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FEATURE MOTION CONTROL


Royal transportation research project reaches new efficiency levels


Given the ease with which we can manoeuvre vehicles around winding roads and tight corners, it may seem that there is little room for improvement of modern power steering systems. However, with the help of Thomson Industries, engineers at Sweden’s KTH Royal Institute of Technology and Integrated Transport Research Lab (ITRL), think otherwise


Also, whenever a pothole or other


trauma throws it off, adjustment of the camber, the vertical alignment of the wheel in relation to the car, must be done with special equipment. If this could be done more easily, even automatically, it could save time and money, makes for a smoother drive with less tyre wear and increased lateral grip.


K


TH and ITRL are working to determine whether replacing


conventional hydraulic steering technology with compact electromechanical technology could bring performance, cost, maintenance, sustainability and flexibility advantages.


WHAT’S WRONG WITH TODAY’S STEERING? The power steering system in most cars today works well up to a point but has many limitations. For one, the steering post takes up a significant amount of space and adds weight to a mechanical system. If the steering system is damaged in a collision, cost of repair or replacement is a major expense. Hydraulic steering systems also require regular maintenance. Energy efficiency is also an issue. Peter


Georén, director of ITRL, says: “The traditional steering system is powered by direct connection to the engine, so when idle, you are giving up power to the systems, but when you drive at high revs, you over-power.” Another limitation of the conventional steering rack is that only the front wheels turn, and do so with limited degrees of movement. This, for example, makes squeezing into a tight parking space challenging. It would be much more efficient if a car could enter a parking spot perpendicularly.


18 FEBRUARY 2017 | DESIGN SOLUTIONS


The Thomson actuator modification was able to deliver a high- powered system that produced twice the force of its predecessor


REPLACING TUBING WITH WIRES Traditionally, pressurised fluid delivered via hydraulic tubing provides the steering force. Whereas, in the ITRL’s research concept vehicle (RCV), electric wires replace hydraulic tubing and activate the small motor built into to the actuator housing. Drivers control the electrical signals using a steering wheel, but this works more like the controller of a video game than the conventional post and tubing system. To equip the system for testing, the


ITRL team determined that the RCV would need eight actuators, two on each wheel, one for controlling the steering and another for controlling the camber. Finding the right linear actuator took some time and only Thomson Industries could meet all of the required specifications. The company supplied four high-speed Max Jac MX24- B8M10E0 actuators to perform steering functions, and four Pro Series actuators to change the wheel camber inclination. As expected, the new electric linear actuators were more responsive than the hydraulic actuators because of the speed with which electricity travels across the wire. The digital control, combined with the specially engineered steering architecture, provided a level of flexibility not possible with conventional actuation. It is this capability, for example, that could replace parallel parking with perpendicular parking, or enable automatic realignment to reduce rolling resistance after hitting a pothole.


TRIAL BY FIRE The testing regiment involved running the car with full load and making sustained turns. Static turning puts more


stress on the system than turning while the car is in motion, and it wasn’t long before the windings on the actuator motors overheated and began to burn. Using different actuators was not an


option as it had already been established that only the Thomson actuators could meet the required specifications. Attention turned to the stock motor that came with the actuator. The motor was not designed for conditions as extreme as those used in the ITRL test. Georén and his team tested the idea that a motor wound differently might deliver the required performance. Under a load of 1,080N, the alternative


motor used required only 2.8A of current to achieve the required torque, keeping it well within its thermal limits. KTH intern Bruno Verde calculated that the motor would reach a steady temperature of 80°C at the winding when consuming 2.7A constantly, which was well with in specification. This confirmed the feasibility of using the alternative motor for the steer-by-wire application.


GOING FORWARD The actuator modification delivered a high-powered system that met the improved performance targets and produced twice the force of its predecessor, as well as greater responsiveness without overheating. The researchers believed that even higher forces (200N at five amps, for example) were possible for short bursts. However, more extensive testing and analysis of the system would be needed to determine the mechanical safe limits. Georén estimates: “It could be five years before steer-by-wire technology actually hits the street, with the first instances likely to be in industrial applications in which high manoeuvrability is important, such as forklifts and cargo carriers. “Nonetheless, Thomson actuators have contributed to the creation of an important milestone.”


Thomson Industries www.thomsonlinear.com


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