Patent Description:
Nowadays, systems incorporating MEMS sensors (e.g. accelerometers, gyroscopes, etc.) are used in more and more applications. Among the reasons for their wide use is the fact that they are cost-effective, have miniature size and consume low power.

There are many sources of error that affect the behavior of the MEMS sensors. One of the sources is sensor bias error, which is the difference between the ideal <NUM> output and the <NUM> output reported by the sensor. Think of a perfectly horizontal surface with an accelerometer sitting on it. If there was no bias error, then the sensor output would read the ideal <NUM> offset voltage on the x and y-axis, +<NUM> output voltage on the z-axis. However, the sensor will read something different than the ideal output on a perfectly horizontal surface because of many factors including mechanical tolerances in the component parts (PCB, screws, standoffs, solder pads, etc.).

Calibrating accelerometers is mostly a factory test problem. Users purchasing accelerometers and assembling them into products can also have factory test issues in regards to accelerometer calibration. Because there can be assembly issues that result in accelerometers that are rotated or tilted relative to the desired position for the application, it may need to make re-zero the <NUM> offset or level the pitch and roll in their production line.

The MEMS accelerometers can be calibrated by using multiple calibration methods. However, the current methods are time-intensive and inconvenient for users. Therefore, calibration technique with less complexity and lower cost is desired in the field.

<CIT> provides apparatus and methods for the calibration of a three axis vector field sensor that provides options for simpler and less expensive calibration of offset and gain for all three axes X,Y,Z. <CIT> provides an automatic calibration method for use in an electronic compass.

One of the objects is to provide methods and apparatus for zero-g offset calibration for a MEMS-based accelerometer, which allows a convenient and low-cost calibrating process for use.

According to one aspect of the present invention, a method for zero-g offset calibration for a measurement apparatus including a MEMS-based accelerometer is provided as defined by independent claim <NUM>. The method comprises:.

the MEMS-based accelerometer is included in a horizontal collimator, and the method further comprises: emitting a reference beam using a laser when the measurement apparatus is in a proper position determined by the MEMS-based accelerometer, where the reference beam is parallel to a horizontal direction when used as the horizontal collimator.

Optionally, at step c), the zero-g offset for the second axis is determined as an average of the one or more pairs of acceleration peaks and acceleration valleys.

Optionally, the first axis is substantially perpendicular to the gravity vector.

Optionally, the MEMS-based accelerometer is a tri-axis accelerometer, the first axis is a Z-axis of a rectangular coordinate system established on the MEMS-based accelerometer and the second axis is an X-axis or a Y-axis of the rectangular coordinate system.

Optionally, the MEMS-based accelerometer is a dual-axis accelerometer, and the second axis is an X-axis or a Y-axis of a rectangular coordinate system established on the MEMS-based accelerometer.

Optionally, the method as described above further comprises: d) outputting a notification on whether the zero-g offset calibration is successfully done.

According to another aspect of the present invention, a measurement apparatus is provided as defined by independent claim <NUM>. The apparatus comprises:.

According to another aspect of the present invention, a computer program product for zero-g offset calibration for a Micro-Electro-Mechanical-System(MEMS)-based accelerometer is embodied in a computer readable storage medium and comprises computer instructions to cause the apparatus as described above to perform the method as described above.

The foregoing and other objects, features, and advantages would be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which:.

In general, the order of the steps of disclosed processes may be altered within the scope of the invention as claimed. As used herein, the term "processor" refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions.

It will be further understood that the terms "comprises" "comprising," "includes" and/or "including" when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Also, use of ordinal terms such as "first," "second," "third," etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood.

<FIG> is a block diagram illustrating a measurement apparatus according to one exemplary embodiment of the present invention. As an example, the measurement apparatus is a horizontal collimator or an instrument for measuring tilt angle.

With reference to <FIG>, the measurement apparatus <NUM> comprises a MEMS-based accelerometer <NUM>, a storage device <NUM>, an output device <NUM> and a processor <NUM>. As shown in <FIG>, the processor <NUM> is coupled to the MEMS-based accelerometer <NUM>, the storage device <NUM> and the output device <NUM>, and the MEMS-based accelerometer <NUM> is coupled to the storage device <NUM>.

