has to be consistent on the same panel: red cannot mean stop on one panel and running on another.
- First screen: the running status indication, the vacuum and temperature readings, and the key flow and pressure indications.
- Within reach of the right hand: the stop button and the reset button.
- Second screen and below: low-frequency settings such as the temperature setting, the time setting and the manual and automatic changeover.
- Lamp grouping: states of the same kind together, green for normal and red for abnormal, so that no two lamps carry overlapping meanings.
What the layout means for the operator
The most direct change is that the path of the inspection round gets shorter. Going round the equipment, the operator has to come up to the panel fewer times, because the readings used daily are gathered on the front screen and there is no need to walk round to the side or crouch down to find them. Once the infrequently used settings have been pushed down, only a handful of lamps appear to be lit on the panel, and an abnormal state is easier to spot as a result: a row in which only the lamps that should be lit are lit is much faster to read than hunting for one lit lamp among ten.
| Area | What is placed there | Frequency of use | Basis for the position |
|---|---|---|---|
| First screen | Running status, vacuum and temperature readings | Several times per shift | In the centre of the line of sight, readable from a normal standing position |
| Within reach of the right hand | Stop, reset | Used at once when something is wrong | Shortens the reaction time and prevents accidental operation |
| Second screen and below | Settings, mode changeover | Low frequency | Pushed down to keep the first screen clear |
One point has to be made: however well the panel is laid out, it cannot replace the operating procedure. The indicator lamps only display the state, and deciding what a lit lamp means for the machine at that moment still relies on the logic covered in the training. Keeping the two apart makes it clearer: the panel is there to put the state where it can be seen, and the procedure is there to tell the operator what to do after seeing it.
Lay the panel out by frequency of use, not by electrical circuit: put what is used daily in the centre, the stop button on the right, and what is adjusted once a year down at the bottom.