Method and device for control of a mobility device using an estimated gait trajectory

1. A method of controlling a pair of mobility devices comprising:

receiving data from at least a first sensor in a first mobility device or a second sensor in a second mobility device;

estimating a gait trajectory;

calculating a reference acceleration;

calculating a reference velocity based on the reference acceleration and a gait phase determined by a gait state machine, wherein the gait state machine classifies a user gait into three phases consisting of a double stance phase, a swing stance phase, and a mid-stance phase; and

commanding a motor and at least one wheel connected to the motor in each of the first mobility device and the second mobility device to the reference velocity.

2. The method of claim 1 , wherein at least one of the first sensor and the second sensor is an inertial measurement unit.

3. The method of claim 1 , wherein estimating a gait trajectory comprises:

identifying the gait phase as either a stance phase or a swing phase;

determining a swing velocity through integration of acceleration,

obtaining a corrected swing velocity, using a Kalman filter, by offsetting the swing velocity with an error term; and

determining a swing distance through integration of the corrected swing velocity,

wherein determining the swing velocity and determining the swing distance occur only during the swing phase.

4. The method of claim 3 , wherein identifying the gait phase comprises:

running a generalized likelihood ratio test on an array of linearized acceleration and gyro sets for a likelihood of gait phase;

comparing angular velocity in at least one dimension to an angular velocity threshold; and

comparing a current of the motor to a current threshold.

5. The method of claim 3 , wherein identifying the stance phase comprises:

comparing at least one of roll angular velocity and yaw angular velocity to a velocity threshold;

identifying whether pitch angular velocity is within a pre-determined range; and

comparing a current of the motor to a current threshold.

6. The method of claim 1 , wherein calculating a reference acceleration comprises:

normalizing a swing velocity identified in the gait trajectory by subtracting a drag constant and multiplying with a proportional gain;

determining a swing distance difference as a difference between an estimated swing distance and a baseline swing distance; and

calculating the reference acceleration as a product of the swing distance difference and the normalized swing velocity.

7. The method of claim 6 , wherein the baseline swing distance is a pre-determined value.

8. The method of claim 6 , wherein the baseline swing distance is estimated from a weighted average of previous estimated swing distances.

9. The method of claim 1 , wherein calculating a reference velocity comprises:

deriving the reference velocity from the reference acceleration, wherein the reference velocity is set to zero if the gait phase identified by the gait state machine is default or double stance.

10. The method of claim 1 , wherein the double stance phase is identified when each mobility device is in a stance phase for a period of time.

11. The method of claim 3 , wherein the swing stance phase is identified if and only if the swing distance is sufficiently large and if a swing distance difference between the first mobility device and the second mobility device is sufficiently large.

12. The method of claim 1 , wherein the mid-stance phase is identified if the first mobility device enters a stance phase and the second mobility device has not yet entered a swing phase.

13. The method of claim 1 , further comprising:

identifying a cross-leg motion from the data and reducing the reference velocity.

14. The method of claim 1 , further comprising:

identifying a stopping motion from the data and rapidly increasing an electrical current to the motor to reduce rotation of the at least one wheel.

15. The method of claim 1 , further comprising:

reducing the reference velocity when the gait state machine identifies the double stance phase and a sudden change in electrical current is detected from the motor, wherein the reference velocity is reduced proportionally to a magnitude of the sudden change in electrical current.

16. The method of claim 1 , further comprising:

preventing rotation of the at least one wheel when a magnitude of a height difference between a beginning and an end of a stride is equal to or greater than an average stair step height.

17. The method of claim 1 , further comprising:

overriding the controller using a remote.

18. The method of claim 1 , further comprising:

identifying a sudden turn when the data indicates that each of the first mobility device and the second mobility device are turning above a threshold angle; and

reducing the command velocity in proportion to a difference between the angle and a baseline angle.