By Roy Coulson* of Enerpac
CONTROLLING and monitoring movement during the hydraulic lifting or positioning of very heavy objects such as road and rail bridges, buildings, drilling platforms and mining shovels is still predominantly done manually.
However, with manual control, differences arising between the lifting points are unavoidable, because measurement of the movement and control of the lifting points are never optimal.
Too great a variance in the lifting of the various parts of the object can result in stress in the object's construction, even though there may not be visual damage.
This type of internal stress has a very negative influence on the construction and will ultimately cause problems such as cracking in concrete.
In reinforced concrete, for instance, hairline cracks can occur, allowing moisture to get through, resulting in a greatly reduced life of either the reinforcing bar or strand in the construction.
In other words, if stress is prevented during movement, the life of the relevant object will not be affected.
Engineers, construction and maintenance staff are consequently more interested in accuracy than in speed, as accurate lifting or lowering movements have a significant effect on the likelihood of stress occurring.
The problem with this is that very precise manual control and monitoring are also very time-consuming. The smaller the tolerance, the lower the speed of lifting or lowering. In such cases, a lifting movement of 8 inches/203mm may take one or two days but with an accuracy of 10 thousandths of an inch.
In response to such problems, high-force hydraulic specialists Enerpac has developed a Synchronous Lifting System by which lifting, lowering or positioning can be carried out completely automatically to a high degree of accuracy.
The computer-controlled system, now available in Australia and New Zealand, is a combination of digital switching and digital control. It offers users significant advantages, such as considerable time savings and virtually no internal stress in the object.
Enerpac's development of a synchronous lifting system began more than six years ago and now has more than 25 systems proving their worth worldwide. Enerpac recently received a European Engineering Award for the system.
Simplicity is key: just specify the tolerance, specify the target, and start lifting. After that, all users have to do is ensure everything is proceeding smoothly.
The Enerpac Synchronous Lifting System meets the requirement for accuracy to the highest degree and offers computer controlled, highly accurate positioning, lifting and lowering of objects.
The operating system receives electronic signals from movement sensors that are attached to the load in the vicinity of the lifting cylinders. These signals are processed and, based on the results, the computer switches the hydraulic control valves on and off.
In theory the cylinder movements can also be controlled via the hydraulic flow, whereby each cylinder receives the same amount of oil. The idea behind this is that if each identical cylinder receives the same flow the movement will also be identical.
In practice, however, this adversely affects the hydraulics, because no account is taken of the differences in force that arise. A load does not react homogeneously and exerts different forces at different lifting points.
With the manual method, control and monitoring of the flow at a pressure of 10,000 psi/700 bar is relatively expensive and not straightforward, as it requires special proportional valves with extremely accurate internal workings. Furthermore, the smallest amount of dirt in the oil can cause problems.
Enerpac therefore developed a much more accurate method with an 'easy' personal computer and sophisticated software. The computer program controls the cylinder movements by switching the valves on and off.
The speed of the PC is used to transmit a short pulse to the valves at a rapid rate. As a result, the individual movements can be kept much smaller than is possible with manual control.
The digital system provides a good combination: digital (hydraulic) connection with digital control.
As the valves are digitally connected (switched on/off) and every cylinder movement is digitally controlled and monitored, we can speak of digital hydraulics. Naturally, the whole hydraulic system has to be geared to this.
Every lifting point must be connected to its 'own' valves and every circuit must be able to work independently of the other circuits. To achieve this, Enerpac uses 2/2 directional valves and a hydraulic rectifier.
The rectifier ensures the oil can only flow in one direction and each circuit can therefore be controlled independently, without interaction between one lifting point and another. Thus, cylinders with different capacities and different forces can be applied to the different lifting points, regardless of the force.
The Enerpac valves work almost like a perfect electrical circuit, with a very short switching time. After 18 milliseconds there is already a flow and after 30 milliseconds the valve is fully open.
A measuring system that measures independently of the cylinder movement is used to calculate the load movement.
The reason for this is that in the first instance, the load does not rise, but the base of the cylinder drops slightly. On the computer screen this could give a positive but incorrect value that cannot be regulated from the screen.
The measuring system consists of a thin cable that comes out of a movement sensor. The end of the cable is attached to the load and the sensor is attached to an immobile surface close to the cylinder.
The movement sensor gives the absolute value of the cable movement, thus giving the actual movement of the load to the greatest degree of accuracy, even to .04 inches/1mm. The electronic signal from the sensor is changed by the computer from an analog to a digital value.
With the measuring system, a new incremental zero level can be set at any time. This is important, for instance, for lifting in stages, depending on the stroke length of the cylinders. Another example is where an object first has to be righted, and then lifted from the new zero level.
Apart from the actual load movement, the pressure of the hydraulic oil is also measured at each lifting point with a sensor. This offers added control of the load movement, while the weight and the center of gravity can therefore also be calculated. The value of the pressure is displayed on the screen in psi or in bar.
Before the start of the lifting, lowering or positioning process (or of a stage in the process) and before the automatic mode is switched on, the target stroke is entered into the PC. This is the distance from the zero level (which may have been reset) to the target level.
A protocol file is used to indicate how often the measured values have to be achieved. This can be done precisely to a fraction of a second and the inch. In each circuit, the cylinder is then put against the load with a specific force, after which the automatic mode can be switched on.
If the first (highest) lifting point has reached the target level, it is switched off. The other lifting points are then adjusted. The result is that the whole load is perfectly aligned, with minimal deviation.
During lifting, the software meticulously follows each lifting point and each cylinder.
The software also constantly monitors which lifting points are in the highest and ... lowest positions and whether these are still within the set tolerances. If a correction has to be applied, the 2/2 movement valve opens briefly and the relevant (lowest) cylinder receives a short hydraulic impulse, instantly followed by a new reading.
Monitoring via the computer screen
The course of the lifting process can be followed on the screen (per lifting point). The software also contains an extra control function, by which the system can temporarily be stopped and manually corrected.
An emergency switch is also built into the system. If something goes wrong, the system will stop automatically and block all points of support.
Multiples of eight lifting points (8, 16, 24) are used as standard. The required hydraulic components for these eight lifting points (pump, valves, etc.) are housed in one cart. Two carts are used for 16 lifting points, and three for 24.
Considerable time savings
Apart from the exceptionally accurate and virtually stress-free movement of an object, saving time is another important feature of the Enerpac Synchronous Lifting System. The process does not have to be stopped after any incremental value, checked, manually re-measured and corrected.
These actions are carried out by the software during the entire lifting process, thus providing considerable time savings, even with low lifting or lowering speeds.
*Roy Coulson is managing director of the Australasian operations of Enerpac.