Off-highway and construction equipment engineers are getting control of their designs with electromechanical systems.By Len Vermillion, Editor in ChiefAs Al Wroblaski tells it, it’s a way to take it to the next level — an easier way to coordinate control functions. He’s talking about electromechanical actuators and their use in off-highway equipment. Wroblaski, a product manager at Danaher Motion, sits in an office at the company’s Marengo, IL, facility, speaking about road pavers. He’s telling the story of a paver manufacturer he knows that recently began designing its equipment with electronic controls. “Historically what they’ve done, for one of the axes going up and down, they used hydraulics,” he says. “What they found was that they wanted to put laser sensors on the paver to sense the height of the pavement and adjust the movement of the machine as the sensor sensed the height. The hydraulics were difficult to interface with that, so they actually went to electromechanical [actuators]. When I asked them why they did it, they said it was so they could interface more readily.” Wroblaski continues to explain how electromechanical parts were being used throughout the paver’s construction, including the seat the operator sits on, as Mark Maybee, a business unit director at Danaher Motion, chimes in: “It’s more cost-effective and it’s easier to get a reliable result,” he adds.
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“Electromechanical systems are designed for things that need control
on a vehicle, such as driving it, shifting it or controlling the functions that the vehicle is designed to do.” |
In the off-highway and construction equipment industry, electromechanical actuators are making ever more in-roads as engineers discover ways to add sophisticate movements to their designs in order to help operators get their jobs done more efficiently. Wroblaski believes there are three significant factors contributing to the influx of electronic controls: they can reduce fuel consumption by making vehicles lighter and more efficient; they maintain a cleaner environment because components don’t leak fluids; and they create a simpler system that requires less installation and maintenance costs.
Electromechanical actuators can either replace or work with hydraulic systems for the purposes of vehicular function control. “Electromechanical systems are designed for things that need control on a vehicle, such as driving it, shifting it or controlling the functions that the vehicle is designed to do,” Wroblaski says. “That could be throttle control. That could be shifting a transmission. It could be shifting a power take-off. It could be enacting a chute or a valve on a machine.”The list of reason that off-highway and construction equipment manufacturers are turning toward electro-mechanics goes on. The reasons range from environmental issues to the difficulty of working with hydraulics to the growing sophistication of designs that take advantage of advanced microprocessors now available.
“Particularly on the hydraulics, you’ve got the environmental issues with the fluid. You’ve got a lot of costs associated with it,” Wroblaski says. “[Hydraulics] are costly to do the installation. They’re costly to maintain. They’re difficult to communicate to central microprocessors that they’re starting to put on these machines.
These days, tiny microprocessors can do the work of vehicular function control.
Another reason, Wroblaski says, is that “the guys that are working with hydraulics have a limited amount of interfaces or ports they can use.” Back in story telling mode, Wroblaski tells of a tractor manufacturer that wanted to add a great deal of different functions to its machine. He says the company found it difficult and expensive to continue to add more hydraulic capability. He says that 10 or 15 actuators could be placed on a plug-in module that could easily be used.
Pros and Cons
While it may sound like electromechanical actuators are the answer to all mobile off-highway equipment design dilemmas, as with any technology, there are pros and cons. Not all off-highway vehicles are created equally.
Debunking the Myths Many manufacturers of off-highway equipment are considering replacing hydraulic actuators with electric actuators but certain myths have arisen about electric drives that have slowed their adoption in many applications. The following takes a look at common myths and Al Wroblaski, product manager at Danaher Motion offers reasons why they can be debunked.
1. Electric actuators can’t handle a harsh environment Hydraulic technology has been used in off-road vehicles for many decades so engineers have become very comfortable with its ruggedness. But many off-highway engineers aren’t aware that over the past decade the power density and ruggedness of electric drives has been substantially improved while in hydraulic drives the improvements have been much smaller or nonexistent.
The power density of hydraulic drives is largely dependent upon the pressure of their drive systems and these pressures have evened out over the past decade. On the other hand, the power density of electric motors has substantially increased over the same time frame because of advances in magnetic materials, construction, manufacturing techniques and electronics. The biggest improvements have come from control systems that are able to deliver substantially more power while maintaining high levels of efficiency. Additional improvements have come in the power transmission side, largely through movement away from off-the-shelf gearboxes to power transmission systems that are optimized to the requirements of electric drive applications. As a result, the power density of the two leading drive contenders are now approximately equal.
2. Electric drives aren’t as reliable as hydraulicsThe myth that hydraulic drives are more reliable may also be due to the fact that many engineers are familiar with a previous generation of electric drives that often did experience reliability problems. But electric drives have ridden the wave that has dramatic improved the reliability of all electronic and electrical products. Based on highly reliable electronics technology and with just a few possible points of failure, the reliability of electric drives has improved in recent years to the point that in the vast majority of cases they will outlive the equipment they are installed on.
