How to Manage Your Mac's Sudden Motion Sensor (SMS)

Enable or Disable the SMS Using Terminal

Managing the Sudden Motion Sensor
The sms item will only be listed if your Mac supports the Sudden Motion Sensor. Screen shot courtesy of Coyote Moon, Inc.

Since 2005, portable Macs have included a Sudden Motion Sensor (SMS) to protect their hard drives. The SMS uses motion-detecting hardware in the form of a triaxial accelerometer that can detect movement in three axes or directions.

The Mac uses the SMS to detect sudden motion that may indicate that the Mac has been dropped, knocked about, or is generally in danger of receiving a severe impact. Once this type of motion is detected, the SMS protects the Mac's hard drive by moving the drive's heads from their current active location over the spinning magnetic disk platters to a safe location retracted into the side of the drive mechanism.

This is commonly referred to as parking the heads.

With the drive's heads parked, the hard drive can endure a pretty substantial blow without experiencing damage to the platters or any loss of data.

When the SMS detects that your Mac has returned to a stable condition, that is, it's no longer being knocked about, it reactivates the drive mechanism. You can get back to work, with all of your data intact and no damage to your drive.

The downside to the Sudden Motion Sensor is that it can experience false trigger events. For example, if you're using your Mac in a noisy venue, such as a concert, night club, airport, construction site, or just about anywhere with recurring low-frequency noise that has enough energy to move your Mac about, even if the movement is imperceptible to you, the SMS can detect these motions and shut down your drive by parking the heads.

The only thing you may notice is a bit of stuttering in your Mac's performance, such as a movie or song pausing ever so slightly during playback.

If you're using your Mac to record audio or video, you may see a pause in the recording.

But the effects aren't limited to multimedia apps. If the SMS is activated, it can cause other apps to pause, beach balls to spin, and more than a little aggravation on your part.

That's why it's a good idea to know how to manage your Mac's SMS; how to turn it on, turn it off, or just check whether it's working or not.

Checking SMS Status on Your Mac

Apple doesn't provide an app specifically designed to monitor the Sudden Motion Sensor system, but OS X does include the ever-handy Terminal app, which we've previously used to delve into the internal workings of our Macs.

  1. Launch Terminal, located at /Applications/Utilities/.
  2. When the command line prompt appears, enter the following (you can copy/paste the text rather than type it, if you prefer):

    sudo pmset -g
     
  3. Press the enter or return key on your keyboard.
  4. You'll be asked for your administrator password; enter the password and press enter or return.
  5. Terminal will display the current settings of the Power Management (the "pm" in pmset) system, which includes the SMS settings. There will be quite a few items listed. Locate the sms item and compare the value to the list below to learn its meaning:
  • sms – 0: Sudden Motion Sensor is disabled.
  • sms – 1: Sensor is turned on.
  • No sms entry: Your Mac is not equipped with an SMS system.

Enable the SMS System on Your Mac

If you're using a Mac portable that's equipped with a hard drive, it's a good idea to have the SMS system turned on. A few exceptions are noted above, but in general, if your Mac has a hard drive, you're better off with the system enabled.

  1. Launch Terminal.
  2. At the command prompt, enter the following (you can copy/paste):

    sudo pmset -a sms 1
     
  3. Press enter or return.
  4. If you're asked for your admin password, enter the password and press enter or return.
  5. The command to enable the SMS system doesn't provide any feedback about whether or not it was successful; you'll just see the Terminal prompt reappear. If you want reassurance that the command was accepted, you can use the "Check the SMS Status on Your Mac" method outlined above.

Disable the SMS System on Your Mac

We've already mentioned a few reasons why you might want to disable the Sudden Motion Sensor system on your Mac notebook.

To that list of reasons, we're going to add one more. If your Mac is only equipped with an SSD, there is no advantage to attempting to park the drive's heads, because there are no drive heads in an SSD; in fact, there are no moving parts at all.

The SMS system is mostly a hindrance to Macs that only have an SSD installed. This is because in addition to attempting to park the SSD's nonexistent heads, your Mac will also suspend any writes or reads to the SSD while the SMS system is detecting motion. Since the SSD has no moving parts, there is no reason to shut it down because of a bit of motion, or to incur a bit of stuttering while the SMS waits for your Mac to return to a stable condition.

  1. Launch Terminal.
  2. At the command prompt, enter the following (you can copy/paste):

    sudo pmset -a sms 0
     
  3. Press enter or return.
  4. If you're asked for your admin password, enter the password and press enter or return.
  5. If you would like to ensure that the SMS is off, use the procedure outlined above in "Checking the SMS Status on Your Mac."

By the way, the SMS system is also used by a few apps that make use of the accelerometer. Most of these apps are games that use the SMS to add a "tilt" feature to the gaming experience. But you can also find some interesting scientific uses for the accelerometer, such as the Seismac app that turns your Mac into a seismograph, just the thing if you live in earthquake country or near a volcano.

One last note: If the SMS doesn't seem to be working, your Mac's SMC may need to be reset.

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