Method and apparatus for zero-g offset calibration of a MEMS-based accelerometer

Abstract

This application generally relates to microelectromechanical systems (MEMS). More particularly, this application relates to methods and apparatus for zero-g offset calibration of MEMS-based accelerometers. This application also relates to computer program products suitable for the same purpose. According to one embodiment, a method for zero-g offset calibration of a MEMS-based accelerometer includes: a) receiving acceleration measurements performed during at least half of a full rotation of the MEMS-based accelerometer, wherein at least half of a full rotation is accomplished by rotating the MEMS-based accelerometer about a first axis that is not parallel to the gravity vector; b) obtaining, based on the accelerometer measurements, one or more pairs of acceleration peaks and acceleration valleys along a second axis that is perpendicular to the first axis; and c) determining a zero-g offset of the second axis based on the one or more pairs of acceleration peaks and acceleration valleys.

Description

Technical Field

[0001] This application generally relates to microelectromechanical systems (MEMS). More particularly, this application relates to methods and apparatus for zero-g offset calibration of MEMS-based accelerometers. This application also relates to computer program products suitable for the same purpose. Background Art

[0002] Today, systems incorporating MEMS sensors (such as accelerometers, gyroscopes, etc.) are used in an increasing number of applications. One of the reasons for their widespread use is the fact that they are cost-effective, have a miniature size, and consume low power.

[0003] There are many sources of error that affect the performance of MEMS sensors. One of these sources is sensor bias error, which is the difference between the ideal 0g output and the 0g output reported by the sensor. Imagine a perfectly horizontal surface with an accelerometer placed on it. If there is no bias error, the sensor output will read the ideal 0g offset voltage on the x and y axes and a +1g output voltage on the z axis. However, due to many factors - including mechanical tolerances in the component parts (PCB, screws, mounts, pads, etc.), the sensor will read differently from the ideal output on a perfectly horizontal surface.

[0004] Calibrating accelerometers is mainly a factory test issue. Users who purchase accelerometers and assemble them into products may also have factory test issues regarding accelerometer calibration. Due to possible assembly issues that can cause the accelerometer to rotate or tilt relative to the position expected by the application, it may be necessary to re-zero the 0g offset or level the pitch and roll on its production line.

[0005] MEMS accelerometers can be calibrated by using a variety of calibration methods. However, current methods are time-consuming and inconvenient for users. Therefore, calibration techniques with less complexity and lower cost are desired in this field. Summary of the Invention

[0006] An object is to provide methods and apparatus for zero-g offset calibration of MEMS-based accelerometers that allow for a calibration process that is convenient to use and of low cost.

[0007] According to one aspect of the present invention, a method for zero-g offset calibration of a MEMS-based accelerometer includes:

[0008] a) receiving acceleration measurements performed during at least half of a full rotation of a MEMS-based accelerometer, wherein at least half of the full rotation is accomplished by rotating the MEMS-based accelerometer about a first axis that is not parallel to the gravity vector;

[0009] b) Obtain one or more pairs of acceleration peaks and acceleration valleys along a second axis perpendicular to the first axis based on the accelerometer measurement; and

[0010] c) Determine the zero-g offset of the second axis based on the one or more pairs of acceleration peaks and acceleration valleys.

[0011] Optionally, at step c), the zero-g offset of the second axis is determined as the average of the one or more pairs of acceleration peaks and acceleration valleys.

[0012] Optionally, the first axis is substantially perpendicular to the gravity vector.

[0013] Optionally, the MEMS-based accelerometer is a triaxial accelerometer, the first axis is the Z-axis of the rectangular coordinate system established on the MEMS-based accelerometer, and the second axis is the X-axis or Y-axis of the rectangular coordinate system.

[0014] Optionally, the MEMS-based accelerometer is a biaxial accelerometer, and the second axis is the X-axis or Y-axis of the rectangular coordinate system established on the MEMS-based accelerometer.

[0015] Optionally, the method as described above further includes:

[0016] d) Output a notification on whether the zero-g offset calibration is successfully completed.

[0017] Optionally, the MEMS-based accelerometer is included in a horizontal collimator or an instrument for measuring the tilt angle.

