The invention relates generally to an apparatus and method for measuring tilt using an accelerometer sensing a minimal number of degrees of freedom.
Accelerometers and gyroscopes belong to a class of devices known as motion detection inertial sensors. In general, a motion detection inertial sensor provides information about the movement/orientation of a device. An accelerometer provides information about the movement/orientation of a device by measuring its own acceleration as opposed to measuring the acceleration of a remote device. Accelerometers are often used along with gyroscopes in inertial navigation and guidance systems. A common use of accelerometers is in airbag deployment systems for automobiles. Another common use of accelerometers is for detecting the tilt of a device. Depending on the information of interest a 2D or 3D accelerometer may be used for detecting tilt.
For the most part, the cost and size of an accelerometer depends on the total number of axes that the accelerometer can measure. For example, an accelerometer that is sensitive to accelerations in the Z-axis (perpendicular to the plane of the silicon chip), will cost much more than an accelerometer that measures only X and Y accelerations (in the same plane as the silicon die). Moreover, the noise level of the Z-axis is typically much higher than that of the X and Y axis, the reduction of which can increase costs. It is therefore apparent that in order to reduce costs, it is desired to eliminate as many sensor axes as possible in the construction of an accelerometer.
A need therefore exists for an accelerometer and associated method for measuring 1D, 2D and 3D tilt sensing only a minimal number of degrees of freedom to minimize costs.
Therefore, the present invention has been made in view of the above problems. Accordingly, the present invention provides a system and method for calculating the tilt from a minimum set of measurements. In the described embodiments, one or more accelerometers are used to sense tilt in fewer degrees of freedom than would otherwise be required in a conventional measurement apparatus. In this regard, the cost and size of the accelerometers is reduced. In one embodiment, a single axis accelerometer measures 2D tilt by taking into account a constant value of the earth's gravitational field in a direction generally perpendicular to the earth.
Components of the apparatus may be individually capable of inertially sensing or determining the direction of gravity. One of the accelerometers may, for example, advantageously, be a MEMS accelerometer.
These and other objects, features and advantages of the invention will be apparent from a consideration of the following detailed description of the invention considered in conjunction with the drawing Figures, in which:
a & 3b are illustrations of, respectively, use of a prior art leveling instrument and a leveling instrument of the present invention.
In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, those skilled in the art will appreciate that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device and a method.
For the purposes of illustration, it is assumed that there is a tilt in both “pitch” and “roll” in the device 10 of
Where:
sAmy: Measured acceleration in the y-axis.
sAmx: Measured acceleration in the x-axis.
sAmz: Measured acceleration in the z-axis.
TiltAnglePitch: Angle with respect to the y axis
TiltAngleRoll: Angle with respect to the x axis
As described, the measurement of tilt with respect to the three co-ordinate axes requires a conventional 3D accelerometer. As will become apparent in the description to follow, the invention provides methods and apparatus to make measurements in three co-ordinate axes using fewer degrees of freedom. In this manner, both a cost, power and space savings of measurement apparatus may be realized.
In accordance with a first embodiment, to sense tilt using fewer degrees of freedom than would otherwise be required in a conventional measurement apparatus, as described above, the inventors have recognized that the measured acceleration in the z-axis of a world coordinate frame, 0Aez, is a constant and is equal to 9.8 m/s2. In other words, it is recognized by the inventors that it is well established that the gravitation field is always essentially perpendicular to the earth, i.e., the vertical component of tilt in both the pitch and roll directions. This information can be used advantageously in equations 1 and 2 above, which allows for a reduction in the number of sensing degrees required to make tilt measurements. In the present embodiment, a conventional 3D accelerometer may be replaced by a two single-axis accelerometers for measuring tilt in the x and y axes, respectively. Thus, a reduction in sensing degree from a single 3D accelerometer to two single-axis accelerometers is realized. Accordingly, a method for measuring tilt in the x and y axes is achieved by measuring tilt in the x-axis using a first accelerometer. Then, measuring tilt in the y axis using a second accelerometer and using the two measurements in equations 3 and 4, as follows:
sAmy: Measured acceleration in the y-axis.
sAmx: Measured acceleration in the x-axis.
0Ae: Constant of 9.81 m/s2
TiltAnglePitch: Angle with respect to the y axis
TiltAngleRoll: Angle with respect to the x axis
The previous embodiment described the separate computations of roll and pitch of a device 10 in the x and y directions respectively. These two results are quantified in equations 3 and 4 above. In the present embodiment, it is contemplated to compute a single tilt angle α that represents both the roll and the pitch of the device 10.
Referring now to
V=√{square root over (sAmx2+sAmy2+9.812)} Eq. [5]
Equations (6)-(8) describe computational steps for computing the angle α from the normal vector V,
This can be re-written as:
Which is equal to:
Where
The advantage of producing a single tilt angle α is that it represents both the roll and the pitch of the device 10. This is shown by way of example with reference to
Referring now to
It will now become apparent that this advantage translates to other applications including, for example, wireless sensors attached to the limbs of a patient. The wireless sensors may be shifted, tilted, rotated and so on without impacting the measurement of the tilt angle α. Hence, sensor placement inaccuracy can be largely neglected.
Recall from the previous embodiment that the normal vector V was calculated to derive the angle α, which is the angle that is formed by the device 10 with respect to the z-axis. In the present embodiment, instead of calculating the normal vector V, it is calculated in the manner to be described.
Referring again to
One disadvantage of this approach is that the accuracy of the angle α is influenced by the real angle of the device 10 with respect to the gravitational field. This is further explained with respect to the graph of
Finally, the above-discussion is intended to be merely illustrative of the present invention and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Each of the systems utilized may also be utilized in conjunction with further systems. Thus, while the present invention has been described in particular detail with reference to specific exemplary embodiments thereof, it should also be appreciated that numerous modifications and changes may be made thereto without departing from the broader and intended spirit and scope of the invention as set forth in the claims that follow. The specification and drawings are accordingly to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims.
In interpreting the appended claims, it should be understood that:
a) the word “comprising” does not exclude the presence of other elements or acts than those listed in a given claim;
b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements;
c) any reference numerals in the claims are for illustration purposes only and do not limit their protective scope;
d) several “means” may be represented by the same item or hardware or software implemented structure or function; and
e) each of the disclosed elements may be comprised of hardware portions (e.g., discrete electronic circuitry), software portions (e.g., computer programming), or any combination thereof.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/IB08/55568 | 12/29/2008 | WO | 00 | 6/21/2010 |
Number | Date | Country | |
---|---|---|---|
61017193 | Dec 2007 | US |