Electronics - Devices

Nov 15, 2013 (4 years and 5 months ago)

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12/6/04

BAE 3023

1

Lecture 14 Implementation of a PID controller

BAE 5030
-

003

Fall 2004

Instructor: Marvin Stone

Biosystems and Agricultural Engineering

Oklahoma State University

12/6/04

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2

Goals for Class Today

(
CAN Implementation
)

Zigbee and 802.14.5

(Kyle)

PID implementation
(Stone)

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Elements of a feedback control system

Review elements and
variables

Gc
G
2
G
3
Error

Manipulated
Variable
D
Controlled
Variable

out
+
+
G
1

out
-
+

out

in
Setpoint

set

out

out_measured
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)
1
(
)
1
(
2
3
2
2
3
1
G
G
G
G
G
G
G
G
G
c
c
set
c
in
out

)
(
_
measured
out
set
c
G
D

2
1
DG
G
in
out

3
_
G
out
measured
out

Output (

out
) is readily calculated as a function of:

in
) and

Setpoint (

set
)

Manipulation is a simple function of the controller TF and error.

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Digital form of a classic feedback controlled system

If sampling rate is fast and holds are employed, this
system approaches the analog system

Gc
G
2
G
3
Error

Manipulated
Variable
D* D
Controlled
Variable

out
+
+
G
1

out
-
+

out

in
Setpoint

set

out

out_measured

out_measured
*
Computer based controller
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One of the conventional models used to express a PID controller is:

dt
de
edt
e
K
M
d
t
t
i
c

0
1
rate
Derivitive
rate
Reset
signal
Error
e
gain
Controller
K
on
Manipulati
M
d
i
c

Time Domain PID Controller Equation

Where:

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Derivitive Form of a PID Controller

A convenient way to implement this equation in a controller is as the
derivative of manipulation known as the velocity form of the equation
as shown below:

2
2
dt
e
d
e
dt
de
K
dt
dM
d
i
c

In a practical system this equation will work well and does not require
-
state references, but eliminating the

i

and

d

term
completely results in:

dt
de
K
dt
dM
c
de
K
dM
c

or,

This equation has no positional reference and error accumulation is a
problem. Use velocity form only for PI or PID modes.

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Conversion of the DE to a Difference Equation

To begin the conversion of the PID equation to a difference
equation, the equation is multiplied by
dt
.

dt
de
d
edt
de
K
dM
d
i
c

Note that since
M

is a differential and
e
ss

is zero, this equation
conveniently applies to the absolute variables as well as the
deviation variables.

For small
D
t
, the equation can be approximated as:

D
D
D

D

D

D
t
t
K
m
d
i
c

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Representation with Discrete Time Variables

Each of the differences (
D
⤠捡渠扥⁥b灲敳獥搠慳摩獣牥瑥⁶慬略a潦o

m

and

) at the times 0, 1, and 2 as shown
below:

t
0
t
2
t
1
M

1

2

0
M
0
M
1
M
2
The equation can be simplified with the assumption that
D
t

is
constant:

D
D
D

D

D

D

t
t
K
m
d
i
c
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Discrete form of PID controller

Replacement of the differences (
D
⤠)楴h瑨攠e楳捲i瑥

v慲楡扬敳e⠠
m and

) results in:

D

D
D

D

1
2
2
1
2
1
2
)
(

t
t
K
m
m
d
i
c

D

D

0
1
1
2
2
1
2
1
2
)
(

t
t
K
m
m
d
i
c

D

D

0
1
2
2
1
2
1
2
2
)
(

t
t
K
m
m
d
i
c
Note that
D

D
t
varies, the equations
should be derived with that in mind.

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Discrete form of PID controller

This equation can be solved for the current manipulation,
m
2
, in terms
of values known at time
t
2
:
m
1

,

e
2

,
e
1
, and
e
0
.

The other parameters in the equation are constants.

D

D

D

D

0
1
2
1
2
2
1
1

t
t
t
t
K
m
m
d
d
d
i
c
Where
C
1
,
C
2
, and
C
3

are constants and the current manipulation is
expressed in terms of known values, the current and past errors.

0
3
1
2
2
1
1
2

c
c
c
K
m
m
c

or,

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Translation of PID Equation into Algorithm

This equation may be translated directly into a computer language, for
example:

m2 = m1 + k*(C1*e2

C2*e1 + C3*e0);

Within a computer program, the current error

is calculated from the current measurement of

the controlled variable and the setpoint, for example:

e2 = T_setpoint

T_measured;

The current manipulation
m2

is then computed using the previous
controller equation, and finally, at the end of the time step, each of the
variables is shifted forward for the next calculation.

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Translation of PID Equation into Algorithm

For example in C, the code might look like:

measure_and_manipulate() //Call once per delta T

begin

T_measured = measure_T(); //Get the measured temperature

e2 = T_setpoint

T_measured //Calculate the current error

m2 = m1 + k*(C1*e2

C2*e1 + C3*e0); //Calculate the manipulation

set_manupilation(m2); //Output the manipulation

e0 = e1; //Shift the error and manipulation

e1 = e2; //forward one time step

m1= m2;

end;

Note that the time step
D
t

is controlled by the time required to execute
the loop. C1,C2 and C3 are all functions of
D
t
. The equation will
probably be executed as floats! (Or very special care must be taken
with scaling.

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Assignment

Complete CAN message demo

Turn in course portfolio by 5:00 PM Wednesday Dec.
8th