Touchscreen Calibration
What’s the Problem?
- You press on an icon, but the one next to it is selected as shown in the image on the right.
- Even worse, you try to close a window by pressing on the [X] but cannot get the system to recognize the press correctly. You cannot close the window!
Mis-Calibrated Touchscreen
Why Does This Happen?
Resistive-type touchscreens are normally used in cost-sensitive designs, or in situations where gloved operation or a physical touch point is desired, vs. a capacitive proximity touch. Their construction is simple, their operation is well understood, and the hardware and software required to support them is readily available from multiple manufacturers. Despite the advantages of resistive-type touchscreens, they are subject to linearity deviations and unit to unit tolerances, so devices equipped with them almost always require some type of calibration. Calibration is necessary because it is difficult to perfectly align a touchscreen to the display behind it. If a button on the display is to be properly activated, the coordinates of the area touched on the screen must be sufficiently close to the coordinates of the feature on the display. Otherwise, the software may not correctly act upon the button presses.
A couple of good articles on touchscreen calibration can be found at these locations:
Mis-Calibrated and Calibrated Touchscreen
What Can You Do About It?
We can Calibrate (or adjust) the touch location reported by the touchscreen to where the touch actually occurred on the display. To do this, we do the following:
- Calibration software puts up targets
- User touches the targets (very accurately)
- Calibration software captures the locations reported by the touchscreen
- Calibration software calculates “Adjustment”.
- Software knows where the targets were on the display and where the touchscreen reported the touches were.
- Adjustment value is “saved”.
When calibration data is available, all reported touches are “Adjusted” before they are used by the system.
Types of Touchscreens
Resistive-type touchscreens are normally used in cost-sensitive designs, or in situations where glovedLet’s compare the two most common types of touchscreens that FDI uses based on their advantages and disadvantages.
*There are some exceptions, but all are costly, and only some types of gloves are supported.
Here are some resources to learn more about types of touchscreens
Methods of Calibration
We can either calibrate ELI directly (1) at the Device level or we can calibrate the Host Operating System (Windows or Linux) (2).
- Calibrate ELI at the Device level
- FDI enables our customers to calibrate resistive-touch ELI units. There are two software utilities that FDI provides to calibrate our resistive-touch ELI units. Refer to the table to determine which utility is right for your ELI.
- Calibrate in the host OS
- FDI provides our customers detailed instructions for OS calibration in Windows 10, Windows 7 and Linux.
- Windows:
- A touchscreen must report as a digitizer in order to be calibrated at the OS level in Windows. As of April, 2019, most ELI units ship from the FDI factory pre-set as digitizers. Some earlier revisions of ELIs reported as a HID Mouse device. Refer to the table below to find the earliest revision an ELI was shipped as a digitizer.
- NOTE: ELI43-CR and ELI43-CP are 480 by 272 pixel displays. When Windows displays its calibration instructions on these ELIs, the instructions overlap with several of the targets, preventing the user from completing calibration. In the image to the right, the user’s touch on this target is never registered by Windows.
- Linux:
- All ELI’s support OS level calibration in Linux
- Linux does not automatically save the calibration data – We do it in the procedure
For Developers – Button Size and Spacing
The following FDI forum post shares some handy tips for developing applications on small displays (like ELI displays).
