The Hardware Foundation of Haptic Feedback: From Mechanical Motors to Solid-State Actuators

1. Introduction
2. The Physics of Haptic Sensation
3. Core Actuator Technologies
4. Key Performance Metrics
5. Hardware Integration and Engineering Issues
6. Conclusion

Introduction
Haptic feedback is the feeling of touch through mechanical force, vibration or motion. It mimics the physical feelings that humans naturally get when they interact with objects. When you push in on a physical button you have some resistance, some movement and a satisfying click. These sensations are the confirmation of your action to be about to interact with.
The role of haptic feedback has become extremely important in the touchscreen age. Modern devices have done away with using physical buttons to capacitive glass surfaces. This leaves an interaction vacuum. Users can no longer depend on tactile feedback
that their input registered. Haptic feedback helps to fill price across this gap by redesigning bodily sensations by way of software-control hardware.
This blog offers an overview on Haptic Feedback Technologies. We study the passage of time from the traditional mechanical motor to the fine state-of-the-art motor. We explore the mechanics of different hardware approaches to produce different experiences of touch. Understanding these technologies is critical to anyone who is designing today's interactive devices.

The Physics of Haptic Sensation
Human touch perception works on two separate systems. These systems are sensitive to different forms of mechanical stimulation. Tactile sensation takes place in the surface layers of the skin. Specialized receptors feel vibrations, pressure and texture. These receptors are very sensitive to high-frequency vibrations. They react to minute displacements that are measured in micrometers. When you touch the texture of fabric or the buzzing sound of a notification, then you're getting a sense of tactile sensation.
The origin of the kinesthetic sensation is in muscles, tendons and joints. These receptors detect force, resistance and large-scale movement. They respond to lower frequencies and movements of a larger scale. When you experience the feeling of pushing a button or the weight of something you are feeling kinesthetic feedback.
There are sensitivity to frequencies of the human perception. The lower frequencies are perceived as slow rumbles. Higher frequencies can sound on the sharp or buzzy side. Also, the perception threshold is important. Humans need 0.01 millimeters of vibration for vibrations to be detectable at the optimal frequencies. This limitation helps to understand the limits when designing an actuator.

Core Actuator Technologies
1. ERM (Eccentric Rotating Mass Motor)
ERM motors are the simplest way of haptic feedback. The design is more simple. An asymmetrical weight is spun on a small DC motor. As the weight turns it causes cyclical force to produce vibration.
The advantages are clear. ERM motors are very cheap to produce. They are simple to drive circuit wise. They generate powerful vibrations that the users are easily able to perceive. These factors explain their prevailing use in the basic devices.
ERM motors have a slow response time. The motor has to go from zero to the operating speed for the mass. This takes 50 to 100 milliseconds. Similarly, coming to a stop takes time as the mass takes time to come to a stop. This latency makes a nice, crisp and accurate feedback impossible.
Control limitations add to these problems. You cannot adjust vibration intensity and frequency by yourself. These two are mechanically linked by their rotation speed. The frequency vibration is always proportional to motor RPM. This avoids subtle effects that are familiar to the touch. Users only get generic buzzing sensations.
2. LRA (Linear Resonant Actuator)
LRA technology has tremendous improvements over ERM motors. The design is based on a different mechanical principle. A magnetic mass is located on a spring suspension. A voice coil is used to drive the mass park and to back along a range linear axis.
The major advantage is the operation in resonance. The system is tuned to a certain natural frequency often between 150-200 Hz. It is at this frequency that the actuator has maximum efficiency and acceleration. The small input power generates high vibration.
Response time is vastly improved. LRA actuators can go start and stop in 10 to 20 milliseconds. This allows sharper and clearer haptic effects. Users have distinct clicks and not mushy buzzes.
Reliability also improves. There are no brushes that can wear out. The simple spring-mass system can go on for billions of cycles. Power consumption is less compared to ERM motors for equal output.
However, LRA actuators have a small operating bandwidth. They work best for the frequency that they are designed for (resonant frequency). Driving them in other frequencies decreases the efficiency and the resulting force output. This limits the range of haptic effects that they can produce. The palette of sensations is still relatively poor.
3.Piezoelectric Haptics
Piezoelectric actuators are a complete change of strategy. Instead of moving masses, they have material deformation. Piezoelectric ceramics are those which expand or contract when a voltage is placed upon them. This is the reverse of the piezoelectric effect.
The advantages in terms of structure are immediately obvious. Piezoelectric elements are very thin materials. They are possible to measure in fractions of a millimeter. There are not any separated moving parts. The whole actuator is a solid state component.
Response time is superior. Piezoelectric materials respond to electrical signals in microseconds. This makes effects impossible with mechanical actuators. Sharp clicks, rapid taps and high frequency textures all open up.
The range of the frequency band is extraordinary. A single piezoelectric actuator can run from 50 Hz up to 1000+ Hz+ upwards. This great broad area allows complex haptic effects. Designers can simulate various textures, materials and feelings. Users can touch the difference between glass, metal and fabric on a flat screen.
The amount of force produced is proportional to voltage. Higher voltages give greater displacements and greater sensations. Modern piezoelectric actuators have become able to ridicule >2G of acceleration. This has the advantage of providing clearly perceptible feedback in even noisy environments.
Performance is not the only benefit of integration. Piezoelectric elements can be glued to the displays or device housings directly. They do not require any space for mass movements. This makes them ideal for devices with ultra-thin properties, such as smartphones, wearables, etc.
The biggest difficulty is drive electronics. This needs relatively high voltages to access smaller current tournament, usually between 100 to 200 volts. Lithium-ion batteries, calcium-ion batteries, lithium-sulfur batteries will generate these voltages need specialised circuitry. However, the power consumption is not high due to low continuous current consumption by capacitive loads.
4.EAP (Electroactive Polymers)
EAP is an emerging technology which having unique potential. These materials are sometimes referred to as artificial muscles. They are electrically deformable on a large scale.
There are two major categories: Ionic EAPs work as a result of ion transport. They require low voltages in exchange for being slower to respond. Electronic EAPs make use of electric field forces. They are fast on their response but require higher voltages.
The key advantages are large displacement. Mode of straining of some EAP material is as high as 100% or more. This allows you to have totally new haptic effects. Devices could alter their shapes or surface texture dynamically.
Current limitations preclude widespread adoption. Most EAPs have a subpar force output. They require for complex drive signals. Durability is also an issue with many formulations. 