The MEMS-based accelerometer <NUM> is configured to measure the accelerations or forces in two or three axes orthogonal to each other. In the context of this description, these axes are denoted as x, y, and Z axis in a rectangular coordinate system established on the MEMS-based accelerometer. When a force is applied to the apparatus or the accelerometer along one of the axes, the accelerometer <NUM> takes the value or reading of force and then converts it into the value of acceleration. This force is converted into acceleration by using Newton's Second Law. Because the mass of the object in the MEMS is known, the processor <NUM> can convert the force reading into an acceleration reading. If an acceleration reading is going in the same direction as the axis, then the accelerometer will output a positive value, and it will output a negative value if the acceleration is in the opposite direction of the axis.

This inertia force reading technique implies that the accelerometer will always read an acceleration value even if the apparatus is stationary. When the accelerometer is stationary, the accelerometer will read the acceleration value due to gravity. This means that if one axis is parallel to the direction of gravity, the acceleration read on that axis will be equal to <NUM>/s<NUM>. In the context of this description, all accelerations measured are with respect to g's or all accelerations are a fraction or multiple of g.

In the present embodiment, the MEMS-based accelerometer may be a tri-axis accelerometer or a dual-axis accelerometer.

The storage device <NUM> is configured to store the acceleration values measured by the accelerometer <NUM>. The storage device <NUM> is further configured to store a computer program comprising computer instructions for calculating tilt angle and performing zero-g offset calibration for the accelerometer <NUM>.

The output device <NUM> may comprise a display and/or a speaker and is configured to output the calculating result and the calibrating result made by the processor <NUM>.

The processor <NUM> is configured to execute the computer instructions stored in the storage device <NUM> to calculate tilt angle and perform zero-g offset calibration for the accelerometer <NUM> based on the acceleration measurement carried out by the accelerometer <NUM>. In particular, to calibrate zero-g offset along X axis and Y axis, the apparatus <NUM> or the MEMS-based accelerometer <NUM> is made to rotate around Z axis, which is set as one not parallel to a gravity vector, for at least half of one complete revolution. Optionally, Z axis is perpendicular to the gravity vector. In an illustrative example, an aperture or a hole is formed on the surface of the case of the apparatus <NUM>, and thus the apparatus <NUM> may be installed on a wall by hanging the apparatus on a hook or nail, which is fixed to the wall, and thus is rotatable around the hook or nail, i.e., is rotatable around Z axis. At the beginning of a calibration procedure, one may provide an initial momentum to the apparatus <NUM>, which, in turn, rotates around the hook or nail. The accelerometer <NUM> measures the accelerations in X axis and/or Y axis during the rotation. As noted above, these measured accelerations are stored in the storage device <NUM>. For each of X axis and Y axis, there is a pair of acceleration peak and acceleration valley due to the gravity during half of one complete revolution.

<FIG> illustrates an example describing acceleration readings of an accelerometer versus a rotational angle or orientation of the accelerometer. As shown in <FIG>, with the rotation of the accelerometer, the acceleration in Y axis has a maximum or peak, i.e., <NUM>, at T0 and has a minimum or valley, i.e., -<NUM>, at T2; on the other hand, the acceleration in X axis has a maximum or peak, i.e., <NUM>, at T1 and has a minimum or valley, i.e., -<NUM>, at T4, where T0, T1, T2 and T4 represent the orientation of the accelerometer.

As noted above, the acceleration readings or values will change with the rotational angle of the apparatus during the rotation, and for each of X axis and Y axis, there is a pair of acceleration peak and acceleration valley during half of one complete revolution. If the rotation is made for one or more revolutions, several pairs of peaks and valleys may be observed. <FIG> illustrates an exemplary curve representing a relationship between the acceleration in X axis or Y axis and the rotational angle. As shown in <FIG>, the number of the pairs of peaks and valleys will increase with the number of the revolutions.

The processor <NUM> is configured to retrieve in the acceleration measurement one or more pairs of acceleration peaks and acceleration valleys for each of X axis and Y axis and determine a zero-g offset for each of X axis and Y axis on the basis of the respective pairs of acceleration peaks and acceleration valleys. Optionally, the zero-g offset for X axis or Y axis may be determined as an average of the respective pairs of acceleration peaks and acceleration valleys. The processor <NUM> is further configured to output the calibration result or the zero-g offset to the storage device <NUM> and/or the output device <NUM>. Optionally, the processor <NUM> is configured to output to the output device and/or the storage device <NUM> a notification on whether the zero-g offset calibration is successfully done.

