26.404 Process Control: Project 2
Study of Closed Loop Control in a Simple Plastics Process
Abdul Moussaoui, Abderrahim Fakiri, Daniel Brooks
Plastics Engineering Department, University of Massachusetts Lowell
Summary:
The closed loop control was studied closely using a simple Thermal Lithographic credit card
Imprinter. The imprinter was controlled via 404 controller program created by the professor on
Lab View Controller 5 software. A polycarbonate (PC) card was stamped on the imprinter using
parameters set to create optimal replication of a selected bump feature on the card. Set-points
were used to control the temperature of the upper and lower brass stamps (independently), as
well as the stamp pressure force applied by a pneumatic cylinder to press the card into the hot
lower stamp containing the stamp pattern. Proportional Integral Derivative (PID) gains were then
adjusted to optimize the control signal while minimizing the average error between the set-points
and observed values.
The final bump replication for the two best resulting cards was as follows:
Card#: 26404_2009-12-17-23-31-21
Bump 1: 0.47mm
Bump 2: 0.40mm
Card#: 26404_2009-12-17-20-48-52
Bump 1: 0.43mm
Bump 2: 0.35mm
The final average error values obtained for the two best resulting cards were as follows:
Card #: 26404_2009-12-17-23-31-21
ave_e_Upper =
0.0405
ave_e_Lower =
0.0429
ave_e_Force =
0.0490
grade =
95.5890
Card #: 26404_2009-12-17-20-48-52
ave_e_Upper =
0.0456
ave_e_Lower =
0.0566
ave_e_Force =
0.0555
grade =
94.7422
Equipment and Materials
Polycarbonate cards were cut from 1/8in sheet to fit the card Lithographic imprinter
stamp cavity.
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The user interface for this closed loop controlled process was provided by 404 Controller
Program created using Lab View Controller 5 software, running on a computer system adjacent
and interfaced to the stamping machine.
The Lithographic stamping mold used in this project and detailed in Figure 1 is made of
machined brass plates. Upper and lower brass stamps have five heater cartridges with one
thermocouple per stamp located placed between heater cartridges holes. The cavity has 0.5mm
deep impressions where the bump replication on the stamped cards will be measure from.
Thermocouple
Bump 1
(Negative)
Bump
Bump22
(Negative)
(negative)
Heater
Cartridges
Figure 1: Lower Stamp Plate with features of note.
Experimental approach
Every aspect of machine control was done through the 404 Controller Program located on
the systems desktop. Set-point inputs were entered into the setpoints file found in the Temp
folder also on the desktop. The setpoints file was written in WordPad with values written as:
time, upper stamp temperature, lower stamp temperature, force of air cylinder. Multiple rows of
records correlated to time points along the course of the process. Kp, KI, and KD values were
adjusted to control the Control signal as a function of error for upper and lower stamp
temperatures as well as the pneumatic air cylinder.
Set-point parameters were tested in trial runs without a card in the stamp in order to
qualify the process. However, before every stamping process (and trial stamping process), a
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temperature pre-set profile was run in order to increase the stamp temperature for an
advantageous process cycle time. A temperature pre-set profile is detailed in Table 1, with the
temperature set gain values and heating profile shown in Figure 2.
Table 1: Set Temperature Setpoints
Upper Stamp Lower Stamp Pressing
Time Temperature Temperature Force (lbf)
0
150
120
0
500
150
120
500
Figure 2: Temperature Preset Gains, and Heating Profile.
Once the temperature preset timed out at 500 seconds the mold would open, the
emergency stop was activated and a card was fitted into the cavity. Processing parameters that
were saved in an alternative location for the purpose of record keeping and continual tweaking
were then pasted and saved in the setpoints file. The run button on the stamping machine could
then be pressed, connecting it to the graphical interface on the computer monitor. Gain values
were entered as desired and “run” selected on the user interface.
The processing parameter setpoints used for to produce the best cards are expressed in
Table 2, with entered gains and a resulting process profile snapshot shown in Figure 2.
