INERTIAL MEASUREMENT UNIT

Abstract

An inertial measurement unit comprising at least one inertial sensor that is arranged to output an inertial measurement and a primary temperature sensor spatially associated with each inertial sensor that is arranged to output a temperature measurement, and a processor that receives the outputs; wherein the processor is arranged to differentiate the temperature measurement with respect to time so as to determine a temporal temperature gradient output. Existing temperature sensor(s) can be used to observe not only absolute temperature, but also thermal gradients, to further improve performance of the inertial measurement unit (IMU). This approach is distinct from the conventional calibration approach adopted for inertial sensors and IMUs in that the temperature sensor(s) in the device are used to determine temporal temperature gradients, in addition to a temperature output alone, one or both of which can be used for parametric compensation.

Claims

1. An inertial measurement unit comprising at least one inertial sensor that is arranged to output an inertial measurement and a primary temperature sensor spatially associated with each inertial sensor that is arranged to output a temperature measurement, and a processor that receives the outputs; wherein the processor is arranged to differentiate the temperature measurement with respect to time so as to determine a temporal temperature gradient output.

2. The inertial measurement unit of claim 1, wherein the processor is arranged to filter and/or smooth the differentiated temperature measurement so as to determine the temporal temperature gradient output.

3. The inertial measurement unit of claim 1, further comprising one or more secondary temperature sensors that each has a different spatial location to the primary temperature sensor and is arranged to output a different temperature measurement, wherein the processor is arranged to process the different temperature measurements so as to determine a spatial temperature gradient output.

4. The inertial measurement unit of claim 1, wherein the processor is further arranged to determine a compensation for the inertial measurement, or an associated parametric error, based on the temporal temperature gradient output and/or spatial temperature gradient output.

5. An inertial measurement unit comprising at least one inertial sensor that is arranged to output an inertial measurement, a primary temperature sensor spatially associated with each inertial sensor, and one or more secondary temperature sensors that each has a different spatial location to the primary temperature sensor, wherein each of the primary and secondary temperature sensors is arranged to output a different temperature measurement, and wherein the processor is arranged to process the different temperature measurements so as to determine a spatial temperature gradient output.

6. The inertial measurement unit of claim 1, wherein the inertial sensor comprises a vibrating structure gyroscope driven to resonance and the primary temperature sensor is arranged to output a temperature measurement based on the resonance frequency.

7. The inertial measurement unit of claim 1, wherein the inertial sensor comprises an accelerometer and the primary temperature sensor is arranged to measure temperature at, or near, the spatial location of the accelerometer.

8. The inertial measurement unit of claim 1, wherein the at least one inertial sensor comprises a MEMS substrate and at least one secondary temperature sensor is located on the MEMS substrate.

9. The inertial measurement unit of claim 1, wherein the at least one inertial sensor comprises an integrated circuit and at least one secondary temperature sensor forms part of the integrated circuit.

10. The inertial measurement unit of claim 1, wherein the processor is located on a printed circuit board and at least one secondary temperature sensor is located on the same printed circuit board.

11. The inertial measurement unit of claim 1, further comprising an electrical connector for a host system, wherein at least one secondary temperature sensor is located at or on the electrical connector.

12. A method of compensating for thermal gradients in an inertial measurement unit, comprising: receiving an inertial measurement output by at least one inertial sensor; receiving a temperature measurement output by a primary temperature sensor spatially associated with each inertial sensor; and differentiating the temperature measurement with respect to time so as to determine a temporal temperature gradient output.

13. The method of claim 12, further comprising: receiving a different temperature measurement output by one or more secondary temperature sensors that each has a different spatial location to the primary temperature sensor; and processing the different temperature measurements so as to determine a spatial temperature gradient output.

