Sonja Brown, TDK
Our five senses keep us alive: we instinctively draw back from things that are too hot or loud that cause pain. Our environment has defined how these reactions developed over time.
Non-language communication that takes advantage of innate perception
Our sense of balance and other innate mechanisms have allowed us to develop new technologies like simulators that fool our brains into thinking we are flying even when we know we are sitting in an auditorium watching a movie.
The science of haptics describes the way things feel, move, or react to applied force and extends to electromechanical switch design. Pressure actuates a push-button switch. The point at which the switch mechanism yields to that pressure can be tuned to provide an aesthetically pleasing action where feedback is genuine.
Haptic actuators were developed to augment or emulate this effect. Once users accept that feedback can be artificial, opportunities to enhance the user experience open. In the previous example, viewers know the auditorium is not moving but are willing to suspend disbelief to experience the sensation of flying. The effect can be particularly impactful when emulating real-world actions using virtual technologies.
Most smart phones now use haptic feedback for emphasis. Forced feedback, a type of haptic feedback, started the same way in the automotive industry and provides information to the driver several ways like vibrating the chair, wheel or pedals to provide spatial information.
Integration of haptic feedback into the control panel is growing, reducing the need for drivers to look away from the road and increasing safety. As a software-defined capability, haptic feedback offers flexibility and potential for increased user personalization and creates the opportunity for extensibility through over-the-air updates.
The OEM view of haptic actuation
Haptic feedback is generally applied as a surface technology, providing tactile feedback to actions like pushing a button. Combined with display technology, it allows the development of innovative and adaptive user interfaces. Buttons and sliders on a screen react like the real thing when pressed or swept with a finger. The sensation is compelling and even though we know it is not real, users find the two actions relatable in a meaningful way.
Studies show consumers react well to haptic feedback. While manufacturers are eager to adopt it, the automotive industry is still trying to identify the best solution. Many current examples combine haptic feedback with software-defined soft buttons. In this case, the feedback relates to the current function of the button, rather than its action. A soft button might control a window and provide haptic feedback by resisting a user’s finger to indicate when it is fully open or closed.
Although innovative, evidence shows haptic feedback can also confuse users. A window switch that provides resistance when it isn’t expected can increase the mental effort needed to interact with it. A major feature of haptic feedback is that it can simplify the control process rather than augmenting conventional controls with physical feedback, reducing the mental stress associated with actions. This could mean an operative is less fatigued at the end of a shift or a driver at the end of a long journey.
Technology needs to be integrated in a way that brings real change to a vehicle’s interior. In practice, this means new concepts around the center console’s design. Integrated touch-sensitive single display could displace conventional controls by providing access to navigation, climate controls and more to anyone sitting in the cockpit area. Drivers would identify menus using haptic touch-feedback, allowing them to concentrate on the road instead of the display. The user experience would be just as immersive for passengers, but they would have the freedom to look more intently at the screen.
Replacing electromechanical controls with a single display is a significant change that will take time to proliferate. Part of the challenge includes creating a feasible supply chain for original equipment manufacturers (OEMs). The foundation for any supply chain includes standards that allow multiple suppliers to provide interoperable solutions. Today, there are no standards for haptic technology.
Several suppliers joined together to form the Haptics Industry Forum whose mission is to help accelerate adoption through hardware, middleware and application layer standardization. This applies to everything from how an actuator performs to guidelines and best practices for designing-in haptic effects. The Forum’s current efforts include integrating haptic encoding into MPEG standards which would allow MPEG files to include signals that could drive haptic actuators if present in the hardware.
The Technology Inside
The haptic effect is created by moving a mass typically generated electronically, either electromagnetically or through a piezoelectric effect. Electromagnetic devices work by spinning an unbalanced mass to create vibration or by shifting a mass in a linear direction. The former is generally referred to as an eccentric rotating mass (ERM), while the latter is referred to as a linear resonant actuator (LRA).
The piezo effect is the main alternative to electromagnetic displacement technologies. The piezo bender, or bimorph, can be considered the forerunner to modern piezo haptic actuators. A bimorph device deflects under the application of an energizing signal. However, the ceramic material used is susceptible to physical and environmental stresses and limited in the amount of force it can deliver.