[0018] According to another aspect of the present invention, a measuring device includes:

[0019] A microelectromechanical system (MEMS)-based accelerometer;

[0020] A storage device configured to store a computer program including computer instructions; and

[0021] A processor coupled to the storage device and configured to execute the computer instructions to perform:

[0022] a) Receive acceleration measurements performed during at least half of a full rotation of the MEMS-based accelerometer, where at least half of a full rotation is completed by rotating the MEMS-based accelerometer around a first axis not parallel to the gravity vector;

[0023] b) Obtain one or more pairs of acceleration peaks and acceleration valleys along a second axis perpendicular to the first axis based on the accelerometer measurement; and

[0024] c) Determine the zero-g offset of the second axis based on the one or more pairs of acceleration peaks and acceleration valleys.

[0025] According to another aspect of the present invention, a computer program product for zero-g offset calibration of a microelectromechanical system (MEMS)-based accelerometer is embodied in a computer-readable storage medium and includes computer instructions for performing the method as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The foregoing and other objects, features, and advantages will become apparent from the following more particular description of the preferred embodiments as illustrated in the accompanying drawings, in which:

[0027] Figure 1 is a block diagram illustrating a measuring device according to an exemplary embodiment of the present invention.

[0028] Figure 2 illustrates an example of the acceleration readings of an accelerometer plotted against the rotational angle or orientation of the accelerometer.

[0029] Figure 3 illustrates an exemplary curve representing the relationship between the acceleration on the X-axis or Y-axis and the rotational angle.

[0030] Figure 4 is a flowchart illustrating a method for zero-g offset calibration of a MEMS-based accelerometer according to another exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0031] The present invention may be implemented in many ways, including as a process; an apparatus; a system; a composition of matter; a computer program product embodied on a computer-readable storage medium; and / or a processor, such as a processor configured to execute instructions stored on and / or provided by a memory coupled to the processor. In this specification, these implementations or any other form that the invention may take may be referred to as techniques. In general, within the scope of the present invention, the order of the steps of the disclosed processes may be altered. Unless otherwise stated, components such as a processor or a memory described as being configured to perform a task may be implemented as a general component temporarily configured to perform the task at a given time or a specific component manufactured to perform the task. 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.

[0032] The terms used herein are for the purpose of describing particular embodiments only and are not intended to be limiting. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly dictates otherwise. It will be further understood that the terms "comprises", "comprising", "includes" and / or "including", when used herein, specify the presence of the 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.

[0033] Moreover, the use of ordinal terms such as "first", "second", "third", etc. in the claims to modify a claim element itself does not mean any priority, precedence or order of one claim element with respect to another or the temporal order in which acts of a method are performed, but are merely used as labels to distinguish one claim element having a particular name from another element having the same name (but using an ordinal term) for the purpose of distinguishing claim elements.

[0034] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0035] A detailed description of one or more embodiments of the invention is provided below along with the drawings that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is defined only by the claims, and the invention encompasses many alternatives, modifications and equivalents. To provide a thorough understanding of the invention, many specific details are set forth in the following description. These details are provided for purposes of example, and the invention may be practiced according to the claims without some or all of these specific details. For the sake of clarity, technical materials known in the art related to the invention are not described in detail so as not to unnecessarily obscure the invention.

[0036] Figure 1 is a block diagram illustrating a measuring device according to an exemplary embodiment of the present invention. As an example, the measuring device is a horizontal collimator or an instrument for measuring an inclination angle.

[0037] Referring Figure 1 , the measuring device 10 includes a MEMS-based accelerometer 110, a storage device 120, an output device 130, and a processor 140. As Figure 1As shown, the processor 140 is coupled to the MEMS-based accelerometer 110, the storage device 120, and the output device 130, and the MEMS-based accelerometer 110 is coupled to the storage device 120.

[0038] The MEMS-based accelerometer 110 is configured to measure acceleration or force on two or three axes that are orthogonal to each other. In the context of this specification, these axes are labeled as the x, y, and z axes in the rectangular coordinate system established on the MEMS-based accelerometer. When a force is applied to the device or accelerometer along an axis, the accelerometer 110 obtains the value or reading of the force and then converts it into a value of acceleration. This force is converted into acceleration by using Newton's second law. Since the mass of the object in the MEMS is known, the processor 130 can convert the force reading into an acceleration reading. If the acceleration reading enters in the same direction as the axis, the accelerometer will output a positive value, and if the acceleration is in the opposite direction to the axis, it will output a negative value.

[0039] This inertial force reading technique implies that the accelerometer will always read an acceleration value, even when the device is stationary. When the accelerometer is stationary, the accelerometer will read an acceleration value due to gravity. This means that if an axis is parallel to the direction of gravity, the acceleration read on that axis will be equal to 9.81 m / s 2 . In the context of this specification, all measured accelerations are relative to g, or all accelerations are fractions or multiples of g.

