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page 763

42. MOTION PLANNING AND TRAJECTORY CONTROL

• When there are multiple control loops that must be controlled as a group we must add higher level control techniques to plan motions and then guide single controllers.

42.1 TRAJECTORY CONTROL

• Given a set of points, we must move at given speeds.

42.1.1 Resolved Rate Motion Control

When the robot motion is important (such as welding applications), the resolved rate method may be useful

This method requires a good model of the manipulator.

The figure below shows an incremental interpolator, based on resolved rate motion control.

Xd'

+

 

 

J–1( θ )

θ 'd

controller

θ ,ω

 

 

 

 

 

 

and robot

 

 

 

 

 

 

 

-

 

 

 

 

 

(PID, etc.)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

X'

J( θ )

42.1.2 Cartesian Motion System

• The figure below illustrates a system for making motions in cartesian space (straight line).

page 764

Xd, Xd', Xd''

 

θ d, θ d', θ d''

 

θ ,θ ', θ ''

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

inverse

 

 

joint control

 

 

 

 

 

 

kinematics

 

 

of robot

 

 

 

 

 

 

 

 

 

 

 

X, X', X''

forward kinematics

42.1.3 Model Reference Adaptive Control (MRAC)

• To compensate for overly complex, or changing systems we can use an adaptive controller

 

 

inverse dynamic

 

τ m

 

 

 

 

 

 

 

 

 

 

model

 

 

 

 

 

 

adaptation

 

 

 

 

 

 

equations

 

 

 

 

 

 

 

 

+

θ ,θ

', θ ''

 

 

 

 

 

 

 

+

controller

τ c

τ

joint controllers

 

d, θ

d', θ d''

e, ed', ed''

 

+

and robot

 

 

 

 

-

 

 

 

 

42.1.4 Digital Control System

• It is more common to use computers to control operations

page 765

Control Program

updated every Ta seconds

 

 

actuator 1

D/A

actuator 2

Computer

etc.

 

sensor 1

A/D

sensor 2

sensor 3

 

updated every Tb seconds

etc.

 

The computer must examine sensors, then decide how to change outputs to actuators. But, computers can only do one thing at once, so the approach is limited by CPU speed.

Note the controlling computer still needs a control algorithm such as PID, resolved rate, MRAC, etc.

e.g. to control a simple robotic arm with proportional control

page 766

θ desired

Start

 

 

 

read position from sensor

 

 

 

 

 

θ actual = 36.0Vactual

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

subtract for error

 

 

 

 

θ

error = θ desired θ actual

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

multiply for torque

 

 

 

 

 

τ = 100.0θ error

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

write to actuator

 

 

 

 

 

Vtorque = 0.5τ

 

 

 

 

 

 

 

 

 

 

 

 

check for

 

 

 

 

 

error or stop

 

no problem

e.g., θ actual < 180

 

turn off torque

stop

digital

 

voltage

 

number

 

 

 

A/D

 

 

 

 

 

 

 

from sensor

 

 

 

 

 

 

 

 

 

 

(e.g. potentiometer)

 

 

 

 

digital

 

 

 

 

 

 

 

 

 

 

 

 

number

 

D/A

voltage

 

 

 

 

 

 

 

 

 

 

 

to actuator

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

e.g.,

 

(e.g., motor)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

θ

desired

θ actual

τ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

time

 

 

 

 

 

 

 

 

 

(sec)

(deg)

(deg)

(Nm)

 

 

 

 

 

 

 

 

 

 

0.0

 

10

0.0

1000

 

 

 

 

 

 

 

 

 

 

0.1

 

10

0.5

950

0.2

 

10

0.95

905

0.3

 

10

1.40

860

0.4

 

10

1.75

850

0.510 etc.... etc...

0.610

0.710

0.810

0.910

1.010

1.110

Some sensors, such as encoders can return binary numbers to the computer, instead of analog voltages, this leads to more accurate, noise resistant position measurements.

The main functions of the control computer are,

1.plans and interpolates a path from the start, to the endpoint in world coordinates.

2.Transforms path points from world to joint coordinates

3.Receives a signal about a completed move, and signals next moves on a path.

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