Hardware, software for dynamic electric vehicle (EV) testing

All photos courtesy of Siemens

The shift to eMobility in the automotive industry has changed many test methodologies in use, with heightened attention to energy efficiency and solutions for providing clean, manageable, and predictable DC voltage.

In the macro view of the end-of-line test stand development for the emerging electric vehicle (EV) sector, four key areas emerge – dynamometer drive systems, DC supply, engineering challenges, and overall machine safety.

The core of the dynamometer drive system centers around a variable frequency drive paired with a high-performance drive motor. Second, the test stand requires a dynamic, configurable DC power supply to simulate the battery. Third, using a single engineering platform, all the aspects of the programmable logic controller (PLC), human-machine interface (HMI), motion, safety, and security factors come together for project planning, machine commissioning, and system programming. Finally, the fourth concern of overall machine safety will look to integrate safety concepts over a network without additional, external hardware.

In an example test stand application for EV powertrain systems, we are confronted with the need to test a hybrid transmission, electric transmission, battery or fuel cell, and motor, as well as differential gearing and transfer case. For roller rig and chassis dynamometers in the test lab, electric or hybrid powertrains present new challenges in performance and noise, vibration, and harshness (NVH). Conventional cold and hot engine test stands remain in use, as do brake, drive shaft, component, and fatigue test stations plus balancing, tire, and steering test and evaluation systems.

Motors

New test stands can include a range of high-performance drive motors for higher speed options, often up to 40,000rpm, plus a variety of torque motors for evaluations at low speeds, and linear motors for vibration testing components. When linear motors are configured in parallel or in a series, the force is nearly limitless, while achieving extreme dynamic accelerations and velocities. Such motors also offer substantial energy efficiencies compared to conventional fluid power systems.

In addition, a conventional internal combustion engine runs red lines at 8,000rpm to 9,000rpm, while EV powertrains typically run with much smaller gear ratios, so the gears and drive shafts are running at much higher speeds. This condition requires heavy-duty spindle motors, which often run >40,000rpm.

Mating drive

After selecting motors, address the mating drive design. Often, applications demand power ratings ranging from 0.12kW to 1MW with clean, efficient energy regeneration capability. A higher degree of drive dynamics with corresponding motor speed/torque control features is required, as is a high degree of scalability and flexibility in the design. And, with the need to capture and transmit substantial amounts of data, multiple communication options in Profinet, Ethernet IP, and EtherCAT protocols are possible. Test stand developers require drives that can be engineered, parameterized, and commissioned with standard software and protocols to minimize build times, simplify user startup, and reduce time to market.

The best drive systems today offer current controllers clock cycles capable of reaching 31.25ms and pulse frequencies up to 32kHz for superior torque, force, and speed control.

Safety concerns in new test stand development projects benefit from corresponding and ongoing improvements in drive technology seen on gantries, numerous other materials handling applications, machine tools, and robotics. All have the common goal of protecting people and equipment. Drive integrated safety functions must ensure safety in the event of an emergency stop (see sidebar, bottom of the page).

Another emerging solution in drive design is cogging torque compensation for torque or linear motors, which offsets the magnet’s tendency to follow the next coil in very low speed applications. The drive learns the phase sequencing and identifies the onset of cogging. This feature permits higher performance and optimal test results at various speeds. The drive can provide a constant torque at lower speeds plus the reduction of speed ripple in synchronous or asynchronous motors. Through a pre-controller in the drive, adjustable current is provided to eliminate cogging.

A set-point generator, integrated in today’s advanced drives, consists of three signal generators – sine-wave, saw tooth, square-wave – which pulse the speed or torque based on a defined waveform. Inputting the amplitude, frequency, and any offsets allow the drive function to achieve high resolution, even at high speeds. Applications for this feature typically include balancing, tire, and wheel tests.

Simulation

A further development in dynamic speed and torque simulation has emerged. This can generate a complex, pre-defined waveform and pulse the speed or torque set-points with resolution up to 10,000 interpolation points per cycle. Duplication of pre-defined waveforms improves test procedure outcomes for various engine components, transmission, and drivelines. This new function can replicate realistic testing conditions with excellent torque precision and faster set-point function.

Learning error compensation allows the testing procedure to mute disturbances within the part. Usually learned after four-to-six cycles, this automatic disturbance compensation functions in two modes: learn and capture, as well as continuous learn and compensate. Optimal bad engine recognition is the chief benefit here, as the drive function learns quickly, which is especially useful on new engine and driveline tests. The function is available on the fly, so disturbance control can monitor a consistent factor or be disabled for other aspects of the test.

As electric motors have much higher rotation speed, often in the 20,000rpm-to-40,000rpm range, there are new challenges for achieving much higher pulse frequencies in the drive system. As higher frequencies occur, the amount of current produced by the drive system is derated. This might normally favor large capacity drives, but through a servo coupling function, multiple motor modules of lower individual amperage can be used without derating current at higher pulse frequencies. This function also eliminates encoder splitters, as the speed controller with the encoder connection is positioned only on the master axis.

Communications

Higher-level drive systems require higher-performing communications systems for data logging and external control. An isochronous Profinet system for cycle times up to 250ms can achieve deterministic behavior without data collisions or jitter. Using ring topology for the higher-level equipment, this software provides users with reduced interface converters, no electromagnetic compatibility (EMC) influences, simple integration, higher performance due to more rapid memory access, and more consistent process data.

An EtherCAT interface is also available, which uses the CANopen over EtherCAT drive profile. It features an integrated web server function accessible through onboard Profinet interface, so different motion control modes of position and speed are possible. A two-port switch supports daisy chain topologies. User benefits include easy migration from CANbus to EtherCAT, user-defined web page flexibility, and easy wiring for synchronous operation on multiple axes applications.

eMobility-specific testing

EVs require a much higher DC power source on dynamic test systems. Testing electric drive units (EDU) requires a configurable, dynamic DC power source to emulate the battery. Energizing the EDU with proper voltage, 0 to 1,000VDC, powers the vehicle inverter and motor which turns the test article’s drive line. This provides mechanical power to the adjoining motors, which convert that mechanical energy into electric power and regenerate to the DC bus. The absence of filtering hardware is the result of a true DC-DC converter.

A battery test can be performed with Siemens Sinamics drives and motors with testing from 72VDC to 1,030VDC and 1,200A possible with clean energy regeneration satisfying IEEE519 standards. A more traditional roller rig, using regen energy provided by the shaft motors running on the vehicle’s wheels, produces constant torque with no ripple, improving the stiffness of the system to eliminate inaccuracies. Energy is exchanged via a common DC bus, and noise is minimized by variable switching frequency.

Other end-of-line, non-destructive testing and process monitoring systems are rapidly emerging as testing tools for electric motors and assemblies based on fault identification, resonance tests, breakage, and possible crack-detection monitoring.

Siemens Industry Inc. https://www.siemens.com

About the author: James Ellis is Motion Control Business development manager for Siemens Industry Inc. He can be reached at james.ellis@siemens.com.