MR Technology, Tactile Feedback and Application of By-Wire Systems
by Douglas F. LeRoy, Market Manager LORD Corporation
Original Equipment Manufacturers (OEMs) are re-engineering traditional vehicle hydraulic systems with intelligent electrical systems that reduce complexity and improve performance. Hydraulic systems are often inefficient, noisy, complex and poise certain environmental concerns in the event of leakage. That said, these systems have been around for many years, so OEMs are comfortable with their implementation. By contrast, non-hydraulic or electro-hydraulic systems are inherently clean (no fluids), quiet to operate, efficient and allow for more ergonomic design and less complex installation during the manufacturing process.
Application Success Design engineers are looking at a host of potential applications for MR technology, especially in the area of driver feedback, or “feel,” that is typically not optimized in electric, by-wire systems. This trend toward control-by-wire – including steer-by-wire, shift-by-wire, throttle-by-wire and brake-by-wire -- has created a need for highly controllable, rugged, cost-effective devices to provide realistic force-feedback sensations to the operator, whether the manual device is a wheel, joystick, pedal, or a lever. For example, commercialized products for forklift vehicles using the LORD Tactile Feedback Device (TFD™) are able to sense steering wheel position, control over-steer when the vehicle is moving quickly, and simulate the end-stop limit of driving wheel turn. With MR material inside the LORD TFD, the rotor turns freely inside the housing in the absence of a magnetic field. When the coil is energized, the magnetic field generated causes the iron particles in the MR material to form a chain, creating a resistive torque that is proportional to the current flow in the coil and reacts quickly to changes in the strength of the magnetic field. To date, LORD TFDs are most prevalent in industrial utility vehicle applications, notably forklift trucks. United Kingdom-based lift truck manufacturer Linde Material Handling UK has taken the technology of programmable tactile feedback the furthest in these vehicular applications. In 2000, Linde Material Handling was the first to incorporate an advanced, fully integrated TFD system developed by LORD, which provides high-fidelity tactile feel in Linde’s all-electric active reach trucks with steer-by-wire control. During development, Linde recognized that their vehicle was unsafe to operate without high-quality tactile feedback to the steering wheel. Although Linde considered using electric motors, they rejected the idea based on high cost, size, weight and energy usage. As such, Linde selected a LORD MR device because it produced a high-fidelity sense of “feel” to the operator with a much lower cost, size, weight and energy usage than electric motors. Further, the LORD TFD is 60 percent smaller than comparable devices and produces the same amount of torque at a fraction of the cost and power. Sister-company Fenwick-Linde S.A.R.L., based in France, also has expanded the unique steer-by-wire technology into a full line of its all-electric lift trucks for a wide range of materials handling applications. Linde incorporated the LORD TFD to improve operator control and safety. The LORD TFD simulates the resistive torque of wheel end-of-turn so the operator can “feel” where the wheels are located. The LORD TFD also increases rotational resistance as vehicle speed increases to reduce the likelihood of vehicle rollover during over-steer. Once again, the LORD TFD produces the same amount of torque in a package size 60 percent smaller than an electric motor. LORD commercialized its most advanced TFD device in 2004 for a safety critical application in marine propulsion. As an integral component in the Volvo Penta IPS product, the LORD TFD provides CANbus communication, health monitoring, and double redundant sensing in addition to providing the typical force feedback to the operator. A similar vehicular steer-by-wire system using MR fluid technology from LORD Corporation is now in the works with European heavy off-highway vehicles. Further, LORD Corporation’s MR technology-enabled steering devices will be included on an asphalt compactor manufactured and built in Italy by a major international heavy equipment maker. |
A natural consequence of electrification of vehicles is the switch from standard hydraulic steering to all electric or by-wire steering systems. In many of these applications, LORD tactile feedback devices (TFD™) for by-wire steering systems are gaining acceptance with design engineers who are seeking new solutions to today’s macro trends: adopt vehicle designs based on electrical technology while maintaining the operator machine interface that can mimic the feel of traditional hydraulics. In by-wire-steering systems, sensing of the steering command is required; however, many engineers are finding that safety and control are enhanced with controllable steering feedback. These engineers view tactile steering feedback as a requirement in any by-wire implementation since it allows the operator a heightened sense of vehicle control and improves safety in operation. So, how does one get started in applying steer-by-wire systems to tomorrow’s designs without starting from scratch? The key to success is identifying industry suppliers who are leaders in by-wire implementation and by applying lessons learned from other applications in order capitalize on the benefits of by-wire systems.
