Choosing an HMI panel size before you draw anything

The first decision in building an operator screen is the one most often left until last: which panel.

It matters more than it sounds, because unlike a web page, a panel does not reflow.

Fixed pixels, fixed layout

A panel is a fixed number of pixels. A graphic drawn at 1280 by 800 and deployed to an 800 by 480 panel does not shrink to fit. The bottom and right of it are off the glass.

Some tools offer to scale a project between sizes. What that produces in practice is text below the readable threshold and objects overlapping, and the work of fixing it is comparable to redrawing.

This is why every HMI tool asks for the panel before the first object is placed, and why the answer should come from the specification rather than from what was in the cupboard.

Resolution, not inches

The diagonal measurement is a marketing number. Two 7 inch panels can be 480 by 234 and 1024 by 600, which is more than twice the usable area for the same stated size.

The rough ladder on current hardware:

• 4 inch: 480 by 272. One or two values and a couple of buttons. A local station, not a control screen.
• 7 inch: 800 by 480. The workhorse for machine level control.
• 10 inch: 1280 by 800. Room for a mimic with real detail.
• 15 inch: 1280 by 800 or 1366 by 768. A line level operator station.
• 21 inch and up: 1920 by 1080. Plant overviews and control rooms.

Viewing distance decides everything

The calculation nobody does, and it settles the size argument faster than any discussion of feature lists.

Minimum comfortable character height in millimetres is roughly three times the viewing distance in metres. Standing at the panel, half a metre away, that is about one and a half millimetres, which is a small font and fine.

Reading it from the operator's normal position four metres down the line, it is twelve millimetres. On a 7 inch panel at 800 by 480, twelve millimetres is a very large fraction of the screen height, and about six lines of text fit on the whole panel.

That is the real constraint, and it is why panels that look adequate on a desk are useless in position.

Portrait, and the machine it is bolted to

Two practical points that come up late and cost money.

Portrait orientation is supported by most panels and suits a tall narrow machine, but it has to be decided at the start, and some tools handle it poorly.

Cut-out dimensions vary between manufacturers for the same nominal size. Changing brand after the panel door is cut is a fabrication job, not a swap.

What to do about it

Specify the resolution, the orientation and the viewing distance in the functional design specification, before any screen is drawn.

Then draw at that size, at one to one, and check the text at the real distance rather than on a monitor. The HMI builder in LADX lists fourteen real panel sizes by resolution rather than by model number, and draws a scale outline of each, because 480 by 272 and 1920 by 1080 should look like different decisions rather than two numbers.