PhD in hydraulics

Improving press velocity control and performance data logging with PhD: pump-modeling hydraulic drive technology

Hydraulic systems play an indispensable role in press operation, enabling the transfer and concentration of enormous force to press metal into desirable shapes in very hostile environments. Unfortunately in doing so, conventional hydraulic systems can be highly inefficient and lack the precise control necessary to achieve the highest levels of quality and productivity.

Press velocity control
Press forming of metal is a predictable process, and many deformation simulation programs exist to assist the pursuit of an optimal pressing cycle. The key to optimizing the press cycle, both in terms of productivity and part quality is the control of press velocity, which results in the intended material flow characteristics during deformation.

Traditional hydraulic systems control press ram velocity in one of two ways: multiple discreet fluid flow transitions by adding or subtracting flow provided by multiple pumps, or by using combinations of servo valves and proportional throttling valves with associated control cards and feedback sensors.

A third alternative, offering the highest level of ram velocity control and precision, is now available using PhD (pump-modeling hydraulic drive technology). The advantages of this technology for energy savings and elimination of oil cooling have been previously well documented, but a recently filed U.S. patent application cites two additional advantages of applying this technology to Press Applications. The PhD offers more precise control of press velocity, as well as unique “sensorless” diagnostic data logging of both press and hydraulic power supply performance.

Smart pump control system
Well-known to the forging and metalworking industries, UNiGY is a positive displacement pump modeling solution that uses the PhD, motor and pump as an instrument to precisely control fluid pressure and flow, and provide “sensorless” data for process control and diagnostics. UNiGY accepts incoming control signals, sent from either a computer or traditional PLC controls, and as an instrument, accurately matches pressure and flow curves deduced from material deformation predicted by FEM programs, such as DEFORM. The press ram velocity profiles can be accurately controlled based on the real time hydraulic control of the UNiGY system.

The digital nature of the UNiGY system enables press cycles to be highly consistent and offers high resolution, even over long periods of use and for the duration of press life.

The system achieves high efficiency by applying power only as necessitated by the press process, and does not require the complicated valve and cooling systems present on traditional hydraulic drive systems. A wide variety of pumps and motors can be used with the UNiGY controller, and many successful retrofits on antiquated hydraulic presses have been performed.

Recently, Kadant Inc. filed a patent for the third generation UNiGY system, which includes DiaLog software that allows for advanced system diagnostics. DiaLog actually maintains an electronic log of the pressures, flow, timing, events and some 60-plus other variables during each cycle of a hydraulic press. This data provides valuable insight into the deformation physics of the workpiece as it is formed to the die.

The patent also covers the system’s PhD (Pump-modeling hydraulic Drive), which provides reliable force control, velocity control and position control with use of the prime mover as an instrument.

Achieving precise process control
Velocity of the ram correlates directly with the flow rate of the fluid delivered to the ram cylinder, and can be varied by accelerating or decelerating the rotational speed of the pump. Certain press cycles require very specific velocities at different positions during the cycle, and UNiGY's precise control of flow volumes and velocities gives faster, smoother transitions without extensive sensors and external controls that are required with traditional presses. This eliminates the feedback lag common to downstream mounted device type of control.

UNiGY also monitors the torque of the motor, factors in the self learned mathematical pump model, and therefore directly controls the pressure and flow of the hydraulic fluid. The pressure of the fluid, combined with the diameter of the ram piston, provide instantaneous knowledge of the forces experienced during a cycle without extraneous feedback instrumentation. DiaLog records this information for later review and optimization, or displays the process in real time.

The UNiGY system calculates the exact speed of the ram based on its mathematical model of the pump, automatically compensating for predictable variations in pump slip that occur at various combinations of pressure, fluid viscosity and pump speed. Figure 1 illustrates the slip characteristics of one of the commercially available pumps analyzed by UNiGY PhD. UNiGY’s smart pump control capabilities means that any complex velocity profile can be easily duplicated in the press cycle without the time consuming analytical and iterative processes.

While conventional hydraulic systems employ proportional control valves, control cards, PID loops and sensory feedback for system tuning. This enables a certain level of press control, but the results can be imprecise, slow and most importantly inconsistent over a period of time. By employing real time, automated hydraulic control that compensates for mechanical system and process variations such as temperature, and making decisions based on algorithms, the UNiGY PhD technology negates the need for additional feedback devices and provides consistent press cycles with minimal instrumentation.

UNiGY is unique in that because it is a pump control technology for hydraulics, it allows seamless integration with any logic center for process specific hydraulic control. It interfaces and relays information in engineering units understood by the operator specific to the process. As shown in Figure 2, for a press fit part deformation, the DiaLog software records all hydraulic information for later review and optimization or real time process control.
Utilizing its precise mathematical model of the pump, UNiGY PhD also has a Volumetric observer which allows positioning information for a particular ram. For coarse ram positioning (± .060), UNiGY combines this figure with the known diameter of the ram to determine ram position without the use of a linear transducer.

Diagnosing pump health and system anomalies
UNiGY further employs its pump modeling capability to provide hydraulic compensation of pump output due to variations resulting from pump and/or system wear and tear. Bounded by parameters set by the system designer, UNiGY can “make-up” for hydraulic losses due to normal pump wear, such that the end application does not see a difference in performance. Rather than employing multiple, discrete devices and operator supervision to ensure the health of the hydraulic system, the DiaLog’s “data mining” capabilities and the PhD’s real time monitoring and control capabilities work together to achieve this result. While the DiaLog software maintains a consistent log of operations, the PhD algorithms and observers in its microcomputer continuously monitor the health of the hydraulic system to include the pump. This helps ensure that the pressure characteristics of the first press cycle and the ten thousandth cycle are identical.

Figure 3 shows a comparison of a healthy, four piston pump compared to a problematic pump. As long as the pump anomaly does not create a hydraulic loss beyond the tolerance set by the system designer or user, UNiGY manages the pump to provide consistent press performance for both velocity and pressure control. Any deviation from the set boundaries can be reacted to in many ways, such as warnings, faults or system shutdown. This has a direct influence on end part quality as the press can produce consistent parts even with partially problematic pumps.

When anomalies occur, such as a clogged filter or a poorly sealing check valve in a piston pump, UNiGY PhD immediately identifies a problem and executes procedures to protect the system, alert the operator and whenever possible adjust applied torques and speeds on the pump to maintain commanded pressures and flows. UNiGY PhD samples 8,000 datapoints per pump revolution, thus allowing for instantaneous and high resolution control of the pump behavior.

Custom solutions
However, these procedures are customizable, depending on the severity of the problem. A burst pipe, for instance, can result in an alarm and the immediate shut down of the system. A partially clogged filter, on the other hand, can result in simply a log entry while the system makes up for the change in flow or pressure levels.

Due to the extensive modeling of the pump, DiaLog can even identify down to which piston is failing in the case of a pump failure long before it becomes catastrophic. A simple “snap shot” by maintenance technicians periodically reveal minor issues before they become major. Traditional hydraulic systems provide little or no definitive error feedback, so trained technicians must spend extensive time troubleshooting the machine to identify and repair the problem.

With UNiGY’s simple system integration, data-logging and diagnostic capabilities, press applications can now experience precise press control and intelligent system diagnostics with the already established energy savings that UNiGY has provided in the past.

This article appeared in the August/September 2008 issue of MRO Today magazine. Copyright 2008.

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