Accelerometers

Question 1

Accelerometers

Answer 1

(devices that measure the acceleration of a strucure).
All current accelerometers consist of a proof or seismic mass supported by a suspension to a frame. Under the influence of an acceleration, the proof mass generates a force that results in a displacement or strain in the suspension element.

Question 2

Bulk vs surface micromachining

Answer 2

We can classificy accelerometers based on the fabrication technology.
-BULK MiCROMACHINING: uses the full thickness of a wafer and is a subctractive process. Silicon is removed by wet or dry etching techniques and forms a proof mass and a suspension system.
Often these sensors consist of a sandwich of several wafers bonded together to provide electrical contacts and form an enclosure for the proof mass.
This technology is not very suitable for monolithic integration and is often used in conjunction with a separate integrated circuit in the same package or even with discrete electronics.
-SURFACE MICROMACHINING: suitable for integrating the electronics and the mechanical structures on the same chip.
It is an additive process in which thin films of typically poly-silicon-oxide are grown on a wafer. The oxide acts as a sacrificial layer and is removed by a release step by a wet etchant such as HF. This results in free-standing beams and plates.
The sensing elements are tipically an order of magnitude smaller than bulk micromachined devices. This technology is compatible with a standard CMOS process and has led to monolithically integrated devices.

Question 3

Piezoresistive accelerometers

Answer 3

These sensors contain a seismic mass suspended by 1,2 or 4 silicon beams. The beams carry
piezoresistors which, in order to achieve maximum output signal, are placed at the edges of the beams where the maximum bending occurs.
An example would be the HITACHI piezoresistive-type triaxial acceleration sensor, which has the ability to simultaneously detect acceleration in three axial directions and with a single chip.
The drawback of these piezoresistive accelerometers is that the output is not very high (100mV for a 10V drive) , the temperature coefficient is relatively large and intrinsically thermal noise is generated in the piezoresistors. To solve this Capacitive detection is employed.

Question 4

Capacitive accelerometers

Answer 4

Differential capacitors are formed by using the suspended mass as the common contact of a capacitive half bridge. For small deflections the differential change in capacitance is proportional to the deflection of the suspended mass and can be converted into a voltage by a charge amplifier and a synchronous demodulation technique.
Pros:
- Avoids temperature dependence
Cons
- Typically more complex fabrication process and interface circuitry

Question 5

Sensitivity and dynamic range trade-off

Answer 5

-Sensitivity is limited by the stiffness of the mechanical suspension
-Dynamic range limited by nonlinearity of the suspension and of the transduction (a stiffer suspension improves the dynamic range by limiting the displacement)
A TRADE-OFF IS NECESSARY

Question 6

Closed loop accelerometers

Answer 6

In these devices the strain or displacement is sensed, amplified and fed back to counteract the displacement of the mass due to acceleration. This means that during operation the structure is virtually stationary and the linearity and effective spring constants are determined by the electronic control.
——> Suspension can be made very compliant without sacrificing dynamic linear range.

• Accelerometers designed with a force-feedback actuator that can provide a restoring force to the proof mass can operate as a closed-loop sensor with a proof mass that moves negligibly in response to inertial forces. The electrical signal applied to the actuator is the sensor output. For capacitive accelerometers the same structure can often be used in both open and closed loop modes, with the latter requiring additional signal processing circuitry to close the loop (the resulting cost increase can be reduced by integrating the electronics on the same chip with the sensing element).
• The piezoresistive sensing technique is rarely used in force balance accelerometers due to its large temperature sensitivity and complex design and assembling.
• Mechanical properties are dominated by the electronic feedback control.
• Performance is strongly influenced by the electronic system characteristics, which are more controllable and reliable than mechanical properties.

Question 7

Resonating accelerometers

Answer 7

We can also employ a resonant microstructure as a strain sensing element to achieve high sensitivity and large linear dynamic range.
The frequency of a resonant structure can be made highly sensitive to compressive or tensile strain and the resonance frequency can be measured with extreme precision.
Pros:
-Continuous self testing
-High Accuracy
-No need for overload protection

Question 8

Transduction techniques

Answer 8

Another characteristics is the type of transduction from the mechanical to the electrical domain by means of measuring the deflectio of the proof mass;

• piezoeletric
• piezoresistive
• capacitive
• resonating
• tunneling