Test stand delivers

Dr Rainer Müller, Schenck Pegasus GmbH

The unique flexibility of a range of test stands from a German company enables them to provide the right solution for test applications at the end of the automotive production line.

Combined roller/brake/ABS test stands are used to check the general functionality and electrical control unit (ECU) parametrisation of front-, rear- and four-wheel-drive vehicles - large and small - at the end of a car assembly line.

The vehicle mass is simulated on the test stand by rotating roller sets that can be coupled with each the other. This enables different driving conditions, such as acceleration, deceleration and braking, to be simulated under road conditions. The combination of roller and brake testing reduces testing time and saves space. Thanks to automated test sequences, repeatable results can be obtained. Test sequences can be matched to specific applications.

X-road test stand
In the past few years, the demands made of roller test stands have gone sky-high: today, features like ABS, ASR and ESP all have to be checked at the end of the production line. While the ever-increasing complexity of testing also increases the complexity of the test stand, there is a contradictory demand for higher availability and improved serviceability in test stands. Schenck Pegasus has responded to these demands with the development of the flexible x-road test stand, which has the widest applicability while being far less complex than the classical test stand.

The flexibility of the new roller/brake/ABS test stand is due to the absence of mechanical components for the coupling of the roller sets. No coupling elements, disc brakes and flat belt drives have been used. Every single roller set is driven, stopped and disengaged by a separate motor controlled by a control unit with specially designed software. This type of control ensures the high dynamics of the test stand. In addition, the roller speed is controlled with the utmost accuracy. The integral decentralised measuring system computes the braking force with a high degree of precision, so that there is no need for a separate measuring device. Self-diagnostic capabilities and early fault recognition are additional advantages of this novel system.

The x-road system has several features that make it ideally suited to meet the demand for flexibility:

With the use of the roller system, the vehicle wheels can be coupled and decoupled at will (for speed synchronisation).
Roller speed and thus wheel speed can be controlled on every wheel.
Braking forces can be determined individually on every wheel.
Defined driving or loading of single wheels is possible.
The x-road test stand is designed to simulate any driving condition required and report the current status by feedback signal. When designing the test stand, its ability to adjust to various test requirements was an important consideration.

Excellent flexibility
The flexible control capabilities of the test stand enable different vehicle types to be tested on the same test stand. Different driving cycles can be realised through a variable load characteristic for warming-up, uphill and downhill drives. Varied generation of speeds or loads on individual wheels - for sensor testing, for example - is also possible. The adjustment of the system to new testing requirements has been well taken care of by incorporating a high degree of module standardisation. Schenck Pegasus is thus able to offer a standard unit the functionality of which can be expanded using additional modules. The latest example of the extended applicability of the system is the company's test stand variant equipped with different roller systems.

As a rule, two different types of roller system are available: the double roller and the single roller. With the double roller, the single vehicle wheel is positioned between two rollers of 500mm diameter. The two rollers keep the single wheel in place and protect the vehicle from the impact of static and dynamic longitudinal forces during testing. On the single roller, the wheel stands on the apex of a single big roller of 900mm diameter, as under road conditions. Two small retaining rollers, placed directly before and behind the wheel, prevent the vehicle from driving off the test stand.

The x-road base unit can be supplied as a DR variant equipped with four double roller sets. This variant covers almost any standard application:

Uphill and downhill driving at definable, varying loads
Infinitely varied simulated vehicle masses
All-wheel drive tests and definable load states
Static and dynamic braking force measurement
Testing of all ABS sensors in one step
In addition, the double rollers of the DR variant also permit braking values of up to 80 per cent of the maximum braking force related to vehicle weight to be tested.

Single and double roller variants
In response to a special request from the automotive industry, x-road has been re-designed for the integration of a single roller system. Thanks to its modular design and the total mechanical decoupling of the single roller sets, Schenck Pegasus has been able to comply with this request: x-road is now available as an SR variant with four single roller sets and as an XR variant with combined single and double rollers.

The demand for a double roller system cropped up when a carmaker decided to use its test stand for both line-end testing of braking and driving manoeuvres and also for the checking of the acoustics. The noise inside and outside the vehicle is checked in order to localise abnormal noise. One short test, and out-of-tolerance components are promptly identified. With this type of test, the single roller can play to its strengths: the average noise level is reduced by approximately 3dbA. With a double roller, software filters have to eliminate the louder noise frequencies of the test stand when running an acoustic check.

To safely keep the vehicle in place, certain tricks have been employed. When the vehicle drives onto the roller, its front wheels contact the front retaining roller. This movement causes the rear retaining roller to fold up so that the front wheel is safely secured. The contact pressure of the retaining rollers is reduced for optimal retention and the gentlest tyre handling. The special spring-loaded design of the retaining rollers ensures permanent contact between tyre and retaining roller, allowing the customer to perform the tests at a maximum speed of 200km/hr - although, on request, even higher speeds are possible.

With the single roller, the slip of the roller under the tyre occurs earlier than with double rollers. The amount of the braking force that can be transferred to the roller along with the tyre is limited to 50 per cent - related to the vehicle weight - which is only 60 per cent of the braking force that can be transferred to a double roller without slipping. This is also the case in acceleration conditions. As a consequence, the better suitability of the single roller test stand for acoustic checks is matched by the superior performance of double roller test stands in dynamic uphill and downhill drive simulation.

The third x-road variant, the XR type, combines a single roller set for the front wheels with a double roller set for the rear wheels. This variant has performed well in acoustic checks, since this kind of check is primarily performed on the front vehicle-accommodating motor, gearbox and drive (when testing a front-wheel-drive vehicle), so that the relevant noise generators, the front wheels, are positioned on the silent roller set. Since the rear wheels are safely retained on the double roller, the front retaining rollers can be folded away for acoustic checks, which further reduces the running noise.

Another advantage of the rear double roller is that vehicle fixation is speeded up, since the exactly defined position of the rear wheels automatically determines the optimal contact point of the front wheels. This helps increase vehicle throughput whilst reducing the noise level.

The versatility and rich modularity of the x-road family of test stands offers the right solution for any application at a superb price/performance ratio.

Author

Dr Rainer Müller has a PhD from the department of mechanical engineering at the RWTH-Aachen, Germany, where his research interests included many areas of robotic research, including kinematic and dynamic parameter identification as well as trajectory planning and generation. Dr M竫ler started his career at Schenck Pegasus in 1996, where he worked in the developmental department and was responsible for the product management of assembly systems and for the development of roll/brake/ABS test stands. Since 2000, Dr Müller has been vice-president of Schenck Pegasus GmbH.
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