Steer-by-wire: the next generation of machine design

Learn some tips for designing safe, scalable steering for modern machines.

Contributed by David Schulte, Industry Market Manager—Agriculture For Parker Hannifin, and Jonathan Thomas, Market And Business Development Manager for Parker Lord

In today’s highly competitive market, OEMs are constantly looking for new technologies, approaches and products that can significantly reduce costs, support faster vehicle launches and provide them with a competitive edge. Steer-by-wire technologies are creating a lot of excitement and attracting interest because they can do all that, plus more, including serving as a key investment toward safe “semi-autonomous operation.”

Evolution of steer-by-wire technology

In the past century, the off-highway market has seen many different approaches to steering, especially in figuring out the best way to steer large equipment. Early machines started with mechanical steering systems consisting of gears and linkages that required a lot of human effort to steer the machine.

A major change occurred in the 1950s when hydraulic power steering was introduced, effectively launching a new era for steering design that continues today for different types of off-highway machinery.

These conventional hydraulic steering systems typically consist of a hydraulic orbital valve that regulates the flow of hydraulic fluid directly to the steering cylinders which turn the vehicle. A newer approach is a hybrid type of system which includes the addition of an electric valve that receives a signal from a GPS system and runs in parallel with the conventional system.

Most recently is the migration to steer-by-wire technology which removes the hydraulics from the cab altogether such that steering signals are communicated exclusively “by wire.” These systems consist of several critical components, including a steering input device (typically either a joystick or a steering wheel) which sends a steering signal to a controller (typically the vehicle controller). Then the controller commands some sort of output device (possibly an electric actuator, though typically an electrohydraulic valve) as hydraulic cylinders still remain a practical method of controlling steering axles.

Electrohydraulic valves used in steer-by-wire systems often need to comply with functional safety regulations and may include diagnostics and redundant controls. These were not as prevalent in traditional hydraulic steering systems. Many off-highway OEMs are choosing fully redundant dual channel steering systems that allow an operator to continue to steer even when a fault is detected.

Parker’s experience with steer-by-wire dates back to the late 1990s, when the company supported forklift manufacturers’ efforts to shift from gas/diesel engines to fully electric powertrains and, concurrently, from mechanical/hydraulic steering systems to fully electric steer-by-wire systems. Since that time, steer-by-wire technology has been deployed in other markets beyond material handling, including turf, agriculture, mining, precision construction, marine, and for an array of niche off-highway vehicles.

Why the interest in steer-by-wire?

A primary driver of increased steer-by-wire adoption is the off-highway market’s interest in fully autonomous machines in order to automate a process for optimal performance, efficiency and reliability; and to reduce the reliance on costly highly skilled human labor. Until fully autonomous machines become more viable, many OEMs are investigating the feasibility of automating specific machine functions, including steering. Steer-by-wire allows for software-based systems to directly control the steering, with those inputs typically coming from a GPS signal instead of the operator.

Greater machine design flexibility is another goal. By removing the mechanical steering column and orbital valve, the operator has better line of sight. Plus, space is cleared within the cab to deploy more flexible cab interiors, new operator control designs, or even things like flexible 180-degree swivel seating.

By disconnecting the steering wheel from the mechanical system, steer-by-wire systems tend to isolate the operator from jolts, noise and vibration, which can reduce operator fatigue over long shifts. It also allows OEMs to select between various steering input devices to suit their target market (i.e. steering wheel versus joystick), or to even relocate the steering wheel to the seat armrest, for example.

Also, with steer-by-wire, OEMs can more easily design a base steering system architecture that can be used across several machine platforms and then fine-tune each platform with its own steering performance or steering identity via software changes. This approach could significantly reduce the engineering development time for future vehicles and allow OEMs to better match steering performance with customer expectations.

Of course, cost is always a consideration when deciding whether or not to change technologies. With steer-by-wire systems, there are typically fewer components — especially a lot less hydraulic hose routing — which results in a lower assembly cost and, potentially, lower warranty costs for hydraulic-related leaks.

The right architecture for safety

Since the level of functional safety varies with these types of systems, two different steer-by-wire system architectures have evolved. The first one could be referred to as a “fail-safe” system. When a failure is detected, the vehicle alerts the operator with some kind of warning and then safely slows the machine to a stop so that repairs can be made.

The other is a “fail-functional” system, where two full channels of independent and redundant systems exist. In the event of a failure, the vehicle again alerts the operator that something’s wrong, but it’s still possible to steer the vehicle.

