The whole assembly is made up of two things, a force sensor and a lateral vibrator. It is able to provide haptic feedback which is able to trick your fingers into feeling different textures through different oscillation settings. To understand how it works, we need to understand the basics of the individual components.
Shen Ye is a developer and MSci graduate in Chemistry from the University of Bristol. Author of the popular Smartphone Futurology series on Mobile Nations, as part of our Experts series, he takes a look at the Taptic Engine and Force Touch technology contained in Apple's new MacBook and the Apple Watch.
The most common vibration motor found in the modern day smartphone is what is known as an Eccentric Rotating Mass vibration motor, an unbalanced mass on the driveshaft of an electric motor.
Image credit: GoDo electronics
See the ERM motor on the top left corner of the iPhone 5, credit: iFixit
The motor is screwed into the frame of the device and, when turned on, it spins rapidly. The uneven centripetal force causes the motor to generate a vibration perpendicular to the axis of the driveshaft. You will find this in almost every Android device and every iPhone before the iPhone 6 (with the exception of the iPhone 4S, which used an LRA vibrator.)
LRA is a new type of vibrator unit which has recently been adopted into smartphones, it stands for Linear Resonant Actuator. It uses a completely different mechanism compared to ERM. The general shape is similar to that of a small button cell (found in the iPhone 4S), but can come in all shapes and sizes (rectangles in the iPhone 6 and iPhone 6 Plus).
A Linear Resonant Actuator, credit: Precision Microdrives
The spring keeps the central mass under slight tension. There is a neodymium magnet attached to the mass that sits in a voice coil, which controls the oscillation of the magnet through electromagnetic signals. If you're familiar with how a speaker works, this essentially uses the same mechanism. Both ERM and LRA vibrators convert electricity into kinetic energy via electromagnetism. LRA has a few advantages over ERM: it uses less energy when vibrating and has a lower latency between turning on and actually producing a vibration. Texas Instruments claims LRA has up to 2x more force and uses 50% less power. The ERM motor has a spin up time where it needs to build up inertia on the unbalanced weight, and it's high overdrive and braking makes it hard to control precise oscillations (see graph).
Vibration comparison between ERM and LRA, credit: TI
The Taptic Engine in the new MacBook uses something very similar, except it is spread out across the touchpad with force sensors integrated at the corners. The spring isn't pressed against the weight but is at the sides where the assembly is connected. With the incredible speed that it takes to power on the Taptics Engine, it allows what feels like instantaneous feedback which can be calibrated to mimic responses such as clicking.
All of the implementation of lateral vibration haptic feedback came from a researcher's thesis published at MIT 20 years ago. It was originally tested on a joystick where the surface of the head needed to be a "haptic neutral" surface — a reinforced ping pong ball though they did consider using an egg shell as well. The vibrations were programmed through wavefunctions which describe amplitude (how strong the vibration is) and frequency. They were able to simulate numerous different textures on that single surface, including various grits of sandpaper and ridged surfaces.
The integrated Force Touch system involves a capacitive pane of glass over force sensors. The capacitive glass works exactly like the touchscreen on modern Multi-Touch devices. The glass contains an embedded fine grid of thin wires connected to electrodes at the four corners. These electrodes generate a constant voltage across this grid of wires thinner than a human hair. When a material that can hold an electric charge (e.g. human skin) touches the screen, the tiny voltage on the screen transfers to the material and the controller chip records the coordinates of the position of voltage drop on the grid.
MacBook Force Touch Trackpad, credit: Apple
The four Force Touch sensors at each of the corners of the trackpad are able to detect the force applied to the trackpad. Without knowing how it works, there are two likely components which can be used to achieve this. The most likely is a strain guage, which is a thin layer of strain sensitive circuity. When the circuitry is pressed on, it becomes thinner which increases its resistance and this is recorded by the microcontroller which translates it into a force value using a calibration curve.
Strain gauge, source: WikiCommons
Another likely candidate is a thin piezoelectric force sensor. A material with piezoelectric properties has the ability to generate a voltage when its shape is deformed. You can find them widely used in flint-less spark lighters which generates the spark to ignite the fuel through by pressing down the clicker. For force sensing there's no need for such a high voltage that a spark is generated, just needs enough for the microcontroller to record the change in voltage. Similar to strain gauges, the microcontroller would need to have a programmed calibration curve to convert the observed voltage from the piezo force sensor into a force value.
Apple may never release information on how their Force Touch works, but this article summarised the mechanisms behind how the Taptic Engine and the science behind the individual components.