Accelerometers: Taking the Guesswork out of Accelerometer Selection

Posted by Steve Hanly on March 11, 2016

selecting-the-right-accelerometer

You know you need an accelerometer; but how do you select the right type for your application?  You’re not alone if you don’t know much about the different types of accelerometers and how they can impact your results.  Most engineers I talk to are surprised to learn just how important accelerometer selection is; and some didn’t even know that there were different types to begin with!

There are three main sensing technologies or types (capacitive MEMS, piezoresistive, and piezoelectric).  But before you can even start considering what accelerometer type works best, it's important to first identify exactly what you are looking to measure.

So let’s first briefly discuss different measurement applications for accelerometers and then break down the different types of accelerometers so that you can select the right accelerometer for your application.  I’ll summarize with a table at the end of the post and some helpful links to keep this as short and effective as possible.  I know the feeling of needing all this information yesterday and wanting to be testing by tomorrow; so let’s dive in and get you back in the field doing the fun stuff, testing!


Accelerometer Measurement Applications

Accelerometers measure acceleration levels (as the name implies!); but there are a number of different applications that use accelerometers.  For simplicity I've summarized all the different applications into three main types: 

Motion

Motion is defined as "slow" changes in position or velocity.  Some examples include human motion, orientation tracking, waves, and sustained accelerations like rocket takeoffs.

Vibration

Vibration is defined as oscillating motion about a position of equilibrium.  Some examples include an electric motor, turbine or bearing monitoring, health monitoring, and resonance detection.

Shock

Shock is defined as a sudden change in acceleration that generally excites a structure's resonance.  A few examples include drop testing, automotive crash testing, and dampeners/shock absorbers testing.


The Three Accelerometer Types - Capacitive MEMS, Piezoresistive, and Piezoelectric

Before I discuss the different accelerometer types it's important to define DC and AC response.

A DC-response accelerometer means that it can measure down to zero hertz which is required to measure the gravity vector and other sustained accelerations.  It's also required for shock applications where you want to integrate the acceleration data for velocity or displacement.  Accelerometers that do not have a DC response will have an intrinsic decay function that will result in significant error during numeric integration, especially over long duration events.

An AC-response accelerometer means that they are AC coupled and therefore can't measure static accelerations like gravity and sustained accelerations.  They generally also can't measure slow vibrations (below a few hertz); but there are high sensitivity accelerometers that go down to 0.1 Hz.  AC-response accelerometers are the preferred option for all vibration testing due to their wide frequency response and high signal-to-noise ratio.

If the terminology gets a bit confusing (I don't blame you if it does!) check out my blog on accelerometer specifications for more information on frequency response and other parameters used to quantify an accelerometer's performance. Now that we've gotten that out of the way, let's explain the three main types!

1. Capacitive MEMS Accelerometer

Micro-Electro-Mechanical Systems (MEMS) is a fabrication technology that can be used to manufacture accelerometers.  When people refer to MEMS accelerometers they are likely referring to capacitive accelerometers although this technology can be used for piezoresistive accelerometers as well.  Capacitive MEMS accelerometers operate based upon capacitance changes in a seismic mass under acceleration. 

MEMS fabrication technology has brought lower manufacturing costs and smaller sizes (as the name implies!) to accelerometers.  This has made them much more prevalent in our everyday life (iPhones, Wii etc.).  Because of their low cost and small size, capacitive MEMS accelerometers often come as surface-mount devices (SMDs) to be directly mounted to printed circuit boards (PCBs).  Almost all mobile devices have a MEMS accelerometer in it and many other electronic devices have an accelerometer for motion tracking and even disk drive protection (for detecting drops).  

Capacitive MEMS accelerometers are DC coupled and therefore best suited for measuring low-frequency vibration, motion, and steady-state acceleration; but they suffer from a poor signal to noise ratio, a limited bandwidth, and mostly restricted to smaller acceleration levels (less than 200g).  Capacitive MEMS accelerometers are very low cost and easy to integrate into your electrical system though so they have become quite popular.

2. Piezoresistive Accelerometer

Piezoresistive is the other commonly used sensing technology for DC-response accelerometers. A piezoresistive accelerometer produces resistance changes in strain gauges that are part of the accelerometer’s seismic system.  Piezoresistive accelerometers have a very wide bandwidth which allows these to be used for measuring short duration (high frequency) shock events such as crash testing.  Piezoresistive accelerometers can be gas or fluid damped which protects the accelerometer; but also further widens the dynamic range by preventing the accelerometer from reaching its internal resonant frequency.

Piezoresistive accelerometers measure down to zero hertz so they can also be used to accurately calculate velocity or displacement information.  Piezoresistive accelerometers typically have a very low sensitivity which makes them less useful for accurate vibration testing.  Piezoresistive accelerometers are also sensitive to temperature variation so a temperature compensation will be required but many now include this compensation internally.  Piezoresistive accelerometers are much more expensive than the capacitive MEMS accelerometers so they’re generally not used for lower frequency and amplitude testing.  Piezoresistive accelerometers are by far the best type for impulse/impact measurements where the frequency range and amplitude are typically high; examples include automotive crash testing, and weapons testing.

