GB2357148A

GB2357148A - Determining a spatial position using an array of single axis transducers

Info

Publication number: GB2357148A
Application number: GB9928770A
Authority: GB
Inventors: Paul Thomas Faithfull
Original assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Current assignee: Bayerische Motoren Werke AG; MG Rover Group Ltd
Priority date: 1999-12-07
Filing date: 1999-12-07
Publication date: 2001-06-13
Also published as: GB9928770D0

Abstract

A spatial position eg for a vehicle is determined from one or more gravitational offsets detected by an array of single axis transducers spaced apart substantially symmetrically in pairs about one or more axes eg about a centre of gravity CoG. Position determination may include using points derived from functions of longitudinal, lateral, and vertical acceleration, which themselves can be derived as a function of initial position and a change in one or more of roll, pitch and yaw angles eg as an integral of rotational acceleration about a centre of gravity . Change in angle can be derived from the difference between signals in one or more pairs of transducers. Gravitational offsets can be cancelled out by subtracting signals of the transducers in a pair from each other. The transducers may operate down to zero frequency and may comprise linear accelerometers.

Description

2357148 A Measurement Method This invention relates to measurement methods

and in particular to a measurement method which is suitable for determining the position in space of a vehicle.

A non-contact method of determining the position of a vehicle in space (as opposed to its geographical position) is sometimes required as an input to a control system, e.g. for vehicle tracking purposes. Some sensors used to provide this input have been known to experience interference from their installation, e.g. sensors based on magnetics and installed in an electrically driven vehicle. Furthermore, some suggested solutions use one or more gyroscopic type transducers, which can prove expensive.

It is an object of this invention to provide an improved measurement method.

Accordingly, the invention provides a measurement method suitable for determining the position in space of a vehicle using an array of singleaxis transducers spaced apart substantially symmetrically in pairs about one or more axes of the vehicle, the method including the steps ofdetermining any gravitational offsets imposed on the transducers; and determining, from one or more of the gravitational offsets, the initial position of the vehicle in space.

The method may include determining the position in space of the vehicle at one or more further points as a function of the substantially true longitudinal, lateral and vertical accelerations of the vehicle.

The method may include determining the substantially true longitudinal, lateral and vertical accelerations as a function of the initial position and a change in one or more of roll, pitch and yaw angles.

The method may include determining a said change in one or more of said roll, pitch and yaw angles as a function of the integral of a rotational acceleration of the vehicle about its centre of gravity.

The method may include determining a said change in angle from the difference between the signals of the transducers in one or more transducer pairs.

The method may include, within a pair, cancelling out the effects of the gravitational offsets by subtracting the signals of the transducers in the pair from each other.

The transducers may comprise single-axis transducers or linear accelerometers and may be arranged to operate down to zero frequency. The array may comprise at least six transducers spaced apart in pairs substantially symmetrically about X, Y, and Z axes of the vehicle.

The invention also provides a measurement system suitable for a vehicle and arranged in use to determine the position in space of the vehicle using a measurement method according to the invention.

The invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a measurement arrangement arranged in use to operate using a method according to the invention; Figure 2 is an illustration of a measurements convention employed by the measurement arrangement of Figure 1; and Figures 3a to 3c are schematic representations of a manner in which the outputs of a position determining means of Figure 1 vary for changes in orientation of a vehicle to which the measurement arrangement of Figure 1 may be fitted.

Referring to the figures, a vehicle tracking system includes a position determining means in the form of an array 10 of non-contact linear acceleration transducers Ai, A.2; Ayi, Ay2; A.1, A,.2 positioned symmetrically in opposing pairs on the X, Y, Z axes respectively about the centre of gravity CoG of a vehicle (not shown further).

The transducers Ai, A,2; Ayi, Ay2; Ai, A.2 used in this embodiment comprise single-axis accelerometers, are capable of zero frequency detection and include offsets due to the effects of gravity. This is illustrated by the angles (roll), 0 (pitch), xy (yaw) for the respective axes X (longitudinal), Y (lateral) and Z (vertical).