The category architecture defines, among other things, the input, the logic device, and the output. Common questions to ask when deciding on a particular architecture address whether or not you need redundancy (which is commonly used to ensure a fail-functional architecture), and do you need a certain level of diagnostics and sensors associated with them.

Since steer-by-wire systems do not have a direct mechanical connection between the driver and the front wheels during normal operation, a key component of the functional safety concept is that the driver retains a minimum level of steering capability following any fault in the system.

There are multiple standards that specify the requirements for the functional safety of steer-by-wire systems. Safety standards IEC 61508 and ISO13849 are two prominent functional safety standards. They provide the safeguards to mitigate risks methodically and transparently, establishing trust with regulators and customers. Among other things, the standard provides functional safety guidelines for the design, deployment, maintenance and application of automatic protection systems. It is applied to safety-related systems that use electrical, electronic or programmable electronic devices.

Key features of IEC 61508 include safety integrity levels (sils) which indicate how well a system meets its safety functions. Another feature is hazard analysis which evaluates potential hazards and the likelihood of their occurrence.

IEC 61508 also defines software-related requirements based on SILs, including techniques for verification and development. Under these requirements, it’s important to document what the system should do in detailed software specifications and then document the test procedure and the fact that the test procedure was tested. Finally, confirm that what has been validated matches the requirements.

ISO 13849 also provides for a method of determining risk that is related to a SIL rating. While not specific to steering, many OEMs utilize ISO13849 because it defines requirements for safety-related control systems. Most designers would place steering in this category. The standard helps designers in selecting architectures, component robustness and diagnostic strategies for matching system design with appropriate levels of performance determined by the risk assessment.

Customizing the operator’s steering experience

Some early adopters of steer-by-wire technology initially complained about the overall steering feel for operators since the steering wheel is no longer mechanically or hydraulically connected to the system. The steering wheel would continuously spin without ever reaching an end stop, and the steering resistance was constant for all operating conditions leaving operators feeling disconnected from the vehicle and, ultimately, hindering their productivity. However, over the last 25+ years, input devices have evolved to reproduce controllable tactile feedback that simulates the feel of legacy hydraulic systems while remaining stable, safe, and quiet.

To recreate this tactile feedback, most equipment manufacturers have deployed “resistive feedback” control using an electric rotary brake. The steering resistance can be controlled in real time by modulating current to the rotary brake to produce a specific torque resistance, which can be customized for unique operating conditions. The rotary brake can be tuned to enhance the human-to-machine interface (HMI) in ways that would otherwise be challenging to implement in legacy steering systems.

For example:

• Adjusting the steering resistance for different operating conditions, including higher sensitivity for precise, low-speed maneuvering, and lower sensitivity for stable, high-speed travel.
• Adjusting the steering ratio (i.e. turns lock-to-lock) for different operating functions.
• Adjusting the steering resistance with the steering cylinder pressure to account for potential latency in the downstream hydraulic circuits.
• Creating detents during steering wheel motion to inform the operator of specific steering wheel positions, or potential vehicle faults/warnings.

Parker produces a steering input device called a Tactile Feedback Device (tfd) that leverages the company’s proprietary magneto-rheological (mr) fluid technology, which is a type of smart fluid that, when subjected to a magnetic field, greatly increases its sheer strength to the point of becoming a semi-solid. The properties of the MR fluid in the active (“on”) state can be controlled very quickly and accurately by varying the magnetic field intensity with an electro-magnet. As a result, electrical current to the TFD can be modulated to change the properties of the MR Fluid and alter the steering resistance in real time, as well as to simulate end stops at full left and full right of a steering turn.

Parker is also currently developing a rotary input device known as a Force Feedback Device, or FFD, which adds a small 2 to 3 Nm servo motor to a 12 to 16 Nm MR rotary brake, along with the steering sensors and integrated controls. This product offers an automotive-like steering feel for an industrial vehicle. The motor is direct drive, so there’s no gear mesh noise or backlash. Using a small motor also significantly helps with the functional safety analysis, because a large motor could inadvertently fail and overpower the operator.

Conclusion

Steering a tractor is very different from steering forklifts. That’s why there is no one-size-fits-all design when converting to steer-by-wire. The type of vehicle, application and performance requirements, as well as the necessary level of functional safety, must all be taken into consideration.

Steer-by-wire systems are gaining in popularity because of their design flexibility, operator comfort, increased safety and lower overall costs. But safety concerns persist due to their autonomous nature. These newer systems must meet multiple safety standards, and have redundant warning systems to ensure machine failures are flagged in advance to minimize user and OEM liability.

Parker Hannifin
parker.com