POWER & ENERGY EFFICIENCY HANDBOOK
Driving modern haptics Piezo haptics now can provide a realistic sensation of a click or the resistance of a pushbutton. Sonja Taylor Brown • TDK
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the movie Ready Player One, teenager Wade Watts regularly enters a virtual reality simulator and virtual gaming environment. For citizens in this future dystopian civilization, the currency of the simulator is more stable than that of the real world, and, as a result, what happens there is more important than what happens on the outside. Wade eventually purchases a haptic suit so he can experience the physical sensation of touch from other users in the simulation. To many engineers, good haptic technology seems as far-off as what they see in the movies. However, automobiles, gaming controllers and cell phones all use haptic technology today. And there are some misconceptions about topics that include the differences between haptic actuators and piezo benders, costs of using piezo haptic technology, customization that allows realistic feedback, and how haptics technology stands up in harsh environments. Many users are familiar with the old-style haptic feedback on cell phone screens. Advantages of today’s haptic devices become evident from a quick review of older haptic methods. Cell phones generally have employed electromechanical haptic devices, either eccentric motors that spin an unbalanced
mass to create vibrations or linear resonant actuators (LRAs) containing a springmounted mass that vibrates back and forth linearly. For many haptic applications like gaming, these electromechanical devices are battery hogs. They are also slow. Start-up time for an eccentric motor, defined as the time to reach 90% of the rated acceleration, is usually about 50 to 100 msec, and stopping takes a similar amount of time. LRAs tend to have slightly quicker start-up times, but applications with repeated haptic events can still have problems with LRA latency. In contrast, today’s haptic devices are based on piezo technology and can provide a realistic sensation of a click or the resistance of a pushbutton. Devices and surfaces can be customized and miniaturized for different applications. Additionally, piezo haptics aren’t just actuators. They can also sense pressure. Thus the devices can be set up to sense varying levels of pressure to, for example, add an element of safety in industrial settings. Here, the humanmachine interface could require workers apply significant pressure to turn a system on or off so accidental “touches” don’t cause problems. On the other hand, piezo haptic actuators can be set up to register the most minute changes in movement for quick actuation of machinery where
Piezoelectric unimorph structure
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DESIGN WORLD — EE NETWORK
10 • 2020
precision is imperative, as with a joystick in a forklift. Though haptics technologies vary, it is not unusual to find them all using the same driver ICs. One example of a modern haptic driver chip is the BOS1901 from Boréas Technologies. This device provides drive voltages up to 190 Vpp working from a 3-V supply, allowing it to drive both piezo haptic actuators as well as the electromechanical haptics that require higher peak-to-peak signals. To accommodate piezo actuators that can double as sensors, the chip incorporates a high-speed SPI communication bus to allow an MCU to monitor data such as actuator voltage for sensing applications. Despite commonalities in their drive electronics, there are differences in piezo technologies employed in haptics applications. Piezo haptic actuator technology is not the same as piezo bender technology. This is perhaps the biggest misconception about haptic actuators. As a quick review,
An example of a modern piezo haptic device is the TDK PowerHap 0904H014V060 actuator which has a unimorph structure in which ceramic piezoelectric elements having electrodes on both sides are bonded to one side of a metal plate. An ac voltage applied to the electrode causes the piezoelectric element to expand and contract, which causes warpage on the bonded metal plate. This unimorph structure enables the whole metal plate to vibrate with high efficiency. eeworldonline.com | designworldonline.com
