Ever wonder how your “silent” phone ringer can be heard from across the room as it dances on the table top? Or why your iPhone’s screen button doesn’t press in when it’s powered off? didn’t notice that? …try it! Or how you can feel the racecar go off the track on your game controller? Haptics!
What are Haptics?
Haptics are the little devices that add the sense of touch to the sights and sounds of the gadgets we interact with. Simple haptic systems give you a simple buzzing alert. More sophisticated systems are able to generate touch sensations that emulate their physical world counterparts and they’re getting better every day. In fact, some of the latest developments use sound waves to provide a sensation of touch without any physical contact at all.
We’ll cover the most common systems in this article. The ones most widely available today. This will provide a good primer to understanding which direction to head in if you’re thinking about adding haptic feedback to any of your products,
Types of Haptics Actuators
There are three main types of haptics widely in use today:
ERMs – “Eccentric Rotating Mass” Actuators
This is the most basic type of haptic device.
You get your tires rotated and balanced so the car rides smooth (every 3,000-5,000 miles like clockwork, right?). If any wheel is just a bit unbalanced, the tire shop will put tiny weights on your rims to even things out.
ERMs are designed to the opposite of that. You WANT vibration, so the weight is intentionally clumped together on one part of the wheel. That way when it spins, it shakes relentlessly. Where your car has big wheels with precise little offset weights, ERMs are small wheels with a relatively huge offset weight.
This is about as simple as it comes for haptic devices. ERMs are put together in large crude units for things like massage chairs, smaller less-crude units for gaming controllers or restaurant “your table is ready” buzzers, down to enclosed precision units smaller than a watch battery that spin at over 10,000rpm inside smartphones. ERMs are the most common haptic device and their simplicity is a good reason for that.
LRAs – Linear Resonant Actuators
These devices move a mass back-and-forth in a straight line, rather than in rotation. They’re actually quite similar to audio speakers.
In speakers, an electrical current is put through a coil which induces a magnetic force either attracting or opposing a permanent magnet. The magnet is stationary and the coil is attached to a lightweight cone-shaped flexible membrane, which moves air as it is pushed or pulled by the magnetic field induced in the coil.
High frequency signals = rapid movement = high pitch tones.
Lower frequency signals = slower movement = low bass tones.
LRAs use the magnetic force to move a small weight instead of moving a lightweight membrane. They are usually enclosed units with the weight attached to a spring. The magnetic force and the spring work together to move the weight back and forth. This oscillating pattern sets up a resonating vibration pattern. It has a peak strength at a specific frequency tuned to the physical mass-spring assembly. These are typically very small – sub watch battery size.
A third category of haptic devices becoming more popular utilizes the piezoelectric effect. These devices rely on the stretching and compressing of specialized piezo materials when a voltage is applied.
Piezo devices don’t require a specific oscillating frequency or rotational speed to work and they’re very responsive. This allows more freedom in the type of vibration patterns it can create.
To get from a stretching/compressing piezo material to a moving mass, these devices are typically designed as thin cantilevered strips with weights. The thin film itself is made of piezoelectric materials and directly bends when electricity is applied.
Another version uses a stack of piezoelectric material set parallel to the thin strip with perpendicular elements bonded to both. When the length of the piezoelectric stack changes, the length of the thin strip stays the same, so it causes the strip to bend.
How are Haptics Actuators Controlled?
Each of these types of devices have characteristics that make them more suitable to specific applications. ERMs are very easy to drive. Just apply a DC voltage and the little motors spin. Vary the voltage to vary the intensity of the vibration. Simple.
LRAs are a bit more complicated. They need a specific frequency of alternating current to get the mass-spring system vibrating resonantly. This can be done with a tuned sine wave, but is more effectively controlled by a driver IC chip specific for the purpose. The voltage and frequency can be modulated to change the intensity of the vibration.
Piezo elements offer more flexibility in the types of vibration since they’re not bound to a regular oscillating pattern. Motion is directly proportional to voltage, so any voltage/time profile will produce a useable deflection and quite complex sensations can be emulated. These elements require a much higher voltage too – on the order of hundreds of volts (versus 2-5V for LRAs and up to 12V for ERMs). The software and ICs required for piezoelectric haptic actuators are far more sophisticated.
The complexity of these devices is directly proportional to energy efficiency. Although the required voltage is high for piezo actuators, the current is exceedingly low. They are actually the most energy efficient type of haptic actuator. LRAs are next, followed far behind by ERMs as the least energy efficient.
Response time can be critical for human interface feedback applications and the performance likewise follows complexity. Since ERMs are really just little motors with a big weight on the end, it is clear to see how it can take some time to build up speed from a rest position. They work great for sustained vibration alerts, but are poor substitutes for the type of sharp “click” or “tap” sensations under your fingertips that LRAs and piezo devices can produce, the latter being superior at that. ERMs can’t change quickly so if you are looking for a precise response, look elsewhere.
This leads to another notable factor – direction of vibration. LRAs cause vibration exclusively in one direction. Piezo haptics effectively do too, though a purist would note that a flexing beam has a small force component along the beam itself. ERMs, on the other hand cause oscillation in two directions as the eccentric weight oscillates on a plane perpendicular to the motor’s axis.
1-Axis Vibration: LRAs & Piezo
2-Axis Vibration: ERMs
If you’re touching a surface like an LCD screen and want to feel a “click” under your fingertip, you only need that vibration in the one direction facing your fingertip. In other cases, you might want to be certain to notice a safety alert. Perhaps on a handle or with a device shoved in your pocket when you’re in a noisy environment. The multidirectional vibration of an ERM is more suitable there.
What Haptics Actuators Should I Use?
If you need strong vibrations and can afford the size, but don’t need to take particular care for battery life, ERMs are the best choice.
If you need to precisely replicate a sensation as for remote surgical robotics, or need to squeeze every last bit of uptime out of your battery and can afford the higher cost, piezoelectric haptics are the clear winner.
For many IoT and human interface haptic applications, LRAs bridge the gap well. They provide a good balance of responsiveness and energy efficiency with widely available and low-cost driver ICs. They’re more similar in performance to piezoelectric haptics than to ERMs.
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