ISSN 1068-798X, Russian Engineering Research, 2017, Vol. 37, No. 7, pp. 651–654. © Allerton Press, Inc., 2017.
Original Russian Text © A.V. Yudin, A.V. Baranov, 2017, published in STIN, 2017, No. 1, pp. 37–40.
Optimal Automatic Maintenance of the Arc Voltage
in the Plasma Cutting of Metal
A. V. Yudin* and A. V. Baranov
Solov’ev Rybinsk State Aviation-Engineering University, Rybinsk, Russia
Abstract—A method is proposed for regulating the unit that automatically maintains the arc voltage in the
plasma cutting of metallic sheet. The method is based on optimal control of a mechanical system modeled by
means of a second-order differential equation. This permits faster correction of the control error and hence
increases the cutting speed.
Keywords: plasma cutting, regulator, optimization, cutting speed
Plasma cutting of metal is undergoing rapid devel-
opment and broad acceptance at present. It is cur-
rently more efficient and economical than any other
method of metal cutting. Its main advantage is high
cutting speed. The heat source quickly ensuring the
required temperature is an electric arc, as a rule. The
distance between the workpiece and the cutter tip is
critical to the cutting quality and to the operating time
of the components. Even small changes in the cutter
height may affect the angularity of the cutting surface.
The cutter height may be tracked by three basic
methods: mechanically; by means of bearings; or in
terms of the arc voltage. In the mechanical method, a
motor controls motion over the vertical Z axis to the
height specified in machining, without taking account
of the curvature of the metal.
To this system, we may add a sensor for locating
the metal. Then, in each arc cycle, the nozzle will
correspond to a new zero coordinate on the Z axis. A
deficiency of this method is that, with considerable
deformation of the metal, the burner may advance
into the metal during transit or cutting, resulting in
cutter fracture. This method also results in poor qual-
ity of the cut.
The bearing method is similar to the mechanical
approach, except that the cutter is prevented from
contact with the metal by a supporting bearing. With
change in metal height, the bearing raises the cutter,
and the cutting height of the metal remains
unchanged. A deficiency of this method is that, if the
part descends, the bearing may collide with it, thereby
resulting in displacement of the XY coordinates or
fracture of the Z axis.
Today, plasma-cutting machines often track the
cutter height by means of the arc voltage. This system
automatically maintains the cutter height by ensuring
constant arc voltage. Essentially, in this approach, the
voltage between the cutter electrode and the sheet
being cut is measured.
As we know, the arc voltage is directly related to its
length—that is, the distance to the workpiece . As
the cutter is raised above the sheet, the arc length
increases and hence the arc voltage is increased. With
decrease in distance between the cutter and the sheet,
the arc length is reduced and hence the arc voltage
falls. The electrode’s feedback loop compares the
actual voltage with the specified value and then sends
a signal to the motor, which raises or lowers the cutters
so as to maintain constant voltage. The operator trans-
mits the specified voltage to the control module
through the regulator or through the numerical con-
trol system of the cutting machine.
In practice, the unit automatically maintaining the
arc voltage does not always ensure the required stabil-
ity of the cutter position. Such errors are especially
likely when the sheet to be cut is curved and when high
cutting speeds are employed, since the rate at which
the drive processes the incoming command is compa-
rable with the rate at which the distance between the
sheet and cutter is changing. Another difficult opera-
tion is passage through a right angle without stopping.
When the cutter speed is reduced at the right angle, the
cutting spot will be enlarged unless the cutter height is
increased. A similar problem arises in milling .
To improve the efficiency and precision in the con-
tour machining of parts with a complex profile, more
thorough use of control software was proposed in :
specifically, the use of a correction system for the ini-
tial position and of adaptive control systems.