Courtesy: Avnet
It’s not the first time we’ve talked about the phenomena of the ‘ghost touch’ or ‘false touch’, where a touch screen responds, seemingly without human interaction. Fortunately (or unfortunately, depending on how you view it!), there is nothing spooky going on here. There are quite a few circumstances where it could happen – electrical noise or even water spilling on the screen can trigger an unwanted response in a standard projective capacitive touch screen. Simply put, the screen just can’t tell what’s human and what is not.
If your screen is being used for tasks which have safety implications, as many of our customers do, this is far from ideal. So how do we tackle the problem?
We use force. (No, not that kind of force.)
By adding a pressure detection solution under the touch screen, we can remove all fear of false triggering. Based on electromagnetic induction, ‘force touch’ creates a waterproof, mistake-proof environment that will even allow for continuous clicking without lifting your hand. But how does this work?
Eddy Current Pressure Sensors
These sensors operate based on the principle of electromagnetic induction and are a spiral planar coil made from a printed circuit board (PCB). When an alternating current (AC) flows through the coil, it generates an alternating magnetic field around it.
If a conductive material (such as a metal target) is brought near this magnetic field, eddy currents are induced within the material. These then create an opposing magnetic field, which reduces the inductance of the sensor. The inductance changes as a function of the distance between the sensor and the conductive surface.
Why we use Eddy Current Pressure Sensors in TFT Projects
ECP Sensors can measure pressure over a really large surface area. In TFT (thin-film transistor) projects, these sensors can be very useful for touchscreens or interactive displays and have many benefits:
Accurate Measurement of Distribution Force: This means that, in a TFT display, you can easily and precisely detect variations in pressure across the screen.
Button Replacement: Eddy current sensors can even be an alternative to physical buttons. Plus, they don’t need any cutouts or holes in the display, which makes for a sleeker design.
Unaffected by debris, liquids or magnetic interference: Unlike mechanical buttons, these sensors are immune to these problematic external factors.
Into the assembly – how pressure sensing sits in the stack
Pressure sensitive coils are made on a Flexible Printed Circuit (FPC) and laminated to the metal frame and touch screen with double-sided adhesive. Micro-deformation occurs between the inductive layer and the metal frame when the touch screen surface is pressed, and the pressure-sensitive chip detects this electromagnetic change.
Design benefits of a Pressure Sensing Solution
This approach reduces structural design difficulties by using the metal frame for touch screen assembly and there is no need for additional sensors when you can just use the PCB metal alignment.
The pressure-sensitive chip has a built-in algorithm which directly outputs the press force level, according to the commissioning parameters, after the structure has been assembled, making it easy for our customer to adjust.
The IO port can drive LEDs directly, creating an integrated solution from pressure sensing to light output.
Subdivision of 1024 levels of force within 0.1mm pressure deformation range.
Eddy Current Sensors are reliable and offer good linearity and repeatability in sizes up to 12.1”.
And finally – they even work with gloves! Which is superb news for those working in extreme environments. Because Eddy Current Sensors aren’t affected by temperature variations, unlike some other pressure sensors. In fact, you could go as far as to say that we’ve ‘forced’ out ghost touches for good.
