Hydraulic Presses

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The hydraulic Die Cushion is comprised of a movable table (Cushion Box), supported at the four corners by hydraulic cylinders. The position of the table is controlled hydraulically, via electric proportional direction control valving and LVDT position feedback. Each of the four cylinders is fitted with a pressure transducer and electric proportional pressure relief valve. The developed tonnage at each cylinder is a resistive force, opposing the downward movement of the press ram. This tonnage is variable, and controlled by a command-input signal to the proportional releifed valve amplifier cards. The amplifiers in turn output a variable voltage to the valve coil. As this voltage is increased, the pressure setting of the valve is increased proportionally. Knowing the basic relationship of command signal to pressure setting and the cylinder bore area, an equivalent tonnage setting is easily obtained. In an effort to maintain simplicity and “user friendliness” the inputs and display of the Cushion tonnage at the operator station is expressed in Tons.

System Setup

When the Die-set is positioned in the press, the Die-setters must be “zero” and the press ram relative to the new Die-set. This function is carried out via a customer furnished LVDT mounted to the press side frame. This zero point, where the press ram initially contacts the die, is used as an input to the Red Stag control system for per-acceleration control. The press operator displays the menu screen on the HMI and the user inputs the desired tonnage settings for each of the cushion cylinders. This input is used for different points through the working stroke of the cushion. The cushion box is also fitted with an LVDT (25-inch stroke). As the cushion is forced downward by the press ram, the control system changes the command output to the proportional relief valves as specified by the operator during set up. In the operator, the change in tonnage is “ramped” up and down (rather than digitally “stepped”) as a function of the command input. This reduces system shock. These operator specified step changes are made based upon position feed back from the LVDT. The preceding example is a simplified sequence of operation. In actual operation, the operator has the ability to set each cylinder at a distinct tonnage value relative to cushion position. For the current project, each cylinder has a maximum setting of 37.5 tons. Prior to initiating a press cycle, the cushion must be raised up completely.

Single Cycle Sequence

Once the operator has input the desired settings, and the press ram has been “zeroed” to the die-set, the cushion is ready for operation. The cushion is then raised up fully. The operator initiates the press cycle (a separate control system) and the ram travels downward. When the punch is 1 inch above the die-set, the cushion is driven downward by the cushion cylinders. This is accomplished by the combination of (an adjustable) minimum command signal to the electric proportional direction control valve. The combination of these pre-accelerates the cushion downward at a rate of 4in/sec. Given that the press ram travels at a rate of 6in/sec, the press ram overtakes the already moving cushion. At 0.5 sec after the press ram passes the pre-acceleration point, it overtakes the cushion and continues downward through the balance of the stroke.  Note: The pre-acceleration point must be adjustable, and specified by the operator. Once established, it will likely be unchanged.

As the cushion travels downward through the press stroke, the pressure transducers continuously output a voltage equivalent to the load induced pressure. At a specified number of distinct points through the stroke, these outputs are ready and recorded as the equivalent tonnage developed. These values are displayed on the HMI in a simple, easily understandable graphics display. A plot of tonnage versus position will be stored and, upon request from the operator, is output to a customer-furnished printer/plotter.

This is a video of Red Stags Intelligent Hydraulic Cushion in a Mechanical Press!

As the press ram returns upward, the cushion table remains down for an adjustable, preset delay of approximately 30 seconds, then returns upward to the pre-set up position point. Upon command output to the “up solenoid” of the electric proportional directional valve. As the cushion returns, the cycle is complete.

Drift Control

In addition to the above sequence, Red Stag Engineering & Automation, Inc. uses a drift control circuit, which keeps the cushion in position while the press is idle. The control system monitors the position of each cushion relative to the specified position. If the cushion drifts an operator-selected amount from the specified position, the control system outputs a signal to shift the appropriate solenoid directional valve and raises the cushion (a separate circuit from the raise/lower circuit). This signal is removed as the cushion returns to the specified position.