Hydraulics make the most of function, workability of today’s utility equipment.
Published in: Diesel Progress North American Edition
Date: 8/1/1999
By: Henke, Russ
The functions of hydraulic systems in present-day utility equipment are described. One of the most common applications is to allow installers or servicemen to reach out-of-the-way job sites using equipment such as aerial work platforms, personnel lifts or boom lifts. The early models of these equipment were all hydraulic but later models employed oil-oil pilot valve systems. The latter paved the way for the use of joystick controls in single or two-axis models.
What is utility equipment? It depends on what the utility is doing at any given time – stringing wire, burying cable or pipe, building a power plant or a dam. The equipment required could range from small rider-type machines to massive construction, earthmoving or haulage machines. Whatever it might be, “utility equipment” all has one thing in common – its relies on hydraulics to facilitate operation and control of the functions the machine must perform to get the job done.
One of the most common of these functions is to enable an installer or serviceman to reach out-of-the-way job sites. A family of machines has evolved to deliver personnel to site such as power lines, telephone lines, overhead lighting, etc., efficiently and safely. These fall under the general category of aerial work platforms, personnel lifts, boom lifts or as they are widely known in the field, “cherry pickers.” If you think hydraulics hasn’t played a vital role in the development of this equipment, try to visualize such machines operated by cables, chain and sprocket drives, or other mechanical transmission methods. It’s not a pretty picture.
Early generations of cherry pickers were all-hydraulic. In order for the operator to run the machine, control valves had to be located in the bucket where he was. Direct lever operation was the technology of the day. Hydraulic hoses had somehow to be run up the boom to the bucket and back down to the various cylinders used to move the boom.
The next step in the development of these machines was the use of oil-oil pilot valve systems. The main power valves were located down in the base unit. Small hydraulic pilot valves were located in the platform or bucket. Thus smaller, more manageable hydraulic hoses could be used in the pilot circuit.
The use of pilot valves led to joystick controls. These were available in single- or two-axis models. Each axis controlled a single, three-position directional control valve, which in turn, would operate a double-acting cylinder on the boom or a bi-directional hydraulic motor in the propel system.
Fig. 2 shows a basic oil-oil-piloted, single-cylinder circuit. Note the parallel lines drawn on each side of the directional control valve symbol. These lines signify that this valve is an infinitely positionable – i.e. proportional – valve. Shifting the lever on the single-axis pilot valve will produce a differential pressure signal to be applied across the two ends of the directional control valve element – spool or whatever. This [Delta][Rho] will cause the element to shift to compress the spring on the other end. When the spring force acting in one direction exactly equals the pressure force acting in the opposite direction, the spool will stop, force equilibrium having been established. This will be the throttling position commanded by me position of the pilot valve joystick.
Which side of center the joystick is moved determines the direction the spool/element shifts and, thus, whether piston rod extends or retracts. With this technology we could control both direction and speed of the piston rod or motor. During this period on-off solenoid(electrical) operated valves were available but their two-state capability did not lend itself to the speed control function required by cherry picker applications. Speed (or velocity) control requires flow rate control which dictates either variable orificing (throttling) or variable displacement pumps. (In a rotary application a variable displacement hydraulic motor might be used if its characteristics better matched application requirements.)
Keep in mind that electrohydraulic proportional control was already well established dating back, in fact, to World War II days. Most proportional systems of this era were “closed loop,” i.e. servo systems used in military, naval and aircraft applications. One of particular interest, because it was a precursor of modern open loop proportional controls, were d.c. proportional solenoid valves for operating tank turret rotation drives and (maybe) gun elevation. This WWII vintage technology was slow to migrate into civilian market areas, such as mobile equipment, including cherry pickers. It was the search for remote control alternates to levers, shafts, flexible cables and oil-oil hydraulic or air-to-air pneumatic pilot systems which led the mobile equipment industry to electrohydraulic proportional control (EHPV).
The first iteration of EHPV was the adaptation of existing proportional electrical/mechanical technology to existing commercial mobile equipment hydraulic valves. Such devices as d.c. proportional solenoids, d.c. linear force motors, torque motors (a favorite of servo valve manufacturers) were mated to mobile equipment valves which did not have servo valve characteristics. Internal clearances were too great so leakage was high. Such valves were originally designed for manual operation and (hopefully) to hold loads in intermediate positions so spools were overlapped, thus increasing the valve’s deadband. There were other problems but the bottom line was that the mobile industry was faced with basically the same old valves with “new” electrical operators for remote control applications. This was not intended as a criticism of how first generation EHPV’s worked – it’s the way most products evolve.
The one critical issue, which the industry overlooked, was the people factor. Industry knew that there were mechanics and there were electricians – we had hardly gotten to electronics yet. They didn’t appreciate that fluid power – hydraulic and pneumatics – and electronics were on the verge of hybridizing into a multiphase technology. Back in the ’70s I coined the term “Fluitronics” to characterize it. It means fluid power with electronic control. At the outset, I referred to the oil-over-oil pilot systems which were standard with the cherry picker industry. When the fluitronic technology emerged it looked like an answer to a cherry picker’s prayer. Get rid of all that hose and run wire instead. And don’t forget to ground it properly or watch your operator’s eyes light up if he happens to hit a power line. Also don’t forget to shield electronic controls and cable against spurious RFI or MI. (I had one client, in the early days, whose boom’n bucket took off on its own due to RFI from an unshielded alternator on the vehicle’s engine – picked up by unshielded electronics mounted near it.)