A piezo haptic actuator also uses the piezoelectric effect to distort material when an electric signal is applied across it. It uses a unimorph structure with electrodes on both sides and bonded to a metal plate, available as either single-plate or multilayer; a multilayer device can provide larger displacement.
The physical displacement dictates the amount of force that can be generated. Unlike an ERM or LRA, a piezo haptic actuator can control the amount of displacement and the frequency of vibration independently, allowing these effects to create more complex waveforms that mimic a variety of effects.
Click image to enlarge
Figure 2. Differences between an ERM, LRA, and Piezo Haptic actuator
Middleware running on a host microcontroller (MCU) or processor generates the waveforms. There is a growing number of commercial solutions available based on libraries of waveforms that represent buttons, sliders, and other devices. When coupled with a driver and the right piezo haptic actuator, these effects can be reproduced to provide the ideal sensation for specific applications.
In terms of start-up time, piezo is the fastest of the three technologies and can move a larger mass in more complex ways. Its small size is a benefit in wearables and its faster response time consumes less power.
The System Solution
Achieving high definition and high performance with haptics requires a system-level approach, comprised of an actuator, driver, and middleware. Depending on the device type, the driver must produce voltages as high as 150 V peak to peak. Some piezo haptic actuators can operate as sensors by detecting the pressure applied to a surface. In this mode, the driver is required to detect and communicate this data back to a host controller. The interface between the driver and the host MCU is often a serial bus like SPI (serial peripheral interface) or I2C (inter-integrated circuit). The MCU would run the middleware that controls the type of effect generated by the actuator.
Specialists like Boréas Technologies developed piezo drivers that include force sensing. Technology domain experts like Boréas and Immersion work closely with TDK and other haptic manufacturers to produce total system solutions optimized for the automotive market
Space requirement is key element to consider when developing a haptic solution. An electromagnetic solution requires more space than a piezo solution but is more limited in the types of effects it can create. Some applications that might only need vibration feedback on a low mass object, and they may not be space-constrained, in which case an ERM or LRA may be ideal. In other applications,the space is more limited, the mass is larger, and/or the desired effects more diverse. In this case, a piezo haptic actuator is a better choice.
Miniature piezo haptic devices like the PowerHap from TDK measure less than 1.5mm thick but can accelerate a mass of 100 grams at over 6 g. The displacement is just 18 µm but the actuator only requires 0.35 mj of energy. Larger actuators are also available that can accelerate a mass of 1200 grams at 12g. The devices are scalable and programmable to optimize acceleration and displacement based on mass and size of end products, making it suitable for a variety of applications; a wearable device may be optimized at 3-5 g of acceleration, an automotive application would require around 6-8 g of feedback acceleration, while an industrial application might need as much as 10-12 g.
Because piezo haptic actuators can also sense pressure, they can be used in place of conventional buttons on surfaces that can deflect when pressed. The feedback provided reassures the user that their input has been detected. Actuators of this kind can detect up to 20 N of pressure. By combining pressure detection and haptic feedback, users get the impression they are interacting with a mechanical button.
When using a piezo haptic actuator, feedback is not restricted to the click of a button. The device structure is more controllable than ERM or LRA devices, and the shape of the signal can dictate the type of effect. For example, the actuator can generate a click and give the impression of sliding a potentiometer.
TDK worked with Boréas Technologies and Immersion to create a platform that demonstrates the next generation of automotive console design. It uses a single center display to provide all the features normally found in a vehicle. Using haptic feedback, the driver or front passenger can easily navigate between menus and adjust the settings of the infotainment system, navigation, and climate control.
TDK also developed two evaluation kits based on its PowerHap piezo actuators that integrate technology from Boréas Technologies.
Click image to enlarge
Figure 3. PowerHap evaluation kits for haptic feedback
The Future of Haptics
Piezo haptic actuators produce a range of feedback sensations that automotive manufacturers are keen to exploit. As well as being small and robust, the system-level benefits of piezo haptic actuators include a response time of less than 1 ms and energy consumption of less than 1 mj per click.
With a growing portfolio of piezo haptic actuators, TDK’s compact and power-efficient technology provides industry-leading acceleration, force, and response time. As the technology evolves, key parameters will further improve, including making the devices even smaller.
User expectations will soon be like the way we expect a display to be touch-sensitive. Users will anticipate force feedback from displays and other surfaces designed for interaction. With piezo haptic actuators, these expectations can be met.