HMI touch button only responds to half: recalibrate with a stylus

Why the same button acts differently at each side

A resistive touch panel reports raw analog values for every press, and a calibration map converts those values into screen coordinates. The map is only as good as the calibration that set it. Two errors can live in it. An offset shifts every reported point by a fixed distance. A scale error starts small in one region and grows across the screen, and that is the kind behind this pattern: near the region where the calibration targets were touched, objects respond anywhere; far from it, a narrow object's outer half reports outside its own bounds.

A button fires when the reported point lands inside its rectangle. Give the map a drift equal to half the button's width and every press on the outer half reports past the far edge. An object spanning the full screen width still catches a drifted point anywhere along it, which is why the full-width button and the screen of selector objects kept working while the edge button died. A resistive panel senses pressure, so the pointed plastic end of an ordinary pen is a valid contact for both the test and the calibration targets; the touch panel technology comparison explains how the panel types differ.

Symptoms and what each points to

• The same button answers anywhere on it at one side of the screen and only on its left half at the other: registration drifts with position, so calibration is the first suspect.
• A full-width button or a screen of selector objects answers everywhere: presses register across the whole panel, and the object's width is absorbing the drift.
• A slightly narrower object at the extreme edge never answers at all: the drift is widest there.
• Recalibration with a fingertip leaves the fault unchanged: the broad contact centers beside the small target, so the new map comes out as wrong as the old one.
• Response is sluggish everywhere but the positions are right: a different fault, covered in Fix a slow HMI panel: touch, entry and screen update delays.

Run the touch test before you edit the project

On the panel used as the example here, the setup menu opens when you press and hold the upper-left-most corner of the screen for 5 seconds. Inside the menu, Test brings up a screen that shows a marker wherever a touch registers, and Settings holds the recalibration function. Other panels reach the same two functions by their own route; the panel's manual prints the sequence. The same menu holds the display settings and the password protection, and Password protect the HMI setup menu covers locking it down.

1. Hold the upper-left corner for 5 seconds with the pen's pointed end to open the setup menu.
2. Open the Test screen.
3. Touch three spots with the pen tip: left edge, center, right edge. At each one, note where the marker sits relative to the pen.
4. Watch the left edge, where the dead button lives. A marker sitting beside the pen by a consistent margin confirms calibration as the fault.
5. If the marker sits under the tip at all three spots and one object still misbehaves, stop recalibrating. That object's size or position in the project is the problem, and the editor is the place to fix it.

Example: a 60 pixel drift kills the right half of a 120 pixel button

Take a small panel with a 320 × 240 pixel screen. A momentary pushbutton spans pixels 10 to 130, so it is 120 pixels wide and sits with its left edge 10 pixels in from the border. The numbers below are illustrative; they show the arithmetic any calibration fault follows, not a measured value from a specific panel.

Say the map in this region reports every touch 60 pixels to the right of the true contact, tapering to zero at the right side of the screen. The button answers while the reported point lands between pixels 10 and 130.

• Touch at pixel 40: reported at 100. Inside the button, so it answers.
• Touch at pixel 100, the middle of the button: reported at 160, which is 30 pixels past the right edge, so nothing answers.
• The boundary: the report is the true position plus 60, and the button answers while the report stays at or under 130, so true touches up to pixel 70 answer. Pixels 10 to 70 are exactly the left half of the button, which is the behavior on the bench.
• The narrower test object, 45 pixels wide at pixels 10 to 55, never answers: a touch at its own left edge, pixel 10, reports at pixel 70, which is 15 pixels outside.

After recalibration the drift sits at a pixel or two, and all 120 pixels answer again.

Recalibrate with a pointed tip

1. Open the setup menu again with the corner hold, pen tip down.
2. Go into Settings and start the touch recalibration.
3. The routine shows its targets one at a time. Touch the center of each target with the pointed end of the pen, and land the contact on the target, not beside it. Follow the routine's own sequence; the number and order of targets belong to it.
4. Let the routine finish before you judge it. This panel's setup screen gives no message when a calibration attempt goes wrong, so the test screen is the only verdict.
5. Return to the Test screen and touch left edge, center and right edge. The marker should sit under the pen tip at all three.
6. Press the repaired button at several points, including the half that was dead, and confirm it answers only inside its area.

If the routine reports an error or will not complete, stop there and route the panel through the manufacturer's support channel. Until a calibration completes cleanly, the map cannot be trusted.

Verify, then leave the layout alone

Repeat the three-spot check, then repeat the checks that exposed the fault: press the left-side button across its full width, press the full-width object, and watch that touches between objects answer nothing. The tempting production workaround is to widen the button to the screen's full width or park it on the right side. That hides the symptom for the day and leaves the wrong map in the panel, ready to swallow the next narrow control you draw at the edge. Reverse any size or position changes you made while diagnosing, once calibration is fixed.

If the panel starts or stops a machine, treat the repaired mapping as a control change: confirm the touch test before the panel goes back into service and follow the site's own procedure for the restart. When the panel is new, the same bench habit applies; Choosing an HMI panel: test startup and runtime first is the routine to run before a panel enters a project.

Which fault you have, and the fix for each

• The marker drifts beside the pen tip at the edges: recalibrate with the pointed tip and verify at three spots. Recalibration ends this fault.
• The marker is accurate everywhere and one object stays dead: correct that object's size and position in the project editor.
• Calibration completes cleanly and one region of the screen stays dead: the touch layer is damaged, and the panel goes to the manufacturer's service channel.

Recommendation: when a button's dead zone moves with the button's position on the screen, treat it as calibration until the touch test says otherwise. Open the setup menu, run the test at the left, center and right, recalibrate touching each target with a pointed pen tip, and re-run the test before the panel returns to a machine. Keep fingertips for operating buttons; calibration targets are small, and a broad contact is how the wrong map got there. More fixes for machine and control screens sit under shop-floor screens.

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