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Paul Goble

10/15/2012 12:16 PM EDT

It looks like everybody else calls these linear resonant actuators, not ...

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WKetel

10/13/2012 9:05 PM EDT

It did take a bit to understand what the actual subject was, it is quite ...

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# The ups and downs of linear resonant acceleration

## 10/10/2012 2:57 PM EDT

Who doesn't like accomplishing a goal? Sometimes it is hard work and you may deviate from the path. But in the end, when you hit your target and complete your task, you can look back at what you've done and take enjoyment from the hard work – but also look forward towards progressing to the next level. Goals can be in all shapes and sizes, as big as moving a mountain or as small as maintaining a specific oscillating frequency. Either way, the payoff should be worth the effort.

Most people familiar with linear resonant accelerators have the goal of driving it accurately, but due to variables outside their control, that goal isn't always accomplished. As with every goal, obstacles appear that are known and unknown. In this article we explain what's going on behind the curtain as we discover the ups and downs of linear resonant acceleration (and why that pun is funny.)

Basically, linear resonant accelerators are a spring-mounted mass that move up and down, particularly at a specific frequency (hence the earlier pun). Oscillating the mass creates vibration in a linear motion rather than rotational omnidirectional vibration, which is seen in an eccentric rotating mass (basically a dc motor) actuator.

The principles of operation behind a linear resonant accelerator (LRA) include leveraging its resonant frequency to increase vibration strength and minimize power consumption. Most LRAs come with a specification for their resonant frequency to ensure that drive is met.

However, since the setup includes a spring, it is subject to Hooke’s Law of elasticity. Thus the frequency changes based on multiple external variables including but not limited to life of use, spring temperature, loading the spring (for example, if a device is held in ones hand versus on a table or left in a pocket), process variations during manufacturing, and so on. All these things have influence over a spring’s resonant frequency.

Right now you are probably saying, “Okay, so I am off by a little bit. What's a few Hz between friends?” My answer to you is, “A small delta ‘Hertz’” (I promise it’s the last pun). Normally it may not be an issue, but being as little as 2.5 Hz off can cause performance degradation with your actuators by as much as a 40 percent reduction in terms of vibration strength, and a 50 percent increase in terms of power consumption. The correlation between system performance and spring all stems from the basics of resonance operation, and specifically the high-quality factor, or “Q factor.”

This means that as energy is added into the system it slowly dissipates out; this energy compounds in such a way that it creates a greater output by leveraging previous inputs. As a result, LRAs are a great device for creating high-powered vibration effects with very low-current consumption. To leverage this savings you just need a way to find that resonant frequency, otherwise you are shooting in the dark. The good news is that auto resonance tracking can help.

Click next page to find out how >>>

D.Billington

10/12/2012 6:34 PM EDT

Good intro article. I have tried your DRV2603 LRA driver and it works very well. I would be interested in a follow up article that goes into detail about how auto-tune resonant frequency detection works.

WKetel

10/13/2012 9:05 PM EDT

It did take a bit to understand what the actual subject was, it is quite interesting. I can visualize at least one way to track the actual resonant frequency, I am sure that there are a few more. The simplest way is to use the device in the feedback loop of it's driving oscillator, like the tuning fork oscillators used in the past. That would be the cheap and easy approach and it would not need a microcontroller or any software to make it work. So probably nobody would want to do it that way.

Paul Goble

10/15/2012 12:16 PM EDT

It looks like everybody else calls these linear resonant actuators, not accelerators. Once I figured out the terminology difference, it was much easier to find more information about their applications.