Model-Free Adaptive Control
of Metal Galvanizing Lines
| |
|
| • Anti-delay
and Feedforward
MFA effectively handles large or
varying delays. |
• Thickness of precious metal (tin or zinc)
is automatically controlled during product changeovers
and disturbances. |
| • Prevents over-plating. |
• Tin or zinc consumption is reduced. |
| • Prevents under-plating. |
• Number of defective parts is sharply reduced.
|
| • Increases production flexibility. |
• Production throughput is improved. |
| • Improves efficiency & productivity. |
• Full investment is returned in weeks if
not sooner. |
Speed changes due to product changeovers
cause major quality problems in tin thickness.
On the display at right, two Anti-delay
and
Feedforward
MFA controllers manipulate the
electrical current (red) quickly to tightly
control the tin thickness (green)
on both top layer and bottom layer.
|
 |
Case History:
MFA to control quality variables at Wuhan Iron &
Steel Corporation
In a rolling mill, a galvanizing process
produces tinned plating. Plating thickness is key to
product quality. Plating that is too thick or thin causes
an economic loss.
The tinning processing line of Wuhan Iron & Steel
(Group) Co. (WISCO) performs large-scale continuous,
high-speed tinned plating. Like on most plating lines,
thickness could only be controlled manually. Differences
in operator skills led inevitably to unstable product
quality.
To keep plating continuous, buffers that store strip
up to 150 meters long are arranged on both sides of
the tank. The outlet buffer stores strip when cutting
while the inlet buffer supplies strip when welding.
Thickness of the plate is manipulated by the galvanizing
current that must be calculated according to the thickness,
width and speed of strip, depending on the concentration
and temperature of the galvanizing liquid, etc. Behind
the outlet buffer, a sensor with a range of 0~15 g/m2
measures plating thickness online.
A Modicon PLC is used for logic control and a Windows
based PC with SCADA software monitors the system.
Plating thickness could only be controlled in open loop.
Large disturbances of strip speed affected thickness
even though current was adjusted based on a process
model. The buffer between the tank and the sensor creates
a large and random time delay that changes from 30 to
150 seconds and cannot be predicted. A variation in
sizes of products (over 20 types per day) causes frequent
changes in process dynamic behavior. On-site disturbances
affect product quality and also make automatic control
extremely difficult.
An Anti-Delay
MFA controller was chosen for closed
loop control of plating thickness. A Feedforward
controller was used to overcome the speed
disturbance. A CyboCon software package was installed
in the PC and connected to the Modicon PLC by using
CyboLink for Modbus, a serial communication driver.
MFA control succeeded in reaching the objectives defined
by WISCO:
• Control of plating thickness to within +/-0.5
g/ m2. MFA controllers maintain thickness within +/-0.3
g/m2 even if strip speed changes severely.
• Ease of installation and operation. Thirty minutes
after connecting to the PLC, MFA controllers for Top-Layer
and Bottom-Layer were launched and controlled plating
thickness immediately. Commissioning was completed in
two days; no maintenance is required.
• Return-on-investment is achieved in a short
period of time due to automatic control of the quality
variables.
Read
PDF File
|
