CHAPTER 5
CONCLUSION
5.1 Future of the Control System Demonstrator
The Control System Demonstrator is a device which has been created by a student for
students. In all probability, the CSD is not an optimal system. It is a first attempt at
building a working control system by a person who previously had no experience in
building control systems. That being the case, the CSD probably has room for
improvement in terms of its ability to control temperature using closed-loop feedback
control. Any student who has an interest in improving the system performance should be
encouraged to do so.
Here are a few ideas that the student may wish to explore when attempting to increase the
system’s performance:
1. Try to reduce the radiation mode of heat transfer and see if this has any affect on the
steady state response. One possible way of doing this may be to coat one side of the
light bulbs with an opaque material so that light is not transmitted directly to the face
of the RTD. See if this has any effect with respect to reducing the jittery behavior of
the bulbs at steady state.
2. Determine a way of eliminating the fact that Derivative action acts on the initial error
caused by a set-point change. The effect of this is an initial bright flash of the bulbs
when D-action is on. See if the controller output spike due to D-action acting on a
set-point change can be eliminated. This will probably require the student to alter the
LabVIEW program in some way. Be sure to make a copy of the original program.
3. Investigate whether or not a mesh screen which is placed between the light bulbs and
the RTD might be useful for mixing the air and creating more uniformity in the air
temperature. It is undesirable for the controller to act on a temperature measurement
of a hot or cold pocket in air which is not well-mixed. As a bonus, the screen may
also act as a radiation shield.
4. Replace the PID Drive subroutine VI with another routine based on a different PID
numerical algorithm. For example, the rule used to approximate the integral of the
error for the PID Drive VI is the Trapezoidal Rule. Try a numerical method for
approximating the integral of the error which has a higher accuracy.
5. Learn how to control the system with a PLC. Learn how to use the RS-Tune
software to analyze a process. Rockwell Automation’s Allen-Bradley PLCs are the
industry leader in North America (Morriss 3). As such they provide the de facto
standard that that other PLC manufacturers must conform to in terms of features and
performance.
119
Many students who will become control engineers will work with Allen-Bradley
PLCs and software. Using RS-Tune on a educational process such as the CSD will
help the student to get an early start in learning how to use the type of software
packages and computer control equipment which is currently being used throughout
industry.
6. Derive the differential equation(s) for the CSD open-loop process, and from these
equations determine the linear transfer function for the process. If possible compare
the derived transfer function to the transfer function of the process determined
experimentally with the RS-Tune software.
7. Using the experimentally or analytically determined transfer function for the CSD
open-loop process, use the root-locus and/or frequency response techniques taught in
class to determine a set of optimum PID parameters (to obtain minimum overshoot
and settling time). Using PID DRIVE subroutine in the LabVIEW program, change
the CSD’s compensator parameters to those which have been analytically determined
to be optimum. Run the CSD using the optimum controller parameters. For a setpoint
or load change, compare the actual closed-loop response to the response predicted by
the closed-loop model.
8. Determine the effect which a change in sampling time can have on the closed-loop
control of the system. Increase the sampling time and determine the result which this
has on the controllability of the system. How low must the sampling rate be before
the system becomes uncontrollable? Likewise, increase the sampling frequency to
some value greater than its default value (of 3 samples per second). Does this have
any significant effect?
5.2 Concluding Remarks
The Control System Demonstrator is a device which has been designed to help
engineering students learn more about the implementation of control systems course
principles. This report provides the student with a source of information for learning
about how electronic hardware and software may be used to construct a working
control system, as well as the programming of digital controllers and data acquisition
software for computerized closed-loop feedback control.
The CSD may be used by the course instructor to illustrate concepts of PID
control. Namely,
•
An increase in Proportional gain can decrease the steady-state error in the system
response to a set-point change or disturbance input. However, too much P gain can
cause the system to become unstable.
• The addition of Integral action can eliminate the steady-state error after a set-point
change or disturbance input is introduced, but at the cost of a slower response time.
Too much Integral action can cause instability.