Electric drives also require far less maintenance and are therefore much less prone to fail due to lack of maintenance. Today’s electric drives, on the other hand, generally require no maintenance at all, not even lubrication. Electric drives run independently with every axis being powered by a different motor. A failure in an electric drive affects only that single drive, which makes it much easier to troubleshoot and repair. Finally, electric drives can easily be configured with a manual override that can be used if something should go wrong.
3. Electric drives are more expensiveThe idea that electric drives are more expensive probably arises from the fact that each electric axis requires a motor, leadscrew and a gearbox while adding a hydraulic axis only requires a cylinder. But the hydraulic cylinder is only one component of the hydraulic system that is required to support the axis. Valves, hoses, and fittings will also be required and in many cases existing hydraulic pumps will not have capacity to accommodate the new axis.
Drive economics depend to a significant degree on how many axes are driven by a particular hydraulic system. Electric drives are usually considerably less expensive in applications where an additional hydraulic pump must be added to handle another axis. As a general rule of thumb, when a hydraulic system drives one, two or three axes of motion, it can generally be replaced with electric drives at a lower cost. On the other hand, when hydraulic systems drive four or more axes of motion, then hydraulic drives are generally less expensive.
The operating cost of electric drives is usually considerably less than hydraulic drives because electric drives only require power when they are actually being used while hydraulic drives generate continuous losses. The simplicity of electric drives also makes them substantially less expensive to install. And electric drives require zero maintenance as opposed to hydraulic drives whose fluid and filter needs to be changed at regular intervals.
4. Electric drives increase design complexity The idea that electric drives increase the complexity of the design process probably arises from the fact that many off-highway vehicles do not currently use electric drives. So adding electric drives requires working with two types of drives instead of one. In addition, many off-highway equipment engineers have not had much experience with electric drives. Finally, engineers may be acquainted with the earlier generation of electric drives that required the selection and assembly of various components such as motors, gearboxes and controls.
Today, electric drives have been simplified to the point that they are considerably easier to specify and design than hydraulic drives. Electric drives are now provided as integrated systems which require nothing more than hooking up two wires and a double-pull double-throw (DPDT) switch. It takes only three steps to determine the size of an actuator for an application: measure the load, determine the duty cycle and specify stroke and retract length. The precise load on an actuator may not be known because of the effects of intermediate linkages. Loads can be determined with software packages that simulate mechanical systems or by performing measurements with a load cell on the actuator. Electric drives can easily be configured by the manufacturer to fit the requirements of any application by changing gear ratios, leadscrew and motor parameters to predictably affect the key performance variables. |
Wroblaski says that not every piece of equipment is a perfect candidate for electric actuators. For example, he says, hydraulic cylinders may still be better suited for loads of more than 2,000 lbs or applications for moving loads with extremely short strokes at 100 percent duty cycle.
Many of those cons keep engineers weary of moving from hydraulics to electro-mechanics. After all, hydraulic controls are more than viable options for off-highway equipment. They are a proven commodity. But, Wroblaski says there many reasons to explore the alternative.
(See “Debunking the Myths” in adjacent box.)For instance, he says, “Electric systems eliminate bulky, heavy power packs and use the existing battery power and replace cumbersome and dangerous hoses with relatively small wires.” Electro-mechanic actuators can also run faster than hydraulics while developing the same force and stroke. Moreover, they run quieter.
Making it Easier
“[Electro-mechanics] take difficult, manual or tedious jobs and make them easier,” Wroblaski says. “A lot of this equipment, if it had a device that you didn’t have to control continuously, but you sometimes had to open or adjust a chute, or a gate, or a valve, these guys would have to stop the equipment. Sometimes they’d have to put themselves in harms way.”
One example would be in stump removal. When operators use a stump cutter, using electromechanical actuators to engage the transmission, prevents the operators from having to approach the dangerous machine. “It makes it safer. It takes away the manual function,” Wroblaski says.
Electromechanical systems, in effect, automate the process that controls the machine. That can lead to higher efficiency. “On some of these devices, let’s say it’s a combine or a forest harvester, they want to have airflow coming through to separate the wheat and they want to optimize that airflow for the moisture content or type of grain. They can have a device that measures all that, feeds it back to the microprocessor, and tells the actuator what they want to do as far as opening and closing,” Wroblaski says. “So they can automate it so no one has to go there and measure it and say ‘It’s 18 percent moisture, I think I better dial it into this.’”
