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page 22

• Assume the response time constant should be 1.0 seconds, and the sampling time is 0.2 seconds, develop the final form of the controller.

τ = 1.0

T = 0.2

4.2 TRACKING

In tracking controllers we assume that the setpoint will be moving. As a result we will focus on a desired error function, instead of an overall transfer function.

To do this we must first develop a function that relates the elements of the control loop, to the desired error response,

page 23

First we look at the basic relationships for a feedback control system,

en = rn cn

cn = GcGpen

en = rn GcGpen

 

en( 1 + GcGp) = rn

Next we relate in the desired error response, and rearrange to separate the controller,

Gde

=

en

=

 

1

 

----

----------------------

 

 

 

 

rn

 

1 + GcGp

 

( 1 + GcGp)

=

1

--------

 

 

 

 

 

1

 

Gde

 

GcGp

=

 

 

 

-------- – 1

 

 

 

 

 

 

Gde

 

 

 

G

 

=

1

1 – Gde

 

------

-----------------

 

 

c

 

G

 

G

de

 

 

 

 

 

p

 

 

 

Note: The error is now treated as a position, and we want to control the error instead. A small error will correspond to a small difference between the changing input and the resulting output.

Specific controllers can be developed using the relationships used for the positioning controllers.

As an example tracking control would be needed in your hand for curved letters, positioning would result in block letters.

4.2.1 Minimum Error

• The method behind this controller is very similar to the deadbeat controller, we try to eliminate the error in a single step,

page 24

The desired error function should have a finite value at only one point in time,

e

n

= S

n

( … + 0B0

+ ATB + 0B2 + 0B3 + 0B4 + … )

 

 

 

 

en

= SnATB

(A is the largest error value)

This has to be manipulated into the desired transfer function,

Gde

=

en

=

SnATB

 

 

 

 

 

 

 

 

----

----------------

 

 

 

 

 

 

 

 

 

 

 

rn

 

 

 

rn

 

 

 

 

 

 

 

 

 

G

 

=

1

 

1 – Gde

=

1

 

rn

ATBSn

 

------

 

-----------------

------

 

---------------------------

 

c

 

G

p

 

G

de

 

 

G

p

 

ATBS

n

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

At this point we would be given or assume an input forcing function, and use it also.

• Find a controller for the transfer function given below, assuming a ramped input with a sampling time of 0.2 seconds. Use a maximum error of 0.02.

Gp( B) =

B

r( t) = 0.2t

1------------------- 0.5B

 

 

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