Control matters: large forces in hydraulic systems

By Patrick Hill06 May 2008

Electronics provide precise control of hydraulic actuators, such as this motor from Sauer Danfoss' H

Electronics provide precise control of hydraulic actuators, such as this motor from Sauer Danfoss' H1 series.

Effectively controlling large forces in hydraulic systems to make access equipment effective and safe is critical. Patrick Hill has talked with platform manufacturers about how they do that.

Pumps pressurise hydraulic fluid and cylinders convert it to motion. In between are control valves, and the mechanisms to control them are a fundamental issue of machine design and use.

Basically, there are currently three ways to control the hydraulic valves (see box). Manufacturers' choices, as MEC's Steve Citron, director of business development, points out, are; "dependent on a number of factors including achieving cost targets, meeting customer's interface needs, type of environments the machines are utilised in, safety and compliance standards"

Considering first machine functionality, hydraulic controls - the simplest of the three approaches - usually provide proportional control of machine movement. "The degree of movement of the control lever determines...the movement of the slide valve in the distributor," says a spokesperson for Haulotte Group. If precision is required, then you use smaller and slower lever movements.

However, because the displacement of valve spools is directly linear to joystick movements, the geometries of linked cylinder, knuckle, and boom can cause varying speeds throughout the range of motion. This variation can be a problem in some MEWPs.

One advantage of electronic controls, says Haulotte, is the ability of computers to compensate for such variations. "[Computers] offer a significant range at the level of the signal (linear angle, parabolic, etc.) that improves the responsiveness and [graduated response of the system]."

Physical configurations of MEWPs can have other effects. Large distances between controls and cylinders can cause, says Haulotte, "parasite" response times in hydraulic controls.

Also, says JLG Industries, "These distances...can increase [the] risk of impact damage, leaks, and sometimes compromise accessibility."

JLG is quite clear about another effect: "the requirement to run a large number of hoses form the base of the machine to the platform [has] consequent weight increase within the boom as well as hydraulic system losses. This limits the size of machine that can be equipped with full-pressure hydraulic controls."

The US manufacturer says the major drawback to the use of pilot operated hydraulic controls on AWP's "is the deterioration in operating performance as the distance between the operating joysticks and the main control valve increases."

According to Haulotte, "Direct electronic control is the preferred solution for [spool] valves and is ideal for relatively low outputs (up to 25 l/min), i.e. as long as the force required remains limited." It provides a "good level" of precision, says the company.

When needing flows above 25 l/min, Haulotte likes to use electro hydraulic valves, which "offers an excellent degree of precision." Such controls are "repetitive, precise (no signal fluctuations)" and have "... limited sensitivity to the heating of the solenoid because of their considerably lower energy consumption."

JLG sees similar benefits, saying, "A high level of operator control is provided by electro-hydraulic systems, despite the lack of direct feedback on the controls."

On the topic of reliability and safety, the company says, "There is little to go wrong with hydraulic controls." On the other hand, ""...hydraulic components will wear, causing degradation in performance." It points out that hydraulic systems are more affected by temperature, both of the oil and components, and that users must "get the balance right during set up and maintenance activities."

Counter to some user opinion, Haulotte says that electrical or electro-hydraulic devices are not ‘less reliable' than the other solutions, but emphasises that they need "quality electric channels and connectors (routing, insulation, etc.)."

Flexibility when creating the signal to electrical controlled valves is important to Haulotte, which likes their " ability to accept a considerable range of [input] signals" They also have "a high range of management capabilities for security functionalities".

To Haulotte, they "provide a degree of high security by re-reading the control signal." On the negative side, "they may be subject to ‘bugs' if the manufacturer does not have sufficient expertise in programming and/or validation of the software. "

One advantage of hydraulic control systems, much appreciated by some users, is what Haulotte describes as the "sensation of control. The forces on the slide valves are retransmitted back to the joystick."

However, JLG points out drawbacks of such feedback; "The effort needed to control the levers can vary with the loads in the system and increase fatigue. The operator is also exposed to any heat and vibration generated by the hydraulic system."

Pilot operated controls separate the operator from the direct contact with high-pressure components, which JLG says " reduces exposure to leaks, vibration, as well as benefits of lower activation forces and reduced joystick travel." This also results in reduced operating effort as well as the benefit of integral system security."

JLG says, "little that can go wrong" with hydraulic systems; "In basic format...[they] are simple to troubleshoot... but increased functionality coupled with regulation induced interlocks introduce many add-on components that increase complexity and decrease reliability."

Some say electronic systems complicate diagnosis and make it more difficult to repair. Haulotte's opinion is that "It is certain that onboard intelligence [the computer] deprives the end users of direct access to the control, though in exchange, guarantees better precision and security in using the machine."

JLG sees the computer as positive; "Maintenance and trouble shooting are other areas where electro-hydraulic controls can provide enhanced functionality. Although they demand engineers with both hydraulic and electrical skills the latest designs can also offer integral diagnostic systems."

Hydraulic systems are relatively simple to design and build and use components manufactured in large volumes. Therefore, they can be low cost, although pilot operated systems are more expensive than full-pressure ones, says JLG; "Although electro-hydraulic controls are generally considered to be more expensive they are becoming increasingly popular as the market drives improved functionality and increased features and options."

The central role of the computer, points out Haulotte, offers an "enormous advantage in being configurable (hence adaptable to the needs of specific customers)."

JLG sees other advantages; "Electro-hydraulic controls are particularly suited to machines with many build configurations, modes of operation and regulatory induced interlocks. In this respect, electro-hydraulic controls are more flexible by allowing system expansion, adding interlocks and operator features with relative ease as well as a reduced requirement for additional hardware."

Precision, cost, flexibility, ease of maintenance -- manufacturers have lots to consider when designing controls. However, as Mr Citron of MEC says, "The most important aspect of any control system is that it is durable to withstand the rigors of the construction and rental markets and most importantly, highly reliable to ensure safety in use."

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