Initializing an inertial sensor using soft constraints and penalty functions

• US 8,913,134 B2
• Filed: 04/22/2014
• Issued: 12/16/2014
• Est. Priority Date: 01/17/2012
• Status: Active Grant

• Current Assignee: Newlight Capital, LLC
• Original Assignee: Blast Motion Incorporated
• Inventors: Goree, John, Bose, Bhaskar, Gupta, Piyush
• Primary Examiner(s): BAILEY, FREDERICK D
• Application Number:
  US14/258,291
• Publication Number:
  US 20140229138A1
• Time in Patent Office: 238 Days
• Field of Search: None
• US Class Current: 348/157
• CPC Class Codes: A63F 13/00 Video games, i.e. games using an electronically generated display having two or more dimensions gaming systems which provide a financial reward G07F17/32
• CPC Class Codes: G01C 25/005 initial alignment, calibration or starting-up of inertial devices
• CPC Class Codes: G01P 15/02 by making use of inertia forces using solid seismic massesG01P15/14 takes precedence
• CPC Class Codes: G06Q 10/0833 Tracking
• CPC Class Codes: H04M 1/72412 using two-way short-range wireless interfaces
• CPC Class Codes: H04M 2250/12 including a sensor for measuring a physical value, e.g. temperature or motion telephones including GPS signal receivers H04M2250/10
• CPC Class Codes: H04N 7/18 Closed-circuit television [CCTV] systems, i.e. systems in which the video signal is not broadcast

Abstract

An initialization method for an inertial sensor that estimates starting orientation and velocity without requiring the sensor to start at rest or in a well-known location or orientation. Initialization uses patterns of motion encoded as a set of soft constraints that are expected to hold approximately during an initialization period. Penalty metrics are defined to measure the deviation of calculated motion trajectories from the soft constraints. Differential equations of motion for an inertial sensor are solved with the initial conditions as variables; the initial conditions that minimize the penalty metrics are used as estimates for the actual initial conditions of the sensor. Soft constraints and penalty metrics for a specific application are chosen based on the types of motion patterns expected for this application. Illustrative cases include applications with relatively little movement during initialization, and applications with approximately periodic motion during initialization.

Claims

1. A method for determining an estimate of the initial orientation Q₀ and initial velocity v₀ of an inertial sensor, comprising the steps:

collecting sample data from an inertial sensor over a capture time interval [t_i, t_f] that includes an activity of interest, said sample data including accelerometer data s(t) and rate gyro data w(t);

selecting an initialization interval [t₀, t_n]⊂

[t_i, t_f] within said capture time interval;

defining a set of soft constraints {C_i} on the position r(t) and velocity v(t) of said inertial sensor during said initialization interval, where each C_i⊂

{r;

[t₀, t_n]→

³}×

{v;

[t₀, t_n]→

³}, where said soft constraints represent approximations of typical patterns of position and velocity trajectories for said activity of interest during said initialization interval;

defining penalty metrics m_j(r(t), v(t)) to quantify the deviation of calculated trajectories of said position r(t) and said velocity v(t) from said soft constraints {C_i}; and

calculating estimates of the initial orientation Q₀=Q(t₀) and initial velocity v₀=v(t₀) of said inertial sensor at the start of said initialization interval to minimize said penalty metrics when said initial orientation and initial velocity are used as initial conditions to integrate said sample data into orientation, position and velocity over said initialization interval via the coupled differential equations

2. The method of claim 1, further comprising:

including among said soft constraints a requirement that the change in position between the start and end of said initialization interval be zero, r(t_n)=r(t₀);

setting said estimate of initial velocity to zero, v(t₀)=0; and

including among said penalty metrics the absolute value of the difference in position between the end of said initialization interval and the start of said initialization interval, m(r(t),v(t))=|r(t_n)−

r(t₀)|.

3. The method of claim 1, further comprising:

including among soft constraints a requirement that the position and velocity of said inertial sensor be periodic during said initialization interval;

calculating frequency domain signals for each axis of the rate gyro by applying a discrete Fourier transform to the corresponding axis of the rate gyro data time series during said initialization interval,

Ω

_x(f)=DFT(w_x(t)),Ω

_y(f)=DFT(w_y(t)),Ω

_z(f)=DFT(w_z(t);

selecting the dominant frequency f_max as the frequency with the largest transform magnitude across the 3 axes, f_max=argmax(max(|Ω

_x(f)|, |Ω

_y(f)|, |Ω

_z(f)|)), and calculating the dominant period T from said dominant frequency as T=1/f_max;

including among said penalty metrics a velocity penalty metric that is the absolute value of the change in velocity over one period of motion, m_v(r(t), v(t))=|v(t+T)−

v(t)|, and a position penalty metric that is the absolute value of the change in position over one period of motion, m_r(r(t), v(t))=|r(t+T)−

r(t)|;

selecting a designated period within said initialization interval, [t₁, t₁+T]⊂

[t₀, t_n], for calculation of said estimates of initial velocity and initial orientation;

calculating said estimate of initial orientation by minimizing said velocity penalty metric over said designated period; and

calculating said estimate of initial velocity by minimizing said position penalty metric over said designated period, using said estimate of initial orientation.