[0040] In this embodiment, the MEMS-based accelerometer can be a triaxial accelerometer or a biaxial accelerometer.

[0041] The storage device 120 is configured to store the acceleration values measured by the accelerometer 110. The storage device 120 is further configured to store a computer program that includes computer instructions for calculating the tilt angle and performing zero-g offset calibration of the accelerometer 110.

[0042] The output device 130 can include a display and / or a speaker and is configured to output the calculation results and calibration results made by the processor 140.

[0043] The processor 140 is configured to execute computer instructions stored in the storage device 120 to calculate the tilt angle based on the acceleration measurements performed by the accelerometer 110 and to perform a zero-g offset calibration of the accelerometer 110. In particular, to calibrate the zero-g offset along the X-axis and Y-axis, the device 10 or the MEMS-based accelerometer 110 is rotated at least half a full turn about the Z-axis, which is set to be an axis not parallel to the gravity vector. Optionally, the Z-axis is perpendicular to the gravity vector. In an illustrative example, a hole or aperture is formed on the surface of the housing of the device 10, and thus the device 10 can be mounted on a wall by hanging the device on a hook or nail fixed to the wall and can thus rotate about the hook or nail, i.e., can rotate about the Z-axis. At the start of the calibration process, one can provide an initial momentum to the device 10, which then rotates about the hook or nail. The accelerometer 110 measures the acceleration on the X-axis and / or Y-axis during the rotation. As noted above, these measured accelerations are stored in the storage device 120. For each of the X-axis and Y-axis, during half of a full turn, there is a pair of acceleration peaks and acceleration valleys due to gravity.

[0044] Figure 2 Illustrated is an example depicting the acceleration readings of the accelerometer versus the rotation angle or orientation of the accelerometer. As Figure 2 shown, as the accelerometer rotates, the acceleration on the Y-axis has a maximum or peak value, i.e., 1g, at T0 and a minimum or valley value, i.e., -1g, at T2; on the other hand, the acceleration on the X-axis has a maximum or peak value, i.e., 1g, at T1 and a minimum or valley value, i.e., -1g, at T4, where T0, T1, T2, and T4 represent the orientations of the accelerometer.

[0045] As noted above, during the rotation, the acceleration readings or values will change with the rotation angle of the device, and for each of the X-axis and Y-axis, during half of a full turn, there is a pair of acceleration peaks and acceleration valleys. If the rotation is performed one or more full turns, several pairs of peaks and valleys can be observed. Figure 3 Illustrated is an exemplary curve representing the relationship between the acceleration on the X-axis or Y-axis and the rotation angle. As Figure 3 shown, the number of peak and valley pairs will increase with the number of full turns.

[0046] The processor 140 is configured to retrieve, for each of the X-axis and the Y-axis in the acceleration measurement, one or more pairs of acceleration peaks and acceleration valleys, and determine, for each of the X-axis and the Y-axis, a zero-g offset based on the corresponding pairs of acceleration peaks and acceleration valleys. Optionally, the zero-g offset for the X-axis or the Y-axis can be determined as the average of the corresponding pairs of acceleration peaks and acceleration valleys. The processor 140 is further configured to output the calibration result or the zero-g offset to the storage device 110 and / or the output device 130. Optionally, the processor 140 is configured to output a notification to the output device and / or the storage device 110 regarding whether the zero-g offset calibration has been successfully completed.

[0047] When used as a horizontal collimator, the measuring device may further include a motor for rotating the measuring device and a laser for emitting a reference beam. The processor is configured to control the laser to emit the reference beam when the measuring device is in a proper position in which the reference beam is parallel to a specific direction (e.g., the horizontal direction). In addition, the processor is configured to control the rotation of the measuring device 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 collimation indicator.

[0048] Figure 4 is a flowchart illustrating a method for zero-g offset calibration of a MEMS-based accelerometer according to another exemplary embodiment of the present invention.

[0049] For illustrative purposes, the following description is made with reference to the device as shown in Figure 1 However, those skilled in the art will recognize that the present invention is not limited to any particular device.

[0050] As shown in Figure 4 In step S410, a set of acceleration values during at least half of a complete rotation of the measuring device 10 is measured by the accelerometer 110 and stored in the storage device 120.

[0051] Then, in step S420, the processor 140 receives the acceleration measurement, e.g., a set of acceleration values, from the accelerometer 110 or the storage device 120.

[0052] At step S430, the processor 140 retrieves, for each of the X-axis and the Y-axis in the acceleration measurement, one or more pairs of acceleration peaks and acceleration valleys.

