Servo drives provide fine control in tough jobs

If the two hydraulic rams were allowed to operate without a control system to keep them in balance, the whole system could easily tip over.

Construction machinery does not come much more heavy duty than that which extracts driven steel piles from the ground.

But in order to work safely, delicate hydraulic balancing circuits are needed to keep the rams level.

Moog's direct drive servo valves provide the accuracy and reliability essential to make this possible.

Pressure within the foundation industry is increasing to find means to extract previously driven steel piles without creating any noise or potentially damaging vibrations.

Such a device needs to be compact, versatile and very powerful.

Dawson Construction Plant's X1000 Universal Pile Extractor consists of two powerful hydraulic cylinders, each capable of generating up to 4000kN of pile extraction force through a set of hydraulically actuated taper-wedge jaws.

These jaws are engineered so as to generate increasing clamping force in direct proportion to any increase in extraction force.

The clamp has been designed to accommodate most steel H and U-sheet pile sections without modification.

"Contractors do not plan to use our equipment until they are desperate, preferring instead to try alternative methods of extracting piles - there are a variety of these, based on vibratory methods".

"However, we believe our system, in addition to being the most reliable solution and silent, is almost always the most economical as well", said Robin Dawson, Chairman of Dawson Construction Plant.

In operation, two small hydraulic cylinders are activated so as to close the taper-wedge clamping system onto the pile.

Once a predetermined clamping force has been reached the UPE electronic control system automatically begins the second "pile extraction" stage of the process, whereby the two massive 4000kN extraction cylinders are actuated.

These cylinders can extract the pile at a rate of up to 3m/min, depending on the level of resistance and the nature of the power supply.

Once full stroke has been achieved, the clamping system automatically releases and the extraction cylinders retract to their original staring position ready to take the next bite.

Hydraulic power is supplied from a fully portable diesel engined power pack, which also houses the electronic control system.

This control system can be operated with manual or full automatic control.

With such great forces at work, it is easy to see how the system could quickly become unbalanced.

In fact, if the two hydraulic rams were allowed to operate without a control system to keep them in balance, the whole system could easily tip over.

To prevent this from happening, two independent safety systems are incorporated in the equipment.

At the macroscopic level, a simple emergency safety cut-out is in place, based on a valve.

A mercury switch is used to measure the angle of tilt and should the whole assembly tip beyond five degrees, the power system will shut down.

However, this is a last resort safety option and the design calls for a much more accurate and sophisticated balancing system based on the use of an electro-hydraulic control system.

This is used to keep the positions of the two levelling hydraulic rams synchronised, so that a potentially dangerous scenario cannot occur.

The balancing system is based on a Moog direct drive proportional valve (DDV), an alternative to a conventional hydraulic piloted proportional valve.

The DDV is more tolerant of the harsh environments found in the construction industry.

The use of the DDV allows delicate accuracy and control of up to 5um to be achieved in heavy duty equipment, such as the X1000 Universal Pile Extractor, or in metal forming machines.

At the heart of most modern hydraulic control systems is the proportional valve and at the heart of the best proportional valves is the bi-directional, permanent magnet linear force motor.

Thanks to the inclusion of a pair of high-energy rare earth magnets, the direct drive valve (DDV) from Moog is able to offer significantly higher performance than other valves.

The combination of permanent magnets and electromagnetic coils results in spool driving forces considerably larger than those that can be obtained by solenoid coils alone, with very low power consumption.

In operation, the electromagnetic coils act as switches that divert the permanent magnetic flux paths, effectively acting as an amplifier, but without drawing additional current from the drive electronics.

The result is that the spool position control is stiff and well able to counter the flow forces that are created on the spool when the valve is passing at a high flow rate and at high pressure.

In contrast, conventional proportional valves face a conflict between stiffness and drive current that cannot always be resolved.

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