When used as a horizontal collimator, the measurement apparatus may further comprise a motor for rotating the measurement apparatus and a laser for emitting a reference beam. The processor is configured to control the laser to emit the reference beam when the measurement apparatus is in a proper position where the reference beam is parallel to a particular direction, e.g., horizontal direction. Moreover, the processor is configured to control the rotation of the measurement apparatus by means of the motor based on the calibration result. Since the proper position is substantially determined by the MEMS-based accelerometer, it can provide a reliable and accurate collimating indicator.

<FIG> is a flow diagram illustrating a method for zero-g offset calibration for a MEMS-based accelerometer according to another exemplary embodiment of the present invention.

For illustrative purpose, the following description is made with reference to the apparatus as shown in <FIG>. However, one skilled artisan in the art would recognize that the present invention is not limited to any specific apparatus.

As shown in <FIG>, at step S410, a set of acceleration values during the rotation of the measurement apparatus <NUM> for at least half of one complete revolution are measured by the accelerometer <NUM> and are stored in the storage device <NUM>.

Then at step S420, the processor <NUM> receives the acceleration measurement, e.g., the set of acceleration values, from the accelerometer <NUM> or the storage device <NUM>.

At step S430, the processor <NUM> retrieves in the acceleration measurement one or more pairs of acceleration peaks and acceleration valleys for each of X axis and Y axis.

Then, at step S440, the processor <NUM> determines a zero-g offset for each of X axis and Y axis on the basis of the respective one or more pairs of acceleration peaks and acceleration valleys. As noted above, the zero-g offset for X axis or Y axis may be determined as an average of their respective pairs of acceleration peaks and acceleration valleys. For example, the zero-g offset for X axis or Y axis may be calculated according to the following formulas: <MAT> <MAT> where ZG_Offsetx and ZG_Offsety represent the zero-g offsets for X axis and Y axis respectively, m represents the number of the pairs of acceleration peaks and acceleration valleys in X axis, n represents the number of the pairs of acceleration peaks and acceleration valleys in Y axis, <MAT> represents the acceleration peak in the ith pair of acceleration peaks and acceleration valleys in X axis, <MAT> represents the acceleration valley in the ith pair of acceleration peaks and acceleration valleys in X axis, <MAT> represents the acceleration peak in the jth pair of acceleration peaks and acceleration valleys in Y axis, and <MAT> represents the acceleration valley in the jth pair of acceleration peaks and acceleration valleys in Y axis.

At step S450, the processor <NUM> outputs the zero-g offsets determined at step S440 to the storage device <NUM> and/or the output device <NUM>.

Optionally, at step S460, the processor <NUM> outputs to the output device <NUM> and/or the storage device <NUM> a notification on whether the zero-g offset calibration is successfully done.

It should be noted that the aforesaid embodiments are illustrative instead of restricting, substitute embodiments may be designed by those skilled in the art without departing from the scope of the claims enclosed. The wordings such as "include", "including", "comprise" and "comprising" do not exclude elements or steps which are present but not listed in the description and the claims. It also shall be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. Embodiments can be achieved by means of hardware including several different elements or by means of a suitably programmed computer. In the unit claims that list several means, several ones among these means can be specifically embodied in the same hardware item. The use of such words as first, second, third does not represent any order, which can be simply explained as names.

Claim 1:
A method for zero-g offset calibration for a measurement apparatus (<NUM>) including a Micro-Electro-Mechanical-System, MEMS,-based accelerometer (<NUM>), comprising:
a) receiving (S420) acceleration measurements performed during at least half of one complete revolution of the MEMS-based accelerometer (<NUM>), wherein the at least half of one complete revolution is accomplished by making the MEMS-based accelerometer (<NUM>) rotate around a first axis not parallel to a gravity vector;
b) from the accelerometer measurements, obtaining (S430) one or more pairs of acceleration peaks and acceleration valleys along a second axis perpendicular to the first axis; and
c) determining (S440) a zero-g offset for the second axis on the basis of the one or more pairs of acceleration peaks and acceleration valleys,
wherein the MEMS-based accelerometer (<NUM>) is included in a horizontal collimator, and
further comprising: emitting a reference beam using a laser when the measurement apparatus (<NUM>) is in a proper position determined by the MEMS-based accelerometer, where the reference beam is parallel to a horizontal direction when used as the horizontal collimator.