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Techniques for implementing haptics

Posted: 01 Aug 2011     Print Version  Bookmark and Share

Keywords:Eccentric rotating mass  Linear Resonant Actuator  Piezo actuators 

If you have already used a cellular phone, there's a huge 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 neighbours. 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 utilises 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
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-centre 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.

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Figure 1: The construction of the eccentric rotating mass (ERM) haptic actuator .

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
The next leap in Haptic technology is the Linear Resonant Actuator (LRA), 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.)

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Figure 2: The linear resonant actuator (LRA) haptic actuator .

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