• Derivative action has the ability to offset the negative effects due to integral action.
Derivative action cannot be used when a significant amount of noise is present in the
feedback signal, however. When this is the case, the controller output is erratic.
120
After many trial runs, the CSD has proven itself to be capable of consistently
demonstrating these ideas regarding PID control. In addition, the CSD meets all of its
intended specifications as summarized in Table 3.1. The total cost for the project was
less than $380, a minimal cost when considering the device’s functionality. A cost
breakdown is given in Appendix I.
In conclusion, hopefully sometime in the future Western Michigan University will
commission a control systems laboratory. A well-equipped controls laboratory would
provide students the opportunity to get hands on experience using control hardware
and software for implementing theoretical controller designs. A controls laboratory
would provide the resources to help each student to better understand the relationship
between course theory and design implementation. If the Control System
Demonstrator is well-received by students, then this may be an indicator that there is
a need for such a laboratory. Until then, it is hoped that the Control System
Demonstrator will get much use, and that it may serve its purpose of helping students
to better understand the relationship between control systems theory and
implementation.
121
APPENDIX A : LabVIEW (SCADA) Program Flowchart
START
PROGRAM MAIN
A
READ
SERIAL
PORT
READ INPUTS
FROM CONTROL
PANEL
SERIAL
PORT DATA
= 0?
false
DISPLAY DATA ON THE
THERMOMETER
B
122
true
B
B
sub-routine
IS OPEN-LOOP
SWITCH ON
AND START
BUTTON ON?
true
AUTOMATE THE
OPEN-LOOP
PROCESS
false
sub-routine
IS CLOSEDLOOP SWITCH
ON AND START
BUTTON ON?
true
AUTOMATE THE
CLOSED-LOOP PROCESS
false
DISPLAY THE DATA STORED
IN ARRAYS FOR
COMPARISON
A
A
123
Start Subroutine
"Automate the Closed-Loop
Process" Subroutine
send filter and fan status
to Stamp Data Open-Loop;
Loop: i = 0 to
simulation
time
Get,display and store
temperature data
False
A
True
Loop: j = 0 to 30
(collect 30 temp.
measurements)
i=0?
False
True
True
j < or = to
30 ?
j= 0 ?
clear
stripchart
j = j+1
False
False
i < or = to
sim time?
return to MAIN
True
True
i = sim.
time?
False
change fan meter
from 50% to 75%
i = i+1
True
count = 1
?
send the values of Proportional Gain,
Integral and Derivative control
status; set-point, filter status, count
and fan-speed to Stamp Data
Comm C-loop. From this
subroutine get temperature and
controller output data. Display this
data and store in the Count row, 0th
column of Array Global Var and
Array Global Var 2
initialize data storage arrays with
zeros; place last temp. value in 1st
row, 0th column of Array Global
Var , and 50(%) in same position in
Array Global Var 2
False
update data storage arrays with
currents readings; place data in row
0 and column Count in both Array
Global Var and Array Global Var2
These arrays store the temperature
and controller output history data
True
i = sim.
time?
True
False
signal"closed-lo
op control
deactivated"
disturbance
button on?
signal"disturban
ce deactivated";
fan 50%
False
blink: "closed-loop
control
de-activated"
A
124
blink"disturbance
activated"; sound
alarm
increase fan from
50 to 75(%)
i = i+1
Start Subroutine
"Automate the Open-Loop
Process" Subroutine
send filter and fan status
to Stamp Data Open-Loop;
Loop: i = 0 to
simulation
time
Get,display and store
temperature data
False
A
True
Loop: j = 0 to 30
(collect 30 temp.
measurements)
i=0?
j < or = to
30 ?