Key Performance Metrics
Evaluating haptic actuators requires knowledge of a few important specs.
Response time refers to the speed with which the actuator reacts. This includes even startup latency as well as settling time. Fast response allows crisp and well defined haptic events. Slowness of response produces mushy, sloppy sensations. Response times of less than 10 milliseconds makes real sharp feedback quite possible.
Vibration acceleration is a measure of the output strength. It's usually measured in terms of so called G-forces or meters per second squared. Higher acceleration gives higher sensations. For clear perception, most applications require at least 1G. Premium devices need 2G or higher.
Frequency bandwidth identifies the assortment of vibrations achievable. Narrow bandwidth actuators (like LRA) are good on one frequency and bad on others. Diverse effect possible with wide bandwidth actuators (like piezoelectric). A range of 50 and 500 Hz is most haptic applications.
Power consumption is very important for mobile devices. Haptic feedback is a feedback which may be activated hundreds of times in a day. Inefficient actuators give a significant drain on batteries. Modern actuators should only consume milliwatts when they are running normally.
Size and weight limit the options for design. Smaller actuators will be able to fit in more devices but may not be as strong.

Hardware Integration and Engineering Issues
Implementing proper haptic feedback involves solving a lot of engineering challenges. 
Space allocation becomes increasingly difficult as devices get smaller and smaller. Foldable phones, sleek tablets and smart rings have little internal volume. Actuators have to fit next to batteries, cameras and other components. 
The mechanical isolation prevents the unwanted vibration transmission. Haptic feedback should be on the user's hand not the camera or microphone. Poor isolation in terms of image blur while taking photos and audio artifacts while speaking thus affects the call. Engineers have tools to dampen and isolate to take care of these problems, namely dampening materials, isolation mounts and careful placement of actuators.
Vibration focusing allows the focussing of the feedback to certain locations. Large devices such as tablets can have several actuators. Each of them drives feedback in the local region, This results in spatially different haptic zones. Users are able to feel where on the screen they're touching.
Environmental stability is ensured to have uniform performance. Temperature changes have an impact on the properties of the actuators. Cold conditions may cause springs to become stiffer or piezoelectric response to be less. Hot conditions may soften adhesives or alter resonant frequencies. Robust designs make up for these variations.
Electromagnetic interference is a problem for high voltage actuators. Electromagnetic noise can be radiated by piezo-electric drivers. This may interfere with cellular radios, WiFi or other sensitive electronics. Careful shielding and circuit design ensure that all these problems are addressed.
Mechanical durability determines product life time. Actuators can exceed some billions of cycles in a device's life. Fatigue failures, adhesive degradation and wear should be avoided. Testing protocol speeds years of use up to days or weeks.

Conclusion
Haptic technology is progressing from mere buzzes to high definition touch. Early devices only had rudimentary buzzes. Modern systems are able to simulate textures, materials as well as complex interactions.
The hardware underpinning for this transformation. Traditional electric rotary motors (ERM) offered crass vibration at cheap rates. LRA actuators enhanced the response and efficiency. Piezoelectric systems are now providing unprecedented system precision and bandwidth. Newer technologies such as EAP hold out the promise of even more radical functionality.
The trend towards solid state actuation is evident. Piezoelectric and other similar technologies negate mechanical complexity. They provide higher response speed, broader bandwidth and integration. These advantages become more important as devices become slimmer and interactions become richer.
Haptic systems of the future will blur the line between the digital and physical. Users will experience the touch of texture on glass screens. They will feel fake button clicks that do not exist physically in reality. They will be given feedback that will help guide them without looking at displays.
This future is all down to hardware innovation. Software can only utilise those capabilities that actuators offer. Advances in materials, miniaturization and control electronics give way to new possibilities. The hardware basis for haptic feedback continues to make changes. We see each generation move us closer to true natural intuitive human-computer interaction.