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Table 2: Stamping Process Setpoints
Upper Stamp
Temperature
Time
0
50
100
150
200
250
300
400
500
600
145
161.25
177.5
193.75
210
191.75
172.5
135
97.5
60
Lower Stamp
Pressing Force
Temperature
(lbf)
125
200
170
300
200
400
210
800
210
800
191.75
1000
172
1100
135
1200
97.5
1300
60
1400
Figure 3: Gains and Process Profile for a Stamping Process
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At the end of the stamping process the mold automatically opens. After pressing the
emergency stop, the card is removed. The “.csv” file automatically saved in the “Temp” file on
the desktop, and was saved on a thumb drive for additional analysis.
Once allowed to cool to room temperature, bumps 1 and 2 were measure to gauge
replication. The “.csv” file saved from the process was analyzed on Mat Lab using a program
written and supplied by the professor called “Analyze_Output.m”. This program takes the data
in the “.csv” file and calculates a portion of the project grade based on the average error between
the observed process feedback and the setpoints.
Analysis & Results
The initial processing setpoints resulted in adequate bump replication. However, not
understanding how the set-point conformance would be calculated to determine a portion of the
grade, the setpoints were not set optimally. In a futile effort to promote faster heating the
temperature setpoints were set directly to the desired optimum stamp transfer temperature. Then
once the set temperature was reached, temperature set-point was rapidly dropped, intending to
encourage cooling. However, as may be expected the lack of set-point conformance to observed
temperature data, once analyzed by the “Analyze_Output.m” program did not yield an acceptable
grade. Figure 4 reflects the set-point strategy, with the set-point values towards the right side of
the figure.
Figure 4: Initial Set-point Strategy
The resulting average errors and grade resulting from this process would have been as follows:
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ave_e_Upper = 0.6827
ave_e_Lower = 0.6783
ave_e_Force = 0.0621
grade = 52.5646
Although the set-point conformance is not acceptable, notice that the gains seem to be
adequate. It should be noted that the Lower stamp temperature was capable of heating much
faster than the Upper stamp temperature. At first it would appear that this is due to greater
volume of brass in the upper stamp (reference Figure 1), however heat transfer calculations do
not support this reasoning.
Notice that in an effort to help the upper stamp to come up to temperature faster it was
pre-heated to 150deg Celsius while the lower stamp was pre-heated only to 120deg Celsius.
Note that it was acceptable to raise the temperature on the upper stamp because does not need to
be touched by the operator in order to place and locate the card.
Heat transfer calculations were made assuming 5 heaters supplying 100W each to the
upper stamp, and the same to the lower stamp. Change in stamp temperatures were calculated
using the top temperature set-point for the stamps minus the preset temperatures for each stamp.
Equation 1 demonstrates how the theoretical heating time may be calculated. Table 3 is a
resulting calculations table for the top stamp versus that for the bottom stamp using the
previously stated assumptions. Volume of each stamp was also calculated for each stamp
disregarding the absence of material in the empty heater bore holes. Other constants used in this
calculation were the density (ρ) of brass (8.75 kg/m3), and the specific heat (Cp = 0.38).
𝑃=
𝑀𝐶𝑝 ∆𝑇
∆𝑡 =
∆𝑡
𝑀𝐶𝑝 ∆𝑇
𝑃
Eqn. 1
Table 3: Calculating Theoretical Heating Time for Upper and Lower Stamps
Top
Bottom
Volume
[m3]
0.0001794
0.00015032
ρ [kg/m3]
8500
M [kg]=ρ x
V
1.5249
1.27772
Cp [J/kg K]
380
Δ Temp
[K]
62
85
Power
[W]
500
500
Theoretical Heating
Time [sec]
71.853288
82.540712
As may be observed despite the additional material of upper stamp, it should heat to
210degrees faster than the lower stamp if it is given a pre-set temperature head start.
It was determined after Project 1 that two heaters may have been dead. The new
theoretical heating times are calculated in Table 4, assuming that only three heaters are supplying
the upper stamp at 100W a piece.
