(Editor's note: Touch-sense interfaces are increasingly popular and common in consumer products, appliances, and even test instruments, as well as medical products. To see a list of all articles we have published on this topic, click here.)
If you’ve ever touched a cellular phone, there’s a good chance you’ve been exposed to haptics. Before you start reaching for the hand sanitizer and calling your doctor (it’d be a bit late for anything at this point anyway, don’t you think?), know that the only infectious thing about haptics is its amazing ability to take gameplay, touch screen devices, and portable electronic user experiences to an entirely different level.
Why in the world would someone call such a cool, enabling technology like haptics such a weird word? It’s all Greek to me…literally. Haptics comes from the Greek word “ἅπτω” which means "I fasten onto, I touch.” Basically, a Haptics-enabled system is any system that incorporates feedback via vibrations through the sense of touch. After the Greeks invented the word, not much happened with it until modern times, where Haptic technology has manifested itself in a multitude of industries.
First applications were seen in aviation to allow pilots to “feel” simulated vibrations in the stick when stalling out was imminent. In older aircraft, this vibration occurred naturally but due to improvements in control systems it had to be detected and the feedback was forced into the system.
Over the years, Haptic systems have spread to simulation and electronic environments. Devices that allow a user to sense and feel objects in a remote (or virtual) environment have been used in excavation, building design, education, and even remote medicine.
On a more personal level, Haptics is the reason you can (or at least should be able to) enjoy silence at the movies and still get reminded of that meeting you “forgot” about, or get that text message saying you won the lottery (why you’d be text messaged that I don’t know), and not alert your neighbors. In the gaming world, Haptics lets you know when your car is starting to veer off the road or when you are taking damage in a Halo grudge match, due to the embedded actuator in your controller and the programming in the game that utilizes it.
But enough about how meaningful it is to you, let’s talk about how it works. In essence, there are really two types of Haptic actuator technologies in the market today. One is old school and one is new school, but both are essentially motor-based. Each topology has its own pros and cons and unique offerings. Let’s take a closer look at each one.
Eccentric rotating mass (ERM) – old school
The Eccentric Rotating Mass is the oldest and most mature Haptic technology in the market. When you think of any vibration-enabled device from your, the vibrations were most likely caused by an ERM. As pictured Figure 1, ERMs are comprised of an off-center rotating mass which, as it spins, creates an omni-directional vibration which propagates throughout the entire device, like the vibration alerts you get when your cell phone is on silent or vibrate mode.
Figure 1: The construction of the eccentric rotating mass (ERM)
Unfortunately, due to the construction of the ERM, the ability to create sophisticated wave profiles is limited. The frequency and amplitude of each wave is coupled together to the input-control voltage, leaving you only one variable to play with to create different effects. Generally, you’re only able to create different combination of pulsing or speed, not too far removed from the dots and dashes of Morse Code.
Along those same lines, getting the motor up and running and subsequently stopping it creates a bottleneck compared to newer technologies, making the ERM one of the slower options when it comes to speed and response time. However, one good thing about the technology is that since it has been around for so long, it’s one of the more cost-efficient options available.
Linear resonant actuator (LRA] – new school
The next leap in Haptic technology is the Linear Resonant Actuator, which has become very popular with a lot of new handset companies. The LRA is a magnet attached to a spring, surrounded by a coil and housed in a casing, Figure 2.
The magnet is manipulated and moves in a linear fashion and eventually is brought up to the resonant frequency. This operation at the resonant frequency allows the driver to operate at a lower power-consumption rate, about 30% better than the ERM; however, you are locked in on that frequency.
Efficiency and performance drop off considerably as the LRA’s drive frequency moves outside of that resonant band. This can be a design concern, because the spring constant can change due to wear and tear, temperature fluctuations, or other environmental factors such as if the LRA’s device is being gripped or not (though if it’s not being gripped, you probably won’t care about a lack in performance.)
