Introduction to MEMS Accelerometers

  Introduction to MEMS Accelerometers

MEMS stands for micro electro mechanical system and applies to any sensor manufactured using microelectronic fabrication techniques. These techniques create mechanical sensing structures of microscopic size, typically on silicon. When coupled with microelectronic circuits, MEMS sensors can be used to measure physical parameters such as acceleration. Unlike ICP® sensors, MEMS sensors measure frequencies down to 0 Hz (static or DC acceleration). PCB® manufactures two types of MEMS accelerometers: variable capacitive and piezoresistive. Variable capacitance MEMS (VC MEMS) accelerometers are lower range, high sensitivity devices used for structural monitoring and constant acceleration measurements. Piezoresistive (PR) MEMS accelerometers are higher range, low sensitivity devices used in shock and blast applications.

  Structure of Variable Capacitive Accelerometers

The sensing element in VC MEMS accelerometers is comprised of a micro-machined proof mass that is suspended between two parallel plates. The mass is suspended on flexures that are attached to a ring frame. This configuration forms two air gap capacitors between the proof mass and upper and lower plates. As the proof mass moves when acceleration is applied, one air gap decreases and the other gap increases creating a change in capacitance proportional to acceleration. The upper and lower plates are laminated to the proof mass sensing element with a glass bond. This creates a hermetic enclosure for the proof mass and provides mechanical isolation and protection. A selection of full scale measurement ranges are attained by modifying the stiffness of the suspension system of the proof mass. A high natural frequency is accomplished through the combination of a lightweight proof mass and suspension stiffness. Ruggedness is enhanced through the use of mechanical stops on the two outer wafers to restrict the travel of the proof mass. The sensor elements use squeeze-film gas damping to mitigate high frequency resonant inputs that cause mechanical saturation. This occurs when the travel of the proof mass exceeds its displacement limits. Damping helps prevent saturation by reducing resonant amplification and extends the flat portion of the frequency response. Gas damping is minimally affected by temperature changes. The sensing element is connected as a bridge circuit to the rest of the electronics in the accelerometer. This minimizes common mode errors and improves linearity. All PCB VC MEMS accelerometers contain conditioning circuitry that provides a high sensitivity output. This integrated circuit also compensates for zero bias and sensitivity errors over temperature. See Figure 1 for VC MEMS accelerometer construction. The sensing elements are typically mounted on a circuit board that is placed in titanium or aluminum housings.

Figure 1. MEMS variable capacitive DC accelerometer construction

  VC MEMS Accelerometer Function and Operation

To take advantage of the DC response of VC MEMS accelerometers, the readout device must be in a DC coupled state. If a signal conditioner is used for power it should also be DC coupled. Consult the appropriate manufacturer or product manual for specific details. Since most PCB VC MEMS accelerometers contain a built-in voltage regulator, they may also be powered from any 6 to 28 VDC power source without adversely affecting performance. Consult individual model product manuals for additional powering details. Because VC MEMS accelerometers can measure static (constant) acceleration, the DC offset voltage will be affected by the positional alignment relative to the Earth’s gravity. When the sensitive axis of the accelerometer is not aligned with gravity, the output will equal the zero-g offset voltage on the PCB calibration certificate. If the sensitive axis of the accelerometer is aligned with gravity, the output will be equal to the bias voltage plus 1g of output. Figures 2 and 3 show sensor orientation examples.

Figure 2. Sensor/sensitive axis not aligned with gravity (zero-g output condition)
Figure 3. Sensor/sensitive axis mounted in +1g output condition with respect to Earth’s gravity

The electronics inside VC MEMS sensors contain a voltage regulator. This allows the sensor to be powered by any unregulated DC voltage source. PCB® offers signal conditioner models 482C27 (4-channels) and 483C28 (8-channels) as VC MEMS power sources. Other acceptable power units include automotive or marine batteries, DC voltage laboratory supplies and low-voltage PC board voltage supplies. The cable shield needs to be terminated at one end to ensure ground loops are not induced. Typically, the shield of the cable is tied to the sensor housing. If the sensor is mounted with an isolation pad (or other form of electrical isolation) separating it from the test structure, then the shield can be tied to signal ground at the signal conditioner or data acquisition end. Otherwise the cable shield should be left floating (not connected) at the instrumentation end.

