GB2625258A - Improvements in accelerometers - Google Patents

Abstract

An accelerometer comprising a piezo electric component – piezo stack 22 - and an accelerometer component 10, which may be a MEMS accelerometer. The two components are mounted one on top of the other, and the piezo stack 22 is configured to receive a sinusoidal voltage supply 24 so that in use the combined components undergo corresponding sinusoidal acceleration and deceleration, so that when a system using the combined components to measure acceleration experiences such an acceleration, it can be deduced by subtracting the measured output signal from the known sinusoidal acceleration due to the piezo stack.

Description

Improvements in Accelerometers This invention relates to accelerometers, including MEMS accelerometers. MEMS (Micro Electro-Mechanical Systems) accelerometers are well known in electronics-small blocks of silicon, etched into a mount, are displaced by inertia when the mount is subject to a force. The block may have a series of plates extending from it which move towards and away from a second set plates in the mounting block.

Widening or closing the gaps between the two sets of plates cause a change in capacitance between them which will vary proportional to the displacement, so measuring the change in capacitance allows the acceleration in that specific direction to be measured.

MEMS accelerometers may not have sufficient dynamic range or linearity for certain applications, and often have problems with noise.

An alternative approach is to use a Servo Pendulous Accelerometer-Such devices are usually larger more complex electromagnetic devices which use a restoring force to re-centre the pendulum.

Although such a set up might increase the dynamic range, and give better performance, such devices are usually larger more complex electromagnetic devices which use a restoring force to re-centre the pendulum and are thus prohibitively expensive-in certain applications, such as space launch where the sunk costs are already high, the additional investment may be absorbed, but it would be useful to drive costs down.

It is an object of the present invention to provide a lower cost accelerometer capable of measuring accelerations into the hundreds of g, with a long linear range. Accordingly, the present invention provides for accelerometer comprising an accelerometer component and a piezo stack, wherein the accelerometer component and the piezo stack are in fixed positions relative to one another, said piezo stack being configured to be connected to a sinusoidal voltage source, and the accelerometer component being configured to provide an output signal.

The accelerometer in this case should be lightweight so as not to inertially perturb the displacement of the piezo stack. For this reason, it is preferable to use a MEMS accelerometer.

Normally, the accelerometer would be mounted on the Piezo stack so that they must move together. This would be best generally to minimise the volume of the Combined Components, but it may be beneficial for the two components to be separated, and such separation will not compromise the way in which the invention functions if the components are fixed relative to one another so there is no lag or loss of movement between the two components, should it be necessary to have a gap between them.

Piezo stacks are well known in the art. They extend in a linear fashion when subject to a voltage. With a sinusoidal voltage they will expand and contract at a predictable rate, and so the acceleration of the accelerometer, sitting on top of the stack, is known. If the article holding the combined components [ComCo] is at rest, the MEMS output will be the sinusoidal output of the Piezo stack. However, if the ComCo is subject to a force, this will be revealed as an additive component as measured by the output, which will show as a displacement of the output signal.

Measurements are taken at the point where the acceleration measured is zero. By knowing the applied voltage and therefore the acceleration due to the acceleration of the piezo stack, the overall acceleration can be deduced. For accelerations where a high dynamic range is required, this ComCo system will provide a much lower cost means of providing higher fidelity readings, which are less susceptible to off-axis distortions.

The invention enables higher linearity within the range of interest due to the fact that measurements are only made when the proof mass is at its central position.

Measurements are most linear with the mass is this position for two main reasons.

Firstly the spring upon which the mass is suspended is not deflected and therefore does not influence the position of the mass and therefore the measurement. Secondly the lack of extension of the spring renders the proof mass less susceptible to accelerations that are not in the axis of interest (ie. Measurements are made in a state of minimal cross axis sensitivity).

This higher linearity means that there is no need to introduce a correction factor into the output which further makes the use of the accelerometer simpler, and thus cheaper.