Top
Bottom
Volume
[m3]
0.0001794
0.00015032
ρ [kg/m3]
8500
M [kg]=ρ x
V
1.5249
1.27772
6
Cp [J/kg K]
380
Δ Temp
[K]
62
85
Power
[W]
300
500
Theoretical Heating
Time [sec]
119.75548
82.540712
These theoretical results appear more accurate. Thought the heating time is low, it may
be accounted for by the lower temperature environment of the room. Heat is being dissipated
through the sides of the stamps into the room, as well as into and through the insulation plate into
which the cooling path is cut. The fact of this heat dissipation may be proven upon looking at
the temperature of the upper platen. The upper platen is the aluminum block supporting the
brass stamp and has a thermocouple in the back. The observed temperature profile is observed in
the user interface and shows us that it is absorbing heat during the heating cycle. Although only
the top platen temperature is measured it can be inferred that the bottom platen is likewise
absorbing heat. These losses of energy can account for the longer than expected heating times in
the process.
Emphasis for this closed loop control process was put on optimizing the bump
replication, and set-point conformance as was calculated by the “Analyze_Output” program
discussed earlier. To select the best card in these metrics a Pareto optimal set was constructed,
Figure 5.
Figure 5: Pareto Optimal Set
Ineffective heating of the upper platen made it clear that it was the limiting variable in the
process. It was here decided that the upper stamp would be heated as quickly as possible and the
setpoints adjusted so that cooling began as soon as the intended top temperature was reached.
Lower stamp temperature was allowed to come up to temperature more quickly so that its earlier
high temperature would encourage heating of the upper platen. The higher temperature of the
lower stamp earlier only aids in the softening of the bottom face of the card, promoting stamp
feature replication. The Kp and KI for the lower stamp were chosen such that it would have
adequate heating and reach a steady state process until the predicted process. Once the upper
platen reached temperature the cooling could be turned on, and both stamps allowed to cool. The
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setpoints were optimized by calculating the slopes of temperature ramp-up and cool down for the
upper and lower stamp and applying them to the set-point profile. Unlike heating, the lower
stamp has only a small cooling advantage due to its slightly smaller volume.
Card 26404_2009-12-17-23-31-21 seen below in Figure 6 resulted in both the best
bump replication, and set-point conformance. The conformance could have been even better if it
had been cooled to only 500degreas, as the heat dissipation capability of the cooling air reduces
as the stamp temperature reduces.
Figure 6: Profile for Card No. 26404_2009-12-17-23-31-21
The bump replication was:
Card#: 26404_2009-12-17-23-31-21
Bump 1: 0.47mm
Bump 2: 0.40mm
The analog output program was run with the below graphs showing set-points, observed
process and signal process Card No. 26404_2009-12-17-23-31-21. The output grade for this
card was 95.5890.
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ave_e_Upper = 0.0405
ave_e_Lower = 0.0429
ave_e_Force = 0.0490
The second best process was displayed by Card No. 26404_2009-12-17-20-48-52 shown
in Figure 7. It can be recognized that a rough temperature ram-up and cool down were
programmed in the set-point resulting in a rough observed temperature profile, particularly in the
upper stamp temperature. It is after this card that the technique of setting the temperature based
on a calculated predicted.
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Figure 7: Profile for Card No. 26404_2009-12-17-20-48-52
The bump replication was:
Card#: 26404_2009-12-17-20-48-52
Bump 1: 0.43mm
Bump 2: 0.35mm
The analog output program was run with the below graphs showing set-points, observed
processes and the control signal in each process for Card No. 26404_2009-12-17-20-48-52. The
output grade for this card was 94.7422.
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ave_e_Upper =
0.0456
ave_e_Force =
0.0555
ave_e_Lower = 0.0566
Project Recommendations
Bump tended to come out lop-sided. It is expected that this is caused by air being trapped
in the bump negative. If some kind of venting could be implemented better bump replication
may be achieved. Venting along the edges may prevent flash from causing separation of the
stamps allowing for more accurate dimensions. It is also suggested that a digital temperature
reader be available, so that temperature can be known without having to run a closed loop
process.
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