By-Wire Basics
Without the traditional mechanical steering link, developers of by-wire controls found early on that the loss of tactile feedback often results in a degradation of control via over or under steer, thus compromising safety and reducing productivity. To overcome this problem, a controllable and proportional force feedback device is connected to the operator steering control to produce useful tactile information, often mimicking the feel of the conventional system being replaced. The controller is programmed with a tactile feedback algorithm that modulates torque feedback to the operator as a function of steering wheel rotational position, velocity, acceleration, steered wheel position, vehicle speed, or other parameters -- singly or in combination.
It is less expensive for an OEM to route wires than hydraulic hoses and from an ergonomic standpoint, reduced effort, variable rate/ratio steering can have the benefit of reducing whole arm and wrist fatigue that can occur from operating traditional multi-turn steering. By-wire steering is a great advantage in “man-up” vehicles where the operator is hoisted up to manually pick stock from high racks in the warehouse because it eliminates complex telescoping mechanical steering linkages or hydraulic hoses.
Incorporating tactile feedback with steering sensing provides the option or possibility to communicate information to the driver regarding vehicle conditions. For example, tactile pulsing (high torque to low torque) while turning the wheel may indicate a vehicle fault or increased steering effort may indicate a vehicle obstruction. Vehicle warning systems, end-stops (end of travel indication), and variable friction or variable steering effort (depending on machine mode of operation) provide more precise control by minimizing the possibility of oversteer/understeer and offer improved safety by providing more precise vehicle control. Furthermore, product standardization around common by-wire controls may decrease the cost associated with operator training and common vehicle interfaces can be adopted more easily.
The Market Need
According to Dr. Markus Plankensteiner, Consortium Coordinator for the TTA-Group (TTA stands for time-triggered architecture), the time is right for the development of reference architectures for on-road use. The TTA Group, a cross-industry consortium for highly dependable time-triggered systems, is working to develop such certifiable architectures for by-wire systems without mechanical backup for on-road use. The objective of the TTA-Group is to develop a global guideline to enhance reliability, interoperability and safety of steer-by-wire systems, which should result in faster adoption and higher cost efficiency for suppliers and manufacturers.
“One of the biggest challenges and hindrances to growth and acceptance has been the cost, or perceived cost, of steer-by-wire,” said Plankensteiner. “The goal of the working group is to optimize and to facilitate technology development on the platform level to minimize costs. This will result in higher cost efficiency for suppliers and manufacturers.”
And, if growth in the TTA-Group is any indication, the interest in TTA-Group's activities in this area is high. More than 20 companies and organizations have teamed up in the steer-by-wire working group to gain competitive advantage together. Manufacturers and suppliers of leading tractor, construction machinery and forklift industries have committed significant resources – time and money – to cooperate on development of common platforms.
Tactile Feedback Using MR Technology
Steering tactile feedback based on proportional Magnetically Responsive (MR) technology enjoys applications ranging from major OEM lift truck manufacturers and OEM marine propulsion system companies, proving that the technology is no longer experimental. MR- enabled devices offer distinct advantages over other types of torque generating devices such as electromagnetic friction brakes or motors. For example, MR feedback produces smooth torque with no stick-slip or cogging. Furthermore, power consumption on electric vehicles is a huge concern. More energy efficient means extended machine run time, less recharge cycles, and extended battery life. Steering devices engineered with MR technology are smaller and consume less power.