Possible examples of deployment are in aerospace, space, automotive use, sports science and in industrial processes with moving parts from centrifuges, conveyors, etc. For example, it may be used in space launch, in motorsport or as a diagnostic for a human athlete or a racehorse to record running performance.

The invention will now be described with reference to the following drawings Figure 1 shows a conventional MEMS Accelerometer Figure 2 shows an accelerometer according to the present invention Figure 3 shows various output signals at different accelerations Figure 4 shows outputs when filtering the signal using a low pass filter Figure 5 shows the use of a comb filter to process the signal In figure 1 we see a conventional MEMS accelerometer (10). Plates (12) extending from a proof mass (18) will move towards or away (16) from a fixed plate (14) as the mass (18) is displaced in the correct direction. As the plates move, the capacitance between them will change proportionally, and thus the acceleration is measured.

In figure 2 there is a piezo stack (22) with an accelerometer (10) on top of it. The 5 chosen accelerometer here is a MEMS accelerometer as they are lightweight and simple to integrate, but other sorts of accelerometer could readily be substituted.

The Piezo stack is subjected to a sinusoidal piezo stack excitation signal (24) and expands and contracts at a known and measurable rate (26). The piezo stack must 113 have a highly linear displacement/voltage relationship (like a mono-crystal) but must also generate enough displacement (like a ceramic) to keep the frequency of oscillation within the bandwidth of suitably sensitive MEMS accelerometers. In the present invention, This is achieved by using a ceramic piezo stack and a closed loop piezo driver circuit. Other sorts of stacks could be used. For example, a mono-crystal stack could be employed if a lower range was required.

Where there is no external acceleration of the system, the acceleration measured by the MEMS accelerometer will be zero plus the acceleration of the Piezo stack due to the excitation voltage.

This is shown in figure 3. For various accelerations of the system, the acceleration of the MEMS accelerometer is equal to the sum of the acceleration of the Piezo stack and the acceleration of the system. Because the input voltage can be known exactly for when the MEMES output is measured to be Og, the real acceleration of the system is thereby deduced. The measurement of acceleration of the system is equal and opposite to the acceleration expected from piezo-stack at the applied voltage.

Because at each cycle the system is rebalanced, this ComCo gives the benefits of a servo-pendulous accelerometer for the lower cost of a MEMS based system.

The outputs then need to be filtered. In the present invention, a comb filter may be used to limit bandwidth, thereby increasing the signal to noise ratio of the system.

The output of a commonly used low pass filter is shown in figure 4 whilst the output from using a comb filter is shown in figure 5. In using the comb filter the first peak is aligned with the excitation frequency. The peaks are set to correspond to multiples of the excitation frequency and the invention uses a low pass filter to limit the number of otherwise infinite peaks from a comb filter to the desired number, which will usually be the initial peak and one other.

Claims (11)

1. CLAIMS1. An accelerometer comprising an accelerometer component and a piezo stack, wherein the accelerometer component and the piezo stack are in fixed positions relative to one another, said piezo stack being configured to be connected to a sinusoidal voltage source, and the accelerometer component being configured to provide an output signal.
2. 2. An accelerometer as claimed in claim 1 in which the accelerometer component is a MEMS accelerometer.
3. 3 An accelerometer as claimed in claim 1 or claim 2, applied to a system wherein the sinusoidal voltage source is used to determine the acceleration of the system when the accelerometer component measures Og.
4. 4. An Accelerometer as claimed in any preceding claim in which a comb filter is used to process the output signal.
5. An accelerometer in any preceding claim for use in a space vehicle.
6. 6. An accelerometer in any of claims 1 to 4 for use in a centrifuge.
7. 7. An accelerometer in any of claims 1 to 4 for use in an automotive application.
8. 8. An accelerometer in any of claims 1 to 4 for use in an aerospace application.
9. 9. An accelerometer in any of claims 1 to 4 for use in a maritime vehicle.
10. 10.An accelerometer in any of claims 1 to 4 for use in sports analysis equipment for humans.
11. 11. An accelerometer in any of claims 1 to 4 for use in sports analysis equipment for animals.