14. The method of claim 12, further comprising: determining a compensation for the inertial measurement, or an associated parametric error, based on the temporal temperature gradient output and/or spatial temperature gradient output.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] One or more non-limiting examples will now be described, with reference to the accompanying drawings, in which:

[0030] FIG. 1 provides a schematic block diagram for a temporal thermal gradient compensation scheme in an IMU;

[0031] FIG. 2 shows the variation of inertial sensor temperature with time in an exemplary IMU;

[0032] FIG. 3 shows the differentiated temperature measurement from a primary temperature sensor spatially associated with the inertial sensor;

[0033] FIG. 4 shows smoothing of the temporal temperature gradient output;

[0034] FIG. 5 provides a schematic block diagram for a spatial thermal gradient compensation scheme in an IMU; and

[0035] FIGS. 6a-6c show examples of some spatial locations for the secondary temperature sensors in an IMU.

DETAILED DESCRIPTION

[0036] There is seen in FIG. 1 an inertial sensor 2 arranged to output an inertial measurement to a processor 4 in an IMU. A primary temperature sensor 6 spatially associated with the inertial sensor 2 is arranged to output a temperature measurement to the processor 4. As illustrated by block 8, the inertial measurement and the temperature measurement may be used by a conventional parametric compensation scheme to determine parametric errors during operation based on stable calibration and test conditions. According to examples of the present disclosure, the processor 4 carries out additional steps using the temperature measurement output by the primary temperature sensor 6. In block 10, the temperature measurement is differentiated with respect to time so as to determine a temporal temperature gradient output. This temporal temperature gradient output is fed to block 12 where a “thermal ramp” parametric compensation scheme additionally determines a compensation for the inertial measurement, or an associated parametric error, based on the temporal temperature gradient output. Block 10 may optionally include filtering and/or smoothing of the differentiated temperature measurement. The processor 4 then provides a compensated parametric output at block 14.

[0037] FIG. 2 shows how the temperature of the inertial sensor 2 may vary in time due to thermal gradient effects in the IMU. FIGS. 3 and 4 provide examples of the differentiated temperature measurement output by block 10, either as a raw measurement of temporal temperature gradient (FIG. 3) or after smoothing e.g. using a moving average filter (FIG. 4).

[0038] There is seen in FIG. 5 an inertial sensor 2 arranged to output an inertial measurement to a processor 4 in an IMU. A primary temperature sensor 6 spatially associated with the inertial sensor 2 is arranged to output a temperature measurement to the processor 4. As illustrated by block 8, the inertial measurement and the temperature measurement may be used by a conventional parametric compensation scheme to determine parametric errors during operation based on stable calibration and test conditions. According to examples of the present disclosure, the IMU further comprises secondary temperature sensors 16a, 16b, 16c each having a different spatial location to the primary temperature sensor 6, and ideally different spatial locations to one another. Each of the secondary temperature sensors 16a, 16b, 16c is arranged to output a different temperature measurement to block 18, where the processor 4 determines a spatial temperature gradient output. Block 18 may optionally include filtering of the different temperature measurements. At block 20 the processor 4 runs a “thermal ramp” parametric compensation scheme to additionally determine a compensation for the inertial measurement, or an associated parametric error, based on the spatial temperature gradient output. The processor 4 then provides a compensated parametric output at block 22.

[0039] FIGS. 6a to 6c illustrate an inertial sensor 2 located on one side of an inertial measurement unit (IMU) 100. The inertial sensor 2, which may be a MEMS-based vibrating structure gyroscope or an accelerometer, comprises a primary temperature sensor 6 having the same spatial location. In addition to the primary temperature sensor 6, three secondary temperature sensors 16a, 16b, 16c are positioned at various different spatial locations around the inertial sensor 2. For example, the secondary temperature sensors 16a, 16b, 16c may be placed on the PCB to which the inertial sensor 2 is mounted. As seen from FIG. 6c, further secondary temperature sensors 16d, 16e may be positioned in other spatial locations on other sides of the IMU. These locations will typically be chosen on known thermal pathways that provide greatest assistance in observing the thermal gradients being applied to the IMU during operation e.g. in an unstable environment.

[0040] It will be appreciated that the two thermal ramp compensation schemes seen in FIGS. 1 and 5, respectively, may of course be combined in a single processor 4.