True
True
j= 0 ?
clear
stripchart
j = j+1
False
False
i < or = to
sim time?
return to MAIN
False
True
count = 1
?
send filter and fanspeed =75(%) data
to Stamp Data Comm O-Loop. From
this subroutine, get and display temp.
and controller output data; store data
in column i and row Count of Array
Global Var (temp) and Array Global
True
False
Var 2 (controller output)
update data storage arrays with
currents readings; place data in row
0 and column Count in both Array
Global Var and Array Global Var2
These arrays store the temperature
and controller output history data
True
i = sim.
time?
disturance status
"de-activated"
False
actuate the open-loop status
message to the control panel;
increase the fan meter display from
50 to 75%; sound alarm; display
"disturbance activated'
i = i+1
change fan meter
from 50% to 75%
i = i+1
A
125
initialize data storage arrays with
zeros; place last temp. value in 1st
row, 0th column of Array Global
Var , and 50(%) in same position in
Array Global Var 2
APPENDIX B: Control System Demonstrator Schematic
Bulb
0 - 115VAC
115VAC
Dimmer
Circuit
+- 15 VDC
power supply
0 - 10 VDC
control signal
+15 VDC
GND
9.1
1/2 W
13.5Vmax
1K
1 turn
F
A
N
Raw Temp
sensor inputs
RTD
conditioning
circuit
0 - 5 VDC
A/D serial data
A/D & D/A
circuit
STAMP II
PWM
+5 VDC Ref
+15 VDC
-15 VDC
GND
126
APPENDIX C: I/O Circuitry
A2D & D2A Circuit
0.1uF
5K
15 VDC
4
2
10K
STAMP
PWM
output
-
4.7M
+
3
10uF
Light
Dimmer
Control
1
U3
(X 2)
11
4.8K
STAMP 5VDC
CS’
STAMP pin 0
Temp Sensor 1
Conditioner
Output
10uF
CH0
Temp Sensor 2
Conditioner
Output
GND
STAMP pin 2
STAMP pin 1
127
Vcc
CLK
1K
1/4 W
CH1
Dout
GND
Din
Appendix C: Dimmer Circuit
Dimmer Schematic
128
Specifications
Supply voltage: 110V of 220-240V 50 or 60Hz
Can be used for: 24V AC
Max. load: 750W/220V 380W/110V 80W/24V
Continue adjustable from 0 to max.
Adjustable zero offset
DC control voltage between 0 en 10VDC (-1V +2V)
Maximum control current: 2.25mA at 12V
Optical isolated DC input
Isolated triac is used
Dimensions: 50x75mm
Dimmer Parts List
Label
C1
C2
C3
C4
D1 D2
D3
IC1
IC2
J1
J2
L1
R1
R2 R3 R6
R4
R5
R6
R7
R7
R8
R8
RV1
RV2
TR1
6P
8P
BT30100
H8003B
H8003P
HS5002
MR3
P8003
PV/1Z
RM3K
Artnr
5MK4.7
100J0D
.1/4/P'PHS
100N0M
1N4148
1N4007
TIL111
TEA1007
SCREW02
SCREWL02
SELFDEP
RA4K7
RA100K0
RA470K0
RA1M0
RA15K0
RA220K0
RA39K0
RC1K5
RE15K0
K220SH
M002SH
L6008L8
6P
8P
BT30100
H8003B
H8003P
HS5002
MR3
P8003
PV/1Z
RM3K
Qty
1
1
1
1
2
1
1
1
1
2
1
1
3
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
1
Description
4.7nF CAPACITOR
ELCO PRINT 100µF-25V
MKT-C 100nF/400V pitch:15mm
100nF MULTILAYER CAPACITOR
1N4148 small signal diode
1N4007 DIODE 1A-1000V
4N27 OPTO CP - VDC=1500 - CTR 30
TFK TEA1007 POWER CONTR.FOR COND.MOTORS
SCREW CONNECTOR PRINT 2 CONT. (CMM5/2
SCREWTERMINAL 2P PITCH 7.5mm 1.5mm2
DEPARASITE SELF 50UH/1 KHz - 26mOhm
RESISTOR 1/4W 4K7
RESISTOR 1/4W 100K
RESISTOR 1/4W 470K
RESISTOR 1/4W 1M
RESISTOR 1/4W 15K
RESISTOR 1/4W 220K
RESISTOR 1/4W 39K
RESISTOR 1W 1K5
RESISTOR 5W 15K
PIHER TRIM 220K SMALL HOR
PIHER TRIM 2M2 SMALL HOR
TRIAC 8A 600V Igt 10mA TO220 ISO TECCOR
6P DIL IC SOCKET 300MIL
8P DIL IC SOCKET 300MIL