Using the array 10, in which the direction of the sensitive linear acceleration axis of each transducer Axl, Ax2; Ayi, Ay2; Azi, Az2 is shown in Figure 1 by an arrow, the initial orientation of the vehicle in space is determined, assuming static initial conditions.

The axes X, Y, Z are translated relative to the action of gravity and the offset imposed by gravity on the signals is represented by the letters A, B and C, as outlined below:

ameasured-vertical'-- vertical-acceleration + A ameasured-lateral = lateral-acceleration = B; and ameasured-longitudinW = longitudinal-acceleration = C.

The angles, 0, xy can be calculated for an initial state from the transducers A.i, A.2; Ayi, Ay2; Ai, Az2, assuming the vehicle to be in a static state:

=tan-'(A) C) (B) Wo = tan C) 0 = tan -1 A (B) From this initial state, the position of the vehicle at a point in time can be calculated as a function of the initial conditions and a change in angle (i.e. a change in roll, pitch or yaw) as illustrated with particular reference to Figures 3a to 3c. The change in angle is determined as a function of the integral of the rotational acceleration about the CoG. This is determined from the difference between two accelerometer signals and is unaffected by gravitation offsets, which are cancelled by the subtraction of the two signals.

Hence:

+ 0 = 00 + AO Given that, E), and Mi are known at any point in time, the offset due to gravity can be calculated as outlined below:

using the identity; A2+ B2 + C2 = g2 and substituting the identities developed; A' + (WAO) 2 2 + AJ2 =g (7an 0 =>A2 1+ +JY] = g 2 wn 2o W2 => A 9 rian 1+ - 1 +_ 2 tan 2oj Similarly:

B= 9 2 1 + t 20 1 an + 2 tan 2 C= -9 - 1 + tan2E) + &M 2 Hence, the accelerations due both to the motion and the inclination of the vehicle with respect to gravity can be determined.

To calculate the roll, pitch and yaw angles:

tan-' a,,, f a,,2- a.,, f + 11 a:I" 0 tan f), f a.2 - a,,, 0)+ f a.,,. 12 V/ = tan- a,,,, f a.,, - ax2 I(a + f 13 Yj" where a.i., aylo and azio represent the initial values readings taken under static conditions.

The true longitudinal and vertical accelerations can be calculated from:

a., + ax2 g2 ax 2 + tan 2()+ tan 2 4f a.2) 9 2 a Y 2 20+_ L 1 tan tan a,, + az2 2 92 rl+ a.

2 + tan

Claims

1. A measurement method suitable for determining the position in space of a vehicle using an array of single-axis transducers spaced apart substantially symmetrically in pairs about one or more axes of the vehicle, the method including the steps of determining any gravitational offsets imposed on the transducers; and determining, from one or more of the gravitational offsets, the initial position of the vehicle in space.

2. A method according to Claim 1, including determining the position in space of the vehicle at one or more further points as a function of the substantially true longitudinal, lateral and vertical accelerations of the vehicle.

3. A method according to Claim 2, the method including determining the substantially true longitudinal, lateral and vertical accelerations as a function of the initial position and a change in one or more of roll, pitch and yaw angles.

4. A method according to Claim 3, including determining a said change in one or more of said roll, pitch and yaw angles as a function of the integral of a rotational acceleration of the vehicle about its centre of gravity.

5. A method according to Claim 4, including determining a said change in angle from the difference between the signals of the transducers in one or more transducer pairs.

-9"

6. A method according to Claim 5, including, within a pair, cancelling out the effects of the gravitational offsets by subtracting the signals of the transducers in the pair from each other.

7. A method according to any preceding claim, the transducers comprising single-axis transducers.

8. A method according to any preceding claim, the transducers comprising linear accelerometers.

9. A method according to Claim 8, the transducers being arranged to operate down to zero frequency.

10. A method according to any preceding claim, the array comprising at least six transducers spaced apart in pairs substantially symmetrically about X, Y, and Z axes of the vehicle.

11. A method substantially as described herein and with reference to the accompanying drawings.

12. A measurement system for a vehicle and arranged in use to determine the position in space of the vehicle using a measurement method according to any preceding claim.