The Case for Electrification on Mobile Machines

By Simon Nielsen, Danfoss
Scissor lift construction

The increase in electrification is nothing new — even consumer markets have seen the transition from gas-powered to battery-powered in items like lawn mowers and power tools. Many are enthralled with Tesla after the rapid growth of its electric vehicle business over the last decade. This same trend is occurring in the off-highway realm, albeit at a slower pace.

Danfoss has traditionally been known as a hydraulics business, the benefits of electrification cannot be overlooked. We aim to use the benefits of electrification where it is technically feasible and where the value is appreciated. First, though, we must explain what we mean by electrification. There are some that see electrification as completely doing away with engines and hydraulics. From a more nuanced perspective, however, electrification of the mobile machine industry may be seen as a continuum of possibilities, including hybridization of power sources and integration of mechanical, hydraulic, and electrical technology.

One application segment that is already rapidly experiencing the trend of electrification is the aerial work platform industry. During the modification of a scissor lift at our Application Development Center (or ADC) in Ames, Iowa, we experienced firsthand some of the opportunities and challenges that come with electrification of conventional vehicle systems.

Real-Life Machines

Traditional scissor lift machines, also known as a mobile elevated work platforms or MEWP, may already be electrified to some degree. Most are already battery powered and have a single electric motor that drives a hydraulic pump. That hydraulic pump then powers all the actuating functions on the machine through a system of valves and hoses so that the hydraulic power can be routed to propel and steer the vehicle or lift and lower the work platform. Traditionally, there are two hydraulic orbital motors that propel the vehicle. Steering of the vehicle and lifting and lowering of the work platform are performed with hydraulic cylinders.

We have further electrified our scissor lift in the sense that we have replaced some hydraulic functions with electric ones. First and foremost, we replaced the hydraulic propel system with a brushless DC electric motor system with integrated brakes. Rather than converting energy from the battery and through the electric motor, hydraulic pump, valves, and hydraulic motors as before, we go straight from the battery to the drive controllers and electric motors. The design and power flow are significantly simplified, the potential for hydraulic oil leakage is eliminated, and the system efficiency increases.

In general, functions that are rotary in nature are relatively easy to convert from hydraulic or mechanical actuation into electric because electric machines are also most commonly rotary in nature.

One caveat of the electric propel motors in this application is that they typically require a gearbox in order to match their speed and torque to the requirements of the vehicle. This is compared to the traditional hydraulic orbital motors, which have enough displacement and torque potential to drive the scissor lift directly without any gearbox and which fit in a space smaller than the new electrical drive assembly. This speaks directly to the incredible power density of hydraulic machines.

After designing the propel system conversion from hydraulic to electric motors, we kept our minds open to further improvements that could be made. Could there be opportunities to improve the steering and lifting functions as well?

Electrifying Steering

Today, there is usually a small cylinder that performs the steering function on a conventional scissor lift vehicle. In a more conceptual change, we chose to replace the steering cylinder on our scissor lift with a standard, 24 V electric linear actuator with integrated position sensor and similar performance capabilities. The linear actuator contains a small electric motor that converts rotary motion into linear motion through a mechanical gearset.

The form factor of the electric actuator enabled it to be integrated relatively easily into the space normally occupied by the hydraulic cylinder. The electrification of this function further simplified the vehicle design and eliminated multiple leak points.

Eliminating Leak Potential

The work platform of the scissor lift is raised and lowered linearly to position operators at their desired working heights. Traditionally, there is a long hydraulic cylinder inside of the scissor assembly that raises and lowers the platform. We also investigated the feasibility of replacing this function with some type of electric linear actuator.

We ultimately determined that although there is a growing market of electric linear actuators for mobile applications, it was more difficult to identify a solution that met the force and speed requirements of the lifting function and fit within the space constraints of the scissor assembly. There is an established market of electric linear actuators for industrial use and niche mobile applications, but they tend to require higher voltages than the 24 Vdc system on the conventional scissor lift. Replacing the hydraulic lift cylinder was not an easy option.

