Machine Design

Steer-by-wire challenges hydraulics

Advanced tactile-feedback devices give electric drives the control and feel of hydraulic steering.

Tactile feedback devices give vehicle operators realistic road feel with steer-by-wire control. The devices are well established on forklifts such as the Linde Material Handling UK's activereach lift trucks, and are gaining acceptance on forestry tractors and offhighway vehicles.

In an MR-TFD, the rotor turns freely in the housing if there is no magnetic field. When the coil energizes, the magnetic field instantaneously changes the magnetically responsive material. The resulting torque on the output shaft is proportional to the input current.

Douglas F. LeRoy
Lord Corp.
MR Solutions
Cary, N.C.

Edited by Kenneth Korane

OOEMs are replacing their traditional vehicle hydraulic systems with intelligent electrical systems that reduce complexity and improve performance. Compared with hydraulics, electromechanical drives are more efficient, quieter, and do away with environmental concerns about leaks. Nonhydraulic systems also permit more ergonomic designs and are simpler to install.

Hydraulics has been around for years and OEMs are comfortable with it. But the potential advantages of electric systems, and the fact that their performance has dramatically improved in recent years, are pushing vehicle manufacturers to take a closer look at this alternative.

One growing trend is the switch from standard hydraulic steering to all-electric or bywire steering systems. The systems rely on electrical technology but maintain the feel of traditional hydraulics, at least from a driver's perspective. This approach has several benefits. For one, it is less expensive to route wires than hydraulic hoses. Electric systems also tend to be more compact, lighter, and more efficient than their hydraulic counterparts. For instance, by-wire steering is a great advantage in "man-up" vehicles that hoist operators to high racks in warehouses because it eliminates complex mechanical steering linkages. And from an ergonomic standpoint, electrical steering takes less effort and can reduce arm and wrist fatigue.

In a traditional steering column with a mechanical linkage, the steering wheel turns a shaft, which moves a rack and pinion or gears that physically turn the wheels. Hydraulic systems modulate flow to control steering-cylinder movement. Bywire systems accomplish the same motions electrically.

In most electrical systems, an encoder or similar sensor detects steering-wheel movements — direction, magnitude, speed, and so on — and sends inputs to an onboard controller. It interprets the signals and then commands electromechanical actuators to turn the wheels accordingly.

Position sensors on the actuators close the feedback loop with the controller, ensuring the vehicle wheels match steering commands.

Without the traditional mechanical steering link, however, developers of by-wire controls discovered early on that the loss of tactile feedback often degraded control. It caused problems such as over or understeer and compromised safety. Overcoming this drawback often requires a tactile-feedback device (TFD) that connects to the steering wheel and mimics the feel of mechanical systems.

For example, a TFD can increase steering effort to tell the operator he's hit an obstruction. Pulsing while turning the wheel may indicate mechanical failure. And TFDs can incorporate vehicle warning systems, endstops (endof-travel indication), and variableeffort steering to enhance control, minimize oversteer and understeer, and improve safety. Further, ongoing efforts to develop standards and common vehicle interfaces may cut engineering and operator-training costs.

In by-wire systems, optical or magnetic Hall-effect encoders or potentiometric-resistive sensors near the wheels or suspension sense motion. The controller receives the data and, in turn, drives the TFD. Tactile-feedback algorithms modulate the torque an operator feels as a function of steering-wheel rotational position, velocity, acceleration, wheel position and speed, and so on.

Design engineers have several options when it comes to TFDs. For instance, feedback based on proportional magnetically responsive-(MR) technology is used in lift trucks and marinepropulsion systems. MR-based TFDs offer advantages over torque-generating devices such as electromagnetic friction brakes and motors. For example, MR feedback generates smooth torque with no stick-slip or cogging, and the units tend to be smaller and consume less power. Better energy efficiency improves fuel economy on conventional vehicles and extends run time and battery life on allelectric ones.

Inside MR-based TFDs, a coil generates a magnetic field that spans the gap between a rotor and stationary pole (stator). A magnetically responsive material fills the gap between rotor and pole.

The rotor turns freely inside the housing if there is no magnetic field. When the coil energizes, however, the magnetic field causes the iron particles to form chains. The result is a torque on the output shaft proportional to the input current. Response takes about 10 msec, permitting fine control of torque output. This gives designers wideranging control possibilities. For instance, steering "feel" can be fairly light, almost free spinning, or offer moderate resistance. Vehicle manufacturers also specify the number of turns from full left to full right.

When the steering actuators reach their travel limits, the controller increases current and torque in the TFD so the driver cannot turn the wheel any further.-Or the system can generate a different tactile feel, such as pulsing.

Another option is variablerate steering. It offers more-precise control at low speeds. An industrial truck, for instance, might offer two or three steeringwheel turns, lock to lock, for maneuvering at low speeds. The TFD automatically adjusts the range to six or more turns for less sensitivity at high speeds. This helps prevent violent turns and rollovers.