As a design engineer, it is crucial to understand the components and how they come together. For example, some suppliers bundle the components for tactile feedback devices (TFD™) by integrating all of the necessary hardware for the operator interface—force feedback, position sensing, shaft and bearings—in a single plug-and-play device, eliminating duplication. Comprehensive “all in one” units, which include the sensing and feedback, may eliminate the conventional steering column by permitting a direct attachment of the steering wheel to the TFD. Environmental considerations include Industrial Protection (IP) rating, salt spray/corrosion, sensing requirements (Input/Output requirements for vehicle controller and redundancy), as well as feedback (torque range required).
The benefits in of MR technology in by-wire applications include:
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LORD Corporation offers TFDs in a product family approach, which results in improved economies for customers of all sizes. To this end, LORD collaborated with sensor suppliers to standardize an encoder package that is flexible and cost-effective. LORD Corporation offers a variety of resolutions and redundancies thus providing customers with many options. And, since the LORD TFD, in typical applications, is mounted directly to the steering wheel, the devices are built to withstand normal steering loads without the use of secondary columns, couplings or shafts. This integration enhances implementation with customers and eliminates components. Further, the product families are engineered around two nominal torque ranges that serve as the “center” point for each product family – 5 N-m and 12 N-m. In real world applications, these nominal torques have proven to be both popular and effective.
Key to success, however, is for design engineers to have an open mind. Although there is a misconception that by-wire systems are still experimental and expensive, as the adoption rate increases, the costs will continue to decrease. And, the technology is proven. There are examples of by-wire systems in place for more than 20 years on 20-ton tractors for forestry applications.
Getting to Market - Quickly
Given the possibilities of by-wire enabled systems, the next step is the integrated design and development phase. Whether you are in the conceptual or prototyping stage, it is crucial to select a component supplier that knows your business and is truly interested in tailoring a solution for your application. Ask your supplier how they can assist you in getting to market sooner and becoming more profitable. For example, what is the best approach to combine a shaft, column, torque unit, and sensor all in one in order to eliminate components and create an integrated unit? Balancing such variables requires expertise in order to adapt a solution for your needs.
Another crucial consideration is selecting a supplier with a track-record of delivering high quality and high performing products since the systems are often integrated into or near safety critical applications. For example, look for a partner with an appreciation for design validation and application-specific life testing. Further, it is important to ensure that your supplier has the ability to test extensively in order to certify the product for any applications.
How a TFD Operates: A LORD TFD generates a proportional torque output in response to electrical current input. Inside the device, a coil is provided to generate a magnetic field. The magnetic flux traverses a gap formed between a rotor and a stationary pole (stator). A gap formed between the rotor and pole is filled with magnetically responsive material. This material contains particles that, when exposed to a magnetic field, align to form chains. The relative motion between the rotor and stator develops a shear action between the magnetic particles. The result is a torque on the output shaft that is proportional to the input current. With millisecond response, the fast-acting, high-fidelity (high control bandwidth) unit allows for very fine control of torque output, enabling system engineers maximum control possibilities. |
Timing also is key. Selecting a supplier that has the resources to ensure a successful launch of your application is paramount. While vendor certification and qualification programs often uncover such discrepancies early in the development cycle, selecting a development partner with a long history of manufacturing and innovation will greatly minimize your risks. Be sure to ask your supplier for examples of past production launches and their schedule, as well as how any development challenges were overcome. Be cautious of any supplier that only has one option for your unique application. By partnering and collaborating with a supplier with an extensive portfolio, you’ll also benefit from benchmarking and value engineering – ensuring a custom solution without having to start from scratch.
Preparing for the Future
If the diversity in end-use applications is any indictor, the acceptance of tactile feedback devices for by-wire systems will continue to grow in acceptance not only as a means to improve safety but also to improve operator comfort and convenience. However, since these components are safety critical, it is important to partner with a supplier that offers a variety of solutions as well as the resources and expertise to test and validate your application.
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