Control system retrofits can save manufacturers up to 50
percent of the cost of a new machine.
Most older hydraulic machines, especially press equipment, use
limit switches and pressure limiters to control two-position
hydraulic valves. These valves are often called “bang-bang” valves
because of the shock and noise that result from the coarse
back-and-forth cylinder operation. Adapting these machines so that
they can manufacture new products requires adjusting pressure
limits with knobs and moving the physical location of the limit
switches. This takes time and operator skill.
With the marketplace moving to low-volume production, machine
changeover times and the variability of operator skills and
accuracy can become limiting factors. In addition, companies can’t
afford to waste material while they adjust their processes.
Machine tuning also becomes harder as the machine ages. Old
equipment is typically worn from years of use, making precise
adjustments harder to accomplish.
Is there hope for these old machines? Absolutely. Hydraulic
machines make particularly good candidates for control system
retrofits, which can save the owner up to 50 percent over the cost
of a new machine. They can also provide a better product.
A case in point is the upgrade of a line of powder metal
presses for the Hydraulic Press Div. of Alpha Lehigh (Alpha, NJ).
Concept Systems Inc. (Albany, OR), a control systems integrator,
retrofitted these presses.
Powder metal presses form complex, molded metal parts with
uniform density throughout a varying cross-section. The presses
can handle a range of different powder materials. Uniform material
transfer is key to powder press operation. Multiple hydraulic
cylinders and pistons are typically used to form a mold cavity. A
mixture of metal powder and binders is pushed into the cavity. It
is then compressed by the cylinders into the required shape. After
compression, the part is baked to achieve the hardness of the
final product. Using this method to produce parts allows
manufacturers to avoid the expensive machining step that was
traditionally used to make small metal parts.
Presses are good examples of complex force-position control
applications that benefit from a combination of PLC-based control
and event-step control executed by a special-purpose motion
controller. The PLC and motion controller are tightly integrated,
with the motion controller executing complex motion steps under
instruction from the PLC. This simplifies PLC programming and
allows the programmer to use the motion controller for sequences
requiring tight coordination or that require tight status
monitoring so operating modes can be adjusted quickly.
In the upgraded press control system, the motion controller
controls each hydraulic axis through a proportional servo valve,
allowing the system to smoothly accelerate and decelerate.
Position feedback is provided via magnetostrictive displacement
transducers (MDTs). Analog transducers installed in the cylinders
as part of the retrofit operation provide hydraulic pressure
feedback. These transducers interface directly to the motion
controller to support precise closed-loop control of the
compression process. Unlike limit switches and pressure release
valves that activate only when the limit has been reached, the new
transducers provide progressive information throughout a move
operation.
The motion controller (an RMC100 model) was supplied by Delta
Computer Systems Inc. (Vancouver, WA). This controller samples
data from the transducers every 10 to 20 milliseconds, enabling it
to produce complex, smooth motion profiles. The transducers also
allow the controller to slow the actuators gracefully before
changing direction.
With fully programmable motion control capability, the speed of
powder loading into the mold cavity can be varied, as can the
powder compaction pressure.
Alpha Lehigh has added a sophisticated new application. This
application involves moving the compression cylinder up and down
rapidly to “fluff” the powder to improve the end product. This
action would be impossible using the machine’s old bang-bang valve
control system. The new control system supports press parameter
tuning using a graphical interface.
Besides configuring and operating the press differently for
different product “recipes,” the control system must also deal
with the variability from one batch of powder to the next.
Environmental variability, such as changes in temperature and
humidity, can affect the quality of the process. This variability
is managed by tight, closed-loop process control.
Because the motion controller can be connected to a factory
network, the new control system enables automated tracking of
product quality data. Process data is fed to other computer
systems at the plant level for statistical process control.
The development, maintenance and operation of production
recipes are facilitated through a new operator screen interface,
implemented by Concept Systems. Once the machine operator selects
a particular recipe, the PLC automatically configures the press to
support it. Through complete control of all process variables,
machine changeover times are greatly reduced, and operational
accuracy is improved. This has improved product consistency and
quality, and increased machine throughput.
Preliminary data since the control system retrofit suggest that
a 50 percent increase in production throughput is possible with
the new control system. Production part quality has also improved.
Before the control system retrofit, machine operators used to
perform time-consuming inspections on 100 percent of the machine’s
output to weed out defects. Now, the quality of parts is
consistently high enough that the parts can be sent to the next
operation without additional inspections.
Machine vendors are quick to suggest that new machines are more
efficient and can provide higher throughput. However, machine
owners are wise to consider a new option, made possible by the
continuous improvement of electronic control systems. By employing
this strategy, some machine owners have been able to obtain the
same or higher productivity of brand new machines at half the
capital cost?or even less.
For more information on motion control systems, call Delta
Computer Systems at 360-254-5435 or visit www.deltacompsys.com.