Likewise, the system can be programmed to limit the rate-ofchange of steering. For example, regardless of how quickly the operator might turn the steering wheel, the controller limits how fast the wheels respond to prevent loss of control.

TFDs can also simulate inertia. Quickly turning from full left to full right requires some effort to overcome inertia in mechanical systems. Electrical steering without TFD doesn't have that sensation. But applying a ramping, varying current to the TFD will simulate the inertia found in mechanical systems.

For vehicle design engineers, it is essential to understand TFD components and how they work together. Considerations when selecting TFDs include sensing and I/O requirements, built-in redundancy, and torque-feedback range, as well as environmental concerns such as industrial-protection (IP) rating and resistance to salt corrosion.

Some suppliers bundle TFD components, combining the hardware for operator-interface force feedback and position sensing with the shaft and bearings in a single plug-and-play device. Comprehensive-all-in-one units that include-sensing and feedback can even eliminate conventional steering columns and let steering wheels directly attach to TFDs. Many off-the-shelf by-wire systems are also available. For example, Danaher Motion has developed two products that simplify installation. One, Advanced Power Steering, consists of a permanent-magnet ac motor with a speed reducer and an ac drive. It interfaces directly to steeringsystem sensors and operates alone or in a CAN network. It is suited for everything from large Class-1 counterbalanced trucks to small Class-3 trucks, replacing mechanical and hydraulic steering systems

The company's Direct Drive Steering is another option. The DDS only runs when changing direction, so overall efficiency is substantially higher than with traditional hydraulic systems. It also offers engineers a great deal of flexibility in positioning input devices such as steering wheels, joysticks, and wire-guidance components.

Putting by-wire to work

According to officials at Danaher Motion Systems, a manufacturer of electric-vehicle controls, there is a common misconception that by-wire systems are still experimental and expensive. The reality is that the technology is proven, says the company's Dwayne Roark. For instance, by-wire systems have been used for more than two decades on 20-ton forestry tractors. And as more people use them, costs continue to decline.

Lord Corp. recently worked with Linde Material Handling UK to develop programmable TFDs for high-fidelity "feel" in all-electric active reach trucks with steerby-wire control. During development, Linde recognized that the vehicles were unsafe without tactile feedback to the driver through the steering wheel. Although the company considered using electric motors, they rejected the idea based on cost, size, weight, and energy-consumption. Instead, it selected an MR device that could send realistic feedback to the operator. And, the TFD is 60% smaller than electric motors that generate the same torque.

Sister-company Fenwick-Linde S.A.R.L., based in France, also relies on steer-bywire technology for a full line of all-electric lift trucks. According to company officials, Lord Corp.'s TFD improves control and safety by simulating the resistive torque of wheel end-of-turn, so the operator "feels" where the wheels are located. The TFD also increases rotational resistance as vehicle speed increases to reduce the likelihood of rollover during oversteer.

Advanced TFDs are also found in safety-critical marine-propulsion applications. As an integral component in the Volvo Penta IPS steering system, the Lord TFD provides CANbus communication, condition monitoring, and double-redundant sensing, in addition to sensory feedback to the operator. A similar steer-by-wire system using MR fluid is now in the works for heavy off-highway vehicles in Europe. For instance, MR steering devices will be incorporated in an asphalt compactor built in Italy by a major international heavy-equipment supplier.

According to Peter Taube, Danaher's engineering manager, there is huge potential for by-wire systems that eliminate hydraulic oil from vehicles. Commercial lawn mowers are one example, he says, because oil leaks on golf courses can be exceedingly expensive to repair. The interest in hybrid systems will also drive the growth for by-wire, says Taube, as will the passenger-car trend toward electric steering.

If the range of applications is any indictor, acceptance of tactile feedback devices for by-wire systems will continue to grow not only to improve safety, but driver comfort and convenience as well.

Driving down costs

One of the biggest challenges to the growth and acceptance of steer-by-wire systems has been the cost, or perceived cost, according to Markus Plankensteiner, consortium coordinator for the TTA-Group based in Vienna, Austria. The group, a cross-industry consortium researching highly dependable time-triggered systems, is developing certifiable architectures for by-wire systems that will not need mechanical backup for on-road use. The objective is to develop global guidelines that enhance reliability, interoperability, and safety of steer-by-wire systems, which should speed adoption and lower costs.

Leading tractor, construction machinery, and forklift manufacturers and suppliers have committed significant time and money developing common platforms. More than 20 companies and organizations have teamed up in the steer-by-wire working group, including vehicle manufacturers such as John Deere, Liebherr, and Volvo Wheel Loader, and system suppliers such as Dana, Eaton, and Sauer Danfoss.

Lord Corp.'s magnetically responsive TFD family have nominal torque outputs of 5 and 12 N-m, along with a range of resolutions, redundancies, and encoders. The units typically mount to a steering wheel and withstand normal steering loads without reinforcing columns, couplings, or shafts.

Danaher Motion Systems,
(866) 993-2624,
Lord Corp.,
(877) 275-5673,
+43-585 34 34-0,

TAGS: Hydraulics
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