3D Touch Versus PCAP Touch

Large multi-touch displays and video walls are becoming the norm at trade shows these days as almost every booth will have these types of displays.  We will also see them at retailers, hotels, museums, business conference rooms/lobbies, medical facilities, and subways.  Companies use digital signage to engage and interact with their customers to expand their branding.  At Premio, we sell hospitality digital signage, PoE digital signage, and smart mirror digital signage with multi-touch functionality for our displays.  Our projected-capacitive (PCAP) can have 10 finger multi-touch while a custom project can have up to 40-point touch.   We only have 10 fingers, so why would we need 40-points of touch?  Well by expanding the surface area of the display, multiple people can use the touch screen and interact with it.  Instead of a single user application you can have multiple people interact with the same display. In most cases, projected capacitive touch sensors are made up of three major components: protective glass, PCAP film with flexible printed circuit board connecting to the touch controller, and the display itself.  The PCAP film is made up of a series of electrodes, composed of a transparent conductive coating, that are configured into rows and columns.  Each electrode is routed back to a connected point where the circuit can be bonded and the flexible printed circuit typically contains the controller for the touch sensor. The protective glass buries the electrodes within a stack of lamination, and serves as a dielectric between your touch and the electrodes of a PCAP film.  The protective glass also serves as a barrier layer-protecting the sensitive electrodes from environmental and human damage.  PCAP touch screens technology works off an electrostatic field that is created by layers of conductive material that are individually etched to form a grid pattern of electrodes.   When a finger touches the screen, the electrical current changes.  A touch event occurs when a finger or conductive stylus interferes with the electric field projective above the surface.  This change in capacitance is detected by the controller which interprets the X and Y coordinates of the event.  Multiple fingers can be tracked simultaneously and the controller supports common multi-touch gestures.  Multi-touch gestures include flick, pinch, expand, tap, click, and rotate.   The next biggest thing to hit the touch technology is 3D touch from the iPhone.  3D touch has the same features as multi-touch like tapping, swiping, pinching and stretching , however, it adds another element to measure the different levels of pressure.  You might think that 3D touch is nothing new, as the Apple watch had it.  While the Apple did have a feature called force touch, force touch distinguishes between different levels of force applied to the display.  The electrodes around the display can differentiate between taps and harder presses and perform a specific action.  However, in reacting to your touch, the force touch is not as fast as 3D touch.  The lighting fast response of the 3D touch is because of the fusion of capacitive sensors and strain gauges.   3D touch adds two new gestures: peek and pop.  Peek is a light press gesture that can be used within apps like Safari, Messages and Mail to preview content.  A harder press is used for Pop gestures to open content.  Moreover, on the phone’s home screen, pressing softly on an app’s icon opens a menu of shortcuts to frequently used programs whereas pressing harder on an app executes that action.   HOW DOES PCAP WORK All projected capacitive touch screen designs have two key features in common—the sensing mechanism that lies behind the touch surface and a lack moving parts.  Mutual capacitance is now the more common projected capacitive approach and makes use of the fact that most conductive objects can hold a charge if they are very close together.  If another conductive object, such as a finger, bridges the gap, the charge field is interrupted and detected by the microcontroller unit.  In other words, when a PCAP film detects a touch, the finger changes the electrical current.  PCAP touch screens are scanned, which consist of a matrix of rows and columns that are read one by one to get a reading, thus this composition creates a three dimensional electrostatic field. To get an exact coordinate, the results from several row/column intersections are read and the counts are used to triangulate the exact touch location.   Its advantage is that it has high precision, multi-touch functionality, and high response speed.  The disadvantages of PCAP is that when the hand is covered with a heavy duty glove, such as in industrial settings, it will usually will not detect a touch. (http://www.sky-technology.eu/en/displays/touch-screens/projected-capacitive-touch-screens-how-they-work.html) Indium Tin Oxide (ITO) (http://www.medicalelectronicsdesign.com/article/choose-right-touch-technology-your-display)\ (https://www.forbes.com/sites/jvchamary/2015/09/12/3d-touch-iphone-6s/#2bb958b04cee) Capacitive sensors are often made of indium tin oxide (ITO), an optically transparent material that conducts electricity.  You can think of ITO as wires in a circuit because that’s how the material behaves, although ITO sensors are actually arranged in a grid of dots across the screen. (http://www.sky-technology.eu/en/displays/touch-screens/projected-capacitive-touch-screens-how-they-work.html) HOW DOES 3D TOUCH WORK Some might suggest that 3D Touch already exists by long-holding on the home screen to select text or move/delete apps from the homescreen/apps menu.  When you finger touches a conductive surface such as glass, it completes a circuit with capacitive sensors on the film, causing a detectable change in the circuit’s electrical signal.  Again, 3D touch is not the same as force touch found in the Apple watch.  Unlike being able to sense if a surface is being pushed using force touch, 3D Touch detects two levels of pressure: soft and hard.  iFixit has conducted a full teardown on the iPhones with 3D touch sensor technology.  The 3D Touch capability is made possible by a capacitive sensor layer that is located at the very back of the display panel.  The 3D touch sensor layer looks like capacitor plates that look similar to a grid of golden rectangles that is connected to the controller chip.  iFixit found another separate layer of capacitive pressure sensors.  To make 3D touch work from touchscreen to sensor plate, Apple also worked with the makers of Gorilla Glass to create a pliable glass that bends ever so slightly by the force that is detected by a sensor called strain gauges.  This means that the deeper you press down on the glass, the closer your finger will be to the corresponding capacitor plate right beneath the display.  According to Apple, “the technology behind 3D Touch starts with the display, which recognizes the pressure you apply.”  Strain gauges in the glass causes the electrical signal to change.  This is what differentiates standard PCAP and 3D Touch.  The strain gauges’ job is not to pinpoint the location of your finger on the iPhone screen but to measure the distance between your finger and the capacitor when you press on the glass.  The PCAP film is still present for precision tracking, however, strain gauges detects where the flex in the glass occurs. (https://www.ifixit.com/Teardown/iPhone+6s+Display+Teardown/49951) Patents shows how 3D Touch works.  Capacitive sensors have one of several twisting traces that match the orientation of forces that will be detected by the strain gauges.  3D Touch includes two layers of strain gauges: one to sense force and another to compensate for temperature.  The one for temperature is there because heat can cause properties in materials to broaden and possibly generate improper signals.  At the corners of the grid, sensors have traces at 45° in order to distinguish strain gauges along that angle, for example, while sensors in the middle have traces that run parallel to the edges of a display.   (https://www.forbes.com/sites/jvchamary/2015/09/12/3d-touch-iphone-6s/#2bb958b04cee) Changes in electrical signals caused by physical forces are calculated at each strain gauge – what Apple’s patents call the “force centroid” and then compared to neighboring sensors which reveals the location and level of pressure.  Or as the patents put it, the touchscreen will “relate measure force difference and force centroid to electronic device.”  If the iPhone receives more information from electrical signals on the left than the right, for instance, you’re obviously pushing down on one side of the display.