[0053] Then, at step S440, the processor 140 determines a zero-g offset for each of the X-axis and the Y-axis based on the corresponding one or more pairs of acceleration peaks and acceleration valleys. As noted above, the zero-g offset of the X-axis or the Y-axis can be determined as the average of their corresponding pairs of acceleration peaks and acceleration valleys. For example, the zero-g offset of the X-axis or the Y-axis can be calculated according to the following formula:

[0054]

[0055] where ZG_Offset x and ZG_Offset y represent the zero-g offsets of the X-axis and the Y-axis respectively, m represents the number of pairs of acceleration peaks and acceleration valleys on the X-axis, n represents the number of pairs of acceleration peaks and acceleration valleys on the Y-axis, represents the acceleration peak in the i-th pair of acceleration peaks and acceleration valleys on the X-axis, represents the acceleration valley in the i-th pair of acceleration peaks and acceleration valleys on the X-axis, represents the acceleration peak in the j-th pair of acceleration peaks and acceleration valleys on the Y-axis, and represents the acceleration valley in the j-th pair of acceleration peaks and acceleration valleys on the Y-axis.

[0056] At step S450, the processor 140 outputs the zero-g offset determined at step S440 to the storage device 120 and / or the output device 130.

[0057] Optionally, at step S460, the processor 140 outputs a notification to the output device 130 and / or the storage device 120 regarding whether the zero-g offset calibration has been successfully completed.

[0058] It should be noted that the above embodiments are illustrative and not restrictive. Without departing from the scope of the appended claims, those skilled in the art can design alternative embodiments. Phrases such as "including", "comprising", "containing" and "having" do not exclude elements or steps that exist in the description and claims but are not listed. It should also 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 implemented by means of hardware including several different elements or by means of a suitably programmed computer. In a unit claim listing several components, several of these components can be embodied in the same hardware item. The use of words such as first, second, third does not denote any order and can simply be interpreted as names.

[0059] It will be clear to those skilled in the art that, with the progress of technology, the concept of the present invention can be implemented in various ways. The above embodiments are given for the purpose of description and not limitation of the present disclosure, and it should be understood that modifications and variations can be made without departing from the scope of the present disclosure, as will be readily understood by those skilled in the art. Such modifications and variations are considered to be within the scope of the present disclosure and the appended claims. The scope of protection of the present disclosure is defined by the appended claims.

Claims

1. A measuring device for zero-g offset calibration of a microelectromechanical systems (MEMS)-based accelerometer, wherein the measuring device is a horizontal collimator and comprises: A MEMS-based accelerometer; A storage device configured to store a computer program including computer instructions; And A processor coupled to the storage device and configured to execute the computer instructions to perform: a) Receive acceleration measurements performed during at least half of a full rotation of the MEMS-based accelerometer, wherein at least half of the full rotation is accomplished by rotating the MEMS-based accelerometer about a first axis that is not parallel to the gravity vector; b) Obtain one or more pairs of acceleration peaks and acceleration valleys along a second axis perpendicular to the first axis based on the accelerometer measurements; and c) Determine the zero-g offset of the second axis based on the one or more pairs of acceleration peaks and acceleration valleys, wherein the measuring device further comprises a laser for emitting a reference beam when the measuring device is in a proper position in which the reference beam is parallel to the horizontal direction, and the proper position is determined by the MEMS-based accelerometer.

2. The measuring device according to claim 1, wherein, At step c), the zero-g offset of the second axis is determined as the average of the one or more pairs of acceleration peaks and acceleration valleys.

3. The measuring device according to claim 1, wherein, The first axis is substantially perpendicular to the gravity vector.

4. The measuring device according to claim 1, wherein, The MEMS-based accelerometer is a triaxial accelerometer, the first axis is the Z axis of a rectangular coordinate system established on the MEMS-based accelerometer, and the second axis is the X axis or the Y axis of the rectangular coordinate system.

5. The measuring device according to claim 1, wherein, The MEMS-based accelerometer is a biaxial accelerometer, and the second axis is the X axis or the Y axis of a rectangular coordinate system established on the MEMS-based accelerometer.

6. The measuring device according to claim 1, wherein the processor is configured to execute the computer instructions to further perform: d) Output a notification as to whether the zero-g offset calibration has been successfully completed.

7. A method for zero-g offset calibration of a MEMS-based accelerometer performed by the measuring device according to any one of claims 1 to 6.

8. A computer program product embodied in a computer-readable storage medium and including computer instructions for performing the method according to claim 7.

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