Haptics & Clicks | Seek n' Geek VIII
Figure 1: The Apple Tactic Engine [Images]
Humans perform best with sensory input from multiple means. Great design of human interfaces include feedback to the user, in the form of visual, auditory, and increasingly haptic feedback.
Haptic (kinesthetic) feedback provides felt responses to a user when they interact with an interface. The kinesthetic feedback can be coupled with a mechanical or digital product. Mechanical systems are more common, like pressing keys on a laptop or on a piano. For digital systems, the vibration you feel in your pocket from your cellphone is basic feedback saying "something has happened", but designers are pushing the bounds to provide more nuanced haptic feedback in digital devices.
The best state of the art demonstration is the iPhone 7's new Tactic Engine, which on the highest level uses force sensors and a LRA linear resonance actuator (shown above) to create a spectrum of vibration. Figure 1 shows magnets or electromagnetic coils -- gold on the left, and darkest part in the x-ray on the right. The key factors allowing different types of feedback are the amount of time vibration is relayed to the user, and the frequency and amplitude of the wave profiles generated. The mechanical design below allows the actuator to start and stop within one revolution leading to millisecond level control. 
As a brief overview, the three types of electronic haptic feedback are:
(1) old school ERM eccentric rotating mass systems | 1 DOF changes in voltage
(2) LRA linear resonance actuators | 2 DOF changes in voltage and amplitude of signal
(3) piezo actuators | 3 DOF, voltage, amplitude and wave frequency
For more of an overview check out this EETimes article.
Figure 2: Haptic feedback mechanisms [Images]
Although most of the haptics hype these days is related to high-tech vibrations, low-tech haptics are cleverly embedded in our everyday lives. Think about driving in the fast lane of the highway and you drift a little out of your lane. You feel and hear the vibration from your car wheels driving over the grooved surface on the edge of the lane. Similarly, when you use a zip-tie you can also feel and hear the clicks as it cinches down.
So, what's going on behind these mechanisms? How do we proactively design low-tech haptic feedback?
These are particularly interesting questions in the context of my thesis which needs two haptic feedback mechanisms to give information back to the user that they have completed a task. This is constrained furthermore by the the requirement that these mechanisms be injection molded with the already existing components. Achieving some haptic feedback isn't difficult - tuning it to be just the right amount requires more analysis.
I designed a ratchet and pawl mechanism for this application using the spreadsheet and sketches below. I changed the length, height and width of the beam to reduce the max bending stress, such that it is at least a factor of 4 less than the max bending stress.