  VC MEMS Accelerometer Verification

Since these accelerometers are intended for use in critical measurement applications, the sensitivity should be verified to be certain that it is within specification. An accurate static calibration can be performed using Earth’s gravity as the acceleration reference. First, place the accelerometer in a +1g orientation so that the base is resting on the mounting surface and the model number is facing up (Figure 3). The accelerometer is experiencing +1g acceleration in this orientation. Record the DC output voltage by using a DVM. Invert the sensor by flipping it 180°. Rest the top on the mounting surface with the model number facing down (Figure 4). The sensor is experiencing -1g acceleration. Record the DC output voltage. To calculate the sensitivity of the accelerometer, use the equation: Sensitivity = [(+1g) – (-1g)] / 2

Figure 4. Sensor/sensitive axis mounted in -1g output condition with respect to Earth’s gravity

  Structure of Piezoresistive (PR) Accelerometers

The sensing elements in PR accelerometers are comprised of flexures on a middle wafer sandwiched between an upper and lower wafer (Figure 5). The bending of these flexures causes a measurable change in resistance that is proportional to the applied acceleration. A selection of full scale measurement ranges are attained by modifying the stiffness of the flexures or the seismic mass. The upper and lower wafers are laminated to a middle wafer using a glass bond. This provides a hermetic enclosure for the flexures as well as mechanical stops for over-range protection. Gas damping lowers resonant amplification when PR accelerometers are used in high shock applications. Damping reduces the response to high frequency energy. Air is used rather than a liquid to reduce thermal effects on damping.

Figure 5. MEMS piezoresistive DC accelerometer construction

The sensing elements are arranged in a fully active Wheatstone bridge configuration. A fully active bridge (Figure 6) uses two resistors that increase with the input acceleration or force, and two that decrease. These are called tension and compression gages, respectively. The difference in voltage of these output lines will be proportional to the excitation voltage applied. The excitation voltage used during application should be the same as was used during the calibration process.

Figure 6. Wheatstone bridge

The sensing elements are typically mounted on circuit boards that are placed inside titanium or aluminum housings. Surface mount packages are also available. Surface mount MEMS sensors are typically soldered or epoxy attached at the next level of assembly. Figures 7, 8 and 9 are examples of packaging styles.

Figure 7. PR MEMS sensors in titanium housings
Figure 8. PR MEMS sensor in aluminum housing
Figure 9. Surface mount PR accelerometer packaged in leadless chip carrier

  MEMS PR Accelerometer Function and Operation

To take advantage of the DC response of MEMS accelerometers, the readout device must be in a DC coupled state. If a signal conditioner is used for power it should also be DC coupled. Consult the appropriate manufacturer or product manual for specific details. PR accelerometers should be powered with a regulated voltage source because sensitivity is proportional to excitation voltage. The recommendation is to use the excitation voltage listed on the calibration certificate to obtain the calibrated sensitivity value. Consult individual product manuals for additional powering details. PCB signal conditioner models 482C27 and 482C28 can be used to power piezoresistive MEMS sensors. Most titanium-packaged PR MEMS accelerometers are supplied with and integral cable attached to the sensing elements. The terminating end is pigtailed and ready for connection to a bridge conditioner (Figure 10). The ground wire and the cable shield should be connected appropriately at the instrumentation end to avoid ground loops. Internal isolators keep the sensing element electrically isolated from the housing and mounting structure.

Figure 10. PR MEMS accelerometer with integral cable and pigtail termination

  MEMS PR Accelerometer Verification

MEMS PR accelerometers are intended for use in critical measurement applications. The health of the sensor is verified by checking the output offset voltage and bridge resistance. To check the output offset voltage connect the +Exc and –Exc leads to an appropriate power supply. Connect the +Sig and –Sig leads to a volt meter that is set to read VDC. Mount the sensor in a +1g orientation so that it is resting securely on a flat, level and stable surface. Measure the differential voltage output of the sensor. Check the calibration certificate for verification of the measured offset voltage. To check the bridge resistance use an ohmmeter or set a digital multimeter to measure ohms. Rest the sensor on a flat and level surface. There is no need to apply an excitation voltage for this test. The input resistance is measured between the +Exc and –Exc wires. The output resistance is measured between the +Sig and –Sig wires. Check the calibration certificate for verification of the measured resistance values.

  Comparison
Variable capacitive MEMS DC Accels for low G applications Piezoresistive MEMS DC Accels for high G applications
Sensing Technology -Micromachined capacitive elements -Titanium or aluminum housings -Micromachined resistive elements -Titanium or aluminum housings -Surface mount packages available
PCB model series 3711 – Structural monitoring, Driveability, Ground vibration testing 3713 – Structural monitoring, Driveability, Ground vibration testing 3741 – Structural monitoring, Driveability, Ground vibration testing
3743 – Structural monitoring, Driveability, Ground vibration testing		 3501 – High G shock/blast 3503 – High G shock/blast 3991 – High G shock/blast

Click for more information on VC MEMS Accelerometers, 3711/3713/3741/3743 series.

Click for more information on  Piezoresistive Shock Accelerometers.