The entire industry embraced fluitronics systems – indeed it was the first to go that way. And then they ran into the people factor – where are and where do we find the trained and educated people to design, install and maintain this new technology in our plant and in the field? In the early going some companies – maybe all switched back to oil-piloted systems. Over the past decade or so vendors and some technical schools have helped fill the educational void. But the engineering colleges have not risen to the challenge. Very few colleges even mention fluid power during the course of a baccalaureate program.
It is obvious that the problem has been addressed as it appears that the industry is currently firmly committed to fluitronics systems. Not coincidentally this has coincided with fluitronics manufacturers and vendors having responded to early problems with improved generations of EHPVs and state-of-the-art electronics to master the problems of the utility equipment market.
In order to develop an appreciation for the scope of fluitronics technology and a better feel for potential problems we’ve discussed, refer to Fig. 4, The Fluitronic Technology Algorithm.
Primary load variables are listed next as functions of position and reaction – both linear and rotational. And secondary variables may have to be considered. I do not believe that anyone can competently design or troubleshoot a system without this information.
Moving down from the load section in the diagram, a major bifurcation is shown. The left-hand path is labeled Conventional and the right hand path Fluitronic. Under conventional (circuits) are listed the traditional types of controls used. The algorithm diagram shows a direct connection to the Fluid Power Systems-Load Interactions section at the bottom of the diagram. Note that it bypasses all of the Fluitronic sections shown to the right. Outputs from conventional fluid systems – controls, actuators, motors – interact with the loads, driving and controlling them. One might view a traditional hydraulic or pneumatic circuit diagram as an algorithm of sorts, for the fluid power solution corresponding to the cycle profile defining the load.
Turning to the Fluitronic side of the accompanying algorithm diagram, for a given application the load cycle profile would be the same as for the conventional solution. The first step in the Fluitronic path is interfacing – i.e., sensors/transducers to produce controller-compatible signals to either servosystem electronics, depending on which technology is to be used.
Note that sensor/transducers are designed to interface with load variables and produce output signals representing the state of the monitored variables. These output signals are electronic – volts or mA being typical. This ‘raw data’ must be ‘conditioned’ to make it compatible with the type of electronics being used with the system – servo electronics or [[micro]processor] [similar to] [[micro]controller] control.
Ultimately, the electronics outputs either analog or digital signals are fed to the fluid power control(s) in the final section. And these controls, in turn, operate the actuators or motors to drive the load.
While I used aerial lift equipment to illustrate my points, they are not the only kind of utility equipment we could look at. Fig. 5 illustrates how far the industry has come since the early trials and tribulations I described at the outset.
Operator input for the boom is through a single joystick and touch pad controls with raised buttons, so the system is simpler and easier to use and delivers smoother operation. The Guardian System continuously monitors machine functions for safer, smoother operation.
With fewer wires and connectors, and LED diagnostics on the platform control panel, there is reduced downtime with the Guardian System. Maintenance is less too, since there are fewer components, and they are protected in environmentally sealed.containers.
Another manufacturer, Calavar, describes its hydraulic system as a “closed center, load sensing system using a pressure compensated piston pump.”
Controls called “load-lock” or “loadholding” valves, Fig.7, have evolved from the pilot-operated check valve. Load-holding valves are used where loads are bi-directional – i.e., can act in either extend or retract direction. They consist of two-pilot-operated check valves – typically enclosed in a single valve body. These valves are used with double acting cylinder or bidirectional hydraulic motors.
We sometimes see these valves integrated into the directional control valve for a given circuit branch. While they accommodate the bidirectional load condition, they do not offer any protection against the failure modes previously discussed occurring between the control valve and the actuator/motor. The generally accepted solution is to integrate these load-lock/holding valves into the actuator or motor they are intended to protect. Manifold mounting of the valve package on the actuator or motor is one technique. Using cartridge type valves inserted into appropriately machined cavities in the output device is another. In either case these are not “field fix” approaches. The actuators or motors must be supplied as a complete package.
Perhaps the oldest hydraulic device for holding an elevated load in position is the two-position, two-way (port) directional control valve, Fig.8. There are some problems with this approach. Because of clearances within the valve there is always some internal leakage resulting in load drift – as opposed to free fall. The two-way valve allows fluid to enter the cylinder, thus extending the piston rod and raising the load. But it doesn’t provide for getting the oil out of the cylinder in order to lower the load. A second valve would have to be added to the circuit to enable the lowering function. A cumbersome solution at best.
A solution to the problem discussed above is the two-position, three-port (way) directional control valve, Fig. 9. In the left end position the valve passes oil to the cylinder thus extending the piston rod and raising the load. When shifted to right position the valve provides a passage for returning oil to tank thus lowering the load.
One problem remains – what if you don’t want the load all the way up or all the way down? What if you want to stop it in any intermediate position? The solution is to add a third position, which blocks the load ports whenever the valve is shifted into center (neutral), Fig. 10. Problem solved? Not exactly. We still have the potential for load drift due to internal leakage in the valve. Most important we have not addressed the problems of component failures occurring between the valve and the actuator or motor – hoses, tubing, fittings, etc. If system integrity is breached in this area, there is no interaction between the valve and the actuator/motor. And down comes the load.
COPYRIGHT 1999 Diesel & Gas Turbine Publications