BOLT 3X10mm
MANUAL
PARTSLIST
ML73/1.5NO PINS WITH TO220 MODIFICATION
NUT M3
PRINT
SCREW 2.9X6.5
LOCK WASHER M3 (DIN6798AZ)
129
APPENDIX D: RTD Sensor Circuitry
RTD-1 Conditioning Circuit
-15VDC
40.2K
1/4W
1%
10K
1/4W
1%
3
+
1K
15 turn
U1
9
1
-
2
1K
1/4W
1%
U1
10
1K
1/4W
1%
+15VDC
2.2K
1/4W
1%
10K
1/4W
1%
5
+
22K
1/4W
1%
8
13
+
1.5K
1/4W
5%
U1
7
RTD 1
5K
1/4W
1%
10K
15 turn
6
130
(X 10)
-
U1
14
+
12
1K
1/4W
1%
(X 22)
APPENDIX E: Test Program
A Simple PBASIC Test Program To Acquire A Temperature Measurement
Test Program 1
' Test Program 1
' Jake Sauer, October 2000
' This program demonstrates use of the Shiftout and Shiftin
instructions
' to communicate with an LTC1298 serial ADC. Shiftout is used to send
' setup data to the ADC; Shiftin to capture the results of the
conversion
'Declare Variables
CS
con
CLK
con
DIO_n
con
config
var
AD
var
startB
var
sglDif
var
oddSign var
msbf
var
high CS
high DIO_n
0
1
2
nib
word
config.bit0
config.bit1
config.bit2
config.bit3
' Main Routine
' Presents the results on the PC screen using the debug instruction
' to display the value of the variable AD (the digital value)
again:
gosub convert
debug "channel",DEC oddSign,": ",DEC AD,cr
debug home
goto again
' Here's where the conversion occurs. The Stamp first sends the
' config bits to the 1298, then clocks in the conversion data.
' The SHIFTOUT instruction sends data bits to pin DIO and the clock
' CLK pin. Sending the least significant bit first, it shifts out the
' four bits of the variable config. Then shiftin changes DIO to
' input and clocks in the data bits--most significant bit first,
' and shifts in 12 bits to the varaiable AD.
convert:
config = config|%1011
low CS
shiftout DIO_n,CLK,lsbfirst,[config\4]
shiftin DIO_n,CLK,msbpost,[AD\12]
high CS
return
131
APPENDIX F: LabVIEW Program and VIs
VI (subroutine) hierarchy
List of SubVIs
Serial Port Init.vi
reads data at serial port
Beep.vi
generates beep (default Windows beep sound, any wave file can be used)
Comparison Graph Array Format VI_3
stores temperature data from multiple runs in an array
Count 1 to 3
keeps track of the number of runs, and resets every fourth run
132
Comparison Graph Array Format Global VI
temporary array storage
GET DATA_Stamp Data Comm Open Loop VI_2
automates the open loop demonstration
GET DATA_Stamp Data Comm Closed Loop VI_with winup
automates the closed loop demonstration
Comparison Graph Array Format VI_4
stores data from multiple runs in an array
Comparison Graph Array Format Global VI_2
stores from multiple runs in an array
Y Minimum Global Var
controller output plot min value
Y Maximium Global Var
controller output plot max value
List of Sub-VI’s (Continued) These VIs are called by VIs in the Main Program
PID Controller Output Calculating VI
calculates what the controller output should be
Data Transmission VI
Works with Stamp to synchronize data transmission through the PC serial port
Character String Parsing VI
receives an ASCII integer from the Stamp, and converts it to decimal floating point num
Data Averaging VI
averages temperature data
133
Main 3
This is the main program,
Connector Pane
Block Diagram
134