That’s not to say similar functions can’t be done with hydraulics. “You can have a microprocessor that will talk to hydraulics,” Wroblaski admits. “but the interface between the two is not as easy.”
“It’s very expensive,” Maybee adds. “You have to be able to do distributed hydraulics. That’s where they have electro-hydraulic components and a centralized controller that’s going to be sending signals and they’re all going to be communicating with one another. The technology’s there, but the cost of making an electro-hydraulic system is generally higher.”
Often times, electromechanical systems are more repeatable and reliable, Maybee also points out.
Why Go Electric?
So when should an engineer of off-highway equipment take the leap to electronic control? In addition to heavy loads discussed earlier, engineers must look at the entire system and function.
Wroblaski says the decision to go electric hinges on considerably more than just the actuator. A basic electrical system can be beneficial when you want to make the machine much simpler. A basic electric system comprises the actuator, a three-position DPDT switch and a few wires. Such a system can be put together by almost anyone who can read a simple wiring diagram, according to Wroblaski.
“Compare this to a hydraulic system that requires special technical expertise to fabricate hoses and lines and install fluid power components so they leak only minimally,” Wroblaski says. “Hydraulic systems need many more components, including pumps, valves, hose fittings, a pressure regulator and a manual joystick-type control valve. Moreover, wires can be installed anywhere, especially in places that not as easy to reach or as safe for cumbersome hydraulic lines.”
Electric actuators are easy to control when the application calls for more than a simple two-way manually operated motion. Some systems, for example, require an actuator to cycle for a specific number of times at various strokes and then halt. According to Wroblaski, this type of scenario is an easy task for a PLC, and it’s just as easy to interface to an electric system.
Actuators inherently hold loads in position either under power or without, making them safer than one-way cylinders, according to Wroblaski. “Nothing more needs to be added to the electric system to accomplish this, but the hydraulic cylinder needs safety check valves to hold the load in the event a hose breaks,” he says. “In comparison, a cut, removed or damaged wire in the electric system does not affect it.”
Electric actuators are also easy to adapt to a variety of power supplies. The most common power used in off-highway vehicles is 12 VDC, but 24, 48 and 90 VDC or 115 VAC supplies are just as easy to handle since a wide range of motors are readily available that fit the same actuator envelope. Wroblaski says moving up to 24 or 48 VDC can be a benefit since the current draw becomes less without decreasing the peak torque delivered to the load. Lower current at a higher voltage also means that smaller diameter wires can handle the same power, lowering the system’s installation cost.
Costs, Weight and Space
Electo-mechanical systems have advantages in reducing part costs, saving costs and reducing the weight of off-highway machinery — an important task as the size of machinery continues to shrink. And, they accomplish those tasks in more ways than always meet the eye.
”There’s a cost advantage,” Wroblaski says. “The real cost advantage isn’t just the purchase of the material. The real cost advantage, which people never really talk about, is the labor expense or putting this thing together. And, then there’s the maintenance side of that.”
“You’ve got the inventory of all the different components, potential operational disruption when you go out of stock on a certain fitting size, putting it together, filling it with oil, purging the system,” Maybee says of the task involved with creating non-electric systems. “Another thing that we’re starting to see more of is [hydraulic] systems with all the oil and all the components on board to support them have a lot of weight. SO the advantages for electromechanical systems are in real estate and weight, which as some of these machines are getting smaller, battery-operated, hybrid, real estate is at a premium. Also as they’re starting to put in more functionality on these vehicles, real estate becomes a premium.”
Environmental Issues
When speaking of electromechanical actuators in off-highway equipment, the first thing Wroblaski usually points out is that they provide an environmental advantage over hydraulics.“Having a smaller package at a lighter weight allows the vehicle to become more fuel efficient,” Maybee says.
“I think the environmental aspect of using electric system is a benefit. People will argue with me, saying there are already hydraulics on the machine, ” Wroblaski says. “My answer to that is, ‘but you’re not adding more.”
While hydraulics are always a part of off-highway machinery at this point, some machines outside of that realm are turning to all electrical systems. One example is a specialized mower for cutting the grass on the greens at golf courses. “The way they work is that they have three reels in front and two in back. The way they work are the reels are operated by hydraulic motors. But they’re noisy,” Wroblaski says.
Unfortunately, a great number of golf courses are near swanky homes and greens are generally cut in early morning hours. To quiet those operations, the manufacturer turned to electro-mechanics to operate the reels.
While electromechanical systems are making in-roads into off-highway and construction equipment for several reasons, hydraulics still play their part. As new design develop more and more off-highway equipment manufacturers, however, are embracing the benefits electric systems can offer.
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