Drives And Motors In Automotive Test Rigs

Test rigs are used extensively in the automotive and aerospace industries, testing a variety of vehicles. Andy Pye looks at how drives & motors are used within these systems.

Typical applications in the automotive industry for which drive systems are used include 

• Rolling road test stands for the car and motorcycle industries
• Engine test stands – for the production of electric and combustion engines
• Transmission/Gearbox test stands for suppliers to the automobile and machine manufacturing industry
• Electric vehicle test stands – with DC supply to power the vehicle inverter 

Automotive manufacturers often randomly select engines from their production lines and subject them to rigorous testing to confirm that they meet all of the key design requirements and parameters.

In addition to in-house testing by car manufacturers and first tier suppliers, automotive test rigs are also used by specialist test houses. For example, Horiba Mira has extensive experience in providing test engineering services to the global automotive industry, testing to regulations and standards, to client-specific requirements and in the development of appropriate test procedures and methods. This organisation boasts no less than 37 major test facilities, including a comprehensive set of safety laboratories (crash, impact simulation, pedestrian); vehicle and component environmental facilities; a full-scale aerodynamic wind tunnel; vehicle and component EMC (electromagnetic compatibility); and component and structures test labs.

AC variable speed drives are very well suited to test rig applications, as they can be used to simulate real conditions in a highly dynamic, accurate, linear and repeatable way. They are very responsive and can stop and start quickly to replicate high speed events.

Regenerative drives are often used, so that the drive is able to regenerate electric power when the motor is absorbing energy or providing a load, returning this energy to the supply and reducing operational costs.

All drives used in these applications should be optimised for energy efficiency and high control precision, with low ripple voltage and current. Excellent torque response is vital, because drives are often used as torque amplifiers, so the time from the torque reference to the torque on the motor shaft needs to be minimised – the more closely the actual torque follows the torque reference, the easier it is for the control system to control and therefore the better the overall test rig performance.

The chassis dynamometer

The most common types of automotive test rig are chassis dynamometers, engine dynamometers and transmission rigs. Chassis dynamometer testing usually employs one drive and motor per test unit axle or wheel, depending on the type of testing being performed.

CP Engineering manufactures engine and chassis dynamometer test systems, transmission test systems and other test rigs for the automotive industry. It has supplied systems to many leading firms in this sector, including Castrol, Cosworth Technology, Delphi and Shell.

CP’s ‘Cadet’ Windows NT based control and datalogging system’s analogue interface is synchronised to the drive’s closed loop vector control. The test systems require real time control and processing that is precisely synchronised to give the same load/speed profile as a real vehicle. The response of the control loop must thus remain consistently within a given cycle time – typically 3.25ms. The drive also has to be capable of motoring to simulate over-run conditions.

Cranking software provides protection for the drive shaft. As the engine fires and accelerates, the drive switches to zero torque to simulate engine idling. Apparently, this cannot be done with conventional dynamometers and gives CP a competitive edge.

Rolling road and brake test stands

Here, different test profiles and tracks are simulated and pre-programmed to reproduce driving resistances as realistically as possible, including braking, start-up or driving round bends, cross-country and off-roading. Internal and safety-related vehicle functions such as Anti-lock braking systems (ABS) and Electric power steering (EPS) are also tested. Through fast-response compensation, highly accurate and reproducible measurements are obtained by taking into account the friction, electrical and thermal dependencies, and moments of inertia over the entire powertrain.

Engine test stands

Drive systems are used in engine test stands, both in development centres and in motor manufacturing. As with any test stand, the key is to accurately simulate everyday operating conditions.

There are specific requirements for the quality assessment of combustion and electric engines, such as different test patterns and speeds, torque and counter torque cycles, endurance tests or short-term loads.

Frequency converters can accurately create the required torque curve, while also recycling the energy generated in the combustion engine, thereby sparing the electrical mains supply from sinusoidal mains currents.

Test engineers can also subject the test sample to specific speeds and torques that expose resonances and technological limits.

Transmission/Gearbox test stands

Here, the torque pulses and running characteristics of an internal combustion engine are applied to the transmission/gearbox being tested. By networking all drive controllers on real-time Ethernet, the necessary synchronization of the inverter current and speed control circuits ensures that the test results reflect real conditions. This avoids the need for unwanted balancing adjustments in the control system. Power electronics serve as input and output drives for a wide variety of transmission/gearbox types. Four load machines replace the wheel/road system and represent the driving profile, while an input drive simulates the internal combustion engine.

Engine Torque Pulsation Simulation (ETPS) recreates the internal combustion engine on a development test stand. To meet the high requirements for this type of test stand, low mass inertia permanent-magnet synchronous motors and asynchronous motors are used.

Testing equipment for electric vehicles

New developments require new testing techniques. The power train on hybrid and electrically powered vehicles comprising the vehicle inverter, motor/engine and transmission/gearbox, can be tested as a complete system, all connected in a common DC bus configuration which allows regenerated power (during braking for instance) to be re-circulated.

For a given installed motor power, this means that the grid connected inverter’s power rating is minimised, saving capital as well as energy costs. A key feature of the system is that all control loops are synchronised which significantly reduces the risk of system resonances.

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