Figure 2: The linear resonant actuator (LRA) haptic actuator
Although you may be locked in terms of frequency, you can modulate the amplitude of the input signal that’s being sent out, to add an extra degree of freedom and unique waveform profiles that you can’t achieve with an ERM. With respect to response time, LRAs have a leg up on ERMs, as they can be used to keep up with entering multiple letters per second for button confirmation, making them well suited for texting or any sort of typing application on a handset.
We’ve covered both old and new school in terms of haptics actuators, but there still remains one more actuator that I haven’t covered. This type of actuator is not motor based, has incredible response time, is energy-efficient, and is much smaller than both the ERM and LRA. These fabulous new devices are known as piezo actuators.
Piezos aren’t exactly a cutting-edge technology; they’ve been around for decades and consist of a film that acts as a transducer between vibration and voltage. Previously, they have been used in energy-harvesting applications and for driving speakers, but have added a new line in their resume by giving you the most sophisticated haptics experience available.
The standard piezo actuator technology involves either a thin strip or a round disk that goes from flat to bent and back, creating vibrations by applying a voltage across the ends (Figure 3). One setup with the strips includes fixing the piezo strip ends to the touch screen itself and then attaching the center of the strip to the case of the device. The touch screen then is housed in a case where it can “float,” allowing piezo vibrations to be felt predominantly on the screen.
This experience is known as “localized haptics.” You will still feel some vibrations in the device itself but the majority is felt on the screen only. If the floating screen is forgone, another topology exists as a drop-in module. This will give similar but reduced functionality: the level of sophistication is not quite as high as localized haptics, but it does reduce design complexity by a significant amount.
Figure 3: Piezo haptics actuators usually involve
either a thin strip (a) or flat disk (b) that creates vibrations
when a voltage is applied
Piezo-based haptics are not bound by any frequency or amplitude constraints, allowing the designer to create wave profiles not attainable by LRAs and ERMs. For example, though you could not replicate the exact tactile feedback felt from pushing a mechanical button, with piezo-based haptics you can come extremely close.
With multiple piezo modules embedded in a design, one can create a high definition haptics experience, allowing individual sections of the touch screen to vibrate. In the case of a capacitive touch driven application, each touch point (finger) could feel its own unique wave response rather than the entire screen shaking.
One drawback to piezo-based actuators is that most systems require around 100-200 Volts peak-to-peak (Vp-p) to be driven through the device. Multiple-layer piezo actuators can reduce that system voltage down to 50 Vp-p, but these multiple-layer piezo actuators can get costly.
From a speed and response-time perspective, look at Figure 4. ERMs and LRAs run in the 30-60 millisecond range, whereas piezo actuator response times run typically less than 2 milliseconds. This attribute makes them extremely power efficient compared to ERMs and LRAs. With piezos, while you can get up to speed, run your waveform and go back to stasis faster, you’ll also be using less energy.
Figure 4: Piezo haptics technology possesses significantly faster
start-up times than either ERM and LRA technology.
As cool as these actuators are, they’re only one component in the whole mix. There are many other products contributing to the actuator’s “greatness.” One component directly behind the actuator is the physical driver. There are many on the market, but only a few are specifically designed for piezo actuator driving.
For example, TI’s DRV8662 is a 200-Vp-p piezo haptics driver with an integrated boost converter. With a fast start-up time of 1.5 ms, this piezo driver is versatile and ready for whatever high-end piezo haptics system you are designing. The input voltage can be single ended or differential, and is useable with a 3.0-5.5V power supply.
All this value in a small package means you can use less board space and reduce your overall system cost, thanks to the lack of a transformer due to the integrated power switch and diode. Piezo haptics is a game-changer for today’s current haptics implementations, and can help ensure your customers are getting the fullest, richest user experience possible.
For more information on TI’s haptics solutions, visit the following links:
About the author
Eric Siegel is a product marketing engineer for touch screen controllers and haptics drivers at Texas Instruments. He has an M.S. in Electrical Engineering from the University of Florida, and has also worked as a test engineer for digital signal processors and in marketing for microcontroller solutions. In his spare time, Eric is a huge movie buff and also practices mixed martial arts.
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