A New Cornerstone

The appearance of Electrically Assisted Steering systems on motor vehicles has created new challenges for automotive test rig manufacturers

by Simon McBeath

The steering gear on a vehicle is fundamental, safety-critical equipment. Simple cyclic durability testing is no longer sufficient as more, increasingly demanding test scenarios have become prevalent. Product validation, durability, characterization, and quality control testing are now required, as well as a number of specific protocols such as life testing, which combines physical and environmental loading. These complex requirements demand flexibility in the design of test equipment.

Electrically Assisted Steering (EAS) is a relatively recent development. The different packaging and functionality of EAS systems, compared to their hydraulic predecessors (and electrohydraulic counterparts), has required new approaches to the design and construction of appropriate test rigs. IST's background in servohydraulic test equipment, digital controllers, and test software inevitably placed it in a prime position to provide rigs for these new steering systems.

New problems, flexible solutions: Most EAS systems are installed in-line with the vehicle's steering column. But whatever the system's particular installation might be, an appropriate test rig must be capable of reproducing the geometry of that installation, for example, to test lubrication of mechanical components in the EAS system. It is also necessary to test the steering rack and tie-rods in their respective installation geometries to simulate in-vehicle conditions.

Rigs must permit the required degrees of freedom and reproduce realistic load conditions; wide-ranging environmental conditioning must accurately represent vehicle operation across diverse worldwide markets; and control software must enable the implementation of varied test regimes.

As Dr Chris Berg, TRW Automotive Chassis Systems (EAS) test manager, remarks; "The test rig from IST had to accommodate not just known business, but potential business as well."

Not only is the packaging of EAS systems different to traditional hydraulic PAS systems, its functionality is also different. Instead of an engine-driven pump providing continuous hydraulic pressure to assist steering effort, EAS systems use an electric motor to provide steering torque-amplification on demand and in response to the angle and speed of steering input (and other inputs such as vehicle speed and driver preference).
Therefore, an EAS test rig has to: provide power to turn the steering (via a hydraulic motor); measure input and output torque (via transducers); measure steering angle, and the rate of change of that angle (via an encoder); and measure loads in the tie-rods (via in-line load cells) generated by the movements of hydraulic actuators. Therefore, both driver and road loads can be simulated.

"One of the problems associated with hydraulic servo actuators used for testing EAS systems is their inherent mass and friction," explains IST's Bob Sheppard. "We use geometric corrections enabled by our Labtronic 8800 Controller to minimize load errors. We also use iteration software to improve tie-rod load signals." The steering actuator bellcranks can also be mounted on very low friction bearings.

IST's EAS test rigs can be run at different suspension positions, by raising and lowering the steering actuators, or can include dynamic jounce (wheels oscillating up and down) via controlled vertical hydraulic actuators. Rigs must also allow for forward or rearward steering rack locations (i.e. ahead of or behind the wheel axle line), left- or right-hand drive and also for different steering column angles (those existing between a coupé and an MPV).

Steering racks are frequently housed on separate mountings or subframes to prevent, or at least to reduce, road vibrations being transmitted through the teering. Nevertheless, EAS systems were found to be subject to end-loads arising from these vibrations. As a result, a further axis of movement can therefore be incorporated into EAS test rigs with the addition of another hydraulic actuator designed to simulate 'rack wobble'.

Inclement conditions

Depending on the vehicle, EAS systems may either be installed within the driver/passenger compartment, or if space is at a premium, down on the pinion within the forward compartment. If packaging issues dictate the latter location then the EAS, like other components in the underhood region, will have to endure a harsh environment. The major environmental influences are likely to be heat (depending on the geographical region of operation) and moisture, but other factors include salt and grit. IST's latest EAS test rigs allow the inclusion of environmental chambers to further increase the scope of testing that can be performed, and include subjection to all the above environmental factors.

Instron SFL, a UK subsidiary of Instron, specializes in the manufacture of furnaces and environmental systems. It was therefore a logical and convenient partner to create integrated environmental test chambers for IST's EAS test rigs. "Automotive customers are very demanding," explains Instron SFL's Steve Horrex. "Performance is the key with these test systems. There are tight specifications on temperature, its control, and on heating and cooling rates. Tests also proceed over long durations, so the equipment has to be fully automated. Humidity control is quite demanding, and salt and grit sprays are also difficult to control and keep reliable." Environmental chambers control is provided by dedicated software.

Variations on a theme

Some of the more specific tests that EAS rigs are required to perform include varieties of impact test. For example, it is possible to simulate a kerb-strike, either by propelling a weight at the tie-rod end of the steering system, or providing a more effective simulation by using real-time data from the vehicle while measuring how the EAS system is affected. Such a test, when simulating a kerb-strike at speed, generates large forces, quite different to those encountered in normal use.

It is also possible to measure what happens when the steering is spun rapidly against its end-stop. This type of test requires a high rate of data acquisition if short-lived load peaks are not to be missed. And 'wind-up' tests, which simulate what happens when the steering is turned whilst a road wheel is up against a kerb, can also be performed.

Clearly, control systems and bespoke software play a major role in these steering trends, as in any type of similar testing, and IST's versatile Labtronic 8800 Controller, RS LabSite^® software suite and dedicated 'Steer Test' software are the company's response. Real-time track load data can be replayed, or block-cycle fatigue tests can be run, and data acquisition and analysis tools are included.

Future trends: Philip Vere, vice-president sales and marketing, IST, described EAS test rig usage; "EAS systems are safety-critical components, so early validation and abuse tests have to be done. The next step is durability, which is currently one of IST's main applications for steering gear testing, using track data and looking at accumulated damage and overall performance.

"Rigs are also available for product characterization, looking for instance at distortions, the effects of different levels of power assistance and control settings. From this characterization data, the steering manufacturers can assess performance and quality, and attempt to interpret this data as 'feel', at which point the testing is becoming increasingly subjective. An ongoing development is to simulate different steering setups."

Conclusion

EAS systems have so far been fitted to a few small, low-powered vehicles. However, forthcoming changes to vehicle electrical systems, together with other inherent benefits, will see a proliferation of EAS systems in the near future. This, in turn, will lead to a demand for test rigs for R&D and quality control.

Published with kind permission of Testing Technology International May 2001.