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page 25

4.3 DISTURBANCE RESISTANT

In real systems we expect that certain events will occur that are not part of our system model.

In this case we assume that the system control is happening as expected, and we add in a new disturbance input.

The block diagram below shows one of these systems, with a disturbance injected between the controller and the process.

dn

 

 

+

 

 

 

 

 

 

cn

 

 

 

Gp

rn

+

en

+

mn

 

 

 

Gc

 

 

-

 

If we neglect the effects of the setpoint, we can rearrange the loop as shown below,

dn

+

 

 

mn

 

 

 

cn

 

 

 

Gp

 

 

Note: we can do this

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

because the sys-

 

 

 

-

 

 

 

 

 

 

 

 

tem is linear.

 

 

 

 

mn

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Gc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Notice that in the above form we are reducing the problem by finding differences (basically a partial differential solution) which will be a good approximation when the disturbance is not too fast or large.

We can develop an equation for the controller, based on the desired system response to the disturbance,

page 26

Gdd =

cn

Gp

--------dn

= ----------------------

 

1 + GcGp

Gc

Gp Gdd

= ---------------------

 

 

GpGdd

• The closed form expression can be calculated by replacing the desired transfer function,

Gdd

cn

 

 

 

 

 

 

 

 

 

= --------

 

 

 

 

 

 

 

 

 

 

 

 

dn

 

 

 

cn

 

 

 

 

 

 

 

 

Gp

 

 

– ∆ cn

 

G

p

G

dd =

--------d

n =

dnG

p

Gc

 

 

= ---------------------

 

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

 

 

 

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

 

 

GpGdd

 

cn

Gpcn

 

 

 

 

 

 

--------

 

 

 

 

 

 

 

 

Gp

d

n

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• These systems are often called regulators, and can be used when a system is subject to unexpected noise. Examples of possible applications would include plumbing systems, electrical power supplies, etc.

4.3.1 Disturbance Minimization

• We can use an approach similar to the deadbeat controller, but we still need to know the type of disturbance expected to develop a controller.

cn = Sn( 0B0 + AB1 + 0B2 + 0B3 + 0B4 + … ) = ABSn

• Consider a case where the disturbance is a step function

page 27

Given the example transfer function and input disturbance,

 

Gp( B)

=

 

 

B

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

0

 

1

 

2

 

3

 

 

 

 

 

1

 

 

dn = ( DB + DB + DB + DB + … ) Sn =

 

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

Sn

 

1 – B

 

Next we assume

 

 

 

 

 

 

 

 

 

 

 

 

 

 

c

n

= S

n

( 0B0 + 5B1

+ 0B2 + 0B3 + 0B4 + … ) = 5BS

n

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

And these are used to calculate the controller equation

 

 

 

 

 

 

 

 

 

 

 

 

1

 

 

 

B

 

– 5BSn

 

 

Gc

=

d---------------------------nGp – ∆ cn =

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

1----------- B

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

=

1------------------------------------------------------ 5( 1 – 0.5B) ( 1 – B)

 

 

 

 

 

Gpcn

 

 

 

 

 

 

B

 

5BSn

 

 

 

5B( 1 – B)

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

Gc

=

1 – 5( 1 – 0.5B) ( 1 – B)

=

1 – 5 + 7.5B – 2.5B2

 

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

 

 

5B( 1 – B)

 

 

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

 

5B – 5B

2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Now we put this in discrete equation form,

 

 

 

 

 

 

 

mn

=

1 – 5 + 7.5B – 2.5B2

 

 

 

 

 

 

 

 

 

 

------en

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

 

 

5B – 5B2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5mn – 1 – 5mn – 2 = – 4en + 7.5en – 1 – 2.5en – 2

 

 

5mn – 1

= 5mn – 2 – 4en + 7.5en – 1 – 2.5en – 2

 

 

 

 

mn

= mn – 1 – 0.8en + 1 + 1.5en – 0.5en – 1

 

 

 

 

Note: this controller has a reference to an ‘n+1’ error term. This term refers to an error value that is in the future. So.... unless you have a time machine, this controller is not REALIZABLE.

• We can examine the previous controller for stability as well,

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