3. Piezoelectric Accelerometer

Piezoelectric accelerometers typically use lead zirconate titanate (PZT) sensing elements which under acceleration produce a proportional electric charge or output.  Piezoelectric accelerometers are the most widely used accelerometer for test and measurement applications; and are the first choice for most vibration measurements due to their wide frequency response, good sensitivity, and easy installation.  They are also incredibly popular so they are available in a great number of different sensitivities, weights, sizes and shapes.  Piezoelectric accelerometers have very low noise levels and should be considered for both shock and vibration testing of all types.  The only exclusion is for applications where velocity and displacement data are needed because they are AC coupled.  They can also not measure static accelerations and generally can't measure vibrations below a few hertz.  Piezoelectric accelerometers require a charge amplifier which can become saturated when the accelerometer experiences acceleration levels outside its measurement range; so it’s important to select an accelerometer with a measurement range slightly larger than the acceleration levels you expect.

Charge mode piezoelectric accelerometers can be considered the most durable sensor type due to its ability to tolerate hostile environmental conditions, including extreme temperatures (-200°C to +400°C), and some can even operate in nuclear environments.  Due to a charge mode piezoelectric accelerometer’s high impedance special cabling is needed to shield from noise.  You will also need a charge amplifier so your system will typically be a little more complicated and expensive with these accelerometers.

Voltage mode Internal Electronic Piezoelectric (IEPE) accelerometers have become the most commonly used accelerometer type.  IEPE accelerometers are basically charge mode piezoelectric accelerometers with a charge amplifier built-into the accelerometer.  Because of this, IEPE accelerometers require no special cabling and are very easy to integrate with your system.  They will require a constant DC power source but many data acquisition systems now include this.  The included microelectronic circuit in an IEPE accelerometer limits their ability to tolerate hostile environments when compared to charge mode accelerometers. IEPE accelerometers still will often have a temperature range of at least -40° to +125°C which is plenty good enough for most applications.

We delved deeper into piezoelectric accelerometers in a separate blog, including an overview on their construction, and benefits and drawbacks.


Different Applications Require Different Accelerometers

The following table summarizes which accelerometer types work for a number of general test applications.  I can't stress enough that if you are planning to perform shock testing and want to integrate your acceleration data for velocity or displacement, you must use either a capacitive MEMS or piezoresistive accelerometer.  Piezoelectric accelerometers take the cake though for vibration testing; but special high sensitivity accelerometers are needed for lower frequency applications. 

 

Application Piezoelectric Capacitive MEMS Piezoresistive 
Static Acceleration
(0 Hz, 1 g)
Gravity, Sensor Orientation
  orange-tick-50.png orange-tick-50.png
G- Force
(0 Hz, <25 g)
Rocket, Centrifugal, Aircraft
  orange-tick-50.png orange-tick-50.png
Seismic
(<1 Hz, <1 g)
Earthquake, Waves, Bridges
orange-tick-50.png    
Low Frequency Vibration
(<5 Hz, <25 g)
Human Motion, Robotics
orange-tick-50.png orange-tick-50.png orange-tick-50.png
General Vibration
(5 Hz to 500 Hz, <25 g)
Electric Motor, Car Suspension
orange-tick-50.png orange-tick-50.png  
High Frequency Vibration
(>500 Hz, <25 g)
Gear Noise Analysis, Turbine Monitoring
orange-tick-50.png    
General Shock
(<100 Hz, <200 g)
General Testing, Shock Absorber Testing
orange-tick-50.png orange-tick-50.png orange-tick-50.png
High Impact Shock
(<250 Hz, >200 g)
Drop Testing
orange-tick-50.png   orange-tick-50.png
Extreme Shock
(>1,000 Hz, >2,000 g)
Vehicle Crash Testing, Metal on Metal
orange-tick-50.png   orange-tick-50.png

Where to buy

Wired Accelerometers

I’ve provided a few links to companies that sell “traditional” accelerometers below to help you get started.  These accelerometers are of very high quality but they can admittedly be difficult to buy (you’ll need to go through a sales person) and have long lead times, typically over 6 weeks.

Surface Mounted Device (SMD) Accelerometers

The capacitive MEMS accelerometers that come as SMD components are much easier to purchase (you can go through any electronics component distributor such as DigiKey or Mouser).  They will require some electronic design on your part though, and are not as high quality as the accelerometers from the previously listed companies.  Here are a few companies and parts though.

Accelerometer Systems

If time and simplicity is important for your application, check out our blog that rates the top 11 vibration data loggers.  This lists a few different options out there; including, Midé’s vibration data loggers (full disclosure - I work at Midé) that incorporate all of these accelerometer types into a simple, easy-to-use accelerometer logger to meet a host of different motion, vibration, and shock measurement applications.


Additional Resources

If you'd like to learn a little more about various aspects in shock and vibration testing and analysis, download our free Shock & Vibration Testing Overview eBook.  In there are some examples, background, and a ton of links to where you can learn more.  And as always, don't hesitate to reach out to us if you have any questions!

 

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Steve Hanly
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Steve Hanly on March 11, 2016
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Topics: Data Acquisition and Analysis


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