Civil Engineering Reference
In-Depth Information
Chapter 7
Control of Electrically Assisted Forming
In this chapter, several control approaches are described for forming a metal under
an electrical current field. In addition, the approaches are demonstrated and poten-
tial applications for these control schemes are discussed. The specific examples
where closed-loop control is used to determine the process output are for constant
force forming, constant stress forming, and constant current density (CCD) form-
ing. Last, the feasibility toward model-based control (MBC) is discussed for the
For most direct current sources, the control of the output current is based off of
a supplied feed voltage to the unit. Thus, the first step in controlling current to the
process is to define the relationship between the signal voltage to the current sup-
ply and resultant output current. An example is shown in Fig.
7.1
.
The relationship can be used to derive the required signal voltage to elicit a
desired current:
V
FEED
=
I
+
47.83
809.74
(7.1)
where
V
FEED
is the feed voltage required by the power supply and
I
is the desired
direct current output.
To control the feed voltage that is supplied to the power supply, a reconfigur-
able embedded control and acquisition system such as a National Instruments (NI)
CompactRIO (cRIO) should be used to provide precise control during the forming
process. An example of GUI is given in Fig.
7.2
for constant force forming.
To allow for material forming under a constant force, a control system that
provided continuous current during EAF testing was modified to incorporate a
proportional-integral-derivative (PID) control block, where the
I
and
D
gains
were set to zero. Thus, only the proportional component was used. The GUI is
shown in Fig.
7.2
, where material dimensions are set along with the PID control-
ler gains and the desired set point. Additionally, limits were imposed such that a