As an improvement upon the conventional system which is located in the scissor lift base and routes flow up to the cylinder via long hoses, we built and attached a dedicated “power pack” hydraulic system onto the cylinder. This miniature hydraulic system is bolted right onto the side of the cylinder and includes a reservoir, pump, valves, sensors, and electric prime mover. Steel hydraulic lines are still required, but we have eliminated the use of hydraulic hoses and fittings, which has tremendously diminished the potential for leakage. The main point here is that we kept a function hydraulic that made sense to really be hydraulic but found reasonable opportunities for improvement as part of the overall vehicle design change.

Reducing the leakage points means that safety in the environment is improved and environmental impact is reduced, as many of these machines are used indoors and in sensitive areas that cannot tolerate leaks. Some users of conventional hydraulic machines even resort to the use of what are called “diapers” to absorb leaking hydraulic fluid from components within the scissor lift. Reduction of leak points reduces maintenance time and costs.

Ensuring Safety with Hydraulics

Critical to scissor lift design is the requirement for emergency lowering should there be a problem with the machine when the platform is elevated. Emergency lowering on a purely electromechanical type of actuator is not always straightforward and may require a special braking assembly or external override tool.

With the hydraulic lift cylinder, it is only necessary to pull a cable that manually displaces an existing control valve and enables fast, but controlled, lowering.

Questions of Efficiency

Efficiency is a standard discussion point in the engineering of mobile machinery, but it is especially relevant on vehicles with limited energy storage or longer recharging times that must make more optimal use of the energy onboard in order to complete the work required. Although many of today’s battery-powered aerial work platforms are capable of storing enough energy for a full shift due to the lower average power level of their operations, any improvement in system efficiency can save energy and potentially increase their operational range or enable reduction of battery capacity.

Hydraulic pumps and motors have continued to increase in efficiency over the last decades due to improvements in their design and manufacturing, prompted by continual calls for improved fuel efficiency and performance. However, cost-sensitive applications like scissor lifts normally utilize more “commoditized” hydraulic components and simplified system designs that are not necessarily optimized for efficiency. In particular, the use of fixed-displacement gear pumps and orbital motors in an open-circuit configuration including many cartridge valves results in relatively low system efficiencies.

The use of an electric propel system, in particular, can improve overall system efficiency. The transformation of battery power into mechanical power at the wheels for propelling the vehicle simply has fewer power losses than the conventional, hydraulic counterpart. The only power loss unique to the electric propel system comes from the required use of a final-drive gearbox. Reducing the electric motor shaft speed to levels matching the vehicle speed range usually requires at least two gear-reduction stages.

Overall, the use of an electric propel drive on a conventional scissor lift can increase the operational range or enable the reduction of installed battery capacity.

Adding in Higher Controllability

The electrified machine has some control advantages as well. In test comparisons of the controllability of both the electric version versus the hydraulic, the differences become apparent. In particular, the simplified control design and advanced control software improve responsiveness and reduce the feeling of delay or lag in the controls. Operators feel more in control, especially as they maneuver within tight areas or in sensitive environments.

On grades up to 30 percent, the electric propel system achieves minimal rollback on both starts and stops through intelligent control of both the motors and brakes. The speed control accuracy and stability on grades is also improved over conventional hydraulic designs.

Electrifying Traditional Hydraulic System Applications

Due to continual improvements in technology and ever-increasing customer demands, the feasibility of electrifying traditional hydraulic system applications is greater than ever before. The aerial work platform market has been part of this conversion trend in recent years, and other applications are now coming into play. In some cases, the battle is no longer between hydraulic and electric technology, but between different types of electric drive technology. Where the feasibility is growing and the value is appreciated by both OEMs and customers, we believe this trend will continue.

For more information on the future of hydraulic powertrains, listen to Simon Nielsen’s interview on NFPA's Fluid Power Forum podcast.

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