Method for Generating Trajectories for Motor Controlled Actuators

• US 20130035773A1
• Filed: 08/05/2011
• Published: 02/07/2013
• Est. Priority Date: 08/05/2011
• Status: Active Grant

• Current Assignee: Mitsubishi Electric Research Laboratories, Inc. (Mitsubishi Electric Corporation)
• Original Assignee: Mitsubishi Electric Research Laboratories, Inc. (Mitsubishi Electric Corporation)
• Inventors: Bortoff, Scott A., Wang, Yebin, Zhao, Yiming
• Granted Patent: US 8,700,190 B2
• Time in Patent Office: Days
• US Class Current: 700/33
• CPC Class Codes: B25J 9/1664 characterised by motion, path, trajectory planning
• CPC Class Codes: G05B 2219/40454 Max velocity, acceleration limit for workpiece and arm jerk rate as constraints
• CPC Class Codes: G05B 2219/40465 Criteria is lowest cost function, minimum work path

Abstract

A method generates trajectories for motor controlled actuators subject to dynamics, acceleration and velocity constraints. The method solves a constrained optimal control problem with dynamics, acceleration, and velocity constraints. The motor control problem is formulated as an optimal control problem using an energy cost function which is based on numerical optimization results. A solution to the two-point boundary value problem (TBVP) for the unconstrained case of the optimal control problem is obtained. The energy efficient motor control trajectory generation solver is designed for real time energy efficient trajectory generation. The solver converts a difficult multi-point boundary value problem (MBVP) associated with the state and acceleration constrained optimal control problem into an iterative solution for the TBVPs with updated boundary conditions.

Claims

1. A method for generating a trajectory for a motion controlled actuator, comprising the steps of:

initializing data for solving a two-point boundary value problem (TBVP);

solving the TBVP associated with a first cost function that represents energy consumption of a motion control system using the data and an analytical solver to obtain an analytic solution for unconstrained motor optimal control subject to boundary condition (BC);

updating the BC and iterating beginning at the solving step if an acceleration constraint is violated;

otherwise

updating the BC and iterating beginning at the solving step if a velocity constraint is violated; and

otherwise

setting the trajectory to the solution of the TBVP if the acceleration constraint and the velocity constraint are satisfied, wherein the steps are performed in a processor.

2. The method as in claim 1, further comprising:

approximating the energy consumption of the motion control system by a second cost function including a copper loss and a mechanical work of the motor.

3. The method as in claim 1, wherein the initialization further comprising:

precomputing a set of matrices based on different parameters in a motor model;

determining the BCs of TBVP;

4. The method as in claim 1, further comprising:

identifying the violation of the acceleration constraints using a tangential condition on the analytic solution.

5. The method as in claim 1 wherein the acceleration and speed constrained minimum energy problem is solved using the analytic solution to the unconstrained minimum energy problem, an analytic expression of an acceleration constrained arc, an analytic expression of a deceleration constrained arc, and an analytic expression of a speed constrained arc.

6. The method as in claim 1 wherein the junction conditions in the multi-point boundary value Problem (MBVP) associated with the acceleration constrained problem are converted to equivalent tangent conditions.

7. The method as in claim 5 wherein switching times for exiting the acceleration constrained arc and for entering the deceleration constrained arc are updated to guarantee the convergence of the switching times to the optimal values.

8. The method as in claim 1 to identify the violation of velocity constraint by checking the violation at a single point only.

9. A method of claim 6 wherein the MBVP is solved by solving a converging series of TBVPs.

10. The method as in claim 9 further comprising reducing the dimension of the TBVP by utilizing the structures the optimal solutions, wherein the structures are obtained analytically using an optimal control theory.

11. The method as in claim 10 further comprising:

simplifying the MBVP with both velocity and acceleration constraints by removing the velocity constraint, and forming another equivalent problem with the acceleration constraint only.

12. The method as in claim 10 further comprising:

solving the acceleration and velocity constrained minimum energy motor control problem by solving acceleration constrained minimum energy motor control problems iteratively, and recovering the solution to the acceleration and velocity constrained minimum energy motor control problem using the iteration result.

13. The method as in claim 10 further comprising:

combining a Newton'"'"'s method and the solver for the acceleration constrained minimum energy motor control problem to compute the optimal switching times for entering and exiting the speed constrained arc.

14. A method for producing a minimum energy trajectory for motor control without going through the online numerical integration process. Rather, the integration is accomplished by matrix operations, which are inevitable to obtain the control time history. The matrices used in these matrix operations are computed off-line and stored for each time in a preselected time grid. Such a storage can be reused each time a new trajectory is computed, hence, this off-line computation arrangement help with reducing the amount of on-line computation.